14. ASSIGNMENT TOPICS WITH MATERIALSkgr.ac.in/beta/wp-content/uploads/2018/09/Thermal... · 2018-10-10 · If you are using the water cooled engine, ... Explain the principle of working

MANISH VYAS ASSISTANT PROFESSOR

14. ASSIGNMENT TOPICS WITH MATERIALS 1.What are the merits and demerits of two stroke I.C. engines over the four stroke I.C. engines? Answer:

4 STROKE ENGINE 2 STROKE ENGINE

4 stroke engine completes 2 rotations of crankshaft after complet ing one cycle.

2 stroke engine completes 1 rotation of crankshaft after complet ing one cycle.

Power is produced once every 4 strokes of the piston.

Power is produced once during 2 strokes of the piston.

Engine design is a bit complicated due to valve mechanism which is operated through gear & chain mechanism.

2 stroke engine has ports which makes it 's design simpler.

No need of adding oil or lubricant to fuel. Addit ion of oil is required.

Top side of the piston is flat. A bump or protuberance may be needed on top side of piston.

Mixture remains only in the combust ion chamber.

Air- fuel mixture enters through inlet port & travels to combust ion chamber pass ing through crankcase.

4 stroke engines are heavier. 2 stroke engines are lighter comparat ively.

4 stroke engines make less noise. 2 stroke engines are louder comparat ively.

2.Differentiate S.I. and C.I. engines. Answer: S.no Parameter SI Engine CI Engine

1. Definition It is an engine in which the spark is used to burn the fuel.

It is and engine in which heat of compressed air is used to burn the

fuel. 2. Fuel used Petrol is used as fuel. Diesel is used as fuel. 3. Operating cycle It operates on Otto cycle. It operates on Diesel cycle. 4. Compression ratio Low compression ratio. High compression ratio. 5. Thermal efficiency High thermal efficiency. Less thermal efficiency.

6. Method of ignition Spark plug is used to produce spark for the ignition.

Heat of compressed air is used for the ignition.


7. Engine Speed High speed engines. Low speed engines.

8. Pressure generated Low pressure is generated after combustion.

High pressure is generated after combustion.

9. Constant parameter during cycle Constant volume cycle. Constant pressure cycle.

10. Intake Air + fuel. Only air. 11. Weight of engine Si engine has less weight. CI engine are heavier. 12. Noise production It produces less noise. It produces more noise.

13. Production of hydrocarbon

Less Hydrocarbon is produced. More hydrocarbon is produced.

14. Starting Starting of SI engine is easy. Starting of CI engine is difficult. 15. Maintenance cost Low High 16. Vibration problem Less Very High 17. Cost of engine Less cost High cost

18. Volume to power ratio Less High

19. Fuel supply Carburetor Injector

20. application It is used in light commercial vehicles like motorcycle, cars

etc.

It is used in heavy duty vehicles likes bus, trucks, ships etc.

3.What is the need and requirement of cooling in I.C. engines? Answer: In addition to overheating, large temperature differences may lead to distortion of the engine components due to set up of thermal stresses. If the cooling system is not provided to an internal combustion engine, the lubricating oil film would break down and the lubricating oil will decompose to give gummy and carbon deposits.

In lack of cooling system, a complete seizure of the piston, bearing and other important parts will occur.This requires more frequent replacements of the components

.It will also increase the repairing cost and breakdown period.

It reduces engine life considerably.

The volumetric efficiency of the engine lowers with an increase in temperature.

This promotes pre-ignition and tendency of the engine to detonate.

Cooling can be achieved by Air or Water.


So, the two main properties desired of an efficient cooling system are, 01) It must be capable of removing only about 30% of the heat generated in the combustion chamber. Too much heat removal will lower the thermal efficiency of the engine.

02) It should remove heat at a fast rate when the engine is hot.

It is also required to be very slow cooling at the starting of the engine so that the different working parts of the internal combustion engine reach their operating temperature in a short time period. If you are using the water cooled engine, then are little chances of freezing of water in cold weather conditions.

It happens when we keep engine without use for very long time.

To overcome this problem, we have to mix anti freezers in cooling water. 4. Explain the principle of working of a four stroke S.I. engine with a neat sketch. Answer: The Four Stroke Petrol Engine uses a cycle of four strokes and petrol as the fuel. Each cycle includes 2 rotations of the crankshaft and four strokes, namely: 1.An Intake Stroke 2.A Compression Stroke 3.A Combustion Stroke also called Power Stroke 4.An Exhaust Stroke The steps involved are as follows: 1.Intake Stroke: As the name suggests in this stroke the intake of fuel takes place. When the engine starts, the piston descends to the cylinder's bottom from the top. Thus the pressure inside the cylinder reduces. Now the intake valve opens and the fuel and air mixture enters the cylinder. The valve then closes. 2.Compression Stroke: This stroke is known as compression stroke because the compression of the fuel mixture takes place at this stage. When the intake valve closes (exhaust valve is already closed), the piston forced back to the top of the cylinder and the fuel mixture gets compressed. The compression is around 1/8th of the original volume. An engine is considered more efficient if its compression ratio is higher.


3.Combustion/Power Stroke: Now in case of petrol engine when the fuel mixture compresses to the maximum value the spark plug produces spark which ignites the fuel mixture. The combustion leads to the production of high pressure gases. Due to this tremendous force the piston is driven back to the bottom of the cylinder. As the piston moves downwards, the crankshaft rotates which rotates the wheels of the vehicle.

4.Exhaust Stroke: As the wheel moves to the bottom the exhaust valve opens up and due to the

momentum gained by the wheel the piston is pushed back to the top of the cylinder. The gases due to combustion are hence expelled out of the cylinder into the atmosphere through the exhaust valve. The exhaust valve closes after the exhaust stroke and again the intake valve opens and the four strokes are repeated.

5. . Answer: The various lubrication systems used for lubricating the various parts of engine are classified as

1. Mist lubrication system 2. Wet sump lubrication system, and 3. Dry sump lubrication system.


1. Mist lubrication system: Mist lubrication system is a very simple type of lubrication. In this system, the small quantity of lubricating oil (usually 2 to 3%) is mixed with the fuel (preferably gasoline). The oil and fuel mixture is introduced through the carburetor. The gasoline vaporized and oil in the form of mist enters the cylinder via the crank base. The droplets of oil strike the crank base. The droplets of oil strike the crank base, lubricate the main and connecting rod bearings and the rest of the oil lubricates the piston, piston rings and cylinder. The system is preferred in two stroke engines where crank base lubrication is not required. In a two-stroke engine, the charge is partially compressed in a crank base, so it is not possible to have the oil in the crank base. This system is simple, low cost and maintenance free because it does not require any oil pump, filter, etc. However, it has certain serious disadvantages. Therefore, it is not popular among the lubrication system. Its disadvantages are the following:

1. During combustion in the engine, some lubricating oil also burnt and it causes heavy exhaust and forms deposits on the piston crown, exhaust port and exhaust system.

2. Since the lubricating oil comes in contact with acidic vapours produced during the combustion, it gets contaminated and may result in the corrosion of the bearings surface.

3. When the vehicle is moving downhill, the throttle is almost closed, and the engine suffers lack of lubrication as supply of fuel is less. It is a very serious drawback of this system.

4. There is no control over the supply of lubricating oil to the engine. In normal operating conditions, the two-stroke engines are always over-oiled. Thus consumption of oil is also more.

5. This system requires thorough mixing of oil and fuel prior to admission into the engine. It requires either separate mixing or use of some additives.

2. Wet-sump lubrication system: In the wet-sump lubrication system, the bottom of the crank case contains an oil pan or sump that serves as oil supply, oil storage tank and oil cooler. The oil dripping from the cylinders, bearings and other parts, fall under gravity back into the sump, from where it is picked up by pump and recirculated through the engine lubrication system. There are three varieties in wet-sump lubrication system. They are:

1. Splash lubrication system 2. Splash and pressure system and 3. Pressurized lubrication system.


2.1 Splash lubrication System: Splash lubrication system is used on small, stationary four-stroke engines. In this system, the cap of the big end bearing on the connecting rod is provided with a scoop which strikes and dips into the oil-filled through at every revolution of the crank shaft and oil is splashed all over the interior of crank case into the piston and over the exposed portion of the cylinder is shown in the figure below. A hole is drilled through the connecting rod cap

through which the oil passes to the bearing surface. Oil pockets are provided to catch the splashed oil over all the main bearings and also the cam shaft bearings. From these pockets oil passes to the bearings through drilled hole. The surplus oil dripping from the cylinder flows back to the oil sump in the crank case.

2.2 Splash and pressure lubrication system: Splash and pressure lubrication system is combination of splash and pressure system as shown in below figure. In this system, the lubricating oil is supplied by a pump under pressure to main and cam shaft bearings. the oil is also directed in the form of spray from nozzle or splashed by a scoop or dipper on the big end to lubricate bearings at the big end of the connecting rod, crank pin, gudgeon pin, piston rings and cylinder.

2.3 Pressurized lubrication system:

In pressurized lubrication system, the lubricating oil is supplied by a pump under pressure to all parts requiring lubrication as shown in below figure. The oil under the pressure is supplied to main bearings of the crank shaft and camshaft. Holes drilled through the main crank shaft bearings journals, communicate oil to big end bearing and small end bearings through the hole drilled in the connecting rod. a pressure gauge is provided to


confirm the circulation of oil to various parts. This system provides sufficient lubrication to all parts and is favored by most of the engine manufacturers. This is used in most heavy duty and high-speed engines 3. Dry-sump lubrication system: In dry-sump lubrication system, the oil supply is carried from an external tank. The oil from the sump is pumped by means of a scavenging pump through filters to the external storage tank. the oil from the storage tank is pumped to engine cylinder through and oil cooler. The oil pressure may vary from 3 to 8 bars. The dry-sump lubrication system is generally used for heavy-duty engines. 6. Explain the principle of working of a battery ignition system with a neat sketch. Answer: Working of Battery Ignition System: In the battery ignition system ignition coil stores the energy in form of magnetic field and deliver it at the instant of ignition, in form of high voltage current with high tension wire to correct spark plug. The diagram of four cylinder battery ignition system is as follow.

First low voltage current flow form

battery to the primary coil through ignition switch and ballast resistor.

Ballast resistor regulates the temperature of ignition coil by regulating current passing form it.

The ignition capacitor connected in parallel with contact breaker. One end of secondary winding is also grounded through contact breaker.

When the ignition switch is closed, the primary winding of the coil is connected to the positive terminal, and current flow through it known as primary current.

The current flows form primary coil produces a magnetic field which induces an EMF in secondary coil.

The cam regulate the contact breaker. Wherever the breaker open, current flows into condenser, which charged the condenser.

As the condenser become charger the primary current falls and the magnetic field collapses. This will induces a much higher voltage in condenser.

Now the condenser discharge into the battery which reverses the direction of both primary current and magnetic field. This will induce a very high EMF in secondary winding.

Now this high voltage EMF produce spark at correct spark plug through distributor. 7. What is knocking and what causes knocking.


Answer: Knocking, in an internal-combustion engine, sharp sounds caused by premature combustion of part of the compressed air-fuel mixture in the cylinder. In a properly functioning engine, the charge burns with the flame front progressing smoothly from the point of ignition across the combustion chamber. However, at high compression ratios, depending on the composition of the fuel, some of the charge may spontaneously ignite ahead of the flame front and burn in an uncontrolled manner, producing intense high-frequency pressure waves. These pressure waves force parts of the engine to vibrate, which produces an audible knock. Causes of Knocking 1. Low Quality Fuel 2. Carbon Deposits in the Cylinder Wall, and 3. Advanced or Delayed Sparking 4. Improper fuel supply. 8. Explain the phenomenon of knock in a S.I. engine. Answer: Knocking, in an internal-combustion engine, sharp sounds caused by premature combustion of part of the compressed air-fuel mixture in the cylinder. In a properly functioning engine, the charge burns with the flame front progressing smoothly from the point of ignition across the combustion chamber. However, at high compression ratios, depending on the composition of the fuel, some of the charge may spontaneously ignite ahead of the flame front and burn in an uncontrolled manner, producing intense high-frequency pressure waves. These pressure waves force parts of the engine to vibrate, which produces an audible knock. Knocking can cause overheating of the spark-plug points, erosion of the combustion chamber surface, and rough, inefficient operation. It can be avoided by adjusting certain variables of engine design and operation, such as compression ratio and burning time; but the most common method is to burn gasoline of higher octane number.

9. Explain the term premature combustion. Answer: Premature combustion: However, at high compression ratios, depending on the composition of the fuel, some of the charge may spontaneously ignite ahead of the flame front and burn in an uncontrolled manner, producing intense high-frequency pressure waves. These pressure waves force parts of the engine to vibrate, which produces an audible knock. Knocking can cause overheating of the spark-plug points, erosion of the combustion chamber surface, and rough, inefficient operation. It can be avoided by adjusting certain variables of engine design and operation, such as compression ratio and burning time; but the most common method is to burn gasoline of higher octane number.


10. Discuss the factors which promote pre-ignition Answer: Following are factors which cause pre-ignition:

Carbon deposits form a heat barrier and can be a contributing factor to pre-ignition. Other causes include: An overheated spark plug (too hot a heat range for the application). Glowing carbon deposits on a hot exhaust valve (which may mean the valve is running too hot because of poor seating, a weak valve spring or insufficient valve lash)

A sharp edge in the combustion chamber or on top of a piston (rounding sharp edges with a grinder can eliminate this cause)

Sharp edges on valves that were reground improperly (not enough margin left on the edges)

A lean fuel mixture An engine that is running hotter than normal due to a cooling system problem (low

coolant level, slipping fan clutch, inoperative electric cooling fan or other cooling system problem)

Auto-ignition of engine oil droplets (Can be solved by using an oil catch tank)[3] Insufficient oil in the engine Ignition timing too far advanced Excessive amount of oxygen in the combustion chamber

11.Distinguish between fans, blowers and compressors and mention one application area

for each. Or, Classify centrifugal compressors based on pressure developed.

Answer: Centrifugal compressors work very much like centrifugal pumps except that they

handle gases instead of liquids. Compressors as well as blowers and fans are the devices used to

increase the pressure of a compressible fluid (gas).

A fan usually consists of a single rotor with or without a stator element and causes only a

small rise in stagnation pressure of the flowing fluid, perhaps as low as 20 to 30 mm of water and


very rarely in excess of 0.07 bar. Fans are used to provide strong circulating air currents or for air

circulation and ventilation of buildings.

A blower may consist of one or more stages of compression with the rotors mounted on a

common shaft. The air is compressed in a series of successive stages and is often led through a

diffuser located near the exit. Blowers may run at very high shaft speeds and cause overall

pressure rise in the range 1.5 to 2.5 bar. Blowers are used in ventilators, power stations,

workshops, etc.

A compressor is a device used to produce large pressure changes ranging from 2.5 to 10

bar or more. Centrifugal compressors are mainly used in turbo-chargers.

The advantages of centrifugal compressor over the axial flow compressor are smaller

length and wide range of mass flow rate of gas. The disadvantages are larger frontal area and

lower maximum efficiency.

12. Explain the various components of typical centrifugal compressors with the help of a

schematic diagram. Discuss the actual pressure and velocity variations of flow across the

impeller and diffuser.

Answer: The principal

components are the impeller and

the diffuser. When the impeller

is rotating at high speed, air is

drawn in through the eye of the

impeller. The absolute velocity

of the inflow air is axial. The air

then flows radially through the

impeller passages due to

centrifugal force. The total

mechanical energy driving the

compressor is transmitted to the fluid stream in the impeller where it is converted into kinetic

energy, pressure and heat due to friction. The function of the diffuser is to convert the kinetic

energy of air that leaving the impeller into pressure. The air leaving the diffuser is collected in a

spiral casing from which it is discharged from compressor. The pressure and velocity variation

across the centrifugal compressor is shown in figure


13.With a neat sketch and velocity triangles, explain different vane shapes of the

centrifugal compressor. Draw the inlet velocity triangle assuming Vu1 = 0.

Answer: There are three types of vane shapes in centrifugal machines namely, (i) Backward

curved vane (ii) Radial vane (iii) Forward curved vane.

The vane is said to be backward curved if the angle between the rotor blade-tip and the

2<90o2=90o) the blade said to be

radial and if it is greater than 90o, the blade is said to be forward curved. Here the blade angles

measured with respect to direction of rotor (clockwise direction). The velocity triangles at the

outlet of centrifugal machines are shown below.

For centrifugal machines usually the absolute velocity at the entry has no tangential

component (i.e., Vu1= 0), thus the inlet velocity triangle for all the 3 types of vanes is same.

13.1 Different vane shapes in centrifugal compressors.

14.Discuss with velocity diagram why backward curved vanes are preferred for radial flow

(centrifugal) compressors.

Answer: Figure 13.1gives the velocity diagram for different vane shapes. For compressor which

absorb a specific amount of energy and runs at a given speed, the diameter (D2) and whirl

velocity (Vu2) can be varied to maintain U2Vu2 2 is large (as in


forward curved vane), Vu2 is also large and U2 has to be small that is diameter should be

2 is small (as in backward curved vane), Vu2 is small. Hence U2 has to

be large and the diameter should be increased appropriately to provide the required performance.

This implies that compressor with backward curved vanes are larger in size than those with

forward curved (or radial) vanes of the same capacity.

Now consider fluid flow through compressors running at the same tip speed (U2) and

with the same radial velocity (Vm2 2) increases exit fluid

velocity (V2) consequently, a very efficient diffuser is needed to obtain a pressure rise using all

to of the kinetic energy at the exit. Because of irreversibilities due to adverse pressure gradients

and thick boundary layers, complete diffusion of the exit kinetic energy with a pressure rise

corresponding to the theoretical, is impossible. Therefore the compressors with large exit angles

will be less efficient overall than compressors with small exit angles. So when high compressor

efficiency is desired, machines with backward curved vanes must be used. This is one of the

reasons that compressors with backward curved vanes are preferred. In some cases, where a large

pressure rise is needed with a compressor of small size, radial blades are used though the

efficiency may not be as high as that of a compressor with backward curved vanes of similar

capacity. Compressors with forward curved vanes are even less common than those of radial

type.

14.Briefly explain the slip and slip coefficient in centrifugal compressors.

Answer:

Fig. 14.1 Velocity and pressure distribution over impeller


uniform over the impeller cross-sectional area. But in actual practice this assumption is not

correct, because the velocity and pressure are not uniform over an impeller cross-sectional area

as shown in figure 8.3. Due to uneven pressure distribution and hence the velocity distribution

the tangential component of the velocity (whirl velocity) reduces, thus head developed by the

machine is always less than that developed at the ideal condition.

Thus slip may be defined as the phenomenon observed in centrifugal machines due to

uneven pressure distribution and the velocity distribution, which results in the reduction of

tangential component of the velocity (whirl velocity).

If V is the tangential component of the velocity without slip and (Vu2) is the tangential

component of the velocity with slip, then slip (S) is

The ratio of ideal head (Hi He) without slip is called the

slip co-efficient (µ).

Then theoretical work done on gas by a centrifugal compressor with slip is given as:

Although above equation modified by the slip coefficient, it is still the theoretical work done on

the gas, since slip will be present even if the fluid is frictionless (ideal fluid).

15.Explain Power Input Factor and Pressure Coefficient for centrifugal

compressor

Answer: The losses that occur in a compressor are due to:

(i) Friction between air and the sides of the passages of flow or between disks.

(ii) The effects of shock (due to improper incidence), separation in regions of high adverse

pressure gradients and turbulence.

(iii)Leakage between the tip of the rotor and casing.

(iv) Mechanical losses in bearings etc.

The frictional losses in the rotor and leakage losses between the tip of the rotor and

casing make the actual work absorbed by the rotor, lesser than the theoretical. This fact is


expressed by a quantity called the power input factor or the work factor, which is defined as the

ratio of actual work supplied to the compressor to the theoretical work supplied to the same

machine.

Then actual work supplied to the compressor is,

The frictional losses in the diffuser and the loss due to exit kinetic energy make the total

static pressure rise, less than the theoretical maximum specified by the impeller speed. This fact

is expressed by a quantity called the pressure or loading coefficient, which is defined as the ratio

of the isentropic work needed to cause the observed rise to the isentropic work specified by the

impeller tip-

But,

Then pressure coefficient is,

16. Derive an expression for total-to-total pressure ratio in terms of impeller tip speed for a

radial vanes centrifugal compressor.

Answer: Figure 8.4 shows the enthalpy-entropy diagram for a centrifugal compressor stage.

State points with superscript are correspond to isentropic compression processes. Air enters the

impeller vanes with lower absolute velocity V1 and leaves with large absolute velocity V2. This

absolute velocity reduces to V3 when it passes through the diffuser vanes, which is slightly higher

than V1. It can be observed that stagnation pressure Po1 will be higher than static pressure P1by

an amount . Similarly Po2 is much higher than static pressure P2 by an amount .


given as:

(Because, )

For backward curved vanes,

Then,

For forward curved vanes,

Then,

For radial vanes,

Then,

The stage efficiency of the compressor based on stagnation conditions at entry and exit is given

by,

Since, no work is done in the diffuser, .


Then pressure ratio of centrifugal compressor is,

For backward curved vanes,

For forward curved vanes,

For radial vanes,

The interesting part of the above equation is that they permit direct evaluations of

pressure ratio and work output, once the initial conditions and the rotor tip-speed are given and

slip-coefficient, power input factor and efficiency are estimated.

17: Discuss the compressibility for a centrifugal compressor. Answer: The Mach number is responsible for the compressibility of a flow. The higher the Mach

number the greater will be the compressibility effect and hence it reduces the compressor

efficiency. The Mach number at the inlet of the impeller eye is mainly depending on the relative

velocity of the impeller at the inlet. It is therefore necessary to keep the relative velocity value as

low as possible.

For a given flow rate, impeller eye may be either large or small. For large eye, velocity

V1 is low and eye tip speed U1 is high. For small eye it is opposite. Both these conditions result

in higher value of Vr1, but it is minimum in between these two. Figure 8.5 shows the variation of

relative Mach number with the eye tip diameter.


18. What is the necessity of providing the pre-whirl at the inlet of the centrifugal

compressor?

Answer: When the relative velocity is too high for efficient operation of a compressor and if

flow rate and speed cannot be altered, still the relative velocity can be reduced by giving the fluid

some initial positive pre-rotation. This is known as Prewhirl. This is usually done by providing

inlet guide vanes installed directly in front of the eye as shown in figure 8.6. Sufficient prewhirl

at the eye tip can avoid reaching the condition of critical Mach number leading to a prewhirl

component. Positive prewhirl is disadvantageous because a positive inlet whirl velocity reduces

energy transfer by an amount equal to U1Vu1.

Fig. 18.1 Variation of Mach number with the eye tip diameter


18.2Prewhirl at impeller eye

19.What is the function of a diffuser? Name different types of diffusers used in centrifugal

compressor and explain them with simple sketches.

Answer:Diffuser plays an important role in the overall compression process of a centrifugal

compressor. The impeller imparts energy to the air by increasing its velocity. The diffuser

converts this imported kinetic energy into pressure rise. For a radial vanned impeller, the diffuser

does compress and increase the pressure equal to 50 percent of the overall static pressure rise.

Fig. 19.1Types of diffusers

1 Vaneless Diffuser: In this type, the diffusion process will take place in the vaneless space

around the impeller before the air leaves the compressor stage through volute casing. A vaneless

diffuser is shown in figure 8.7 (a). A vaneless diffuser has wide range of mass flow rate. But for

a large reduction in the outlet kinetic energy, diffuser with a large radius is required. Because of

long flow path with this type of diffuser, friction effects are important and the efficiency is low.

2 Vanned Diffuser:In the vanned diffuser as shown in figure 8.7 (b), the vanes are used to

diffuse the outlet kinetic energy at a much higher rate, in a shorter length and with a higher

efficiency than the vaneless diffuser. A ring of diffuser vanes surrounds the impeller at the outlet,

and after leaving the impeller, the air enters the diffuser vanes


The diffuser efficiency defined as the ratio of ideal enthalpy rise to the actual enthalpy

rise in the diffuser.

20 Explain the characteristics of Centrifugal Compressor

Answer: An idealized centrifugal compressor characteristic curve is shown in figure 8.8. Consider

a centrifugal compressor delivering through a flow control valve situated after the diffuser. There

is a certain pressure head, even if the valve is fully closed and is indicated by state 1. This

pressure head is merely due to the churning action of the impeller vanes. The pressure head so

diffuser increases the pressure head. Thus, at state 2, the maximum pressure head is reached but

the efficiency is just below the maximum efficiency.

Fig. 8.8 Characteristic curve of centrifugal compressor

Further increase in mass flow reduces the pressure head to state 3. But at this state, the

efficiency is maximum compared with state 2. Thus the value corresponding to state 3 is said to

be design mass flow rate and pressure head.

Further increase in mass flow decreases the pressure head and reaches zero at state 4.

Corresponding to this state, all the power absorbed by the compressor is used to overcome the

internal friction and thus the compressor efficiency is zero.


21. Explain the surging phenomena in centrifugal compressors with the help of head-

discharge curves.

Answer:The phenomenon of a momentary increase in the delivery pressure resulting in

unsteady, periodic and reversal of flow through the compressor is called surging.

Consider compressor is operating at the state 3 as shown in figure 8.8, if the mass flow is

reduced by gradual closing of the flow valve, the operating point move on to the left. Further

reduction in mass flow increases the pressure head until it reaches the maximum value. Any

further decrease in flow will not increase the pressure head and hence reduces the pressure head

to state 6. At this condition there is a large pressure in the exit pipe than at compressor delivery

and the flow stops momentarily, and may even flow in the reverse direction. This reduces the

exit pipe pressure, then compressor again starts to deliver the air and the operating point quickly

shifts to 3 again. Once again the pressure starts increasing and operating point moves from right

to left. If the exit pipe conditions are remain unchanged then once again the flow will breakdown

after state 2 and cycle will be repeated with high frequency. This phenomenon is called surging.

If the surging is severe enough then the compressor may be subjected to impact loads and

high frequency vibration leads to failure of the compressor parts.


16. Unit wise-Question bank

UNIT-I Two marks questions with answers

1.What is value timing diagram? Answer: An automotive engine le/exhale) process. The engine's camshaft opens and closes the valves at a specific interval. The timing of opening & closing of valves is specified in degrees corresponding to the position of engine's pistons. Engine valve timing is the most critical process of IC engines. 2. why the inlet valve is opened before TDC and closed after BDC? Answer: The inlet valve usually opens few degrees before the piston reaches TDC in its exhaust stroke. It closes after quite a few degrees of piston reaching the BDC, i.e. when the piston starts to move up the cylinder in the compression stroke. In suction stroke, the air-fuel mixture or charge gets sucked into the cylinder very rapidly. This is because the downward movement of the piston creates the vacuum (or negative pressure) in the cylinder and the air-fuel mixture gets filled in the empty space. 3. What is scavenging?


Answer: It is a process of removing exhaust gas in the engine cylinder by means of incoming charge air. As a result cylinder is filled with fresh pressurized air for next compression stroke. The passage of scavenge air will also assist cooling of the cylinder, piston and valves. Time available for scavenging process in 2 stroke engine is less than 4 stroke engines.

4. Define firing order? Answer: The firing order is the sequence of power delivery of each cylinder in a multi-cylinder reciprocating engine. This is achieved by sparking of the spark plugs in a gasoline engine in the correct order, or by the sequence of fuel injection in a Diesel engine. 5. What is meant by compression ratio?

Answer: The static compression ratio of an internal combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity. It is a fundamental specification for many common combustion engines.

In a piston engine, it is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke.

Three marks questions with answers 1. What is combustion efficiency? Answer:Combustion or burning,isahigh-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion in a fireproduces a flame, and the heat produced can make combustion self-sustaining. Combustion is often a complicated sequence of elementaryradical reactions. Solid fuels, such as wood and coal, first undergo endothermic pyrolysis to produce gaseous fuels whose combustion then supplies the heat required to produce more of them. Combustion is often hot enough that incandescent light in the form of either glowing or a flame is produced. 2. What are the products formed during combustion process? Answer: Combustion is a chemical process. It is a reaction between the fossil fuels such as coal or natural gas and oxygen. The main products of the combustion process are carbon dioxide and water. The combustion process is always exothermic and it liberates heat. The actual reaction taking place due to combustion can be written down as- Fuel + O2 -> Product + Heat Due to the combustion of fuel and air, heat energy is generated where the internal energy of the fuel species is the chemical energy. This energy is associated with the chemical bonds and intermolecular attractions. The heat of reaction is the quantity of heat exchanged between a


system and its surroundings during a chemical reaction at a specified temperature. The heat of combustion is defined as the quantity of heat released in combustion of a fuel with oxygen. C + O2 ->CO2 + 8,084 kcal/kg of carbon 2C + O2 -> 2CO +2430 kcal/kg of carbon S+ O2 -> SO2 + 2224 kcal/kg of sulphur 2H2 + O2 -> 2H2O + 28922 kcal/kg of hydrogen The process of combustion can be completely carried out only in the presence of adequate amount of oxygen. Rapid fuel oxidation results in large amount of heat. Almost 79% of air which is not oxygen is nitrogen. Nitrogen is considered as temperature reducing diluter. Nitrogen absorbs heat and reduces the combustion efficiency of the fuel. The amount of heat which is available for use is also reduced. At high temperatures, nitrogen combines with oxygen to produce oxides of nitrogen, which are toxic pollutants. 3. What do you mean by SFC? Answer: Specific fuel consumption is the amount of fuel consumed by a vehicle for each unit of power output. A vehicle's specific fuel consumption is more or less independent from its nitrogen oxide emissions per kilometer. The specific fuel consumption of an engine is the rate of fuel burnt to produce a unit of thrust. Improvements in fuel efficiency mean that the specific fuel consumption per kilometer driven decreased by more than 30 percent. 4. Define mean effective pressure? Answer: The Mean Effective Pressure (MEP) is a theoretical parameter used to measure the performance of an internal combustion engine (ICE)not an actual pressure measurements within the engine cylinder. The cylinder pressure in an ICE is continuously changing during the combustion cycle. For a better understanding of the pressure variation within the cylinder, read the article The pressure-volume (pV) diagram and how work is produced in an ICE. The mean effective pressure can be regarded as an average pressure in the cylinder for a complete engine cycle. By definition, mean effective pressure is the ratio between the work and engine displacement: pme=W/Vd(1)

where:


pme [Pa] mean effective pressure W [J] work performed in a complete engine cycle Vd [m3] engine (cylinder) displacement 5. explain the working of fuel atomizer? Answer: Working of Fuel Atomizer

The pressure of the fuel, supplied from fuel feed pump through the fuel passages lifts the nozzle valve.

The fuel passes down to the nozzle and injected into a cylinder in the form of a very fine spray.

When the fuel pressure drops, the nozzle valve moves down under the force of the spring and rests on its seat. This closing the nozzle inlet to the passage, thus the fuelsupply is cut off.

Any fuel leaked past the plunger of nozzle valve is sent back to the fuel tank through return passage.

The adjusting screw enables the tension in the spring, to be adjusted. Whether the valve is working properly or not can be checked with the help of a feeding pin.

Hence to obtain various types of fuel spray, there are numerous types of injection nozzles (fuel atomizer) are used

Single hole nozzle, Conical ended single hole nozzle have just one hole, through which the fuel is sprayed.

In order to achieve greater penetration of the fuel spray, the multiple hole nozzles are quite useful. Somewhere long stem nozzles are also in use.

The pintle nozzle, which is used in engines employing swirl or pre-combustion chamber. Where the fuel is not required to penetrate the air.

The stem of the valve is extended to form a pintle protruding through the mouth of the nozzle body.

Hence such nozzles tend to give hollow conical spray varies from 40 to 60 degree, depending upon the type of the pintle.

The delay nozzle is similar in action to the pintle nozzle. Except that the rate of fuel injection increases towards the end of the delivery.

Due to the reduced quantity of fuel sprayed at the beginning of the combustion, there are reduced chances of diesel knocks.

An injection nozzle should be mounted in such a manner that it is not subjected to intense heat.

Five marks questions with answers 1. Explain lubrication system for IC engines?


Answer: The various lubrication systems used for lubricating the various parts of engine are classified as

Mist lubrication system Wet sump lubrication system, and Dry sump lubrication system.

1. Mist lubrication system: Mist lubrication system is a very simple type of lubrication. In this system, the small quantity of lubricating oil (usually 2 to 3%) is mixed with the fuel (preferably gasoline). The oil and fuel mixture is introduced through the carburetor. The gasoline vaporized and oil in the form of mist enters the cylinder via the crank base. The droplets of oil strike the crank base. The droplets of oil strike the crank base, lubricate the main and connecting rod bearings and the rest of the oil lubricates the piston, piston rings and cylinder. The system is preferred in two stroke engines where crank base lubrication is not required. In a two-stroke engine, the charge is partially compressed in a crank base, so it is not possible to have the oil in the crank base. This system is simple, low cost and maintenance free because it does not require any oil pump, filter, etc. However, it has certain serious disadvantages. Therefore, it is not popular among the lubrication system. Its disadvantages are the following:

6. During combustion in the engine, some lubricating oil also burnt and it causes heavy exhaust and forms deposits on the piston crown, exhaust port and exhaust system.

7. Since the lubricating oil comes in contact with acidic vapours produced during the combustion, it gets contaminated and may result in the corrosion of the bearings surface.

8. When the vehicle is moving downhill, the throttle is almost closed, and the engine suffers lack of lubrication as supply of fuel is less. It is a very serious drawback of this system.

9. There is no control over the supply of lubricating oil to the engine. In normal operating conditions, the two-stroke engines are always over-oiled. Thus consumption of oil is also more.

10. This system requires thorough mixing of oil and fuel prior to admission into the engine. It requires either separate mixing or use of some additives.

2. Wet-sump lubrication system: In the wet-sump lubrication system, the bottom of the crank case contains an oil pan or sump that serves as oil supply, oil storage tank and oil cooler. The oil dripping from the cylinders, bearings and other parts, fall under gravity back into the sump, from where it is picked up by pump and recirculated through the engine lubrication system. There are three varieties in wet-sump lubrication system. They are:

4. Splash lubrication system


5. Splash and pressure system and 6. Pressurized lubrication system.

2.1 Splash lubrication System: Splash lubrication system is used on small, stationary four-stroke engines. In this system, the cap of the big end bearing on the connecting rod is provided with a scoop which strikes and dips into the oil-filled through at every revolution of the crank shaft and oil is splashed all over the interior of crank case into the piston and over the exposed portion of the cylinder is shown in the figure below. A hole is drilled through the connecting rod cap

through which the oil passes to the bearing surface. Oil pockets are provided to catch the splashed oil over all the main bearings and also the cam shaft bearings. From these pockets oil passes to the bearings through drilled hole. The surplus oil dripping from the cylinder flows back to the oil sump in the crank case.

2.2 Splash and pressure lubrication system: Splash and pressure lubrication system is combination of splash and pressure system as shown in below figure. In this system, the lubricating oil is supplied by a pump under pressure to main and cam shaft bearings. the oil is also directed in the form of spray from nozzle or splashed by a scoop or dipper on the big end to lubricate bearings at the big end of the connecting rod, crank pin, gudgeon pin, piston rings and cylinder.

2.3 Pressurized lubrication system:

In pressurized lubrication system, the lubricating oil is supplied by a pump under pressure to all parts requiring lubrication as shown in below figure. The oil under the pressure is supplied to main bearings of the crank shaft and camshaft. Holes drilled through the main crank shaft bearings journals, communicate oil to big end bearing and small


end bearings through the hole drilled in the connecting rod. a pressure gauge is provided to confirm the circulation of oil to various parts. This system provides sufficient lubrication to all parts and is favored by most of the engine manufacturers. This is used in most heavy duty and high-speed engines 3. Dry-sump lubrication system: In dry-sump lubrication system, the oil supply is carried from an external tank. The oil from the sump is pumped by means of a scavenging pump through filters to the external storage tank. the oil from the storage tank is pumped to engine cylinder through and oil cooler. The oil pressure may vary from 3 to 8 bars.

The dry-sump lubrication system is generally used for heavy-duty engines.

2. Discuss the importance of cooling system for an IC engines. Describe different cooling system? Answer: There are mainly two types of cooling systems :

(a) Air cooled system, and

(b) Water cooled system.

Air Cooled System

Air cooled system is generally used in small engines say up to 15-20 kW and in aero plane engines.

In this system fins or extended surfaces are provided on the cylinder walls, cylinder head, etc. Heat generated due to combustion in the engine cylinder will be conducted to the fins and when the air flows over the fins, heat will be dissipated to air. The amount of heat dissipated to air depends upon :

(a) Amount of air flowing through the fins.

(b) Fin surface area. (c) Thermal conductivity of metal used for fins.

Fin

Cylinder


1 : Cylinder with Fins Advantages of Air Cooled System

Following are the advantages of air cooled system :

Radiator/pump is absent hence the system is light.

In case of water cooling system there are leakages, but in this case there are no leakages.

Coolant and antifreeze solutions are not required.

This system can be used in cold climates, where if water is used it may freeze. Disadvantages of Air Cooled System

Comparatively it is less efficient. It is used in aero planes and motorcycle engines where the engines are exposed to air

directly. WATER COOLING SYSTEM In this method, cooling water jackets are provided around the cylinder, cylinder head, valve seats etc. The water when circulated through the jackets, it absorbs heat of combustion. This hot water will then be cooling in the radiator partially by a fan and partially by the flow developed by the forward motion of the vehicle. The cooled water is again recirculated through the water jackets . Types of Water Cooling System

There are two types of water cooling system : Thermo Siphon System

In this system the circulation of water is due to difference in temperature (i.e. difference in densities) of water. So in this system pump is not required but water is circulated because of density difference only

Pump Circulation System

In this system circulation of water is obtained by a pump. This pump is driven by means of engine output shaft through V-belts.

Engine

Radiator Engine


3 : Pump Circulation System

3. Sketch and explain the valve timing diagram of a four stroke petrol engine? Answer: We consider that the valves open and close at the dead centre positions of the piston. But, in actual practice they do not open and close instantaneously at dead centers. The valves operate some degrees before or after the dead centers. The ignition is also timed to occur a little before the top dead centre. The timings of these sequence of events can be shown graphically in terms of crank angles from dead centre position. This diagram is known as valve timing diagram. Valve timing diagram for four-stroke petrol engine: diagram shows the valve timing diagram for a four-stroke cycle petrol engine. The inlet valve opens 10-30° before the top dead centre position. The air-fuel mixture is sucked into the engine cylinder till the inlet valve closes. The inlet valve closes 30-40° or even 60° after the bottom dead centre position. The air-fuel mixture is compressed till the spark occurs.

The spark is produced 20-40° before the t.d.c. position. This gives sufficient time for the fuel to burn. The pressure and temperature increases. The burning gases expand and force the piston to do useful work. The burning gases expand till the exhaust valve opens. The exhaust valve opens 30-60° before the b.d.c. position. The exhaust gases are forced out of the cylinder till the exhaust valve closes. The exhaust valve closes 8-20° after the t.d.c. position. Before it closes, again the inlet valve opens 10-30° before the t.d.c. position. The period between the inlet valve opening and exhaust valve closing is known as valve overlap period. The angle between the inlet valve opening and exhaust valve closing is known as angle of valve overlap.

Valve timing diagram for four-stroke diesel engine: The actual valve timing diagram for four-

stroke diesel engine is shown in figure the inlet valve opens 10-25° before the top dead centre


position. Fresh air is sucked into the engine cylinder till the inlet valve closes. The inlet valve

closes 25-50° after the bottom dead centre position. The air is compressed till the fuel is injected.

The fuel injection starts 5-10° before the t.d.c. position in the compression stroke. The air fuel

mixture burns. The

temperature and pressure

increases.

The burning gases expand till the exhaust valve opens. The exhaust valve opens 30-50° before the b.d.c. position. The exhaust gases are forced out of the engine cylinder till the exhaust valve closes. The exhaust valve closes 10-15° after the t.d.c. position. Before the exhaust valve closes, again the inlet valve opens 10-25° before the t.d.c. position. The period between the inlet valve opening the exhaust valve closing is known as

valve overlap period. The angle between these two events is known as angle of valve overlap.

4.DESCRIBE THE FUEL SYSTEM FOR DIESEL ENGINE? Answer: Fuel supply system is a seperate system used to deliver diesel at correct time in correct quantity, to a diesel engine (or C.I engine), for smooth and efficient operation. The operation of a diesel engine is different from that of a petrol engine. In a petrol engine, air-fuel mixture is supplied by a carburetor to the engine, at the beginning of the suction stroke. But in a diesel engine, fuel (without air) is supplied at the end of the compression stroke, by means of

a fuel supply system. Diagram of fuel supply system in diesel engine: Components:


Fuel supply system in diesel engine is made of the following components: 1. Diesel tank or reservoir 2. Low pressure pump 3. Filter 4. Fuel injection pump 5. Fuel injectors

1. Diesel tank or reservoir: Whenever you supply fuel to a diesel engine vehicle, it is stored in the diesel tank. Diesel tank temporarily stores diesel that is to be supplied to the engine. 2. Low pressure pump: It pumps the diesel at a low pressure to the fuel injection pump through a filter. 3. Filter: Before diesel is supplied to an engine, it must be filtered to remove any unwanted impurities. Filter is used for this purpose. 4. Fuel injection pump: This is the most important component of the fuel injection system. Fuel injection pump pressurizes the fuel to the required level and injects it correctly at the end of the compression stroke, during each cycle of operation of the engine . 5. Fuel injectors: Injectors are devices used to inject the fuel to the cylinder. In diesel engine, when fuel is injected, it is automatically atomized. Working:

1. Diesel is pumped from the diesel tank by a low pressure pump. It is passed through a filter. The filter removes any unwanted impurities in the diesel.

2. Filtered diesel is supplied to the inlet port of the fuel injection pump. The fuel injection pump automatically pressurizes the diesel to the required level and supplies it to the fuel injector. The fuel injector forces the fuel into the cylinder at the end of the compression stroke, during each cycle of operation of the engine.

3. Fuel injection pump is operated by means of a cam shaft. CAV fuel injection is the most common fuel injection pump used in diesel engines.

4. Any leak-off diesel from the fuel injection pump is supplied back into the filter as shown in the diagram above.


Characteristics of a good fuel supply system: 1. A good fuel supply system should be able to deliver the fuel correctly at the end of the

compression stroke. 2. It must be able to properly atomize the fuel. 3. It must operate smoothly and sharply during each cycle of operation of the engine. 4. It must be able to supply the fuel above atmospheric pressure. 5.Explain with the help of line diagram the working of simple carburettor? Answer: A carburetor is a device that blends air and fuel in the proper ratio for the combustion in an internal combustion engine. it is most important to supply proper air-fuel ratio in inlet manifold of the internal combustion engine. A good Carburetor will do this for your engine. Internal combustion engine has very low comparison ratio. It also uses highly volatile liquid fuel such as petrol, paraffin etc. Hence the carburetion process is must requirement. The process of breaking up the fuel into minute particles and mixing it with air is known as carburetion,In the carburetor, the fuel is completely broken into the minute particles, vaporized and mixed with the air in a proper ratio. The homogeneous mixture of fuel and air thus obtained is supplied to the engine cylinder. This device is used in petrol engine or similar liquid fuel engines. By means of which the fuel mixed with air is supplied into the induction manifold of the engine. The carburetor controls the required quantity of fuel and breaks it up into minute globules. This afterward being mixed with the correct quantity of air. The main purpose of a carburetor is to supply the required quantity of petrol and air mixture. The strength must be correct as per requirement of load conditions of the engine. The ratio should not be more than 20:1 and not less than 8:1. The ideal ratio is 15:1

Construction of Simple Carburetor Simple carburetor consists of a venturi and a fuel jet located therein. For maintaining the level of fuel in the jet, a float chamber is usually required. A throttle valve in the form of a flat circular metal disc mounted on a spindle. This is

provided for controlling the flow of air-fuel mixture to the induction manifold. The level of fuel is just kept slightly below the top of the jet to prevent the leakage when not in operation. Normally 1.5 mm difference is kept between the top of the jet and the surface of the fuel in the float chamber. A needle valve controls the passage of fuel from the fuel pump when the air begins to flow past the jet; a low-pressure zone is created in the venture because of the increased velocity of air. The fuel begins to rise because of the difference in the air pressure on the fuel which is equal to the pressure of the atmosphere and on the fuel in the jet at the venturi and issue out from the jet


in the form of a fine spray.Minute petrol particles present a large surface area being exposed to the air stream.The fuel is not completely vaporized in a carburetor and some globules of fuel still enter the induction manifold. Those are vaporized during the compression stroke in an engine cylinder.A check valve controls the flow of air into the carburetor. The gas-tight connection is provided between the carburetor and the induction manifold. When two concentric venturi is provided, the discharge end of the inne

A higher velocity of air which aids in the atomization of the fuel is obtained at the throat of primary venturi. As there is a lower pressure compared to the main venturi. The main advantages of multiple venturi are, these keep the fuel away from the carburetor walls. Hence obtain the considerable reduction in fuel consumption. The Simple Carburetor may be of following three types 01) Updraft Type: In this, the air enters the carburetor at the bottom and leaves at the top 02) Downdraft Type: In this, air enters the carburetor at the top and leaves at the bottom. This is used in most of the passenger cars.Because it can be installed at a higher level in the engine. Hence makes it more accessible for the purpose of inspection and repair. 03) Side draft Type: It has more width and mounting of oil bath air filter is clumsy.

Multiple choice questions

1. Advantage of reciprocating IC engines over steam turbine is (a) Mechanical simplicity (b) Improved plant efficiency (c) Lower average temperature (d) All of the above 2. The intake charge in a diesel engine consists of (a) Air alone (b) Air + lubricating oil (c) Air + fuel (d) Air + fuel +lubricating oil 3. Disadvantages of reciprocating IC engine are (a) Vibration (b) Use of fossil fuels (c) Balancing problems (d) All of the above 4. Gudgeon pin forms the link between


(a) Piston and big end of connecting rod (b) Piston and small end of connecting rod (c) Big end and small end (d) Connecting rod and crank 5. Engines of different dimensions, power and speed are compared on the basis of (a) Maximum pressure (b) Fuel consumption (c) Mean effective pressure (d) Unit power 6. In a four-stroke IC engine cam shaft rotates at (a) Same speed as crank shaft (b) Twice the speed of crank shaft (c) Half the speed of crank shaft (d) None of the above 7. Thermal efficiency of CI engine is higher than that of SI engine due to (a) Fuel used (b) Higher compression ratio (c) Constant pressure heat addition (d) None of the above 8. SI engines are of (a) Light weight (b) High speed (c) Homogeneous charge of fuel and oil (d) All of the above 9. Compression ratio in diesel engine is of the order of (a) 5-7 (b) 7-11 (c) 11-12 (d) 12-20 10. Main advantage of two stroke engine over four stroke engine is (a) More uniform torque on the crank shaft 11. (b) More power output for the cylinder of same dimensions (c)Absence of valves (d)All the above

Fill in the blanks questions 11. 12. An IC engine gives an output of 3kW when input is 10,000 J/s. The thermal efficiency of the

13. In a reciprocating engine with a cylinder diameter of D and stroke of L, the cylinder volume

14. if L is the stroke and N is the rpm, mean piston speed of two-stroke engine


15. 16. The range of volumetric efficiency of a diesel engine is 17. Relative efficiency is the ratio of 18. Brake specific fuel consumption is defined as

19. Carburettor is mainly employed in 20. Keys :

Multiple choice questions Fill in the blanks questions 1.d 11. 2S CI engine 2.a 12.30% 3.d 13. D2/L+clearence volume 4.b 14.2LN 5.c 15.Actual fuel-air ratio/stiochiometric fuel air ratio 6.c 16.85-90% 7.b 17.Actual thermal efficiency/Air standard efficiency 8.d 18.Fuel consumption per brake power hour 9.d 19.SI engine 10.d 20.Ps=bp/A

UNIT-II

Two marks questions with answers

1. What is called flame front and flame velocity? Answer: Flame front: Boundary or front surface of the flame that separate the burnt charges from the unburnt one. Flame speed/velocity: The speed at which the flame front travels.

Flame speed affects the combustion phenomena pressure developed and power produced.

Burning rate of mixer depends on the flame speed and shape/contour combustion chamber.

2. What is the normal combustion and abnormal combustion in SI engine? Answer:


Normal Combustion: When the flame travels evenly or uniformly across the combustion chamber.

Abnormal Combustion: When the combustion gets deviated from the normal behavior resulting in loss of performance or damage to the engine.

3. What is knocking in both SI and CI engines? Answer: Detonation or engine knocking is when the air-fuel mixture does not burns in the cylinder at the time of firing from spark plug but still some air-fuel mixture burns in some other part of cylinder, normally outside the normal fuel burn envelope. This is not acceptable and causes harm to the engine. Diesel knock is slightly different in the aspect, the mixture burnt in cylinder is due to compression, but there is a small time lag between fuel injection and fuel burning. This is more or less unavoidable and does not impact the engine much.

4. What is pre ignition? Answer: Pre-ignition (or preignition) in a spark-ignition engine is a technically different phenomenon from engine knocking, and describes the event wherein the air/fuel mixture in the cylinder ignites before the spark plug fires. Pre-ignition is initiated by an ignition source other than the spark, such as hot spots in the combustion chamber, a spark plug that runs too hot for the application, or carbonaceous deposits in the combustion chamber heated to incandescence by previous engine combustion events.

5. What is ignition timing? Answer: In a spark ignition internal combustion engine, Ignition timing refers to the timing, relative to the current piston position and crankshaft angle, of the release of a spark in the combustion chamber near the end of the compression stroke.

Three marks questions with answers 1. What is ignition delay period?

Answer: An ignition delay in a CI engine is the time taken by the fuel to auto-ignite after being injected into the engine cylinder. It can be called as the preparatory phase during which the fuel prepares to undergo auto-ignition.

Ignition delay can be divided into two parts. Both these occur simultaneously.

a. Physical delay, during which the fuel gets mixed and atomized inside the cylinder and is raised to auto-ignition temperature. Heavier viscous fuels have longer delay period. It can be reduced by increasing injection pressure or by increasing the combustion chamber temperature or the turbulence.

b. Chemical delay, during which the air fuel mixture undergoes pre-ignition reactions. This is similar to the ignition lag in SI engines.

High ignition delay is not favorable. It will cause knocking in CI engines. Some factors which affect ignition delay are:

Compression Ratio: Higher the CR, lower the delay.


Engine Speed: Higher engine speed leads to lower delay. Injection timing: Delay increases with increase in ignition advance. Intake temperature: Higher intake temperature results in lower delay. Output: Higher output causes lower delay. Atomization: Better atomization helps reduce ignition delay. Auto-ignition temperature: With a low auto-ignition temperature, the fuel starts to

burn early, meaning lower delay. Intake Pressure: Higher pressure reduces the delay period.

2. Define suction induced swirl and combustion induced swirl? Answer: Suction swirl: During suction, air is admitted into the engine cylinder in a tangential direction. The entering air is deflected by the cylinder wall. Air thereby assumes a rotary motion i.e. swirl about the cylinder axis. This swirl is called suction swirl. Helical ports produce swirl upstream of the valve and directed ports have it downstream. In diesel engines, tangential entry of air is affected by one of the following methods: 1. By masking a portion of the inlet valve. 2. By angling the inlet port in the desired direction. 3. By providing a lip in the inlet port, over one side of the inlet valve.

Compression swirl: The combustion chamber cavity tends to modify the swirl as the piston approaches the Top Dead Centre (TDC) position during the compression process. As the piston approaches TDC the rotating air is forced into the piston bowl. The rotational force is magnified by the reduced diameter of the piston bowl. Thin, deep bowls have a higher swirl rate.

3. What are anti knock agents? Answer: An Knocking additives is a chemical that, when added to gasoline, raises the octane value of the gasoline which, in turn, raises the temperature and pressure at which gasoline will auto-ignite. It allows the gasoline/air mixture to wait until the spark plug ignites the fuel, reducing pre-detonation which can be harmful to the engine.

The first widely used knocking additive was lead. It worked quite well and had the side effect of being a good lubricant for valves and valve seats. But lead is dangerous to humans and the

elsewhere. Lead also destroys catalytic converters which take out more impurities from the exhaust stream before they reach the atmosphere.

Methylcyclopentadienyl manganese tricarbonyl (MMT) Ferrocene Iron Carbonyl Iron Pentacarbonyl toluene Isooctane

All commonly used as antiknock agents.


4. What is a combustion chamber? Answer: The combustion chamber is the space bounded by cylinder head, piston top and cylinder walls in which combustion of fuel occurs. Combustion chambers are designed to give high thermal efficiency and smooth operation of the engine.

The specific objectives of combustion chamber are: To regulate the rate of pressure rise. To prevent the possibility of detonation at all times. To burn the maximum amount of charge after ignition.

5. Define Auto-ignition? Answer: spontaneous ignition of fuel-air mixture under certain conditions of temperature, pressure and density without aid of initiating flame or spark is called auto-ignition. It may be noted that the ignition by hot spots in combustion chamber which are act as a regular spark is called pre-ignition.

Five marks questions with answers 1. State and explain different combustion stages in SI engine?

Answer: Combustion process in SI engines is divided mainly into three phases:

1. Ignition lag or delay period- This first phase is the duration between the occurrence of spark at spark plug and the deviation of combustion curve from motor curve (AB in figure). This period tends to be very nearly constant in time. Factors influencing this phase are:

i. Fuel- Higher the self-ignition temperature of fuel, longer is the ignition lag.

ii. Mixture ratio- Ignition lag increases as the relative fuel air ratio increases or decreases from 1.2

iii. Initial temperature- Ignition lag increases rapidly with increase in temperature of the air-fuel mixture.

iv. Flame temperature- As the temperature of the flame between the spark plug electrodes increases, the reaction time decreases and so the ignition lag decreases.

2. Rapid combustion phase- This second phase (BC) starts when combustion curve deviates from the motor curve and extends till the maximum pressure is reached. In this phase mean temperature of gases in the engine cylinder continues to increase beyond the


maximum pressure point. The time required mainly depends on the intensity of turbulence or state of agitation of the air-fuel mixture. The duration of the phase is approximately constant in terms of the crank-angle movement.

3. Afterburning- This third and final phase (CD) occurs between points of maximum pressure and maximum temperature. After the flame front ahs reached the cylinder walls about 25% of the charge is still not completely burnt. But at this stage it is difficult for the remaining oxygen in the charge to react with the petrol vapours which slows down the rate of combustion. Simultaneously heat is liberated due to chemical interaction caused by reassociation of the combustion products throughout the expansion stroke.

However it has to be noted that no clear distinction can be made between these three phases as no abrupt change is noticed.

2.State and explain different combustion stages in CI engine?

Answer:

There are four stages of combustion in CI engine as follows:

1. Ignition delay- During this stage there is a physical delay period which is the time from beginning of injection to the attainment of chemical reaction conditions. The fuel is atomized and mixed with air and its temperature is raised. This

period is followed by a chemical delay period in which preflame reactions start and accelerate until local ignition takes place.

2. Rapid or uncontrolled combustion- This is second stage in which pressure rise is rapid since during delay period the fuel droplets have had time to spread themselves over a wide area and have fresh air around them. This phase extends from end of delay period to point of maximum pressure.

3. Controlled combustion- The very high temperature and pressure at end of second stage cause the fuel droplets injected during last stage to burn instantly and any further pressure rise can be controlled by purely mechanical means that is injection rate. This period ends at maximum cycle temperature. The heat evolved by end of this phase is 70 to 80 percent of total heat of fuel supplied.

4. After burning- This fourth stage may not be present in all cases but due to poor distribution of fuel particles combustion may continue in the expansion stroke. Its duration may be 70 to 80 degrees of crank travel from TDC.

3. Explain knocking, properties and its effects in CI engine?


Answer: Knock is the result of the rapid combustion of the end-gas ahead of the flame front which undergoes chemical reactions prior to normal combustion. These cause high frequency pressure oscillations inside the cylinder that produce the sharp metallic noise called knock. Two theories have been postulated to explain the origin of knock:

1. The Auto ignition theory which states that when the air/fuel mixture in the end- gas region is compressed to sufficiently high pressure and temperature the fuel oxidation process - starting with the preflame chemistry and ending with rapid energy release - can occur spontaneously in parts or all of the end-gas region.

2. The Detonation theory which holds that,under knocking conditions, the advancing flame front accelerates to sonic velocities and consumes the end-gas at a rate much faster than would occur with normal flame speeds.

In CI Engine: There is both physical delay (time taken for atomisation, vaporization of fuel and

intermixing of fuel and air) and chemical delay (i.e; time taken to complete chemical reaction). As these delay period increases more and more fuel is accumulated and as the pressure and temperature reaches the optimum value for combustion a sudden blast

. 4. Explain different types of combustion chambers in SI and CI engines? Answer: Types of combustion chamber in SI engines: The location of spark plug and values in T-type combustion chamber is shown in fig. It is the earliest type of combustion chambers suitable for SI engines with valve and spark plug locations. (I)T-type chamber: The location of spark plug and values in T-type combustion chamber is shown in fig .it is earliest type combustion chamber and is obsolete now. Its chacteristics are: 1. Good turbulence. 2. Short cylinder block. 3. Long flame travel. 4. Greater tendency knock.

5. Unsatisfactory fuel and air utilization. (II)F-type chamber: In this type of combustion chamber the values are placed one over the other as shown in fig. It is better than T-type .Its characteristics are: 1. Higher compression ratio can be used. 2. The value mechanism is complicated. (III)L-type chamber: the features of L-head chambers are shown in fig. In this type the valves are located in one side of the cylinder. Its characteristics are:


1. Lesser tendency to knock. 2. High turbulence 3. Reduced flame travel. 4. High power output. (IV)Overhead valves: The valves are located in the cylinder head and they are opened by cam mechanism. The most of the engines use overhead valve arrangement. The characteristics of these values are: 1. Very high turbulence 2. Smooth operation with high volumetric efficiency 3. High output COMBUSTION CHAMBER FOR CI ENGINES

The shape of the combustion chamber and the fluid dynamics inside the

chamber are of great importance in diesel combustion. Diesel engines are divided into

two basic categories according to their combustion chamber design.

Direct-Injection (DI) engines: This type of combustion

chamber is also called an open combustion chamber. In this type, the

entire volume of the combustion chamber is located in the main cylinder

and the fuel is injected into this volume.

Indirect-Injection (IDI) engines: This type of combustion

chambers, the combustion space is divided into two parts, one part in the

main cylinder and the other part in the cylinder head. The fuel injection is

effected usually into that part of the chamber located in the cylinder head.

These chambers are classified further into: Swirl chamber in which compression swirl is generated.

Precombustion chamber in which combustion swirl is induced. Air cell chamber in which both compression and

combustion swirl are induced.

The details of these chambers are discussed in the following

sections.


Direct Injection Engines

These engines have a single, open combustion chamber into which the entire

quantity of fuel is injected directly. An open combustion chamber is one in which the

combustion space incorporates no restrictions that are sufficiently small to cause large

differences in pressure between different parts of the chamber during the combustion

process.

The main advantages of this type of chambers are:

Minimum heat loss during compression because of lower

surface area to volume ratio and hence, better efficiency.

No cold starting problems.

Fine atomization because of multihole nozzle.

The drawbacks of these combustion chambers are:

High fuel injection pressure required and hence complex design

of fuel injection pump.

Necessity of accurate metering of fuel by the injection system,

particularly for small engines.

Open combustion chamber

The open combustion chamber is one, in which all the air for combustion is

confined in one space. These chambers mainly consist of space formed between the flat

cylinder head and a cavity in the piston crown in different shapes. The fuel is injected

directly into this space. The injector nozzles used for this type of chambers are generally

of multihole type working at a relatively high pressure (above 200 bars). Combustion of

the entire fuel takes place within this space. Figure 3.3 shows the schematic diagram of


open combustion chamber. In DI diesel engines, a single combustion chamber with

different piston bowl shapes such as square, cylindrical, hemispherical, shallow depth,

toroidal etc. have been used. There are many designs of open chamber, some of which are

shown in figure 3.4.

Shallow depth chamber

In the shallow depth chamber the depth of the cavity provided in the piston is

quite small. This chamber is usually adopted for large engines running at low speeds.

Since the cavity diameter is very large, the squish is negligible.

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Schematic of different shapes of open combustion chambers Hemispherical chamber

This chamber also gives small squish. However, the depth to diameter ratio

can be varied to give any desired squish to give better performance.

Cylindrical chamber

This design was attempted in recent diesel engines. This is a modification of the cylindrical chamber in the form of a truncated cone with a base angle of 30o. The swirl was produced by masking the valve for nearly 180o of circumference. Squish can also be varied by varying the depth. Toroidal chamber

The idea behind this shape is to provide a powerful squish along with the air

movement, similar to that of the familiar smoke ring , within the toroidal chamber. Due to

powerful squish the mask needed on inlet valve is small and there is better utilization of

oxygen. The cone angle of spray for this type of chamber is 150o to 160o.

Re-entrant combustion chamber

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The re-entrant combustion chamber is a type of open combustion chamber

which has a smaller diameter (opening) at the entry than at the middle. In this combustion

chamber, the lip of the combustion chamber protrudes beyond the walls of the bowl

provides a substantial improvement in performance and emissions over the previous open

straight sided bowl designs. The bowl lip prevents the air squish motion pushing fuel

above the piston crown, so that the majority of the fuel charge is mixed and burnt within

the bowl. The lip also creates further micro turbulence within the bowl. Deep re-entrant

chambers should be more effective than shallow chambers. Figure 3.5 shows the

schematic diagram of the re-entrant combustion chamber.

Schematic diagram of re-entrant combustion chamber Indirect Injection Engines

In these engines, the combustion space is divided into two parts. A divided

combustion chamber is defined as one in which the combustion space is divided into two

or more distinct compartments connected by restricted passages. The fuel is injected to

the auxiliary chamber, which is connected to the main chamber via a nozzle or one or

more number of orifices. The main advantages of the indirect injection combustion

chambers are:

Injection pressure required is low.

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Direction of spraying is not very important.

These chambers have the following serious drawbacks which have made its

application limited.

JJ Poor cold starting performance requiring heater plugs.

KK Specific fuel consumption is higher.

Swirl chamber

A swirl chamber consists of a spherical shaped chamber separated from the

engine cylinder and located in the cylinder head. Into this chamber, about 50% of the air

is transferred during the compression stroke. A throat connects the chamber to the

cylinder which enters the chamber in a tangential direction so that the air coming into this

chamber is given a strong rotary movement inside the swirl chamber and after

combustion , the products rush back into the cylinder through the same throat at much

higher velocity. The schematic diagram of swirl chamber is shown in Figure 3.6.

Pre-combustion chamber

A Typical pre-combustion chamber consists of an antichambers connected to

the main chamber through a number of small holes. The precombustion chamber is

located in the cylinder head and its volume accounts for about 40% of the total

combustion

space. During

the compression

stroke the piston

forces the air

into the

precombustion

chamber.

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The fuel is injected into the prechamber and the combustion is initiated. The resulting

pressure rise forces the flaming droplets together with some air and their combustion

products to rush out into the main cylinder at high velocity through the small holes. Thus,

it creates both strong secondary turbulence and distributes the flaming fuel droplets

throughout the air in the main combustion chamber, where the bulk of combustion takes

place. Figure 3.7 shows the schematic diagram of pre-combustion chamber Air-cell chamber

This chamber is divided into two parts, one in the main cylinder and the other called

energy cell. The energy cell is divided into two parts, major and minor, which are

separated from each other and from the main chamber by narrow orifices. A pintle type

of nozzle injects the fuel across the main combustion chamber space towards the open

neck of the air-cell.

5. Explain the need for air motion and types? Answer:

AIR MOTION WITHIN THE CYLINDER The air motion inside the cylinder greatly influences the performance

of diesel engines. It is one of the major factors that controls the fuel-air mixing in diesel engines. Air-fuel mixing influences combustion, performance and emission level in the engine. The air motion inside the cylinder mainly depends on manifold design, inlet and exhaust valve profile and combustion chamber configuration. The initial in-cylinder intake flow pattern is set up by the intake process, and then it is modified during the compression process. The shape of the bowl in the piston and the intake system, control the turbulence level and air-fuel mixing of the DI diesel engine. The variation of shape of intake system, shape of piston cavity, etc. lead to a change in the flow field inside the engine.

EFFECTS OF AIR MOTION The air motion inside the cylinder 1. Atomizes the injected fuel into droplets of different sizes. 2. Distributes the fuel droplets uniformly in the air charge. 3. Mixes injected fuel droplets with the air mass. 4. Assists combustion of fuel droplets.

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5. Peels off the combustion products from the surface of the burning drops as they are being consumed. 6. Supplies fresh air to the interior portion of the fuel drops and thereby ensures complete combustion. 7. Reduces delay period. 8. Reduces after burning of the fuel. 9. Better utilization of air contained in the cylinder.

TYPES OF AIR MOTION The air motion in a diesel engine is generally caused by either by the

intake port during the suction stroke or by combustion chamber geometry during the compression stroke. Three different elements of the air motion present during intake to expansion strokes in a diesel engine cylinder have been classified as

1. Swirl 2. Squish 3. Turbulence

Swirl

Swirl is defined as the organized rotation of the charge about the cylinder axis. It is created by bringing the intake flow into the cylinder with an initial angular momentum. Swirl is generated during the intake process in DI diesel engines by the intake port and subsequently by combustion chamber geometry during the compression stroke. The swirl intensity increases the tangential component of the velocity of air inside the cylinder, which aids in the mixing of fuel and air, and significantly affects the combustion and emission characteristics of diesel engines. Schematic of swirl air motion

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Suction swirl During suction, air is admitted into the engine cylinder in a tangential direction. The entering air is deflected by the cylinder wall. Air thereby assumes a rotary motion i.e. swirl about the cylinder axis. This swirl is called suction swirl. Helical ports produce swirl upstream of the valve and directed ports have it downstream. In diesel engines, tangential entry of air is effected by one of the following methods:

By masking a portion of the inlet valve.

By angling the inlet port in the desired direction.

By providing a lip in the inlet port, over one side of the inlet valve.

Compression swirl

The combustion chamber cavity tends to modify the swirl as the piston approaches the Top Dead Centre (TDC) position during the compression process. As the piston approaches TDC the rotating air is forced into the piston bowl. The rotational force is magnified by the reduced diameter of the piston bowl. Thin, deep bowls have a higher swirl rate.

Squish The squish motion of air is brought about by a recess in the piston crown. At the end of the compression stroke, the piston is brought to within a very small distance from the cylinder head. This fact causes a flow of air from the periphery of the cylinder to its center and into the recess in the piston crown. This radial inward movement of air is called squish by Ricardo. The combustion recess, into which the air mass is squeezed in, is located either in the piston crown or in the cylinder head. The former arrangement is preferred and is widely used. In this case, heat losses from the compressed air will be lesser. This is because the piston crown is not cooled to that extent as the cylinder head which is cooled by the coolant. The figure 3.2 shows squish motion during compression.

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Turbulence Turbulence contributes to the dispersion of fuel and the micro mixing of fuel and air respectively. As such, they greatly influence the diesel engine performance. The flow processes in the engine cylinder are turbulent. In turbulent flows, the rates of transfer and mixing are several times greater than the rates due to molecular diffusion. This turbulent diffusion results from the local fluctuations in the flow field. It leads to increased rates of momentum and heat and mass transfer, and is essential to the satisfactory operation of Spark Ignition and Diesel engines.

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Multiple choice questions 1. Hydrocarbons are decomposed into smaller hydrocarbons by (a) Reforming (b) Refining (c) Cracking (d) Polymerization

2. The molecular structure of the straight-run gasoline is changed by (a) Cracking (b) Reforming (c) Refining (d) Boiling

3. For SI engines fuels most preferred are (a) aromatics (b) (c) olefins (d) napthenes

4.for CI engine fuel most preferred are (a) aromatics (b) paraffins (c) olefins (d) napthenes

5.In Si engines maximum flame speed is obtained when the equivalent ratio is between (a) 1.1 and 1.2 (b) 1.0 and 1.1 (c) 1.2 and 1.3 (d) Less than 1

6.In SI engines flame speed increases (a) With turbulence (b) With air-fuel ratio (c) Both (a) and (b) (d) None of the above

7.With increase in compression ratio flame speed (a) Increases (b) Decreases (c) Remains the same

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(d) None of the above 8.With increase in speed the crank angle required for flame propagation (a) Increases (b) Decreases (c) Not affected (d) None of the above

9.Increasing the compression ratio in SI engines the knocking tendency (a) Decreases (b) Increases (c) Not affected (d) None of the above

10.Decreasing the cooling water temperature in SI engines the knocking tendency (a) Increases (b) Decreases (c) Not affected (d) None of the above

Fill in the blanks questions

11. Detonation in SI engine occurs due to 12. 13. In CI engines with increase in compression ratio the delay period 14. In CI engines knocking tendency increases with 15. In CI engines by increasing inlet air pressure the knocking tendency 16. Open combustion chambers in CI engines require 17. The advantages of the indirect combustion chambers are 18. In CI engines the delay period is affected by 19. For a four cylinder vertical engine, the commonly used firing order is

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Keys :

Multiple choice questions Fill in the blanks questions 1.c 11.auto ignition of the charge after the spark in struck 2.b 12.decreases 3.a 13.during combustion 4.b 14.decrease in compression ratio 5.a 15.decreases 6.c 16.accurate metering of fuel by injection system 7.a 17.low injection pressure and direction of spray 8.c 18.compression ratio and engine speed 9.b 19.1-3-4-2 10.b 20.Accelerating

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UNIT-III

Two marks questions with answers

1. Define brake power? Answer: Brake power can be defined as the power available at the crankshaft.Since it us measured with the help of brake dynamometer ut is referred as brake power. It may be input power or output.In case of IC engine it is output power and in case of compressor it is input power. In its raw form it is difference between indicated power and friction power. BP = IP - FP . 2. Define mechanical efficiency? Answer: Mechanical efficiency measures the effectiveness of a machine in transforming the energy and power that is input to the device into an output force and movement. Efficiency is measured as a ratio of the measured performance to the performance of an ideal machine

Mechanical efficiency={BP/IP}x100%.

3. What is an indicated power? Answer: Indicated power is the power generated inside the engine cylinder and is measured from the indicator diagram. For single cylinder engine,

I.P=Pm*a*l*n/60 For multi cylinder engine

I.P=k*Pm*a*l*n/60.

Where k=number of cylinders.

4. Define mean effective pressure? Answer: Mean effective pressure is defined as mean pressure acting on a piston during powere stroke that would produce the same network as a actually developed in one cycle. The mean effective pressure is given by the equation.

m.e.p=network for one stroke/stroke volume. Pm=Wnet/Vs.

5. Define clearance volume? Answer:

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Three marks questions with answers

1. Define Brake mean effective pressure? Answer: The brake mean effective pressure is a theoretical constant pressure acting on the piston which would give BP of this engine. Thus it is the mean effective pressure which is based on the BP of an engine and is obtained as given below. BP=Pbm*l*a*n Therefore, BMEP Pm=BP/lan kN/m2 2. Define specific fuel consumption? Answer: The amount of fuel consumption by an engine in a given time is always measured during a trial. The specific fuel consumption is defined as the consumption of fuel per unit time per unit of power developed. An engine with a lower specific fuel consumption is the more efficient engine. Specific fuel consumption is one of the most important parameter used in the comparison of engines. It is usually expressed in kg/KWh.if the fuel supply is expressed in litres,it may be converted into kilograms by multiplying its value with specific gravity(relative density). Thus,10litres of petrol having specific gravity 0.72 has a mass of 10x0.72=7.2kg. The specific fuel consumption may be stated for either the IP or the BP of an engine. Based on I.P Specific fuel consumption=fuel consumption per unit time/indicated power. 3. Define clearance ratio? Answer: The clearance ratio (k) may be defined as a ratio of the clearance volume (Vc) to the piston displacement (V1-V3), Clearance ratio= [V3/V1-V3] = [Vc/Vs] Vc=V3=kVs

4. What is volumetric efficiency in case of compressor? Answer: volumetric efficiency is defined as the ratio of actual volume of air drawn per stroke at STP to the piston displacement. Volumetric efficiency =Actual volume of air drawn per stroke at STP/stroke volume The actual volume of air drawn into the compressor, measured at STP conditions is called free air. The volume of free air delivered(FAD) will be less than the swept or stroke volume of the piston. The volume of free air compressed and delivered per minute is known as capacity of compressor. Thus the volumetric efficiency may be expressed as

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Volumetric efficiency = compressor capacity/piston displacement. . 5. What is the function of air compressor? Answer: Air compressor is a machine which increases pressure of air and delivers it at a higher pressure. It is driven by a prime mover(engine or electric motor).Air compressor takes in atmosphere air, compresses it to desired pressure. High pressure air is delivered to a receiver (storage vessel) from which it may be used for required purpose. The volume of air is delivered by the compressor per unit time capacity (output) of a compressor.

Five marks questions with answers 1. a two stroke cycle, 210 mm bore x 280 mm stroke, single cylinder at engines gives the following results on test. Speed=350Rpm; Net brake load=620N; diameter of brake drum=1m; oil consumption=4.25kg/hr;i.m.e.p=2.75bar,heating value of fuel used=45000kj/kg; Air fuel ratio by mass=32; Room temperature=3700c. Calculate(i)IP,(ii)BP,(iii)Indicated thermal efficiency,(iv)Brake thermal efficiency and (v)percentage heat loss of exhaust. Solution: d=210mm=0.21m, l=280mm=0.28m Pm=2.75bar=275KN/m2, N=350RPM (W-S)=620N=0.62KN,Rm=D/2=1/2=0.5m Piston area, a= 0.21)2/4=0.03462m2

(i) IP=Pm*l*a*n/60= (275x0.28x0.03462x350)/60 = 15.55kW Ans (ii) BP=(W-S)Rm 11.356kW Ans (iii)B.Th.E = BP x 3600 / mf x Cv = [11.356x 3600/4.25x45000] x 100

= 21.37% Ans (iv) I.Th.E = IP x3600 / mf x Cv = [15.55x 3600/4.25x45000] x 100

= 29.27% Ans

(v) Percentage of heat lost in exhaust gases: Heat to exhaust =mg x Cp (tg-tf) Where mg= (mass of air +mass of fuel) = (ma+mf) =mf[ma/mf+1]=mf(AFR+1)=4.25(32+1) =140.25kg/hr=0.03896kg/s Therefore, Heat to exhaust=0.03896x1.007 (370-20) =13.73 kJ/s Heat supplied in fuel = mf x Cv = 4.25/3600 x 45000 = 53.125 kJ/s Percentage heat lost to exhaust gases, = {13.73/53.125} x 100=25.84% Ans

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2. A six cylinder single acting I.C engine 12 cm bore and 15 cm stroke has a piston speed of 480metres per minute. it develops 45kW BP and has mechanical efficiency of 75%.The mean effective pressure is 4.42bar.The specific fuel consumption is 0.25 kg per kW of BP hour. If the fuel has a heating value 42000 kJ/kg; determine (a) Whether this is a two or four stroke engine (b) The brake thermal efficiency Solution: d=12cm=0.12m l=15cm=0.15m BP=45kw

mech=75%;Pim=4.42bar=442KN/m2 bsfc=0.25kg/kwh;Cv=42000kJ/kg

mech=BP/IP IP= mech=45/0.75=60kW Also IP=k*Pim*a*l*n/60

2x 0.15) =800 Therefore, Number of working strokes per min=800 Also,2lN=480 N=480/2 x 0.15=1600RPM Since n=N/2 it is four stroke engine Brake thermal efficiency: B.TH.E=BP x 3600/ mf x Cv =1x 3600/(mf/BP x Cv)=34.28% 3. A 4-cylinder,2-stroke petrol engine develops 40kW at 40 RPS. The m.e.p on each piston is 8 bar and the mechanical efficiency is 80%.calculate the diameter and stroke of each cylinder if stroke to bore ratio is 1.5.calculate BSFC if brake thermal efficiency is 24% CV of fuel is 44 MJ/kg. Solution: BP=40kN, N=40RPS, mech=80%,Pm=8bar=800kN/m2 IP=BP/ mech=40/0.8=50kW IP=k*Pm*a*l*N 50=4 x 800 x (d2) x 1.5d x40 d = 0.069 = 69mm and l=1.5 x 69 =103.5m B.Th.E= BP X 3600 / mf x CV mf=1 x 3600 / 0.24 x 44000 =0.341 kg/kwh. BSFC=0.341 kg/kwh

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4.A double acting air compressor of 50kW in which air is drawn at 1 bar ,500c and compressed according to the law PV1.2=constant to 6 bar. The compressor runs at 150 RPM with an average piston speed of 175 m/min. neglecting clearance, find the sizes of the cylinder. Solution: P = 50kW, P1=1bar,T1=50+273=323k,P2=6bar N=150rpm,speed=2lN=175m/min,n=1.2 Wnet=n/n-1 x P1V1[(P2/P1)n-1/n 1] Interms of Pm(MEP), Wnet=PmVs= n/n-1 x P1V1[(P2/P1)n-1/n 1] Neglecting,VS=V1 Pm= n/n-1 x P1 [(P2/P1)n-1/n 1] =1.2/1.2-1 x 1x[(6/1)1.2-1/1.2-1] =2.088bar =208.8KN/m2 Also, P= Pm

2/4)X175/60 50=478.3d2 Or d= =0.323m=323mm L=175/2N=175/2x150=0.583m=583mm 5. Acompressor discharges 0.5m3 per min free air at 6.5 bar .free air is measured at 1 bar and 150c.Isothermal compression efficiency is 0.72. Mechanical efficiency is 0.9.determine the shaft power. Solution: P1=1bar P2=6.5bar Volume of free air = 0.5 m3/min Wiso=P1V1loge(P2/P1) =100 x 0.5 loge(6.5/1)=93.59 kJ/min Piso=93.59/60=1.5598kW Considering the compression efficiency Indicated power=isothermal efficiency/isothermal efficiency IP=1.5598/0.72=2.166KW Shaft power (BP)=IP/ mech=2.166/0.9=2.407KW

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Multiple choice questions 1. Mechanical efficiency is ratio of (a) Fp to bp (b) Fp to ip (c) Bp to ip (d) Ip to fp 2. If N is the rpm, number of power strokes/min in a four stroke engine is (a) 2N (b) N/2 (c) N (d) 4N 3. If N is the rpm, number of power strokes/min in a four stroke engine is (a) N (b) 2N (c) N/2 (d) 4N 4. An indicator from an engine has a length of 100mm and an area of 2000 mm2. If the

indicator pointer deflects 10mm for a pressure increment of 2bar, the mep is (a) 2bar (b) 4bar (c) 8bar (d) 1bar

5.The spark timing and combustion rate should be such that (a) Peak pressure occurs at TDC (b) One half of the total pressure occurs at TDC (c) Ignition delay is reduced (d) None of the above 6. Volumetric efficiency is a measure of (a) Speed of the engine (b) Power of the engine (c) Breathing capacity of the engine (d) Pressure rise in the cylinder

7.Indicated power is directly proportional to (a) Torque (b) Air consumption (c) Cylinder peak pressure (d) None of the above

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8. Turbocharger engines are those in which charge density is increased by (a) Separate air compressors (b) Compressors driven by exhaust gas turbine (c) Cooling inlet air (d) None of the above 9. Brake thermal efficiency of SI engine is in the range (a) 35%-60% (b) 25%-35% (c) 60%-80% (d) None of the above

10. Performance mep shows (a) indicated power vs speed (b) bmep vs piston speed under various conditions

bth vs speed under various conditions ith vs speed under various conditions


11.The bp of a four cylinder engine is 30 with all cylinder firing and 20 with one cylinder cut. The mechanical efficiency is \ 12.. The bore and stroke of a single cylinder four stroke engine are 100 mm and 160 mm respectively. If the brake torque is 50 NM the bmep is 13.The volumetric efficiency of a well designed engine is in the range 14.The normal efficiency of petrol engine as compared to diesel engine is 15. For SI engine with engine speed, torque

16.For SI engine, air consumption with engine speed 17.Charge efficiency depend on 18.Indicated mean effective pressure is given by 19. Screw compressor belongs 20.The compressor capacity of a reciprocating compressor is directly proportional to

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Keys :

Multiple choice questions Fill in the blanks questions 1.c 11.75% 2.b 12.5bar 3.a 13.75-90% 4.b 14.Lower 5.b 15.Increases the decreases 6.c 16. Increases the decreases 7.b 17. compression ratio 8.b 18.Imep=Fmep+bmep 9.b 19.positive displacement compressor 10.b 20.speed

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UNIT-IV

Two marks questions with answers 1. Define complete (or) Perfect Intercooling? Answer: If the temperature of air leaving the intercooler 3 is equal to the original inlet temperature of air 1 , then the intercooling is known as perfect intercooling. If T3 is not given, assume 3 1. Then the intermediate pressure P2 1 3

2. Define imperfect intercooling? Answer: If the temperature of air leaving the intercooler 3 is greater than the original inlet temp. of air 1 , then the intercooling is imperfect intercooling. 3. Name different types of rotary positive displacement compressors? Answer: Different types of rotary positive displacement compressor, they are 1. Roots blower compressor 2. Vane type blower compressor 3. Lysholm compressor. 4. Name different types of rotary non- positive displacement compressors? Answer: Different types of rotary non-positive displacement compressor, they are 1. Centrifugal compressor 2. Axial flow compressor 5. Define Stagnation State? Answer: The state of a fluid attained by isentropically decelerating it to zero velocity at zero elevation is referred to as the It is often used as a reference state.

Three marks questions with answers Stagnation Properties?

Answer: The properties of the fluid at the stagnation state are the stagnation properties of the gas. 2. Define stagnation enthalpy? Answer: Stagnation enthalpy of a gas or vapour is its enthalpy when it is adiabatically decelerated to zero velocity at zero elevation. 3. Define slip factor? Answer: Slip factor is defined as the ratio of actual whirl 2

2

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4. Compare between reciprocating and centrifugal compressor? Answer:

5. Explain the term work factor? Answer: The actual work input to the air is greater than the theoretical value due to friction between the casing and the air carried around by the vanes. In order to take this into account

w is introduced, so that the actual work done on the air become Work factor Actual work supplied

Theoretical work supplied

act 2 2 02 01

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Five marks questions with answers 1. Explain secondary flow losses in an axial flow compressor. Answer: Secondary flows are produced by combined effects of curvature and boundary layer. Secondary flow is developed when the components of

velocity are developed from the deflection of an initially sheared flow. Such secondary flow occurs when there is a bend, when a sheared flow passes over an aerofoil shape with finite lift or when a boundary level meets an obstacle.

Secondary flow loss occurs when boundary layers are growing on the casing and hub walls of the machines are deflected by rows of blades - stator and rotor.

2.Comparison between reciprocating and rotary compressors? Answer:

Reciprocating air compressors Rotary air compressors Suitable for low discharge of air at high pressure.

Suitable for handling large volumes of air at low pressures.

Low speed (RPM). High speed (RPM). Pulsating air supply. Continuous air supply. More cyclic vibrations occur. Less vibrations occur. Complicated lubricating system. Simple lubrication system. Air delivered is generally contaminated with oil.

Air delivered is relatively more clean.

Large compressor size for the given discharge.

Small size for same discharge

250 300 m3/min Free air Delivery. 2000 3000 m3/min FAD. High delivery pressure. Low delivery pressure.

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3. Comparison between centrifugal and Axial compressors? Answer:

Centrifugal compressors Axial flow compressors Radial flow Axial flow (Parallel to the direction of axis of

themachine) Pressure ratio per stage is high, about 4.5:1. This unit is compact.

Low pressure ratio per stage about 1.2:1. This is due to absence of centrifugal action. Less compact and less rugged.

Isothermal efficiency is about 80 to 82%

With modern aerofoil blades, iso is about 86 to 88%.

Frontal area is larger Frontal area is smaller. Hence the axial flow compressor is more suitable for jet engines due to less drag.

More flexibility of operation due to adjustable prewhirl and diffuser vanes.

Less flexibility of operation.

Low starting torque required. High starting torque required.

Multistaging is slightly difficult. More suitable for multi-staging. Upto 400 bar delivery pressure is possible.

Delivery pressure is only upto 20 bar.

It is used in application of blowing engines in steel mills, low pressure refrigeration, big central air conditioning plants, fertiliser and industry, supercharging I.C. engines, gas pumping in

Mostly used in jet engines due to higher efficiency and smaller frontal area. Also used in power plant gas turbines and steel mills.

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4. Discuss about centrifugal compressors? Answer: Centrifugal Compressor: Principle: The compression principle of centrifugal compressor is quite different from that of reciprocating or rotary type compressor. When the air passes through the rotating impeller it experiences force or work which is performed by centrifugal forces. The work input takes place as an increase in pressure and

entering in the diffuser section. The diffuser is actually a fixed or static component that escorts the air flow when it leaves the impeller. This loss in velocity eventually results in an additional increase of pressure. The impeller and the diffuser contributes about 65% and 35% of the total pressure developed or produced in the compressor. Construction: A centrifugal compressor generally consists of four components named inlet , impeller,diffusor and collector.

long distance pipe lines etc.

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1.Casing and inlet . The above mentioned components are usually protected or guarded by a casing or housing . A case house consists of number of bearings in order to provide radial and axial support of the rotor. The case also contain nozzles along with inlets and discharge flow connections in order to introduce and extract flow from the compressor. Casing are of two types :-

Horizontally split Vertically split

2. Impellers. The impellers are assembled or mounted on a steel shaft and this assembly is known as compressor rotor (mostly in multi stage compressors). The rotor provide velocity to the gas with blades that are attached to a rotating disc. These blades can be forward-leaning , radial or backward-leaning depending upon the desired output . Most of the multistage compressors use backward-leaning blades as they provide the widest range of efficiency. 3. Diffuser. The impeller extracts the gas with great velocity into a diffuser passage. The diffuser usually compromise two walls which form a radial channel. Because of these arrangements the velocity of the gas decreases and dynamic pressure is converted into static pressure. The diffuser passages are small space between adjacent diaphragms which generally turns the gas flow 180° in order to direct it towards the next impeller. 4.Collector. Following the last stage impeller the gas must be collected and delivered to the discharge flange. The component used to collect the gas discharged through the diffuser is called as collector. It may also be termed as volute or scroll. The collector may also contain valves and other instrumentation in order to control the compressor.

Types: There are two types of it :-

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Single stage centrifugal compressor Multi stage centrifugal compressor

Both of these compressor work on the same principle but they do have some drastic difference in their construction and working. Single stage compressor. Single stage compressors consist of only a single impeller and it is use for moving the air or other gases up to 3 to 1 compression ratio for either pressure or vacuum duty. These type of compressors are considered to have a beam design or an overhung impeller arrangement. In this type of arrangement the impeller is at the non-driving end of the shaft. One major advantage of it over the multistage compressor is that it provides high efficiency and the delivered gas is totally oil and surge free. Multistage compressor. Multistage compressors consists of 1-10 impellers and it can be arranged in a variety of flow path configurations. Throughout each and every stage the temperature and the compression ratio are assumed to be constant. Multistage compressor can be arranged in straight-through, compound, and double flow configurations. Multistage compressor are also considered to have beam-type design but the impellers are located between the radial bearings . Applications of centrifugal compressor: Compressed gas or air: Centrifugal compressor is one of the most simple and efficient way to obtain or produce compressed air. They are best suited when the demand of air or gas is constant and excessive. Food Industry: Food processing industry depends highly on this type of compressor as it can provide oil free compressed air which are necessary for some sensitive petitions. Gas turbines: Gas turbines use either or both axial and centrifugal compressor to provide the necessary compression. Centrifugal compressors are mostly used in gas turbines such as :-

Turboshaft Turboprop Micro turbines and Auxiliary power units

Oil refiners, petrochemical and chemical plants: The centrifugal compressors used for the above purposes generally have a horizontally split casing and most of them are multistage compressors. These type of compressors are generally operated by over-sized steam engines and gas turbines.

Refrigeration and air control: Centrifugal Compressors support a wide variety of refrigerants and thermodynamics and are also able to supply compression in water chiller cycles due to which it has a high demand for usage in refrigerators and air conditioners.

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Advantages and Disadvantages: Advantages a.) When compared to other compressors, it is relatively agile and easy to manufacturer. b.) As this compressor does not require any special foundation it is highly energy efficient and reliable. c.) They consist of a small number of rubbing parts and are absolutely oil free in nature. d.) It generates a higher pressure ratio per stage than the axial flow compressor. Disadvantages. a.) They produce a limited amount of pressure and are not suitable for very high compression. b.) As they work at relatively high speed an enlightened or worldly mounting is required. c.) They are very sensitive towards problems such as stalling and choking. 5. Explain the working of screw compressor? Answer: Rotary compressors are the another type of famous compressors.It uses two Asymmetrical rotors that are also called helical screws to compress the air.

The rotors have a very special shape and they turn in opposite directions with very little clearance between them.The rotors are covered by cooling jackets.Two shafts on the rotors are placed that transfer their motion with the help of timing gears that are attached at the starting point of the shafts/compressor

Working principle-Air sucked in at one end and gets trapped between the rotors and get pushed to other side of the rotors .The air is pushed by the rotors that are rotating in opposite direction and compression is done when it gets trapped in clearance between the two rotors.Then it pushed towards pressure side.

Rotary screw compressors are of two types oil-injected and oil-free.

Oil-injected is cheaper and most common than oil-free rotary screw compressors.

Advantages: Less noisy. These are called the work-horses as they supply large amount of compress air. More energy efficient as compared to piston type compressors.

The air supply is continuous as compared to reciprocating compressors. Relatively low end temperature of compressed air.

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Disadvantages: Expensive then piston-type compressors. More complex design. The maintenance is very important Minimum one day use is important in a weak to avoid rusting.

Multiple choice questions 1. Which of the following type does Screw compressor belongs to? a) Positive displacement compressor b) Dynamic compressors c) Both a & b d) None of the above

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2. The compressor capacity of a reciprocating compressor is directly proportional to __ a) Speed b)Pressure c) Volume d) All

3. The specific power consumption of non lubricatedcompressor compared to lubricated type is ____ a) Lesser b) Same c)Higher d) None 4. The discharge temperature of two stage compressor compared to single stage one is ____ a) Lesser b) Same c) Higher d) None

5.The compression ratios for axial flow compressors are ____. a) Lesser b) Higher c) moderate d) None

6.The volumetric efficiency of the compressor ______ with the increase in altitude of place a) increases

b) decreases c) does not change d) None 7.The ratio of isothermal power to actual measured input power of a compressor is known as: a) Isothermal efficiency b)Volumetric Efficiency c) Barometric efficiency d) None 8.The basic function of air dryer in a compressor is: a. prevent dust from entering compressor b. storage and smoothening pulsating air output c. reduce the temperature of the air before it enters the next state to increase efficiency d. to remove remaining traces of moisture after after-cooler

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9.For every 4°C raise in air inlet temperature of an air compressor, the power consumption will increases by_____

a)2% b)1% c)3% d)4% 10. The percentage increase in power consumption of a compressor with suction side air filter and with the pressure drop across the filter of 200 mm Wc is ____ a)1.0% b) 3% c)2.4% d)1.6%


11. The compressor shou 12.Reduction in the delivery pressure of a Compressor working at 7 bar, by 1 bar would reduce the power consumption by 13.The acceptable pressure drop at the farthest point in mains header of an industrial compressed air

14.The likely estimate on equivalent power wastage for a leakage from 7 bar compressed air system through 1. 15.From the point of lower specific energy consumption, which of the following compressors are suitable for part load operation 16.From base load operation and from achieving best specific energy consumption point of view, which of the fol

are not required for evaluating volumetric efficiency of the compressor 18.If the compressor of 200 cfm loads in 10 seconds and unloads in 20 seconds, the air leakage

19. For every 40C rise in the inlet air temperature, the Increase in energy consumption is by 20.Vertical type reciprocating compressors are used in the capacity range of Keys :

Multiple choice questions Fill in the blanks questions 1.a 11.surge point 2.a 12.6 to 10% 3.c 13.0.3bar 4.a 14.0.8kW 5.b 15.Two stage reciprocating compressor 6.b 16. centrifugal compressor 7.a 17. power

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8.d 18.67CFM 9.b 19.1% 10.d 20.50-150 CFM

UNIT-V Two marks questions with answers

1. Define refrigeration? Answer: Refrigeration is a process of removing heat from a low-temperature reservoir and transferring it to a high-temperature reservoir. The work of heat transfer is traditionally driven by mechanical means, but can also be driven by heat, magnetism, electricity, laser, or other means.

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Refrigeration has many applications, including, but not limited to: household refrigerators, industrial freezers, cryogenics, and air conditioning. 2. What is unit of refrigeration? Answer: A unit of refrigeration (TR), also called a refrigeration ton (RT), is a unit of power used in some countries (especially in North America) to describe the heat-extraction capacity of refrigeration and air conditioning equipment. It is defined as the rate of heat transfer that results in the melting of 1 short ton (2,000 lb; 907 kg) of pure ice at 0 °C (32 °F) in 24 hours 3.What is the effect of sub cooling? Answer:This is a process of cooling the refrigerant liquid below its condensing temperature at a given pressure .Subcooling provides 100% refrigerant liquid to enter the expansion device, preventing vapor bubbles from impeding the flow of refrigerant through the expansion valve. If the subcooling is caused by a heat transfer method external to the refrigeration cycle, the refrigerant effect of the system is increased, because the subcooled liquid has less enthalpy than the saturated liquid. Subcooling is accomplished by refrigerating the liquid line of the system, using a higher temperature system. Simply we can state, subcooling cools the refrigerant more and provides the following accordingly:

usage,

4.What is the Effect of superheating? Answer:

During the evaporation process the refrigerant is completely vaporized partway through the evaporator

. As the cool refrigerant vapor continues through the evaporator, additional heat is absorbed to superheat the vapor. Under some conditions such pressure losses caused by friction increase the amount of superheat.

If the superheating takes place in the evaporator, the enthalpy of the refrigerant is raised, extracting additional heat and increasing the refrigeration effect of the evaporator. If it is provided in the compressor suction piping, no useful cooling occurs.

In some refrigeration systems, liquid-vapor heat exchangers can be employed to superheat the saturated refrigerant vapor from the evaporator with the refrigerant liquid coming from the condenser.

the heat exchanger can provide high system COP. Refrigerant superheating can also be obtained in the compressor. In this case, the saturated refrigerant vapor enters the compressor and is superheated by increasing the pressure, leading to the temperature increase.

Superheating obtained from the compression process does not improve the cycle efficiency, but results in larger condensing equipment and large compressor discharge piping.

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The increase in the refrigeration effect obtained by superheating in the evaporator is usually offset by a decrease in the refrigeration effect in the compressor. Because the volumetric flow rate of a compressor is constant, the mass flow rate and the refrigeration effect are reduced by decreases in the refrigerant density caused by the superheating. In practice, it is well known that there is a loss in the refrigerating capacity of 1% for every 2.5°C of superheating in the suction line. Insulation of the suction lines is a solution to minimize undesirable heat gain.

The superheating is a process to remove excess heat from superheated refrigerant vapor, and if accomplished by using an external effect it will be more useful to the COP. superheating is often considered impractical, owing to the low temperatures (less than 10°C) and small amount of available energy.

5. What is the function of Expansion valve? Answer: Expansion valve reduces condensing pressure and temperature to evaporation temperature and pressure to get cooling.

Three marks questions with answers 1. Define (i) actual COP (ii) Theoretical COP (iii) Relative COP? Answer: coefficient of performance (cop): the effectiveness of a refrigerator is determined by coefficient of performance. It is defined as the ratio of refrigerating effect(useful effect) to the energy expanded in the cycle. Coefficient of performance = refrigerating effect/energy spent COP=N/W (i)Actual COP is the ratio of N and W which are actually during the test. (ii)Theoretical COP is the ratio of N and W which is obtained by applying laws of thermodynamics to the refrigerating cycle. (iii)The ratio of actual COP to theoretical COP is called as Relative COP. Relative COP=Actual COP/Theoretical COP. 2. What is the difference between wet compression and dry compression? Answer: If the Carnot vapour cycle follows the path 1-2-3-4, then there is dry compression of the refrigeration vapor since the refrigerant is dry saturated at state 1. This type of compression is desirable in the compressor. But, in this case we see that the refrigerant now has to be compressed isothermally from state 2 to state 3, which is impossible to achieve in practice. The alternate path 1'-3-4-5 involves a wet compression of the vapor from state 1' to state 3. Wet compression is highly undesirable as the compressor now has to deal with two different fluid phases. Besides, the liquid droplets present in the vapor would now react with the lubricant in the compressor which is highly undesirable. Thus, we see that both the paths of the Carnot vapor cycle are not suitable for use in practical refrigeration systems.

3. Write short notes on p-h chart? Answer: The refrigeration industry did not always have the analysis tools that are available today. For many decades, the manufacturers and technicians relied on the

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graphical and tabulated values of refrigerant properties and expected equipment performance. One of their favorite tools was the pressure-enthalpy diagram which defines the thermodynamic properties for the refrigerant in use and the performance of the equipment. This chart shows the pressure expressed in bar along the vertical axis. The energy content or enthalpy of the refrigerant is shown along the horizontal axis in kJ/kg. The above pressure-enthalpy chart is typical of the refrigerant R22, a common refrigerant in small refrigeration systems. A closer analysis of the chart shows that there are distinct regions separated by

is a liquid-vapor mixture. If the liquid is at the boiling point, but just hasn't begun to boil, it is defined as saturated liquid. Adding any heat to this liquid will vaporize a portion of it. Adding more heat to the liquid-vapor mixture eventually evaporates all of the liquid. At some precise point (G), the vapor is fully saturated. Adding any more heat to the vapor will cause it to rise in temperature further; this is referred to

always the case. Superheated vapours can be cold. By the term superheated, we simply mean that they are above the corresponding saturated vapor point. Similarly, subcooled liquid can be generally warm. It just means that the liquid is cooler than the saturation line at that pressure. We now present a detailed study of the pressure-enthalpy diagram. We consider the refrigerant to be initially at point A. To reach this point after leaving the evaporator at G, the refrigerant is heated slightly and crosses the compressor suction valve to point A. The compressor elevates the refrigerant's pressure to a point at which it can push the discharge valve open and flow into the condenser. The refrigerant vapor leaves the compressor at point B, desuperheats to point C, and then begins to condense. After the vapor is completely condensed at point D, it is subcooled a bit further (E), at which time it is still at a much higher pressure than the evaporator. Controlling the flow to the evaporator and throttling to the pressure of the evaporator is the job performed by the expansion device, a capillary tube or a throttling valve in small refrigeration systems. This pressure reduction step 73 Refrigeration Systems vaporizes a portion of the liquid which cools (called flash gas) the remaining liquid going to point F. The "average" mixture of vapor and liquid crossing the valve doesn't change in energy content. It simply separates into liquid and vapor at the reduced temperature and pressure according to its precise thermodynamic properties. The liquid at point F is then ready to pick up heat in the evaporator and form vapor at point G where the cycle repeats itself. 4.What are the different components of vapour compression refrigeration system? Answer: There are six main components in a refrigeration system: (i) The Compressor (ii) The Condenser (iii) The Metering Device or expansion valve (iv) The Evaporator (v) Piping material (vi) Refrigerant 5.What is the function of capillary tube in vapour compression refrigeration system? Answer: When the refrigerant leaves the condenser and enters the capillary tube its pressure drops down suddenly due to very small diameter of the capillary. In capillary the fall in pressure of the refrigerant takes place not due to the orifice but due to the small opening of the capillary. The decrease in pressure of the refrigerant through the capillary depends on the diameter of the capillary and the length of the capillary. Smaller is the diameter and more is the length of the capillary more is the drop in pressure of the refrigerant as it passes through it.

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In the normal working conditions of the refrigeration plant there is drop in pressure of the refrigerant across the capillary but when the plant stops the refrigerant pressure across the two sides of the capillary emuch load on it. Also, due to this reason one cannot over-charge the refrigeration system with the refrigerant and no receiver is used.

Five marks questions with answers 1. Describe a simple vapour compression cycle giving clearly its flow diagram?

Answer: Simple Vapour Compression System

shows a simple vapour compression refrigeration system which consists of

(a) Compression : The vapour at low pressure and low temperature (point 2) enters to compressor where it is compressed reversible adiabatically (isentropically). The pressure and temperature increase and pass through discharge line (point 3). (b) Condensation : The vapour at high pressure and temperature enters to condenser where it is condensed to high pressure liquid (point 4) and is collected by receiver tank. (c) Expansion : The liquid from receiver tank passes through the expansion valve where it is throttled (isoenthalpy) and passed at a controlled rate after reducing its pressure and temperature. (d) Vaporisation: In evaporator the liquid at low pressure and temperature evaporates and is changed into vapour. During vaporisation the liquid absorbs latent heat of vaporisation from the medium (air, water or brine) which is to be cooled.

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Vapour compression cycle on T-S and P-h is shown Coefficient -of Performance of Refrigerator for Vapour Compression Cycle Coefficient of Performance of refrigerator for vapour compression cycle is given by COP = h2 h1 / h3 h2 ( COP = heat rejected evaporator / work input to compressor) The Quantity of Refrigerant (m r) Circulated in the Refrigeration Plant The Quantity of refrigerant is given by m r = 210 T / h2 h1 kg / min where T = Load in tons of refrigeration on the refrigerator Power Required to Drive the Compressor Power = m r (h3 h2) / 60 KW Where h is in KJ / kg Quantity of Cooling Water (me) Circulated in the Condenser Quantity of cooling water circulated the condenser per minute can be found out from the equation m, Cc (t0 t;) = m, (h3 h4).

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where mc = mass of cooling water circulated per minute Cc = specific heat of cooling water t0 =outlet temperature of cooling water t1 = inlet temperature of cooling water. 2. What are desired properties of refrigerants? Answer: Desired Properties of a Refrigerant:

1. Vapor density: To enable use of smaller compressors and other equipment the refrigerant should have

smaller vapor density.

2. Enthalpy of vaporization: To ensure maximum heat absorption during refrigeration, a refrigerant should have high enthalpy

of vaporization.

3. Thermal Conductivity: Thermal conductivity of the refrigerant should be high for faster heat transfer during

condensation and evaporation.

4. Dielectric strength: In hermetic arrangements, the motor windings are cooled by refrigerants vapor on its way to the

suction valve of the compressor. Therefore, dielectric strength of refrigerant is important

property in hermetically sealed compressor units.

5. Critical temperature: In order to have large range of isothermal energy transfer, the refrigerant should have critical

temperature above the condensing temperature.

6. Specific heat: To have minimum change in entropy during the throttling process, the specific heat should be minimum. For this, liquid saturation line should be almost vertical.

7. Leak tendency: The refrigerant may leak out of the system. The problems with leakage are wearing out of joint

or the material used for the fabrication of the system. A denser refrigerant will have fewer

tendencies to leak as compared to higher density refrigerant. The detection of leaks should be

easy to loss of refrigerant. Leakage can be identified quickly if the refrigerant has distinct color

or odour.

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8. Toxicity: The refrigerant used in air conditioning, food preservation etc. should not be toxic in nature as

they will come into contact with human beings. refrigerants will affect human health if they are toxic.

9. Cost of refrigerants: The quantity of refrigerant used in industries is very less. The cost of the refrigerants is generally

high when compared to other chemicals in the industry. Very low industry professional will not

take necessary action to control the leaks.

10. Availability: Refrigerants should be available near the usage point. It must be sourced and procured within a

short period to enable the user in case of leaks, maintenance schedules etc.

Properties of Commonly Used Refrigerants:

1. Carbon dioxide: Carbon dioxide is widely as refrigerant in mechanical systems refrigerant, marine services,

hospitals etc. due to its excellent safety properties. It is odourless, non-toxic, non-flammable, non-explosive and non-corrosive.

2. Sulphur dioxide: Sulphur dioxide was widely used as refrigerant during early 20th century. However its use has

been restricted now-a-days because of its many inherent disadvantages. It is highly toxic, non-

flammable, non-explosive, non-corrosive and works at low pressures

3. Ammonia: Ammonia is one of the earliest type of refrigerants which is still widely used in many

applications due to its inheritance excellent thermal properties, It is toxic in nature, flammable

explosive under certain conditions, it has low specific volume¸ high refrigerating effect, low

piston displacement in case of reciprocating compressors make it an ideal refrigerant for cold

4. Freon-11: Freon-11 (Trichloro fluoromethane) is used under low operating pressures; it is non-toxic, non-

corrosive and non-flammable. Due to low operating pressure and high displacement, it is used in

systems employing centrifugal compressors. It is used for air-conditioning applications.

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5. Freon-12: Freon-12 (Dichloro difluoromethane) is non-flammable, non-toxic and non-explosive. It is

highly chemically stable. If it is brought into contact with open flame or heater elements, it decomposes into highly toxic constituents. It has not only excellent safe properties but also

condenses at moderate pressure under normal atmospheric conditions.

6. Cryogenic refrigerants: Cryogenic refrigerants are those refrigerants which produce minus temperature in between range

-157°C to -273°C in the refrigerated space. The cryogenic refrigerants have exceptionally low

boiling point at atmospheric pressure. Some of the widely used cryogenic refrigerants are

Helium, Nitrogen, Oxygen, and Hydrogen. 3. Explain air refrigeration system? Answer: The simple air refrigeration cycle works on Reversed Brayton or Joule cycle. The four processes of the cycle are:

1. Isentropic compression process (1-2): Cold air from refrigerated space is drawn into the compressor and compressed to the cooler pressure. The air temperature increases from T1 to T2.

2. Constant pressure cooling process (2-3): The high pressure and high temperature air is now passed into the cooler where it is cooled at constant pressure.

3. Isentropic expansion process (3-4): Air from cooler is drawn into the expander where it is expanded isentropically from cooler pressure to refrigerator pressure which is equal to atmospheric pressure. The temperature of air also falls from T3 to T4 i.e. much below the cooling water temperature T3.

4. Constant pressure heating process (4-1): This cold air from expander is now sent to the refrigerated space where it absorbs the heat at constant pressure.

4.Explain reversed carnot cycle on T-S diagram? Answer:

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Reversed Carnot cycle ,It consists of the following processes. Process a-b: Absorption of heat by the working fluid from refrigerator at constant low temperature T2 during isothermal expansion. Process b-c: Isentropic compression of the working fluid with the aid of external work. The temperature of the fluid rises from T2 to T1. Process c-d: Isothermal compression of the working fluid during which heat is rejected at constant high temperature T1. Process d-a: Isentropic expansion of the working fluid. The temperature of the working fluid falls from T1 to T2. COP of Refrigerator=Heat absorbed /Work supplied =Heat absorbed /(Heat rejected - Heat absorbed) Practically, the reversed Carnot cycle cannot be used for refrigeration purpose as the isentropic process requires very high speed operation, whereas the isothermal process requires very low speed operation. 5. Explain with neat sketch the working of a vapour absorption system?

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Answer: The vapor absorption refrigeration system comprises of all the processes in the vapor compression refrigeration system like compression, condensation, expansion and evaporation. In the vapor absorption system the refrigerant used is ammonia, water or lithium bromide. The refrigerant gets condensed in the condenser and it gets evaporated in the evaporator. The refrigerant produces cooling effect in the evaporator and releases the heat to the atmosphere via the condenser. The major difference between the two systems is the method of the suction and compression of the refrigerant in the refrigeration cycle. In the vapor compression system, the compressor sucks the refrigerant from evaporator and compresses it to the high pressure. The compressor also enables the flow of the refrigerant through the whole refrigeration cycle. In the vapor absorption cycle, the process of suction and compression are carried out by two different devices called as the absorber and the generator. Thus the absorber and the generator replace the compressor in the vapor absorption cycle.

The absorbent enables the flow of the refrigerant from the absorber to the generator by absorbing it. Another major difference between the vapor compression and vapor absorption cycle is the method in which the energy input is given to the system. In the vapor compression system the energy input is given in the form of the mechanical work from the electric motor run by the electricity.

In the vapor absorption system the energy input is given in the form of the heat. This heat can be from the excess steam from the process or the hot water. The heat can also be created by other sources like natural gas, kerosene, heater etc. though these sources are used only in the small systems. Condenser: Just like in the traditional condenser of the vapor compression cycle, the refrigerant enters the condenser at high pressure and temperature and gets condensed. The condenser is of water cooled type. Expansion valve or restriction: When the refrigerant passes through the expansion valve, its pressure and temperature reduces suddenly. This refrigerant (ammonia in this case) then enters the evaporator. Evaporator: The refrigerant at very low pressure and temperature enters the evaporator and produces the cooling effect. In the vapor compression cycle this refrigerant is sucked by the

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compressor, but in the vapor absorption cycle, this refrigerant flows to the absorber that acts as the suction part of the refrigeration cycle. Absorber: The absorber is a sort of vessel consisting of water that acts as the absorbent, and the previous absorbed refrigerant. Thus the absorber consists of the weak solution of the refrigerant (ammonia in this case) and absorbent (water in this case). When ammonia from the evaporator enters the absorber, it is absorbed by the absorbent due to which the pressure inside the absorber reduces further leading to more flow of the refrigerant from the evaporator to the absorber. At high temperature water absorbs lesser ammonia, hence it is cooled by the external coolant to increase it ammonia absorption capacity. The absorption refrigeration system comprises of condenser, expansion valve, evaporator, absorber, pump and generator. The refrigerant leaving the evaporator enter the absorber, where it is absorbed by the absorbent. The strong solution of refrigerant-absorber enters the generator with the help of the pump. The refrigerant then enters the condenser while the remaining weak solution enters back to the absorber and the cycle is repeated. he initial flow of the refrigerant from the evaporator to the absorber occurs because the vapor pressure of the refrigerant-absorbent in the absorber is lower than the vapor pressure of the refrigerant in the evaporator. The vapor pressure of the refrigerant-absorbent inside the absorbent determines the pressure on low-pressure side of the system and also the vaporizing temperature of the refrigerant inside the evaporator. The vapor pressure of the refrigerant-absorbent solution depends on the nature of the absorbent, its temperature and concentration.

When the refrigerant entering in the absorber is absorbed by the absorbent its volume decreases, thus the compression of the refrigerant occurs. Thus absorber acts as the suction part of the compressor. The heat of absorption is also released in the absorber, which is removed by the external coolant.

When the absorbent absorbs the refrigerant strong solution of refrigerant-absorbent (ammonia-water) is formed. This solution is pumped by the pump at high pressure to the generator. Thus pump increases the pressure of the solution to about 10bar . The refrigerant-ammonia solution in the generator is heated by the external source of heat. This is can be steam, hot water or any other suitable source. Due to heating the temperature of the solution increases. The refrigerant in the solution gets vaporized and it leaves the solution at high pressure. The high pressure and the high temperature refrigerant then enters the condenser, where it is cooled by the coolant, and it then enters the expansion valve and then finally into the evaporator where it produces the cooling effect. This refrigerant is then again absorbed by the weak solution in the absorber. When the vaporized refrigerant leaves the generator weak solution is left in it. This solution enters the pressure reducing valve and then back to the absorber, where it is ready to absorb fresh refrigerant. In this way, the refrigerant keeps on repeating the cycle.

The pressure of the refrigerant is increased in the generator, hence it is considered to be equivalent to the compression part of the compressor.

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Multiple choice questions

1.Freon group of refrigerants are (a) Inflammable (b) Toxic (c) Non-inflammable and toxic (d) Nontoxic and non-inflammable 2.The boiling point of ammonia is (a) -10.5°C (b) -30°C (c) -33.3°C (d) -77.7°C 3.For obtaining high COP, the pressure range of compressor should be (a) High (b) Low (c) Optimum (d) Any value 4.A reversible engine has ideal thermal efficiency of 30%. When it is used as a refrigerating machine with all other conditions unchanged, the coefficient of performance will be (a) 1.33 (b) 2.33 (c) 3.33 (d) 4.33 5.Cooling water is required for following equipment in ammonia absorption plant (a) Condenser (b) Evaporator (c) Absorber (d) Condenser, absorber and separator (rectifier) 6.The freezing point of sulphur dioxide is (a) -56.6°C (b) -75.2°C (c) -77.7°C (d) -135.8°C 7.Mass flow ratio of NH in comparison to Freon-12 for same refrigeration load and same temperature limits is of the order of (a) 1: 1 (b) 1: 9 (c) 9: 1

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(d) 1: 3 8.In a refrigeration system, the expansion device is connected between the (a) Compressor and condenser (b) Condenser and receiver (c) Receiver and evaporator (d) Evaporator and compressor 9.The vapour compression refrigerator employs the following cycle (a) Rankine (b) Carnot (c)Reversed rankine (d) Reversed carnot 10.In actual air-conditioning applications for R-12 and R-22, and operating at a condenser temperature of 40° C and an evaporator temperature of 5° C, the heat rejection factor is about (a) 1 (b) 1.25 (c) 2.15 (d)5.12

Fill in the blanks questions 11.Rating of a domestic refrigerator is of the order of 12.A human body feels comfortable when the heat produced by the metabolism of human body is equal to the 13.The bank of tubes at the back of domestic refrigerator is 14.In a lithium bromide absorption refrigeration system,water is used 15. The condition of refrigerant after passing through the condenser in a vapour compression system is 16.The COP of a vapour compression plant in comparison to vapour absorption plant is 17. The fluids used in Electrolux refrigerator are 18. Domestic refrigerator working on vapour compression cycle uses the following type of expansion device 19.The condition of refrigerant after passing through the expansion or throttle valve, in a vapour compression system is 20.An important characteristic of absorption system of refrigeration is

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Keys :

Multiple choice questions Fill in the blanks questions 1.d 11.0.1ton 2.c 12.heat stored in human body to dissipated surroundings 3.b 13.condnser tubes 4.b 14.refrigerant 5.d 15.saturated liquid 6.b 16. more 7.b 17. Ammonia water and hydrogen 8.c 18.capillary tube 9.d 19.very wet vapour 10.b 20.Quiet operation

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