Chapter 4 -Cooling and Lubrication

8/12/2019 Chapter 4 -Cooling and Lubrication

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Engine Cooling System & Lubrication System

ENGINE COOLING SYSTEM

3.1 Necessity for cooling

In an internal combustion engine, the fuel is burned within the engine

cylinder. During combustion, high temperatures are reached within the cylinder,

for example in a compression ignition engine as high as 2000-2500 0C is reached.

About one third of the heat energy liberated b the burning fuel is converted into

power. Another one third goes out through the exhaust pipe unused. The

remaining one third flows into the various components of the engine, namely,

cylinder, cylinder head, valves, spark plug (in SI engines), fuel injector (in CIengines) and pistons. This heat flow takes place during combustion and

expansion processes.

3.2 Effects of over heating of the engine components:

1. Evaporation of lubricating oil that lubricates the piston and cylinder

wall. This will result in metal to metal contact of the piston and cylinder

wall leading to piston scuffing and piston seizure.

2. Setting up of thermal stresses in the cylinder, cylinder head and piston.

This may lead to cracking of them.

3. Sticking of piston rings in the ring grooves, due to carbonization of the

oil. Ring sticking will result in inefficient sealing of the cylinder,

increased blow by of gases and loss of thermal efficiency.

4. Burning of piston crown.

5. Burning and warping of exhaust valves.

6. Reduction in volumetric efficient i.e. reduced weight of charge retained

in the cylinder.


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3.3 Optimum cooling

To avoid overheating, and the consequent ill effects mentioned above, the heat

transferred to an engine component (after a certain level) must be removed as

quickly as possible and be conveyed to the atmosphere. It will be proper to say

the cooling system as a temperature regulation system.

It should be remembered that abstraction of heat from the working medium by

way of cooling the engine components is a direct thermodynamic loss.

Effects of excessive cooling:

1. Reduction in thermal efficiency.

2. Increased corrosion of engine parts.

3. Reduced mechanical efficiency.

4. Improper vaporization of fuel.

Hence, if high thermal efficiency is desired the quantity of heat abstracted shall

not be more than that necessary to prevent overheating.

3.4 Air cooling

In air cooling, large quantities of air is circulated around the hot enginecomponents. In two wheeler engines, air flows over the engine components due

to the movement of the vehicles. In other types, where the engine is totally

enclosed by the vehicle body, air is forced by a fan or blower of generous

capacity. The fan or blower is fixed to the flywheel.


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The air is delivered through the ducts and is directed along the spaces between

the fins. Typical blower cooling system of an inline engine can be seen in figure

below.

Baffles or cowling surround thecylinder and direct air well over the

cylinders and thereby improve

cooling.


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3.5 Advantages of air cooling:

1. Cylinder and cylinder head casting are less complicated.

2. Engine weight is reduced, because engine jackets are not there.

3. Cheaper to manufacture- both labor and material.

4. Volume or size (overall) may be reduced, as no device such as

radiator is required for re-cooling the coolant.

5. Engine warms up more quickly, and delivers its full power in lesser

time than the liquid cooled engines.

6. Quick starting of the engine is possible even in frosty weather.

7. Rate of coolant frost (ice) damage of the engine (jackets) is not

there.


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8. Risk of coolant frost (ice) damage of the engine (jackets) is not

there.

9. Engine is less liable to break down. Radiator unit and joints tubing

and other sources of coolant leakage.

Disadvantages of air cooling:

1. Greater mechanical noise, particularly because of the fan.

2. Not suitable for multi-cylinder engines, unless a fan (which absorbs

some power) and suitable cowling are used.

3. Not suitable for engines to be mounted on vehicles meant for

agricultural and construction applications. This is because the

space between the cooling fins are likely to be blocked, either partly

or fully with vegetable matter i.e. grass, chaff, straw and weeds,

and mud and other dust particles.

3.6 Liquid Cooling:

In the present day automotive engines, liquid cooling is widely used for the

following reason:

The resistance to heat transfer from the cylinder wall to a liquid in contact with it

is low. When the velocities of the liquid are fairly high, this resistance is very low.

Due to this, the film heat transfer coefficients are high. Also heat transfer is

greater. This fact results in low temperature differences between the cylinder

wall and the coolant when liquid cooling is used instead of air cooling.

The most commonly used substances for cooling is water. Other liquids are

ethylene glycol or Preston or glycerin. These have boiling points much higher

than water. But they have corrosive effects on engine parts.

In liquid cooled engines, spaces are provided around the cylinders and on the

cylinder heads. These spaces are called water jackets. The coolant is circulated


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through these jackets. In most of the automotive engines, the coolant jacket is

cast integral.

3.6.1 Thermosyphon cooling system:

When a vessel of cold water is heated, the hot water will tend to rise (by virtue of

its lower density) and its place will be taken over by cold water. This causes a

definite circulation within the water mass from top to bottom and vice versa. This

is called natural convection. The Thermosyphon, circulation cooling system

shown in fig.1, works on this principle.

In the thermosyphon circulation cooling system, when the engine is cold the

whole water is at the same temperature and is at rest. When the engine is


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operating, the water around the cylinder heads and cylinder walls gets heated

and flow up to the top header tank of the radiator. Now the cold water from the

lower part of the radiator flows and fills the coolant spaces in the cylinder head

and the cylinder block. The hot water flows downward through the radiator tubes.

This water is cooled by the stream that flows past the tubes. Air is sucked by the

fan which is driven by the engine crankshaft. In this system, the water circulation

through the cylinder, cylinder head and the radiator is by natural means.

For success in operation, the passages through the jackets and radiator should

be free and the connecting pipes large. Further the jackets should be placed as

low as possible relatively to the radiator, in order that the hot leg shall have as

great a height as possible.

During operation the water level must on no account be allowed to fall below the

level of the delivery pipe to the radiator top. If this happens, water circulation will

cease.


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In general the thermo-syphon system requires a larger radiator and carries a

greater body of water than the pump circulation system. Further, a somewhat

excessive temperature difference is necessary to produce the requisite

circulation. On the other hand, to some extent it automatically prevents the

engine being run too cold.

The thermo-syphon circulation cooling system is simpler in construction and

operation. This system is used in some motor cars

3.6.2 Forced circulation cooling system (or Pump circulation cooling

system):

Present day automotive engines

are of larger power output and

operate at higher speeds. As such

they tend to operate at higher

temperatures. The rate of coolant

circulation in these engines must

be greater in order to increase the

cooling effect. This necessitates

the use of a water pump in theseengines. The pump ensures forced

circulation of the coolant. The

pump draws cold water from the

radiator bottom tank, circulates it

through the engine coolant jacket

and pushes it back to the hot water

tank (top tank of the radiator). Thepump assisted forced circulation

system can be seen in figure 2.


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1. Radiator:

In mobile units, the hot water let out by the engine must be cooled by some

method and then re-circulated by the cooling system. Radiator is widely used for

this purpose in transport vehicles. The radiator presents a large amount of

cooling surface to the air so that the water passing downward through it in thin

steams is cooled efficiently. The practical factors that govern the choice of the

available radiator are available space, air flow resistance and cost. The details of

the radiator are seen in the figure.


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Details of the radiator and thermostat

Radiators constructed at present are mainly of two types:

1. Vertical water tube type (Down flow) and,

2. Cellular type (Cross flow) .

In the vertical water tube type, the water falls through a nest of tubes from an

upper header tank to a lower tank and air flows horizontally between the tubes. In

the cellular type, the air flows horizontally through short tubes assembled

honeycomb fashion with small clearances between them through which the water

circulates vertically from top to bottom.

The amount of heat that can be dissipated by a radiator depends upon the

following:

1. Relative wind velocity,

2. Air density and humidity,


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3. Water and air temperature,

4. Cooling (or heat transfer) surface provided,

5. Ratio of tube depth to diameter,

6. Conductivity of the metal used,

7. Design of radiator and its disposition.

3. Thermostat

Types of Thermostat

1. Bellow type (Figure 1),

2. Wax type (Figure 2)

A Pump, B - Thermostat


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3.7 LUBRICATION SYSTEM

Lubrication circuit is one of the most important ones in the engine. The enginecannot run smoothly for more than a few minutes without the lubricating oil.Whenever two metallic surfaces move over each other under direct contact, dryor solid friction is produced. This is due to the irregularities on the two surfacesinterlocking each other. The dry friction thus. created produces a lot of heat andresults in wear of the metal surface.

3.8 Purpose of lubrication

1. To reduce friction between the moving parts.

2. To reduce wear of the moving parts by way of lubricating the surfaces andkeeping them apart.

3. Remove heat from engine parts by acting as a cooling agent. To keep

down the temperature of the moving parts and thus prevent scuffing and

seizure.

4. To act as a seal and prevent leakage between the parts such as pistons,

piston rings and cylinders.

5. Absorb shocks between bearings and other engine parts, thus reducing

engine noise and extending engine life.

6. Acts as a cleaning agent. To wash away acidic accumulation and abrasive

metal worn from the friction surfaces.

3.9 Crankcase ventilation

In the case of four stroke engines, air must circulate through the crankcase,

when the engine is running. This removes the hot lubricating oil vapors and blow

by from the crankcase. For entry of air and exit of the gases, the crankcase is

provided with a breather tube. The air entry into the crankcase also cools the oil

to some extent. However, discharging the crankcase vapors into the atmosphere


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causes air pollution. To prevent atmosphere pollution, modern engines have a

positive crankcase ventilation (PCV) system. In this case, the crankcase is

connected by a pipe to the inlet manifold. This enables the oil vapors to be

sucked into the inlet manifold during engine suction and burned.

3.10 Methods of lubrication:

The different parts of an engine are lubricated by anyone combinations of the

following methods.

1. Mechanical system Splash lubrication

2. Pressure lubrication system.

a. Wet sump lubrication system

b. Dry sump lubrication system.

3. Semi pressure lubrication system.

3.10.1 Splash lubrication:

In the splash lubrication system, the oil retained in the oil pan is churned

and splashed up by the internal parts of the engine (connecting rod big end andcrankshaft) into a combination of liquid and mist. This oil mist is sprayed over the

interior of the engine i.e. on the cylinder walls and on the underside of the piston

crown. Some of the oil splashed gets collected in pockets over the main

bearings. The collected oils then flow through the bearings by means of oil hole

and grooves in the bearing surfaces.

In one design, the connecting rod big end caps have scoops. These

scoops pick up oil from the oil pan, during the lower ends of the connecting rodtravel. The connecting rod caps have drilled holes. Through these holes, part of

the oil picked up by the scoop reaches the end bearings. The remaining oil

lubricates the rest of the engine parts by splash i.e. the throwing of oil by the

connecting rods, crankshaft and other oiled moving parts.


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In another design, the connecting rod big end cap has thin projections.

These projections improve oil splashing.

3.10.2 PRESSURE LUBRICATION SYSTEM

In the pressure lubrication system, the lubricating oil is pumped under

pressure to the various engine bearings. The oil is delivered by an oil pump

under a pressure of 1.30 to 1.40 kscm into an oil gallery or distributor duct. This

gallery distributes oil to the various engine parts and bearings. The oil pump is

driven by the engine crankshaft or camshaft. This system can be seen in figure.

The crankshaft main bearing (each one) is pressure fed from the main oil gallery.

Internal drilled holes (or ducts) in the webs of the crankshaft conduct oil from the

main bearings to the connecting rod big end bearings. The oil flows from the big

end bearing through the long hole drilled in the shank of the connecting rod to the

wrist. pin.

The lubricating oil flow through a bearing is controlled by maintaining

limited clearance all around between a round bearing and a round shaft. The

clearance between a round bearing and a round shaft. The clearance between

the two is called oil clearance. Proper clearance must exist and this varies with

different engines, but 0.0375 mm is a typical clearance.


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When there is excessive clearance, the oil pump cannot build up normal

pressure. Excessive clearance can also cause some bearings to fail due to oil

starvation.

On the other hand, if the oil clearances are insufficient, then there will be

metal contact between the bearing and the shaft journal. Extremely rapid wear

and quick failure will occur. Further, there will not be enough oil throw off for

adequate lubrication of the cylinder walls, pistons and rings.

In 1 head engines oil is fed under pressure to the valve mechanisms in the

head. For example, some engines have the rocker arms mounted on hollow

shafts. These shafts feed oil to the rocker arms. Some engines withindependently mounted rocker arms have hollow mounting studs. These studs

feed oil gallery in the hed to the rocker arm ball pivots. On other engines, the oil

flows up through hollow push rods to lubricate the valve stems and other valve

train parts. The oil spills off the rocker arms and provides lubrication for the valve

stems and other valve train parts, the oil spills off the rocker arms and provides


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lubrication for the valve stems and push rod and valve stem tips. Therefore, all

valve mechanism parts are adequately lubricated.

Cylinder walls are lubricated by splashing oil thrown off from the

connecting rod bearings. Some engines have oil spit holes or groove in theconnecting rods that index with drilled holes in the crankpin journals with each

revolution. As this happens a stream of oil is spit, or thrown, onto the cylinder

walls. When oil spit that holes are not used, the engine relies on oil that flows

through the side clearance between the side of the connecting rod and the

crankshaft for cylinder wall lubrication.

In many engines, the piston pins are lubricated with oil scraped off the

cylinder walls. The pistons have grooves, holes, or slots to feed oil from the oilring groove to the piston pin bosses.

Lubrication of piston pin, camshaft bearings, rocker arm shaft, rocker arm,

cam and tappet surfaces can be seen in figure.


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3.10.3 Semi pressure lubrication system

The semipressure lubrication system is a combination of splash and

pressure lubrication system. Most automotive engines use this system. This is

more simple and less costly than the complete pressure lubrication system. This

system also enables bearing loads and engine speeds than for the splash

system.

3.10.4 Wet sump lubrication

In the wet sump lubrication system, the main oil supply is kept in the sump

which is below the engine cylinder in the crankcase. The oil pump draws oil from

the crankcase and forces it through the lubricating oil filters to the various parts of

the engine. This system is widely used incars and trucks.


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In some engines, the oil pump is submerged in the sump oil. Some

designs use tiling intake pipe and strainer unit.

Where there is no oil hole in the connecting rod shank, the cylinder walls

and piston pins are lubricated by the spray of oil that is forced out of the

connecting rod bearings and thrown by the revolving cranks. This spray also

lubricates many exposed internal parts.

A separate line supplies lubricant to the accessories and their drive shafts,

valves and rocker arms. Used oil drains back into the crankcase by gravity for

recirculation.

3.10.5 Dry sump lubrication

The dry sump lubrication is used in more expensive cars. In the dry sump

pressure lubrication system, there is no oil sump in the crankshaft chamber. In

this system, the oil is kept either in a separate tank or reservoir, provided with

cooling fins. Two pumps are used in this system. One pump sucks the oil from

the reservoir and forces it under pressure to the various bearings of the engine,

as in the wet sump system. The other pump (also called scavenger oil pump) is

of large capacity. This pump sucks oil which drains down to the bottom of the

crankshaft chamber, and returns it to the oil reservoir.


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The main advantage of the dry sump lubrication system is that the oil is

cooled during its circulation. As such, the oil has better lubricating value. Drysump system is suitable for engines fitted to vehicles which may have to work in

inclined positions.


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Full flow and partial or by-pass flow pressure system

The full flow system and the partial or bypass flow system can be seen in

figure. The full flow lubrication system is one in which the entire quantity of oil

delivered by the pump is forced through the oil filter before the oil filter reaches

the various parts of the engine. It is the most common and widely used type.

The bypass lubrication system does not filter all of the oil that enters the engine

bearings. It filters some of the extra oil not needed by the bearings.


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Exercise 3

1. What is the necessity of Cooling?2. What are the effects of engine overheating?3. What are the effects of Over cooling and under cooling?4. What are the advantages and disadvantages of air cooling?5. What is thermosyphon cooling? Explain with a neat sketch working

themosyphon cooling?6. Write a short note on with a neat sketch working of i) Forced circulation

cooling systemii) Thermostat cooling

7. What is the purpose of lubrication?8. What are the methods of lubrication? Explain with a neat sketch working

of splash lubrication.9. With a neat sketch explain working of a Pressure lubrication system10. Explain briefly with a neat sketch working of Dry sump lubrication

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Chapter 4 -Cooling and Lubrication