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8/10/2019 FDP_Internal Combustion Engines - II
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LUBRICATINGSYSTEM
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LUBRICATINGSYSTEMS
Mist Lubrication system
Wet sump lubrication system
Dry sump lubrication
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MISTLUBRICATIONSYSTEM
Used in two stroke cycle engines which are usually
crank charged.
Usually 2 to 3% of Oil is added with the fuel.
Oil and fuel mixture is inducted through thecarburetor.
The gasoline vaporizes and the oil in the form of
mist passes through the crank case and lubricates
the bearings and connecting rods.
Rest of the oil passes through the cylinder,
lubricates the cylinder piston and piston rings.
Simple and low cost because of absence of oil
pump
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DISADVANTAGES
Oil burns in combustion chamber, leads to heavy
exhaust emissions.
Oil starvation of the working parts, when the throttle
is closed.
No separate prior mixing of fuel and oil to provide
effective lubrication.
5 to 15 % of high lubrication consumption for two
stroke engines as compared to four stroke engines
of same size.
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WETSUMPLUBRICATION
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WETSUMPLUBRICATION- TYPES
Splash lubrication
Used in small engines
Striking of Drippers on the connecting rod over the oil in
the oil trough leads to the supply of oil for the
lubrication.
This system is not sufficient for high bearing loads.
Pressure feed lubrication
An oil pump is used to lubricate.
Drilled passages are used to lubricate the connectingrod bearings.
Cylinder, piston, rings are lubricated by the sprays
thrown from the crank shaft and connecting rod.
This system is used for heavy engine loads.
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Dry sump
Used for four stroke and large two stroke engines that
uses external pumps and a secondary external reservoir
for oil, as compared to conventional set wet sump
system. High performance sports cars uses dry sump equipped
engines because they prevent oil starvation at high
loads.
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COOLINGSYSTEM
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TYPESOFCOOLINGSYSTEM
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 generateddue 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.
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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.
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TYPESOFCOOLINGSYSTEM
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 coolingin 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.
Two types of water cooling systems Thermo Siphon System
Pump Circulation System
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THERMOSIPHONSYSTEM
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.
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PUMPCIRCULATIONSYSTEM
In this system circulation of water is obtained by a
pump. This pump is driven by means of engine
output shaft through V-belts.
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COMPONENTSOFWATERCOOLEDSYSTEM
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Advantages
Uniform cooling of cylinder, cylinder head and valves.Specific fuel consumption of engine improves by using
water cooling system. If we employ water cooling system, then engine need
not be provided at the front end of moving vehicle.
Disadvantages The water pump which circulates water absorbs
considerable power.
If the water cooling system fails then it will result insevere damage of engine.
The water cooling system is costlier as it has morenumber of parts. Also it requires more maintenance andcare for its parts.
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IGNITIONSYSTEM
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TYPESOFIGNITIONSYSTEM
Battery Ignition System
Battery ignition system was generally used in 4-
wheelers, but now-a-days it is more commonly used in
2-wheelers also (i.e. Button start, 2-wheelers like Pulsar,
Honda-Activa, Scooty, etc.). In this case 6 V or 12 Vbatteries will supply necessary current in the primary
winding.
It mainly consists of a 6 or 12 volt battery, ammeter,
ignition switch, auto-transformer (step up transformer),
contact breaker, capacitor, distributor rotor, distributorcontact points, spark plugs, etc.
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The ignition system is divided into 2-circuits
Primary Circuit : It consists of 6 or 12 V battery, ammeter,
ignition switch, primary winding it has 200-300 turns of 20
SWG (Sharps Wire Gauge) gauge wire, contact breaker,
capacitor.
Secondary Circuit : It consists of secondary winding.
Secondary Ignition Systems winding consists of about
21000 turns of 40 (S WG) gauge wire.
Bottom end of which is connected to bottom end of primary
and top end of secondary winding is connected to centre of
distributor rotor.
Distributor rotors rotate and make contacts with contact
points and are connected to spark plugs which are fitted incylinder heads (engine earth)
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BATTERYIGNITIONSYSTEM
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MAGNETOIGNITIONSYSTEM
In this case magneto will produce and supply the
required current to the primary winding. In this, we can
have rotating magneto with fixed coil or rotating coil with
fixed magneto for producing and supplying current to
primary, remaining arrangement is same as that of a
battery ignition system.
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MAGNETOIGNITIONSYSTEM
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FIRING ORDER The order or sequence in which the firing takes place, in
different cylinders of a multi cylinder engine is called Firing
Order. In case of SI engines the distributor connects the spark plugs
of different cylinders according to Engine Firing Order.
Advantages A proper firing order reduces engine vibrations.
Maintains engine balancing.
Secures an even flow of power.
Firing order differs from engine-to-engine. Probable firing orders for different engines are :
3 cylinder = 1-3-2 4 cylinder engine (inline) = 1-3-4-2 ; 1-2-4-3
6-cylinder in line engine = 1-5-3-6-2-4
8 cylinder in line engine =1-6-2-5-8-3-7-4
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DRAWBACKS (DISADVANTAGES) OF
CONVENTIONAL IGNITION SYSTEMS
Because of arcing, pitting of contact breaker point
and which will lead to regular maintenance
problems.
Poor starting : After few thousands of kilometers of
running, the timing becomes inaccurate, which
results into poor starting (Starting trouble).
At very high engine speed, performance is poor
because of inertia effects of the moving parts in the
system.
Some times it is not possible to produce spark
properly in fouled spark plugs
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PERFORMANCECALCULATIONS
The basic performance parameters are the
following :
Power and Mechanical Efficiency.
Mean Effective Pressure and Torque.
Specific Output.
Volumetric Efficiency.
Fuel-air Ratio.
Specific Fuel Consumption.
Thermal Efficiency
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The power developed by an engine and measured at the output shaftis called the brake power (bp)
where, T is torque in N-m and N is the rotational speed inrevolutions per minute.
It is the power developed in the cylinder and thus, forms the basisof evaluation of combustion efficiency or the heat release in thecylinder.
where, pm = Mean effective pressure, N/m2
, L = Length of the stroke, m,
A = Area of the piston, m2,
N = Rotational speed of the engine, rpm (It is N/2 for four strokeengine), and k = Number of cylinders.
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Mechanical efficiency = bp/ ip
The difference between ip and bp is called friction power
(fp).
Fp = ipbp
Mean Effective Pressure and Torque
Mean effective pressure is defined as a
hypothetical/average pressure which is assumed to be
acting on the piston throughout the power stroke.
Pm = Mean effective pressure, N/m2
Ip = Indicated power, Watt, L = Length of the stroke, m,
A = Area of the piston, m2
N = Rotational speed of the engine, rpm (It is N/2 for
four stroke engine), and k = Number of cylinders.
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The torque is related to mean effective pressure by
the relation
Specific output of an engine is defined as the
brake power (output) per unit of pistondisplacement and is given by,
Specific output = bp/A L
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Volumetric efficiency
v = (Mass of charge actually sucked in) /(Mass ofcharge correspondingto the cylinder intakeP and T conditions)
Brake Specific Fuel bsfc = Actual fuel Air ratio / Stoichiometric fuel Air ratio
Thermal Efficiency
Brake thermal efficiency = bp / mfxCvwhere
Cv = Calorific value of fuel, kJ/kg, and
mf =Mass of fuel supplied, kg/sec.
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