FDP_Internal Combustion Engines - II

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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Documents

FDP_Internal Combustion Engines - II