Types Of Internal Combustion Engines
Heat engines are used to convert heat into mechanical work
. Sadi Carnot (French) conceived a theoretical engine which is free from all the defects of practical engines. The efficiency is maximum and it is an ideal heat engine.
There are different types of internal combustion engines which include the steam engine, Otto cycle, petrol engine, Steam and gas turbines, spark-ignition and diesel engines.
Internal Combustion Engine:
In internal combustion engine, the fuel is burnt in the cylinder of heat engine itself. The products of combustion act as the working substance in an internal Combustion engine. Two different types of internal combustion engines are (i) Diesel engine (ii) Petrol engine. Here we will discuss the working mode of petrol engine.
Example of Internal Combustion Engine
Petrol Engine:
Petrol engine is an example of internal combustion engine, which are used in Scooters, motor cars, auto rickshaws, buses and aero planes. A mixture of petrol vapours and air acts as the working substance.
This type of internal combustion engine (petrol engine) consists of a cylinder having an inlet valve I and an outlet valve O. The cylinder is fitted with an air tight moving piston, which is connected to a shaft. A spark plug is connected in the cylinder to cause the fuel to ignite an electric spark.
Petrol Engine
The petrol vapour and air are mixed in a device known as Carburretor. The combustion is completed in four successive piston strokes, so called four stoke engine.
The four strokes which are cyclically repeated are explained below:-
1. Intake Stroke( charging stroke, Suction stroke)
During this stroke, the piston moves outwards. The inlet valve opens and the mixture comes in the cylinder at atmospheric pressure.
2. Compression Stroke
During this stroke, the piston moves downwards. The mixture is compressed adiabatically to nearly one-eighth of the original volume, which results in the increase of temperature. It may be noted that higher compression gives higher efficiency.
3. Power Stroke (Working Stroke)
During this stroke, the ignition plug produces spark as a result of which fuel is ignited and combustion takes place. The chemical energy is converted into heat energy. Both pressure and temperature become very high. An explosion takes place and the piston is pushed outwards. It is only during this stroke that the engine performs useful work.
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4. Exhaust Stroke
During this stroke, the piston moves downwards and the combustion gases are forced out of the cylinder. After this stroke, the cycle starts all over again.
The P-V diagram of the petrol engine is shown below. The part AB represents the suction stroke, BC represents adiabatic compression, CD represents the combustion process in which heat Q1 is evolved .The portion CD shows a considerable increase in the pressure of the mixture. The portion DE represents the adiabatic expansion when both pressure and temperature decreases. EB represents the fall of pressure accompanied by loss of heat and BA represents the exhaust of the combustion products.
P-V diagram of the petrol engine
Efficiency
= net work done/Q1
= 1- (Q2/Q1)
Or
= 1-(V2/V1) -1
Or
= 1-(1/) -1------------- (1) where = V1/V2 is known as adiabatic compression ratio
Here equation (1) gives the efficiency of the internal combustion engine
Solved Examples on Internal Combustion Engine
Solved Examples
1) A Carnot cycle is performed by air initially at 327C.Each satge represents a compression or expansion in the ratio 1:6.Calculate(i) the lowest temperature (ii) efficiency of the cycle, given = 1.4?
Solution:
T1= 327+273=600K
V1/V2 = 1/6
= 1.4
(i) T2V2 -1 = T1V1 -1
T2= T1(V1/V2) -1
T2= 293K
(ii) Efficiency, = 1- (T2/T1)= 0.512
%age = 0.512* 100=51.2%
2) The efficiency of otto cycle for petrol engine is 50% and the adiabatic constant is 1.4.Calculate the compression ratio?
Solution:
Efficiency = 50%
Adiabatic constant, = 1.4
Compression ratio, =?
= 1-(1/) -1
50/100= 1-(1/) 1.4-1= 1-(1/) 0.4
Taking log on both sides, we get
0.4 log [1/] = log 0.5
On solving, we get, = 5.7
by: mathqa
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