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Heat and Mechanical Energy
We know that heat is a form of energy. Energy can be converted from one form to another. Thus, heat can be converted to mechanical energy and mechanical energy can be completely converted to heat.

Heat Engine
Heat engine is a device that converts heat into mechanical energy continuously. A heat engine basically consists of
  1. a cylinder fitted with a smooth piston and
  2. a working substance enclosed within it.
Depending on the method of producing and transferring heat energy to the working substance, heat engines are classified into two types.

Description: Description: 73409.png


Efficiency of Heat Engine The efficiency of the heat engine is defined as
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If the heat absorbed is Q1 and the heat rejected is Q2, then W = Q1 – Q2.
Description: 71866.png

η < 1. If heat is absorbed at T1 (K) and rejected at T2 (K), then it can be shown that Description: 71911.png Thus,
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Note: 100% of work (mechanical energy) can be converted into heat, but 100% of heat absorbed cannot be converted into work. h can be large if T1 – T2 is large.


Internal Combustion Engines

The fuel of an internal combustion engine is inflammable. It can be petrol or diesel or gas. The fuel is burnt inside the main cylinder connected with piston. Thus, it is called internal combustion engine.

Petrol Engine (Four-stroke)
It is also called as Otto engine. The petrol from the tank goes to a device called carburettor. The petrol–air mixture is produced in it.

First stroke (intake stroke)
The inlet valve (V1) is open. The descending piston draws fresh petrol–air mixture into the cylinder from the carburettor (Figure 8.4a).

Second stroke (compression stroke) Both the valves are closed. The rising piston compresses the mixture to a pressure of about 8 atm. The mixture is ignited by the electric spark produced by the spark plug (Figure 8.4b).


Third stroke (power stroke) Both the valves are closed as in the second stroke. The combustion of fuel produces high temperature (~2000°C) and pressure (~15 atm). This forces the piston downwards. The moving piston rotates the crankshaft and the wheel connected to it (Figure 8.4c).


Fourth stroke (exhaust stroke) The exhaust valve (V2) is open. The rising piston discharges the gases from the cylinder (Figure 8.3d). Just before the piston moves downwards, V2 is closed and V1 is opened, stroke 1 continues and the next cycle begins.
First stroke (intake stroke)

Diesel Engine

The operation of a diesel engine is similar to that of a petrol engine. A fuel injector is present instead of a spark plug and ignition takes place spontaneously by the expansion of the air–fuel mixture and not by an external spark.

Difference Between Petrol Engine and Diesel Engine

Petrol Engine
Diesel Engine
1. Fuel ignition is due to electric spark produced by the spark plug.
Fuel ignition is due to high temperature created by compression.
2. Fuel is mixed with air in carburettor. This mixture is taken through valve V1 during intake stroke.
Fuel in injected into the cylinder at the end of the compression stroke.
3. Starts easily in cold conditions also.
In cold conditions, this requires pre-heating of the cylinder.

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