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Combustion Engine

Heat Engine

A device by which heat energy is converted into mechanical work is called heat engine.  Heat engines are of two types :

1.  External combustion Engine : Combustion of fuel takes place outside the cylinder of the engine. e.g. STEAM ENGINE

2.  Internal combustion engine : Combustion of fuel takes place inside the cylinder of the engine. e.g. - PETROL ENGINE - DIESEL ENGINE

Steam/External Combustion engine

Steam Engine is an external combustion engine, because steam is produced outside the cylinder. In an external combustion engine, coal is burn to produce heat. This heat is used to boil water in a boiler to produce steam. The stream occupies larger volume as compared to the volume occupied by the same amount of water. Therefore, steam can cause greater pressure on the walls of the container.

When the hot and compressed steam is introduced into a cylinder with movable pistons, it expands and exerts a push on the pistons. As a result, the piston moves forward.

When the steam condenses to water due to loss of heat energy, it causes the piston to fall back to its original position. Therefore, the pistons can be made to move forward and backward repeatedly by introducing fresh stream and taking out the cooked steam. The motion of the piston can be used to turn the wheels of an engine.

When the piston moves out then the piston rod pushes the crankshaft. The crankshaft then rotates the wheel. In this  way heat energy is converted into mechanical energy.


The main reason for low efficiency of steam engines/external combustion engine are:

(1) There is a loss of heat in steam engine because of large exposed surface.

(2) It is not possible to maintain steams at a very high temperature.

Exhaust Stroke:

When the piston in the expansion stroke has moved sufficiently outwards, the intake valve gets closed and the exhaust valve opens automatically. Because, the steam loses its heat energy and condenses to water, the piston then moves back into the cylinder. As the piston moves inwards, it pushes out all the spent steam through the exhaust valve. This stroke is called the exhaust stroke because the spent stream is expelled from the cylinder in this stroke.

The spent steam goes into the condenser. The water formed in the condenser is sent to the boiler with the help of a pump.

Main limitations of external combustion engine: 

-Very heavy and bulky: Due to its big boiler and furnace it cannot be used for running small vehicles like cars etc.

-Does not start at once: Production of heat through coal fires takes a long time.

-Unsafe: Steam has to be maintained at very pressure to increase the efficiency of the engine. Due to excessive pressure boiler can burst.

-Low efficiency: The thermal efficiency is only about 20% which means that it can convert only 20% of heat into mechanical work.

Steam Engine for the generation of Electricity:

Figure 1 shows the principle behind the production of electrical energy in a power station.  Most of our power stations use coal as their source of energy.  When coal is burnt its stored chemical energy is released as heat energy.  This heat energy boils water at high pressure to make superheated steam at temperatures of about 7000C.  The kinetic energy in the superheated steam is used to drive turbines.  These are connected to the electricity generator by large coils rotating inside a strong magnetic field.

You can see in Figure 2 a chart showing how energy is used in this process.  There is a lot heat lost in the power sta.  The electrical energy itself is also converted to heat energy in factories and houses.

There are two problems that arise from the production of electricity that worry a lot of people.

  • Pollution.  Burning coal makes the gases carbon dioxide and sulphur dioxide. These pollute the atmosphere.  When sulphur dioxide dissolves in water an acidic solution is formed containing sulphurous acid.

Water + sulphur dioxide + sulphorous acid

So when sulphur dioxide gets into rain, the rain becomes acidic.  We call this acid rain.  Acid rain damages stonework in buildings.  It is thought that acid rains kills trees also.

The large industries that we rely on to make our lives easier and more comfortable often cause large amounts of pollution and use a great deal of energy. Huge power stations are needed to provide electricity to power them, and these use up valuable deposits of coal and oil. Pollution is the price that we have to pay.

Internal Combustion Engine:

Diesel engines

A four-stroke diesel engine works in a similar way to the petrol engine you just read about.  The differences are these:

  • During the intake stroke only air is taken into the cylinder.
  • The compression ratio of diesel engines is higher than that of petrol engine.  At the end of the compression stroke the air has been squashed to about its original volume.  At this point the temperature of the air is about 7000C its pressure is about 35 atmospheres (3.5 MPa).

  • The diesel engine has no sparking plug.  At the end of the compression the fuel is forced into the cylinder under high pressure.  The temperature is so hot that the fuel burns as soon as it has mixed into the air.  When the burns, the temperature inside the cylinder reaches about 25000C and the pressure is about 100 atmospheres (10 MPa).

  • Such an engine using diesel oil has a higher efficiency than a petrol engine

Petrol engines

If you put your finger over the end of a bicycle pump, and pump the cylinder up and down a few times, you find that the air inside the cylinder gets hot.  As the piston moves it collides with moving molecules.  The molecules rebound off the moving piston at a greater speed.  This means the gas gets hotter.  The pressure inside the bicycle pump cylinder increases for two reasons.  First the air has been compressed, and then this compression warms the air up.  If you compress the air slowly the temperature will stay the same because heat will escape through the sides of the cylinder (Figure 1).

Figure 2 shows how the idea is used in a four-stroke petrol engine.

(1)  On the intake stroke a petrol vapour and air mixture is fed into the cylinder

(2)  On the compression stroke both the inlet and exhaust valves are closed.  The piston moves up rapidly to compress the air/petrol mixture to about 1/10 of its original volume: the compression ration is 10:1.  The pressure inside the cylinder will now be about 20 atmospheres (2 MPa) and the temperature about 3500C.

(3)  The working stroke.  Once the gas bas been compressed the sparking plug produces a spark which starts the fuel burning.  The temperature in the cylinder now rises to about 10000C.  The pressure now rises to about 50 atmospheres (5 MPa).  The higher pressure now forces the piston back.  Although energy is used to squash the gas during the compression stroke, the working stroke gives out far more energy.

(4)  In the exhaust stroke, the exhaust valve opens to allow the high pressure gases to escape.






Shows a simple two stroke diesel engine; which could be used in lawn mower. In this engine the intake, expression and exhaust strokes are all combined into one upward movement of the piston. 




Sources of energy

Types of energy

Forms of energy

Energy Conservation

Transformation of energy

Measurement of energy



Combustion engines

Joule's experiment

Fun Facts