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Sources of energy!sources_of_energy2.htm;
Types of energy!types_of_energy2.htm;
Forms of energy!forms_of_energy2.htm;
Transformation of energy!transformation_of_energy3.htm;
Measurement of energy!measurement_of_energy4.htm;
Combustion Engine!Combustion engine1.htm;
Joule's Experiment!joule's experiment.htm;
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
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
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.
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
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.
reason for low efficiency of steam engines/external combustion
is a loss of heat in steam engine because of large exposed
(2) It is
not possible to maintain steams at a very high temperature.
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.
steam goes into the condenser. The water formed in the condenser
is sent to the boiler with the help of a pump.
limitations of external combustion engine:
heavy and bulky: Due to its big boiler and furnace it cannot be
used for running small vehicles like cars etc.
start at once: Production of heat through coal fires takes a
Steam has to be maintained at very pressure to increase the
efficiency of the engine. Due to excessive pressure boiler can
efficiency: The thermal efficiency is only about 20% which means
that it can convert only 20% of heat into mechanical work.
Engine for the generation of Electricity:
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.
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.
two problems that arise from the production of electricity that
worry a lot of people.
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.
sulphur dioxide + sulphorous acid
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.
four-stroke diesel engine works in a similar way to the petrol
engine you just read about. The differences are these:
the intake stroke only air is taken into the cylinder.
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
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
engine using diesel oil has a higher efficiency than a
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