WORKING OF A PETROL ENGINE
The working of an internal combustion engine is divided into four stages called four strokes of the engine and hence the engine is called a four stroke engine.
The intake stroke :
When the engine starts, the piston moves downwards in the cylinder, because of which a region of low pressure is created in the cylinder, above the piston. At this moment, the intake valve opens and the fuel mixture(petrol vapour and air mixture) is sucked into the cylinder from the carburettor.
The compression stroke :
When the sufficient amount of the fuel mixture (petrol vapour and air mixture) has entered the cylinder, the intake valve gets closed. The piston is then forced to move upwards which compresses the fuel-mixture to about one-eighth of its original volume. Higher the compression ratio, more will be the efficiency of the engine.
The power stroke :
Before the piston completes its upward movement, compressing the petrol vapour and air mixture, the spark plug produces a little electric spark inside the cylinder and this spark sets fire to the petrol-air mixture. The petrol vapour burns quickly in a little explosion, producing a large volume of gases and enormous heat. The heat thus produced expands the gases rapidly. The pressure of rapidly expanding hot gases pushes the piston downward with a great force. The piston pushes the piston rod and the piston rod pushes the crank shaft. The crank shaft is joined to the wheels of a car. When the crank shaft turns, the wheels rotate and move the car.
The exhaust stroke:
When the piston has been pushed to the bottom of the cylinder by the hot expanding gases in the power stroke, then the exhaust valve opens. After that, due to the momentum gained by the wheels, the piston is pushed upwards. The upward movement of the piston, expels the spent gases through the exhaust valve into the atmosphere, carrying away the unused heat. The exhaust valve then closes, the intake valve opens up, and the above four strokes of the engine are repeated again and again.