Combustion is utilised in the petrol engine by converting it into motive power. The petrol engine is often referred to as the internal combustion engine. This only means that the fuel burns within the engine. Rocket and jet engines are other forms of internal combustion engines.
The petrol engine operates by burning a combination of petrol and air inside a cylinder. This cylinder encompasses a piston. The heat generated causes the air to expand and force the piston downwards. This in turn rotates a crankshaft which is linked via a gearbox and clutch to the wheels.
Most cars today have a four-stroke engine. A stroke is the name given to the movement of either the piston shifting up, or shifting down. Within the four-stroke engine a cycle of actions is repeated, in which the piston moves four times. The cycle is described as follows:
INDUCTION STROKE
The piston travels downwards and the inlet valve opens. This causes the fuel and air mixture to be drawn into the cylinder.
COMPRESSION STROKE
The inlet valve closes as the piston rises upwards. The fuel and air mixture is compressed.
COMBUSTION STROKE
The electric spark plug positioned near the top of the cylinder generates a spark which ignites the fuel. This action drives the piston back down the cylinder.
EXHAUST STROKE
The outlet valve opens and once more the piston ascends. This movement results in the exhaust gases being ejected out of the cylinder.
This design is somewhat ingenious, in as much as the fact that there are always at least two cylinders which are out of phase by 180 degrees. As all pistons are attached to the same crankshaft, the downward forced action of one cylinder forces the piston up the other. This is then reversed and the repeating cycle goes on with both cylinders working in tandem.
