A motor-car engine that runs on petrol has a number of cylinders that are fitted with pistons. When the engine is working, the pistons move up and down inside the cylinders and each upward or down ward movement of a piston is known as a stroke. The motor-car engine works on a four-stroke energy cycle. This means that the fuel-air mixture is ignited at each cycle to provide the needed energy for the engine to work properly. The efficient working of the motor-car engine is dependent on the fuel-air mixture which (i) must ignite at the correct stage in each cycle and (ii) must be completely burnt.
The fuel should posses the correct volatility and temperature of ignition for it to be burnt at the right time of the cycle. If the fuel is too volatile and has too low an ignition point, it would be burnt prematurely. When this happens, a condition called knocking, or pinking, results. Knocking is a characteristic metallic sound caused by vibrating pistons. This knocking reduces the efficiency of the motor-car engine and shortens its life. On the other hand, a fuel which is not volatile enough and has too high an ignition temperature may not be burnt completely. Excessive black smoke and unburnt hydrocarbons are expelled from the car via the exhaust, causing air pollution.
Petrol is the only fuel that meets all the requirements necessary to power a motor-car engine. Therefore, motor-car engines are known as petrol engines. Nowadays, more efficient petrol engines have been designed, however, even these newly designed petrol engines have a tendency to knock if low quality fuel is used.