Petrol Fuel Engine

As a piston moves from the bottom to the top of a cylinder, it compresses the air and petrol mixture. A number, called the compression ratio, tells how much the mixture is compressed. A high-compression engine may have a compression ratio of 10 to 1. Such an engine compresses the mixture to a tenth of its original volume. A low-compression engine may have a ratio of 8 to 1.

High-compression engines burn petrol more efficiently than do low-compression engines. But high-compression engines require high-octane petrol. Until the 1970's, the octane level of petrol depended on the amount of lead additives--the more lead, the higher the octane. In the mid-1970's, manufacturers began to equip cars with devices called catalytic converters that reduce the pollutants in car exhausts. Lead was found to interfere with the effectiveness of catalytic converters. Cars with catalytic converters had to use low-octane petrol because high-octane lead-free petrol was costly to produce. As a result, the car industry reduced compression ratios so that engines could burn lower octane lead-free fuels efficiently.