Why is a diesel engine more efficient than the petrol engine?

The diesel engine is more efficient than the petrol engine because of its greater thermal efficiency. This means that the ratio of the work done by the engine to the amount of heat supplied is higher. Since the heat in this case comes from fuel-oil, it is not possible to convert all the heat energy in the fuel into useful work. There are losses due to friction, exhaust and radiation.

The compression ignition, or diesel engine was invented by the German engineer Rudolf Diesel in 1897. It is much simpler in construction than the petrol, or spark ignition engine. Pre-ignition troubles are avoided because only the air in the engine cylinder in compressed and the fuel is introduced only at the instant of combustion. The engine requires no spark plugs and burns fuel oil which is less expensive than petrol.

The basic construction of the diesel engine is similar to the petrol engine. The main different is the way in which the fuel is introduced and ignited. With an average compression ratio of 16:1 the air in the cylinder during the compression stroke reaches a pressure of about 500 pounds a square inch. At the right instant a precise amount of fuel passes from the injector pump to the injector at high pressure and enters the cylinder as a fine spray. Owing to the high temperature of the air, the fuel starts to burn without the need for a spark. But the high pressure means that diesel engines have to be stronger than petrol engines. They are, therefore heavier and more expensive.