Diesel vs Gasoline

The most distinguishing feature of the diesel engine is that it uses compression ignition to burn the fuel, which is injected into the combustion chamber during the final stage of compression. In a diesel engine, fuel is injected at high pressure into the hot, compressed air in the cylinder, which causes it to burn and no spark is required for this. Thus, “compression ignition" is done rather than "spark ignition". The petrol engine is known as a “spark ignition” engine.

The Petrol engine uses the Otto cycle in which a fuel/air mixture is ignited by a spark plug. The air and fuel mixture when ignited by a spark burns and thereby expands to force the piston down. In case of a petrol engine, fuel and air are pre-mixed usually before compression. Earlier the pre-mixing used to be done in a carburetor but now (except in the smallest engines) electronically-controlled fuel injection is used for this. The pre-mixing of fuel and air makes a petrol engine to run at a much higher speed than a diesel. However, it severely limits their compression, and thus efficiency.

Diesel engines offer better fuel efficiency when compared to petrol due to the fact that they have higher compression ratio. Another advantage is that, a diesel engine can be more easily turbocharged than a petrol engine because of the fact that if the compression ratio and the pressure in the cylinder are high during the inlet stroke, the mixture starts to burn to soon, while the piston is on its way up. The diesel engine has no fuel in the cylinder and thus allows the turbocharger to suck as much air as it can without creating any problem.

The higher compression ratio is helpful in raising fuel efficiency. Diesel engines are much more efficient than petrol engines when at low power and at engine idle. Diesel engines, unlike the petrol engine, lack a butterfly valve (throttle) in the inlet system, which closes at idle. This creates parasitic loss and destruction of availability of the incoming air,...