Petrol Engine Versus Diesel Engine

The choice between Petrol cars and Diesel cars is highly debated right from the inception of these two engine types. There have been endless discussions on this topic among car enthusiasts across the world including India. This article is dedicated to comparing these two most famous engine variations.

Differences Between Petrol and Diesel Engines

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, reducing the efficiency of petrol/gasoline engines at idle. This mechanism makes the diesel engine an attractive choice for many. Diesel engines are left idle for many hours or sometimes days in many applications, such as marine, agriculture, and railways. The engines are more efficient when compared to petrol engines of the same power. They consume significantly lower fuel and offer better mileage.

A diesel engine can relatively last longer, due to the fact that petrol destroys lubrication and diesel doesn’t. While all the above support diesel engines, a major advantage of petrol engines is that they are relatively lighter than diesel engines. Engine weight is an important factor which affects speed and performance of a car. This is one of the reasons why some of the fastest cars in the world run on petrol. It should be noted that even though diesel engines are heavier and make the automobile heavy, they have more torque.

Engines and Emissions

Diesel engines consume around 30% less fuel than petrol engines and this results in much lesser carbon dioxide emissions. The diesel engines produce virtually no carbon monoxide and are much safer than petrol engines. Tests done on car emissions reveal that while Nitrogen Oxides are higher in a new diesel engine when compared to a new petrol engine. But by the time they cover 50,000 miles or so, they are the same and after that the petrol engine produces more Oxides than the diesel engine. Hydrocarbon emissions contained in petrol engine emissions are considerably more than that in diesel engine emissions. However, diesel is certainly more dangerous from the point of view of Suspended Particulate Matter (SPM). SPM refers to solid particles suspended in open air, such as soot generated by combustion of various fuels. They might cause respiratory problems because of their tendency to deposit themselves in the lungs. Though much has been done to improve the fuel efficiency and reduce emissions from the petrol engine, still more needs to be done.

In India, diesel is much cheaper than petrol. Ironically, diesel cars are more expensive than petrol cars.