A petrol, or gasoline, engine is very similar to a diesel engine but is generally lighter and uses a spark ignition system rather than the diesel's compression ignition system. Diesel engines are exposed to greater pressures and must be built more heavily than petrol engines for the same capacity. Both petrol and diesel are refined crude oil products, but diesel is less refined and is very similar to heating oil.
A diesel engine is exposed to higher pressures during operation. All components of a diesel engine of a given capacity are heavier than those in a corresponding petrol engine. A petrol engine works by pumping a mix of air and fuel into its cylinders, compressing it and then lighting it with an electric spark. A diesel engine first pumps in air, then compresses just the air to a very high degree, and finally pumps in fuel, which is spontaneously ignited in the high-temperature air.
Diesel engines and petrol engines each have their advantages. Diesel engines are more efficient, using less fuel to produce power. They are, however, more polluting. Gasoline engines are more suited to high-speed applications because they are able to safely support higher revolutions per minute. Diesel, due to its greater efficiency, is frequently used for heavier loads at lower speeds.