Most engines need a system called an ignition system – one that uses an electric spark – in order to burn the mix of fuel and air and make power. Other types of ignition systems use compressed air from an outside source, such as an air compressor. A diesel does not. It burns a diesel fuel (similar to heating oil) through very high compression or pressing together of the mix. A tiny bit of fuel is injected, or forced, into the engine's cylinders at just the right moment. Since gases heat when they are pressed together, the compression of the air and fuel mix cause the mix to explode within the cylinder.
Diesel engines make very good use of the fuel they burn. They also make lots of torque (pronounced "tork"), or twisting power. An engine with a lot of torque will be able to spin its shaft even if it is very hard to do so. This makes a diesel engine a good choice in heavy equipment like trucks, trains and construction machines. The very big trucks on the road have diesel engines in them. So do train locomotives, unless they are electric or the old steam-powered kind.
Sometimes, even the torque of a diesel engine is not enough to run such big machines. To boost power, large diesels often have a device called a turbocharger attached. A turbocharger is a type of turbine – something used to move a lot of air very quickly. Jet engines also contain a turbine. In the diesel, pressure from the exhaust spins the turbocharger at a very high speed. Fresh air is then forced back into the engine. Since the engine works by pumping air, the more air you can put through it, the more power it makes. That is where the turbocharger helps. A diesel engine that has a turbocharger is called a turbodiesel. The whistling sound sometimes heard near one of these engines is caused by the turbocharger, or "turbo" for short.
A diesel engine can also be made to run on vegetable oil made from old cooking oil. This type of fuel is called biodiesel. Running a diesel engine on biodiesel fuel makes the exhaust smell like food. Vegetable oil as fuel is not a new idea. The engine that Rudolf Diesel used to demonstrate his new idea ran on peanut oil.