Gasoline engine:

Another variation among a gasoline engine and a diesel engine is the manner in that engine speed is controlled. Within any engine, speed (or power) is a direct function of the amount of fuel burned in the cylinders. A Gasoline engines are self-speed-limiting, because of the method the engine uses to control the amount of air entering the engine. Engine speed is indirectly controlled through the butterfly valve in the carburetor. The butterfly valve in a carburetor limits the amount of air entering the engine. Within a carburetor the rate of air flow dictates the amount of gasoline which will be mixed along with the air. Limiting the amount of air entering the engine limits the amount of fuel entering the engine, and, thus, limits the speed of the engine. Through limiting the amount of air entering the engine, adding more fuel does not raise engine speed beyond the point whereas the fuel burns 100 percent of the available air (oxygen).

Diesel engines are not self-speed-limiting since the air (oxygen) entering the engine is always the maximum amount. Thus, the engine speed is limited solely through the amount of fuel injected into the engine cylinders. Thus, the engine always has enough oxygen to burn and the engine will attempt to accelerate to meet the new fuel injection rate. Since of this, a manual fuel control is not possible since these engines, in an unloaded condition, could accelerate at a rate of more than 2000 revolutions per second. A Diesel engine needs a speed limiter, generally known as the governor, for control the amount of fuel being injected into the engine.

Unlike a gasoline engine, a diesel engine does not needs an ignition system since in a diesel engine the fuel is injected into the cylinder as the piston comes to the top of its compression stroke. While fuel is injected, it vaporizes and ignites because of the heat created through the compression of the air in the cylinder.