The main differences between the gasoline engine and the Diesel engine are:

• A gasoline engine intakes a mixture of gas and air, compresses it and ignites the mixture with a spark. A Diesel engine takes in just air, compresses it and then injects fuel into the compressed air. The heat of the compressed air lights the fuel spontaneously.
• A Diesel engine uses a much higher compression ratio than a gasoline engine. A gasoline engine compresses at a ratio of 8:1 to 12:1, while a Diesel engine compresses at a ratio of 14:1 to as high as 25:1. The higher compression ratio leads to better efficiency.
• Diesel engines use direct fuel injection, in which the Diesel fuel is injected directly into the cylinder. Gasoline engines generally use either carburetion, where the air and fuel is mixed long before the air enters the cylinder, or port fuel injection in which the fuel is injected just prior to the intake valve (outside the cylinder).

Note that the Diesel engine has no spark plug, that it intakes air and compresses it, and that it then injects the fuel directly into the combustion chamber (direct injection). It is the heat of the compressed air that lights the fuel in a Diesel engine.

In this simplified animation, the green device attached to the left side of the cylinder is a fuel injector. However, the injector on a Diesel engine is its most complex component and has been the subject of a great deal of experimentation - in any particular engine it may be located in a variety of places. The injector has to be able to withstand the temperature and pressure inside the cylinder and still deliver the fuel in a fine mist. Getting the mist circulated in the cylinder so that it is evenly distributed is also a problem, so some Diesel engines employ special induction valves, pre-combustion chambers or other devices to swirl the air in the combustion chamber or otherwise improve the ignition and combustion process.

One big difference between a diesel engine and a gas engine is in the injection process. Most car engines use port injection or a carburetor rather than direct injection. In a car engine, therefore, all of the fuel is loaded into the cylinder during the intake stroke and then compressed. The compression of the fuel/air mixture limits the compression ratio of the engine - if compressed too much, the fuel/air mixture spontaneously ignites and causes knocking. A diesel compresses only air, so the compression ratio can be much higher. The higher the compression ratio, the more power is generated.

Some Diesel engines contain a glow plug of some sort that is not shown in this figure. When a Diesel engine is cold, the compression process may not raise the air to a high enough temperature to ignite the fuel. The glow plug is an electrically heated wire (think of the hot wires you see in a toaster) that helps ignite the fuel when the engine is cold so that the engine can start. According to Cley Brotherton, a journeyman heavy equipment technician, "All functions in a modern engine are controlled by the ECM communicating with an elaborate set of sensors measuring everything from R.P.M. to engine coolant and oil temperatures and even engine position (i.e. T.D.C). Glow plugs are rarely used today on larger engines. The ECM senses ambient air temperature and retards the timing of the engine in cold weather so the injector sprays the fuel at a later time. The air in the cylinder is compressed more, creating more heat, which aids in starting." Smaller engines and engines that do not have such advanced computer control use glow plugs to solve the cold-starting problem