Diesel engines are unique in that there are no spark plugs ¹. Ignition is produced by introducing diesel fuel into highly compressed, and therefore hot, air.

Diesel engines operate on the Otto cycle, the same series of steps as four-stroke engines. Unlike other engines, the fuel and air are not mixed prior to entering the cylinder. Air is injected and compressed, and at the peak of compression, the fuel is injected directly into the cylinder. The fuel combusts, which pushes the piston as in the other engine types.

Because only air is being compressed, the compression ratio in the cylinder can be much higher than in a four-stroke engine. Compressing the mixture in a four-stroke to the same amount as in a diesel engine can cause premature ignition. This higher compression ratio results in much better efficiency and more power.

Diesel fuel is also more viscous ² and unrefined than gasoline. For this reason, diesel fuel is often referred to as diesel oil. Because it is less refined than gasoline, it is also often cheaper than normal gasoline.

Starting an engine with no ignition system can be a problem. To solve this, most modern diesel engines use a computer system to time the compression and fuel injection in order to increase the chance of ignition. In smaller and less sophisticated engines, a glow plug is often employed. Glow plugs are often used in remote-controlled, gas powered vehicles and is essentially a heating element much like one that can be found in a toaster. This heats up the mixture enough to cause ignition.