How Diesel Engines Work

By: Marshall Brain & Kristen Hall-Geisler  | 

Diesel Engines vs. Gasoline Engines

In theory, diesel engines and gasoline engines are quite similar. They are both internal combustion engines designed to convert the chemical energy available in fuel into mechanical energy. This mechanical energy moves pistons up and down inside cylinders. The pistons are connected to a crankshaft, and the up-and-down motion of the pistons — known as linear motion — creates the rotary motion needed to turn the wheels of a car forward.

Both diesel engines and gasoline engines convert fuel into energy through a series of small explosions — or combustions. The major difference between diesel and gasoline is the way these explosions happen. In a gasoline engine, fuel is mixed with air, compressed by pistons and ignited by sparks from spark plugs. In a diesel engine, however, the air is compressed first, and then the fuel is injected. Because air heats up when it's compressed, the fuel ignites.


The following graphic shows the diesel cycle.

The diesel engine uses a four-stroke combustion cycle just like a gasoline engine. The four strokes are:

diesel engine animation
Diesel engine
© 2019
  1. Intake stroke: The intake valve opens up, letting in air and moving the piston down.
  2. Compression stroke: The piston moves back up and compresses the air.
  3. Combustion stroke: As the piston reaches the top, fuel is injected at just the right moment and ignited, forcing the piston back down.
  4. Exhaust stroke: The piston moves back to the top, pushing out the exhaust created from the combustion out of the exhaust valve.

Remember that the diesel engine has no spark plug that intakes air and compresses it; instead it 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 the next section, we'll examine the diesel injection process.