Gasoline engines and diesel engines both work by internal combustion, but in slightly different ways. In a gasoline engine, fuel and air is injected into small metal cylinders. A piston compresses (squeezes) the mixture, making it explosive, and a small electric spark from a sparking plug sets fire to it. That makes the mixture explode, generating power that pushes the piston down the cylinder and (through the crankshaft and gears) turns the wheels. You can read more about this and watch a simple animation of how it works in our article on car engines.
Diesel engines are similar, but simpler. First, air is allowed into the cylinder and the piston compresses it—but much more than in a gasoline engine. In a gasoline engine, the fuel-air mixture is compressed to about a tenth of its original volume. But in a diesel engine, the air is compressed by anything from 14 to 25 times. [1] If you've ever pumped up a bicycle tire, you'll have felt the pump getting hotter in your hands the longer you used it. That's because compressing a gas generates heat. Imagine, then, how much heat is generated by forcing air into 14–25 times less space than it normally takes up. So much heat, as it happens, that the air gets really hot—usually at least 500°C (1000°F) and sometimes very much hotter. Once the air is compressed, a mist of fuel is sprayed into the cylinder typically (in a modern engine) by an electronic fuel-injection system, which works a bit like a sophisticated aerosol can. (The amount of fuel injected varies, depending on how much power the driver wants the engine to produce.) The air is so hot that the fuel instantly ignites and explodes without any need for a spark plug. This controlled explosion makes the piston push back out of the cylinder, producing the power that drives the vehicle or machine in which the engine is mounted. When the piston goes back into the cylinder, the exhaust gases are pushed out through an exhaust valve and, the process repeats itself—hundreds or thousands of times a minute!
Animation: How a four-stroke diesel engine works.
Like a gasoline engine, a diesel engine usually operates by repeating a cycle of four stages or strokes, during which the piston moves up and down twice (the crankshaft rotates twice in other words) during the cycle.
In a two-stroke diesel, the complete cycle happens as the piston moves up and down just once. Confusingly, there are really three stages in a two-stroke cycle:
Two-stroke engines are smaller and lighter than four-stroke ones, and tend to be more efficient since they produce power once during each rotation (instead of once during every two rotations, as in a four-stroke engine). This means they need more cooling and lubrication and suffer higher wear and tear.
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Woodford, Chris. (2009/2020) Diesel engines. Retrieved from https://www.explainthatstuff.com/diesel-engines.html. [Accessed (Insert date here)]
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