Have you ever gazed in amazement as a giant truck crawls slowly up a hill? Probably not! Such things happen everyday. But stop and think for a moment about what's happening—how a huge, heavy load is being systematically lifted against the overwhelming power of gravity using nothing more than a few cups of dirty liquid (fuel, in other words)—and you might agree that what you're seeing is quite remarkable. Diesel engines are the power behind our biggest machines—trucks, trains, ships, and submarines. On the face of it, they're similar to ordinary gasoline (petrol) engines but they generate more power, more efficiently by working in a subtly different way. Let's take a closer look!
What is a diesel engine?
Like a gasoline engine, a diesel engine is a type of internal combustion engine. Combustion is another word for burning, and internal means inside, so an internal combustion engine is simply one where the fuel is burned inside the main part of the engine (the cylinders) where power is produced. That's very different from an external combustion engine such as those used by old-fashioned steam locomotives. In a steam engine, there's a big fire at one end of a boiler that heats water to make steam. The steam flows down long tubes to a cylinder at the opposite end of the boiler where it pushes a piston back and forth to move the wheels. This is external combustion because the fire is outside the cylinder (indeed, typically 6-7 meters or 20-30ft away). In a gasoline or diesel engine, the fuel burns inside the cylinders themselves. Internal combustion wastes much less energybecause the heat doesn't have to flow from where it's produced into the cylinder: everything happens in the same place. That's why internal combustion engines are more efficient than external combustion engines (they produce more energy from the same volume of fuel).
How is a diesel engine different from a gasoline engine?
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. 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: Like a gasoline engine, a diesel engine usually operates by repeating a cycle of four stages or strokes.
- Intake: Air (light blue) is drawn into the cylinder through the open green air inlet valve on the right as the piston moves down.
- Compression: The inlet valve closes, the piston moves up, and compresses the air mixture, heating it up. Fuel (dark blue) is injected into the hot gas through the central fuel injection valve and spontaneously ignites. Unlike with a gas engine, no sparking plug is needed to make this happen.
- Power: As the air fuel mixture ignites and burns, it pushes the piston down, driving the crankshaft (red wheel at bottom) that sends power to the wheels.
- Exhaust: The green outlet valve on the left opens to let out the exhaust gases, pushed out by the returning piston.
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