In theory, diesel engines and gasoline engines are very similar. They are both internal combustion engines designed to convert chemical energy in the fuel into mechanical energy. This mechanical energy causes a piston to move up and down inside a cylinder. The piston is connected to a crankshaft, and the up and down motion of the piston (called linear motion) creates the rotational motion needed to turn the car’s wheels forward.
Both diesel and gasoline engines convert fuel into energy through a series of small explosions (or combustions). The main difference between diesel and gasoline is the way these explosions occur. In a gasoline engine, the fuel is mixed with air, compressed by the piston, and ignited by the spark of the spark plug. However, in a diesel engine, the air is compressed first, and then the fuel is injected. Since the air heats up when it is compressed, the fuel ignites.
Diesel engines use the same four-stroke combustion cycle as gasoline engines. The four strokes are:
- Intake stroke: The intake valve opens, letting air in, and causing the piston to move down.
- Compression stroke: The piston moves up and compresses the air.
- Combustion stroke: When the piston reaches the top, fuel is injected and ignites at just the right moment, forcing the piston back down.
- Exhaust stroke: The piston returns to the top, expelling the exhaust gases from the combustion out the exhaust valve.
A diesel engine has no spark plug to draw in and compress air; instead, it injects fuel directly into the combustion chamber (direct injection). The heat of the compressed air ignites the fuel in a diesel engine.
A big difference between diesel and gasoline engines is the injection process. Most car engines use a combination of port injection (injecting fuel before the intake stroke (outside the cylinder)) and direct fuel injection. Port injection is used at lower engine speeds because it has a more stable mixture of air and fuel. Direct injection is used at higher speeds to provide more power and less chance of detonation, which is when the air is over-compressed and the fuel ignites on its own.
Diesel engines use only direct fuel injection—diesel is injected directly into the cylinder. The injectors on a diesel engine are the most complex components and are the subject of a great deal of experimentation—in any particular engine, the injectors may be located in a variety of locations. The injector must be able to withstand the temperatures and pressures in the cylinder while still delivering the fuel in a fine mist. Getting the mist to circulate and distribute evenly within the cylinder is also a problem, so some diesel engines employ special intake valves, pre-combustion chambers, or other devices to swirl the air within the combustion chamber or otherwise improve the ignition and combustion process.
Some diesel engines come with glow plugs. When a diesel engine is cold, the compression process may not raise the air temperature high enough to ignite the fuel. A glow plug is an electrical heating wire (think of the hot wire in a toaster) that heats the combustion chamber and raises the air temperature when the engine is cold so that the engine can start. Direct injection technology has advanced enough that glow plugs are generally no longer needed to ignite a spark, but many cars still have them. Their heat helps burn the fuel more efficiently. Of course, mechanical properties aren’t the only difference between diesel and gasoline engines. There’s also the matter of the fuel itself.
