Types of engines and how they work

Engines are machines that convert a source of energy into physical work. If you need something to move around, an engine is just the thing to slap onto it. But not all engines are made the same, and different types of engines definitely don’t work the same.

Probably the most intuitive way to differentiate between them is the type of energy each engine uses for power.

• Thermal engines

  • Internal combustion engines (IC engines)

  • External combustion engines (EC engines)

  • Reaction engines

• Electrical engines

• Physical engines

Thermal engines

In the broadest definition possible, these engines require a source of heat to convert into motion. Depending on how they generate said heat, these can be combustive (that burn stuff) or non-combustive engines. They function either through direct combustion of a propellant or through the transformation of a fluid to generate work. As such, most thermal engines also see some overlap with chemical drive systems. They can be airbreathing engines (that take oxidizer such as oxygen from the atmosphere) or non-airbreathing engines (that have oxidizers chemically tied in the fuel).

Internal combustion engines

Internal combustion engines (IC engines) are pretty ubiquitous today. They power cars, lawnmowers, helicopters, and so on. The biggest IC engine can generate 109,000 HP to power a ship that moves 20,000 containers. IC engines derive energy from fuel burned inside a specialized area of the system called a combustion chamber. The process of combustion generates reaction products (exhaust) with a much greater total volume than that of the reactants combined (fuel and oxidizer). This expansion is the actual bread and butter of IC engines — this is what actually provides the motion. Heat is only a byproduct of combustion and represents a wasted part of the fuel’s energy store, because it doesn’t actually provide any physical work.

IC engines are differentiated by the number of ‘strokes’ or cycles each piston makes for a full rotation of the crankshaft. Most common today are four-stroke engines, which break down the combustion reaction in four steps:

1. Induction or injection of a fuel-air mix (the carburate) into the combustion chamber.

2. Compression of the mix.

3. Ignition by a spark plug or compression — fuel goes boom.

4. Emission of the exhaust.

External combustion engines

External combustion engines (EC engines) keep the fuel and exhaust products separately — they burn fuel in one chamber and heat the working fluid inside the engine through a heat exchanger or the engine’s wall. That grand daddy-o of the Industrial Revolution, the steam engine, falls into this category.

In some respects, EC engines function similarly to their IC counterparts — they both require heat which is obtained by burning stuff. There are, however, several differences as well.

EC engines use fluids that undergo thermal dilation-contraction or a shift in phase, but whose chemical composition remains unaltered. The fluid used can either be gaseous (as in the Stirling engine), liquid (the Organic Rankine cycle engine), or undergo a change of phase (as in the steam engine) — for IC engines, the fluid is almost universally a liquid fuel and air mixture that combusts (changes its chemical composition). Finally, the engines can either exhaust the fluid after use like IC engines do (open-cycle engines) or continually use the same fluid (closed-cycle engines).

How Diesel Engines Work

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:

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.

How Do Diesel Vehicles Work?