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:
Induction or injection of a fuel-air mix (the carburate) into the combustion chamber.
Compression of the mix.
Ignition by a spark plug or compression — fuel goes boom.
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).
