Sep. 28, 2021
An engine is a machine that converts energy into physical labor. If you need something to move, an engine is just something to slap it around. But not all car engines are the same, and different types of engines certainly don't work the same.
Probably the most intuitive way to distinguish between them is the type of energy each engine uses to power it.
Internal combustion engines (IC engines)
External combustion engines (EC engines)
In their broadest definition, these vehicle engines require a heat source in order to be converted into motion. Depending on the way they generate said heat, they can be combustible (burning something) or non-combustible engines. They work through the direct combustion of a propellant or through the conversion of fluids to produce work. Therefore, most heat engines also have some overlap with chemically driven systems. They can be aspirating engines (absorbing oxidants such as oxygen from the atmosphere) or non-aspirating engines (chemically binding oxidants in the fuel).
EURO I Vehicle Engine 6110 Series
Internal combustion engines (IC engines) are very common today. They power cars, lawn mowers, helicopters, etc. The largest IC engines can produce 109,000 horsepower and power a ship moving 20,000 containers. Internal combustion engines derive their energy from the fuel burned in a specialized area of the system called the combustion chamber. The total volume of the reaction products (exhaust gases) produced by the combustion process is much larger than the total volume of the reactants (fuel and oxidizer). This expansion is the real basis of the IC engine - this is what really provides the movement. The heat is just a by-product of combustion and represents the wasted part of the fuel energy store, as it does not actually provide any physical work.
IC engines are distinguished by the number of "strokes" or cycles each piston makes to rotate the crankshaft in its entirety. The most common engine today is the four-stroke engine, which breaks down the combustion reaction in four steps.
Introduction or injection of the fuel-air mixture (carbide) into the combustion chamber.
Compression of the mixture.
Ignition by spark plug or compression - fuel explosion.
Emission of exhaust gases.
For each step, a 4-stroke piston is alternately pushed down or up. Ignition is the only step in the engine that produces work, so for all other steps, each piston relies on energy from an external source (other pistons, electric starter, manual crank, or crankshaft inertia) to move. The reason why your car needs a working battery to start running.
Other criteria that distinguish internal combustion engines are the type of fuel used, the number of cylinders, the total displacement (internal volume of the cylinders), the distribution of cylinders (inline, radial, V-engine, etc.), and the power and weight output.
External combustion engines (EC engines) keep the fuel and exhaust products separate - they burn the fuel in a combustion chamber and heat the working fluid in the engine through a heat exchanger or engine wall. In some ways, EC engines function similarly to their IC counterparts - they both require the heat gained by burning something. However, there are some differences.
EC engines use fluids that have undergone thermal expansion and contraction or phase shift, but their chemical composition remains the same. The fluid used can be gaseous (as in a Stirling engine), liquid (in an organic Rankine cycle engine), or phase-shifted (as in a steam engine) - for internal combustion engines, the fluid is almost universally a mixture of liquid fuel and burning air (changing its chemical composition). Finally, engines can either discharge the fluid after use (open cycle engines) or continue to use the same fluid (closed-cycle engines), just like internal combustion engines.
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