According to HowStuffWorks, internal combustion engines work by igniting a compressed fuel-air mixture in a combustion chamber and harnessing the energy to perform some type of work. Internal combustion engines transfer chemical energy into mechanical motion.
The four-stroke internal combustion engine has a combustion chamber, piston and cylinder, where a fuel-air mixture is drawn in or injected. The mixture is compressed by the piston and ignited, typically with a spark plug or similar device, and the rapid expansion of gases inside the cylinder forces the piston, which is connected to a crankshaft and flywheel, down. The crankshaft transfers the energy of the moving piston to other components, while the flywheel sustains the momentum of the crankshaft until the next combustion cycle takes place. Cams control the intake of fuel, the timing of the ignition and the operation of the valves for the intake and exhaust cycles.
Internal combustion engines come in a variety of sizes and designs, including rotary, in-line and v-blocks. However, the two most common types are the four-stroke engine and the two-stroke engine. The two-stroke engine combines the intake and compression strokes and the exhaust is expelled near the end of the cycle as new fuel is drawn into the cylinder. One of the major drawbacks of the two-stroke engine is that the exhaust is expelled from the cylinder into which fresh fuel is drawn before compression, taking some of the fresh fuel with it.