The internal combustion engine is an amazing machine that has evolved for more than 100 years. It continues to evolve as automakers manage to squeeze out a little more efficiency, or a little less pollution, with each passing year. The result is an incredibly complicated, surprisingly reliable machine.
Other HowStuffWorks articles explain the mechanics of the engine and many of its subsystems, including the fuel system, cooling system, camshafts, turbochargers and gears. One could argue that the ignition system is where it all comes together, with a perfectly timed spark.