The engine has a number of systems that help it to do the job of converting fuel into motion. Most of these subsystems can be implemented using different technologies, and better technologies, and better technologies can improve the performance of the engine. Here's a look at the major subsystems used in modern engines:
Lubrication System - The lubrication system makes sure that everything moving in the engine gets oil so that it can move easily. The two main things needing oil are the piston (so they can slide easy) and bearing that allow things the rotate freely. In most cars oil is sucked out of the oil pan by an oil pump, ran through the oil filter to remove any grit, and then squirted under high pressure bearing and the cylinder walls. The oil then trickles down into the sump, where it is collected, and used again.
Fuel System - The fuel system pumps gas from the gas tank and mixes it with air so that the proper air/fuel mixture can flow into the cylinders. There are three common ways to deliver the fuel: carburetion, port fuel injection, and direct fuel injection. In carburetion a device called a carburetor mixes gas into air as the air flows into the engine. In fuel injected engines the right amount of fuel is injected individually into each cylinder either right above the intake valve or directly into the cylinder.
Exhaust System - The exhaust system includes the exhaust pipe and muffler. Without a muffler you would hear thousands of small explosions coming out your tailpipe. A muffler dampens the sounds.
Valve Train - The valve train consists of the valves and mechanism that opens and closes them. The opening and closing system is called a cam shaft. The cam shaft has loves on it that move the valve up and down. Most modern engines have what are called overhead cams. This means that the cam shaft is located above the valve. The cams on the shaft active the valves directly or through a very short linkage. Older engines used a camshaft located in the sump near the crankshaft. Rods linked the cam below to valve liters above the valves. This approach has more moving parts and also causes more lag between the cam's actuation of the valve and the valve's subsequent motion. A timing belt or timing chain links the crankshaft to the cam shaft so that the valves are in sync with the pistons. The cam shaft is geared to turn at one-half the rate of the crankshaft. Many high-performance engines have 4 valves per cylinder (two for intake, two for exhaust) and this arrangement requires two cam shafts per bank of cylinders, hence the phrase "dual overhead cams."
Ignition system - The ignition system produces a high-voltage electrical charge and transmits it to the spark plugs via ignition wires. The charge first flows to a distributor, which you can easily find under the hood of most cars. The distributor has one wire going in the center and 4, 6 or 8 wires coming out of it. There ignition wires send the charge to each spark plug. The engine is timed so that only one cylinder receives a spark from the distributor at a time. This approach provides maximum smoothness.
Cooling System - The cooling system in most cars consists of the radiator and water pump. Water circulates through passages around the cylinders and then travels through the radiator to cool it off. In a few cars (including the Volkswagen Beetles), as well as most motorcycles and lawn mowers, the engine is instead air-cooled (you can tell an air-cooled engine by the fins adorning the outside of each cylinder to help dissipate heat). Air-cooling makes the engine lighter but hotter, generally decreasing engine life and overall performance. Air intake system - Most cars are normally aspired, meaning that air flows through an air filter and directly into the cylinders. High-performance engines are either turbo charged or super charged, meaning that air coming into the engine is first pressurized to increase performance. The amount of pressurization is called boost. Turbo chargers used a small turbine attached to the exhaust pipe to spin a compressing turbine in the incoming air stream. Super chargers are attached directly to the engine to spin the compressor.
Electrical System - The electrical system consists of a battery and an alternator. The alternator is connected to the engine by a belt and generates electricity to recharge the battery. The batter makes 12 volt power available to everything in the car needing electricity (ignition system, radio, headlights, windshield wipers, power windows, locks, seats etc.) through the vehicles wiring.
There are some other subsystems that I have not mentioned, but are not as important as the one's on this page.