The internal-combustion engine is a nifty little contraption whose purpose is to convert chemical energy to mechanical energy. This process, as the name implies, involves the burning of some sort of fuel inside the device, hence "internal-combustion." Internal-combustion engines are generally one of three types: standard four-stroke engines, two-stroke engines, and diesel engines.

Basic Design

An engine generally turns some sort of driveshaft, as in the case of an automobile, so the problem is getting a fuel to turn a rod. To accomplish this, a cam ¹ is attached at one end to the driveshaft and a piston at the other end. The piston is situated inside a cylinder. A fuel/air (or fuel/air/oil) mixture is injected above the piston; this mixture is then ignited, which pushes the piston down, rotating the cam, which rotates the driveshaft. Automobile engines normally have at least four cylinders to drive the wheels. Oil is added for several reasons, including:

• Creating a tight seal between piston and cylinder.
• Lubricating parts to reduce wear.
• In some cases, cooling the engine.
• Removing sediment from the engine.

The engine is started with the aid of a starter motor. This motor is directly connected to the battery (normally 12 volts). The battery is then used to power the motor, which cranks the engine, eventually starting it. An alternator, attached to the engine, then begins to produce electrical energy. The alternator continually recharges the battery as it is used to power the spark plugs that ignite the fuel in the engine.

Gasoline and diesel, the most common fuels for internal-combustion engines, are hydrocarbons. Thus, in theory, the exhaust should only contain carbon dioxide and water. However, because of the high operating temperatures of this type of engine, other products are created including CO ² and NO_x³, a class of ozone-depleting gases. For this reason, catalytic converters are attached to the exhaust pipes of most automobiles produced today.

Four-Stroke Engines

A four-stroke engine is what can be found in most automobiles, light aircraft, and some generating equipment. The engine operates in four steps, or "strokes" - hence the name. The four steps are:

• Injection
• Compression
• Combustion
• Exhaust

In other words...

• Suck
• Squeeze
• Bang
• Blow

The fuel/air mixture is sent to the cylinder through a valve as the piston is pushed down. The piston then pushes back up, compressing the mixture. At the top of the stroke, the spark plug arcs, igniting the mixture, which pushes the piston back down. The exhaust valve then opens, and the piston pushes back up, expelling the gases.

The mixture normally arrives in the cylinders in one of two ways. First, a carburetor can be used. The carburetor mixes fuel and air before it is sent to the engine. A more advanced method is fuel injection, which normally mixes the fuel and air immediately before it enters the cylinder. Fuel injection is normally more efficient than carburation, as newer systems are computer-controlled.

Airplane Engines

Propellor-driven aircraft often use four-stroke engines; however, airplane engines are slightly different from automobile engines. First, because the atmosphere thins out as altitude is increased, the fuel/air mixture would normally become too rich ⁴ at altitude. To compensate, The pilot can manually adjust the richness of the mixture for a carburated engine. Fuel-injection systems do not have this problem.

The ignition system in an airplane engine is also slightly more advanced than that in an automobile. Reliability is key to the operation of an aircraft; having an engine go out in the air is much more dangerous than having one go out on the interstate. Therefore, the ignition system on an airplane is completely independant of the electrical system. Two engine-driven magnetos, or "mags", power the spark plugs. The alternator and battery could both go out, yet the mags will still power the engine. In addition, each cylinder has two spark plugs, increasing efficiency and adding a redundant system.

Two-Stroke Engines

A two-stroke engine is the little brother of the four-stroke. Whereas in a four-stroke engine the spark plug arcs every other revolution, a two-stroke engine produces power from every revolution.

Start with the cylinder at the bottom of the stroke. Positive pressure in the area surrounding the cylinder (the "crankcase") forces a fuel/air/oil mixture into the cylinder. Note there are no valves in this type of engine. The piston drives upward, and the spark plug arcs, as in a four-stroke engine. In the design of the cylinder, however, the exhaust port is higher up in the cylinder than the injection port. So, as the piston is pushed down, it uncovers the exhaust port; the pressure inside the cylinder expels most of the exhaust. Also, the downward motion of the piston pressurizes the crankcase, which pushes fuel into the cylinder once the injection port is uncovered. The cycle then stars over again.

Two-stroke engines have some major advantages. First, they are very simple and lightweight, which makes them cheap to produce and ideal for small devices, such as lawnmowers, gas-powered trimmers, go-karts, and jet-skis. Also, because fuel is ignited every revolution, rather than every other revolution, the two-stroke engine produces much more power.

Unfortunately, there are some very large disadvantages to the two-stroke engine. In other types of engines, the crankcase is filled with oil, for the reasons previously stated. However, because the crankcase is being used to pressurize the fuel mixture, oil cannot be added there. Instead, it is mixed with the fuel. Two-stroke oil can be very expensive, and because it is ignited with everything else, produces a lot of pollution. Also, the lubrication system is not as effective with the oil mixed with the fuel, so two-stroke engines do not last very long.

Fuel efficiency is also a problem. In addition to other things, the lack of valves in the cylinder means that, during the compression stage, some of the mixture leaks out the exhaust port. This decreases efficiency and contributes to pollution.

Diesel Engines

Diesel engines are unique in that there are no spark plugs ⁵. Ignition is produced by introducing diesel fuel into highly compressed, and therefore hot, air.

Unlike other engines, the fuel and air are not mixed prior to entering the cylinder. Air is injected and compressed, and at the peak of compression, the fuel is injected directly into the cylinder. The fuel combusts, which pushes the piston as in the other engine types.

Because only air is being compressed, the compression ratio in the cylinder can be much higher than in a four-stroke engine. Compressing the mixture in a four-stroke to the same amount as in a diesel engine can cause premature ignition. This higher compression ratio results in much better efficiency and more power.

Diesel fuel is also more viscous ⁶ and unrefined than gasoline. For this reason, diesel fuel is often referred to as diesel oil. Because it is less refined than gasoline, it is also often cheaper than normal gasoline.

Starting an engine with no ignition system can be a problem. To solve this, most modern diesel engines use a computer system to time the compression and fuel injection in order to increase the chance of ignition. In smaller and less sophisticated engines, a glow plug is often employed. Glow plugs are often used in remote-controlled, gas powered vehicles and is essentially a heating element much like one that can be found in a toaster. This heats up the mixture enough to cause ignition.