Combustion-style airplane engines can be found on almost all airplanes from Wrold War II and before ¹, and most modern small aircraft. These engines operate on the same principles as standard four-stroke engines, but are often designed fundamentally differently. In fact, early airplane engines often operated in very strange ways. The three most common types of combustion airplane engines include standard engines similar to a car's, a circular non-rotating engines, and a circular rotating engine.The first type is found in most new light aircraft, while the latter and former are mainly found in historic aircraft.

Radial Engines

Radial airplane engines are very curious-looking devicese. These engines' cylinders are not arranged in rows like a car's engine, but rather in a circle. There are two different types of radial engine: one that revolves, and one that does not.

"Non-Revolving" Radials

These engines operate similarly to car engines, with the obvious exception of the difference in design. The engine is mounted directly to the airplane, with the propellor shaft rotated by the engine. While this may seem to be the normal way things should be, these engines had one minor problem: cooling. These engines were air-cooled, but the problem was the amount of airflow over the cylinders. Because of the lack of airflow, the cylinders were left exposed outside of the engine cowling to increase exposure to moving air. To further aid in cooling, large slats of metal were attached to the cylinders ². These slats act like a microprocessor's heatsink, expanding the surface area of the device to allow still more air to flow over the cylinders.

"Revolving" Radials

Another, novel way of solving the cooling problem was with an engine that actually rotated. In this type of engine, the shaft was mounted to the fuselage of the aircraft, and the propellor was fixed to the engine. The engine turned, rotating the propellor with it. This design ensured constant airflow over the cylinders, as the engine was constantly moving through air.

Throttle the Engine

Early engines, while enabling the plane to get off the ground, were not exactly the most user-friendly. More specifically, there was absolutely no way to control how fast the engine operated; that is, there was no "gas pedal." What the pilot had was a switch similar to the handle on a lawnmower, a "dead man's switch," whereby, if the switch was released, the engine would cut out. So, the engine was either full-on or completely off. However, the engine's speed could be controlled - with extreme difficulty - by continually pushing this button. The engine would run, turn off, run, turn off, etc., making the average number of RPMs³ less than full power. Thus, the plane could slow down, so long as the button was not held for too long; if it was, the engine would completely cut out, requiring a restart. Modern airplanes have a working throttle that controls the fuel/air flow into the eingine. This feature is definitely useful, especially if the pilot wishes to do anything other than fly straight-and-level.

Modern Engines

Modern airplane combustion engines are remarkably similar to normal automobile engines. However, airplane engines are usually much smaller than automobile engines; the engine in a Cessna 152, for example, has only four cylinders and a carbureter rather than fuel-injection. Despite this, however, this airplane has a cruising speed of about 90 knots, almost twice the speed limit of a US highway.

There are two main differencees in the design of an airplane engine compared to an automobile engine which are designed to make the plane safer and more fuel efficient.

Carburetion

While the newest airplane engines do have fuel-injection systems that automatically adjust the fuel/air mixture entering the cylinders, older engines only have carbureters. This creates a problem, as the carbureter only atomizes the fuel to mix it with the air; it does not actually control the amount of fuel entering the mixture. So, an extra knob can be found inside the cockpit of airplanes with carbureted engines: the mixture knob. By adjusting this knob one can manually adjust the amount of fuel entering the mixture. This is very handy when flying higher than a few thousand feet, as the air density drops, decreasing the amount of air in the mixture. Thus, the mixture becomes too rich⁴ which decreases fuel efficiency and could cause the engine to quit if there is not enough air for the fuel to burn. Controlling the amount of fuel entering the cylinders allows for more efficient burning and safer operation.

In addition, carbureters have one other potentially dangerous problem. The process of carburetion, which will be saved for a later article, relies on Bernoulli's Principle, which states that as the speed of a fluid increases, the temperature and pressure decrease. The carbureter must speed up the air to properly mix it with the fuel, so the air's temperature decreases. Air contains a quantity of water vapor, and if the air's temperature decreases too much, icing can form inside the carbureter, decreasing the flow of air. If enough ice builds up, not enough air will flow and the engine will be starved of fuel. To counteract carbureter icing, a "carb heat" mechanism is installed into airplanes. This mechanism connects the carbureter with a shroud around the exhaust pipe. The hot exhaust gases heat up the air in the shroud, which is then ducted directly into the carbureter, melting any ice. When active, the carb heat does reduce performance, but makes the airplane much safer.

Electrical System

In addition to the carburetion system, the electrical system of an airplane is also different than a car's. To insure safety, there is a redundant system, that is, there are two identical, independant electrical systems. Each cylinder has two spark plugs, ,and two different sources provide electrical energy for the spark plugs. The dual spark plugs not only increase safety, but performance as well, as the fuel burns more evenly.

A four-stroke engine's electrical system is powered by the battery, which is continually recharged by the alternator. In an airplane, the battery is used only to start the engine. After the engine starts, two magnetos, or "mags," power the two sets of spark plugs. The mags act like miniature generators that are powered by the turning of the engine. Because of the design of this system, the magnetos will run as long as the engine is running. A complete electrical failure could occur and the engine would still run. This makes the engine highly reliable, which is necessary to continued operations off the ground.