Piston Engines: Basic Principles

This engine is liquid cooled and powered early fighters of WWII including the P-51 Mustang up to 1941.

Reciprocating piston engines are the most common power plants on general aviation aircraft. These engines are virtually identical to automobile engines, with three important exceptions:

1. Most aircraft engines are air cooled. This approach saves the weight of a radiator and coolant and adds a measure of safety. The loss of coolant or a failure of the cooling system in a liquid-cooled engine quickly causes complete engine failure.
2. Aircraft engines have dual ignition systems, with the energy to create the spark generated by magnetos. A magneto, turned by the crankshaft, isn't dependent on the aircraft battery. Each cylinder also has two spark plugs. If one plug or magneto fails, the other provides a spark to burn fuel.
3. Because an aircraft engine operates throughout a wide range of altitudes, the power controls include a manual mixture control that the pilot uses to maintain the proper air/fuel ratio as the airplane climbs and descends.

Carburetors and Fuel Injectors

Most piston engines used in aircraft have either a carburetor or fuel injection system to deliver fuel and air to the cylinders. The carburetor mixes fuel and air before it enters the cylinders. Carburetors are common on smaller engines because they're relatively inexpensive. Larger engines usually have fuel-injection systems, which squirt fuel directly into the cylinders, where it mixes with air during the intake stroke.

The Four-Stroke Cycle

A typical piston engine operates according to a four-stroke cycle.

1. Intake: The piston moves down in the cylinder, drawing in air and fuel through the open intake valve.
2. Compression: The intake and exhaust valves in the cylinder close and the piston moves up in the cylinder, compressing the fuel-air mixture.
3. Power: As the piston nears the top of the cylinder during the compression stroke, a burst of electricity from the ignition system generates a spark in the spark plugs. The sparks ignite the air/fuel mixture, which expands rapidly as it burns. The force of this expansion drives the piston back down in the cylinder. As the piston moves down, it turns the crankshaft, which turns the propeller.
4. Exhaust: When the piston reaches the bottom of the cylinder, the exhaust valve opens. The piston then moves back up in the cylinder, pushing the burned air/fuel mixture out of the cylinder.