An airplane’s powerplant includes the engine, propeller, and the engine accessories. Not only does it provide thrust, allowing the airplane to accelerate and climb without assistance, but it also powers the various aircraft systems by providing electrical power and driving fuel, air, and often hydraulic pumps. At the heart of the powerplant is the reciprocating engine, which produces power by burning gasoline inside its cylinders.
This causes the back and forth, or reciprocating, movement of a piston inside each cylinder.
Each piston is connected to a rod, which translates this back and forth motion into the spinning of the engine’s crankshaft.
The Four Stroke Cycle
The engine operates on a four stroke cycle.
The first is intake. An intake valve opens, allowing a highly flammable mixture of vaporized fuel and air to rush into the cylinder. When the cylinder is filled, the intake valve closes.
Next is compression. As the piston is driven upward by the motion of the rest of the engine, it tightly squeezes the flammable mixture into a small space near the top of the cylinder.
Third is the ignition or power stroke. A carefully timed firing of the spark plug ignites the fuel/air mixture, resulting in a great deal of heat and pressure inside the cylinder, which powerfully forces the piston downward.
The exhaust stroke finishes the cycle, by allowing the spent exhaust gasses to escape through the exhaust valve as the piston rides to the top of the cylinder once again. When the piston reaches the top of the cylinder, the exhaust valve closes, and the cycle begins again.
The Cam Shaft
The intake and exhaust valves are held closed by springs and opened by a cam shaft. As the engine turns, it drives the cam shaft, which is shaped with strategically placed lobes along its length. These lobes open the intake and exhaust valves on the different cylinders as the cam shaft rotates.
Engine Description
Reciprocating engines are described by the number and arrangement of their cylinders. Common cylinder arrangements are radial, in-line, v-type, and opposed. Opposed engines are the most common on small airplanes, since they produce good power, pound for pound, without creating very much drag as the airplane flies.
Engine Cooling
In contrast to automobiles, airplane engines are air cooled by outside air, which is routed by baffles across the cylinders and engine. Each cylinder has fins which allow the air to cool the cylinder more effectively. When an engine is air cooled, a combination of high engine power and low airflow tends to cause the engine to run hotter. This situation is typically encountered during climb out. The engine temperature gauge should be scanned to assure the engine’s temperature limitations are not exceeded.
Excessively high engine temperatures will cause loss of power, excessive oil consumption, and possible permanent internal engine damage. Operating an engine at a higher temperature than it was designed for will cause loss of power, excessive oil consumption and detonation.
Detonation occurs when the fuel explodes suddenly inside the cylinder, instead of burning evenly as designed. This sudden detonation of the fuel is often described as engine knocking. Engine pre ignition occurs when the fuel spontaneously ignites inside the engine prior to the proper time for normal ignition. Pre ignition can be cause by hot spots inside the engine or overheated spark plugs. It changes the timing of the engine and is also often referred to as engine knocking. To prevent detonation, the pilot should keep the temperature of the engine within recommended limits.
Lowering the nose will allow the airplane to gain speed, which will eventually lower the engine temperature.