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 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.
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
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.