Engine
Most light aircraft use a conventional 4 stroke engine. It operates similarly to an automobile engine. There may be 4, 6, or 8 pistons in the engine. The four strokes are:
- Intake
- Compression
- Power
- Exhaust
- Intake Stroke - The piston goes downward during the intake stroke. A valve called the Intake Valve is open, such that an air/fuel mixture enters the cylinder through the carburetor. During this operation a second valve, the Exhaust Valve closed.
- Compression Stroke - During this stroke, the piston is forced upward by the crankshaft. Both the Intake and Exhaust valves are closed. Consequently the air/fuel mixture in the closed cylinder is compressed by the upward movement of the piston.
- Power Stroke - As the piston nears the top of the cylinder, the spark plugs fire under control of the magnetos. There are 2 spark plugs in each cylinder, with a separate magneto supplying the electrical spark current to each plug. The spark ignites the fuel/air mixture, causing an explosion to occur in the cylinder. This forces the cylinder downward, and imparts power to the crankshaft. While one cylinder is performing the power stroke, other cylinders are in some phase of the other three strokes. Therefore the power stroke is the only one contributing to propulsion of the aircraft.
- Exhaust Stroke - When the piston reaches the bottom of the power stroke, the Exhaust Valve opens. The piston is pushed upward by the crankshaft, causing the burned fuel / air mixture to be purged from the cylinder. The exhaust valve closes, and the piston is now in a position for another intake stroke.
The pistons connect to the crankshaft through connecting rods. They attach the piston, which has an up-down motion, to the crankshaft which turns in a rotary motion. The crankshaft is usually directly connected to the propeller. In some aircraft, a gear arrangement connects the propeller to the crankshaft. The crankshaft may also drive auxiliary devices such as the Magnetos, Vacuum Pumps, Alternator and other devices. The connection may be directly or through pulleys, belts, and gears.
The ignition system is comprised of the magnetos and spark plugs, and is independent of the electrical system. Even if the alternator and battery are inoperable, the ignition system continues to function. If there is insufficient battery power to crank the engine, the engine can be started on most small aircraft by “hand propping”. This is a procedure wherein the propeller is turned swiftly by hand to get the magneto system to fire, and start the engine. It is similar to pulling the starter cord on a lawn mower.
EXTREME CAUTION MUST BE EXERCISED WHEN HAND PROPPING AN ENGINE.
A meter within the cockpit called the Tachometer indicates the engine Revolutions per Minute (RPM). Monitoring devices such as the Oil Pressure and Oil Temperature gauge in the cockpit may be attached to the engine.
