Piston engines are the heart of many aircraft, powering flight through a series of controlled explosions. This section breaks down how these engines work, from the four-stroke cycle to key components like cylinders and valves.
Understanding piston engines is crucial for pilots and mechanics alike. We'll explore how fuel and air mix, ignite, and turn into power, as well as how different parts work together to keep planes in the sky.
Four-Stroke Cycle
Cycle Phases and Operations
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Four-stroke cycle operates in a sequence of four distinct phases completing one full cycle
Intake stroke draws air-fuel mixture into the cylinder as piston moves downward
Compression stroke compresses the mixture by moving the piston upward
Power stroke generates thrust when ignited mixture expands, forcing piston downward
Exhaust stroke expels combustion gases as piston moves upward again
Valve Timing and Piston Movement
Intake valve opens during intake stroke allowing air-fuel mixture to enter
Both valves close during compression stroke to seal the cylinder
Spark plug ignites compressed mixture just before top dead center (TDC)
Exhaust valve opens during exhaust stroke to release combustion products
Crankshaft rotates twice (720 degrees) for one complete four-stroke cycle
Engine Components
Core Structural Elements
Cylinder forms the combustion chamber where air-fuel mixture burns
Cylinder head seals the top of the cylinder and houses valves and spark plug
Piston moves up and down within the cylinder, transferring force to the crankshaft
Connecting rod links the piston to the crankshaft, converting linear motion to rotational
Valve Train and Power Transmission
Crankshaft converts reciprocating piston motion into rotational motion
Camshaft controls the opening and closing of valves via lobes and pushrods
Intake valves allow air-fuel mixture to enter the cylinder (typically larger than exhaust valves)
Exhaust valves permit combustion gases to exit the cylinder
Timing chain or belt synchronizes crankshaft and camshaft rotation
Fuel and Ignition Systems
Fuel Delivery Methods
Fuel injection system sprays fuel directly into the intake manifold or cylinder
Electronic fuel injection (EFI) uses sensors and computer control for precise fuel metering
Carburetor mixes air and fuel mechanically using venturi effect and fuel jets
Float bowl in carburetors maintains constant fuel level for consistent mixture
Ignition Components and Timing
Ignition system generates high-voltage spark to ignite the air-fuel mixture
Distributor routes high voltage to each spark plug in firing order (older systems)
Electronic ignition systems use crankshaft position sensors to time spark delivery
Spark plugs create the electric arc that ignites the compressed mixture
Ignition timing advances or retards spark based on engine speed and load
Power Output Measurements
Horsepower measures the rate of work done by the engine
Brake horsepower (BHP) represents the engine's power output at the crankshaft
Indicated horsepower (IHP) includes power lost to friction and accessories
Power curves show how horsepower varies with engine speed (RPM)
Specific power relates engine output to its displacement (horsepower per liter)
Torque Characteristics and Applications
Torque represents the rotational force produced by the engine
Peak torque typically occurs at lower RPM than peak horsepower
Torque curve illustrates how engine torque changes with RPM
Flywheel helps smooth out torque pulses between power strokes
Gearing in transmissions and propeller reduction units optimizes torque delivery