9.1 Piston Engine Operating Principles

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

Engine Performance

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