✈️Intro to Flight Unit 9 – Propulsion: Piston Engines & Propellers

Basic Engine Principles

• Internal combustion engines convert chemical energy from fuel into mechanical energy through combustion
• Four-stroke cycle consists of intake, compression, power, and exhaust strokes
  • Intake stroke draws air-fuel mixture into the cylinder
  • Compression stroke compresses the mixture, increasing pressure and temperature
  • Power stroke ignites the compressed mixture, driving the piston down and generating power
  • Exhaust stroke expels the spent gases from the cylinder
• Two-stroke cycle combines intake and compression into one stroke, and power and exhaust into another, resulting in a power stroke every revolution
• Reciprocating motion of the pistons is converted into rotational motion of the crankshaft via connecting rods
• Thermodynamic efficiency depends on compression ratio, with higher ratios leading to greater efficiency but also increased risk of detonation
• Volumetric efficiency measures the effectiveness of an engine's air intake system, impacting power output
• Engine displacement represents the total volume swept by all pistons, affecting torque and power characteristics

Types of Aircraft Piston Engines

• Horizontally opposed engines (boxer engines) have cylinders arranged in two banks on opposite sides of the crankshaft, providing a low profile and good balance
• Radial engines arrange cylinders in a circular pattern around the crankshaft, offering high power-to-weight ratio but increased drag
• Inline engines have cylinders arranged in a single row, resulting in a narrow frontal area but potential cooling challenges
• V-type engines arrange cylinders in two banks forming a "V" shape, providing a compact design with good power-to-weight ratio
• Rotary engines (Wankel engines) use a triangular rotor instead of pistons, offering high power density and smooth operation but limited application in aviation
• Diesel engines use compression ignition instead of spark ignition, providing high efficiency and reliability but increased weight and complexity

Engine Components and Systems

• Cylinders contain the combustion chamber and provide a sealed space for the piston to reciprocate
  • Cylinder heads seal the top of the cylinders and house the valves and spark plugs
  • Cylinder walls are honed to provide a smooth surface for the piston rings to seal against
• Pistons transfer the force from combustion to the crankshaft via connecting rods
  • Piston rings seal the combustion chamber and regulate oil consumption
• Crankshaft converts the reciprocating motion of the pistons into rotational motion
  • Main bearings support the crankshaft and allow it to rotate freely
• Camshaft operates the valves, controlling the timing of intake and exhaust events
  • Pushrods, rocker arms, and valve springs transmit motion from the camshaft to the valves
• Lubrication system distributes oil to reduce friction and wear on moving parts
  • Oil pump circulates oil from the sump to the engine components
  • Oil filter removes contaminants from the oil to maintain its effectiveness
• Cooling system regulates engine temperature to prevent overheating and maintain optimal performance
  • Air-cooled engines rely on airflow over fins to dissipate heat
  • Liquid-cooled engines use a coolant (usually a water-glycol mixture) circulated through passages in the engine block and cylinder heads

Propeller Mechanics and Design

• Propellers convert rotational motion from the engine into thrust by accelerating a mass of air
• Blade angle (pitch) determines the amount of air displaced per revolution, affecting thrust and efficiency
  • Fixed-pitch propellers maintain a constant blade angle, optimized for a specific flight regime
  • Variable-pitch propellers allow the blade angle to be adjusted in flight, improving performance across a range of conditions
• Blade shape and airfoil selection impact propeller efficiency and noise characteristics
  • Thinner, more curved airfoils are more efficient but have a smaller operating range
  • Thicker, less curved airfoils are more robust and have a wider operating range but reduced efficiency
• Number of blades affects propeller efficiency, vibration, and noise
  • Increasing the number of blades reduces diameter and improves efficiency but may increase noise and complexity
• Propeller governor systems maintain constant RPM by adjusting blade pitch, ensuring optimal performance
• Propeller synchronization systems align the phase of multiple propellers to reduce vibration and noise in multi-engine aircraft

Engine Performance and Efficiency

• Power output is determined by factors such as displacement, compression ratio, and RPM
  • Brake horsepower (BHP) measures the power output at the engine crankshaft
  • Indicated horsepower (IHP) represents the theoretical power generated in the cylinders
• Torque is the rotational force produced by the engine, affecting acceleration and low-speed performance
• Specific fuel consumption (SFC) measures the fuel efficiency of an engine, expressed as fuel flow per unit of power output
  • Lower SFC values indicate better fuel efficiency
• Altitude effects on performance include reduced air density and pressure, impacting volumetric efficiency and power output
  • Turbocharging and supercharging systems compress intake air to maintain performance at higher altitudes
• Detonation (knocking) occurs when the air-fuel mixture ignites prematurely, causing damage to the engine
  • Higher octane fuels and precise ignition timing help prevent detonation
• Engine operating limits, such as maximum RPM and temperature, must be observed to ensure longevity and reliability

Fuel Systems and Combustion

• Aviation gasoline (avgas) is a high-octane fuel designed for spark-ignition piston engines
  • Different grades (e.g., 100LL, 100/130) are used depending on engine requirements
• Fuel injection systems precisely meter fuel into the intake air, improving efficiency and throttle response compared to carburetors
  • Continuous-flow fuel injection provides a constant supply of fuel to the engine
  • Pulsed fuel injection delivers fuel in timed bursts, synchronized with the intake valve opening
• Carburetors mix fuel and air before entering the engine cylinders
  • Venturi effect creates a pressure drop that draws fuel into the airstream
  • Float chamber maintains a constant fuel level for consistent mixture
• Stoichiometric ratio represents the ideal air-fuel mixture for complete combustion (typically 14.7:1 for gasoline engines)
  • Rich mixtures (more fuel) are used for high-power settings and cooling
  • Lean mixtures (more air) are used for fuel efficiency and reduced emissions
• Ignition systems initiate combustion by providing a high-voltage spark at the spark plugs
  • Magnetos generate high-voltage current independently of the aircraft's electrical system for redundancy
  • Dual ignition systems provide two spark plugs per cylinder for increased reliability and performance
• Detonation and pre-ignition can cause engine damage and must be avoided through proper fuel selection, mixture control, and cooling

Engine Maintenance and Troubleshooting

• Regular inspections and maintenance are essential for ensuring engine reliability and performance
  • Oil changes and filter replacements maintain lubrication system effectiveness
  • Spark plug cleaning and replacement ensure proper ignition and prevent fouling
  • Valve clearance adjustments maintain proper valve timing and prevent damage
• Compression tests measure the sealing efficiency of the cylinders, rings, and valves, indicating engine health
• Oil analysis can detect wear particles and contaminants, providing early warning of potential issues
• Common issues include fouled spark plugs, clogged fuel filters, and leaking gaskets or seals
  • Rough running or misfires can indicate ignition or fuel system problems
  • Excessive oil consumption may suggest worn piston rings or valve guides
  • Unusual noises (knocking, tapping, or squealing) can indicate mechanical issues such as worn bearings or damaged gears
• Borescope inspections allow visual examination of internal engine components without disassembly
• Overhaul intervals specify the maximum operating time or cycles before an engine must be disassembled, inspected, and refurbished to maintain airworthiness

Environmental Considerations

• Emissions from piston engines include carbon dioxide ($CO_2$), carbon monoxide (CO), unburned hydrocarbons (HC), and nitrogen oxides ($NO_x$)
  • Incomplete combustion results in higher CO and HC emissions
  • High combustion temperatures promote the formation of $NO_x$
• Noise pollution from piston engines and propellers can impact airport communities and wildlife
  • Mufflers and exhaust systems help reduce engine noise
  • Propeller design and blade count affect noise levels and pitch
• Fuel efficiency improvements reduce fuel consumption and greenhouse gas emissions
  • Lean-burn technologies and advanced fuel injection systems optimize the air-fuel mixture
  • Lightweight materials and improved engine designs reduce overall aircraft weight
• Alternative fuels, such as unleaded avgas and biofuels, aim to reduce environmental impact
  • Unleaded avgas eliminates the need for tetraethyl lead (TEL) additives, reducing lead emissions
  • Biofuels derived from renewable sources can lower net carbon emissions
• Sustainable aviation practices, such as optimized flight planning and continuous descent approaches, minimize fuel consumption and emissions
• Environmental regulations, such as noise and emissions standards, drive advancements in engine and propeller technology to mitigate the impact of aviation on the environment