6.1 Principles of Internal Combustion Engines

Internal combustion engines are the workhorses of aviation, converting fuel into mechanical energy to power aircraft. These engines use a four-stroke cycle - intake, compression, power, and exhaust - to generate thrust, with pistons, connecting rods, and crankshafts working together.

Aviation engines come in two main types: spark-ignition and compression-ignition. Spark-ignition engines use spark plugs and gasoline, while compression-ignition engines rely on high compression and diesel fuel. Each type has its own advantages and disadvantages for aircraft propulsion.

Internal Combustion Engines in Aviation

Principles of aviation combustion engines

• Internal combustion engines convert chemical energy from fuel into mechanical energy through combustion process
  • Fuel burned inside engine cylinders (gasoline, diesel, jet fuel)
  • Expanding gases from combustion drive pistons, which rotate crankshaft to generate power
• Main components of an aviation internal combustion engine:
  • Cylinders: Combustion chambers where fuel-air mixture is ignited
  • Pistons: Reciprocating components driven by combustion pressure, transfer energy to crankshaft
  • Connecting rods: Link pistons to crankshaft, converting linear motion to rotational motion
  • Crankshaft: Rotates to provide power output, driven by connecting rods and pistons
  • Valves: Control flow of air, fuel, and exhaust gases in and out of cylinders (intake and exhaust valves)
  • Spark plugs (spark-ignition engines): Initiate combustion with electric spark at precise timing
  • Fuel injectors (compression-ignition engines): Inject pressurized fuel directly into cylinders for combustion

Four-stroke cycle stages

1. Intake stroke:

   • Intake valve opens as piston moves downward, drawing air (spark-ignition) or air-fuel mixture (compression-ignition) into cylinder

2. Compression stroke:

   • Intake valve closes, piston moves upward to compress air or air-fuel mixture
   • Compression increases pressure and temperature in cylinder, preparing for combustion

3. Power stroke:

   • Combustion initiated by spark plug (spark-ignition) or self-ignition due to high temperature and pressure (compression-ignition)
   • Expanding gases from combustion push piston downward, providing power to turn crankshaft

4. Exhaust stroke:

   • Exhaust valve opens as piston moves upward, expelling burnt gases from cylinder
   • Cycle repeats, starting with intake stroke for continuous engine operation

Spark-ignition vs compression-ignition engines

• Spark-ignition (SI) engines:
  • Rely on spark plugs to ignite air-fuel mixture at precise timing
  • Typically run on gasoline or aviation gasoline (avgas)
  • Lower compression ratio (8:1 to 12:1), suitable for gasoline fuels
  • Throttle valve regulates air intake to control power output
• Compression-ignition (CI) engines:
  • Utilize high compression to self-ignite fuel without spark plugs
  • Operate on diesel or jet fuel, which have higher ignition temperatures
  • Higher compression ratio (14:1 to 25:1) enables self-ignition
  • Fuel injection system controls fuel delivery to regulate power output

Reciprocating engines for aircraft propulsion

• Advantages of reciprocating engines in aircraft:
  • Reliable and well-established technology with extensive operational history
  • Lower cost compared to turbine engines, suitable for small aircraft
  • Efficient at low altitudes and speeds, ideal for general aviation
  • Fuel flexibility, can operate on various gasoline and diesel fuels
• Disadvantages of reciprocating engines in aircraft:
  • Limited power output compared to turbine engines, constraining aircraft size and performance
  • Lower power-to-weight ratio, resulting in heavier propulsion systems
  • Reduced efficiency at high altitudes due to decreased air density
  • Higher vibration and noise levels, impacting passenger comfort
  • More complex maintenance due to numerous moving parts and mechanical systems