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Overview

Most aircraft engines are horizontally-opposed, reciprocating, air-cooled engines.

Pistons​

  • Pistons convert up and down motion to rotation via the crankshaft

Engine piston

Four Strokes of an Engine​

  • Intake: Intake valve opens (exhaust valve closed), fuel + air drawn into cylinder
  • Compression: Fuel + air mixture compressed as piston is pushed down
  • Ignition: Piston pushed up from expansion
  • Exhaust: Exhaust valve opens and burned gasses are expelled

Four stroke of an engine strokes

Carburetor​

Float-type carburetor

Carburetor supplies an intake of fuel and air to the engine cylinders in the correct ratio.

  • The venturi is a small opening that accelerates air and provides suction
  • Note that the fuel-air mixture lines are directed into the cylinders sequentially
    • This means the first cylinder will have the richest mixture, and it gets slightly leaner for the subsequent cylinders

Fuel Injection​

  • Fuel-injected engines do no have a carburetor
  • Fuel is metered in equal amount for each cylinder
  • The fuel metering unit is a mechanical pump that is controlled by the throttle

Intake Manifold​

  • Air intakes should take in clean air for use in the engine

Ignition Systems​

Magneto systems

  • Most aircraft have two independent ignition systems, with two spark plugs for each cylinder
  • Magnetos are small generators attached directly to the crankshaft
  • The current produced by the magnetos are connected to a timing unit, which sends sparks at specific intervals for the engine to fire

Fuel Systems​

  • Wet wings: tanks that are coated surface of the wing structure (as opposed to tanks set within the wing)
  • Low wing airplanes only have L/R fuel selectors
    • This is due to the pressure imbalance between the tanks
    • A both position would draw fuel from the lowest tank, since it has lower pressure, and run the tank dry