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Helicopter engines can be either piston or gas turbine turboshaft. Air is drawn in, compressed, mixed with fuel, ignited, then the rapid expansion of the gas is harnessed to turn a drive shaft which is fed to the main transmission. The engines use gasoline (Avgas) or Kerosine (Jet A1) to power them.
Source: How Do Helicopter Engines Work? Your Ultimate Guide – Pilot Teacher
