The fuel-air ratio in a turboprop engine, also referred to as the “equivalence ratio” (φ), is a measure of how the actual fuel-air mixture ratio in the engine compares to the stoichiometric fuel-air mixture ratio. The stoichiometric ratio is the ideal proportion of fuel and air required for complete combustion. A φ value of 1 indicates that the engine is operating at stoichiometric conditions, which means there is just enough air to completely burn all the fuel.
The formula for calculating the equivalence ratio (φ) in a turboprop engine with SI units is the same as the formula for turbojet and turbofan engines:
the stoichiometric fuel-air mass ratio depends on the specific fuel being used. For hydrocarbon-based fuels like Jet-A or Jet-A1, a typical stoichiometric ratio is approximately 14.7:1, which means 14.7 kg of air is required for every 1 kg of fuel to achieve complete combustion.
In a turboprop engine, the mass flow rate of air includes the air that is used for combustion in the gas turbine core, and this air can be augmented by additional air from the propeller’s slipstream. The mass flow rate of fuel is the fuel supplied to the engine.
Turboprop engines typically operate with a range of equivalence ratios depending on the power settings and operating conditions, and engine control systems are responsible for adjusting the fuel-air ratio to achieve the desired engine performance and efficiency.