Calculating the fuel-air ratio (equivalence ratio, φ) in a turboshaft engine involves determining the ratio of the actual fuel flow rate to the stoichiometric fuel flow rate for a given amount of incoming air. The stoichiometric ratio represents the ideal proportion of fuel and air needed for complete combustion. The formula to calculate φ is the same as for other types of engines:
Where:
- φ is the equivalence ratio.
- m_dot_fuel is the mass flow rate of fuel in kg/s.
- m_dot_air is the mass flow rate of air in kg/s.
- (m_dot_fuel / m_dot_air)_stoichiometric is the stoichiometric fuel-air mass ratio, which depends on the specific fuel being used.
To calculate φ, you’ll need to know the mass flow rates of fuel and air, as well as the stoichiometric ratio for the fuel you’re using. The stoichiometric ratio for hydrocarbon-based fuels like Jet-A or Jet-A1 is typically around 14.7:1, meaning 14.7 kg of air is required for every 1 kg of fuel to achieve complete combustion.
This calculation will give you the equivalence ratio for the given operating conditions of the turboshaft engine. The value of φ can provide insights into the fuel-air mixture’s richness or leanness compared to the stoichiometric conditions and is essential for engine performance and emissions control. The engine control system can adjust the fuel flow rate to maintain the desired φ value for optimal operation.