The fuel-air ratio in a turbofan engine, often 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 turbofan engine with SI units is the same as the formula for turbojet 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.
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 turbofan engine, the mass flow rate of air includes both the bypass air (the air that bypasses the engine core and goes directly through the fan) and the air that passes through the engine core (the combustion chamber). The mass flow rate of fuel is the fuel supplied to the engine.
Similar to turbojet engines, turbofan engines may operate with equivalence ratios greater than 1 (fuel-rich) or less than 1 (air-rich) depending on specific operating conditions and requirements. Engine control systems are designed to manage the fuel-air ratio to optimize performance, emissions, and other operational parameters.