The fuel-air ratio in a turbojet 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 turbojet engine with SI units is:
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 practice, turbojet engines may operate with equivalence ratios greater than 1 (fuel-rich) or less than 1 (air-rich) depending on the specific operating conditions and requirements. A fuel-rich mixture provides excess fuel for cooling and can help control combustion temperatures, while an air-rich mixture can reduce emissions and increase combustion efficiency. However, both extremes can have drawbacks, so the actual operating φ is carefully controlled to balance these factors and optimize engine performance.