The propulsive efficiency (ηp) of a turbofan engine is a measure of how effectively the engine converts the energy from the combustion of fuel into useful thrust for propulsion. Turbofan engines are more efficient than turbojet engines because they harness the energy from both the jet exhaust and the bypass air. In SI units, the formula for calculating the propulsive efficiency of a turbofan engine is as follows:
ηp = (2 * V * (Ve – V)) / (g * (Ve^2))
Where:
- ηp is the propulsive efficiency (unitless, a dimensionless ratio).
- V is the airspeed of the aircraft in meters per second (m/s).
- Ve is the effective exhaust velocity of the engine in meters per second (m/s). The effective exhaust velocity accounts for both the jet exhaust and the bypass air.
- g is the acceleration due to gravity, approximately 9.81 m/s².
The propulsive efficiency equation quantifies how effectively the turbofan engine converts the energy in the fuel into useful thrust for propulsion. It considers the difference between the airspeed of the aircraft (V) and the effective exhaust velocity (Ve) of the engine. Turbofan engines are more efficient than turbojets because they use the bypass air to contribute to thrust, in addition to the jet exhaust. This results in a higher propulsive efficiency.
Turbofan engines are commonly used in modern aviation because of their improved fuel efficiency, lower noise levels, and better propulsive efficiency compared to older turbojet engines. The formula above provides a way to quantify and compare the effectiveness of these engines in converting fuel energy into thrust for propulsion.