A method of comparing engine efficiencies is by comparing

• A. specific fuel consumption.
• B. fuel consumption.
• C. thrust to weight ratio.
• D. thermal efficiency.

Answer: (A)

Specific fuel consumption is the best way to compare engine efficiencies because it relates the amount of fuel used to the useful output the engine provides—thrust. Lower specific fuel consumption means more thrust per unit of fuel, i.e., greater efficiency.

Fuel consumption alone doesn’t tell you how much thrust you got for that fuel; an engine could burn more fuel but deliver much more thrust, or vice versa, leading to a misleading assessment of efficiency. Thrust-to-weight ratio measures how much thrust you get relative to the engine’s weight, which is about performance sizing rather than how efficiently fuel is used. Thermal efficiency looks at how well the engine converts chemical energy to work inside the cycle, but real propulsion efficiency also involves how that energy is converted into thrust and how much fuel is wasted in the exhaust and through other losses. Specific fuel consumption encapsulates these factors into a single, practical metric used to compare engines across operating conditions.