What is brake thermal efficiency and how is it estimated from engine data?

• A. The ratio of useful shaft work to chemical energy in the fuel; estimated from BHP, fuel flow rate, and fuel energy content.
• B. The ratio of fuel energy to heat lost; estimated from exhaust temperature.
• C. The ratio of heat rejected to fuel input; estimated from thermal imaging.
• D. The ratio of engine displacement to fuel consumption; estimated from volume and flow.

Answer: (A)

Brake thermal efficiency is a measure of how effectively the chemical energy in the fuel is turned into useful shaft work. It is calculated by comparing the useful power the engine delivers at the crank (the brake power) to the rate at which energy from the fuel enters the engine. In practice, you estimate it from brake power, the fuel flow rate, and the fuel’s energy content. The typical formula is η_b = P_b / (ṁ_f × q_fuel), where P_b is brake power, ṁ_f is the fuel mass flow rate, and q_fuel is the fuel’s energy content per unit mass (LHV or HHV).

This approach directly ties the output energy to the input chemical energy, showing how much of the fuel’s energy is available as useful work after accounting for all losses in the engine and drivetrain. Conversely, options that rely only on exhaust temperature, thermal imaging, or engine displacement miss the fundamental ratio between useful output and energy input, so they don’t accurately define brake thermal efficiency.