USCG Exam PracticeDiesel engine principles (motor propulsion)
A diesel engine's brake horsepower is 900 and its indicated horsepower is 1,000. What does the 100 hp difference represent, and what is the engine's mechanical efficiency?
- A. Heat lost to the cooling system; thermal efficiency is 90%
- ✓ Friction (mechanical) losses; mechanical efficiency is 90%Correct
- C. Power consumed by the turbocharger; mechanical efficiency is 90%
- D. Friction (mechanical) losses; mechanical efficiency is 111%
Why B is correct
Brake horsepower is power at the output shaft; indicated horsepower is power developed in the cylinders. The difference is friction (mechanical) loss. Mechanical efficiency is the ratio of brake to indicated horsepower: 900/1000 = 90%. Heat lost to cooling is accounted for in thermal efficiency, not mechanical efficiency. (DOE-HDBK-1018 Vol.1 §1-8)
Cited:DOE-HDBK-1018 Vol.1 §1-8