USCG Exam PracticeDiesel engine principles (motor propulsion)
A diesel engine's indicated horsepower is 1,200 HP and its brake horsepower measured at the output shaft is 1,080 HP. Which statement correctly identifies what accounts for the difference and what term describes the ratio of these two values?
- A. The 120 HP difference represents thermal losses to the cooling system; the ratio is thermal efficiency
- ✓ The 120 HP difference represents friction (mechanical) losses within the engine; the ratio of brake to indicated horsepower is mechanical efficiencyCorrect
- C. The 120 HP difference represents scavenging losses in the turbocharger; the ratio is volumetric efficiency
- D. The 120 HP difference represents heat lost to exhaust gases; the ratio is specific fuel consumption
Why B is correct
DOE-HDBK-1018 Vol.1 §1-8 states that brake horsepower is the power delivered at the output shaft, indicated horsepower is the power developed in the cylinders, the difference between them is friction (mechanical) loss, and their ratio is mechanical efficiency. Thermal efficiency and specific fuel consumption are separate performance measures.
Cited:DOE-HDBK-1018 Vol.1 §1-8