USCG Exam PracticeDiesel engine principles (motor propulsion)
In a diesel engine, brake horsepower (BHP) is less than indicated horsepower (IHP). The ratio of BHP to IHP defines which performance parameter, and what accounts for the difference between the two values?
- A. Thermal efficiency; the difference represents heat rejected to the cooling-water and exhaust systems
- B. Volumetric efficiency; the difference represents the air charge lost during scavenging in a two-stroke engine
- ✓ Mechanical efficiency; the difference represents friction losses within the engineCorrect
- D. Specific fuel consumption; the difference represents fuel energy not converted to cylinder pressure
Why C is correct
Indicated horsepower is the power developed in the cylinders; brake horsepower is the power delivered at the output shaft. The difference between IHP and BHP is the friction (mechanical) loss within the engine, and their ratio — BHP divided by IHP — is defined as mechanical efficiency.
Cited:DOE-HDBK-1018 Vol.1 §1-8