Which statement correctly defines indicated horsepower (IHP) and brake horsepower (BHP) for a marine diesel engine?

• A. IHP is the theoretical power derived from cylinder pressures via indicator diagrams; BHP is the shaft power at the crankshaft, measured with a dynamometer or calculated from torque and RPM (BHP = Torque × RPM / 5252).
• B. IHP is measured at the crankshaft; BHP is theoretical from cylinder pressures.
• C. IHP equals BHP under all conditions.
• D. IHP is the same as brake mean effective pressure.

Answer: (A)

Indicated horsepower is the power produced inside the engine cylinders, calculated from the pressures the gas reaches during the cycle as shown on indicator diagrams. Brake horsepower is the shaft power available at the crankshaft after all mechanical losses, and it can be measured with a dynamometer or calculated from torque and RPM using the relation BHP = Torque × RPM / 5252. This distinction matters in marine diesels because the engine’s internal cylinder pressure represents what could be produced, while the brake horsepower reflects what actually leaves the crankshaft to drive the propeller after friction, pumping, and other losses. The other statements mix up where power is measured or equate internal cylinder power with shaft power, and they conflate indicated horsepower with brake mean effective pressure, which is a pressure metric, not a power figure.