How do you relate engine indicated horsepower (IHP) to brake horsepower (BHP) for a marine engine?

How do you relate engine indicated horsepower (IHP) to brake horsepower (BHP) for a marine engine?

• A. BHP equals IHP multiplied by the engine's mechanical efficiency (ηm)
• B. BHP equals IHP divided by efficiency
• C. BHP equals IHP plus throttle loss
• D. BHP equals IHP minus fuel consumption

Answer: (A)

The key idea is that the power available at the crankshaft is reduced from what the engine actually develops inside the cylinders due to mechanical losses like friction, bearing losses, and accessory drive losses. This reduction is described by the engine’s mechanical efficiency, ηm, which is defined as BHP divided by IHP. Therefore the relationship is BHP = ηm × IHP. In other words, indicated horsepower is the power produced in the cylinders, while brake horsepower is the power you can pull from the crankshaft after those mechanical losses have taken their share. Since ηm is less than 1, BHP is smaller than IHP. For example, if IHP is 2,000 hp and ηm is 0.90, then BHP ≈ 1,800 hp. Dividing IHP by efficiency would imply more power at the crank than is produced inside the cylinders, which isn’t correct because losses always reduce usable output. Adding a throttle loss term or subtracting fuel consumption aren’t the standard ways this relationship is defined; the proper link is through mechanical efficiency.

The key idea is that the power available at the crankshaft is reduced from what the engine actually develops inside the cylinders due to mechanical losses like friction, bearing losses, and accessory drive losses. This reduction is described by the engine’s mechanical efficiency, ηm, which is defined as BHP divided by IHP. Therefore the relationship is BHP = ηm × IHP.

In other words, indicated horsepower is the power produced in the cylinders, while brake horsepower is the power you can pull from the crankshaft after those mechanical losses have taken their share. Since ηm is less than 1, BHP is smaller than IHP. For example, if IHP is 2,000 hp and ηm is 0.90, then BHP ≈ 1,800 hp.

Dividing IHP by efficiency would imply more power at the crank than is produced inside the cylinders, which isn’t correct because losses always reduce usable output. Adding a throttle loss term or subtracting fuel consumption aren’t the standard ways this relationship is defined; the proper link is through mechanical efficiency.