What is the difference between shaft power and brake power?

Question

I know little about internal combustion engines and I feel a little confused about brake and shaft powers. I read a book that only talks about shaft power, and other that only talks about brake power, without making a proper definition of the physical meaning of these quantities. I'm now thinking that they both mean the same thing: useful power on the crankshaft (the power produced by the engine cylinders less the losses and the power that is used in accessory components).

For example, in Mechanics and thermodynamics of propulsion (1992) (Hill Peterson), pag. 155, the the brake specific fuel consumption uses shaft power in its definition, and not the brake power (as the name suggests it).

I would really appreciate some clarification, or some book suggestions, since the ones that I read are too vague.

Answer 1

The short answer is these terms are defined by standards organizations based on specific test methods. These methods specify the engine condition, environmental factors, fuel chemistry, inlet and exhaust systems configurations, and the correction factors applied to the measurements. The specific tests and standards used vary by country and by industry. So neither term has a specific meaning unless you call out the document that defines the terms.

In general, though, shaft horsepower measures power after the transmission, brake horsepower measures before the transmission. On a cruise ship, you might have many gensets feeding a couple of high voltage buses that power the prop shaft motors. You want to talk about shaft power here, not brake power. Similarly, turboshaft jet engines have builtin gearsets to drive, say, a helicopter rotor. Shaft power is the relevant term here as well. Shaft power usually makes more sense when comparing different types of power plants that use different transmission arrangements.

Answer 2

You mean BRAKE horsepower. It is the same as shaft horsepower. It is called brake horsepower (abbreviated BHP) because it is measured at the output shaft of the engine while it is running on a test stand with a mechanical device called a brake.

There are other definitions of the power output of an engine which rely not on measurements but on calculations based on things like piston area, which is why shaft or BHP are named this specific way.