SHP vs. BHP

[flyboy]

Guys,

I student recently asked me the difference in brake horse power and shaft horsepower. I really couldn't give him a good answer. I've compared my BE200 POH with my BE76 POH and can't really find my answer (in detail). Any help is appreciated.

 

[enigma]

If memory serves, brake horsepower is the power produced in the cylinders and shaft horsepower is sort of self explanitory, it's the hp delivered to the prop/rotor shaft. Brake hp is sort of a theoretical number.

Or, I could be wrong and I'm certain that someone will set us straight. That's the great thing about this bbs.

enigma

 

[TZRav8r]

BHP is the power delivered to the shaft from the pistons. SHP is the power delivered by the shaft to the prop. SHP and BHP are equal in direct drive engines. The only time the two are different would be for engines using reduction gearboxes and therefore experiencing some power loss due to friction within the gearbox.

 

[EagleRJ]

They are pretty much the same thing, except BHP usually refers to reciprocating engines, and SHP refers to turboprop engines. Both figures refer to the total motive force available at the output shaft, after accounting for friction and accessory losses.

SHP is sometimes further defined as ESHP, or Equlivalent Shaft Horsepower, which includes jet thrust from the gas generator exhaust.

 

[TonyC]

[My] student recently asked me the difference in brake horse power and shaft horsepower.

I googled the matter, and learned quite a bit - - I think.

It seems that most of the world thinks of the two as being essentially equivalent, even to the point of being interchangeble. Whether it be a gasoline powered automobile engine, or a motorcycle, or a diesel-powered truck, or a tractor or some other type of land-borne vehicle, they seem to think of the two as the same.

The best I found from a automobile context was a fellow that says "Shaft horsepower is NOT the same thing. It is supposed to mean BHP, less certain losses."

The American-Heritage dictionary defines brake horsepower as "The actual or useful horsepower of an engine, usually determined from the force exerted on a friction brake or dynamometer connected to the drive shaft."

I found a few other places where a distinction is made, but those usually deal with diesel engines (are diesel enthusiasts more purist?). In one case I found:

The power output of a base or stripped engine is known as gross horsepower;
The power output of an engine equipped with all necessary auxiliaries is known as net horsepower; and
The power available at the output shaft for doing work is called brake horsepower.


Now, when you start talking about boats and ships, they start getting technical and breaking out the difference. At this site: M.I.Link Maritime Dictionary I found that brake horsepower is the power available at the shaft of an engine after minusing the frictional losses in the running component of the engine but before passing through reduction gears or other transmission devices; for direct-connected diesel engine, this power is same as shaft horsepower; this is measured using a brake which gives the name. Shaft horsepower there is defined as the net power available at the propeller shaft after loosing some power to overcome the friction in the engine running gear, the reduction gears, thrust block or other transmission devices; thus, this is always less than the indicated horsepower. It notes that for direct-connected diesel engine, this brake horsepower is same as shaft horsepower. For geared engines, shaft horsepower is also less than the brake horsepower.

One of the few aviation-related references comes from a Plane & Pilot Magazine Article:

Plane & Pilot Magazine | Feature Articles | Muscle Machines & Power Plants
"First, a quick review of horsepower.

[color=dark-blue]Horsepower[/color]
Nineteenth-century inventor James Watt came up with the original definition of horsepower: the power required to lift 550 pounds at the rate of one foot per second, or 33,000 pounds at one foot per minute.

Horsepower can be expressed in different ways:

Brake horsepower—The actual horsepower of a piston engine, measured by a brake attached to the driveshaft and recorded on a dynamometer.

Shaft horsepower—The actual horsepower of a turbine engine, measured on the power turbine shaft, normally mounted at the rear of the engine."

I was about satisifed that I'd found the end of the matter when I came across a few questions on the FAA's Airman Knowledge Test Question Bank for the Aviation Mechanic Powerplant.

Here's one question:
17. A02 AMP
The actual power delivered to the propeller of an aircraft engine is called
A) friction horsepower.
B) brake horsepower.
C) indicated horsepower.


Given what I've encountered so far, I'd be inclined to answer "shaft horsepower" which is not an option. The next best, I'd imagine, is B) brake horsepower.

Then along comes this question, the only one that has both brake horsepower and shaft horsepower as choices.
28. A03 AMP
The horsepower developed in the cylinders of a reciprocating engine is known as the
A) shaft horsepower.
B) indicated horsepower.
C) brake horsepower.

Had I seen this question first, I would have been inclined to answer C) brake horsepower, but since I've already used brake horsepower on something different, I'm confused.


I figured since the FAA is testing on this definition thing, they must surely provide the answer somewhere, so off I trouped to Part 1--DEFINITIONS AND ABBREVIATIONS, and I found some, but not a lot of help.

Sec. 1.1 General definitions.

As used in Subchapters A through K of this chapter, unless the context
requires otherwise:

"Brake horsepower" means the power delivered at the propeller shaft (main drive or main output) of an aircraft engine.

Notice A) it does not distinguish between piston or turbine driven engines, and B) it does not provide a definition of Shaft Horsepower.


So, after all that study, I'm still confused. I still think I got Question 17 correct, but where does that leave me on Question 28?

Does anybody have a source document that will satisfy the questions on the Airman Knowledge Test for the AMP?

Learnin's great, ain't it?

 

[JulietAlphaKiloEcho]

Regarding "but since I've already used brake horsepower on something different, I'm confused.": Is there some rule on the exam that the same answer can't be correct for two different questions?