HP versus Brake Horsepower 3

[Lead Engineer]

I seem to be the only one in my company that believes that BHP is the power available to the system before losses due to appurtenances, e.g. a belt drive for a fan. For simplicity sake,(I may not be using an accurate analogy) I suggest that the BHP is the NEMA or nominal horsepower. For example, Greenheck on their cut sheets for fans shows the "Operating Power" then below in the section that tells you the motor size 10 hp or 5 hp . . .
I talked to an EE friend of mine and he told me that he checked a couple of other sources and he was not sure that he agreed with me.
These are a couple of websites that I used in my discussion;

http://www.wisegeek.org/what-is-brake-horsepower.htm

http://www.differencebetween.net/tehcnology/difference-between-hp-and-bhp/

Seems pretty clear to me.
The term BHP comes from the early days of measurement where they actually would apply a brake to the output shaft and calculate HP from the force on the brake.
I could always not be accurate, or the other case is that the term depends on context of usage . . .

 

[CWB1]

Many folks like myself prefer to run our daily drivers at lower speeds to maximize engine life. I generally could care less about peak power unless I'm on the track, and even then I try to balance available low-RPM power with peak. Because I choose to run slow, peak torque is the more important figure to me than peak power as it speaks to the engine's capability at a useful (to me) RPM.

 

[FACS]

From "Mechanical Engineering Reference Manual for the PE Exam", Tenth edition, Michael R. Lindeburg, PE

Page 20-4 states: "The actual power delivered to a fan from it's motor is the brake horsepower, BHP, or brake kilowatts, BkW.

The mechanical efficiency, ME can be calculated as; ME=AHP/BHP, where AHP is Air Horsepower, which is the power required to move the air.

 

[LittleInch]

All this "brake" business is just legacy wording from the days when horse power was sometimes a theoretical or nominal value and hence something needed to differentiate between that and physical power at a shaft output.

It ("brake") can and should be simply replaced by the location of the power reading.

Hence for cars it could be at "engine crankshaft" or at front or rear wheels or wherever with whatever items are included ( such as water pumps, A/C hydraulic pumps etc etc

In the example above BHP would be "shaft horse power" and AHP would be fan output power (air).

BTW, I have never heard of BkW or AHP before now. and hope I never see it ever again.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[FACS]

LittleiInch,
Chapter 20 is about "Fans and Ductwork." If that is not your field of expertise, or you are not interested in learning outside of your field, then you do not have to worry about seeing it again.

This was taken from material designed around studying for the PE exam, and therefore trumps any "opinions". After all, More details = Better answers.

 

[Tmoose]

" Marketing and emissions requirements for both outdoor power equipment and vehicles have combined to give us what we have today - great big power numbers at rarely used engine speeds ".

I think many if not most mowers, non-electronic controlled standby generators, snowblowers and whatnot are governed to 3600 rpm max, and supposed to be operated there, so reporting HP at 3600 rpm is valid for them. I think the marketing ploy of stating torque is simply to be able to use a "bigger" number in the brochure. Like those tool sets that throw in a bunch of allen wrenches to get the tool piece count up. "96 pieces for $39.99 " "This way to the egress." "But wait, just pay a separate handling fee."

 

[CWB1]

Sure, but modern ratings have become rather irrelevant to useful and/or efficient work. My "new" garden tractor claims 20 hp at 3450 RPM and is gutless unless the engine is run at that speed. The 50 year old version it replaced claimed 10 hp at 3600 but had plenty of low-end torque. Cutting the same 4' swath at the same tractor speed the "10 hp" chugged along at <2000 rpm while getting roughly twice the fuel economy, making half the noise, and without wearing itself out. After <10 years the new mower is well on its way to worn out, which makes me wish I'd taken the time and effort to rebuild the old one.

Vehicles are very similar. Its great that newer pickemup trucks are ~350 hp at 6k+ RPM. Unfortunately most folks try to get the best economy and life out of them so tend to keep engine speed to <3k RPM where many are rather lacking.

 

[LittleInch]

Yes it all comes down to "usable power" and low down torque. I used to take 4 x 4's into the desert for some dune / sand driving. As a group we rapidly noticed that the V6 2.something litre jeeps which could hold their own on the highway and in theory had a decent max power, struggled when the going got sticky compared to the 4 litre V8 / straight 6's everyone else had. We eventually wouldn't go out with them as we were pulling them out of deep sand too often.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Tmoose]

"Cutting the same 4' swath at the same tractor speed the "10 hp" chugged along at <2000 rpm "

I'm curious what the owner's manual for that old mower said about recommended engine ( and blade speed ? ) for mowing.

1 - Running typical fan cooled small engines slow means lots less cooling air flow, just as on an induction motor run on a VFD. Go slow enuff on a VFD, and an auxilliary cooling fan is required.

2 - Rotary mower cutting action depends on a certain blade velocity for blade uplift and cutting action and good grass clipping transport.

"The blade has a sharpened cutting edge at each end with a curved up sail area to create an air flow. This air flow will whip the grass blades exposing them to the blade cutting edge. The air flow also assists in carrying the cut grass blades out the discharge."
"A rotary mower cuts grass by impacting the blade cutting edge against the grass blades at a very high velocity. This cutting action requires that the blade cutting edge is sharp and rotating at an adequate speed."

https://cdn2.toro.com/en/-/media/Fi...hash=5F2079BA8FA04FB4FDE3B94F1134BDBAA8763AB6

 

[CWB1]

I'm curious what the owner's manual for that old mower said about recommended engine ( and blade speed ? ) for mowing.

Ask and ye shall receive. The owner's manual provides instructions on setting low and high idle, and other tune-up info but doesnt get too specific about operation nor blade speed. I kept the blades sharp and am very conscious of lawn health. It cut fine across flat ground at 1/2 throttle, occasionally I'd push it to 2/3 on hills or in deep grass but almost never went full throttle. Interesting side note - Both tractors use the usual cable pull lever next to the steering wheel. The old model has a smooth, non detented action allowing the operator to set a seemingly "infinite" number of engine speeds. The modern one has a light spring return and a slight detent to retain the lever at full throttle/rated speed/power making it effectively two-speed. The power curve drops off rather sharply/noticeably even if you hold it just below the detented "full" position.

Move the throttle lever to the position where the engine operates best for the load to be handled.