# Thread: horsepower / brake horsepower of a motor

1. Senior Member
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## horsepower / brake horsepower of a motor

what the heck is the difference?

thanks much

2. gar
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3. Senior Member
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The wiki answer is comprehensive, if a bit long. For an electric motor, brake horse power is the mechanical horsepower available at the shaft at specified rpm and full load current. That's how we spcecify motors - output horsepower in HP (US) or kW (Euro), where 0.746kW=1 HP. Car and truck engines in Europe are rated in kW too. Horsepower out is less than HP in, regardless of units. Output is less than input by 50% - 95%, depending on motor size, bigger motors usually being more efficient than smaller ones, and construction.

4. Originally Posted by sgunsel
That's how we spcecify motors - output horsepower in HP (US) or kW (Euro), where 0.746kW=1 HP. Car and truck engines in Europe are rated in kW too.
In UK, we mostly use SI (metric) units of measurement. See the Celsius v Fahrenheit thread.
But oddly, for cars and car engines, we use a mixture of units. We bhp for power and lb-ft for torque. The engine capacity is either cubic centimeters (cc) or litres.
Top speed is given in mph,
We buy fuel by the litre but express fuel economy in mpg. Our road signs are in miles, speed limits in mph.....

5. gar
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I tried to look back in some old books for a brake horsepower definition. What I found was insufficient.

My understanding of the use of brake horsepower with respect to an automotive meant that it was the mechanical power out from the crankshaft with the engine block stripped of any other loads. What this means is no water pumps, no fan, no generator, and no muffler, and today no airconditioner compressor, or power steering pump, etc.

In "The Gasoline Automobile", by Hobbs and Elliott, 1924, on page 35 ---
" We can determine the power developed by the explosions in the cylinder, in which case we have what is called the indicated horse power (i.hp.); or we can attach a brake to the flywheel and measure the power that the engine actually produces. This is called the brake horse power (b.hp.)."

When comparing these two measurements it is logical to eliminate all unrelated loads on the engine. Further for evaluation of the engine from a thermal conversion perspective it is useful to not have to subtract a lot of unknown values. Thus, it makes sense that brake horsepower is defined with no accessories on the engine.

In 1929 to 1932 when Ford was developing a low cost monoblock V-8 much of the secret development work was done in the Edison Fort Myers Laboratory in Greenfield Village. To do dyno testing of the engine Zoerlein used the steam engine in this building as the brake load, probably as an air compressor.

In the early 30s after the V-8 was in production and durability tests were run on the regular dyno stands, then with the engines running full throttle the exhaust manifolds would glow red.

When you have an engine in your car you really aren't interested the brake horsepower, but the actual power available with all the extra junk on the engine. So a brake horsepower rating of an engine is greater than what you actually get at the drive shaft .

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6. We see BHP used a lot on pump curves because some pumps can be powered by electric motors or engines, so using BHP tells you exactly what they need at the pump shaft. It becomes your responsibility to then determine the engine size that accomplishes that. But if it's a direct drive electric motor, it's pretty much 1:1

7. Originally Posted by Jraef
We see BHP used a lot on pump curves because some pumps can be powered by electric motors or engines, so using BHP tells you exactly what they need at the pump shaft. It becomes your responsibility to then determine the engine size that accomplishes that. But if it's a direct drive electric motor, it's pretty much 1:1
Other than gearbox losses, the ratio of the drive from the prime mover won't affect the power required from it. Just the rotational speed.

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