BHP

[Jack]

I maybe stupid, but......

 

What is Brake Horse Power all about?

 

Jack.

[oddball]

This should tell you all you need to know, or maybe confuse you even more :shock:

 

http://en.wikipedia.org/wiki/Horsepower

[Jack]

Thanks Oddball.

 

We use to measure HP at the PTO of the a tractor but I have never understood BHP but it looks like this link will sort that out!

 

Cheers.

 

Jack.

[mcspool]

This should tell you all you need to know, or maybe confuse you even more :shock:

 

http://en.wikipedia.org/wiki/Horsepower

 

"Brake Horsepower (bhp) - The measure of horsepower at maximum engine output, minus power lost from heat, friction, expansion of the engine, etc."

 

In other words: the amount of hp measured at the brakes.

 

H.

[Mark]

I may be stupid but, does it not stem from the early days when horses were used to pull machinery etc. and all the automotive industry done was use the phrase to describe the power output of a mechanical device :shock:

[Jack]

"Brake Horsepower (bhp) - The measure of horsepower at maximum engine output, minus power lost from heat, friction, expansion of the engine, etc."

 

In other words: the amount of hp measured at the brakes.

 

H.

 

Perfect, got it now.

 

Well this section works for me!

 

Yours happily.

 

Jack.

[Richard Farrant]

We use to measure HP at the PTO of the a tractor

 

Jack,

 

Back in my apprenticeship days, a long way back , we had to do the HP test on tractor PTO shaft, using a water brake dynamometer. A lot of calculations were involved. That is PTO HP as there is the drag from the transmission to consider. BHP is taken at the flywheel of an engine, on a testbed by a dynamometer, some manufacturers do not run water pumps or dynamo / alternators or fans, in order to register max. power. When I overhauled engines in REME workshops, all diesels were put on there to bed them in prior to a BHP test. Off the top of my head I think that 1 bhp = 33,000 foot/pounds of torque, may be wrong, have not thought about this for so long

 

Richard

[John Pearson]

I started a mechanical engineering degree more than 30 years ago but got thrown out due to wine, women and song. I will scrape my brain and see if I can remember anything:

A horsepower is a horsepower and they are all 33.000 foot pounds of work per something (hour I think). I am a bit more sure it is 0.743 kilowatt hours. You will note that it is a mass, (pound), moved a distance (feet) in a time (hour?). I think James Watt chose it as a simple way of telling purchasers of his engines what a work rate they could expect. He measured it by a horse pulling a weight up a well by use of a pulley. The figure he got was actually about 66% of the current horsepower figure: he added on half as much again so no one could actually find a horse that could work at the rate of one horsepower continuously.

The unit is fixed but the problems come with how you measure it: think about ship steam engines, jets, rockets, bullets and of course petrol engines. It is actually the different methods of measurement that cause the different names: a few examples:

RAC HP. This was a sort of a 'divide by the number you first thought of' method that related to early cars and motorcycles. It was a fixed number, multiplied by the number of cylinders in the engine, multiplied by the bore (in inches) and then divided by something. It was a simple way to guess at the power output in the 1890's and was used as the basis of the road fund licence. Very quickly the engines made were tuned to produce more power but the system stayed in use for years which is why the Bedford RL engine is called a '28hp' but actually produces about 100 horsepower, the Dingo engine is called an '18' hp engine and it is the reason for the '7' in the Lotus super 7 and the Austin seven cars.

IHP: this stands for Indicated Horse power. This is particularly used for big steam engines running mills etc. It is a way of working out the horsepower output of an engine by counting the revolutions the engine runs at and measuring the pressure of the steam going in compared to the pressure of the steam coming out. By calculation, this gives you the output horsepower.

MHP: measured horsepower, I think that this is a bit like IHP but can't remember what the difference is: I think that it relates to a sort of mechanical computer that does the above calculations for you by using pressure recorders and a tachometer and displays the results like a speedometer guage. Again, this was used for big steam engines where the speed varied, say in ships or winding things up coal mines

BHP This is brake horsepower and means the horsepower measured against a brake. In small engines this can be some sort of friction device and you measure the heat output but bigger engines usually pump water against a known water back pressure. Sometimes the friction brakes are water cooled so it is easy to get the systems confused but the concepts are the same, you measure the output 'directly'.

Sadly, this is not the end of the matter as you get BHP (SAE) and BHP (DIN). The first stands for 'Society of American engineers' and the DIN I can't remember. I think the 'I' means 'International' but the D and the N, I do not have a clue.

Both of these are systems rather than different measurements. One of them (DIN I think) measures the power at the crankshaft of the engine with no generator, water pump, air cleaners, exhaust silencer etc. It is a measure of the maximum amount of power an engine can produce. SAE has all the ancilliaries including gearbox but is still the measure of the engine alone and when in use, there will be further power losses in the back axle and wheel bearing for example. Power is sometimes measured 'at the back wheel' which should be the true amount you have to play with. For example the 1970's Triumph Trident motorcycle was listed as giving 58bhp but the first model of the Honda 750 4cylinder was rated at 65 I think. The Trident was SAE and the Honda DIN (or the other way around?) and the Trident was faster. However, faster still was the 1950's Vincent 1000cc V twin. This was rated at 50 I think but that was SAE at the back wheel.

All the same sizes of horses though!

My brain hurts!!

[ArtistsRifles]

D.I.N. = Deutsche Industrie Norm

[AlienFTM]

You may wish to compare BHP with PS. PS = Pferdestärke = Horse power in German. German cars are rated and insured by PS.

 

1PS = 0.9863201 BHP

 

Don't bother trying to pronounce Pferdestärke - it's universally PS.

 

Okay P - FAIR - duh - sh - tear (rhymes with hair) - kuh.

[Richard Farrant]

RAC HP. This was a sort of a 'divide by the number you first thought of' method that related to early cars and motorcycles. It was a fixed number, multiplied by the number of cylinders in the engine, multiplied by the bore (in inches) and then divided by something. It was a simple way to guess at the power output in the 1890's and was used as the basis of the road fund licence. Very quickly the engines made were tuned to produce more power but the system stayed in use for years which is why the Bedford RL engine is called a '28hp' but actually produces about 100 horsepower, the Dingo engine is called an '18' hp engine

 

John,

 

Good, I thought that 33,000 was the right number, just shows, some of the things they teach you in your early years actually sinks in!

 

Just to elaborate on the RAC hp, it was as you say, a formula for taxing vehicles, there being different taxation bands from memory. The formula, and I cannot type it as shown, was Bore diameter (inches) squared X number of cylinders, over 2.5. Manufacturers started to design engines around this formula, to get as a large capacity as possible but in the lowest taxation bracket. This is why so many engines of the 30's and 40's were long strokes and small bores, nothing to do with efficiency or power at all. As you mentioned the Dingo engine, here is an example, the cylinder bore is too small to draw the big end up through, it is quite a job to get the pistons out with the engine still in the chassis. One point, the Bedford RL engine is not the 28hp, the RL had the 300 ci, where as the 28hp was in fact 214 ci. A 28 hp actually developed 72 bhp.

 

Richard

[AlienFTM]

30-something years ago I was reading a car mag and learnt how an Italian had found a rusting lump at the back of his barn. He dug it out, started to remove the rust and was astonished to discover Ferrari markings on the cylinder head.

 

Eventually he found out what he was looking at. The engine dated from the days (mid-50s IIRC) when Formula 1 meant "whatever it takes to win."

 

Monza being a pure-torque circuit, Ferrari had decided they needed a FIVE LITRE TWO CYLINDER engine especially to ensure victory at Monza.

 

They built this thing and started to bench test it ... until the bench went hurtling out of the workshop. They gave up on a 5 litre 2 cylinder engine and quietly hid the evidence.