Horsepower and Brake horsepower

[Jon]

Didn't think this was techy enuf for the Technical forum as it's not entirely bike orientated and I need the answer in an "Enginneering for Dummies"stylee

Then I started typing it and changed my mind

Just been asked by a mate to describe brake horsepower in simple terms and went "erm..............it's...............erm............hang on"

I know horsepower was originally something to do with how many times a horse could turn a treadmill in an hour, cos the guy wanted to know how many he could replace with the steam engine he'd built (isn't progession wonderful )

I had a quick hunt for "brake horsepower" and came up with the fact it was measured with "Dynamometers" , which of course started another thread. What's a dynamometer

Dynamometers measure torque by use of a brake. Brake torque is simply a measurement of how much resistance is needed to hold the engine at a steady rate of speed

So is this a guy holding a brake on an engine, e.g. holding the reins of a horse to stop it going too fast? and the dynamometer is a springy affair a bit like a set of hanging scales that's attached to the reins and shows how much force/weight is being used by turning a needle on a scale?

Surely taking your foot off the accelerator (or showing 'whoa!') would make you slow down? Why would you wind and engine up and then try to stop it?

Anyway, I digress.

'Extreme' idiots guide to horsepower and brake horse power please, I'm confusing myself now!

Cheers

Jon
(I was away the day we did physics )

 

[Panzer Patrol]

Littlererooster....its for you hooo

 

[jnrobert]

One HORSEPOWER is defined as 33000 foot-pounds of work per minute.

(WORK is defined as a FORCE operating through a DISTANCE. - or torque in other words + acceleration).

The difference in Brake Hp and HP is how its measured. But you can also convert it to electrical energy and measure it as watts and then convert it back to HP.

A brake is simply putting a resistance on the rear wheel or output shaft of the engine and measuring the torque (the energy is converted to heat - just like your real brakes).

 

[MikeO]

One HORSEPOWER is defined as 33000 foot-pounds of work per minute.

(WORK is defined as a FORCE operating through a DISTANCE. - or torque in other words + acceleration).

The difference in Brake Hp and HP is how its measured. But you can also convert it to electrical energy and measure it as watts and then convert it back to HP.

A brake is simply putting a resistance on the rear wheel or output shaft of the engine and measuring the torque (the energy is converted to heat - just like your real brakes).

Well, I'm glad we cleared that up......


MikeO

 

[Gecko]

Well, I'm glad we cleared that up......


MikeO

Yep that's much better now

 

[Greg Masters]

What's a dynamometer

A dynamometer (dyno) in this context is usually in the form of a test bed to which an engine is strapped - the output from the engine being connected to a water vane. This vane is parallel to another vane, the distance between them being variable. As the engine turns, a varying amount of power is transferred to the second vane depending on the distance between them (setup similar to a torque converter, but without the t/q elements). From the second vane there is an arm to which weights can be added. The amount of weight that can be lifted is a function of the torque. Power = revs x torque.

You run the engine and increase both throttle and load (by adjusting the water vane) until the engine is running at full throttle. Measure the revs and torque. Decrease the load incrementally, and measure the revs and torque step by step and you have all the data needed for both power and torque graphs.

Except that there's more to it than that. All that is fine for a constant speed engine (like a cement mixer or a diesel generator) but of less use for a bike or car engine. These days, dynos are inertia devices - a massive flywheel of known inertia that the engine is required to accelerate. Not only does the engine have to use its power to accelerate the flywheel, it also has to accelerate its own moving parts. By setting the inertia of the flywheel to equal that of the bike/car (allowing for each available gear), true performance figures can be more accurately assessed. This is a better way of measuring available power and, more important, drivability.

Greg

 

[MikeO]

This is turning into a really informative day.......


MikeO

 

[jnrobert]

Well, I'm glad we cleared that up......


MikeO

Well, I can try to make it clearer. What part confuses you?

 

[Panzer Patrol]

Go on MikeO, tell im. Go on I dare ya

 

[MikeO]

Well, I can try to make it clearer. What part confuses you?

Most of the stuff between, 'One.....' and '.....brakes).'

Making it clearer won't help - I'm stupid.

MikeO

 

[littleredrooster]

Making it clearer won't help - I'm stupid.

Well I'm glad we've cleared that up!

Saw this thread at about 1 in the morning after coming back from the pub and thought.....how do I explain this.......naw; can't do that. Thanks to Greg for the engineering explanation!

 

[Bill]

I'm glad someone explained it!
I work for the Ford Motor Company at their research and engineering centre. I actually test and develop engines on dyno's for a living.
The dyno's we use are fairly complicated electrical affairs.
We can (and do!) test engines at different air pressures and temperatures (-40 / +50°C) so that they will start and run at the highest, lowest, coldest and hotest places in the world.
We can dial in any gear/axle ratio, weight, inertia, wheel size, cd, in fact, you name it and we can do it!
The dynamic dyno's will even run drive cycles from data collected whilst driving a vehicle on the road.
Then there are the chassis dyno's...but thats another story and I really should be working!