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Author Topic: BHP, torque, & Newton Metres.  (Read 10042 times)

Offline DoctorBeaver

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BHP, torque, & Newton Metres.
« on: 27/06/2007 21:13:33 »
Engine specifications quote bhp & torque ratings. I think I understand what bhp is (although I can never remember the exact definition), and I know that a car with 300bhp would be a lot quicker than a similar car with 200bhp. But what about torque? What is it?

And then we have NM. I saw an advert for a Merc C class the other day and the power rating was given in NM. How does that convert to lb/ft?
« Last Edit: 27/06/2007 21:22:30 by DoctorBeaver »


 

Offline daveshorts

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« Reply #1 on: 28/06/2007 00:24:28 »
It is all a bit confusing I agree, there are about 3 different concepts going on...

The thing that makes your car accelerate is Force, this is measured in Newtons (N) or Pounds Force (lb)
F=ma so a=F/m and the larger the force the faster you can accelerate.

Power is the amount of energy you are using in a second,
it is also Force x Speed
It is measured in Watts (W - 1000W =1kW) or horse power - brake horse power is refering to the power that an engine will dump into a brake attached to the front of the engine - not what is getting to the road.

So a car's maximum acceleration could be limited by the force it can produce or the power it can produce, at low speeds the maximum force is important, at higher ones because power is force x speed total power can be the limiting factor.

So what is torque?
Torque is a measure of twisting force, it is closely related to moments, you may have done at primary school - a small force a long way from the fulrum will balance a large force close to it.
Torque = Force x Distace
so it is measured in
Nm or pound feet lbft

Because an engine produces rotation it will produce a torque at at number of revolutions per minute (rpm) which is converted by the transmission and wheels into a forwards force. For the same transmission and gear the larger the torque the larger the force pushing you forwards.

Low gears convert the torque at an rpm into a high force but low speed, high gears low force but at a higher speed. So with enough gears your car should never be limited in it's acceleration by the force it can apply, and the fundamental limit is the power produced.

However it is a right pain to be going up and down the gearbox all of the time so what you really want from an engine is for it to produce lots of torque at low rpm, so you can get away with being in a gear from lowish speeds to highish ones, so you don't have to change gear so often.
 

Offline daveshorts

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« Reply #2 on: 28/06/2007 00:27:19 »
Oh there are about 4.9N / lb force and 0.3m / ft

so 1 lbft is about 4.9x.3 = 1.5Nm

am sure someone could look up the exact values.
 

Offline DoctorBeaver

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« Reply #3 on: 28/06/2007 01:04:50 »
However it is a right pain to be going up and down the gearbox all of the time so what you really want from an engine is for it to produce lots of torque at low rpm, so you can get away with being in a gear from lowish speeds to highish ones, so you don't have to change gear so often.

You mean like the Aston Martin DB7 GT? That can go from a standstill to 135mph in 4th gear!

Thanks for your reply, by the way. I think I'm right in saying that, in general, given an engine of capacity x, there would be more torque from a single cylinder of that displacement rather than if it were a multi-cylinder engine.

A friend of mine has a Buell motorcycle. It's based on a big Harley engine. Although it has quite a lot less bhp than, say, a Honda Fireblade (which would mullah the Buell in a straightline drag), it can accelerate out of corners faster because it has a lot more torque.
 

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« Reply #4 on: 28/06/2007 06:42:38 »
Thanks for your reply, by the way. I think I'm right in saying that, in general, given an engine of capacity x, there would be more torque from a single cylinder of that displacement rather than if it were a multi-cylinder engine.

Not at all.

There are lots of factors pertaining to torque, and the relationship between toque and speed.

Factors such as compression ration, valve timing, stroke and bore, will all effect torque.

Diesels have very good torque at low engine speeds.

In my experience, multicylinder engines tend if anything to have better low revving toque (6 or 8 cylinder engines will have a much better spread of power through the rev range, and will idle at speeds when a 4 cylinder engine will stall).

Harleys were always intended as touring bikes, so emphasised torque while keeping engine revs low; while the Japanese have always gone for high revving, high power, but lower torque engines (even more extreme when they were building 2 stroke engines that had very little torque, but could rev at 12,000 rpm or more, and develop there power at the top end of the rev band).
 

Offline DoctorBeaver

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« Reply #5 on: 28/06/2007 07:01:48 »
I think I'll stick to QM - it's not as confusing!  ???
 

Offline syhprum

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« Reply #6 on: 28/06/2007 11:56:16 »
By no means would a car of 300 BHP always be quicker than one with 200 BHP, the top speed depends on the frontal area and the degree of streamlining (coefficient of pentration) while the acceleration depends on the ratio of power to weight ratio.
 

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« Reply #7 on: 28/06/2007 14:44:02 »
By no means would a car of 300 BHP always be quicker than one with 200 BHP, the top speed depends on the frontal area and the degree of streamlining (coefficient of pentration)

Also on gearing (not so commonly theses days, as most cars have a top gear that is slightly overgeared to improve efficiency, but stioll possible to have an undergeared top gear), and even whether the car has a speed limiter.

while the acceleration depends on the ratio of power to weight ratio.

And even the tyres, and still the gear ratios, and how flexible the engine is - even though a low torque high power engine with the right gear box, and the right driver, can still accelerate fast, but it takes a lot of work to allow it to do so, while a torquey engine is easier to use with less effort going through the gear box.
 

Offline DoctorBeaver

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« Reply #8 on: 28/06/2007 14:48:06 »
By no means would a car of 300 BHP always be quicker than one with 200 BHP, the top speed depends on the frontal area and the degree of streamlining (coefficient of pentration) while the acceleration depends on the ratio of power to weight ratio.

I did say a "similar car" meaning all other things being more-or-less equal.
 

lyner

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« Reply #9 on: 28/06/2007 23:15:15 »
The power of an engine is really the dominant factor; it affects its top speed - the limit being when resistive forces times speed are equal to the max power output. The relevance of torque is to the range of engine revs over which the engine develops a high torque and the number / spacing of the gears. If you could have an infinitely variable and 100% efficient gearbox, the only thing that would count would be POWER OUTPUT.
 

Offline DoctorBeaver

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« Reply #10 on: 28/06/2007 23:20:48 »
I remember DAF trying out a variable ratio gearbox in, I believe, the 60s. It was called the Variomatic. I know it involved elastic bands and centrifugal weights; but that's as far as my knowledge of it extends.
 

another_someone

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« Reply #11 on: 29/06/2007 00:22:17 »
I remember DAF trying out a variable ratio gearbox in, I believe, the 60s. It was called the Variomatic. I know it involved elastic bands and centrifugal weights; but that's as far as my knowledge of it extends.

That lasted for quite a while, and I am not sure the technology is not still used in some vehicles.

On the other hand, hybrid (petrol/electric vehicles, such as the Prius) also have continuously variable transmission, although by a very different rout.
 

Offline daveshorts

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« Reply #12 on: 29/06/2007 09:11:07 »
My brother had an automatic FIAT uno that worked on elastic bands. It had two conical pulleys with a belt between them which could move up and down (possibly controlled by some spinning weight arrangement, I never looked). As the belt moved up and down it changes the gear ratio between the two.



It was extremely strange to drive as the speed of the engine was almost entirely divorced from the speed of the car, so you would put your foot down, the engine would rev really  high, the engine note would stay the same, and you would gradually speed up.
 

lyner

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« Reply #13 on: 05/07/2007 23:55:24 »
The variomatic was very clever; it had separate drives to each back wheel  and acted as a differential at the same time.
Shame about the lack of efficiency.
A conventional gearbox seems to be best in that respect.
 

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« Reply #13 on: 05/07/2007 23:55:24 »

 

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