Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 14/08/2009 09:44:33
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What does Jeremy Clarkson mean when we talks about engine "torque" and engine "power" - how do these two measures inform us about the performance of an engine?
Chris
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Torque is a measure of the effort of turning.
At low RPM, torque is low, it increases as the RPM increases. At 5252 RPM, the torque and horsepower are equal. Beyond 5252 RPM, torque decreases.
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Force and power are related in a similar way.
Power(watts ) = force(N) times speed (m/s)
= work (N m) per second (1/s)
and
power = torque (N m) times angular speed(1/s)
It all ties in nicely.
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About engine performance:
I've been told that Torque relates to acceleration in cars.
Horse Power relates to top speed.
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Thanks - it's this last point I'm after clarifying.
A Ford Cosworth generates more than 400Hp, but so does a Ford Tractor engine! So why put an engine weighing a tonne in a Ford Tractor when you could have a Cosworth engine that's much smaller and lighter yet equally powerful?
Chris
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Well, the difference between a tractor engine and a car engine is that a tractor engine is fitted to power equipment as well as driving the tractor, while a car engine just needs to be able to drive the car.
That's why tractors have such big engines. There's a whole lot of extra mechanics to power up some tools like a mower or w/e
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Torque will tell you the force that the wheels can exert on the road - which gives you the acceleration (once you've taken off the other forces like friction and drag).
Top speed is limited by the power available - when it balances speed times resistive forces.
The relevance of torque is more elusive and is particularly relevant in IC engines which have zero torque at zero revs and an optimum, near their top revs. Given an infinitely variable gear system, the torque would be more or less irrelevant - the engine could be run at a speed which produces maximum power and then the motive force (hence, acceleration) could always be the maximum. With only three gears (common in cars of the 1950s and before) the engine would need a wide range of operating speeds over which the torque would be fairly constant. You would need a 'flat torque curve' - which would probably mean that the maximum torque was more limited.
If you add more gears, the torque curve can be narrower and more 'peaky', giving the opportunity to deliver higher max power over the whole car speed range. Racing cars use many gears so that the 'tuned' engine is always working near its optimum revs.
An engine with high torque over a wide speed range is easier to drive with because you don't constantly need to be changing gear.
Crudely speaking, the max power from an engine is limited by the cylinder volume (all other things being equal) but they do say that the torque tend to be limited by the area of the pistons. I'm not sure how valid the statement about torque is in theory but it may well be true in practice. It may just be a consequence of shorter stroke engines being able to rev higher.
A big, lower performance, engine will be much more flexible and convenient to use in a tractor, which doesn't need good acceleration but does need to be capable of dealing with varying loads and to run at constant speed and without a lot of 'fuss'.
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Thanks - it's this last point I'm after clarifying.
A Ford Cosworth generates more than 400Hp, but so does a Ford Tractor engine! So why put an engine weighing a tonne in a Ford Tractor when you could have a Cosworth engine that's much smaller and lighter yet equally powerful?
Chris
Because the Cosworth has much less torque than a tractor enine, furthermore, its full torque is only at medium/high revs; at low revs, the torque is too low to push a (tractor + resistance); probably you couldn't even start with such an engine in a tractor (which have to plough, for example).
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In a reciprocating IC engine, torque is created by applying a force to the crank, so for a given force it is the radius of the crank that dictates torque. However, increasing the crank radius requires an increase in stroke length and a higher mass crank counter-weight, both of which tend to reduce the practical max speed limit of the engine and narrow its effective usable speed range.
Such engines are found in railway, marine and agricultural applications and tend to be low-revving, long-stroke engines, operating in a relatively narrow speed range for most of the time.
These just aren't suitable for automobiles though, which need to vary their speed over a much wider range.
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The tractor engine is de-reated so that it can deliver full power indefinitely, so all the parts are very robust, and heavy. Also, it probably generates full power at low RPM.
The Cosworth engine is is made as light as possible to reduce the weight of the vehicle. It produces full power at high RPM (which is one way to get a lot of power from a small engine.) The downside is that it can only deliver full power for relatively short periods. Racing engines are only good for a couple or races without a major overhaul.
Regarding torque, if you have enough gear ratios, you can keep the engine at max RPM and full power output all the time. This will enable very rapid acceleration even although the engine has a very narrow "torque band". Of course, such a vehicle is not for everyone!
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Just boy racers!
And, possibly, that Clarkson character.
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Hey! Without "boy racers" we might never have taken to the skies, broken the sound barrier, or landed on the Moon.
Watch your mouth [>:(]
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Did anyone wear a baseball cap sideways on the Moon? Don't tell me you can't wear one in a space helmet.
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I don't think fried chicken and southern accents would fly well with the lunatics up on the moon [;D]
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Geezabrake pal!
With a name like Buzz Aldrin, could he be anything else?
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This has about run its course, I think. I might point out that the answer to the OP is probably "He hasn't really got a clue".
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Kindly define OP
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Sophia - Why so suddenly coy? Did you realize that you just stepped on a landmine?
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Easy guys.
by the way:
OP : noun
Original Poster: The person who begins the selected thread in that particular forum.
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Woops! I thought OP was "Old Person" - as in me! I apologize for my overreaction.
However, it may not be entirely appropriate to use comments like "hasn't really got a clue". Sounds a bit elitist. The "OP" asked a legitimate question. He/she is entitled to legitimate replys.
Abbreviations can be dangerous things.
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I meant Clarkeson has probably not got a clue. As if I would ever slag orf a fellow FM.
(forum member)
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Wait a minute, I thought FM was short for "Flippin' Magic", or something like that.
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Local definitions rule. IISS
(if I say so?)
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I've often wondered about this one. Would it be fair to say that if you imagine the axel as a bar rather than an rod, which was moving in a straight line rather than rotating, torque of the axel would be comparible to force exerted by the end of the bar on an object in it's way?
Sorry if that's a convoluted description of what I mean!
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Crudely speaking, the max power from an engine is limited by the cylinder volume (all other things being equal)
Well yes, if you only keep one thing variable and the rest constant then the variable item will be the limiter. If you kept the fuel injection variable and everything else constant then the amount of fuel injected would be the limiting factor.
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What I wrote was over simplified - I meant that the power available would be proportional to volume (v. obvious, really) in that it represents the amount of air / fuel that can be burned each cycle. However, the torque characteristic can be affected by piston area and allows higher torque at higher revs (which actually implies more power too??), possibly because of the possibility of bigger valves.
It's too complicated for generalisations I expect. But when has that got in the way of a good forum topic?
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Yeah I know, I was just being pedantic [:)]