Energy does NOT take time into account.
Power=Energy / Time
1Nm of torque = 1 Joule of Energy : 1 joule is required to lift 1 N by 1 m.
1 Watt = 1 N moved 1 m in 1 second.
So time only comes into play when talking about power.
e.g. A chocolate bar has energy stored in it, but this is not related to time. If you ate it, the rate at which you used the energy would determine the rate at which the energy you gained was used up; The power. YOu could still move only the same amount the same distance, but you need more power to do it more quickly.
A good analogy for power is as follows:
An adult may lift a 30kg sack of potatos from the floor to a table. A child may not be able to lift the sack, but lifting the potatos out of the sack and putting them on the table individually he/she could do it. The same energy would be used, but the adult would be more powerful because they could do it in one move (say 5 seconds), not several (say 2 minutes)
Hope this helps
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Alan
I'd agree with most of it, since power = work done in a period of time. I'm confused about the description of torque though.
Torque cannot equal joules - a joule is an amount of work, and work is done only when a body is moved against a resistence. Now torque is purely a measure of turning effort (think of its units, lb ft or Nm, and there is no time element in there) so it does not measure work. Watts measure work - a watt being a joule/second) The classic example here is a steam engine, which develops torque when steam is admitted but before it starts moving anything.
I'm always bemused when torque and power for engines are quoted as though they were separate items. The two can't be separated.
I'm still imperial on this and the relationship is:
HP = 2 x pi x N x T/ 33,000
Where N = engine speeed in revs/min and T = Torque in lb ft
So an engine described as 'torquey' is one which, yes, develops plenty of torque at lower revolutions, and so has a flatter power curve. However, it's power that moves vehicles, not torque.
Regards
John S
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Torque cannot equal joules - a joule is an amount of work, and work is done only when a body is moved against a resistence.
It does. Work done (energy) = Force (N) * distance (m)
Which is the same as torque measured in Nm
>>Now torque is purely a measure ofturning effort (think of its units, lb ft or Nm, and there is no time element in there) so it does not measure work.
Not so. Force (N) = mass (kg) * acceleration (m/s²) Hence Nm = kgm/s²
>>Watts measure work - a watt being ajoule/second)
No, watts measure power - the RATE at which work is done.
Not sure where this leaves us really. Power makes cars go along (you must do a given amount of work in a given time to overcome the air and frictional resitance trying to slow you down). However, if we consider the work being done at any given time (kgm/s²), and then differentiate that with respect time (eg the rate of of change of work done) we get (my calculus is a bit rusty but I think this is right) a function of the order kgm/s³, which is power. I THINK this means that the more torque available at a given time the greater the rate of change of work can be, eg acceleration. This is why diesels feel to pull better because without the revving the nuts off the engine they can do a greater rate of change of power. So maybe the key is not the absolute value of the torque, but the engine speed at which is occurs
As you pointed out John, the two are vitrutally indistinguishable, but people like to quote numbers.
RichardW
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Here's a very good analysis of the relationship between power and torque, from first principles:
www.stanford.edu/~voloshin/lhowwhy.html
I think the marketing boys are slowing coming round to realising that there's more to it than just quoting inflated bhp figures at us.
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Richard
Take your point, but I still don’t agree that torque = work. I realise that torque = force x distance. Yes, there’s a distance term in there, but it’s not the distance over which the force moves, it’s there because torque is a twisting force and can only be described by a term which includes a force and a distance from the point about which it’s acting. The fact remains that torque, where no movement occurs, does not equal work done, even though the units are Nm or ft lb. When it's measured on an engine test, the measurement is one of static force on the crankshaft dyno, coupled with a distance from the centre of the dyno - no movement is involved. Consider the steam engine, when steam is first admitted to the cylinder. The pressure acts on the piston, the piston attempts to turn the crankshaft. Until the steam pressure is sufficient to move the crankshaft against the load then no work is being done – the engine is stationary. Nothing’s moved so no work has been done. The Newton is a derived unit but then so are the kg and pound which both include ‘g’. What I’m saying is that the horse power is 550 ft lb per second, the crucial bit being ‘per second’. Yep, watts measure power, or the rate at which work’s being done – badly explained on my part. But, again that crucial element is there the RATE at which works being done.
Whatever, you’re right about the different ‘feel’ of engines. Diesels have flat torque curves and develop maximum torque much lower in their rev range than the average petrol engine. They therefore have a power curve which is fairly linear, and because of the low engine speed for peak torque, then they are developing more power at those lower engine speeds. Acceleration is related to excess power available over that required for constant speed, so they accelerate the car well at lower engine speeds. Alternatively, when you get a high revving Honda unit, for example, the maximum torque occurs at very high engine speeds. The engine has a rising torque curve, and so a peaky power curve – result, the engine doesn’t pull that well at low engine speeds, but the power output rises at a greater rate than the engine speed, so it appears to really fly when the revs are up. The figures neeed to be read with care.
Regards
John S
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In your example above JohnS, the dyno has to apply a force at a distance to counter the torque of the engine, that is what is keeping the rpm of the engine constant at the point you are measuring. If you blip the throttle of your car in neutral, you can increase the revs to the limiter with hardly any throttle, because no torque is being applied to the shaft. you only see the twisting force when you oppose it with a twisting force in the opposite direction. This is when work is being done and it is when the shaft experiences a torque. If the dyno was not doing work resisting the torque of the engine, the revs would rise as in the neutral example. This really is basic physics.
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The dyno is where the concept of "Brake horsepower" comes from. The dyno has to brake the engine, to apply the reaction force to the crankshaft of the engine in order to measure the torque, which is then used with the speed to calculate the brake horse power.
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Epic 80
I realise it's basic physics, and basic physis (or is it mechanics?) says work is done when a body is MOVED against a resistance. If it doesn't move no work is done. The blipping the throttle analogy is a red herring. It's quite clear that revving the engine with no load connected produces little power. What the dyno does is resist the motion of the crankshaft so giving it some resistance to move against. The torque measured isn't work, it's a load. The work is done in the dyno, pumping the fluid. You can then calculate the power output from the torque and the rotational speed. You can't calculate it from the torque alone.
Regards
John S
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Epic 80
Just read your second post, which says exactly what I've just said. Yes, the torque is necessary, but it isn't work. It's an essential component of calculating the work done though. That's why I've said you can't separate torque and work, as some try to.
Regards
John S
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