It is a popular topic of discussion with car enthusiasts - but one that seems to be never-ending and usually unresolved. So, just what is it that best determines a car's acceleration performance - power or torque?
In order to be able to properly deal with the question, it is essential to have an understanding of what the terms really mean, as opposed to the gut level understanding (or misunderstanding) common to enthusiasts.
Torque
Torque is simply a measure of the twisting force that is applied in an attempt to rotate an object. It is a force that is applied to a lever arm, and is measured in Newton Metres (Nm). Newtons are a unit of force, and at the surface of the earth a 1kg object will exert a force on the ground of 9.8N, due to gravitation. Torque is, in effect, the product of the force and the length of the lever arm. Understood in this way, it is clear that there are two ways of increasing torque; either increase the force or increase the length of the lever arm.
Think about a door that you wish to open. You can either apply a small force at the outer edge (where the lever arm is long), or far more force close to the hinge (where the lever arm is short). In this way, you can have the same torque, but it is developed by applying different forces at different lever lengths.
"But what has all of this got to do with cars?" I hear you ask. Well, think about some of the engines you know. You know that increasing the lever length can increase the torque. Now think of those long stroke engines, and how, for their capacity, they seem to generate a lot of torque (such as the Ford 4-litre engine). The capacity, in effect, gives you the force, and the lever multiplied by that force gives the torque. So, to increase torque, increase the stroke, or increase the capacity, or both. Of course, there are negative aspects to increasing the stroke, which we will come to later.
There is one facet that is very important to realise: you can be exerting a lot of torque but not be doing anything. Think of all the time that you have skinned knuckles attempting to loosen a rusted bolt. You have definitely applied a lot of torque, but you have not done any work.
We now come to the aspect that causes a lot of angst among many enthusiasts - power. Most people think that they know what power means; unfortunately, many do not.
Power
Power is defined as the rate of doing work, and has units of Kilowatts (kW - named after James Watt) or horsepower in the old Imperial units. To see what power actually is, let's consider the experiment that James Watt did a couple of hundred years ago. He wanted to know the rate at which draft horses could raise coal from a coal mine. So he measured the mass of coal brought up, the distance that the coal was raised, and divided this by the length of time that it took to do this. He found that the horses would lift 33 000 pounds 1 foot in one minute (or 1 pound 33 000 feet in one minute). He called this unit the horsepower. In metric, the Watt is defined as the power to do one Joule of work per second. One horsepower is equivalent to about 746 Watts.
Now, another aspect to realise is that power and torque are intimately related. Remember how power is the rate of doing work? So, with engines, power is the torque multiplied by the radial velocity. Without getting into the physics in detail (we will forget about radians etc), the power of an engine is given by the following relationship:
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Torque (Nm) x RPM |
| Power (kW)= |
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9549 |
Or, in the old Imperial units:
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Torque (lb/ft) x RPM |
| Power (hp)= |
|
| |
5252 |
So, as you can see, power and torque are very intimately related... but it is important to realise that they are different.
Remember that you can be applying a lot of torque for no result. Well, looking at the fact that power is the rate of doing work, it is obvious that if you are doing no work, you generate no power! So, no matter how hard you push that spanner, if the bolt doesn't turn, you generate no power.
There are two ways of increasing the amount of torque generated by an engine - either increase the capacity (or, more correctly, capacity times volumetric efficiency), or increase the length of the lever arm (or stroke).
For increased power, you can increase either (or both) the torque, or the revs at which that torque is generated.
Here we revisit the issue of stroke. Increasing stroke will increase torque, so theoretically it would be good to have very long stroke engines. The problem is, if the stroke is too long, the volumetric efficiency decreases, particularly with increasing revs (which is why long stroke engines don't like a big rev, apart from the rotating friction and harmonics). Now this decrease in revvability more than compensates for the torque increase, which is why very high power output engines tend to have very short strokes (once again, engine strength issues ignored).
Okay, now we have all that physics stuff out of the way - we understand the meanings of power and torque in their correct contexts. So next week we can get into the interesting stuff - looking at engines, cars, and how power and torque relate to performance in terms of acceleration and top speed.
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