Why the power of a car is meaningless to its driver
The Sunday Nation
Nairobi,
12 February 2012
Motoring enthusiasts are like music junkies: both are preoccupied with meaningless power figures printed on their favourite gadgets. Whereas many manufacturers of music systems have the habit of quoting completely false and misleading quantities, their counterparts in the motor industry are quite honest.
I wrote about the power figures written on music systems a few years ago (2006) and demonstrated that the number of watts (real or fake) cannot tell a user how loud the output will be. The reason is that the loudness is largely determined by the physical design (as opposed to electronic design) of the loudspeakers. So if you really want to know how loud a system can get, you simply pump it to its maximum volume and listen!
The same basic logic can be applied the power of cars: if you want to know how fast it accelerates and its top speed, simply drive off at full throttle.
Nevertheless, it would be nice to know what power means in a car. The physics definition of power is the rate of doing work…and work is said to be done only when a force moves.
In cars, power is mostly quoted as horsepower. One horsepower is roughly equal to the working rate of …eh… a workhorse! So if your car says it has 100hp, then you have 100 horses in the bonnet.
In the metric system of measurements, power is quoted in watts and one horsepower is equivalent to about 746watts, or 0.746kW. Thus a 100hp car produces about 75kW.
Now, a horse can easily run along at about 50km/h. Does it then mean that if we tied two horses to the same carriage we would hit 100km/h? Of course not! But we would probably be able to pull approximately twice as much weight as with one horse.
No wonder your 100hp car cannot get anywhere near the erroneously expected 5,000km/h! Most cars with this sort of power manage only about 120km/h – 150km/h. However, this car can probably pull 100 times the load that one horse pulls.
Unfortunately, however, it is impossible to tell how fast car can accelerate or its top speed by simply looking at it power output. The reason is that the acceleration depends by equal measure on the total weight (mass) of the vehicle – the heavier it is, the more sluggish.
The top speed is determined the rate of revolution of the car’s engine and the net gearing ratio of the whole transmission system; that is, gearbox plus differential plus tyre size.
From the forgoing, it is now clear why motor vehicle manufacturers also give data on acceleration (time to reach, say 100km/h) and maximum speed of their cars. The reason is that, from a driver’s point of view, the power rating of a car is a completely meaningless (and useless) quantity!
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