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A PHYSICIST WRITES . . .
(September 2006)
Glancing back at some of my columns so far this year, I see one or two loose ends that perhaps need tidying up! In March I mentioned Potential Energy as one of the many forms of energy that come from the fuel burnt in the engine of your car. PE is what the car acquires by going uphill, against the pull of gravity.
In simple school-book physics, when something that’s up comes down again, its PE can be recaptured and changed into another useful form of energy, such as kinetic (KE). A swinging pendulum is a good example of PE being converted into KE and then back again. But is your car able to perform the same efficient conversion of energy when you are driving up hill and down dale?
At first sight the answer is yes, more or less. After all, you can coast all the way down a hill consuming very little fuel and (depending on the road and the traffic) you may have acquired a decent speed at the bottom. However, this won’t be nearly enough to take you to the top of the next rise (assuming it’s the same height) without burning more fuel. Why not? Because of friction and drag on the car all the time it’s moving, changing some of its energy into heat.
In any case, you don’t normally drive through hills imitating a pendulum — or rather, its travelling equivalent: a switchback train! It’s more likely that you aim to keep to a constant speed, applying the accelerator or the brakes as required. Unfortunately this will have an adverse effect on your fuel consumption...
Let’s try a rough calculation for a typical family car weighing 1800 kg. I know how much energy is released by petrol when it’s burnt, and I also know that only a quarter of this is made use of in pushing the car (the rest disappears as heat). Suppose then that the car is being driven up a modest 3% gradient, or 1 in 33, at a steady speed. My estimate is that the climbing will consume an extra 1/40 of a gallon per mile.
So if your ‘normal’ consumption (on the level) happens already to be 40 mpg, then on this hill it will double — or do I mean halve? — to 20 mpg. And on steeper hills the figure will be even worse: 9 mpg on a 10% gradient (or 1 in 10), for example. You will save some fuel on the downhill runs as I said, but this is still bad news I’m afraid, if you are planning to head to the mountains for a drive.
Also in March, I mentioned that hybrid cars were supposed to be good for fuel-saving. There were mixed reactions to this in the April newsletter! Referring to the Toyota Prius, an owner praised it but a non-owner was rather dubious.
The big problem with the rapidly expanding available range of ‘green’ cars, it seems to me, is the number of different things to consider and compare, in addition to overall consumption: the fuel economy first in town and then on the open road, the maximum power that’s available, how far a full tank will take you, the amount of CO2 produced (this is effectively zero for biofuel from crops, because CO2 is absorbed as they grow) ... and then the puchase cost. It’s certainly good news that the drive to save fuel has given a boost to the development of light-weight car bodies and of tyres with low rolling resistance. And at last the total dominance of the petrol engine is ending.
In May I suggested (with tongue in cheek) that all centre white lines should be painted out, to encourage drivers to be more careful in the face of oncoming traffic. Later I discovered that in Wiltshire they did this a couple of years ago, on some 30 mph roads at least, and successfully reduced accident rates. So what are the other local authorities waiting for?
Then in June I discussed the dangers of braking on a bend. I said that no braking system could allow for the different amount of grip between left and right tyres on the road (resulting from the heavier loading on the outer wheels as you curve round). I forgot that ABS or the Antilock Braking System, which I referred to later in the column, does exactly that when it’s preventing individual wheels from locking up!
ABS is well suited to drivers who would tend to brake too hard in an emergency. A recent enhancement of ABS is Emergency Brake Assist, which is designed to help the many drivers (apparently) who would not brake hard enough. It does this by detecting sudden braking and immediately reinforcing it, until the normal ABS action kicks in or you lift your foot.
Indeed, there’s a bewildering list of high-tech safety systems being incorporated in new models all the time, for keeping the car on the road (however slippery) and in its lane (however narrow) and at a safe distance from the vehicle in front. But as owners grow used to the protection provided by these features, will they become better drivers or worse? Will they pay more attention to road conditions, their speed and what’s going on around them, or less? Perhaps we should all be polishing up our advanced-driving techniques, in preparation — either for making the purchase or for driving more defensively.
Going back to March again: I pointed out the fuel-saving advantages of making less (late) use of the brakes and more (early) use of the normal drag on the car, when you need to slow down. I’ve just read that the train companies have been told by the Office of Rail Regulation to teach their drivers how to do the same thing as they approach stations! Coincidence ... or is it possible that someone in Rail Regulation is one of our members?
Peter Soul
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