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A PHYSICIST WRITES . . .
(March 2006)
Where’s your get up and go? It got up and went! This old joke just about sums up the way in which most of our energy is consumed. We lay hold of vast quantities of ‘hot’ energy (mainly in fossil fuels), use it and then let it dissipate away as ‘cool’ heat which can never be recovered. And the perfect example of this is driving a car. I must admit, though, that I’ve been putting off discussing energy, mainly because it exists in so many different forms (certainly enough to fill several motoring columns!). First I shall try to list them and link them together.
Let’s start with the chemical energy stored in the fuel that you put into the tank: when the fuel is ignited in the engine, this energy changes to energy of compression within the resulting hot gas. Then it becomes mechanical energy as the gas expands and the pistons move. Thus the car accelerates and acquires kinetic energy (assuming you’re in gear), also potential energy if it happens to be climbing a hill. And then when you slow down, the kinetic energy becomes heat within the brakes.
Meanwhile the engine is also driving the alternator which generates electric current (as well as some magnetic energy). This current feeds the headlamps among other items, producing both light and radiant heat. It also gives the battery its charge (ready for turning the starter motor next time). And don’t forget your audio system, which detects the incoming radio signal and emits sound — as do other parts of the vehicle, of course. In fact, the only genuine sorts of energy I can think of that a car ignores are X-rays and nuclear energy!
Do you see the pattern? The energy that was in the fuel is being converted from one variety into another, until ... well, where does it go? The short answer is that in the long run it all disappears into the environment as heat (even the light and the noise raises the outside temperature very slightly). Apart from the fact that you have arrived at your destination, there is nothing to show for the fuel you’ve used except warm tyres and a hot engine, which will soon cool down. I might mention that much the same is true of central heating at home, which really ought to be called global heating, because all the warmth you enjoy indoors soon leaks away through the walls etc, to the great outdoors.
Each of the transformations I described above obeys the same basic law of physics, Conservation of Energy: “The total amount of energy after the change is exactly equal to the amount there was before”. What the law fails to mention is that at each stage, some heat will be produced even if it isn’t what you want! For example, a typical petrol engine converts only about 25% of the chemical energy in the fuel into useful mechanical energy — the rest appears as heat, hence the need for a cooling system.
The alternator is rather more efficient, about 60%, in changing mechanical energy into current. But when the current passes through standard headlamps (or any of the filament bulbs on the car), barely 5% of the electrical energy becomes visible light. Put together, these three percentages tell us that when the headlamps are on, the engine needs to burn an extra amount of petrol containing more than one hundred times as much energy as you are getting in the form of light. What an appalling inefficiency of energy conversion this represents, and there’s nothing you can do about it.
However, there are some wastages that you can reduce — meaning that you will save fuel. I mentioned back in June 2004 that by switching from tyres with the highest resistance to rolling, to those with the lowest, you could save enough in fuel (over the lifetime of the tyres) to pay for them. But first you will need to find out which tyres are which!
Another idea: suppose you are some distance from a junction at which you will have to stop. Keeping up a good speed until you’re close, you then brake smartly. If you leant out and touched a brake disc you would discover it’s very hot. This heat-energy came almost directly from the fuel you were wasting by maintaining your speed, against the various resistive forces (tyres, friction, air resistance). Next time, why not save fuel by taking your foot off early and letting these forces slow you down gradually? This will only add a few seconds to your journey, on average.
And if you are in a slow-moving queue, I do hope you’re not one of those drivers who, when the vehicle in front advances a little, can’t resist the urge to catch up with it as quickly as possible — hitting the accelerator then the brakes, time and again. What a waste of fuel this is, not to mention putting stress on the car, the passengers and probably the driver in front too. I tend to go to the opposite extreme, hanging back until I can maintain a steady crawl that will iron out the perturbations ahead for the benefit of all the vehicles behind. This doesn’t always work, though, as there’s often someone behind who introduces the perturbations again...
But the best idea I’ve heard for saving fuel is to invest in a ‘hybrid’ car! If you’ve already done so, why not tell the Editor about your experiences with it?
[This suggestion produced some rather mixed views on hybrid cars from readers, in the April newsletter!]
Peter Soul
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