In this article I hope to describe some efficient driving ‘tactics’ and hopefully also explain why they work.
To do this I start with a question… what, exactly, is energy?
Many people think scientists know this, but alas, they don’t. It is one of science’s great mysteries.
On the other hand, they do know an awful lot about how it flows (they call it thermodynamics which is science-jargon for “heat-flows”). And when energy flows, we also know how to harness it.
If we consider the car, we can think of a fuel tank as bottled energy. The engine then turns that bottled energy into motion. But the laws of physics say energy is never destroyed – in only flows. So where does it go after that?
Understanding the answer to this simple question will help us all to drive more efficiently.
Here are some of the outlets for the energy from your petrol tank:
1. Accelerating your car – energy is transferred into the mass of the car. They call this ‘kinetic’ energy – kinetic is just latin for ‘movement’.
Aside for nerds: It’s the kinetic energy in a car that makes it so dangerous – when a car crashes into a tree this energy flows at a speed comparable to a bomb-blast, bending the metal and hurting the people.
2. Going uphill – the energy is also put into the mass of the car. They call this ‘potential’ energy – we’ll see why in a minute.
3. Friction – the friction inside the engine, of the wind on the car – and last but not least the rubbing of your brake shoes on your brake disks – all turn your energy into heat
4. Noise – some goes into people’s ears, but eventually it all just heats the environment.
And that’s it.
So the first thing to notice is that friction and noise are bad. It is not our aim to heat the world up.
So how do we avoid heat and noise? Firstly, keep your car in good nick. Keep your tyres properly inflated too.
Secondly, drive slowly. Air friction is much more significant the faster you go. Doubling your speed quadruples the frictional loss per km and multiplies the energy loss per second (power) by 8! Thus there is about 21% more energy loss to friction at 77mph than at 70mph, despite being only 10% faster. (and it requires 33% more engine power!)
My third tip is a little controversial. Try not to brake.
If you are approaching a stop, try to coast to a halt by taking your foot off the gas far in advance. If you do this, you will avoid heating your brakes and rather spend your energy on air friction, which was inevitable anyway. We will come back to braking in a minute.
Now can we do anything about the energy required to accelerate and go uphill?
Yes, we still ‘have’ this energy- so it can be recovered!
The mass in your car (including you yourself) become a store of energy when you are a) going fast, and are b) at the top of a hill.
The hill energy is called ‘potential’ energy because its got the ‘potential’ to be recovered. We generally recover it without even thinking – when we go back down the other side – gravity does much of the work.
However, we only get it all back if we don’t brake (or use engine compression) to slow ourselves. If failing to brake would lead you to exceed the speed limit, then that’s a pity, as I can’t condone breaking the speed limit, especially if my children are about.
What about the kinetic (going fast) energy? We usually also recover this - but only if we allow ourselves to coast to a stop. Again, if we use the brakes, we turn all that precious energy directly into heat, which is literally burning it.
We also tend to brake when we approach corners, again, it is more frugal to take your foot off the pedal far in advance of the corner such that you are already going slow enough to take it safely when you (eventually!) reach it.
All that might sound complicated, but it all translates to a simple rule of thumb: don’t use your brakes unless you have to. Of course this logic can be taken to its extreme (and occasionally unsafe) conclusion – take a look at the practices of the hypermiler community.
Anyway, that’s all you need to know to get a good 10-20% more miles from each tank.
That concludes this series of articles on greener motoring, I hope it has been of use. Please don’t hesitate to add your own tips in the comments section. Thanks!