How to drive more efficiently

19 05 2009

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

Too much energy ends up in the brakes...

Too much energy ends up in the brakes...

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.

Using potential energy...

Using potential energy...

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!

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5 responses

19 05 2009
The Beginner’s Guide to Green Motoring « The Provincial Scientist

[…] 4. How to drive more efficiently […]

4 11 2011
Steven

What? Nothing about being in the correct gear and the effect that has on efficiency of power distribution?

4 11 2011
jarrodhart

Hey Steven, I left that for a wise and knowledgeable respondent to put in the comments section 😉

I know you know this (and yet still chose to drive a souped-up Opel Kadett with big exhaust pipes in low gear despite the knowledge), but for the benefit of other readers, my two cents worth…

Being the ‘right gear’ is certainly important because every engine has a most efficient speed (I discuss this in the page on ‘what makes an efficient car) and you want to keep it at that speed despite the annoying fact that the car needs to go at all manner of speeds.

However, some people think you should always be in the highest possible gear that doesn’t result in a stall, but that is perhaps less clearly supportable. Yes, the higher the speed of the engine, the more frictional loss, no doubt, but you should never go so slow that the engine is unable to get good quality combustion. This can be detected by shuddering and strange sound from the engine (aka knocking or pinking) which also shortens engine life.

Also, the higher gear you’re in, the more responsive the car it is small movements of the foot – and the constant speed adjustments are not only highly annoying but also inefficient.

So a good point sir!

PS Newsflash, I’m moving back the UK, so I may hunt you down – I know where you live 😉

7 11 2011
Steven

Thanks for the response.
You can hunt me down all you like, but with your green driving and my evident disregard thereof, you will not catch me.

7 11 2011
jarrodhart

Of course, you were complicit in me buying a car with big exhaust pipes too, so my criticism comes right back at me 😉
I’m sure now as we become older fuddy-duddies we ‘re slowing down and being more responsible – we shouldn’t be too quick to criticise those who do as we did 😉
I really am the pot calling the kettle black as I’m presently driving a car that gets terrible mileage and has been implicated in a string of world rally championships – I am feeling bad though and will reform soon, promise! Until then however, maybe I can catch you!

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