# Simple Machines

By Marcus Wilson 26/12/2009

There’s a lot to do while driving.   Look at the road – watch the speedo (98 kmh – OK there), watch the road – look in mirrors – check fuel gauge (half – OK there) – watch road – watch that car at the intersection ahead – check temperature gauge (where it should be) – eyes on road – glance in mirror – eyes on road – check fuel gauge (quarter – OK there) – car ahead slowing, might need to break – check mirror –

Whoaa. Hang on. How did the fuel gauge drop a quarter of a tank in the space of a couple of minutes?  Watch gauge. Visibly dropping. Where’s my petrol going? Car isn’t on fire – can’t see a trail of fuel behind me in  the mirror. Watch gauge again. Stable, no – wait – it’s going up again. Back to half a tank again.

Now, I assume this is a problem with the fuel gauge, not the fuel system itself. Probably with the float in the fuel tank, but I haven’t checked it out. The fuel gauge is a pretty simple machine, but one that works quite well. It’s a bit like what’s in the toilet cistern – there is a float that sits on top of the petrol in the tank; as the level drops the float drops. It is connected to an arm that goes to a variable resistor that controls current to the gauge in the car. The gauge itself is probably a bimetallic strip – the more current flows the more it heats up – and the more it heats up the more it bends, due to the different expansion rates of the two metals. And generally the whole thing works reasonably well, though I know it can fail, particularly in older cars like mine. Low-tech solutions are often quite good enough.

One interesting project a couple of students in my department have been working on recently is a ‘battery gauge that works’. Have you noticed that your battery gauge on your mobile phone, or your digital camera, isn’t really to be trusted. Might say half charged, and suddenly you find yourself out of power. Or switch it off for a few minutes, and suddenly it appears to have charged up a bit?

Making a gauge that is accurate for a battery is actually not a simple task. The way they release their energy isn’t quite as simple as that of a fuel tank.  But, my question is, does it really matter? Given that car fuel gauges are a bit iffy, but every driver seems to cope OK, do we need any high-tech battery gauges?

NB – students – you know who you are – I’m not trying to rubbish your work – just get you thinking about the need for it. As any engineer should.