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Don’t ask that question. There I was, last night, leaving work, thinking that I had nothing to put on a physics blog, when I turned the key in my car ignition to find an engine that was struggling to give me any power. After going a kilometre or so, it was clear it wasn’t a problem with the automatic choke, and I pulled over and called the AA.

It was, of course, a problem that they see frequently – I note it’s on the AA top ten list of reasons for callouts. A degraded high-tension lead. That’s the lead that supplies a spark plug its high voltage – but this one had a tiny hole in it that was giving a short circuit to the cylinder head, meaning the spark plug wasn’t getting enough voltage across it to spark (and consequently, the fuel in the cylinder wasn’t igniting).  So the AA man did a temporary fix with a bit of insulation tape and I got home without incident. Just need to get some new leads now.

Anyway, that reminded me of how the car electrical system works. It’s a pretty neat concept, that allows you to go from a 12 volt d.c. (direct current) battery to many thousands of volts over the spark plugs. The easy way of transforming voltage is to start with an a.c. supply – that’s why the mains supply to our homes is a.c. (alternating current). Transformers, consisting of two coils with different numbers of turns, step up or down the voltage according to the turns on the two coils.  The changing current through the primary gives a changing magnetic flux through the secondary, this changing flux puts a voltage on the secondary.

But how do you do it with a d.c. supply? Nothing is alternating Put a constant current through the primary and there is no voltage on the secondary. The answer is to switch your current on and off. If you put current through a primary solenoid, then switch it off, the lines of magnetic flux will collapse, and cut that solenoid (and a concentric secondary), creating a voltage on the secondary by Faraday’s law of electromagnetic induction. If there are lots of turns on the secondary solenoid, it can give a sizeable voltage for a short period of time. It is this voltage that is fed to the spark plugs, allowing them to spark. Or, in the case of my car last night, not.

A simple application of a capacitor controls the collapse of current so that there is no spark across the switch (in the distributer) supplying current to the primary solenoid.  

All quite simple, really, and that might be one reason why a temporary fix (insulating tape) is actually possible.

This is a nice reference on the electrical system for a car http://www.jetav8r.com/Vision/Ignition/CDI.html