Making a spark

This is the age old problem of making a spark at the right time to ignite the fuel and crate the power stroke.

Almost all early engines are fitted with the Kettering ignition system to do this. A simple and effective idea which works well, but has big draw backs in the form of wearing out its parts as it does its job so needs constancy in servicing to keep in top condition.

The parts needed are the coil, this converts 12 volts DC to 20,000 or more, HT Leads to send the voltage where needed, The distributor cap which sends the voltage down the right lead, The spark plug which turns the voltage into a spark, the points which switch the coil on and off and the condenser which smooths out the voltage spike at the points and the rotor arm which shows the voltage where to go.

All these parts are needed to make the engine run but first how can we make 12volts turn into 20,000 volts or more. This is where the coil comes into its own. The coil is in fact two coils a low tensile coil on the outside and a high tensile coil on the inside. With the points closed the outside coil charges up, then when the points open the charge that is built up in the outer coil collapses inwards charging the inner coils and the number of turns on the inner coil is in the hundreds so acts like a voltage step up transformer and produces a high voltage spike which last for a fraction of a second. In that time the voltage is looking for earth so travels down the main HT lead from the coil to the center of the distributor the passes though the rotor arm and back out down the lead that is needed to the spark plug. Here is the cleaver bit the spark plug has a small gap normally about 20 thou with the bottom of the probe touching earth the voltage is traveling at such a force that it jumps the gap in the plug to reach earth producing a small lightning bolt (Spark). This then lights the fuel but that is another story.

So now we know how the spark is made how do we make sure its at the right time. This is the job of the distributor which runs at engine speed driven by the cam which runs at half engine speed. Being connected to this means that we can tell the rotor arm exactly where the piston is on any of the cylinders so that it can show the voltage which plug to go to. Also at the base of the rotor arm are the points these tell the coil when to charge and discharge. These are the biggest problem with the whole of the Kettering system as they wear and the back EMF (Electro motive force) burns out the faces of the points. As stated earlier when the points turn the coil off from charging the voltage build up which is needed to create the spark also travels down the lead to the distributor to reach the points producing a spark. With the aid of the condenser the spark is reduced but regular checking to keep this system in top shape is needed as at 1000rpm the points have turned on and off 1000 times (on a four cylinder engine). This means that the coil has to have enough charge to discharge every 0.06 seconds and at higher speeds the times are reduced even more so how can we improve this system.

The way improve this design is in the form of Electronic points. There are lots of systems on the market but I refer to the Simon's Best British Classics BBCK4 Powerspark system ( This system does away with the points and condenser and replaces it with a magnetic switch. The trigger is placed at the bottom of the rotor arm so is timed with the engine still switching the coil at the right time but instead of points now you have a small module sitting in their place and with the gap set right with the rotor it is then tightened down and then you can pretty much forget it as there are no moving parts to service. This type of device also stops the back EMF producing a nice clean break to trigger the coil. We have now improved the system on two main accounts first the servicing of the distributor is now less and the spark is now smother allowing the engine to run more evenly. This can lead to better miles per gallon and improved power from the engine.

The next big step from here as with modern cars is to do away with the distributor and HT leads and have coils on each of the plugs and a computer switching them on and off when needed.

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January 2023


Pops Garage