MASCOT March-April 2004

How Does It Work? - No 8 - The Ignition System

The Tool Chest

The ignition system is probably the cause of more breakdowns than any other system on a car, so if we ever need to sort out a problem it is essential we understand how the system should work, then we will have a much better chance of working out what has gone wrong.

The object of the ignition system is to ignite the fuel/air mixture in the cylinders by producing a spark across a gap of about.025" (0.51 mm) at the spark plug electrodes. At atmospheric pressure this requires only a few hundred volts, but when the pressure is raised to that inside the combustion chamber as the piston approaches TDC (Top Dead Centre) about 8,000 volts is required. This high voltage is produced by either a magneto or a coil ignition system.

Whereas a coil ignition system requires a battery to provide a low-tension voltage, a magneto is a self-contained unit which generates its own low tension power to produce the high voltage pulses, which it then distributes to the appropriate cylinders at the correct time. Otherwise it works using the same principles as a coil system and depends basically on the same fact discovered by Faraday in 1831, that when an electrical voltage is applied to a coil of wire, a magnetic field is generated around the coil- see FIG 1. Similarly, when a coil of wire moves relative to a magnetic field, or vice-versa, an electric voltage is generated in the coil. The more coils, or the stronger the magnetic field, the higher the induced voltage. For the Primary circuit, the coils of wire are wound around an iron core, which becomes a magnet when the current flows. In an ignition coil, a Secondary winding of thousands of turns of fine enamalled wire is added around this core, such that when the current is switched off, or the contact breaker opens, the magnetic field created by the Primary coils collapses, and a very high voltage (HT) is induced in the Secondary winding. It is this HT voltage that provides the spark at the plug electrodes - see FIG 2.

It is very important that the spark occurs at exactly the right moment - usually a few degrees before TDC to ensure the fuel is burning fully as the piston starts its power stroke. It may sound pretty noisy when the exhaust pipe is removed, but the fuel does not explode, burning relatively slowly. The timing of the spark and its delivery to each cylinder in turn is the job of the distributor. The layout of a typical ignition system is at Fig 3 and shows the coil, with its Primary and Secondary circuits connected to the battery via the ignition switch. The job of contact breaking in the Primary circuit is done by the aptly named Contact Breaker points. These are opened by a cam which is driven from the camshaft, the cam making one full turn for each two turns of the crankshaft. On a four cylinder engine the cam has four lobes, with six on a six cylinder engine. The cam opens the points about .015", thus causing the magnetic field to collapse, producing the HT current which flows alone the main HT lead to the rotor arm, which by this time will be pointing to the correct distributor segment. From thence it flows along the Plug Lead to the spark plug Unfortunately the collapsing magnetic field not only generates our HT, it also generates a voltage in the Primary circuit, which, were it not for the Condenser, would arc across the CB points. This would not only cause them to burn away, it would maintain a voltage in the primary circuit, thereby slowing down the collapse of the magnetic field and reducing the HT voltage.

The Condenser consists of two sheets of foil or metallised paper, separated from each other by at least two sets of insulating material such as waxed paper. These are wound into a cylindrical shape and inserted in a metal container. One foil sheet is connected to the container, the other to a terminal or wire which connects to the insulated side of the contact breaker - you can see this at the inset in Fig 3. When the points are closed a current flows into, but not through, the condenser, placing an electrical charge on the positive foil sheet. Open the points and the foil retains its charge - close them again and it will discharge in the opposite direction. But having opened the points we now have the unwanted voltage generated in the Primary circuit trying to jump the gap, but this is mostly absorbed by the foil becoming charged, thus all but eliminating arcing across the points. However, some arcing still does occur and this carries a little bit of tungsten with it, creating a pit in one CB point and building a pip on the other.. Eventually the pip gets so large that it prevents a clean break of contact and needs to be cleaned oft. This is best done with a fine file or stone. The pitted contact should also be cleaned, but it is not necessary to completely remove the pit - same new points even have a hole in the middle to try and reduce the burning. Apart from testing for an open circuit there is no easy test for a condenser, but if yours is not working the CB points will appear blackened and there may also be arc splash in the area around the points. If in doubt fit a new one.

Having cleaned the points it is essential the gap is correctly set. For magnetos this is usually .012" and for coil ignitions, .015", but do check the makers instructions if in doubt, as a few thou difference in the points gap can make several degree difference to the ignition timing and dwell angle. Turn the engine slowly in its normal direction of rotation until the heel of the moving CB point is at the top of a cam lobes. Slacken the fixed point screws so that it is just possible to move the contact plate such that a feeler gauge of the correct thickness can just be inserted between the points. Tighten the screws and recheck the gap. The correct gap not only ensures the points open sufficiently to minimise arcing, but also that they stay closed long enough - called the "Dwell Angle" - to allow the magnetic field sufficient time to build up in the primary circuit, as it builds much more slowly than it collapses. And so to the Spark Plugs and their HT leads. There are two main kinds of HT leads - those with copper wires and the later, carbon track leads, which are intentionally high resistance to suppress radio interference. The older, copper cables do not usually give trouble provided their insulation is sound. It is sometimes possible to hear or even see the spark if the insulation is cracked. However, carbon leads do deteriorate with time and are a common cause of misfiring. Generally only one cylinder will be affected initially, but if the problem is in the main HT lead from coil to distributor, the symptoms will probably be difficult starting or cutting out under load. If in doubt, or if they are over 5 years old, renew them.

Again there is no definitive test, as the leads will often convey a spark at atmospheric pressure, failing only when under load, but if you remove the HT lead from the distributor cap and hold it (using insulated pliers recommended) close to the cylinder block or head, then spin the engine with the ignition turned on, you should see a thin, blue spark at least a 1/4" (6mm) long. This is a pretty goad indication that there are no major problems in the Primary circuit and as far as the distributor cap in the HT circuit, whereas a fat, red spark means all is not well in the system. If this test is OK; you can go on to Barry's Test for the Rotor Arm (See `Electrical Gremlins', Mar 03 Mascot). Remove the distributor cap then hold the HT lead about 1/8" from the metal end of the rotor arm and spin the engine again (ignition on). If there is a spark now, you will know that the rotor arm insulation has broken down and that you will have to renew it. Before you refit the distributor cap, ensure that it is thoroughly clean inside and out, and if there is a spring-loaded carbon brush in the centre that this is free. (Some early rotor arms have a leaf spring riveted to the metal part of the arm to convey the HT from the terminal in the cap). Check also that the cap is not cracked and that there are no signs that the spark has "tracked", that is, leaked along dirt or foreign matter on the cap, which will have caused a telltale black line (or track) across the Bakelite.

I will deal with the Spark Plug in more detail next time, but provided they are of the correct type, are clean (inside and out) and have the correct spark gap, they are unlikely to be a major source of trouble.

I hope this longer-than-usual chapter hasn't been too much to absorb all at once, but I couldn't see a suitable place for your editor to divide it into two parts. Please ask Aunty if there is anything more you would like to know on this subject.