MASCOT May-June 2004

A How Does It Work? No 9 - The Ignition System (Pt 2)

The Tool Chest

In Part 1, we left off at the Sparking Plugs, which we said were unlikely to be a major source of trouble, providing they are kept clean inside and out. In the days of Leaded petrol, lead deposits would build up inside the plug. These shorted out the spark by providing an easier path for the spark to take rather than it have to jump the gap, and were a major cause of misfiring and poor starting - at least we have some advantage from running "lead-free"!

The plugs must also be of the correct Reach (thread length) and Heat Range. These are indicated by the plug number, eg L9Y, and it is important to ensure you fit only those recommended in your car Maker's Handbook, or an equivalent. A typical spark plug is at Fig 1. To function correctly, the plug must operate at the correct temperature. If this is too low, oil and carbon will build up on the insulator. This is called "Fouling", which, as with the lead deposits, will also cause the spark to short-circuit, resulting in a misfire. If the plug runs too hot, the electrodes can overheat and cause pre-ignition, which generally manifests itself as a noisy, rough-running engine.

The heat range is governed by the distance between the centre electrode tip and the sealing, or heat transfer washer. Engines that run hot, eg, in high performance cars, require a Cold plug - see Fig 1 A. These have only a short heat flow path, so the plug runs cool. Engines that run cooler require a Hot plug - see Fig 1B. These have a longer heat flow path, so the plug runs hotter. This helps to vaporise the fuel/air mixture around the electrodes and so aids proper combustion, at the same time reducing the tendency to fouling.

When fitting spark plugs it is important to fit the correct sealing washer, as this aids heat transfer from the plug body to the cylinder head. Also, always use a proper spark plug spanner. This will have only a short tommy bar to avoid overtightening.

Having ensured we have a good spark, it is important that it is delivered to the cylinder at exactly the right moment for the conditions under which the engine is operating. In Part 1 we mentioned that this is usually a few degrees before TDC in order to ensure the fuel is burning fully as the piston starts its power stroke. However, as the engine speed increases, the spark needs to occur earlier or it will not have finished burning by the end of the power phase, thus losing efficiency. Or, when cruising with a low throttle opening, the weaker mixture will take longer to burn, therefore this, too, requires the spark timing to be "advanced". Conversely, when the engine is labouring at low revs, the spark needs to occur later, or be "retarded" for maximum efficiency. Modern engines have sensors and electronic management systems to control the timing precisely according to the operating conditions, but up to the early 1930s, most ignition control was done manually by the driver, who was provided with a lever which turned the distributor body relative to the cam, see Fig 2, thereby advancing or retarding the spark, or ignition, timing.

To eliminate this chore, which required a high degree of skill by the driver to get the best out of the engine, automatic systems were introduced. First came the Mechanical Advance, which was built into the distributor below the contact breaker assembly. It consists of two flyweights pivoted on a driving baseplate. See Fig 3. The weights are linked to the contact breaker cam by two small toggles. Two springs, each connected between the flyweight pivot and toggle, pull the weights inwards. This is the "fully retarded" position - See Fig 3A. As the engine speed increases, the weights fly outwards against the pull of the springs, and the action of the toggles moves the cam in the direction of rotation, thereby causing the spark to occur earlier, or "advancing" it - See Fig 3B. The degree of advance is relative to the engine speed, the maximum being limited by pegs in the weights which locate in two holes in the baseplate. As you can appreciate, there is movement of the cam unit on its pivot shaft, and it is important that this is regularly lubricated. There is a felt pad in the recess below the rotor arm, and this should be given a drop of engine oil when the rotor is removed for maintenance of the CB points.

To cater for the additional control needed for the different engine load conditions, a Vacuum Advance mechanism was introduced. See Fig 4 A diaphragm operated rod is connected to the distributor body, or in later distributors, to the contact breaker mounting plate, and the vacuum side of the diaphragm is connected to the engine inlet manifold, usually just on the intake side of the throttle valve. This is so that the ignition is fully retarded at tickover, the vacuum, which operates against a spring, only coming into action when the throttle is partially open. During cruising conditions the vacuum will be high and will pull on the rod, thereby rotating the distributor body, or contact breaker plate, relative to the cam, thus advancing the spark, or ignition timing. As the throttle is opened under load, the vacuum reduces and the spring moves the rod outwards, thus retarding the ignition timing.

So now, with the weights controlling the timing according to the engine speed, and the vacuum controlling it according to the load conditions, it is possible to have the optimum ignition timing for any combination of operating conditions, automatically.