ignition theory

The Components:
In the standard system the ignition circuit consists of spark plugs, plug leads, a coil, a set of points (also known as contacts), a condenser, a points cam and springs and weights that control the mechanical advance system.

Operation:
The coil is the central component in the ignition system. The ignition coil contains two separate circuits the low tension circuit (LT) which usually contains between 100 and 150 turns of heavy copper wire wrapped round an iron core and the high tension circuit containing anything from 15,000 to 30,000 turns of fine copper wire also wound round an iron core. The two circuits are often referred to as the primary and secondary ‘sides’ of the coil. These circuits are placed very close to each other in order to induce a magnetic field, but are insulated so that they do not touch. The current flow in the coil produces a lot of heat and so the coil is filled with a special oil to provide cooling.
In effect, the coil is an amplifier that raises the voltage from the 12.6 or so supplied by the battery to as much as 40,000 volts at the spark plugs. The low-tension circuit wire goes into the coil through the positive terminal, across the primary windings to the negative terminal and is earthed through the points.

When the points open the circuit is broken and the current in the primary circuit is shut off. This causes the magnetic field in the coil to collapse and induces a high voltage in the high-tension circuit of the coil which is released through the coil output terminals (the big ones the leads go onto), along the ignition leads and to the spark plugs.

The points themselves consist of two parts; a fixed part and a spring loaded moving part, both of which have a matching contact faces. The spring-loaded part has a plastic ‘heel’ on it that rides on the points cam. The points cam is driven off the end of the camshaft and is oval in shape. As the camshaft rotates the points cam rotates and (because it is oval) this causes the moving contact of the points to move away from the fixed contact, breaking the circuit. The spring forces the moving contact onto the points cam at all times and it therefore follows the shape of the cam, opening and closing the contact between the fixed and moving parts of the points. A condenser is included in the circuit and this is a large capacitor to ensure that a spark does not jump between the contact surfaces on the points – without this the points would burn out very quickly.

Ignition Timing:
Without going in to a long treatise on ignition timing here’s the layman’s guide to what goes on. Ignition timing refers to the point during the combustion cycle at which the spark plugs fire, and is expressed in degrees of crankshaft rotation in relation to the top dead center (TDC) position of the pistons. Starting at the end, the idea is to have the spark plug ignite the air/fuel mixture just before the piston reaches TDC. The mixture takes a little while to ignite fully and the maximum force is therefore applied on the piston just after TDC to push it back down the bore. Timed in this way the engine will ignite the majority of the fuel contained in the mixture and provide maximum power and efficiency.

All of this is very important because if ignition begins too late (retarded), the piston is already moving down the cylinder and the pressure on it generated by the expanding fuel is reduced. On the other hand, if the ignition timing is too early (advanced), the fuel begins expanding before the piston reaches the top of its stroke. The pressure caused by the expanding fuel pushes down violently on the piston and causes detonation. Detonation is commonly known as pinking and is that funny high pitched rattling noise you will hear when the engine is under load.

Detonation caused by too much advance can cause serious engine damage. The excess pressure will also blow by the seal between the barrel and head and cause poor compression as well. Bad news either way so we need to ensure the timing is right.

Static Timing:
At idle, the spark plug should be ‘triggered’ at 8 degrees before TDC on 602cc engines (12 degrees on old cars).

Dynamic Timing:
As engine speed rises piston speed goes up but the burning and expansion rate of the fuel stays the same. To compensate for this, ignition of the air/fuel mixture needs to occur earlier so the fuel can be ignited at the correct point.

In our case, this advance is provided by the springs and advance weights located behind the points box. There are two of these with two eyelets each. One eyelet on each weight is attached to the peg on the points cam and the other eyelet on each is attached to a similar peg on the end of the camshaft. As the engine revs rise these weights are thrown outwards by centrifugal force, ‘twisting’ the points cam in relation to the camshaft, resulting in more ignition advance. The operation of the points remains the same but the ‘signal’ (contacts opening) from the ignition circuit to fire the plugs arrives a bit earlier. There are stops provided in the mechanism to limit the amount of advance but (depending on the condition of the mechanism) anything up to an extra 30 degrees of advance can be provided.

Dwell:
The contact gap defines the dwell angle which is the period where the two faces of the contact are closed, generally expressed in degrees. The angle is very important, as it is during this period that current will flow through the low-tension side of the coil, building up the energy that is going to ignite the spark plugs. If the gap is too big then the dwell is reduced and the contacts will be closed for too short a time. The voltage in the high-tension circuit will be reduced and the plugs will not produce as good a spark. If the gap is too small the high-tension voltage will be too high and will cause the coil to heat up, reducing its efficiency to provide a good spark. There is also a small effect on the timing – the timing becomes retarded as the contact gap decreases (dwell increases) and vice-versa.

Practice:
OK, so that’s the theory dealt with, what about the practice. The standard system works very well with unworn components and if it is set up correctly, but there are a number of problems that can occur:

1. The heel on the moving part of the points wears out and the contact gap closes up causing an increase in the dwell angle and the resultant loss of energy in the high-tension side of the coil. The most common symptoms are poor starting, performance and fuel economy.
2. The condenser can go faulty and this causes arcing (sparking) across the two faces of the contact. This sparking causes the two faces to become uneven with high and low spots. This results in a poor dwell angle and in extreme cases the two parts can be welded together. Common symptoms are very rough running, or no running at all.
3. The points cam can wear unevenly. This means that the timing to each of the cylinders will be different. This normally manifests itself as a very uneven idle.
4. The springs and weights of the advance mechanism can wear out and upset the timing of the vehicle. Typically the wear causes them to spin out too quickly and supply too much advance throughout the rev range. It is also possible that they could stick and not provide the full advance required at higher revs. In this case the ignition system will be retarded and cause poor performance and fuel economy.
5. The main problem is that the whole mechanism is located behind the fan and adjusting or replacing faulty items is both time consuming and difficult to set up correctly if you don’t have much experience on how to do it. This means that it often get ignored until an actual fault develops.
   As you can see there are a number of areas that can cause problems if the system is not maintained correctly. If you would like to know how to get it perfect see the detailed instructions contained elsewhere in this section.