Basic Points Type Inductive Discharge Ignition Systems
Most LBCs use this type of ignition system. Later cars benefit from the miracles of Electronic Ignition systems which are not described here, but most of the same principles apply. One can think of electronic ignitions as “improved points.”
The basic ignition system consists of; the Ignition Coil, Points, Capacitor (aka Condenser), Distributor and Sparking Plugs. A ballast resistor may also be included in this system. Various bits of wire connect all these parts together and move the electrons to the right place at the right time, hopefully. Without the aid of diagrams, the scope of this is limited, but the function of each component is described briefly below, For simplicity’s sake, no formulas will be used, only descriptions of the various aspects.
- Ignition Coil – This is the part that makes high voltage (approx. 20KV for a stock coil, and up to 40KV for a high performance coil) for the spark plugs from the low voltage (12V) that is supplied to it by the car. It is basically a simple transformer operating on the principle of “mutual inductance”. The coil stores up energy over a relatively long (for ignition systems) period of time and then releases it suddenly to the spark plugs via the distributor and HT wiring.
- Coil operation – when the points close, current through the coil primary increases from zero to a maximum value (determined by circuit resistance) in an exponential manner, rapidly at first, then slowing as the current reaches it’s maximum value. The rate at which the current rises is determined by the coil inductance and the circuit resistance. At low engine speeds, the points are closed long enough to allow the current to reach a level limited only by the total circuit resistance, ie, a DC value. At higher speeds, the points open before the current has time to reach this maximum value. In fact, at very high speeds, the current may not reach a value high enough to provide sufficient spark, and the engine will begin to miss. This current through the coil builds a magnetic field around the coil. When the points open, The current through the coil is disrupted, and the field collapses. The collapsing field tries to maintain the current through the coil. Without the capacitor, the voltage will rise to a very high value at the points, and arcing will occur. The time for the field to collapse will also increase. With the capacitor, the current provided by the collapsing field will discharge through it, limiting the voltage at the points, and the current/field will collapse very rapidly, having a discharge path to ground through the capacitor.The coil, capacitor, and resister form a tuned, oscillator circuit. When the coil is completely discharged, the capacitor is completely charged. Now, the capacitor will try to discharge to the coil. Without resistance, there is nothing to limit the coil or capacitor discharge current, and the cycle will repeat, ie, the coil will charge, then discharge to the capacitor, which will charge, then discharge to the coil, etc. With the resistance, however, the current is “dampened,” and the amplitude of the oscillating current is reduced rapidly, dropping to negligible within 3-4 cycles.When the magnetic field of the primary coil collapses, it cuts through the windings of the secondary coil, producing an output voltage. The magnitude of the output voltage is determined primarily by the windings ratio and by the speed at which the primary field collapses. A slow collapse will produce a lower output than a rapid collapse. Until the arc occurs at the plugs, the output of the secondary is nearly an open circuit, allowing the voltage to reach a peak before current is produced. As soon as the spark occurs, the resistance is reduced, and current flows through the plug gap, maintaining the arc. The primary and secondary windings are isolated from each other, so that no current in one flows through the other. However, the secondary is connected to the primary at the point where the primary connects to the points and capacitor, and there is no direct path for the return of the secondary current other than through the capacitor. As a result, the capacitor is part of the secondary as well as the primary. There is an oscillation in the secondary, just as there is in the primary, for the same reasons. By properly selecting the coil/capacitor parameters, the designer can “tune” the circuit to provide the most effective output voltage, as described below.
Typical Ignition-Coil ParametersTurns Ratio 100:1 Secondary 25,000 turns #41 Primary 250 turns #22 Primary Inductance 6 to 10 mH Primary Resistance about 1.5 ohms Secondary Inductance 40 H Secondary Resistance 10 kilohms
- Points – Ignition points are a set of electrical contacts to switch the coil off and on at the appropriate time. The points are opened and closed by the mechanical action of the distributor shaft lobes pushing on them. The maximum amount of (coil primary) current that can be switched by points is about 4 amps. Above this level points burnout may occur.
- Capacitor (Condenser) – The capacitor performs several functions. It prevents the points from arcing and prevents coil insulation breakdown by limiting the rate of voltage