Understanding the Ignition System

[Ed. Note: this was written for Triumph cars, but the fundamentals are the same for most vehicles; even more modern cars with an electronic distributor (hall sensor) works in essentially the same way, with a computer "opening the points" instead of a mechanical switch. Obviously, none of this will apply to very modern "distributorless" ignition systems, although they will still have an ignition coil.]

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.

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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 [dwell] and then releases it suddenly to the spark plugs via the distributor and HT wiring.

Typical ignition coil from a classic car

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 resistor 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 Parameters

Turns 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.

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Capacitor (Condenser)

The capacitor performs several functions. It prevents the points from arcing and prevents coil insulation breakdown by limiting the rate of voltage rise at the points. It's primary function is to provide for a rapid decay of the primary coil current. The capacitor also "third-harmonic" tunes the coil, raising the peak output voltage and increasing the secondary voltage rise time. This increases the efficiency and the amount of energy transferred to the spark plugs. If the coil secondary voltage rises too quickly, excessive high frequency energy is produced. This energy is then lost into the air-waves by electro-magnetic radiation from the ignition wiring instead of going to the spark plugs where we would like it to go. Voltage rise time should be more than 10 microseconds; a 50-microsecond rise time is OK. Conventional systems have a typical rise time of about 100 microseconds.

Distributor

The electrical "traffic cop" which directs the voltages to their proper places at the correct time. It routes the high voltage generated by the coil to the intended spark plug via the HT wiring. The distributor houses and operates the points, and capacitor (described above). The standard points type distributor can produce ignition timing errors in three ways: 1) wear of the rubbing block, 2) variations in the cam profile, 3) shaft eccentricity.

Rebuilt distributor with cap removed (points and shaft cams visible)
4 distributor shaft cam lobes, 1 per engine cylinder.

Sparking Plugs

These are the business end of the ignition system. The spark plugs take the electrical energy provided to them by the rest of the ignition system and turn this into the (hopefully) optimum spark event which ignites the fuel. Proper polarization of the coil is necessary to provide a negatively charged high voltage to the center electrode of the spark plug, which is hotter than the outside electrode. This enables us take advantage of thermionic emission* which reduces the voltage required by 20 to 50% for a given spark magnitude. The plug gap affects both the voltage and the energy required. As the plug gap is increased, the required voltage increases, but the required energy decreases.

*Thermionic emission - (aka Edison effect) The propensity of some metals to give up their free electrons more easily when heated, actually boiling off of the metal. This is the fundamental operating principle of vacuum tubes, once called thermionic tubes.

A spark plug typically receives 20,000 to 40,000 volts from the coil

Ballast Resistor

This is an electrical resistor which is switched in and out of the supply voltage to the ignition coil. It makes the engine much easier to start by effectively doubling** the voltage provided to the ignition coil when the engine is being cranked, compensating for the reduced battery voltage. This provides a much better spark just when the car needs it most. When starting a cold engine, the plugs and the air are cold, the cylinder pressure is up, and the fuel / air mixture is poorly controlled. The oil is thick, the battery is cold and its voltage drops as much as 60% because of the high current drained by the starter motor. It's a wonder the car starts at all.

** Actually, it cuts the voltage to the coil in half when the car is already running, but it's easier to understand the first way. A nominally 6 Volt coil is used in a ballasted ignition system.

Internal coil resistance

Resistance built into the coil, in addition to the inherent resistance in the copper windings, to limit current through the points at idle and low rpm operation. This resistance is not to be confused with the ballast resistor mentioned above. It serves a completely different function. As stated above, at low engine speeds, the primary coil current can reach a higher value than at high speeds. If a coil is designed to provide sufficient output at high speeds, the primary coil current can reach excessive values at low speed. Conversely, if the coil is designed to limit low speed primary current, it may lack sufficient power at high speeds.

One way to provide for both low and high speed operation is to provide a "ballast" resistor in series with the primary winding. This resistance consists of an iron wire coil. Iron has the property of increasing resistance with temperature. At low speed, the high current heats up the iron wire, increasing its resistance, and reducing current. At high speed, the current, as described above, is less, so the iron wire resistance does not increase, thus the current is not limited. There are other design technique available to provide for wide speed variations, so not all coils will use an internal resistance.

Dwell Angle

The degrees of rotation of the cam/distributor during which the points are closed. During each rotation of the cam/distributor, the points must open and close once for each cylinder. For a 4 cylinder engine, this allows 90 degrees of rotation for each cylinder, (360/4,) for a 6 cylinder, 60 degrees, and for an 8 cylinder, 45 degrees. For reasons stated above, the points must stay closed long enough to allow the coil primary current to reach an acceptable value, and open long enough to discharge and produce a spark. Typically, the ratio of closed to open is on the order of 3 to 1, ie closed for 45 degrees and open for 15 in a 6 cylinder engine.

This really only scratches the surface of ignition systems. You have been spared topics like Cylinder pressure, Ignition-voltage waveshape, Timing, Capacitance, Inductance, formulas with Greek letters and other technical mumbo-jumbo. But then there's always part 2...

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Photos courtesy Nathanael Burton and and Nick Ares, used under the terms of of the Creative Commons Attribution-ShareAlike 2.0 Generic license.