When it comes to engines, mechanical components like pistons, rods, cranks, carbies and cams tend to get all the glory. However, none of these are much use without an ignition system to initiate the combustion process and set everything in motion. In this feature we’re going to cover the theory and workings of various styles of ignition systems.

First published in the April 2005 issue of Street Machine

PLUGS

While there are a number of different types of ignition system, the spark plug is common to them all. Forcing a spark to jump the air gap between the centre electrode and the body of the plug is the whole point of any ignition system. All plugs have metric threads and the same basic design: a central metal core (the electrode, connected to a high tension lead) surrounded by a threaded metal body (usually with a tip attached to its end), with a heavy porcelain-like insulator separating the two.

Plug gaps are extremely important — the larger the gap, the more fuel will pass through it and the more effectively combustion will begin. However, larger plug-gaps also require more electrical energy to get the spark to make the trip. Higher compression ratios (and mixtures that vary from stoichiometric) also require more energy to create a spark. Plugs come in various heat ranges, determined by the shape of the ceramic insulator.

LEADS

As with so many things automotive, high-tension leads are a compromise. They need to pass lots of current (to help the spark jump the plug gap), which calls for low resistance. However, low resistance generally means high radio interference — severe enough to prevent you from listening to your radio. All cars (even Top Fuelers) now run suppression leads.

To suppress the unwanted RFI (Radio Frequency Interference), suppression leads use a thick layer of silicon insulation surrounding a current-carrying wire, which is tightly coiled and then spirally wound around a magnetically permeable material. The fields of the two windings cancel each other out to create a lead that features both low resistance and high suppression of radio interference.

COILS

There are quite a few different types and brands of coil. However, for all intents and purposes they work similarly enough to be covered by one explanation. A coil relies on two electro-magnetic principles: first, when an electric current passes through a conductor (wire) it creates a magnetic field (flux) around that conductor.

Switch off the current flow and the magnetic field collapses, creating a changing magnetic field. And second, when a conductor is placed within a changing magnetic field, a current (voltage) will be induced in that conductor.

Here’s how these two principles work together in the case of an ignition coil. A car’s coil consists of two windings (a primary and a secondary) wound very close to each other around an iron core. The primary winding is charged (creating a magnetic field), then the current is quickly switched off, causing the magnetic field to collapse. The primary’s quickly collapsing magnetic field induces a current in the secondary. Both windings are formed from lacquer-coated wire; the primary is quite thick and usually has a couple of hundred turns. The secondary winding consists of thousands of turns of extremely thin wire. It’s this dramatic difference in the number of turns (the turns ratio) which creates the secondary’s extremely high output voltage — up to 70,000V.

TIMING AND DWELL

For optimum combustion (whether for maximum fuel economy, emissions or outright power), it’s imperative that the spark fires at precisely the right instant — as little as 0.005 of a second has a dramatic effect on the combustion process. Determining the right instant is called ignition timing and is measured in degrees (before top dead centre) of crankshaft rotation.

Firing it earlier is known as advancing the timing, while retarded timing is firing it later. Dwell angle is the amount of time (measured in degrees of distributor shaft rotation) the coil is given to charge up. While not determined by dwell angle, ignition timing is affected by it as a longer dwell period forces the plug to fire later, which retards ignition timing.

WHAT’S THE POINT?

The method used to switch the coil’s primary is one way of categorizing ignition systems. For many years, primary current flow was controlled by a set of spring-loaded breaker points, also known as contact points or simply points. There are still plenty of them around but by modern standards they’re inefficient and ideally should be replaced with an electronic set-up. Their main benefit these days is in helping convey the concept of how ignition systems work in an easily understandable way.

In a points-based ignition system, a cam (with the same number of lobes as the engine has cylinders) is attached to the distributor’s central rotating shaft. The points have a rubbing pad attached to a sprung arm; as a cam lobe passes the rubbing pad, the spring-loaded arm is moved, opening the points and interrupting current flow to the primary winding. The quicker the primary field collapses, the stronger the pulse out of the secondary.

The problem with points is that current interruption isn’t instantaneous because some arcing across the points occurs when the two contacts first start to separate. This arcing also causes pitting and burning of the points, which is why the gap between them needs to be reset periodically (the width of the gap affects dwell angle, hence timing). Ignition timing is adjusted by altering the position of the points relative to the rotating cam.

GETTING SATURATED

When the coil’s primary is fully charged, its magnetic field is at full strength and the coil is said to be fully saturated. In a points system, dwell angle (measured in degrees of distributor rotation) remains constant but as engine speed rises, the time taken to rotate through this arc decreases. Accordingly, the points remain closed for less time, leading to less than full saturation, lower output and a weaker spark. This isn’t a problem at lower revs.

However, if you want to turn your crank to racing speeds, reduced dwell time becomes a big issue. It’s a vicious circle — you want a better, hotter spark the faster you spin the engine! And that’s not the only problem. As engine speeds rise, the rubbing pad on the sprung arm of the points can start moving so fast it begins to bounce across the cam lobes, preventing the points from closing properly, resulting in a disastrous loss of power.

BREAKER BREAKER

One popular solution to diminishing dwell time was to fit a second set of points which were adjusted to close earlier than the first, considerably increasing dwell time and coil saturation at high revs. These are known as dual or twin-point systems. However, with the wide range of superior electronic ignition systems available today, it’s hard to recommend going the dual-point route.

ELECTRONIC SWITCHING

Electronic ignition systems replace problematic points with solid state devices. These electronic systems can be viewed as a two-part set-up in that various low voltage sensing devices are used to trigger beefier solid state devices (such as transistors) which perform the actual switching of the current to the coil’s primary winding.

These solid-state devices can switch a greater primary current (for a better, hotter spark) and can switch this higher current faster and more sharply for an even greater gain in spark output. There are three main types of automotive triggering devices: magnetic, optical and computer.

MAGNETIC ATTRACTION

Magnetic triggering devices differ in appearance but they all have three basic elements: a permanent magnet, a coil for sensing changes in the magnetic field and a toothed wheel (called a reluctor) attached to the rotating shaft of the distributor.

The reluctor has as many teeth, or protrusions, as the engine has cylinders and when these protrusions pass corresponding protrusions on the permanent magnet, a change occurs in the magnetic field (above).

This is detected by the sensor’s coil, sending a signal to the main control unit, which cuts power to the coil’s primary. The Hall-Effect (above) is a variation on the magnetic trigger which generates a signal by diverting a magnetic field by means of tangs on a rotating disc. The advantage of a Hall-Effect trigger is that it puts out a constant voltage irrespective of engine speed.

1. Another type of common magnetic triggering set-up. Magnetic triggers create their strongest signals at higher speeds — they are also extremely accurate.

2. Magnetic crank triggers are a popular choice for high-performance race engines like this one from well-known Melbourne engine builder Fred Camileri.

SEEING THE LIGHT

THE signal from optical triggering devices is also independent of speed. These systems use an infrared light that shines onto a light sensor (a photoelectric cell or photoconductor). Interposed between these is a disc with slots cut into it — as the disc rotates it alternately blocks the light, then allows it to pass. The number of slots in the disc equals the number of engine cylinders.

In Mallory’s Unilite system, when the light is blocked from reaching the photoconductor, current is supplied to the coil’s primary. When light shines through a slot and hits the sensor, the accompanying solid state switch-box is instructed to turn off current flow to the primary.

HEI

An evolution of the electronic ignition system is the HEI or High Energy Ignition system. When current is applied to a coil’s primary, current flow (which determines magnetic field) doesn’t instantaneously go to maximum. Rather, due to the coil’s inductance (resistance to changes in current flow), it takes a few microseconds for current to achieve full saturation. Lowering the resistance of the primary winding can reduce this time but doing that dramatically increases current draw, and excessive current draw by the primary is undesirable.

In an HEI system, a coil with a much lower primary resistance is used. But rather than switch the primary fully on, it’s only switched on for one fifth of the time (much nearer to the time the plug fires) required to reach full saturation. While this never allows the coil to reach full saturation, the primary’s much lower resistance allows enough current to flow to create an equivalent magnetic field as a fully saturated regular coil but in a much shorter time. With the coil needing only one fifth of the time to produce the necessary magnetic field, HEI systems can create powerful, high intensity sparks right up to very high revs (over 10,000rpm!). Be warned though, using an HEI coil in a system designed for a regular coil (i.e. continuous current fed to the primary for much longer) will burn out the points or drive-circuitry due to excessive current draw.

CDI

So far, all the ignition systems described rely on the primary coil being charged up via battery voltage. In a Capacitive Discharge Ignition (CDI) system, solid state circuitry is used to bump up the car’s 12 volts to a couple of hundred volts, which is then stored in a capacitor. When triggered, the CDI system discharges all of this stored voltage into the primary coil extremely quickly. The coil now acts as a transformer rather than a storage inductor, stepping the voltage up even higher.

It’s not uncommon to see claims of 50,000-70,000 volts — with easily enough current to deliver a fatal shock. This results in an extremely strong secondary discharge that can punch a spark through just about anything you can cram into a cylinder. A variation of this system fires multiple shots into the primary. Naturally, this produces multiple bursts from the secondary and thus multiple sparks at each plug. However, the number of sparks during each combustion event drops steadily as revs rise and by the time you’re spinning the engine to fairly serious revs you’re back to one spark during each cycle.

ADVANCE AND RETARD

Regardless of rpm, the time it takes for the air and fuel to burn basically remains constant. Therefore as revs increase, the spark plug has to be fired earlier (advanced) so that combustion can be completed before the end of the power stroke. In a regular distributor, centrifugal weights are used to pull the triggering mechanism around to a more advanced position — the greater the rpm, the greater the advance.

Weights and spring tension can be adjusted to optimise the timing curve for a given engine. With lean mixtures (cruising/light load conditions) the fuel/air charge burns slower, therefore the spark must be advanced to optimise economy. This is achieved using a vacuum advance canister to pull the points around to a more advanced position — vacuum cans are often omitted in high performance applications. The control module or ECU does both these jobs in an electronically controlled ignition system.