The Auto Ignition System

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The Kettering Ignition system.

This is the tried and trusted, very reliable ignition system still used in many cars today – albeit older models. The Kettering system used a contact breaker, known as “points”, a condensor across thes points (condensor = capacitor, British usage) which switched the +12V through the High Voltage or ignition coil to ground. The points opened and closed on the cam lobe of the distributor which was driven by a geared drive to the cam shaft. Kettering systems were later modified to switch a transistor which in turn switched the ignition coil, reducing current and burn of the contacts of the points. The distributor has a rotor which passed the high voltage from the oignition coil through to the spark plugs.

As these units still used conventional contact-breakers or points which from many engineers’ point of view was still the back-bone of the system and carried with it all the inherent problems with mechanical switches so with semiconductor technology they went through many modifications – So, although advertisers could claim that the system would erradicate contact bouce at high revs, sparking at the contact points and would inprove fuel economy and cold starting, the simpler units didn’t improve matters all that much because they still used mechanical switching and the spark pulse was of very short duration which could be deemed unreliable. My own experience was the early CDI or capacitor discharge ignition systems were reliable and did improve cold weather starting but most importantly point burn was eliminated which allowed longer runs between replacement.

Early ignition systems: The Kettering or IDI (Inductive Discharge Ignition) – the contact breaker ignition system.

Above image courtesy Popular Mechanics magazine. Subscribe to Popular Mechanics magazine by clicking on this link.

The figure above gives an elementary description of the working of the Kettering ignition system. This form of ignition relies on three things – the contact breaker or points, the condenser (or capacitor) and the inductance of the ignition coil. The contact breaker, as it opens charges the capacitor which in turn discharges through the ignition coil primary winding which in turn creates a back EMF which again charges the capacitor. This is a ringing LC oscillator (L = inductance, C = Capacitance) – the energy is dissipated quickly but is of such a magnitude that the voltage at the secondary of the ignition coil is stepped up to about 25KV. Of course the capacitor does not charge immediately, it obeys a rule based on time constant. The capacitor is seen as a short circuit across the contact breaker as the contact breaker opens, the voltage rise may take a few microseconds based on this time constant which means the current through the ignition coil is “slowed down” reducing point burn. For those needing further information do please read up on ringing oscillators, LC circuits, RC circuits and resonance.

In a multi-cylinder engine the camshaft drives a cam in the distributor which in turn opens and closes the points – this is synced with the rotor arm inside the distributor which selects which sparkplug or cylinder firing sequence to follow.

The advantage of the Kettering ignition system is it’s simplicity. The disadvantages are contact burn, condenser failure, mechanical wear.

The CDI or Capacitive Discharge Ignition System.

The early electronic ignition systems used a transistor as a switch to eliminate contact burn . This system worked very well but being what they are automotive engineers went one step further, they electronically generated the high voltages necessary which fed a capacitor to store energy before being released into the ignition coil.

The early systems still used a transistor or thyristor to switch the coil but an inverter cicuit produced a voltage of upwards of 400V D.C. to pulse the coil instead of relying on the back EMF/condenser layout.

The CDI system was an improvement on earlier Kettering designs because fundamentally they were immune to the battery voltage whilst hard cranking (as the theory goes in any event) and did allow for easier running as engine wear took it’s toll e.g. oil clogging and richer fuel mixtures. For high R.P.M. the plugs had a high intensity spark of short duration which manufacturers claimed increased the horsepower. However these units were still fed from the mechanical points which although now becoming more reliable, still succoumbed to wear and also, very importantly still used the mechanical distributor to sequence the correct firing order at the plugs. Distributors had their own fair share of problems – high intensity arcing and eventual breakdown. With turbo charged engines however the original CDI showed a serious disadvantage over the Kettering design – the short duration spark was not ideal in a lean burn situation. The next step was to produce a CDI system which had a multiple discharge (MSD) rate over the firing cycle (patented in 1975). This earlier systemx were prone to wearing spark plugs and insulators more rapidly than it’s predecessor because of the higher energy levels but showed a pronounced advantage over the single short pulse units when tested with leaner fuel mixtures, a big plus in racing circles.

MSD-6AL — Multiple Discharge Ignition – Type 6AL Part # 6420

The Multiple Spark Discharge system was designed to get around the failureof the standard CDI units to burn fuel completely at lower RPM. As a rule the MSDunits are designed only to run multiple sparks at lower RPM, possibly in the magnitudeof 3 000 rpm or lower. Interesting write-up and design here. (*.pdf).

Modern Ignition Systems

The modern system of ignition is tied into the ECU (Electronic or Engine Control Unit). The system no longer has a contact breaker/points and distributor, instead using sensors which act as switches and utilise a coil bank comprising of a coil for each spark plug or even better an ignition coil mounted just before the spark plug which erradicates the use of high voltage cables.

Crankshaft position sensor.
Camshaft position sensor (s).
Electronic control module.
Ignition control module
Knock sensor (s).
Ignition coil (s).
Ignition Rotor and Hall effect switch.

The modern ignition system advances and retards spark by obtaining outputs from the various sensors to determine load and RPM – technology has virtually eliminated the use of moving parts to determine dwell and switching.

To be continued…

Back to Part One – ECU