The Auto Ignition System

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.

Kettering Ignition – Contact Breaker Ignition

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.

Distributor Cap

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.

Transitor switched Ignition

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.

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


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Electronic / Engine Control Unit – ECU

First of all the ECU (Electronics Control Unit or sometimes referred to as Engine Control Unit, I prefer the latter, it makes more sense, all control units nowadays would be electronic in nature) is a sophisticated beast.  Reading up on ECU systems and failures on the many forums I was staggered to see the lack of knowledge and dangerous opinions and suggestions.  One person had it right though – if you don’t know, don’t touch, leave it to the professionals. Most avid DIY’ers have a multimeter but most do not know how to apply Ohm’s Law i.e. Current = Voltage / Resistance.  It’s no use ducking and diving under the bonnet not knowing what to search for – a multi-meter set to a current setting is going to cause untold damage (short-circuits) and on diode test sensitive electronic devices can be damaged due to the current generated through this test function.

The ECU is essentially a computer – it has inputs (e.g. mouse, keyboard etc) and it has numerous outputs (e.g. graphics, modem, printers etc).  The inputs are in the form of sensors and the outputs drive low impedance/resistance actuators.  The ECU can take it’s input from a potentiometer (variable resistor), thermistor (temperature sensitive resistor), magnetic pickups (possibly a pulse generator), voltage generator and switches (open and/or closed).  The variable instances of a potentiometer or voltage is analoguous whilst switching is digital, either a low or high. The throttle feedback mechanism in the engine management system is analoguous whilst the radiator fan is switched on by a sensor which kicks in or out at a predetermined temperature setting.  All the inputs in an ECU are processed which in turn monitors and controls fuel mixture and firing angle.

Mazda RX-7 Engine Control Unit
Engine Control Unit – Mazda RX-7

This diagram comes courtesy of – in our endeavours to move images over from our previous CMS certain files were lost. In hindsite there are other great websites offering fantastic material – is one of them.

ECU Inputs of a fuel injected vehicle.

Engine Air Flow Sensing
Vane type air flow sensor (Vs) – situated in the air induction system between air filter and throttle. A potentiometer is set by the air flow which in turn has a variable voltage output.

Vane Air Flow Sensor
Vane Airflow Sensor – courtesy autopartsnetwork

Karman Vortex Air Flow Meter (Ks) – measures the frequency of vortices generated as the air flow increases, using a vortex generator, oscillating mirror and photocoupling device. Output is a low to high frequency digitised train. (constant voltage).

Hot Wire Mass Air Flow Meter – A very simple but effective way of measuring the mass of air flowing through the induction process is by putting resistance wire across the air inlet and applying a regulated voltage to it – thereby passing current through the wire. Air passing over the wire will cool it, causing current to increase.  Dense air is cooler as well which also causes an increase in current. Although simpler than the above two sensors they are easier to modify and are cheaper.

Hot wire mass air flow meter
Hot wire mass air flow meter – courtesy
  • Manifold Absolute Pressure Sensing (MAP) – this sensor is usually a piezzo device (bending a piezzo chip changes the resistance) with a vacuum pipe leading to the intake manifold.
    Engine speed and cranshaft angle sensing – along with the engine air flow sensing, the engine speed and crankshaft angle sensors determine angine load.  Commonly known as the Ne (number of engine revolutions per minute) and G1 (crank angle) signals, G1 determines the injection and ignition timing relative to Top Dead Center (TDC).  Both the G1 and Ne sensors work on magentic pickups, situated in the distributor housing.
    Ignitor pulse – the ECU feeds a regulated DC voltage to the spark ignitor circuit through a pull-up resistor, when ignition is sensed a negative going pulse is returned to the ECU – if these pulses are not sensed by the ECU the injectors are shut down preventing flooding and possible damage to the catalytic converter.
    Water Temperature Sensing (THW) – a thermistor, here a negative temperature coeffiecient sensor, (as temperature rises, resistance drops) in the block near the coolant return to the radiator. This sensor allows the ECU to determine fuel enrichment – cold starting requires a richer air/fuel mxture.
    Air Temperature Sensing (THA) – also a thermistor (also negative coefficient) situated normally in the air intake region or air flow metering system.  With air being more dense at lower temperatures the ECU requires an accurate assessment of inlet air temperture to process the correct air/fuel mixture.
    Throttle Angle – this is usually a linear function, showing various stage of throttle opening as well as when either full or at idle. The throttle angle sensor is connected to the throttle valve.
    Exhaust Oxygen Content – the oxygen sensor is located near the manifold collector / exhaust valves to allow for the best possible fuel/air mixes = 1:14.7 which is usually at about 400 degrees Celsius for optimal results (measured manifold temperature).  A 1:14.7 ratio is only applicable under normal running conditions and not when accelerating, loading, engine starting, cold temperatures.  The exhaust oxygen sensor is heated through a circuit driven by the ECU when temperatures are below the optimum 400 degrees Celsius to prevent erratic processing of fuel/air mixtures – i.e. low exhaust gas temperatures.
    Engine Cranking Signal – the ECU allows for a richer fuel/air mixture when it senses that the engine is being cranked. This is an electrical connection made to the ignition switch.
    Engine Knock Sensor – there are quite a few variants of the knock sensor,  the chief result required is to retard timing if and when timing is too advanced.  A typical sensor would be a piezoelectric device mounted on the cylinder head which picks up unusual detonation vibration.
    Altitude Sensing (HAC, High Altitude) – this is an atmospheric pressure sensor which reduces injector firing when at higher altitudes where air is thinner. This prevents mixture from running too rich.

High Altitude Sensor – Barometric Pressure
High Altitude Sensor – courtesy MechToday

Stop Light sense (STP) – when braking, fuel flow is minimised to cut-off or near cut-off.

Next:  Part Two – electronic ignition – 10th September 2011