Ignition Systems...How Do They Work?

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« on: December 25, 2007, 08:24:07 PM »
We all know what an ignition system does, but how does it do it? In this article I will explain the basic operation of the two different types of ignition systems: inductive and capacitive discharge (commonly called CD ignitions).

An inductive ignition is comprised of a coil, distributor, and either breaker points or a magnetic pickup type trigger. The inductive igntion receives its power directly from the battery and stores all of its spark energy within the coil.

The ignition key switch typically has 3 terminals on it. They are labeled "BAT", "IGN" and "SOL". "SOL" powers the starting circuit while "IGN" powers the ignition and everything else with "switched power" (i.e. circuits that only receive power when the key is turned to the "On" position, such as the gauges). The "BAT" terminal gets connected to a +12 volt source. This terminal is the moving contact of the switch that makes contact with the "IGN" terminal in the "On" position, and both the "IGN" and "SOL" terminals in the "Start" position.

In the "Start" position, both the "SOL" and "IGN" terminals receive +12 volt power from the "BAT" terminal of the switch. This is so that while the starter motor is spinning, the ignition can receive power in order to fire the engine during starting. When the key is released from the "Start" position back to the "On" position, power is removed from the "SOL" terminal and only the "IGN" terminal receives power from the "BAT" terminal. This turns off the starter, but leaves the ignition on to keep the engine running.

On a points type ignition, the "IGN" terminal runs to a ballast resistor near the ignition coil. The other side of the ballast resistor connects to the + terminal of the ignition coil. The purpose of the ballast resistor is to limit the voltage to the coil so that the coil does not overheat. The (-) terminal of the coil gets connected to the breaker points inside of the distributor. The other side of the breaker points gets connected to ground.

The ignition coil is nothing more than a step up transformer. It takes 12 volts from the battery and steps it up to about 15,000 volts to run the spark plugs. The transformer is comprised of two coils: the primary coil and the secondary coil. The + and - terminals are the primary coil while the center contact of the coil is the secondary coil. The other side of the secondary coil is internally connected to the - terminal of the coil.

When voltage is applied to the primary coil, a magnetic field builds up around the primary coil. The secondary coil passes right through this magnetic field, but is not phased by it. When power is taken away from the primary coil, it causes the magnetic field to collapse. It also generates about a 250 volt spike . This generates a "counter voltage" in the secondary coil that is much higher than the spike voltage, but much lower in current. This is because the secondary coil has much more turns of wire than the primary coil. The turns ratio between the primary and secondary coils is typically around 100:1 (1200 turns of wire in the secondary for every 1 turn of wire in the primary). With this ratio, a primary spike voltage of 250 volts will induce a counter voltage of 25,000 volts (250 volts x 100= 25,000 volts).

In order for a voltage to be induced into the secondary coil, current flow through the primary coil must be interrupted momentarily to cause the magnetic field around the primary to collapse. This is the function of the trigger circuit, which on a points type ignition it is comprised of a set of breaker points. The breaker points are nothing more than a switch that is opened and closed by a cam that rotates inside of the distributor. The breaker points are connected between the - terminal of the coil and ground...they provide a ground path for the - terminal of the coil. When the cam passes over the breaker points, the contacts seperate, which interrupt the ground path to the - side of the coil, which interrupts current flow through the primary side of the coil. This causes the magnetic field around the primary to collapse, inducing the counter voltage into the secondary coil. This voltages gets sent to the rotor contact inside the distributor, which routes the high voltage to the correct spark plug.

Something else the breaker points do is send a trigger signal to the tachometer to tell the tach how fast the engine is spinning. By counting the pulses created by the points opening and closing (the points open and close faster as the engine spins faster), the tach can display how fast the engine is spinning.

On an electronic ignition, the points are replaced by a magnetic pickup. The magnetic pickup provides a ground. Everytime the non magnetic trigger wheel passes through the magnetic pickup, it interrupts the ground to the coil, causing the primary's magnetic field to collapse, which induces the higher voltage into the secondary coil. Electronic ignitions do not use a ballast resistor.

An HEI ignition is probably the simplest ignition to wire. You only have 2 terminals on an HEI distributor: + Power and Tach. The "IGN" terminal of the ignition switch supplies power to the + Power terminal while the Tach terminal gets connected to the "sense" terminal on the tach. All ground connections are internally connected to the distributor housing and receive their ground from the engine block since the negative terminal of the battery is connected to the block.

One of the major disadvantages to an inductive ignition is that all of the spark energy is stored within the coil. Because of this, the engine can reach a certain RPM where the coil doesn't have enough dwell time to recharge itself before the next cylinder is fired. Exactly where this RPM point will be depends upon the type of ignition system used and the specific characteristics of the engine in question. When this RPM point is reached, there is insufficient initial spark energy to jump the spark gap and ionize the air/fuel mixture, and the coil will be unable to fire that cylinder, generating a miss. If you increase RPM further, additional misses will be generated and eventually the ignition system will be unable to fire any combustion chamber. This is called "crash," and engine performance hits a wall.

This is where CD ignitions come in. CD stands for "Capacitive Discharge" and relies on the discharging of a capacitor to power the ignition coil.

On a CD ignition, all of the spark energy is stored within a capacitor. A capacitor within the ignition box stores a voltage of around 460 - 480 volts. A capacitor can recharge a lot faster than a typical ignition coil can, and under ideal conditions can receive its full charge without rolloff up to 15,000 RPMs or so.

We'll use an MSD ignition as our example for the operation of a typical CD ignition system.

On a typical CD ignition, you have a coil and a distributor just like an inductive type ignition, however you also have a box called an Ignition Pulse Amplifier. This box pulls power directly from the battery and has a seperate "turn on" circuit internal to the box that receives 12 volts from the "IGN" terminal of the ignition switch to tell it when to turn on or off.

Inside the box you have a step up transformer. This transformer steps 12 volts from the battery up to around 460 - 480 volts. This transformer charges a capacitor inside of the box. The capacitor discharges this voltage through the primary side of the ignition coil when the trigger circuit (typically a magnetic pickup trigger) tells it to. By sending a higher voltage into the primary of the ignition coil, the secondary coil voltage also increases.

With a turns ratio of 100:1, a discharge voltage of 460 - 480 volts will create a secondary voltage of 46000 - 48000 volts...much higher than that of a typical inductive ignition.

The other advantage to CD ignitions is multiple spark discharge. Up to 3000RPMs, the ignition delivers multiple sparks through 20* crankshaft rotation upon each firing of the ignition. Above 3000RPM the ignition cannot supply multiple sparks fast enough so it only supplies 1 spark above this RPM, but the spark duration remains at 20* at any RPM. Below 3000RPMs, the multiple spark feature reduces plug fouling at extended periods of idle such as in the no wake zone.

The following connection information is for an MSD 6M2 Marine ignition box:

There are a total of 7 connections on a typical MSD box. Two of these are the + and - connections to the battery. These wires are hardwired internal to the box and are seperate from the weatherpak connectors on the box. These wires are a heavy gauge red and black wire. These wires are the wires the ignition system draws power from. If you wish to fuse the red wire, the ignition draws 1 amp per 1000RPMs. Since most jet boat motors don't spin higher than 5000 - 5500RPM, I recommend the use of a 10 amp fuse.

The 6M2 box is to be used with the MSD Blaster or Blaster2 coil.

On the 6 pin weatherpak connector you have 6 wires, along with a single pin weatherpak connector which only has 1 wire. This one wire is your tach signal to go to the sense terminal on the tach. The 6 pin connector's connections are as follows:

Orange: + terminal of coil
Black: - terminal of coil


Red: Ignition "On" Signal...goes to the "IGN" terminal of the ignition switch

Green/Purple Connector: This connector is used for the MSD Pro Billet Distributor, or another distributor with a magnetic pickup. Green - +, Purple - (-)

White: This wire is used for a points type distributor or the MSD Ready To Run Distributor. On the R2R, this wire gets spliced onto the orange wire of the distributor

If you run the MSD box I highly recommend the use of the MSD Ready 2 Run distributor. This type of distributor has its own onboard ignition module and does not require the box, but will work with it. The reason behind using the R2R with the box is for redundancy. If the box fails, it can be easily bypassed and the ignition will work direct from the distributor. You won't get the super high voltage spark, but the ignition will work and the engine will run with some sacrafice in performance.
« Last Edit: December 25, 2007, 08:50:12 PM by Jetaholic »
To err is human, to forgive divine...except for running Fords ;D

F - Found
O - On
R - River
D - Dead


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