ignition system basics

Discussion in 'Homework Help' started by eddy123456, Mar 5, 2008.

  1. eddy123456

    Thread Starter New Member

    Feb 4, 2008
    In its simples form ive found a good website on ignition system :
    2.4 Capacitive discharge ignition system (CDI)
    2.4.1 Components of capacitive discharge ignition system and their function

    Components Functions
    1. Primary coil - generates alternating current
    2. Diode - converts alternating current to direct current
    3. Condenser - charges and discharges current
    4. Thyristor or SCR
    SCR = Silicon
    Control Rectifier - closes/opens the circuit, acts like the contact points of the contact point ignition system.
    The current can flow from A to K only when G is activated.
    5. Pulser coil - generates a current to activate the SCR to open the circuit.
    2.4.2 Function of CDI ignition system
    When starting the engine, the magnetic flywheel will rotate.
    As the magnetic field of the permanent magnet cuts the charge coil, alternating current is induced and flows to the diode. The diode will rectify it into direct current and the condenser will store it.

    As the magnetic flywheel turns until the ignition stroke, its magnetic field cuts the pulser coil and current is generated.
    The generated current will trig the SCR, causing the condenser to discharge its stored current to flow through SCR and through the primary coil to ground. This will induce the high current voltage in the secondary coil which flows to the spark plug causing arcing between the electrodes of the spark plug.


    Question :
    Understanding the principle
    - I don’t quite understand what happens after the permanent magnet cuts the charge coil, doesn’t that means it will cause current flow in the primary coil and hence produce a spark first before the pulser coil is activated?
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    Your images are too small to make out.

    I ran a couple of aftermarket CDI kits. The capacitor charged from an internal inverter through a step-up transformer to about 400 volts. The ignition points were only used to trigger an SCR that dumped the capacitor charge through the coil primary. The discharge was quite rapid, and gave a higher voltage out of the secondary.

    The ignition circuit using a permanent magnet on the flywheel is called a magneto. You don't usually find those except on lawnmowers and aircraft engines. The magnet does cause current in the primary. The high voltage gets generated when the points open and the primary field collapses, inducing a much higher voltage in the secondary winding due to the rapid change. No secondary current flows until the spark occurs.
  3. eddy123456

    Thread Starter New Member

    Feb 4, 2008
    sorry about the images, but if you click on it, youll be directed to the actual size.(heres another image that ive modified, its still small :) )


    In terms of basics i do understand it, but the actual circuit confuses me when i think about the electrons.
    From what i understand, there are 2 important parts :

    1) alternator : charging the capacitor, this capacitor will be use in producing high voltage in the secondary coil.

    2) pulser coil : the signal from pulser coil will turn on the thyristor and a change in electromagnetic field will occur in the ignition coil via capacitor discharging and high voltage will be produce in the secondary voltage.

    The problem that im thinking about is when you consider :

    - if its a 1phase alternator, the pulse would go to the capacitor(charge it) but what happens in the primary coil?(because to charge a capacitor you need a complete circuit, pulse-capacitor-primary coil-ground) since its just a pulse, dont you think it will produce a current in the secondary coil? or is it assumed to be negligible?(although +/-50V from the alternator is quite high)

    - next,in order to discharge the (+side)capacitor through the thyristor and primary coil and then back to the capacitor (-side), dont you think the charge will go to ground?

    - does the charge coil suppose to be 1 phase? if its like a 3 phase which will mean that it will constantly be charging the capacitor, What will happen when the thyristor is triggered?
  4. jpanhalt


    Jan 18, 2008
    There is a ground at the "bottom" of the primary coil and the pulser coil to complete the circuits. I am sure there is inductive coupling to the secondary during charge, but there is also a spark plug gap that interrupts that circuit. The secondary pulse from the charging is not enough to cause misfiring of the plug.

    That's right. It goes from the positive side to ground causing rapid collapse of the primary field, which induces sufficient voltage in the secondary to fire the plug.

    Hope this helps. John

    PS: Just noticed this is homework. Be sure to study the differences betweeen the Kettering and CDI.
  5. ecb123

    New Member

    Mar 6, 2008
    Response for your first post-as the magnet passes the first coil, it induces a voltage across the capacitor-charges it with a dc voltage since diode in series. Then as magnet passes the 2nd coil the scr is turned on-then capacitor discharges into spark coil which fires spark plugs at secondary.