How CDI Ignition System Works On Motorcycle? Check This Out

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In the previous article we have discussed the magneto ignition system, where this system is used on small gasoline engines such as on motorcycles.

In addition, there is an ignition system on the motorcycle that is more effective than magneto.

That is CDI (capacitor discharge ignition).

Then what is CDI? How does it work ? and what's the difference with the magneto ignition system? let's discuss together.

CDI Ignition System Definition


CDI consists of two main words, namely capacitor and discharge.

So, the CDI system can be interpreted as an ignition system that utilizes discharging current on the capacitor component.

What is capacitor?

Capacitor is an electronic component that can store electric current, and can release (discharge) the stored current spontaneously.

In conventional ignition systems such as magneto or battery ignition systems, the use of capacitors is only to prevent sparking at the breaker point. But on CDI systems, capacitors have a major role.

The release of current on the capacitor serves to induce the ignition coil.

This is the first difference between conventional ignition systems and CDI. In conventional ignition systems, induction of the coil occurs when the primary current is cut off. But on CDI, induction actually occurs by releasing more current to the primary coil coil.

Please note, the discharge current of this capacitor can reach 400 volts so that the output voltage can still be increased up to 20,000 V by an ordinary step up transformer induction.

For more details, see the diagram below


Components marked above
  • Alternator, as a source of electric current
  • Ignition switch
  • CDI Unit, in which there are several components such as capacitors, series of diodes, SCR (silicon controlled rectifier)
  • Pick up coil, to find out ignition timming
  • Step up / ignition coil transformer, used to increase the secondary voltage up to 20 KV.
  • Spark plug


The alternator used in the CDI ignition system can produce AC voltages between 100 - 400 V. This current is used as the basis for the induction process.

Then how is the process going?

1. Electricity generation on the alternator

Starting when the engine crankshaft is rotated, the energy to rotate the crankshaft get from mechanical energy (kick start) or electric (motor starter).

Previously, in the alternator there were two main components, namely the rotor magnet connected to the crankshaft and the stator coil located around the magnet.

Crankshaft rotation will rotate the rotor magnet on the alternator, the result is the stator coil produces AC voltage. The voltage produced by this alternator ranges from 100 V (low RPM) to 400 V (high RPM). This electricity is then used as an energy source for the ignition system.

2. The process of charging the capacitor

In this process, electric voltage from the alternator is flowed into the CDI unit, inside the CDI unit there are several parts such as;

  • Diode, serves to change the AC current into DC
  • Capacitor, as a storage and electric discharge
  • SCR


The voltage from the alternator will be directly sent to the capacitor, but because the current is still in the form of AC (alternate current), it needs to be changed to DC (direct current).

Therefore, before the current enters the capacitor, it will pass through the diode.

After passing the diode, the current is change from AC to DC and is directly used to charge the capacitor.

3. The discharging capacitor process

The capacitor can release a voltage that was previously stored, if there is no current source that hits it.

Therefore, to make the capacitor discharge its voltage, the primary voltage circuit of the capacitor must be disconnected or at least prevent the voltage from passing through the capacitor.

But how ?

For this matter, we are not talking about breaker points anymore, in the CDI system it is strongly influenced by two components namely the pick up coil and the SCR (silicon controlled rectifier).

Pick up coil is a coil that will conduct electric waves when there is a magnetic field passed it. That means, the pickup coil must be equipped with a magnet as a triger. The pickup coil is mounted near the rotor magnet, while a small magnet is connected to the crankshaft where the angle adjusted to the engine's timming.

When the crankshat rotates, this small magnet will rotate and each rotation this magnet will induce pick up coil oncel. When this process occurs, the pickup coil will generate electrical waves which are sent to input 2 on the CDI unit.

Then the electrical wave from the pickup coil is sent to the SCR. The function of the SCR is to activate the path from the alternator directly to ground.

For this SCR to work, a triger is needed. This triger is provided by the coil pick up. When an electric wave from the pickup coil enters the SCR, it will ignite the SCR to active.

As a result, the voltage from the alternator will directly flow to the ground because the nature of the electric current is always flowing to the ground with the smallest resistance.

When the voltage flows to ground, there is no more supply to fill the capactitor and this makes the capacitor also release its voltage.

4. Induction process on ignition coil

In the circuit above, it appears that the capacitor is connected to the primary coil. When the capacitor releases the voltage, there will be massive voltage flow to the primary coil suddenly, the voltage can reach 400 V.

Through induction on the ignition coil, a primary voltage of 400 V can change to 20 KV on the secondary coil. Then the voltage from the secondary coil is connected to the spark plug.

Induction on the ignition coil will occur for a moment because all voltage in the capacitor will be released at one time. That is why, spark on the spark plug is also only for a moment.

To do the induction in the next cycle, the capacitor need to charge back. It do by turned off the SCR so that the voltage from the alternator can flow back to the capacitor and chagrge it.

The SCR will be active and periodically inactive according to the triger of the pickup coil.

As explained above, the SCR will work when there is an electric wave coming from the pick-up coil while the pick-up coil will generate electric waves when a small magnet passes through the pick-up coil. When the magnet is pulled away it will automatically eliminate electric waves and the SCR is deactivated.