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Many modern motorcycles and scooters have a particular type of ignition system known as “CDI ignition.”

In this blog post, you’ll discover more about this ignition system, including:

  • What it stands for
  • How it works
  • The 5 components of a typical CDI system

Prefer to get this information in podcast form? Listen to the 30 Minute Motorcycling Podcast episode about CDI Ignition:

What is CDI Ignition?

The “CDI” part in “CDI Ignition” stands for “Capacitor Discharge Ignition.”

And before you ask, a capacitor (pictured below) is an electronic component that can store energy, but not create it.

A capacitor also can’t store as much energy as a battery.


image of capacitor
But even so, capacitors can store electric energy, and also release it when needed – often quickly. 

It’s this ability that a CDI system takes advantage of.

If you’re curious to find out more about capacitors, here’s a video from the YouTube channel The Engineering Mindset that explains them brilliantly:

Benefits of CDI ignition

The most significant advantage of CDI ignition is that you can easily get a reliable ignition spark.

Compared to an inductive system (i.e., a system that relies on the ignition coil to do most of the work), the charging time for a CDI system is shorter. 

And because there are few mechanical parts, a CDI ignition system is low-maintenance and requires little extra adjustments.

How Does CDI Ignition Work?

The CDI process works like this:

1: A tiny signal current is transmitted from the pick-up or pulse generator coil to the CDI unit, telling it that a spark is needed 

2: The thyristor switches on, and the electricity flows from the CDI box to the primary windings on the ignition coil and further down to the spark plug

3: Once the capacitor has discharged completely, the thyristor shuts off the flow of electricity

So where does the capacitor get its power from?

In many cases, the power to the capacitor comes straight from the battery.

And as such, the Achilles Heel of the CDI system is that is it requires a fully charged battery.

If the battery isn’t charged, there’s no electricity in the capacitors – which in turn means that your bike won’t start.

5 Common Parts of a CDI Ignition System

Depending on your motorcycle or scooter, the CDI system might be different. 

However, most of these components listed below are common on most bikes that use this system of ignition.

1. Alternator or flywheel magneto

Remember – even though capacitors can store electricity, they can’t create it. 

That’s the job of the alternator, or flywheel magneto if you have an older bike with a kick-starter. 

Either way, there’s a small pulse generator or pickup coil fitted close to the flywheel generator or the alternator’s rotating parts, respectively. 

Depending on the design, the pulse generator or pick-up coil is either mounted separately or integrated into the stator plate assembly (i.e. the housing for all the alternator’s non-moving parts).

2. CDI Box

The CDI ignition system’s center is the CDI box, which is often located under your seat. 

This CDI box contains the capacitors themselves as well as other circuitry components, such as

  • Thrysistor
  • An inverter 

The thyristor (also known as the SCR, or “Silicon-Controller Rectifier”) works as an electronic switch that opens and shuts off the flow of electricity.

The inverter converts direct current into alternating current.

Because the CDI box is often black, it’s sometimes called the “black box.”

3. Trigger mechanism

The trigger mechanism is what sends the signal to the CDI box to discharge the stored electricity inside the capacitor.

Once the capacitor is drained, the thyristor inside the CDI box closes and the flow of electricity is cut off.

4. High Tension coil

Sometimes called the “HT coil”, for short, the High Tension coil is integrated into the ignition control unit.

When the HT coil needs the spark to ignite the fuel, it gets the power from the capacitor inside the CDI unit. 

5. The spark plug

True – the spark plug isn’t unique to the CDI system, but it is the final destination for the electric spark.

Once the spark reaches the spark plug (specifically the spark plug’s electrodes), it:

1: “Jumps” across a tiny gap between the spark plug’s center and ground electrodes, 

2: Ignites the compressed fuel mixture during the ignition