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Have you ever wondered how a CDI system works for a motorcycle or scooter?

Do you even know what it stands for?

Tune in to discover:

  • What the “CDI” in “CDI ignition” stands for
  • 5 common parts of a typical CDI ignition system
  • How a CDI system works


Hello, and welcome to another episode of 30-minute motorcycling – a podcast for those riders who are at the beginning of their own Dual Wheel Journey, where you’ll discover something new about how your motorcycle, moped, or scooter works – in less than 30 minutes.


Have you ever looked at the side panel of your scooter or motorcycle and seen the letters “C” “D”, and “I?”


If you have, you’ve got a common ignition system, but how does it work?


In this episode, we’re going to cover what a CDI system is, how it works, and the 5 most common parts of a CDI ignition system.


First of all, what does “CDI” in “CDI ignition” stand for?


It stands for “Capacitor Discharge Ignition.”


And before you ask, a “capacitor” is an electronic component that can store a small amount of electricity. It can’t store as much as your battery, but it can store enough to release electricity nearly instantly.


This is the principle that CDi ignition uses – it uses the capacitor’s ability to store and release a reliable ignition spark.


Not only that, but since a CDI system has so few moving parts, it’s also low-maintenance.


A CDI system can vary depending on the bike’s design, but there are generally 5 common components:

  • The CDI box
  • The alternator or flywheel magneto
  • The trigger mechanism
  • One of more High tension coils
  • The spark plug


Let’s start with the CDI box. Also called the “black box,” the CDI box houses the capacitors themselves and any other circuitry components, such as a thyristor and an inverter.


Also known as the SCR or “Silicon-Controller Rectifier,” the thyristor can best be described as a switch that shuts the flow of electricity on and off.


As for the inverter, it converts direct current into alternating current.


Simple – but where does the capacitor get its power from? 


In many cases, the power to the capacitor comes straight from the battery. As such, a CDI system relies on a fully charged battery – otherwise, there’s no electricity in the capacitor, which means that your bike won’t start.


The second common CDI component is the alternator or flywheel magneto. 


Remember – a CDI box can store energy, but it can’t generate it. That’s where the alternator or flywheel magneto if you’ve got an older bike with a kickstart mechanism comes in.


Depending on which setup you’ve got, the alternator or flywheel generator will have either a small pulse generator or a pick-up coil fitted to the flywheel generator or the alternator’s rotating parts, respectively.


The pulse generator or pick-up coil can be mounted either separately or integrated into the stator’s plate assembly – AKA the place where the alternator’s non-moving parts are located.


Next, we have the trigger mechanism. This one’s quite simple – the trigger mechanism is what sends the signal to the CDI box to discharge the stored energy inside the capacitor. 


After the energy is discharged, the thryristor inside the CDI box closes and the flow of electricity is cut off.


Then, we have the high tension coil, also known as the “HT coil,” which is often integrated into the ignition system and receives the power to ignite the fuel from from the CDI box – more specifically, from the capacitors inside it. 


And finally, we have the spark plug. And yes, it’s not unique to the CDI ignition system, but it is the final destination for the spark nonetheless. Once the spark gets to the spark plug, it


“Jumps” across a tiny gap between the spark plug’s center and ground electrodes, igniting the compressed fuel mixture during the power stroke.


So to recap, here’s how a CDI system works:


At first, the pick-up or pulse generator coil sends out a tiny signal current to the CDI box, telling it that it needs a spark.


Then, the thyristor inside the CDI box is activated, and the electricity flows from the CDI box to the high tension coils and further down to the spark plug


And finally, once the capacitor inside the CDI box has discharged completely, the thyristor closes and the flow of electricity is stopped


And that’s how a CDI ignition system works in a nutshell. If you’d like to read more about it, I’ve included a link to a blog post about CDI ignition on The Dual Wheel Journey in the podcast episode description, along with a helpful YouTube video from The Engineering Mindset that explains how capacitors work


Until next time, keep your helmet on and your eyes on the road. Bye!