Motorbikes those days used a contact breaker for distributing the spark at right time. But the problem of wear affected the ignition and performance characteristics of the engine and that's why the bike manufacturers jumped towards the electronic ignition system. From early 2000s, electronically controlled ignition systems gained more momentum as the spark is distributed at the perfect timing. This also helped in distributing a spark with more intense and it is achieved with some electronic components such as capacitors, diodes etc.. In this article, let's see how the most popularly known electronically controlled capacitive ignition system works.
Layout diagram
The whole system consists of 3 individual sub-systems such as charging circuit, discharging circuit and triggering circuit. Triggering circuit consists of a small ferromagnetic material and a trigger coil or pulse coil or pickup coil. Charging circuit consists of a magneto unit, capacitor and diode. Discharging circuit consists of a SCR, ignition coil, HT cable and sparkplug.
Firstly, let's see something about the each individual sub-systems. Triggering circuit is present at the motorbike's left side of the crankcase. As we all know, right side of the crankshaft is paired to a clutch assembly where the left side is connected to a magneto unit. Generally, there'll be two parts such as stator and rotor. Coil assembly is a stator and the magnet unit is a rotor. Due to the principle of electromagnetic induction, voltage will be generated which can be utilized. In the magnet or magneto unit, there'll be a small projection of ferromagnetic material. This ferromagnetic material is positioned in such a way it passes the pickup coil. Whenever the ferromagnetic material comes and aligns with the pickup coil, very small amount of EMF will be induced which plays an important role.
Charging circuit consist of a magneto unit which produces power that can be utilized. The voltage produced here will be in the range of 140-200 AC and it is directly sent to the RR unit for regulating and rectifying. After that, it enters the discharging circuit present inside the CDI box. As we all know, capacitor is a charge storage device and it can also discharge. The charged capacitor must not discharge in the charging circuit itself and to achieve this, a diode is present which prevents the charges from flowing in a reverse direction.
In the discharging circuit, there's a SCR, ignition coil and sparkplug. SCR expands as Silicon Controlled Rectifier switch. Basically it's a electronic switch and it'll be at the open position by default. When it is electrically triggered, then the switch comes to the closed position allowing the charges to flow through it. This SCR is directed to the trigger or pickup coil which electrically triggers it. As said previously, a small amount of EMF induced in the pickup coil helps in triggering the SCR. Ignition coil acts as a step-up transformer in which it converts 12V signal into 20,000+V in order to ignite the fuel air mixture present inside the combustion chamber. This high amount of voltage is delivered to the sparkplug via a High Tension cable and plug adapter.
Now, let's see the working. When we try to crank up the engine, magneto unit will also rotate along with the crankshaft. As the ignition key is on, charges starts flow to the CDI box and the RR unit. Inside the CDI box, capacitor will be charged and it is ready to discharge. The ferromagnetic material in the magneto unit is position in such a way it passes the pickup coil when the piston reaches the top of the cylinder that is end of the compression stroke. When the ferromagnetic material in the magneto unit passes the pickup or pulse coil, a small EMF is induced and it is directed to the SCR. Now the SCR is electrically triggered and the circuit closes. Capacitor discharges the voltage and it flows to the primary coil assembly of the ignition coil. Due to the difference in the number of turnings and as per the law of Faraday's electromagnetic induction, the flux in the primary coil transfers to the secondary coil. As a result of this process, high amount of voltage is produced in the secondary coil and it reaches the sparkplug via a HT cable. This high intense spark is enough for igniting the fuel air mixture which is compressed inside the combustion chamber by the piston.
This process repeats continuously as long as the crankshaft rotates and the ignition key is on. I tried to explain the working of the Capacitive Ignition System as simple as I could and this is the basic working. The actual parts and design may vary from manufacturer to manufacturer. If you have any suggestions or corrections or if I conveyed anything wrong, please let me know in the comment section below.
Image courtesy : YouTube/11th Gear
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| CDI Module |
Layout diagram
The whole system consists of 3 individual sub-systems such as charging circuit, discharging circuit and triggering circuit. Triggering circuit consists of a small ferromagnetic material and a trigger coil or pulse coil or pickup coil. Charging circuit consists of a magneto unit, capacitor and diode. Discharging circuit consists of a SCR, ignition coil, HT cable and sparkplug.
Firstly, let's see something about the each individual sub-systems. Triggering circuit is present at the motorbike's left side of the crankcase. As we all know, right side of the crankshaft is paired to a clutch assembly where the left side is connected to a magneto unit. Generally, there'll be two parts such as stator and rotor. Coil assembly is a stator and the magnet unit is a rotor. Due to the principle of electromagnetic induction, voltage will be generated which can be utilized. In the magnet or magneto unit, there'll be a small projection of ferromagnetic material. This ferromagnetic material is positioned in such a way it passes the pickup coil. Whenever the ferromagnetic material comes and aligns with the pickup coil, very small amount of EMF will be induced which plays an important role.
Charging circuit consist of a magneto unit which produces power that can be utilized. The voltage produced here will be in the range of 140-200 AC and it is directly sent to the RR unit for regulating and rectifying. After that, it enters the discharging circuit present inside the CDI box. As we all know, capacitor is a charge storage device and it can also discharge. The charged capacitor must not discharge in the charging circuit itself and to achieve this, a diode is present which prevents the charges from flowing in a reverse direction.
In the discharging circuit, there's a SCR, ignition coil and sparkplug. SCR expands as Silicon Controlled Rectifier switch. Basically it's a electronic switch and it'll be at the open position by default. When it is electrically triggered, then the switch comes to the closed position allowing the charges to flow through it. This SCR is directed to the trigger or pickup coil which electrically triggers it. As said previously, a small amount of EMF induced in the pickup coil helps in triggering the SCR. Ignition coil acts as a step-up transformer in which it converts 12V signal into 20,000+V in order to ignite the fuel air mixture present inside the combustion chamber. This high amount of voltage is delivered to the sparkplug via a High Tension cable and plug adapter.
Now, let's see the working. When we try to crank up the engine, magneto unit will also rotate along with the crankshaft. As the ignition key is on, charges starts flow to the CDI box and the RR unit. Inside the CDI box, capacitor will be charged and it is ready to discharge. The ferromagnetic material in the magneto unit is position in such a way it passes the pickup coil when the piston reaches the top of the cylinder that is end of the compression stroke. When the ferromagnetic material in the magneto unit passes the pickup or pulse coil, a small EMF is induced and it is directed to the SCR. Now the SCR is electrically triggered and the circuit closes. Capacitor discharges the voltage and it flows to the primary coil assembly of the ignition coil. Due to the difference in the number of turnings and as per the law of Faraday's electromagnetic induction, the flux in the primary coil transfers to the secondary coil. As a result of this process, high amount of voltage is produced in the secondary coil and it reaches the sparkplug via a HT cable. This high intense spark is enough for igniting the fuel air mixture which is compressed inside the combustion chamber by the piston.
This process repeats continuously as long as the crankshaft rotates and the ignition key is on. I tried to explain the working of the Capacitive Ignition System as simple as I could and this is the basic working. The actual parts and design may vary from manufacturer to manufacturer. If you have any suggestions or corrections or if I conveyed anything wrong, please let me know in the comment section below.
Image courtesy : YouTube/11th Gear
Related articles
• Fault diagnosis - Ignition system
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