Upgrading a vehicle from a contact breaker (CB) point ignition system to a capacitor discharge ignition (CDI) system

 






Upgrading a vehicle from a contact breaker (CB) point ignition system to a capacitor discharge ignition (CDI) system involves replacing the mechanical CB points with an electronic ignition system. This upgrade provides more reliable and efficient spark generation, leading to better engine performance, reduced maintenance, and improved fuel efficiency.

Understanding the Systems:

  • CB Point Ignition System: This system uses mechanical contact points to control the timing of the spark plug ignition. It is prone to wear and requires regular maintenance.

  • CDI Ignition System: CDI uses an electronic circuit to store energy in a capacitor and then discharge it quickly to generate a high-voltage spark. CDI systems are known for providing strong, consistent sparks even at high engine speeds.

Components Needed for CDI Design:

  1. CDI Module: The heart of the system, it charges the capacitor and discharges it to the ignition coil.
  2. Ignition Coil: Steps up the voltage to a level sufficient to produce a spark at the spark plug.
  3. Flywheel Magnet: Provides the trigger signal to the CDI module by inducing a voltage in a pickup coil.
  4. Pickup Coil (Trigger Coil): Detects the position of the flywheel and sends a signal to the CDI module to fire the spark.
  5. Capacitor: Stores the energy that will be discharged to the ignition coil.
  6. Diode: Prevents the capacitor from discharging prematurely.
  7. SCR (Silicon Controlled Rectifier): Acts as a switch that discharges the capacitor when triggered by the pickup coil.
  8. Resistors and Zener Diodes: Used for voltage regulation and protection.
  9. Spark Plug: The end component that ignites the fuel-air mixture.

Designing the CDI Circuit:

  1. Power Supply Section:

    • Source: The power for the CDI can be taken from the vehicle's magneto (alternator) or a battery (if available).
    • Rectification and Filtering: If using an AC source, rectify the AC to DC using a bridge rectifier and smooth it with capacitors.

  2. Charging Circuit:

    • Capacitor Charging: The rectified DC voltage is used to charge a high-voltage capacitor (typically 1µF to 2µF, rated for 400V or more).
    • Diode: A diode is placed in series to prevent the capacitor from discharging back into the power supply.

  3. Trigger Circuit:

    • Pickup Coil: This is positioned near the flywheel magnet. When the magnet passes by, it induces a small voltage in the pickup coil.
    • Triggering SCR: The voltage from the pickup coil triggers the SCR, which discharges the capacitor into the primary winding of the ignition coil.

  4. Discharge Circuit:

    • SCR: The SCR is triggered by the pulse from the pickup coil, discharging the capacitor through the ignition coil.
    • Ignition Coil: The ignition coil steps up the voltage to several thousand volts, creating a high-voltage pulse that jumps across the spark plug gap, igniting the air-fuel mixture in the engine.

  5. Output to Spark Plug:

    • The high-voltage output from the ignition coil is directed to the spark plug, creating the spark needed to ignite the engine's fuel-air mixture.

Simple CDI Circuit Diagram:


Battery / Magneto -----> Rectifier -----> Filter Capacitor
                               |
                         High-Voltage Capacitor -----> Diode -----> SCR -----> Ignition Coil -----> Spark Plug
                               |
                         Pickup Coil -----> Trigger Signal -----> SCR

Steps to Implement the Upgrade:

  1. Remove CB Points:

    • Disconnect the mechanical contact breaker points and condenser from the ignition system.

  2. Install the Pickup Coil:

    • Mount the pickup coil near the flywheel magnet. Ensure it is properly aligned to detect the correct timing position.

  3. Install the CDI Module:

    • Connect the power supply, pickup coil, and ignition coil to the CDI module according to the circuit design.
    • The output from the CDI module should go to the primary winding of the ignition coil.

  4. Replace or Modify the Ignition Coil:

    • If necessary, replace the old ignition coil with one suitable for CDI use. Some CDI systems require a different type of coil.

  5. Connect the Spark Plug:

    • The high-voltage output from the ignition coil should be connected to the spark plug.

  6. Test the System:

    • Once everything is connected, start the engine and check for proper ignition timing and spark strength. Adjust the pickup coil's position if necessary.

Advantages of CDI Over CB Points:

  • Reduced Maintenance: No moving parts like contact breakers, reducing wear and maintenance needs.
  • Higher Spark Energy: Produces a stronger spark, improving combustion efficiency.
  • Better Performance at High RPM: CDI systems maintain spark consistency even at high engine speeds, improving performance.
  • Improved Fuel Efficiency: More efficient combustion can lead to better fuel economy.

Safety Considerations:

  • High Voltage: CDI systems generate high voltages that can be dangerous. Ensure all connections are insulated, and avoid direct contact with the ignition system while the engine is running.
  • Proper Grounding: Ensure the circuit is properly grounded to prevent electrical faults or damage.

By upgrading to a CDI system, you can significantly improve your vehicle's ignition system's reliability and performance, especially for older vehicles originally equipped with CB points.



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Lakshan hettiarachchi



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