In 1999, I became the proud owner of a car model DACIA 1310, a clone of Renault 12, built with 1960 technology, with 1400 cm³ on gasoline and clasic carburator. I tried at that time to build a CDI - capacitive dischgarge ignition with multispark circuit. It worked for a couple of months. For testing, I used components with normal temperature range (0 to +70 °C), but for final release it is mandatory to use extended range version of the parts (-40 to +125 °C). Unfortunately, at that time, I could not find such components in usual electronic stores, so I stopped the project in protype phase.

The main advantage of CDI is the very fast risetime of the spark, ideal for firing fouled plugs, which is probably why they are almost standard on many engines where oil contamination of the spark plugs can be an issue. The amount of energy delivered in the spark is often higher than the amount of energy delivered from a conventional coil and contact breaker (points) system, but it is delivered in a much shorter time period.

My project uses an oscillator with about 20 kHz frequency, 50% duty cycle, producing alternate switching of two MOSFET transistors. This switching performs a push pull reaction over the connected transformer wherein the two halves of the winding are saturated alternately with the MOSFTs conduction, which results in pumping of the 12V DC across the two half winding of the transformer.

Specification:
- power supply: 12 V from car battery
- input signal from platinum distributor
- minimum modifications to car circuit
- oscillator frequency: 20 kHz
- capacitor charging max voltage: 250 V

The schematic diagram shows the DC/DC converter, signal conditioning for ignition ruptor input, 2.5 V reference circuit, discharge circuit with thyristor and a capacitor for discharge on ignition coil. Transformer has been built on ferite core (from an old power charger), with two coils of 13 turns 0.6 mm and one coil of 360 turns 0.3 mm enameled copper.

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Links:
[1] - IRFI540N datasheet
[2] - CD4047B datasheet
[3] - CD4001B datasheet
[4] - LM293 datasheet
[5] - LM285 datasheet
[6] - VS-25RIA datasheet