Capacitive ignition system

A high-frequency inverter boosts voltage to around 500 volts. This voltage is stored in a capacitor. If the EFIgnition gives a signal to the ignition module, the capacitor will be discharged over the ignition coil. This creates a spark plug with a very high voltage.

Benefits:

  • This spark will be able to ignite very flammable mixtures.
  • The charging time of the ignition coil is very short.

Cons:

  • The spark ENERGY is lower than that of a standard inductive system
  • Sets high demands on high voltage cables and components
  • The inverter is pricey

Because the charging time of the capacitor and ignition coil is so short, a spark can occur shortly in succession. For example, multiple times in the same stroke on the same ignition coil. This way you know for sure that the mixture will ignite.

But there is another advantage. If we use an enhanced V8 as an example. We run this engine 6000 rpm and we use a distributor ignition, then we run into problems with an inductive charging system for the ignition coil. The normal loading time of a conventional ignition coil is approximately 2.3 milliseconds. The spark duration is approximately 1 millisecond.

6000 revolutions per minute / 60 seconds = 100 revolutions per second.

Dividing 1 second by 100 rpm gives 10 milliseconds per revolution.

Because it is a four-stroke V8 engine, we need 8 sparks per 2 full engine rotations.

20 milliseconds / 8 cylinders = 2.5 milliseconds

However, we agreed that for a strong spark, we needed 2.3 millisecond of charging time and 1 millisecond of sparking time. This amounts to 3.3 milliseconds. This time is not there! The ECU reserves time for the spark and therefore reduces the loading time of the ignition coil. The ignition coil is therefore loaded less strongly, namely with only 1.5 milliseconds. The spark will therefore become considerably weaker.

The solution is a cappacitive system. This can charge in less than 1 millisecond and the spark duration is also much shorter. You then have more than enough time to build up a good spark. So in fact only with distributor systems with 6 or more cylinders does capacitive ignition make sense.

MSD 6A

 

 

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