Hi,
I'll give this a go, but don't take my word for it, this is based on what I've read, seen, and a few assumptions.....
Right, in 'normal' ignition, the 12v from the car bettery is 'chopped' (basically turned on/off, to give it an AC component) this is fed into the ignition coil to produce anything from 20-40kV. Now, this is continually providing HV AC from the coil, and the distributor selects which sparkplug gets the HV.
In 'capacitive discharge ignition' exactly when each sparkplug fires, is down to the EMS (Engine management system). Essentially, it steps up the battery voltage to something like 330-500v DC. So now you have a 500v supply with a small current. This supply is used to charge capacitors, one cap per cylinder (sparkplug). When the EMS tells it to (by sending out a pulse) the cap is discharged through another transformer..(again, one transformer per cylinder)..to produce a very quick, sharp HV spike at 40-50kV. This has several advantages:
1. The timing of the sparks can be very accurately controlled, and modified depending on conditions. This can give constant revs when ticking over. Also, can change the performance of the engine while its running.
2. Because we are not simply switching a HV line to the sparkplug, but charging a capacitor over time, and then discharging it VERY quickly (1us?) The power in that spark is alot greater than in standard ignition....if you've done any chemistry you'll know that that more energy used to start a reaction (initiation energy), the quicker the reaction occurs....better fuel economy and more BHP
3. Also, because of the above....we are not producing 40kV constantly, and only using it when the distributor switches it to a sparkplug...instand we use the power form the battery as and when we need it. It uses less current, more efficient (depending on the design, some seem to use MORE power).
I hope this helps, I'm planning on buying an old car and modding it with this idea, just to see how it runs.
BuriedCode.