Capacitive discharge ignition systems are typically abbreviated to CDI. Instead of using the inductance of the ignition coil to store energy, the 12V supply from the battery is stepped up to around 400-500V and this is stored in a capacitor. To fire a spark, the energy of the capacitor is released into the ignition coil that simply acts as a transformer to step the voltage up.

The big advantage of CDI is that the capacitor can be fully charged in a very short time (typically 1ms). This means that CDI is well suited to application where insufficient dwell time is available (i.e high-revving V8 engine using a single distributor).

Although the energy of a good inductive system can be equal to that of a CDI, there is one other consideration. The CDI may release the same amount of energy, but it does so over a much shorter duration. This means that the CDI has a more intense spark, but it does not last as long. This has both advantages and disadvantages.

The short spark duration is not good for lighting relatively lean mixtures as used at low power level. As a result misfire may occur. To help this problem many CDI ignitions release multiple sparks at low engine speeds. This multiple spark discharge method is where the company MSD got its name. The idea is that if the engine misfires on the first spark then one of the subsequent sparks should ignite. Another approach is to use an inductive ignition at low rpm and switch to a capacitive ignition at high rpm (as claimed by the HKS TwinPower).

The intense spark of a CDI is very good at igniting mixtures under high loads. This makes CDI well suited to firing a plug under very high levels of boost pressure, or with water injection or overly rich air/fuel ratios.

CDIs do generate very high levels of electromagnetic noise and this is the main reason why CDIs are rarely used by automobile manufacturers. However if good wiring loom layout and other suppression techniques are observed then CDIs may be used with any engine management system with very good results.

CDI coils
As mentioned earlier, CDI coils do not use the inductance of the coil to store energy. Therefore a good CDI coil has very different properties when compared to a good inductive ignition coil. Very low inductance coils may be used to produce the most intense (but shortest duration) spark. Using a conventional inductive coil with CDI releases the spark energy over a longer duration which can be a good or bad thing depending on the application.

The Ultimate CDI System
As with inductive systems, the ultimate CDI systems use a single coil per cylinder. These coils can be driven quite hard has they only have to fire once per 720 degree engine cycle and have time in-between to cool. Unfortunately, this requires a multiple channel CDI which doesn’t come cheap. Typically it is this added cost which sways many of our customers towards a well-setup inductive system.