In our previous section we had a look at inductive storage-type ignition systems. In this section we'll move on to the next type of ignition system: the capacitor discharge (CD) ignition system.
Capacitor discharge (CD) ignition systems differ from inductive storage-type ignition systems in that they store ignition energy in a capacitor rather than in an ignition coil. They still use an ignition coil, but the coil is used as a pulse transformer to quickly step up the electrical current.
In a CD ignition system, the primary circuit powers a mini-oscillator or a transformer which charges a capacitor to about 400 to 600 volts and relies on the distributor to trigger the system. The distributor can have either a magnetic triggering system, as in a transistor HEI ignition system, or a light-emitting diode (LED) triggering system. If it has a LED triggering system, a tiny infra-red light beam between a LED and a photo transistor is interrupted by a rotor to produce the triggering signal. When the signal is triggered, the capacitor delivers its stored energy to the coil's primary winding. The coil then acts as a pulse transformer and steps up the current from the capacitor to the 30,000 to 40,000 volts that is required to create a spark across the plug gap.
Advantages and Disadvantages
CD ignition systems have a major advantage over inductive storage-type ignition systems in that it is far quicker to charge a capacitor than to saturate a coil. In fact, only 20 microseconds are required to fully charge a capacitor! Theoretically, a CD ignition system should be good for up to 3,000,000 quality sparks per minute. However, CD ignition systems produce a short-duration spark and rely on multiple ignition strikes to effectively extend the spark duration. This reduces the number of quality sparks a CD ignition system can produce to 20,000 RPM on a 4-cylinder engine and 10,000 RPM on a V8 engine, which is still quite a lot!
In our next section we'll discuss distributorless ignition systems (DIS).