Inductive Discharge Coils (STOCK)
Generating the spark on most production automotive systems is accomplished by the coil.
Coils have 2 sets of windings, a primary and a secondary.
The typical coil will have around 250 turns of wire on the primary and about 25,000 on the secondary for a ratio of 100 to 1.
Coil resistance on the primary (winding) will be from .5 to 2.5 ohms usually and on the secondary (winding), between 5000 and 12,000 ohms.
The inductance and resistance of the coil will determine how quickly a coil can be charged and discharged.
Most inductive discharge systems have a spark duration of between 1 and 2 milliseconds.
As cylinder pressure increases, the voltage required to jump the plug gap increases. This is especially true in turbocharged engines under boost. The second problem on high performance engines with high rev limits, is that there is less time to charge the coil with increasing rpm. As such, a high rpm, high output, turbo engine puts greater demands on the ignition system than does a 5000 rpm naturally aspirated engine. Additionally, with a single coil, the more cylinders that you are firing, the less rpm you can run before the spark voltage becomes insufficient to jump the plug gap. A V8 engine would only run to about half the rpm that a 4 would before encountering misfire.
Coil Charge Time and Saturation
The amount of time it takes to charge the coil or bring the current to maximum in the primary windings is called charge time. Input voltage and coil resistance are the main parameters relating to charge time. When the current has reached its maximum value in the primary, it is said to be fully saturated.
If current is applied longer than the time needed to fully saturate the primary, energy is wasted and there is nothing more to be gained. If the current is cut off before saturation is achieved, the maximum spark energy available will be reduced.
Typical coils require charge times of between 2.1 and 6 milliseconds. Obviously, a coil requiring 6 milliseconds to saturate would be unsuitable on a high revving engine as there is not 6 milliseconds available to charge it between discharges at high rpm. For this reason, most performance and racing coils have low primary resistances between .5 and .7 ohms and are fully saturated in less than 3 milliseconds. This permits full coil output at very high rpms.
Most 4 cylinder engines below 200hp/L specific output will run fine below 9000 rpm with a good inductive discharge coil setup.
Capacitive Discharge Ignition (LIKE THE MSD)
on very high output engines,...a single inductive discharge coil is inadequate to supply spark at high rpm and high cylinder pressures. This is where the CD ignition or CDI is used to reduce charge times...
In normal inductive discharge coils, only 12-14 volts is available from the battery to charge the primary.
The CDI charges capacitors to store a high voltage kick to fire to the primary side, putting between 30 and 500 volts onto the primary windings which reduces the charge time substantially.
A coil that would take 3 milliseconds to become fully saturated with 12 volts is now fully saturated in less than 1 with a CDI.
