I am working on a plasma ignition for engines. I use a capacitor discharge and need help finding the right capacitor for the job. I am not sure what value I need. The system uses 110 vac and the capacitor will be discharging every time a cylinder fires. On an 8 cylinder engine at idle that would be 13 times per second. at wide open throttle it could be as much as 100 times per second. I have used a 90uF 250vac cap in the past with good results. Should i use a 120 vac instead and does the 90 uF cap fit my 13 to 100 cycles per second load?

 

CD ignitions work by enabling a sharper current pulse through the ignition coil primary. The capacitor stores a charge from a charge pump, so it functions as a source to drive current through the coil primary when electronically switched. The charging rate is fixed, while the discharge rate is variable.

Keeping a voltage overhead in the capacitor is important. The 250 volt rating is good. The capacity of the capacitor is significant, as it supplies pulses of current at a rate that varies. But, as long as the voltage on the capacitor does not sag significantly at high RPM, then it may be considered to be large enough. If you want to change the capacitor, maintain the voltage rating and increase the capacity.

 

I am not using the capacitor on the primary side of the coil. The original 12 volt coil stays stock. i am adding a second voltage on the opposite side of the spark gap. It is referred to as a dipole. The second voltage is applied to ground using 120 vac. I am trying to match impedance to the load. The load is what I am having trouble determining.
I tried a new cap today of 130-160 uF 120 vac and it works very well. I have some higher value caps on order. Now my problem is overheating a resistor on the output of the cap. I am using a 10 ohm resistor (wirewound) 10 watts. The resistor is necessary to keep from destroying the sparkplug. Any ideas for a resistor that can handle the load? I have tried small motor windings that have about the same resistance, but they get very hot too.