Ignition type capacitor? You mean the little .1 uF caps used in distributors? That would not be large enough to boost the field supply significantly.
The field draws around 2A, and it needs to be boosted for 1/2 of an engine revolution - about 1/40 s at 600 rpm, That will require many 1000's of uF's.
If we keep the voltage constant, how would a drop in frequency cause visible flicker in an incandescent light?
I said type, I did not specify size really... And it will work and it won't require many 1000's of uF's. The capacitance cancels some inductance in the field winding, it does not make much difference how long it stays in the circuit it's job here is not to deliver a pulse of energy but to continiously cancel winding inductance and increase current flow. As far as size goes, you might need something between a standard ignition capacitor and a motor start capacitor... But it's not going to take something huge. I have seen big ignition type capacitors for aftermarket/performance/race coils back in the 60's and 70's that I think would work fine. Same type, same construction, same voltage rating would work here... You would have to play with the size because too big of one will cause surges in place of your drops.
As for voltage/frequency drops in lighting applications: When you have flicker you are seeing the voltage drop because the frequency is lagging during portions of the engine cycle. If you engineer a solution to boost the field voltage during this portion of the cycle, as we have been discussing, the visible flicker is resolved, what remains unresolved is the frequency sag... The energy looks smooth to the eye, and the eye will not resolve the frequency lag in lighting applications... Good enough for government work, but in a power processing research application such as I intend to run, frequency sagging is unacceptable.
I intend to put high energy physics loads on my equipment. Some drift is acceptable (such as rack changes to accomodate loads), but sagging during engine cycles, every cycle, is going to give me headaches. The other problem with the type equipment I run is that it is even harder to load up than say dropping an air compressor motor and an arc welder on at the same time... Huge inductor masses (in fact I have loaded shorted out arc welders on as ballast)...
I need clean, stiff, energy out of a system that would want to drag the engine around by the nose.
I will post some pics, then you will understand better.