Engine ignition AC frequency
Here's an image from a PicoScope probe: -



And here's the article - you can even download the waveform file. I'm not saying this fully answers the question, I'm offering this up as a result of 1 minute googling. I'm sure if 10 minutes were spent you might get maximums and minimums etc..

OK 1 minute more and I got what is someone's impression of the above but including a spark line: -



Still from the same website.