If you have time and want a good read about cylinder pressure and spark read this:
http://www.castensdynojet.com/insiders.htmHow does an electric spark ignite fuel? Actually, nobody is quite sure. Three long-standing theories exist. The first,
the thermal theory, has been around the longest but has recently lost ground to the other two, as new, more sophisticated tests are run. The thermal theory states that ignition is simply a factor of the heat of the spark, and that it occurs in much the same way as if the air/fuel mixture were lit with a burning match. This theory has been somewhat (although not entirely) discounted by recent data compiled on electronic ignitions, which have the ability to tailor a spark's duration (burn time), intensity (strength, measured in amps) and phase angle to dictate when during the spark duration the highest intensity will be delivered. The ability to control ignition through factors other than thermal energy has led to the metal fragmentation and ping pong theories.
The metal fragmentation theory states that highly ionized electrons rip metal fragments from the sparkplug's electrodes as they migrate across the plug gap, collide with the hydrocarbon molecule (gasoline), and act as a catalyst to the burning process. Supporters of this theory allude to tests that show how variations in the electrode's metal composition will enhance certain ignition properties. As a practical example, Champion sparkplug electrodes with a relatively soft metal composition allow for large amounts of metal fragmentation, which enhance the plug's ability to burn through large quantities of fouling contaminants. This formula is useful for engines that have a fouling problem, although the plug's soft electrodes tend to wear down relatively quickly. At the opposite end of the scale are Autolite plugs, which feature a hardened metal composition electrode with long-wear characteristics for engines that don't have a serious fouling problem.
The ping pong theory states that fuel ignition is accomplished when electrons from the spark collide with hydrocarbon molecules as they migrate across the electrode gap, splitting the positively charged protons from the molecule, which in turn collide with other molecules, triggering ignition. Although fuel ignition may well be a combination of all three theories, recent trends lean toward a combination of the ping pong and metal fragmentation concepts.
