It's true that few folks have commented on the facts, so I will.
You used a textbook application of Newtonian physics, converting potential (well, really chemical) energy to kinetic energy. And it was VIOLENLTY WRONG, about what I'd expect from a green freshman just as energy conservation was introduced. It describes the motion of a mass on an ideal massless spring, or dropping a rock in vacuum. It's correct for orbital motion. It's IRRELEVANT here. It's not even correct for the closely related and highly simplified case of a cannon (also known as a cylinder with no connecting rod). And here's why.
It's not a closed system. Energy leaves the cylinder via direct heat conduction (into the oil and water jacket), through the exhaust, and through frictional losses at the rings, connecting rods, and crankshaft. The connecting rod PREVENTS some of the energy from being converted to mechanical energy, in favor of heat. Virtually all gasoline engines runs at about half perfect thermal efficiency (I've done the calculation -- have you?). Energy flow through the radiator, exhaust, and crankshaft are all approximately equal for a conventional gasoline engine. While you might affect that with a BAD spark plug (not in the direction you want), you're strictly bounded above (and not at all far above) by how much you can improve. Thermal efficiency is less than perfect for a number of reasons, not the least of which is geometrical constraints. The piston's location is determined by the rotating crankshaft, and it doesn't line up with the constant-energy lines you would need for perfect thermal efficiency.
Sorry, you get a D.
Also, , if you think the spark contributes significantly to cylinder heat, why do you suppose no one has invented an internal heating engine that works that way? No need for fuel; just heat the air with a spark running off a battery.
And since you seem to have some high school introduction to energy conservation, you ought to be able to calculate the energy deposited into the cylinder by a 30 kV spark. Make the same calculation for stoichiometric gasoline and tell us if that's significant or not. The match analogy, which for some reason you have dismissed, is entirely apt here. The point of the spark is to light off the fuel, nothing more. There are some internal combustion engines that DON'T HAVE SPARKS (at least, not in normal operation). These include major workhorses such as Diesel and gas-turbine (jet) engines.
Oh, and many engines use "waste spark systems" that actually fire the spark plugs a second time, during the exhaust stroke. Why? No, it's not to burn the extra fuel off -- the exhaust valve is open, so it would backfire. And the power stroke is over so it can't affect power, except perhaps by wasting energy (were it actually significant). It's so one can use half as many coils (3 on a 6cyl, 4 on an 8cyl, etc.) in the absence of a distributor.
