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I recently had an idea about the design of an internal combustion engine. Normally, the spark must be timed using some kind of circuit, but if there was a "spike" on the piston:

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then would there be no need for a specialized ignition system?

Of course, there would have to be some kind of insulation between the spike and the casing, and between the spike and the piston.

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    $\begingroup$ Or dump the spark and call it a, ... Diesel engine $\endgroup$ – Carl Witthoft Nov 11 '16 at 13:55
  • $\begingroup$ I'm assuming you're saying you just want to constantly energize the spikes and let proximity cause the arc? Are you saying that the cylinder spike would be at some voltage, the piston spike would be ground, and the rest of the engine would be at some floating (ungrounded) voltage? Have you heard of a Van Ded Graaff generator? If you ground the engine block, then what prevents the spark from arcing between the spike and the block? How does electricity pass to the spike in the cylinder? There is a rotating joint at either end of the connecting rod. $\endgroup$ – Chuck Nov 11 '16 at 16:21
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Well, you'd still need something to generate the high voltage for the spark, so there isn't much overall simplification. A magneto driven by points is about as simple as it gets.

But more importantly, the spark does not always occur at TDC — in fact, in most operating regimes for most engines, it never does. In order to get the best efficiency, the ignition timing must be variable relative to the piston position.

Normally, at idle, ignition occurs a few degrees before TDC. But as the speed goes up, or under hard acceleration, the ignition might be advanced by as much as 40°-50°. Back when we used mechanical points for ignition timing, these were known as "centrifugal advance" and "vacuum advance", respectively.

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    $\begingroup$ Additionally, it's a lot easier to replace a spark plug than a piston! $\endgroup$ – Mark Nov 11 '16 at 23:03

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