Expert: Matt Lyons - 11/30/2008

Question
if the spark plug fails to initiate in an unleaded engine will the fuel mixture compress to top dead center and decompress without ever exploding? if so I presume that it can do this continually until enough heat builds up in the chamber (for example) to cause ignition much like a diesel - can you comment?

many thanks.

Answer
No. The fuel is expelled through the exhaust port each time the engine cycles. Gasoline must be vaporized in order for it to ignite. Simple compression with todays ratios won't do it. A diesel engine has very high compression ratios causing the fuel to heat rapidly above its ignition point.
If anything you would cause an explosion in the exhaust pipe or muffler with the unburned gasoline being ignited by other cylinders in the firing order.