I think our definition of explosion is what is at fault.

From what i have been taught, an explosion is the rapid combustion of everything in a medium at once.

Normal engine operation does not fit my definition of an explosion. In normal engine operation, the spark plug lights the AFM(Air Fuel Mixture), and the AFM burns in an even outward pattern from the spark plug. This takes about .003 seconds on average.

Now if we follow my definition of an explosion, rapid takes on the discerning fashion. If you consider .1 seconds to be rapid, the AFM is indeed exploding. If you start working as an electrical engineer in the .000000001 second intervals, you'd say .003 seconds is an eternity. So really, it depends on what you define as your time constants. Since the flame goes from one side to another in a linear "domino" fashion, and it is not and instantaneous process, I see normal engine running as nonexplosive rapid burning of the AFM.