An engine is basically an air pump.

But it is wrong to consider that air flows through the engine - instead it pulses i.e. a cylinder only draws air in once every 2 revolutions of the crank.

These pulses can be put to good use if you tune the length of the pipe the air is travelling through. An example of this is the beer bottle, as you blow over the top the air molecules inside vibrate - the more beer you drink the deeper the note (the lower the frequency at which the air is vibrating).

Enough?

Alex