Question about the exhaust
 

Was wondering during the process of burning the fuel, once the A/F mix is compressed and the piston is forced downwards, what happens to the exhaust before the piston forces it out through the exhaust valve? Does the burnt fuel stay toward the top of the cylinder or does the force of the piston going downward create a sort of vacuum which brings the burnt fuel down towards the bottom (relatively speaking) of the cylinder, or does it occupy all the space relatively evenly?

The mixture spreads evenly, since it's the expanding/combusting air-fuel mixture that is pushing the piston down rather than the piston pulling it.