Does the piston reach the max height the piston can go without fuel or oxygen I would think it can't because the oxygen molecule being squeezed in the middle. I'd say it almost fully go up but not completely is this correct?
TL DR: You are absolutely correct: the piston does not completely close the gap.
There are two points in the rotation of the crankshaft where the piston stops momentarily as it changes direction:
- TDC - Top dead center - as the piston is traveling upwards in the cylinder
- BDC - Bottom dead center - as the piston is traveling downwards in the cylinder
After the intake cycle, the piston starts traveling upwards on the compression stroke. As the piston travels upwards, it compresses the air/fuel mixture (only air at this point if the engine is direct injected - for simplicity's sake, I'll leave that for another time), which creates heat and helps homogenize the mixture. Just before the piston gets to TDC, the spark occurs and combustion starts occurring. In order for the combustion to continue and power to be extruded from the explosion, there has to be somewhere for the combustion to go. Without the space, something would give. That something would be a head gasket or weaker points in the metal.
Two reasons why there has to be a space left:
- The air/fuel can be compressed quite a bit, but it cannot be compressed to nothing. There has to be a space left.
- If the area were able to be compressed to nothing, some sort of metal-to-metal contact would occur as the piston reached TDC. This would cause a lot of wear to occur in the engine, which would destroy it in quick order.
All engines are designed with a swept volume (the volume the piston moves through) and a clearance volume (the space between the head and the piston once it is at the top of its stroke).
The ratio between the total volume (swept + clearance) and the clearance volume is the compression ratio - around 7 to 12 : 1 for gas / petrol engines and 18 to 24 : 1 for diesel engines.