Trapped above the piston is the air fuel charge inducted from the previous cycle. Below the piston, the rotary disk has started to uncover the induction port that leads to the carburetor and the outside atmosphere. Because the volume of the crankcase beneath the piston is increasing, its pressure relative to the outside air is decreasing. This pressure differential causes outside air to rush into the carburetor where it mixes with available fuel before continuing through the induction port and rotary disk, into the crankcase. Ignition is achieved by the heating effect of compression and the heat retained by the glow plug element from the previous cycle, among other things.
Combustion occurs betweeb the top of the piston and the cylinder head.
This volume is known as the combustion chamber. As combustion progresses and the gas increases, pressure rises proportionally. The momentum of the rotating engine components carries the piston beyond TDC in the direction of crankshaft rotation. The period of crankshaft rotation necessary to move the piston from exhaust opening to transfer opening is called exhaust lead. Exhaust lead is necessary in most engines to provide time for the relatively high exhaust-gas pressure to blow down to atmospheric pressure.