How a Four Stroke Engine Works 2
The compression cycle is when the piston is on its way back up inside the cylinder.
Internal workings of the Compression Cycle
The inlet port has now been closed by the camshaft, as a result the cylinder is now completely airtight inside. The upwards motion of the piston compresses the fuel and air mixture to a fraction of its original size.
This also heats up the mixture and makes it easier to ignite – when any gas is compressed it increases in temperature based on the gas laws. This happens because the individual molecules of the fuel air mixture are confined to a smaller space, as a result they have less room to move around in, this means they hit each other more often and their kinetic energy of motion is converted into heat energy.
As you can see from the image below, the piston has compressed the fuel/air mixture to a fraction of its original size, this in itself will have heated the mixture tremendously, but not to a high enough temperature to cause spontaneous combustion.