When the intake valve is opened while the piston goes down, air will be taken into the cylinder.
When the piston goes up, the air in the cylinder will be compressed.
|Combustion and expansion stroke
When fuel is drawn from the injection nozzle at the time when the piston reaches the top and the air is most compressed, the fuel will automatically ignite with heat (500 to 700 degree Celsius) generated by air compression, igniting and thermal expansion.
When the exhaust valve opens and the piston goes up, the gas inside the cylinder will be given off as exhaust.