In effect, the compression stroke is two discrete cycles : the initial portion when the intake valve is open and final portion when the intake valve is closed.
32.
Starting at 1, the piston is at bottom dead centre and both valves are closed at the start of the compression stroke; the cylinder contains air at atmospheric pressure.
33.
This finds practical application in diesel engines which rely on the lack of quick heat dissipation during their compression stroke to elevate the fuel vapor temperature sufficiently to ignite it.
34.
Ignition occurs as the temperature of the fuel / air mixture is taken over its autoignition temperature, due to heat generated by the compression of the air during the compression stroke.
35.
This produces a "'homogeneous charge : "'a homogeneous mixture of air and fuel, which is ignited by a spark plug at a predetermined moment near the top of the compression stroke.
36.
In the case of the normally aspirated engine, at the start of the compression stroke the cylinder air / fuel volume is very high, this translates into a low starting pressure.
37.
The piston reaches bottom center and begins its climb up the cylinder on the compression stroke _ when the fuel / air is compressed just prior to ignition and the power stroke.
38.
With this design, Atkinson was able to make a more conventional engine yet preserve the efficiency of having a short compression stroke and a long expansion stroke in a rather ingenious way.
39.
Normally during the compression stroke, energy is used as the upward-traveling piston compresses air in the cylinder; the compressed air then acts as a compressed spring and pushes the piston back down.
40.
In addition, as the air is compressed and becomes heated, some of this heat is lost to the cold cylinder walls, further reducing the temperature at the top of the compression stroke.
