Cylinder Pressure in a Spark - Ignition Engine : A Computational Model

an s-curve to describe the gradual transition between exhaust pressure and intake or inlet pressure. The start of compression, which marks the end of the intake process, does not necessarily occur at the same time as the close of the intake valve (IVC). The intake valve closes after bottom center (BC) while the volume in the cylinder is decreasing. The engine speed determines the point at which the fuel/air mixture stops flowing into the cylinder. At lower revolutions per minute (rpm), the start of compression is closer to IVC; at higher speeds, it is closer to BC. An s-curve approximates the angle of the start of compression as a function of engine speed. The volume of the cylinder during intake increases as the piston descends, thereby drawing in the fuel mixture. There is little resistance to gas flow into the cylinder, which causes the pressure in the cylinder to remain relatively constant and equal to the inlet pressure.