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

both valves are open. During this period, the model used 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. Compression. Both the intake and exhaust valves are closed during the compression stroke so that the gases can neither enter nor exit the cylinder. The piston is moving upward , so cylinder volume decreases. Pressure increases as the gas in the cylinder is compressed. Because of the high speed of the piston, the duration of compression is short and negligible heat is lost to the walls of the cylinder. Relatively little energy is dissipated due to internal friction of the gas. Overall, there is little change in entropy during compression , and the gas behavior can be described by the equation: 2 (1) This equation allowed for calculation of the cylinder pressure at any crank angle during compression based on the knowledge of initial pressure and volume, P 0 and V 0 , which determine the constant. The volume of the cylinder is a direct function of crank angle, cylinder geometries, crank radius and connecting rod length (see ref. 1). The ratio of the specific heat of the fuel at constant pressure to the specific heat at constant volume is γ; its value varies from compression to combustion to expansion. During compression, γ is approximately equal to 1.3. 3 Combustion. The combustion process was described by the McCuiston, Lavoie and Kauffman (MLK) model. 4 The mass-burn fraction, χ b , was modeled as a function of pressures and volumes: (2) where …