Cylinder Charge in Spark-Ignition Engines – Fresh Gas, Residual Gas & EGR Diagram

This manual page explains the fundamentals of cylinder charge in a spark-ignition (gasoline) engine as part of an engine management system description. It first concludes an overview of the ignition subsystem, which determines the crankshaft angle for the ideal spark instant, and lists the goals of the closed-loop control system: exhaust emissions, fuel consumption, power, comfort and convenience, and safety. The main section defines the cylinder charge as the gas mixture in the cylinder after intake-valve closure, consisting of fresh air-fuel mixture plus residual gases. The "fresh gas" subsection explains that air enters mainly through the throttle valve (with supplementary gas via the evaporative-emissions system) and that trapped air mass is the prime determinant of engine torque, so power increases usually require increasing maximum cylinder charge. The "residual gases" subsection describes the two sources of residual gas—undischarged exhaust and recirculated exhaust (EGR)—and notes their influence on ignition and combustion, which can be desirable at part throttle. Figure 2 is a labeled schematic, "Cylinder charge in the spark-ignition engine," identifying 12 components including the purge valve, EGR valve, mass airflow points with associated pressures (barometric, intake-manifold, combustion-chamber, back-pressure), intake and exhaust valves, and throttle-valve angle α.