Ignition Finally, the ignttion" subsystem de- termines the crankshaft angle that corresponds to precisely the ideal instant for the spark to ignite the mixture. The purpose of this closedrloop control system is to provide the torque demanded by the driver while at the same time satisfying strict criteria in the areas ol , Exhaust emiSSlons, , Fuel consumption, , Power, , Comfort and convenience, and , Safety. Cylinder charge Elements The gas mixture lound In the cylinder once the intake valve closes is referred to as the cylinder charge, and conSists ol the inducted fresh airaluel mixture along with residual gases. Fresh gas The fresh mixture drawn into the cylinder is a combination ol lresh air and the luel entrained With it. While most of the fresh air enters through the throttle valve, supplementary lresh gas can also be drawn in through the evaporalive- Fig 2 Cylinder charge In the spark-ignition engine 1 Air arid iuel vapor 2 Purge valve Witri variable aperture. 3 Link to evaporaliVe-emlsslcns control system, I Exhaust 935, 5 EGR valve wllh variable aperture 5 Mass ainluw lbarometrlc pressure rm), 7 Mass airtlow (intake-maniralu pressure in), emisSions control system (Figure 2). The air entering through the throttle-valve and remaining in the cylinder alter intake- valve closure is the decisive faclor dellning the amount ol work transferred through the piston during combustion, and thus the prime determinant lor the amount of torque generated by the engine. In consequence, modillcalions to enhance maximum engine power and torque almost always entail increasing the maximum possible cylinder charge, The theoretical maximum charge is deflned by the volumetric capacity. Residual gases The portion of the charge consisting of residual gases is composed of a The exhaustagas mass that is not discharged while the exhaust valve is open and thus remains in the cylinder, and The mass of recirculated exhaust gas (on systems With exhaustagas recucu- latlon, Figure 2). The proportion of residual gas is de termined by the gas-exchange process. Although the residual gas does not participate directly in combustion, it does influence ignition patterns and the actual combustion sequence. The eflects of this residual-gasoomponentmaybethoroughly desirable under partathrottle operation. Larger throttle~valve openings to come pensate for reductions in lreshagas filling a Fresh air charge (combustion-chamber pressure pg), 9 Resldual gas charge (combustlanrchamber pressure [13), in Exhaust gas (hack-pressure ,ii), ii Intake valve, 12 Exhaust valve, it Throttle-valve angle UMMOoualV