Improved
transient response times are obtained while maintaining low emissions with a low pressure EGR
system through methods for quickly obtaining a desired
oxygen concentration for charge-air to be used for
combustion. Under a first method, fuel quantity in the main
combustion event is controlled in the
combustion process to produce exhaust around a relatively constant target exhaust
oxygen concentration value. By keeping the exhaust
oxygen concentration levels at a relatively constant value,
lag time in waiting for low pressure EGR valve adjustments during transients may be avoided, and the
system's air handling response to meet transients may be paced solely by adjusting the
mass of air to be supplied (i.e. boost response). Under a second method, a multiple-stage
combustion process is utilized, in which fuel feed is controlled in a small, preliminary HCCI-type combustion event in order to produce a target oxygen concentration of charge-air to be used for the second, main combustion event. Under a third method, exhaust rebreathing is used to produce a target oxygen concentration of charge-air to be used for combustion.