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A method of controlling exhaust gas recirculation (EGR) in a turbocharged compression-ignition engine system including an engine, an induction subsystem in upstream communication with the engine, an exhaust subsystem in downstream communication with the engine, a high pressure EGR path between the e

A method of controlling exhaust gas recirculation (EGR) in a turbocharged compression-ignition engine system including an engine, an induction subsystem in upstream communication with the engine, an exhaust subsystem in downstream communication with the engine, a high pressure EGR path between the exhaust and induction subsystems upstream of a turbocharger turbine and downstream of a turbocharger compressor, and a low pressure EGR path between the exhaust and induction subsystems downstream of the turbocharger turbine and upstream of the turbocharger compressor. A target total EGR fraction for compliance with exhaust emissions criteria is determined, then a target HP/LP EGR ratio is determined to optimize other engine system criteria within the constraints of the determined target total EGR fraction.

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1. A product comprising: a control subsystem configured to:utilize at least one mass flow sensor to measured the mass flow of at least one gas in an induction plenum of an engine;use the measured mass flow as input to one or more system models;estimate a target total EGR fraction responsive to the o

1. A product comprising: a control subsystem configured to:utilize at least one mass flow sensor to measured the mass flow of at least one gas in an induction plenum of an engine;use the measured mass flow as input to one or more system models;estimate a target total EGR fraction responsive to the one or more engine system models, and at least one other engine system input signal;send a signal representative of the target total EGR fraction to a controller configured to control exhaust gas recirculation (EGR) wherein the controller is also configured to: receive input signals including the target total EGR fraction of an EGR system comprising at least two EGR paths,determine a target HP/LP EGR ratio to control other engine system parameters within the constraints of the target total EGR fraction, andtransmit actuator output signals responsive to the target HP/LP EGR ratio;wherein the controller is further configured to:generate HP EGR and LP EGR setpoints in accordance with the determined target HP/LP EGR ratio, determine target HP and LP EGR valve opening percentages corresponding to the HP and LP EGR setpoints,adjust at least one of the setpoints or opening percentages by processing an estimated total EGR fraction as process variable input and the target total EGR fraction as setpoint input, andtransmit the opening percentages as the actuator output signals to an EGR valve actuator configured to move an EGR valve in response to the actuator output signal. 2. A product as set forth in claim 1 wherein the other engine system parameters include at least one of fuel economy, engine system performance, engine system protection, or engine system maintenance. 3. A product as set forth in claim 2 wherein the other engine system parameters include at least one of induction subsystem temperature control or exhaust subsystem temperature control. 4. A product as set forth in claim 3 wherein the target HP/LP EGR ratio is determined to increase exhaust subsystem temperatures to regenerate a diesel particulate filter. 5. A product as set forth in claim 3 wherein the target HP/LP EGR ratio is determined to reduce induction subsystem temperatures. 6. A product as set forth in claim 3 wherein the target HP/LP EGR ratio is determined to control induction air temperature to avoid condensation. 7. A product as set forth in claim 2 wherein the target HP/LP EGR ratio is determined to avoid turbocharger overspeeding. 8. A product as set forth in claim 2 wherein the target HP/LP EGR ratio is determined to increase torque output in response to driver demand for vehicle acceleration. 9. A product as set forth in claim 2 wherein the target HP/LP EGR ratio is determined to provide a higher percentage of LP EGR to reduce turbocharger lag. 10. A product as set forth in claim 2 wherein the target HP/LP EGR ratio is determined to allow the total EGR fraction to be achieved without closing intake or exhaust throttle valves. 11. A product as set forth in claim 2 wherein the target HP/LP EGR ratio is determined to optimize induction air temperature for maximum fuel economy. 12. A product as set forth in claim 2 wherein the target HP/LP EGR ratio is determined to reduce exhaust emissions. 13. A product as set forth in claim 2 wherein the target HP/LP EGR ratio is achieved by open-loop control of an HP EGR valve opening or an LP EGR valve opening to improve EGR response. 14. A product as set forth in claim 2 wherein the target HP/LP EGR ratio is determined to reduce compressor tip overheating. 15. A product as set forth in claim 1, wherein the at least one other engine system input signal includes at least one of turbocharger boost pressure or induction mass flow, and the output signals include at least one of HP EGR setpoints, LP EGR setpoints, or turbocharger setpoints. 16. A product as set forth in claim 1, wherein the controller also includes HP and LP EGR control modules in communication with an induction control module to receive and process the output signals based on the target HP/LP EGR ratio and transmit actuator command signals. 17. A product as set forth in claim 2 wherein the controller is also configured to estimate total EGR fraction responsive to at least one sensed proxy parameter indicative of total EGR fraction. 18. A product as set forth in claim 17 wherein the at least one sensed proxy parameter is cylinder pressure. 19. A product as set forth in claim 1 wherein the estimated total EGR fraction is responsive to at least one proxy parameter indicative of total EGR fraction and is input to at least one engine system model to generate the estimated total EGR fraction. 20. A product comprising: a control subsystem configured to:utilize at least one mass flow sensor to measured the mass flow of at least one gas in an induction plenum of an engine;use the measured mass flow as input to one or more system models;estimate a target total EGR fraction responsive to the one or more engine system models, and at least one other engine system input signal;send a signal representative of the target total EGR fraction to a controller configured to control exhaust gas recirculation (EGR) wherein the controller is also configured to: receive input signals including the target total EGR fraction of an EGR system comprising at least two EGR paths,determine a target HP/LP EGR ratio to control other engine system parameters within the constraints of the target total EGR fraction, andtransmit actuator output signals responsive to the target HP/LP EGR ratio;wherein the controller is further configured to:generate HP EGR and LP EGR setpoints in accordance with the determined target HP/LP EGR ratio,adjust at least one of the setpoints by processing an estimated total EGR fraction as process variable input and the target total EGR fraction as setpoint input, andtransmit the setpoints as the actuator output signals to an EGR valve actuator configured to move an EGR valve in response to the actuator output signal.