Active control of one or more EGR loops

1. A method, comprising:

operating an internal combustion engine including an intake system with a compressor and an exhaust system with a turbine, a high pressure exhaust gas recirculation (EGR) loop connecting the exhaust system upstream of the turbine to the intake system downstream of the compressor, and a low pressure EGR loop connecting the exhaust system downstream of the turbine to the intake system upstream of the compressor;

recirculating exhaust gas to the intake system through both the high pressure EGR loop and the low pressure EGR loop to combine with an intake air flow and provide a charge flow to an intake manifold of the internal combustion engine that includes an EGR fraction indicative of a ratio of the recirculated exhaust gas to the charge flow;

determining a desired oxygen concentration in the charge flow to the internal combustion engine;

determining a NOx concentration in the charge flow;

determining an engine out NOx concentration in the exhaust gas; and

adjusting the EGR fraction in response to the NOx concentration in the charge flow and in the exhaust gas to adjust an actual oxygen concentration in the charge flow toward the desired oxygen concentration.

2. The method of claim 1 , wherein adjusting the EGR fraction includes controlling a position of a control valve in at least one of the low pressure EGR loop and the high pressure EGR loop.

3. The method of claim 1 , wherein at least one of the high pressure EGR loop and the low pressure EGR loop includes a heat exchanger for cooling recirculated exhaust gas passing therethrough.

4. The method of claim 1 , further comprising determining the EGR fraction of the charge flow in response to at least one of an oxygen concentration and the NOx concentration in the charge flow, at least one of an oxygen concentration and the engine out NOx concentration in the exhaust gas, and at least one of an oxygen concentration and a NOx concentration in the intake air flow.

5. The method of claim 4 , further comprising estimating a portion of the EGR fraction from the high pressure EGR loop in response to a position of a high pressure EGR control valve of the high pressure EGR loop and engine operating parameters.

6. The method of claim 5 , further comprising determining a target EGR fraction and wherein adjusting the EGR fraction includes controlling a position of a control valve in the low pressure EGR loop to adjust the EGR fraction toward the target EGR fraction.

7. The method of claim 1 , further comprising a mass air flow sensor in the intake system upstream of the connection with the low pressure EGR loop operable to measure the intake air flow in the intake system.

8. The method of claim 1 , wherein:

determining at least one of an oxygen concentration and the NOx concentration in the charge flow includes measuring the oxygen concentration with a first oxygen sensor in the intake system downstream of the connection of the high pressure EGR loop with the intake system; and

determining at least one of an oxygen concentration and the engine out NOx concentration in the exhaust gas includes measuring the oxygen concentration with a second oxygen sensor in the exhaust system.

9. The method of claim 1 , wherein:

determining the NOx concentration in the charge flow includes measuring the NOx concentration with a first NOx sensor in the intake system downstream of the connection of the high pressure EGR loop with the intake system; and

determining the engine out NOx concentration in the exhaust gas includes measuring the engine out NOx concentration with a second NOx sensor in the exhaust system.

10. The method of claim 1 , wherein adjusting the EGR fraction includes operating at least one of an intake throttle in the intake system, an exhaust throttle in the exhaust system, and a variable geometry inlet to the turbine.

11. A method, comprising:

an internal combustion engine including an intake system with a compressor and an exhaust system with a turbine, a high pressure exhaust gas recirculation (EGR) loop connecting the exhaust system upstream of the turbine to the intake system downstream of the compressor, and a low pressure EGR loop connecting the exhaust system downstream of the turbine to the intake system upstream of the compressor;

recirculating exhaust gas to the intake system through at least one of the high pressure EGR loop and the low pressure EGR loop to combine with an intake air flow and provide a charge flow to an intake manifold of the internal combustion engine;

determining a charge flow to the internal combustion engine;

determining one of an oxygen concentration and a NOx concentration in the charge flow;

determining one of an oxygen concentration and an engine out NOx concentration in the exhaust gas;

determining an EGR fraction indicative of a ratio of the recirculated exhaust gas to the charge flow from the one of the oxygen concentration and the NOx concentration in the charge flow, the one of the oxygen concentration and engine out NOx concentration in the exhaust gas, and one of an oxygen concentration and a NOx concentration in the intake air flow;

determining an intake air flow from the difference between the charge flow and the EGR fraction; and

correcting an intake air flow measurement from a mass air flow sensor in the intake system in response to a deviation of the intake air flow measurement from the intake air flow determined from the difference between the charge flow and the EGR fraction.

12. The method of claim 11 , wherein:

determining one of the oxygen concentrationand the NOx concentration in the charge flow includes measuring the oxygen concentration with a first oxygen sensor in the intake system downstream of the connection of the high pressure EGR loop with the intake system; and

determining one of the oxygen concentration and the engine out NOx concentration in the exhaust gas includes measuring the oxygen concentration with a second oxygen sensor in the exhaust system.

13. The method of claim 11 , wherein:

determining one of the oxygen concentration and the NOx concentration in the charge flow includes measuring the NOx concentration with a first NOx sensor in the intake system downstream of the connection of the high pressure EGR loop with the intake system; and

determining one of the oxygen concentration and the engine out NOx concentration in the exhaust gas includes measuring the engine out NOx concentration with a second NOx sensor in the exhaust system.

14. The method of claim 11 , farther comprising recirculating exhaust gas through both of the high pressure EGR loop and the low pressure EGR loop.

15. A method, comprising:

operating an internal combustion engine including an intake system with a compressor and an exhaust system with a turbine, a high pressure exhaust gas recirculation (EGR) loop connecting the exhaust system upstream of the turbine to the intake system downstream of the compressor, and a low pressure EGR loop connecting the exhaust system downstream of the turbine to the intake system upstream of the compressor;

recirculating exhaust gas to the intake system through at least one of the high pressure EGR loop and the low pressure EGR loop to combine with an intake air flow and provide a charge flow to an intake manifold of the internal combustion engine;

determining a charge flow to the internal combustion engine;

determining an EGR fraction estimate of a ratio of the recirculate exhaust gas to the charge flow in response to a position of an EGR flow control valve in the at least one of the high pressure EGR loop and the low pressure EGR loop and engine operating parameters;

determining one of an oxygen concentration and a NOx concentration in the charge flow;

determining one of an oxygen concentration and an engine out NOx concentration in the exhaust gas;

determining an EGR fraction indicative of the ratio of the recirculated exhaust gas to the charge flow from the one of the oxygen concentration and the NOx concentration in the charge flow, the one of the oxygen concentration and engine out NOx concentration in the exhaust gas, and one of an oxygen concentration and a NOx concentration in the intake air flow;

providing a diagnostic of the at least one of the high pressure EGR loop and the low pressure EGR loop in response to a deviation of the EGR fraction estimate from the EGR fraction.

16. The method of claim 15 , wherein:

determining one of the oxygen concentration and the NOx concentration in the charge flow includes measuring the oxygen concentration with a first oxygen sensor in the intake system downstream of the connection of the high pressure EGR loop with the intake system; and

determining one of the oxygen concentration and the engine out NOx concentration in the exhaust gas includes measuring the oxygen concentration with a second oxygen sensor in the exhaust system.

17. The method of claim 15 , wherein:

determining one of the oxygen concentration and the NOx concentration in the charge flow includes measuring the NOx concentration with a first NOx sensor in the intake system downstream of the connection of the high pressure EGR loop with the intake system; and

determining one of the oxygen concentration and the engine out NOx concentration in the exhaust gas includes measuring the engine out NOx concentration with a second NOx sensor in the exhaust system.

18. The method of claim 15 , further comprising recirculating exhaust gas through both of the high pressure EGR loop and the low pressure EGR loop.