Exhaust-gas regeneration under rich conditions to improve fuel economy

1. A method for operating a boosted gasoline engine, the method comprising:

diluting an intake air charge of the engine to a first level of dilution within a first dilution tolerance limit when operating at a stoichiometric air-to-fuel ratio; and

diluting the intake air charge of the engine to a second, greater level of dilution within a second, greater dilution tolerance limit while operating at an enriched air-to-fuel ratio, in response to a condition of excessive exhaust temperature downstream of the engine.

2. The method of claim 1 wherein the intake air charge is diluted to the second level of dilution only if such dilution improves fuel economy.

3. The method of claim 1 wherein the enriched air-to-fuel ratio is provided during a tip-in condition.

4. The method of claim 3 further comprising increasing compression of the intake air charge to preserve torque during the tip-in condition.

5. The method of claim 1 wherein diluting the intake air charge includes diluting upstream of an intake-air compressor with exhaust gas trapped downstream of an exhaust turbine, the turbine mechanically coupled to a compressor.

6. The method of claim 5 further comprising cooling the exhaust gas in a heat exchanger prior to dilution of the intake air charge.

7. The method of claim 1 wherein the second level of dilution is such as to cause combustion instability if used at the stoichiometric air-to-fuel ratio.

8. The method of claim 1 wherein the stoichiometric air-to-fuel ratio is provided during a first operating condition, and wherein the enriched air-to-fuel ratio is provided during a second operating condition, the second operating condition including a higher engine load than the first operating condition.

9. The method of claim 8 further comprising diluting the intake air charge to a third level of dilution, lower than the second, during a third operating condition, wherein a catalyst temperature downstream of the engine is cooler in the third operating condition than in the second operating condition, but engine load is substantially the same.

10. The method of claim 1 wherein the intake air charge is diluted with exhaust gas, the method further comprising estimating a fuel content of the exhaust gas.

11. The method of claim 10 further comprising adjusting an amount of fuel injected into the engine based on the estimated fuel content of the exhaust gas.

12. A gasoline engine comprising:

an intake-air compressor fluidically coupled to an intake manifold;

an exhaust turbine fluidically coupled to an exhaust manifold and mechanically coupled to the compressor;

a conduit having an inlet coupled downstream of the turbine, an outlet coupled upstream of the compressor, and a valve to regulate exhaust flow through the conduit to adjust a level of intake-air dilution in the intake manifold; and

a controller configured to open the valve to provide a first level of dilution within a first dilution tolerance limit when operating the engine at a stoichiometric air-to-fuel ratio, and a second, greater level of dilution within a second, greater dilution tolerance limit when operating the engine at an enriched air-to-fuel ratio.

13. The engine of claim 12 further comprising a heat exchanger thermally coupled to the conduit and configured to cool the exhaust therein.

14. The engine of claim 12 further comprising an air-to-fuel ratio sensor arranged in the conduit and operatively coupled to the controller.

15. The engine of claim 12 further comprising an exhaust-temperature sensor.

16. The engine of claim 15 wherein the exhaust-temperature sensor is thermally coupled to an exhaust-aftertreatment catalyst.

17. The engine of claim 15 wherein the controller is further configured to operate the engine at the enriched air-to-fuel ratio only when an output of the exhaust temperature sensor indicates an over-temperature condition.

18. A method for operating a boosted gasoline engine, the method comprising:

providing a lean or stoichiometric air-to-fuel ratio;

diluting an intake air charge of the engine to a first level of dilution within a first dilution tolerance limit;

detecting a condition of excessive exhaust temperature; and

in response to the condition of excessive exhaust temperature, providing an enriched air-to-fuel ratio and diluting the intake air charge of the engine to a second, greater level of dilution within a second, greater dilution tolerance limit.

19. The method of claim 18 further comprising increasing the compression of the intake air charge to preserve torque while operating at the second level of dilution.

20. The method of claim 19 wherein the enriched air-to-fuel ratio is provided during a tip-in condition.