Internal combustion engine control during cold start

1. A method for controlling an internal combustion engine during a cold start operation, comprising:

supplying the engine with an air-fuel mixture having a substantially constant, lean actual air/fuel ratio when the engine is started; and

allowing engine idle speed to vary as a function of the difference between a target air/fuel ratio and said actual air/fuel ratio.

2. The method of claim 1 , further comprising: varying the target air/fuel ratio to control said engine idle speed.

3. The method of claim 2 , further comprising: calibrating said target air-fuel ratio to provide a nominal idle speed during cold start.

4. The method of claim 3 wherein said nominal idle speed (N 1 ) during a cold start is higher than a predetermined nominal idle speed during normal operation of the engine.

5. The method of claim 3 wherein said calibration allows the nominal idle speed to vary as a function of fuel volatility, while maintaining said actual air/fuel ratio substantially constant.

6. The method of claim 5 wherein the calibration allows the nominal idle speed to be reduced if a fuel with lower volatility is used.

7. The method of claim 3 wherein a throttle in an air intake conduit is kept at a substantially fixed opening angle during the calibration.

8. The method of claim 1 wherein an actual air factor (λ A ) is within a range of 1.02<λ A <1.2 during cold start idling.

9. An internal combustion engine having an air intake conduit and a throttle arranged to supply induction air to at least one combustion chamber, at least one fuel injector to supply fuel to the induction air, an outlet for exhaust gas downstream from the engine, and a central processing unit for controlling the operation of said engine wherein the engine is arranged to operate with a lean actual air/fuel ratio during a cold start of the engine and has an idle speed that varies as a function of the difference between a target air/fuel ratio and the actual air/fuel ratio.

10. The engine of claim 9 wherein the central processing unit causes the fuel injectors to vary the target air/fuel ratio of the induction air to control the idle speed of the engine.

11. The engine of claim 10 wherein the target air fuel ratio is controlled by the central processing unit to achieve a nominal idle speed.

12. The engine of claim 11 wherein the throttle in the air intake air conduit is kept at a substantially fixed opening angle during the calibration.

13. The engine of claim 11 wherein the central processing unit maintains said actual air/fuel ratio substantially constant when the nominal idle speed varies due to changes in fuel volatility.

14. The engine of claim 9 wherein the engine operates with an actual air factor (λ A ) in a range of 1.02<λ A <1.2 during cold start idle.