Transient Fuel Modeling and Control for Cold Start Intake Cam Phasing

Advancing intake valve timing shortly after engine crank and run-up can potentially reduce vehicle cold start hydrocarbon (HC) emissions in port fuel injected (PFI) engines equipped with intake variable cam timing (iVCT). Due to the cold metal temperatures, there can be significant accumulation of liquid fuel in the intake system and in the cylinder. This accumulation of liquid fuel provides potential sources for unburned hydrocarbons (HCs). Since the entire vehicle exhaust system is cold, the catalyst will not mitigate the release of unburned HCs. By advancing the intake valve timing and increasing valve overlap, liquid fuel vaporization in the intake system is enhanced thereby increasing the amount of burnable fuel in the cylinder. This increase in burnable HCs must be countered by a reduction in injector-delivered fuel via a compensator that reacts to cam movement. Both modeling of the liquid fuel shedding process using a transient, multi-domain, physical model and empirical studies reveal details about the fuel behavior for cold start intake valve advancement with iVCT. A relatively simple transient fuel compensator has been developed that directly reacts to changes in intake valve timing, or, to be more precise, to the amount of valve overlap. Experimental testing showed that this new structure allows effective compensation of the transient fuel effect caused by early phasing of the intake cam.