Reducing cold start hydrocarbon emissions from port fuel injected spark ignition engines with improved management of hardware & controls

Abstract

An experimental study was performed to investigate strategies for reducing cold start hydrocarbon (HC) emissions from port fuel injected (PFI) spark ignition (SI) engines with better use of existing hardware and control systems. Engine experiments and computer simulations were used for three major phases of the project: the effect of variable valve timing on first cycle mixture preparation, the interaction between fuel injection and valve events, and the development of a flow reactor exhaust manifold for fast catalyst light-off. For the first cycle of cranking, delaying the intake valve opening (IVO) creates a pressure difference across the valve, resulting in strong but brief shear flow to facilitate atomization; delayed IVO also produces a cooler charge due to the expansion process before IVO. It was observed that the in-cylinder equivalence ratio increased with delayed IVO, primarily by displacing the lean portion of the stratified cylinder charge back into the port. However, HC emissions for the first cranking cycle increased with delayed IVO. With closed valve injection, injection timing has no significant impact on mixture preparation or emissions. With open valve injection, however, HC emissions scale with both valve lift and mass flow because of increased cylinder wall wetting.

(cont.) By timing split injection such that the second injection event hits the overlap back flow, a small mixture preparation and emissions benefit was achieved. Earlier IVO results in a longer back flow period, however the impact on mixture preparation is small. The observed reduction in HC emissions resulted from a higher residual gas fraction due to early IVO, which yielded later combustion phasing, which in turn yielded increased post-flame oxidation. Under steady-state cold coolant conditions, operation of a 4-cylinder engine with three cylinders running rich and the fourth used to pump air into the exhaust manifold resulted in near total oxidation of CO and HC at sufficiently retarded spark timing. Exhaust gas temperatures and enthalpy flow rates were significantly higher than for the conventional engine configuration at fast idle. Using this strategy to perform real cold starts proved challenging without the additional hardware needed for sufficient control over air flow to the engine.