Effects of Load on Emissions and NOx Trap/Catalyst Efficiency for a Direct Injection Spark Ignition Engine 1999-01-1528

A 1998 Toyota Corona passenger car with a direct injection spark ignition (DISI) engine was tested at constant engine speed (2000 rpm) over a range of loads. Engine-out and tailpipe emissions of gas phase species were measured each second. This allowed examination of the engine-out emissions for late and early injection. Regeneration of the lean NOx trap/catalyst was also examined, as was the efficiency of NOx reduction. NOx stored in the trap/catalyst is released at the leading edge of regenerations, such that the tailpipe NOx is higher than the engine-out NOx for a brief period. The efficiency of NOx reduction was <50% for the lowest loads examined. As the load increased, the efficiency of NOx reduction decreased to near 0% due to excessive catalyst temperatures. Loads sufficiently high to require a rich mixture produce high NOx reduction efficiencies, but in this case the NOx reduction occurs via the three-way catalysts on this vehicle. The Emissions Index for engine-out hydrocarbons (EIHCs) decreases with increasing load for late injection and is independent of load for early injection. The EIHCs for early injection is four times lower than that for late injection at the lowest load examined, which is representative of a high speed cruise on the FTP driving cycle. The EIHCs for a comparable port fuel injected (PFI) vehicle is lower at all loads than that for the DISI vehicle. For the lowest load examined, the EIHCs for the DISI vehicle is six times that for the PFI vehicle. In contrast, the Emissions Index for engine-out NOx (EINOx) is lower for the DISI vehicle than for the PFI vehicle at all loads examined. At the lowest load examined, the EINOx is four times higher for the PFI vehicle. For the DISI vehicle, the EINOx is decreases slightly with increasing load for low loads, then increases as the load increases further and peaks for a slightly lean mixture.