Hydrocarbon and particulate matter evolution in the exhaust manifold of a spark-ignited engine under cold-start operation

Cold-start, as defined by the time between first engine crank and three-way catalyst light-off, is responsible for a large percentage of NO_x, unburned hydrocarbon, and particulate matter (PM) emissions in light-duty engines. Minimizing emissions during cold-start is a trade-off between achieving faster three-way catalyst light-off, and engine out emissions during that period. In this study, various exhaust gas and PM species were measured at three locations in the exhaust manifold using advanced hydrocarbon and PM speciation methods in addition to standard exhaust-gas-analyzers to characterize how or if these species evolve downstream of the exhaust port during cold-start operation. A gasoline direct injected spark ignited (DISI) single cylinder engine was run at 2-bar net indicated mean effective pressure (NIMEP) at spark timings ranging from normal-SI operation relevant early spark timing of -10 degrees after top dead center firing (dATDCf) to more cold-start-relevant retarded spark timing of 25dATDCf. At the retarded spark timings, the exhaust manifold gas temperature increased downstream of the exhaust port indicating the presence of oxidation reactions that counteract heat transfer losses; a trend also supported by oxidation of total hydrocarbons (THC), CO, and H₂ observed downstream of the exhaust port. Additionally, increase in NO_x emissions downstream of the exhaust port indicated presence of high temperature required to drive NO_x chemistry. Aromatics were the largest exhaust hydrocarbon species group detected at all spark timings, while the paraffins were under-represented from their fuel composition fraction. Organic carbon accounted for more than 95% of the PM mass at exhaust port for all spark timings. At retarded spark timings, the PM mass reduced downstream of the exhaust port primarily driven by organic carbon oxidation. Particle number and size measurements at retarded spark timings indicated a reduction in particle count downstream of the exhaust port with an accompanying increase in particle size possibly due to agglomeration.