<div class="htmlview paragraph">Premixed compression ignition low-temperature diesel combustion (PCI) can simultaneously reduce particulate matter (PM) and oxides of nitrogen (NO_x). Carbon monoxide (CO) and total hydrocarbon (THC) emissions increase relative to conventional diesel combustion, however, which may necessitate the use of a diesel oxidation catalyst (DOC). For a better understanding of conventional and PCI combustion, and the operation of a platinum-based production DOC, engine-out and DOC-out exhaust hydrocarbons are speciated using gas chromatography.</div> <div class="htmlview paragraph">As combustion mode is changed from lean conventional to lean PCI to rich PCI, engine-out CO and THC emissions increase significantly. The relative contributions of individual species also change; increasing methane/THC, acetylene/THC and CO/THC ratios indicate a richer combustion zone and a reduction in engine-out hydrocarbon incremental reactivity.</div> <div class="htmlview paragraph">The DOC is most effective in oxidizing CO, followed by acetylene and olefins, aromatics, non-methane paraffins and methane. DOC conversion efficiency of CO and THC is high for lean conventional and lean PCI but very low for rich PCI. The high CO/O₂ ratio of rich PCI is believed to cause nearly all of the catalyst's active sites to be filled with CO, essentially disabling it until the CO/O₂ ratio is reduced. Lean PCI DOC-out exhaust has the lowest combination of NO_x, PM, CO and THC emissions, and the lowest atmospheric ozone forming potential.</div>