Premixed compression ignition low-temperature diesel combustion (PCI) can simultaneously reduce particulate matter (PM) and oxides of nitrogen (NOx). 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.

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.

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 NOx, PM, CO and THC emissions, and the lowest atmospheric ozone forming potential.