Partial oxidation products and other hydrocarbon species in diesel HCCI exhaust

A single cylinder engine was operated in HCCI mode with diesel-range fuels, spanning a range in cetane number (CN) from 34 to 62. In addition to measurements of standard gaseous emissions (CO, HC, and NOx), multiple sampling and analysis techniques were used to identify and measure the individual exhaust HC species including an array of oxygenated compounds. A new analytical method, using liquid chromatography (LC) with electrospray ionization-mass spectrometry (ESI-MS) in tandem with ultraviolet (UV) detection, was developed to analyze the longer chain aldehydes as well as carboxylic acids. Results showed an abundance of formic and butyric acid formation at or near the same concentration levels as formaldehyde and other aldehydes. Concentrations of all partially-oxidized species were low when the combustion phasing was advanced, but were much higher at retarded combustion phasing when the main combustion event was slower due to the pressure and temperature conditions being less severe. The increased concentrations at retarded combustion phasing were higher for the high CN fuels, which exhibited significantly more low temperature heat release (LTHR) than the low CN fuels. Fuels with high CN had higher concentrations of aromatic aldehydes in the exhaust than low CN fuels, even though they had lower aromatic concentrations in the starting fuel. This study also shows that the high unburned hydrocarbon emissions associated with HCCI combustion often includes high concentrations of partially-oxidized species.