Development of low -temperature premixed diesel combustion strategies and formulation of suitable diesel oxidation catalysts.

Abstract

The successful development of low-temperature premixed compression ignition (PCI) combustion strategies and formulation of suitable diesel oxidation catalysts (DOC) is presented in this dissertation. Low-temperature PCI combustion is shown to reduce engine-out oxides of nitrogen (NO_X) and soot emissions by 74% and 86% respectively at 1500 rpm and 400 kPa brake mean effective pressure (BMEP) over a modern conventional diesel combustion strategy. The PCI load-range is investigated at 1500 rpm between 50 kPa and 720 kPa BMEP. NO_X can be held to 1 g/kg-fuel at all loads. Soot is below 0.015 g/kg-fuel up to 400 kPa BMEP but increases to 1.11 g/kg-fuel at 720 kPa BMEP, thus limiting PCI operation at high engine loads. Excessive carbon monoxide (CO), hydrocarbon (HC) emissions and low exhaust gas temperatures are obtained at 50 kPa BMEP limiting PCI operation at low loads. Eight DOC formulations are investigated on a gas bench reactor using surrogate exhaust gas mixtures containing CO, O₂, H₂O, C₁₁H₂₄. C₂H₄ and N₂. HC speciation of engine exhaust identified n-undecane (C₁₁H₂₄) and ethene (C₂H₄) as the dominant HC species. PCI and conventional light-off and light-down tests are performed. Light-off temperatures are higher in PCI exhaust compared to conventional exhaust. Light-dowel temperatures are lower than light-off temperatures under PCI and conventional conditions. CO and HC are converted to 100% and 98% respectively under fully lit conditions. The addition of ceria to washcoat and an increased washcoat loading show 20°C lower light-off temperature compared to the reference formulation. Formulations containing mixtures of platinum (Pt) and palladium (Pd) outperform Pt-only formulations. Engine experiments agree well with reactor experiments. DOC formulations containing zeolites adsorb HC at low temperatures. HC speciation shows that HCs of carbon number five and higher are trapped on zeolite at low temperatures. CO conversion is 100% through the fully active catalyst; HC conversion is lower compared to the reactor experiments. Methane (CH₄) and HC desorption in the emissions sample system are responsible for the discrepancy. Light-off temperatures are higher in engine experiments compared to reactor experiments due to heat loss and inhibition by CO and unsaturated HC.