Species resolved hydrocarbon emission profiles from advanced diesel combustion and characterization of heat-up diesel oxidation catalysts.

Abstract

This study summarizes the successful characterization of a heat-up diesel oxidation catalyst (DOC) combined with a low-temperature premixed charge compression ignition (PCI) combustion to further reduce exhaust emissions. Because different types of hydrocarbon (HC) species can affect DOC oxidation performance, a direct raw exhaust HC sampling and speciation method with gas chromatography (GC), capable of identifying 70% HC carbon mass, was developed to minimize condensation on particulate matter (PM) of mid or high boiling temperature HC species during exhaust sampling. This HC speciation method provided species resolved HC emission profiles from combustion modes (lean conventional, lean PCI, rich PCI) and engine parameters (injection timing, exhaust gas recirculation (EGR) rate, speed, and load). Highly increased soot precursor concentration as combustion mode changes from lean conventional to lean PCI to rich PCI, reflects a limiting of the soot-forming reactions from the decreased combustion temperature and better fuel-air mixing. As injection timing is retarded and EGR rate is increased in the PCI regime, a higher yield of HC emission was observed mainly due to lower bulk gas temperature. The lower temperature causes increase of HC emission, relative partially burned HCs and relative alkenes+alkynes concentration. Speed and load effect on HC species emission showed more complicated HC emission trends than injection timing and EGR rate because all of the engine control parameters were optimized to compromise fuel economy and emissions. Two representative HC species in typical diesel exhaust are ethene and n-undecane, which are used for reactor tests. Through evaluation of three DOC formulations e.g., platinum (Pt) only, Pt and palladium (Pd), and Pt and Pd with ceria (CeO₂), by a reactor test, the PtPd catalyst shows the lowest light-off temperature, followed by the PtPd+CeO₂ catalyst and then the Pt catalyst, regardless of combustion modes, HC species compositions, and oxygen concentrations. Although in-situ engine test of the PtPd catalyst with post in-cylinder fuel injection shows that PCI is superior to conventional combustion in increasing exhaust gas temperature, PCI produces significant PM as post fuel injection timing is retarded. An extra fuel injection system immediately before the DOC would most likely solve this problem.