Influence of Diesel Post Injection Timing on HC Emissions and Catalytic Oxidation Performance

For diesel emission control systems containing a Diesel Oxidation Catalyst (DOC) and a Catalyzed Soot Filter (CSF) the DOC is used to oxidize the additional fuel injected into the cylinder and/or the exhaust pipe for the purpose of increasing the CSF inlet temperature during the soot regeneration. Hydrocarbon (HC) oxidation performance of the DOC is affected by HC species as well as a catalyst design, i.e., precious metal species, support materials and additives. How engine-out HC species vary as a function of fuel supply conditions is not well understood. In addition, the relationship between catalyst design and oxidation activity of different hydrocarbon species requires further study.

In this study, diesel fuel was supplied by in-cylinder, post injection and exhaust HC species were measured by a gas chromatograph-mass spectrometry (GC-MS) and a gas analyzer. The post injection timing was set to either 73°, 88° or 98° ATDC(after top dead center). The DOC used was either Pt or Pt/Pd based and oxidation characteristics for each HC species were compared. To mimic the CSF regeneration conditions main/post ratio and mass flow rate of intake air were adjusted to create a 300°C inlet and a 550°C DOC outlet condition. Many more HC species were detected with compared to without the post injection. With the post injection, higher concentration of alkane (C₉-C₂₀), alkene (C₂-C₁₅), Formaldehyde (HCHO) and Acetaldehyde (CH₃CHO), were observed compared to without post injection. The Pt/Pd catalyst showed higher HC oxidation performance than the Pt catalyst when the catalyst length was long, but the Pt catalyst showed better performance when the catalyst was short. The Pt/Pd catalyst was assumed to have higher HC cracking functionality compared to the Pt catalyst.