Fuel Dosing on a Diesel Oxidation Catalyst for After-treatment System Heating on a Heavy Duty Engine Powered by Polyoxymethylene Dimethyl Ethers

Abstract

Polyoxymethylene dimethyl ethers (OME) are synthetic fuels, which offer the property of sustainability because the reactants of production base on hydrogen and carbon dioxide on the one hand, and the air pollution control in consequence of a soot-free combustion in a diesel engine on the other hand. High exhaust gas recirculation (EGR) rates are a promising measure for nitrogen oxide (NO_x) reduction without increasing particle emissions because of the resolved soot-NO_x trade-off. However, EGR rates towards stoichiometric combustion in OME operation reveals other trade-offs such as methane and formaldehyde emissions. To avoid these, a lean mixture with a combination of EGR and exhaust after-treatment with selective catalytic reduction (SCR) is useful. The limitation of urea dosing due to the light-off temperature of SCR systems requires heating measures. Besides electrical heating, fuel injection into the diesel oxidation catalyst (DOC) for an exothermic reaction is an effective method for a quick achievement of the catalyst working conditions. A slip of unburned or partially oxidized hydrocarbons (HC) in fossil diesel operation restricts this measure. Conducted investigations of a heavy-duty engine showed a reciprocal proportionality in OME operation between injection quantity and unburned fuel fragment slip over the DOC. Additionally, the NO oxidation increases with rising post injection quantities, whereas fuel dosing in diesel operation shows the opposite effect. These behaviors during OME operation enable potentials for further NO_x reduction in tailpipe emissions because of earlier urea dosing in SCR systems in addition to lower raw emission levels caused by high EGR rates.