Homogenous gas-phase reactions in exhaust-gas tail pipes of internal combustion engines

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Increasing demands on the control of pollutant emissions such as NOx from internal combustion engines, the use of fuels with decreased climate impact, and the development of more efficient engines call for alternate after-treatment strategies such as positioning of catalytic converters closer to the engine. The resulting higher temperature and pressure bear the potential for significant gas-phase reactions in the tailpipe effecting the emission reduction. For instance, in a selective catalytic reduction (SCR) system, the molar ratio of NOx and NH3 could be changed by gas-phase reactions leading to increased NOx emission. Homogeneous reactions among the exhaust gas components, HC/HCO/CO/NOx/O2/NH3/H2O/CO2/N2, have barely been investigated and are far from being understood at tail-pipe conditions. The present joint Sino-German project will study gas-phase reactions for those conditions in the presence of a NH3-SCR system at lean operating conditions. Gas-phase reactions in the raw gases in the engine-near tailpipe and in emission control devices will be characterized, understood, and modelled through studies in the homogeneous gas-phase and in the open channels of catalytic honeycomb structures. The scientific aim is to provide a fundamental understanding of gas-phase reactions that will occur in these raw gases. Only such a fundamental understanding will allow the characterization of exactly those reactive mixtures that indeed reach the catalyst and thus to develop realistic chemical models, and to optimize fuel-engine-associated aftertreatment systems towards reduced emissions with decreased toxicity and chemically less hazardous signature. To achieve these goals, the project will make use of the combined, complementary, and often unique expertise of the partners regarding combustion chemistry diagnostics, kinetic analyses, and model development relevant to oxidation and combustion, and aftertreatment strategies. The project will consider (i) fuel diversity reaching from conventional gasoline and Diesel to current biofuels and to future electrically-derived fuels, (ii) variable engine operation conditions, and (iii) operation of the catalytic converter closer to the engine. This vast combination of conditions is a reason for the current lack of fundamental understanding of gas-phase reactions in the near-engine raw gases. It is thus of utmost importance to identify common, prototypical compositions and variables, including mixture components and temperature as well as potentially pressure sensitivity.This collaboration addresses acute challenges that will also have an impact on air quality issues in China and Germany. The project will build upon and expand an already well-established partnership between German and Chinese groups.