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Hydrogen in Diesel Exhaust: Effect on Diesel Oxidation Catalyst Flow Reactor Experiments and Model Predictions
Abstract:
Engine operating strategies typically geared towards higher fuel economy and lower NOx widely affect exhaust composition and temperature. These exhaust variables critically drive the performance of After Treatment (AT) components, and hence should guide their screening and selection. Towards this end, the effect of H2 level in diesel exhaust on the performance of a Diesel Oxidation Catalyst (DOC) was studied using flow reactor experiments, vehicle emission measurements and mathematical models. Vehicle chassis dynamometer data showed that exhaust from light-duty and heavy-duty diesel trucks contained very little to almost no H2 (FTP average CO/H2 ∼ 40 to 70) as compared to that of a gasoline car exhaust (FTP average CO/H2 ∼ 3). Two identical flow reactor experiments, one with H2 (at CO/H2 ∼ 3) and another with no H2 in the feed were designed to screen DOCs under simulated feed gas conditions that mimicked these two extremes in the exhaust H2 levels. The results from these experiments showed that at moderate space velocities when H2 was present in the feed, DOC light-off temperature was 20°C lower as compared to when H2 was not present in the feed. This large difference in light-off temperature suggested that DOCs should be screened using feed gas conditions that more closely represent actual vehicle exhaust H2 level (which is typically not measured). A DOC model was calibrated using the light-off data from the two flow reactor experiments. The model based on the calibration that included H2 in the feedgas resulted in predictions of light-duty diesel vehicle CO and HC conversions that were up to 32% higher than the actual measured conversions, further confirming the importance of using realistic engine exhaust conditions on AT design and model development.