Validation of Chemical Kinetic Mechanisms in a Motored Engine for n-Heptane and n-Dodecane

抄録

Normal alkanes are important components of real-world and surrogate fuels, but discrepancies exist in their chemical kinetic mechanisms making combustion simulation during the engine design process less reliable. To provide ignition data for mechanism development and improve the fundamental understanding of the combustion process, reactivity and oxidation intermediates for two normal alkanes, n-heptane and n-dodecane, were measured in a modified CFR octane rating engine at an equivalence ratio of 0.25. The in-cylinder conditions of the motored engine covered a temperature range from 400K to over 1000K and a pressure range from atmospheric pressure to 22.9 bar with a compression ratio ranging from 4.0 to 15.7. Concentrations of detailed oxidation intermediate species in the exhaust gas were measured to provide simulation benchmarks. Existing chemical kinetic mechanisms were validated using a multizone model developed to simulate HCCI combustion in the motored engine. Prediction of CO concentration as an oxidation stage indicator was accurate in the negative temperature coefficient (NTC) regime, with less than 8% prediction error with the best-performing kinetic mechanisms. Prediction of the critical compression ratio (CCR, observed by sweeping the compression ratio until the onset of second-stage ignition) for n-heptane was improved from 7.9 to 7.0 by updating the H₂/O₂ sub mechanism, with the measured CCR being 7.1±0.08. Existing mechanisms showed incorrect predictions for n-dodecane reactivity. Although reaction pathways were similar, reaction rates differed between the mechanisms and greatly influenced the simulation results. All three evaluated mechanisms underestimated n-dodecane reactivity, with the best predicted CCR being 0.3 compression ratio too high. C₁₂H₂₄O production might need to be eliminated, and CH₃CHO production enhanced to improve the n-dodecane mechanisms.