科研之友
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摘要
The current work investigates the flame structure and the propagation of premixed laminar flame fronts for mixtures of diluted methanol-air enriched by fuel reforming products under spark ignition engine conditions. Two engine concepts were investigated: one with external fuel reforming (EFR) and one with reformed exhaust gas recirculation (R-EGR). Here, the fuel reformate (or syngas) is a mixture of H-2, CO, and CO2 with a CO selectivity of 6.5%, which is used to represent the products of methanol steam reforming over a Cu-Mn/Al foam catalyst. The simulations were exercised over a wide range of dilution level and unburned gas temperature at 40 bar with a skeletal chemical kinetic mechanism using zero/one-dimensional codes. Two types of dilution-air and EGR-were compared at the same fuel-to-charge equivalence ratio (phi'). The results showed that the knock limit for spark-ignited operation is extended with rising dilution levels, especially when diluted by an R-EGR mixture. Syngas addition also leads to a reduced knock tendency. At the knock limit, the fraction of heat released by autoignition is greater at a higher dilution rate. At the lower flammability limit, the stable flame propagation range is expanded to a lower unburned gas temperature when using air dilution. Under stoichiometric conditions, dilution with an R-EGR mixture is recommended for partial load operation, because it provides a stable flame at high dilution levels. The influence of gas properties were also investigated, where a shorter ignition delay after compression was observed with a leaner mixture. The ignition delay increased with a higher EGR ratio, especially in the case of the R-EGR mixture. Therefore, the knock tendency increases with a leaner mixture. However, the knock ringing intensity decreases if the mixture is diluted by air. A mixture diluted by R-EGR can reduce the knock tendency, compared to the two EGR dilution cases, and can decrease the knock ringing intensity, compared to the two air dilution cases at high dilution ratios.
- 出版日期2017-9
