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The Knock Propensity of Carbon Dioxide-Containing Natural Gases: Effect of Higher Hydrocarbons on Knock-Mitigating Influence of Carbon Dioxide
ISSN: 1946-3952, e-ISSN: 1946-3960
Published December 16, 2020 by SAE International in United States
Citation: Essen, M., Gersen, S., Dijk, G., Dai, L. et al., "The Knock Propensity of Carbon Dioxide-Containing Natural Gases: Effect of Higher Hydrocarbons on Knock-Mitigating Influence of Carbon Dioxide," SAE Int. J. Fuels Lubr. 13(3):265-273, 2020, https://doi.org/10.4271/04-13-03-0017.
To assess the effect of the presence of carbon dioxide (CO2) in natural gases on the knock resistance of fuel, the knock behavior of a lean-burn, high-speed medium Brake Mean Effective Pressure (BMEP) Combined Heat and Power (CHP) engine fueled with CH4 + 8 mole% C3H8 mixtures. The engine experiments are supplemented with ignition measurements and simulations of ignition and cylinder processes for various fuel compositions. The engine results show that increasing the fraction of CO2 results in an increase in knock resistance. The analysis of simulations of cylinder processes shows that for binary mixtures (CH4/CO2) and ternary mixtures (CH4/C3H8/CO2) the increase in knock resistance with increasing CO2 fraction is caused by the reduction in peak pressure/temperature, which consequently increases the autoignition delay time of the mixture. For the ternary mixtures, the presence of 8 mole% propane in the fuel mixture partially compensates the reduction in peak pressure/temperature by decreasing the autoignition delay observed in the Rapid Compression Machine (RCM) measurements and ignition calculations. In contrast, for binary mixtures, the autoignition delay time increases with increasing CO2 content. Consequently, the effect of increasing fractions of CO2 in methane results in a substantially larger increase in the knock resistance as compared to the CH4/C3H8 mixtures studied.
The characterization of the changes in knock resistance using the Propane Knock Index Methane Number (PKI MN) shows an agreement with the Knock-Limited Spark Timing (KLST) measurements within the experimental uncertainty. Traditional methods for computing an MN, such as AVL and MWM, base the computation for CO2-containing gases on extrapolation of measured data for binary CH4/CO2 mixtures. As a result, these methods strongly overpredict the change in MN when CO2 is added to the propane-containing fuel, underestimating the potential risk for an engine. This result shows that caution is advised when extrapolating the binary CH4/CO2 data to gas mixtures containing higher hydrocarbons.