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The effects of air humidity on the knock characteristics of fuels are investigated in a lean-burn, high-speed medium BMEP engine fueled with a CH4 + 4.7 mole% C3H8 gas mixture. Experiments are carried out with humidity ratios ranging from 4.3 to 11 g H2O/kg dry air. The measured pressure profiles at non-knocking conditions are compared with calculated pressure profiles using a model that predicts the time-dependent in-cylinder conditions (P, T) in the test engine (“combustion phasing”). This model was extended to include the effects of humidity. The results show that the extended model accurately computes the in-cylinder pressure history when varying the water fraction in air. Increasing the water vapor content in air decreases the peak pressure and temperature significantly, which increases the measured Knock Limited Spark Timing (KLST); at 4.3 g H2O/kg dry air the KLST is 19 °CA BTDC while at 11 g H2O/kg dry air the KLST is 21 °CA BTDC for the same fuel.

A method is described to characterize the effects of changes in the composition of gaseous fuels on engine knock by computing the autoignition process during the compression and burn periods of the engine cycle. To account for the effects of fuel composition on the in-cylinder pressure and temperature history relevant for knocking, changes in heat capacity of the air-fuel mixture and in the phasing of the combustion process are also incorporated in the method. Comparison between pressure profiles measured in a lean-burn, high-speed medium-BMEP gas engine and the calculated pressure profiles at non-knocking conditions shows that the method accurately computes the in-cylinder pressure history when varying the fuel composition. To characterize gaseous fuels for their resistance to knock, a propane-based scale (Propane Knock Index, PKI) is reported in this study.

In this paper, we present a two-zone thermodynamic model that allows us to predict the time dependent in-cylinder conditions (P, T) in a high-speed medium BMEP engine fueled with different gas compositions. The details of this model rely on the observation that the measured combustion phasing correlates strongly with the (computed) laminar burning velocity under the conditions existing in the cylinder. To account for turbulence effects, a model parameter is introduced in the burning rate model. Calculations show that for Dutch Natural Gas (DNG)/ethane/propane mixtures a single model parameter, independent of the gas composition, is sufficient to predict the pressure profiles accurately. In contrast, the model parameter for DNG/H2 mixtures shows a dependence on the hydrogen content in the fuel. Adjustment of the model parameter resulted in successful prediction of the effect of hydrogen on the combustion phasing.