Both spark ignition (SI) natural gas engines and compression ignition (CI) dual fuel (DF) engines suffer from knocking when the unburnt mixture ignites spontaneously prior to the flame front arrival. In this study, a parametric investigation is performed on the knocking performance of these two engine types by using the GT-Power software. An SI natural gas engine and a DF engine are modelled by employing a two-zone zero-dimensional combustion model, which uses Wiebe function to determine the combustion rate and provides adequate prediction of the unburnt zone temperature, which is crucial for the knocking prediction. The developed models are validated against experimentally measured parameters and are subsequently used for performing parametric investigations. The derived results are analysed to quantify the effect of the compression ratio, air-fuel equivalence ratio and ignition timing on both engines as well as the effect of pilot fuel energy proportion on the DF engine. The results demonstrate that the compression ratio of the investigated SI and DF engines must be limited to 11 and 16.5, respectively, for avoiding knocking occurrence. The ignition timing for the SI and the DF engines must be controlled after −38°CA and 3°CA, respectively. A higher pilot fuel energy proportion between 5% and 15% results in increasing the knocking tendency and intensity for the DF Engine at high loads. This study results in better insights on the impacts of the investigated engine design and operating settings for natural gas (NG)-fuelled engines, thus it can provide useful support for obtaining the optimal settings targeting a desired combustion behaviour and engine performance while attenuating the knocking tendency.
The study of the thermodynamic properties of engine fuels and the in-cylinder gas is involved in the analysis of chemical compound reaction and the thermodynamic analysis of fuel and gas, which is quite important in engine combustion investigations because the fuel chemical energy converts into internal work energy during this stage. Although the research on the thermodynamic properties of fuel and in-cylinder gas has lasted a few decades, with the development of new fuel types, developing general models with sufficient accuracy to calculate the fuel and in-cylinder gas thermodynamic properties for engines remains a challenge. This paper presents a model to calculate the diesel fuel and in-cylinder gas thermodynamic properties based on the mixture composition theory and considering the diesel fuel and in-cylinder gas mixtures in terms of the chemical reaction fundamentals. The diesel fuel and in-cylinder gas thermodynamic property modeling approach for the combustion investigation is then applied in the heat-release calculation model of a marine diesel engine, which is validated by experimental research on heat release. According to the simulation and experimental results, when considering that the diesel fuel and in-cylinder gas thermodynamic properties are affected by the in-cylinder temperature, fuel type, and air excess ratio, the engine combustion simulation results in predictions that are more accurate than those when setting the values constant. This paper provides a general approach for the investigation and application of engine fuels and in-cylinder gas thermodynamic properties, in particular for new fuel substitution in engines.
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