Reduction of exhaust emissions is a major research task in diesel engine development in view of increasing concern regarding environmental protection and stringent exhaust gas regulations. Simultaneous reduction of NOx emissions and particulate matter is quite difficult due to the soot/NOx trade-off and is often accompanied by fuel consumption penalties. Towards this aim, automotive engineers have proposed various solutions, one of which is the use of alternative gaseous fuels as a supplement for the commercial liquid diesel fuel. This type of engine, which operates fuelled simultaneously with conventional diesel oil and gaseous fuel, is called “dual fuel” diesel engine. Among alternative gaseous fuels, natural gas is considered to be quite promising due to its low cost and its higher auto-ignition temperature compared to other gaseous fuels facilitating thus its use on existing diesel engines. Previous research studies revealed that natural gas/diesel engine operation results in deterioration of brake engine efficiency, CO and HC emissions compared to conventional diesel fuel operation. In attempt to curtail these negative effects, various theoretical and experimental studies were carried out examining the influence of various parameters such as pilot fuel quantity, diesel fuel injection timing advance and intake charge conditions on “dual fuel” engine performance characteristics and pollutant emissions. However, there are more to know about the proper combination of these engine parameters to attain the optimum results regarding reduction of CO and HC emissions without further deteriorating, if not improving, brake engine efficiency. Hence, in the present study, a theoretical investigation is conducted using an engine simulation model to examine the effect of the aforementioned parameters on performance and exhaust emissions of a natural gas/diesel engine. Predictions are produced for a high-speed natural gas/diesel engine performance characteristics and NO, CO and Soot emissions at diverse engine speeds and loads using a comprehensive two-zone combustion model. The main objective of this comparative assessment is to elaborate the relative impact of each one of the above mentioned parameters on engine performance characteristics and exhaust emissions. Furthermore, an endeavor is made to determine the optimum combinations of these engine operational parameters. The conclusions of this study may be proven to be considerably valuable for the application of this technology on existing DI diesel engines.

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