Comparative Assessment of the Impact of Water Addition either to the Intake Air or in Diesel Emulsion on the Performance and Emissions of a HDDI Diesel Engine

AbstractA theoretical investigation is performed to comparatively evaluate the impact of water addition to the intake air (humidification) or in water-diesel fuel emulsion on the performance and emissions in a turbocharged (T/C), heavy-duty direct injection (HDDI) diesel engine. Specifically, three water addition strategies are examined, i.e., intake air humidification, combustion of water-diesel emulsion considering increasing the injector nozzle-hole diameter for keeping constant the injection duration with increasing water content, and combustion of water-diesel emulsion considering a variable injection duration with increasing water content. In all cases, three water percentages were considered: 10%, 20%, and 30% by weight. In all cases examined, the engine brake power output was kept constant corresponding to a speed of 1,800 rpm and to 25%, 50%, 75%, and 100% of full load, equal to those corresponding to the baseline case without water addition. From the examination of the theoretical results, it is shown that the use of water-fuel emulsion with increased injection duration results in higher cylinder pressures and bulk gas temperatures during the premixed combustion phase compared to the other two water addition strategies, whereas water-fuel emulsion with constant injection duration promotes the intensification of the diffusion combustion phase compared to the other two water induction strategies. The use of both water-fuel emulsion strategies result in a significant reduction of cumulative soot formation rate compared to intake air humidification. Overall, it can be concluded that the use of water-fuel emulsion with constant injection duration can result in a simultaneous increase of brake thermal efficiency and significant reduction of exhaust soot and NO values at both partial and full engine load; thus, it is the most beneficial water addition strategy of those examined in this study.