Comparison of ethanol/diesel fuel dual direct injection (DI2) strategy with reactivity controlled compression ignition (RCCI) in a diesel research engine

Abstract

The dual direct injection (DI2) strategy is expected to cover the advantages of conventional diesel (CDI) mode while avoiding deficiencies of reactivity controlled compression ignition (RCCI) strategy such as high amount of incomplete combustion emissions and extremely high rate of pressure rise. Therefore, the present study focused on the comparison of combustion characteristics of these combustion modes in a diesel research engine, which was firstly operated under CDI mode at varying loadings and a constant speed of 2400 rpm to establish the reference data. Then, a low pressure port fuel injection (PFI) and high pressure gasoline direct injection (GDI) systems were activated to run on RCCI and DI2 modes, respectively, whereas test engine was already equipped with a common-rail direct injection (CRDI) system for conventional diesel fuel injection. Results showed that both direct and port injection methods increased the in-cylinder pressure (up to 9%) compared to conventional mode. In RCCI combustion, the rate of pressure rise increased more than direct injection and conventional mode. CDI exhibited higher thermal efficiency than RCCI and DI2 modes while both advanced modes were successful in reducing NOx and Smoke Opacity. However, significantly higher CO and unburned HC emissions were observed with both advanced modes in comparison to conventional one. Both CO and unburned HC emissions were reduced by direct injection of ethanol (DI2 strategy) at the rates of 16% and 35% compared with port injection method of RCCI strategy. This is an important finding of this study, which shows that the DI2 strategy has a potential to address a deficiency of RCCI and will bring it closer to its application to practical engines.