Considering the popularity of biodiesel fuels for diesel vehicles, the impacts of rapeseed oil methyl ester (RME), which is the most utilized biodiesel fuel in Europe, on tailpipe emissions from a diesel passenger car was investigated. In this study, 30% RME blended diesel fuel (RME30) was used and the comparison of tailpipe emissions between RME30 and a reference diesel fuel was conducted using a test vehicle with the latest engine and aftertreatment system.
The results of the investigation reveal that RME30 generates about the same amount of NOx in tailpipe emissions as diesel fuel, and less HC, CO, and PM. These phenomena occurred in spite of attaching catalysts to the test vehicle, and therefore suggesting that the NOx conversion efficiency of the catalysts for RME30 is equal to that for diesel fuel.
The injection rate for RME30 was the same as that for diesel fuel. As for spray characteristics, spray penetration for RME30 at high temperature and high pressure condition tended to be slightly stronger than that for diesel fuel, but the effect is thought to be too small to increase tailpipe emissions in vehicle tests.
The results of the combustion analysis of RME30 show no significant difference found in the heat release rates between RME30 and diesel fuel with pilot injections, which are usually operated. Without pilot injections, however, RME30 has a shorter ignition delay and even less HC, CO and PM emission characteristics than diesel fuel. This tendency was furthermore found to intensify under high EGR rate conditions. These phenomena, the superior ignitability of RME30 with the same cetane number as diesel fuel, seem to be due to the oxygen contained in RME30.
To analyze these phenomena, the combustion process was observed by methods of direct image and laser diagnostics. The observation results of vapor mixture by laser-induced fluorescence (LIF) measurement of HCHO and OH confirmed that RME30 exhibits more active LTR and HTR under higher EGR rate condition than diesel fuel. The active LTR and HTR of RME30 are thought to be related to the reduction of HC, CO and PM emissions.