Comparison of the Soot Reactivity of Different Diesel Fuels on a Research Single-Cylinder Engine and on a Multicylinder Production Engine

Abstract

Abstract. This study examines the impact of various diesel fuels on exhaust emissions under representative part-load engine operation. Since differences in NOx emissions between fuels are generally minor, the focus is placed on particulate emissions and the corresponding soot reactivity. The test fuels included fossil diesel (with and without fatty acid methyl ester, FAME), renewable paraffinic diesel (hydrotreated vegetable oils, HVOs), as well as several blends: R33 (diesel/HVO), HVO/OME (oxymethylene ether), diesel/OME with equivalent OME content, and a blend of HVO, OME, and monoaromatics. Experiments were conducted on a single-cylinder research engine (SCE) and in part validated on a Euro 6d-Temp multicylinder production engine (MCE). Regardless of the set operating point, it can be stated that soot emissions are highest both regarding mass and number when fossil diesel fuel without fatty acid methyl ester is used, and that the lowest soot emission occurs when using the HVO/OME blend. The other fuels are in between in terms of soot emissions. This behavior can also be observed in the total particle number concentration. When using HVO or OME, the particle size distribution also shifts toward smaller particles. However, this varies depending on the operating point. This is also confirmed by the tests at the multicylinder engine. Thermogravimetric analysis (TGA) was used to determine soot reactivity. With regard to the soot reactivity on the single-cylinder research engine, it can be determined that this is very dependent on the operating point, and the differences are accordingly more or less pronounced depending on the fuels used. However, the trends are similar regardless of the operating point. But the variance in soot reactivity due to the fuel formulation is reduced at low air–fuel ratios, high exhaust gas recirculation (EGR), and higher engine speeds, so that this is largely determined by the engine's setting parameters. The highest soot reactivities were achieved with both test engines when using fossil diesel with fatty acid methyl ester. In general, soot emissions resulting from oxygenated fuels due to fatty acid methyl ester or OME show significant increases in soot reactivity. On the other hand, additional blending with aromatics reduces the soot reactivity again.