Fischer-Tropsch and FAME Fuels as Alternatives for Diesel Engines; an Experimental Study
Abstract: Research into alternative fuels is necessitated by our heavy reliance on limited oil reserves and increasingly strict emissions legislation. The studies reported in this thesis involved conducting engine tests using alternatives to fossil Diesel fuel in both heavy-duty and light-duty single cylinder engines. The combustion modes considered were conventional Diesel combustion and two more advanced modes: low temperature combustion (LTC) and homogeneous charge compression ignition (HCCI). Both Fischer-Tropsch (FT) and Fatty Acid Methyl Esther (FAME) fuels were considered; in both cases, the formation of emissions, fuel consumption and combustion behavior were compared to those of conventional Diesel fuels. The overall objectives of these studies were to determine whether it is possible to operate conventional Diesel engines with the alternative fuels and to operate the engines in a fashion that will minimize the emissions of species covered by legislation such as soot and NOx. It was also expected that the results obtained would provide new insights into soot and NOx formation when burning alternative fuels in a Diesel engine. The first experiments were performed in a light-duty engine with Fischer-Tropsch fuels, Swedish environmental class 1 (MK1) and European Diesel (EN590), using both conventional and HCCI combustion. In the conventional Diesel combustion mode, it was found that the soot emissions were up to 30 percent lower than those obtained using MK1. It was further seen that the high cetane number of the tested F-T fuels reduced the ignition delay and over-leaning, which resulted in lower HC emissions. Despite the high cetane number of the F-T fuels, HCCI combustion could be achieved with increased levels of EGR. The FT fuels are less dense than conventional Diesel fuel, which may reduce the extent of spray-wall interactions and would thus reduce CO emissions. The FT fuels had heating values that were similar to those for conventional Diesel fuels and therefore exhibited comparable specific fuel consumption. In the heavy-duty engine tests, the FT fuels yielded soot emissions that were up to 60 percent lower than those for MK1 Diesel. However, there was no clear difference between the two in terms of NOx emissions, which appeared to be more sensitive to the engine’s operating parameters than to the nature of the fuel. For example, they could be reduced by up to 90 percent by increasing the level of EGR from 0 to 40%. The FAME fuels also yielded very substantially lower soot emissions than European Diesel in the heavy-duty engine, being up to 90% lower for the FAME fuels. This was attributed to the absence of aromatics (which are precursors in soot formation) in the alternative fuels and to the high oxygen content of the FAME fuels. In addition, the flame lift-off was longer for FAME fuels, which improves the mixing of fuel and air and thus generates lower soot emissions. Unfortunately, the FAME fuel also yielded NOx emissions that were up to 20% higher than those for conventional Diesel, which was partially due to the FAME flame temperature being 40°C higher than that for Diesel fuel. The oxygen content of the FAME fuels also gave leaner fuel-air mixtures and thus higher NOx emissions. When assessing alternative fuels, it is important to consider ethical and environmental factors throughout their entire lifecycle and also to be aware that there is unlikely to be a single alternative that can fully replace all fossil fuels currently in use.
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