An Experimental Study of Fischer-Tropsch Fuels in a Diesel Engine
Alternative fuels for Diesel engines include ester-based fuels (derived from sources such as rapeseed, soybeans and palm oil), dimethyl ether, and various synthetic fuels. Use of these fuels could contribute to a cleaner environment, reduce global warming and improve the human health by reducing levels of harmful emissions. One such fuel is Fischer-Tropsch (F-T) fuel, a synthetic fuel that can be produced from coal, natural gas or biomass, the properties of which heavily depend on the catalyst used in its production.
Various published studies have shown that use of F-T fuels could reduce emissions of soot, NOx, HC and CO, to varying degrees, since they have a number of desirable properties, including low sulfur and aromatic contents, high cetane numbers and low density.
To further explore the performance of F-T fuels, and the emissions generated when they are used, engine tests were performed at Chalmers in both HCCI and conventional Diesel combustion modes. The engine used in the investigations was a single cylinder, 0.5 liter, research engine, and the performance of two F-T Diesel fuels derived from natural gas, produced by Statoil in Trondheim, was compared with that of two conventional Diesel fuels (Swedish low sulfur Diesel and European EN 590 Diesel).
In HCCI combustion mode the soot emissions were too low for meaningful comparison between the fuels. However, HC and NOx emissions were lower when using the F-T fuels. There were no significant between-fuel differences in emissions of CO and CO2, or fuel consumption. In HCCI combustion mode, the combustion was phased by using exhaust gas recirculation (EGR), thus there were no between-fuel differences in ignition delay, despite the early injection (which is a key feature of HCCI combustion). However, higher levels of EGR were needed for the F-T fuels than for the standard Diesel fuels, which reduced the thermal efficiency of the engine.
In conventional Diesel combustion mode the soot emissions were markedly lower (by up to 30 percent) when using the F-T fuels than when using the already clean Swedish low-sulfur Diesel. Reductions were also found in HC and CO emissions, while CO2 and fuel consumption values obtained with the different fuels were similar. The high cetane number of the F-T fuels resulted in shorter ignition delays for the pilot injections.
A challenge that had to be met before widespread use of F-T fuels could be economically viable was to minimize their production costs. However, these costs have fallen sharply, through a combination of intense research efforts and the construction of F-T plants with high production capacities, and they are now competitive with oil from fossil sources, so long as the price of the latter is higher than $25/barrel.
The possibilities of producing Fischer-Tropsch fuels from several feed-stocks and using existing engines without modifications are major advantages with F-T fuels. Moreover, F-T fuels can be blended in any proportions with petroleum-based Diesel, and existing refueling and maintenance infrastructure can be used for vehicles running on them. All these advantages, and the cleanliness of their exhaust gases, make F-T fuels attractive alternative Diesel fuels.