Effects of Indirect and Direct Coal-to-Liquid Fuel on Combustion, Performance, and Emissions in a Six-Cylinder Heavy-Duty Diesel Engine

Coal to liquid (CTL) is one of the potential methods to obtain the alternative fuel used in diesel engines. There are two methods to obtain alternative fuel from coal, namely, indirect and direct liquefaction processes. This paper evaluates the effects of indirect coal-to-liquid (ICL) fuel and direct coal-to-liquid (DCL) fuel on the combustion, performance, and emissions on a heavy-duty diesel engine. The engine was run at 1,200 and 2,000 rpm without exhaust gas recirculation (EGR) and three engine loads (25, 50, and 75%) were operated at each speed. Three injection pressures (80, 100, and 160 MPa) were applied. Results show that the cetane number is the dominating factor in heat-release patterns at the light load; however, the difference of cetane number between CTL fuels and diesel only has a minor effect on ignition delay at high loads. The brake specific fuel consumption (BSFC) of ICL is lower than that of diesel fuel, while the BSFC of DCL is higher than that of diesel fuel. The brake thermal efficiency (BTE) values of two CTL fuels are lower than that of diesel baseline due to their poor lubricity and lower mechanical efficiency. The soot emissions of ICL are always lower than those of diesel fuel at all tested operating conditions. However, the soot emissions of DCL are lower than those of diesel fuel at the low load but a little higher than those of diesel fuel at the high load. The NOx emissions of ICL fuel are lower than those of the other two fuels under low injection pressure, while the NOx emissions of ICL are a little higher than those of diesel fuel at high injection pressure but lower than those of DCL. Unburned hydrocarbons and carbon monoxide emissions of both CTL fuels remain low under both light- and high-load conditions. In general, ICL fuel is more preferable than DCL and diesel fuel from the perspectives of engine performance and emissions.