Effects of Varying Engine Settings on Combustion Parameters, Emissions, Soot and Temperature Distributions in Low Temperature Combustion of Fischer-Tropsch and Swedish Diesel Fuels

It has been previously shown that engine-out soot emissions can be reduced by using Fischer-Tropsch (FT) fuels, due to their lack of aromatics, compared to conventional Diesel fuels. In this investigation the engine-out emissions and fuel consumption parameters of an FT fuel derived from natural gas were compared to those of Swedish low sulfur diesel (MK1) when used in Low Temperature Combustion mode in a single cylinder heavy-duty diesel engine. The effects of varying Needle Opening Pressure (NOP), Charge Air Pressure (CAP) and Exhaust Gas Recirculation (EGR) according to an experimental design on the measured variables were also assessed. CAP and EGR were found to be the most significant factors for the combustion and emission parameters of both fuels. Increases in CAP resulted in lower soot emissions due to enhanced charge mixing, however NO_x emissions rose as CAP increased. In contrast to the effects of CAP, as the EGR level rose from 0% to around 50% the soot emissions increased and the NO_x emissions decreased. However, at approximately 50% EGR the soot emissions started to decrease again because the soot formation zone was avoided since there was less available oxygen (and hence lower in-cylinder temperatures). The effects of increasing NOP were similar, but weaker, to those of increasing CAP. In contrast to previous studies, in which FT yielded lower soot emissions, it was observed in this study that FT fuel yielded higher soot emissions at high EGR levels (above 40%) than MK1.

An AVL visioscope 1.1 was installed in the test engine to measure instantaneous temperatures and soot concentrations in the cylinder during combustion. The results showed that the FT fuel yielded lower soot concentrations than MK1 at low EGR levels.