The results of a comprehensive experimental investigation into the exhaust emission performance and combustion properties of neat and blended Gas-To-Liquids (GTL) diesel fuel are presented. A sulphur-free European diesel fuel was used as the reference fuel, and two blends of the GTL diesel fuel with the reference fuel, containing 20% and 50% GTL diesel fuel respectively, were investigated. The study was based on a Mercedes Benz 2.2 liter passenger car diesel engine and presents emission data for both the standard engine calibration settings, as well as settings which were optimized to match the characteristics of each fuel.
Vehicle emission tests showed that the GTL diesel fuel results in reductions in HC and CO emissions of greater than 90%, while PM is reduced by 30%, and NOx remains approximately unchanged. Engine bench dynamometer tests showed reductions in soot of between 30% and 60%, and NOx reductions of up to 10% with the GTL diesel fuel, depending on the operating point. Results with the GTL diesel blends showed a pronounced non-linear response, with reductions in emissions generally being larger than indicated by the blending ratio, to the extent that the emission performance of the 50% blend was similar to that of the neat GTL diesel fuel.
Optical engine tests were carried out to investigate the combustion properties of the neat GTL diesel fuel in more detail. In addition to reflecting the better ignition quality of the GTL diesel, these tests also showed that it has better vaporization characteristics and a more uniformly distributed flame structure.
A design of experiments (DOE) approach was used to optimize the injection settings and EGR rates for each fuel in order to obtain the most favourable NOx-soot trade-off. By this means it was estimated that simultaneous reductions of 35% in both NOx and PM could be obtained in the vehicle emission test with GTL diesel fuel, with an optimized engine calibration. The corresponding estimated reductions with the 50% and 20% blends of GTL diesel are 30% and 15% respectively, again reflecting the non-linear exhaust emission response of the blended fuels.