Effect of Hydrocarbon Molecular Structure in Diesel Fuel on In-Cylinder Soot Formation and Exhaust Emissions

Exhaust emissions and combustion characteristics from well-characterized diesel test fuels have been measured using two types of single-cylinder HSDI diesel engines. Data were collected at several fixed speed/load conditions representative of typical light-duty operating conditions and full-load performance (smoke-limited maximum torque) points. In addition, in-cylinder soot formation processes of these fuels were investigated via Laser Induced Incandescence (LII) using an optically accessible single-cylinder engine. The test fuels used in this study have been formulated with a sophisticated blending algorithm that systematically varies the hydrocarbon molecular structure in the fuels while maintaining the distillation characteristics of market diesel fuels. The following results have been obtained from this study.

(1) The lowest PM emissions were observed with a fuel containing approximately 55% iso-paraffins and 39% n-paraffins with CN=52.5. Compared with the base fuel (corresponding to average market fuel in Japan), this fuel yields a 40 - 70% PM reduction and an increase in the maximum torque of approximately 8%. (2) A highly n-paraffinic fuel representative of a Fischer-Tropsch liquid did not yield PM reductions as high as expected. This is due to its very high cetane number (CN=80.5), resulting in a decreased ignition delay which initiates combustion before sufficient fuel-air mixing has occurred. This conclusion is corroborated by LII analyses of highly n-paraffinic fuels which show regions of high soot concentration in the burning fuel spray jet near the injector. (3) Under low and medium loads, cyclo-paraffins (naphthenes) have a higher PM formation tendency than iso- or n-paraffins. Under high load conditions, however, paraffin molecular structure has a very small effect on PM formation. (4) Aromatics have a higher soot/PM formation tendency than paraffins under all speed/load combinations investigated. A correlation of PM formation with fuel chemical composition has been developed from a statistical analysis of the data. Expressing the fuel effects in chemical terms allows well-to-wheel analyses of refining and vehicle impacts resulting from molecularly based fuel changes.