Walter Weissman

Effect of the hydrocarbon molecular structure in diesel fuel on the in-cylinder soot formation and exhaust emissions

Abstract Evaluations of diesel fuel effects on combustion and exhaust emissions in single-cylinder direct-injection diesel engines led to the unexpected result that a Swedish ‘class 1’ fuel generated more particulate matter (PM) than a fuel denoted ‘improved’, even though ‘class 1’ fuel had much lower distillation temperatures, aromatic concentration, sulphur level, and density than the ‘improved’ fuel. Little differences were observed in the combustion characteristics between these fuels, but detailed compositional analyses showed that ‘class 1’ fuel contains higher levels of cyclic and/or branched paraffins. Subsequent investigations in a laboratory flow reactor showed that ‘class 1’ fuel produces more soot precursors such as benzene and acetylene than the ‘improved’ fuel. In addition, experiments in a low-pressure laminar flame apparatus and shock tube with model (single-molecule) paraffin fuels showed that isoparaffins and cycloparaffins generate more soot precursors and soot than n-paraffins do. These results strongly suggested that the effect of molecular structure on exhaust PM formation should be more carefully examined. Therefore, a new series of investigations were performed to examine exhaust emissions and combustion characteristics in single-cylinder engines, with well-characterized test fuels having carefully controlled molecular composition and conventional distillation characteristics and cetane numbers (CNs). These investigations revealed the following. Firstly, under low and medium loads, cycloparaffins (naphthenes) have a higher PM formation tendency than isoparaffins and n-paraffins. Under high-load conditions, however, the paraffin molecular structure has a very small effect. Secondly, a highly n-paraffinic fuel does not yield PM reductions as high as expected, due to its high CN and corresponding shorter ignition lag, which initiates combustion under a state of insufficient fuel-air mixing. This finding was corroborated by laser-induced incandescence analyses. Thirdly, the lowest PM emissions were observed with a paraffinic fuel containing 55 per cent isoparaffins and 39 per cent n-paraffins. Fourthly, aromatics give higher soot and PM levels than paraffins do at high and medium load conditions. Smaller differences are observed at lower speeds and loads. Fifthly, the best fit to the PM emissions was obtained with an equation containing the regression variables CN, aromatic rings, and naphthene rings. This expression of the fuel effects in chemical terms allows well-to-wheel analyses of refining and vehicle impacts resulting from molecularly based fuel changes.

Fuel Octane and Composition Effects on Efficiency and Emissions in a High Compression Ratio SIDI Engine

The effects of fuel octane have been assessed on the efficiency and emissions of a high compression ratio (e=13) spark ignition direct injection (SIDI) engine. Under low load stratified operation (1200 rpm, ∼20% load), a low octane fuel (RON=84, comprised of toluene, iso-octane, and n-heptane) yielded higher brake thermal efficiency and significantly lower hyd carbon emissions than a base gasoline (RON=91). The indicator diagram for the low octane fuel showed evidence for two stage heat release, suggesting the presence of spark induced compression ignition (SICI). These results suggest that higher efficiency under low load stratified conditions can be obtained with lower octane fuels that undergo SICI combustion. The effect of fuel octane under high load was assessed at WOT with a high RON model fuel (RON=103, comprised of toluene, iso-octane, and n-heptane). This high octane fuel exhibited a torque benefit compared to pure iso-octane (RON=100) and premium gasoline (RON=99) that is significantly greater than expected based on RON alone. The results suggest that a high RON fuel, in particular one that is high in aromatics, yields significant torque benefits under high load.

Fuel Effects on SIDI Efficiency and Emissions

Spark ignition direct injection (SIDI) engines have the potential to realize significant thermal efficiency improvements compared to conventional port fuel injection engines. The effects of fuel properties on efficiency and emissions have been investigated in a prototype of an Avensis Wagon equipped with a 2.0 liter, 4 cylinder spark ignition, direct injection (SIDI) engine designed to meet US 2000 emission standards. The vehicle employed a close coupled three-way catalyst and a NOx storage and reduction catalyst. Seven matrix fuels were blended to the same RON with varying levels of aromatics, olefins, ethanol, and volatility. Relative thermal efficiency, fuel economy, and tailpipe emissions were measured for the matrix fuels and a base fuel under the FTP LA4 driving cycle. The engine was operated in a lean burn mode in light load condition for approximately half of the driving cycle. Fixed speed/load engine bench tests were also carried out for three of the matrix fuels to complement the vehicle tests. In addition, laminar burning velocity measurements of all seven matrix fuels were made in a constant volume combustion vessel. The vehicle tests showed a 2% spread in relative thermal efficiency, with increased efficiency correlating with higher olefins and lower aromatics. In the bench engine tests, a 2 % increase in peak torque was observed, with the fuel having the highest olefins plus aromatics content yielding the highest torque. The influence of the primary fuel properties autoignition resistance and burn rate (laminar burning velocity) on these results is discussed. Lower aromatic levels directionally correlated with decreased NOx emissions, and lower driveability index (DI) directionally correlated with decreased non-methane hydrocarbon emissions.

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.