GTL fuels, with their high cetane and absence of PAH (poly-aromatic hydrocarbons), are cleaner-burning than petroleum diesel and have the potential to reduce PM, as well as the potential to reduce NO_x emissions by increasing the EGR ratio without any smoke penalty.

The absence of sulfur in GTL can also contribute to a fuel consumption improvement in the case of NO_x adsorber catalyst use by reducing the desulfurization process frequency due to the reduction of deterioration of the aftertreatment catalyst.

However, the lower densities of GTL fuels can result in increased fuel consumption.

Since the total amount of GTL fuels will not drastically increase in the near future, the use as a blend fuel will be on of the likely short-term strategies to penetrate into the market. In that case, there is no need to consider the deterioration of the volumetric fuel consumption in exchange for the most low-emissions potential of the neat GTL fuels. However, if engines are optimized to adapt to the distinctive specifications of the neat GTL fuel, assuming its use in restricted (urban) areas, there could be a possibility to improve not only exhaust emissions, but also fuel consumption.

From their investigation of the combustion and emissions characteristics of the GTL fuels across the three platforms and the single-cylinder engine with optical access, the researchers found:

• There were no apparent differences between all fuels, including the baseline diesel fuel, on NO_x emissions and brake specific energy consumption. However, smoke and HC emissions were reduced by use of GTL. The lower T90 GTL fuel achieved lower smoke emissions, although HC emissions were increased with its use.

• First appearance of luminous flame from the injected fuel sprays was temporary and spatially delayed with the GTL fuels, even though the ignition delay was shortened. This result showed that soot formation was suppressed with the GTL fuels, perhaps caused by the absence of PAH, according to the researchers. Soot oxidation phenomena did not differ much with each fuel. Since the diffusion of pilot injected fuel was narrower in the GTL fuel under the same injection parameters as the baseline diesel, it is necessary to modify the pilot injection parameters for the GTL fuels to suppress the deterioration of smoke emissions.
  

• Toxic mono-cyclic aromatic hydrocarbons were drastically reduced with one of the GTL fuels compared to the baseline diesel.

For the optimization study, the team used GTL Fuel B—because of its lower PM and HC emissions under steady state and transient tests—and modified the 4L engine to achieve the simultaneous improvements in exhaust emissions and fuel consumption.

• They reduced the compression ratio to 15.0:1 to improve the NO_x and smoke emissions trade-off, especially in low- to medium-load condition, without any energy consumption penalties. The purpose of reducing the compression ratio was to maintain the longer ignition delay even with the high cetane number.

  Combustion chamber geometry was modified to obtain better air utilization for the pilot fuel. The bowl diameter was slightly enlarged to accommodate the longer penetration of fuel spray and the bowl center height was decreased to entrain more unburned gas into the main fuel spray.

• By using the smoke tolerance of Fuel B for the low oxygen content, it was possible to reduce the amount of intake fresh air, and concurrently to increase the EGR ratio while maintaining smoke emission. It was therefore possible to apply a smaller turbocharger without the deterioration of fuel consumption.

  By applying the high EGR and the low air excess ratio concept, there are other advantages of reducing emissions and  fuel consumption simultaneously by control of the exhaust aftertreatment system. Since the exhaust gas temperature is increased by the reduction of air quantity, it is possible to maintain the activation of the catalyst even under continuous low load conditions like the JE05 transient mode, without using any control to increase temperature, like the delay of injection timing or the use of intake throttling. The lower oxygen content in the exhaust gas can reduce the quantity of fuel dosing to exhaust oxygen for the NO_x regeneration with the DPNR aftertreatment system.

• The achieved exhaust emissions and the specific energy consumption results with the JE05 Japanese transient test cycle satisfied the project goal, which was to halve the emission levels of current Japanese new-Long-Term Regulation (1.0 g/kWh of NO_x, 0.0135 g/kWh of PM) while maintaining the same volumetric fuel consumption as the base diesel fuel engines.

Resources

• Noboru Uchida, Hiroshi Hirabayashi, Ichiro Sakata, Koji Kitano, Hiroshi Yoshida, Nobuhiro Okabe (2008) Diesel Engine Emissions and Performance Optimization for Neat GTL Fuel (SAE 2008-01-1405)