Annex 18: Future Greener Diesel Fuels
Because vehicles with diesel engines are more fuel efficient than vehicles with spark-ignited gasoline engines, vehicles with diesel engines are becoming more popular in several IEA member nations. Although heavy multi-ton trucks have generally used diesel engines for some time, the use of diesel engines in smaller trucks and automobiles is increasing, and continued design improvements in diesel engines intended for new personal transportation vehicles make it likely that the numbers of diesel engines will continue to increase. However, diesel engines have relatively high emissions of particulate matter (PM) and nitrogen oxides (NOx).
Preliminary tests have shown that the addition of ether oxygenates to diesel fuel can lead to substantial reductions in exhaust emissions of both particulate matter and NOx. For example, in the United States, the Co-ordinating Research Council (CRC) VE-10 study found that increasing the oxygen content of diesel fuel from 0 to 2 percent by adding an oxygenate, decreased PM emissions by 10 to 15 percent. Other tests have shown that using diesel fuels of increased cetane number can reduce NOx emissions and many oxygenates under consideration for blending with diesel fuel have very high cetane numbers and so would be expected to contribute to this effect.
However, the consideration of such oxygenates as motor fuels is relatively recent, and there is a lack of data related to their use as motor fuels and their ability to be blended successfully with various diesel fuels. Specifically,
- The oxygenate must be miscible with various types of diesel fuels over the range of ambient temperatures seen in vehicle service.
- The oxygenate must give a blend with an appropriate volatility range when mixed with various diesel fuel base stocks. The flammability of the fuel in tanks depends on the vapour pressure, so this can be an important safety issue.
- The oxygenate-diesel fuel blend must have adequate or improved ignition quality.
Many different oxygenates for diesel fuel have been suggested, and additional information on properties of oxygenate-diesel fuel blends is needed to focus future R&D in this area.
In order to support the use of oxygenates in diesel fuels, this annex provided data on the miscibility, flash point, cloud point, water tolerance, vapour pressure, and ignition quality over a range of diesel fuel-oxygenate blends and environmental temperatures through laboratory tests with diesel fuel and oxygenate samples.
The diesel fuels included a USA reference diesel, a Fischer-Tropsch diesel, and an oil sands diesel. The oxygenates tested included:
1. dipentyl ether,
2. tripropylene glycol monomethyl ether,
3. glycerol tributrate (tributrin),
4. 2-ethoxyethyl ether (diethylene glycol diethyl ether),
5. dibutyl maleate,
6. dibutoxymethane (butylal), and
7. diethyl maleate [Only limited work because of miscibility difficulties].
Oxygenate blend levels were 0 (diesel only), 5, 10, 30, and 100 (oxygenate only) volume percent. Test temperatures ranged from -30 to 30 C. Vapour pressure measurements were made using a gas chromatographic technique that distinguished fuel and oxygenate contributions to the total vapour pressure. Ignition quality measurements were made using the IQT constant volume combustion apparatus.
Michael J. Murphy
Battelle Memorial Institute, USA