Molecular structure of DMF. Credit: ACS, Zhong et al. Click to enlarge.

With a boiling point of 92-94 °C, DMF is also less volatile than ethanol (bp 78 °C), and is immiscible with water. DMF is also believed to have a high research octane number (119), which will allow for the use of high engine compression ratios for improved fuel economy.

More attractively, DMF consumes only one-third of the energy in the evaporation stage of its production, in comparison to that required by fermentation for ethanol. In fact, the catalytic strategy successfully developed for the production of DMF from building blocks of carbohydrates, cellulose (fructose or glucose), has made the large-scale and low-cost production of DMF possible. The implication of this breakthrough is significant, because DMF shares very similar physicochemical properties to gasoline, as previously discussed. However, its wide application as a main automotive fuel has been prevented by limited historical supply and commercial availability.

Despite the developments in the biomass conversion technology, which may have paved the way for the mass production of DMF as a new biofuel candidate, there are some outstanding issues that remain unresolved before it can be commercialized. Little is currently known about the combustion and emission characteristics of DMF, especially the speciation of the unregulated emissions and their toxicities. However, reports are beginning to emerge on the laminar flame characteristics of DMF combustion. If DMF can be accepted as an alternative transportation fuel, extensive engine performance and emissions research is required.

—Zhong et al.

The UK researchers claim that theirs is the first investigation on the use of DMF as a biofuel in a single-cylinder, spark-ignition, GDI (gasoline direct-injection) engine.

The test engine was a single-cylinder, four-stroke, spark-ignition, spray-guided direct-injection (DI) engine with a swept volume of 565.6 cm³ and a compression ratio of 11.5. The fuel was delivered by a free piston accumulator that was pressurized to 150 bar using bottled nitrogen (oxygen free).

The test fuels were 95 research octane number (RON) gasoline, bioethanol (both supplied by Shell Global Solutions UK), and DMF (99.8% purity from Shijiazhuang Lida Chemical Co., Ltd., China).

Among the findings of the study were:

1. The fuel rate for ethanol is at least 33% more than that of gasoline throughout the entire load range. DMF, however, is very similar to gasoline, which is largely due to the similar gravimetric calorific values and relatively high density.

2. The indicated thermal efficiency of DMF is similar to gasoline. Ethanol, on the other hand, has a consistently high indicated efficiency, which is probably due to its high combustion efficiency and oxygen content (35% oxygen content by mass, 18% higher than DMF), the researchers suggested. The efficiency does not drop off as suddenly as is experienced with DMF and gasoline and remains above 37%. This advantage is partly offset by the higher pumping losses incurred because of its lower stoichiometric air/fuel ratio.

3. The initial combustion duration of DMF is shorter than that of gasoline. When it is compared to ethanol, the difference varies with load, so that it is longer at low-load conditions but shorter at higher load conditions.

4. The constant ignition timing of 34° BTDC induced severe engine knock for DMF at 7.1 bar IMEP and gasoline at 6.5 bar IMEP. Ethanol showed no signs of knock, and the load could be extended safely to 8.5 bar IMEP. The onset of knock for DMF at loads similar to gasoline was somewhat unexpected, the authors said, and this finding justifies a further study into the real octane number for DMF, which is given in early studies as higher than that of ethanol.

5. The emissions of CO, HC, and NO_x using DMF are all similar to those with gasoline. Ethanol combustion produces lower CO emissions because of the lower maximum in-cylinder temperature and possibly more complete combustion. At lower loads, ethanol produces lower HCs, probably because of its relatively high oxygen content, which gives rise to more complete combustion. Ethanol also produces lower NO_x emissions for the whole load range, possibly because of its higher latent heat of evaporation, which leads to a relatively lower in-cylinder temperature.

6. PM emissions of DMF are similar to that of gasoline. DMF actually produced the smallest mean diameter sized particles of the three fuels and a similar concentration to gasoline, which suggests the total mass of PM for DMF is the lowest.

7. Using pure DMF as a fuel on a research engine did not show any apparent adverse effects on the engine for the duration of the experiments.

Overall, the experiments confirm that, because of the similar physicochemical properties of DMF and gasoline, DMF exhibits very similar combustion and emissions characteristics to gasoline. This indicates that DMF may be suitable to use as an existing gasoline-type engine fuel and that no major modifications and adjustments would be needed to produce an equivalent engine performance and emissions level.

To supplement this investigation, detailed engine testing using the MBT timings for the three fuels is being carried out and will follow this report. Modeling and optical studies of the spray behavior and chemical kinetics of the oxidation reactions of DMF are ongoing and will explore, in more detail, the in-cylinder combustion initiation and development. In addition, a critical issue for biofuels and automotive fuels in general is the presence of any toxic components in the engine-out emissions. Therefore, further tests concerning the speciation of emissions for DMF are underway.

—Zhong et al.

Resources

• Shaohua Zhong, Ritchie Daniel, Hongming Xu, Jun Zhang, Dale Turner, Miroslaw L. Wyszynski and Paul Richards (2010) Combustion and Emissions of 2,5-Dimethylfuran in a Direct-Injection Spark-Ignition Engine. Energy Fuels, Article ASAP doi: 10.1021/ef901575a