Direct-Injection Engine Study Finds That DMF Is a Promising Biofuel, With Combustion Performance and Regulated Emissions Comparable to Gasoline

Zhong

Fuel flow rate (gasoline equivalent) under different engine loads for DMF, ethanol, and gasoline at 1500 rpm and ? = 1.0. Credit: ACS, Zhong et al. Click to enlarge.

Researchers from The University of Birmingham (UK) and specialty chemicals company Innospec recently performed a series of experiments in a single-cylinder gasoline direct-injection (GDI) research engine to study the performance of the liquid biofuel 2,5-dimethylfuran (DMF) benchmarked against gasoline and ethanol. Initial results, reported in the ACS journal Energy & Fuels, suggest that DMF is very promising as a new biofuel; not only is the combustion performance similar to commercial gasoline, but the regulated emissions are also comparable.

With recent work improving the high yield conversion of biomass-derived carbohydrates to DMF (earlier post, earlier post), there is growing interest in using this as a bio-based substitute for petroleum-derived gasoline. DMF has a volumetric energy density similar to that of gasoline formulations and 40% greater than that of ethanol.

Dmf

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).

Fuel Properties
Properties   DMF Ethanol Gasoline
molecular formula   C6H8O C2H6O C2-C14
molecular mass kg/mol 96.13 46.07 100-105
density at 20 °C kg/m3 889.7 790.7 744.6
water solubility at 25 °C mg/mL insoluble, ?1.47 highly soluble, ?100 insoluble
gravimetric oxygen content % 16.67 34.78 0
H/C ratio   1.33 3.00 1.865
O/C ratio   0.17 0.5 0
stoichiometric air/fuel ratio   10.72 8.95 14.56
gravimetric calorific value (LCV, liquid fuel) MJ/kg 33.7 26.9 43.2
volumetric calorific value (LCV, liquid fuel) MJ/L 30 21.3 32.2
research octane number (RON)   119 110 95.8
auto-ignition temperature °C 285.85 423 257
latent heat of vaporization at 20°C kJ/mol 31.91 43.25 38.51

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 NOx 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 NOx 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


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