Background. The Secretary of the Navy set a goal of sailing a fully operational “Great Green Fleet” by 2016. All candidate non-petroleum fuels must qualify as drop-in replacements. As part of the fuel qualification program, lab-scale testing is performed to assess preliminary fitness for use in compression ignition applications. The testing compares in-cylinder and brake metrics between the alternative and a baseline fuel.

Metrics include start of ignition, ignition delay, location of peak pressure, and rate of heat release.

DSH. The DSH biofuel is derived from a Direct Sugar to Hydrocarbon (DSH) process in which sugar and yeast produce a farnesene-type hydrocarbon molecule (branched hydrocarbon with multiple double bonds) which is then processed into a moderately branched single alkane molecule (> 98% purity) with a moderately higher cetane number than conventional diesel fuels. (The Navy acquired DSH from Amyris.)

A distinctive chemical characteristic of DSH fuel that differs from any conventional diesel fuel is that it is comprised almost entirely (>98%) of farnesene, a single, C₁₅ branched paraffin [2,6,10-trimethyldodecane]. Specification properties that differ from typical diesel fuels include the absence of aromatics and sulfur, a moderately low density, and a moderately high cetane number.

It has been shown that blending a neat DSH component at a composition up to 50% by volume into current stocks of naval distillate fuel can create a product that meets all physical and chemical requirements in the F-76 procurement specification.

… The study presented in this report provides a characterization of DSH combustion quality and resulting brake metric effects, and it provides a preliminary assessment on 50/50 F76/DSH76 fitness for use in Navy compression ignition systems.

—Hamilton et al.

The “moderate deviation” of DSH properties from those of F76 is the primary reason for qualifying it as a blendstock rather than as a fully 100% synthetic alternative, the researchers said.

The team used three engines in their testing: an AM General Engine Products (GEP) 6.5L V-8 HMMWV (Humvee) turbo diesel; a Waukesha CFR diesel, and a single-cylinder Yanmar L48V.

The team used three acceptance criteria:

• Relative ignition delay (IGD) of the new fuel as compared to the baseline fuel.
• Angle of Peak (AOP) combustion pressure.
• Maximum rate of heat release (ROHR).

The physical property differences lead to later Start of Injection times in three diesel engines (AM General GEP, Waukesha CFR and Yanmar). However, due to the increased reactivity of DSH, ignition delay is reduced across most of the speeds and loads tested.

While engine operation on the neat DSH falls just inside the proposed Navy alternative fuel acceptance criteria at all operating points, they expect that a 50% DSH and 50% diesel fuel blend will be tested for broader application due to acceptable combustion characteristics in this compression-ignition study.

Since relative changes in baseline combustion phasing are understood to decrease as the concentration of DSH decreases, 50/50 F76/DSH76 is considered fit for use in compression ignition engines and an acceptable candidate blend to continue with full scale diesel engine qualification testing.

—Hamilton et al.

Resources

• Hamilton, L., Luning-Prak, D., Cowart, J., McDaniel, A. et al. (2014) “Direct Sugar to Hydrocarbon (DSH) Fuel Performance Evaluation in Multiple Diesel Engines,” SAE Int. J. Fuels Lubr. 7(1):270-282, doi: 10.4271/2014-01-1472