Microstrain  Engineering Design Team

Propulsion Research Could Revolutionize Jet Fuel

Development of a cold flow fuel additive can lower commercial airlines operating costs.

AFRL's Propulsion Directorate, Turbine Engine Division, Fuels Branch, Wright-Patterson AFB OH

In 1989, the Propulsion Directorate began evaluating jet fuel additives to reduce coking and maintenance costs in aircraft engines and fuel systems. In 1994, the directorate selected an additive, designated SPECxAID 8Q462, to test on F-16s with Pratt Whitney F100-PW-200 engines. The resultant additized fuel, designated JP-8+100, raises the fuel's thermal stability up to 100 F hotter without increasing its propensity to form coke. The two-year test program using JP-8+100 resulted in a significant reduction in coke-related maintenance. To date, thousands of Air Force aircraft, as well as numerous aircraft of allied nations, successfully use JP-8+100. The Federal Aviation Administration (FAA) recently approved the use of the Betz Dearborn SPEC×AID 8Q462 fuel additive and its commercial equivalents for all Pratt Whitney commercial aircraft turbine engines. The FAA's approval of the additive for use in commercial engines is a huge step towards wide use in the commercial aviation industry.All Air Force aircraft, with the exception of the high-altitude U-2 reconnaissance aircraft, use JP-8 fuel. The U-2 uses jet propellant, thermally stable (JP-TS) fuel because of the decreased freeze point and elevated thermal stability requirements of the U-2's higher altitude flights. JP-8+100 meets the thermal stability requirements but falls short of JP-TS low-temperature capabilities. The directorate is starting the second of a five-year program to improve the low-temperature properties of JP-8 fuel through the use of additives.

Jet fuel freezes, similar to gelatin, and firms but does not solidify like frozen water. The directorate developed a new fuel, JP-8+100LT (LT for low temperature), that will suppress crystal growth and adherence. Freezing crystals, like building blocks, stack onto each other. As they group together, they become too large to pass through fuel screens and filters (see Figure 1). JP-8+100LT will prevent crystals from building into larger structures and allow fuel to continue flowing through all parts of a system (see Figure 2). Cold flow enhancers prevent crystal growth, enabling small crystals to remain suspended and flowable in liquid fuel.

Jet fuel cost is one driving factor for this research. The price for JP-8 in fiscal year 2000 was $0.61/gal. The cost of JP-TS was $3.25/gal. The cold flow additive package for JP-8 will cost only pennies per gallon, resulting in great savings. This effort promises to reduce U-2 fuel cost by as much as 80%.

Other nations and commercial airlines expressed interest in the fuel additive program. Currently, commercial jet fuel is produced from only the highest grade of petroleum (top 10% of a barrel of oil) to meet temperature range and thermal stability requirements. This makes the price of commercial jet fuels very expensive. Airlines can significantly reduce their operating costs through the use of this cold flow additive, which can extend the temperature range of jet fuel produced from lower grades of petroleum. Due to international interest in this technology, the directorate's research team presented a paper at the International Association for the Stability and Handling of Jet Fuels Symposium in Graz, Austria.

The directorate recently opened a new cold flow fuel laboratory at Wright-Patterson AFB, Ohio. It houses a cold stage microscope, differential scanning calorimeter, and the low-temperature fuel wing tank simulator. A larger scale simulator, currently under construction, will incorporate a model of the U-2 wing, but will also fully adapt to accommodate future testing for other planes, such as the Global Hawk and other technically advanced aircraft.

Transition of this new fuel to operational Air Force units is under way. One method is to mix LT with +100 and distribute the mixture to operational units. JP-8+100LT may also have applications in other military vehicles.


Figure 1. JP-8 crystal growth


Figure 2. JP-8+100LT crystal growth

 

Ms. Cynthia Obringer and 1Lt Kirsten Wohlwend of the Air Force Research Laboratory's Propulsion Directorate wrote this article. For more information contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn/index.htm. Reference document PR-00-08.

 

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