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European Aviation Safety Agency (EASA), for use in general aviation aircraft. Their efforts have
recently proven fruitful with certifications to operate their Centurion 1.7 engine on Cessna 172 aircraft,
and soon this same engine will be certified for the Piper Warrior III. Both the government and industry
are already evaluating an application of this type on the MQ-1 Predator to determine what “actual”
performance results would be realized when installed.
Technology outlook. The use of both motor gasoline and aviation gasoline in small UA is undesirable,
because it is both unsafe (JP fuels have higher flashpoints than gasoline, making them more tolerant of
explosive combustion situations) and logistically difficult to support. There are currently several ongoing
efforts to develop small JP5/8 fuel burning engines in the power classes and power to weight ratios being
discussed here, including lightweight versions for aviation applications. For example, the opposed
cylinder (OPOC) engine development program (FEV Engine Technology, Inc.) is developing a light
weight, high powered diesel engine that is being sized for the A160. In addition, Nivek R&D, LLC, is
developing a lightweight six-cylinder diesel engine for the A-160.
Reliability. Reliability of current low cost two and four-cycle UA engines are on the order of a few
hundred hours, sometimes less. This shortcoming, when compared to turbine engines, is often
overlooked due to the low cost of reciprocating engines. However, good engine reliability has proven
to be a significant factor in user acceptance of UA. Nevertheless, most UA demonstrations, and even
development programs do not stress reliability in the design process, nor prove reliability in their
development, many times resulting in disappointing results in extensive flight and operational testing.
UAS ROADMAP 2005
APPENDIX D – TECHNOLOGIES
Page D-4
Developing reliability in a small HFE will present a large challenge due to the differences in
combustion and lubrication between JP fuels and gasoline, and the duty cycles imposed on them for
UA use.
Efficiency, brake specific fuel consumption, (BSFC) and power-to-weight ratio. One of the most
desirable traits for any UA is persistence, and engine fuel efficiency has a major influence on the
number of UA required for a given time on target coverage. Current gasoline two cycle engines have
relatively poor efficiency, while four stroke engines are better but at the cost of increased engine
weight. Both engines are significantly better than small gas turbines in this power class. As a result,
any effort to develop HFEs will place a large emphasis on efficiency. A HFE that operates on a true
diesel cycle could double the endurance of a given UA, which normally uses a two-stroke gasoline
engine. Currently, two cycle engines tend to be used extensively in small UA, particularly in
demonstration efforts. They provide the UA designer a low cost and lightweight, yet powerful
engine, providing significant capability per dollar. This is known as the power-to-weight ratio. Due
to low cycle efficiency their BSFCs tend to be high, resulting in aircraft with limited endurance
capabilities. Existing gasoline engines converted to operate on heavy fuels would not have
significantly improved BSFCs, but would improve the logistics footprint by operating with a common
fuel. True diesel cycle engines would offer greatly reduced BSFCs, but technological advances are
required to reduce the weight of these engines to get them near the same mass as gasoline engines.
The technological advances to bring two-cycle engine efficiencies up to HFE levels are equally
complex.
Technology challenges. There are two approaches to using JP fuels in UA designed for lightweight
gasoline engines; converting an existing gasoline engine to operate satisfactorily on JP fuels, or
developing a true diesel engine light enough to be substituted for an existing gasoline engine.
Depending on the approach chosen there are different technology challenges associated with each:
• Conversion – This approach will yield an engine of similar efficiency to the current gasoline
engines (no improvement in BSFC) but will be close in power to weight and minimize integration
efforts. Challenges include designing a combustion system that effectively burns JP fuels without
using a diesel cycle, and obtaining acceptable engine reliability while using JP fuels.
• Light-weight Diesel – This approach will yield an engine of much greater efficiency than current
gasoline engines but a significant technology challenge will be weight reduction in order to even
approach that of current gasoline UA engines while maintaining reliability.
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