SONEX Two-Stroke Heavy Fuel Engine (HFE) Technology

The U.S. Department of Defense (DoD) now requires engines used in unmanned aerial vehicles (UAVs) and other military applications for which gasoline storage and use are undesirable, to operate on less volatile, kerosene-based heavy fuels (JP-5, JP-8 and D-2 diesel) to reduce the hazard associated with gasoline.  In the case of two-stroke spark-ignited (SI) engines, Sonex has established a viable heavy fuel engine (HFE) technology baseline by applying its patented Sonex Combustion System (SCS) combustion chamber design and proprietary Cold Starting System (CSS) to the conversion of single and multi-cylinder, lightweight, gasoline engines for use in military (and commercial) applications such as UAVs to start reliably, even in cold temperatures.  The SCS CSS is comprised of a patented heated fuel vaporizer and a solid state control module.

The SCS heavy fuel conversion maintains the gasoline engine’s stock carburetion or fuel injection system, intake and exhaust systems, spark ignition system, and compression ratio.  SCS HFEs^TM running on heavy fuels JP-5, JP-8 and D-2 diesel (with lubricant additive for all fuels) retain the ignition precision of the SI process and knock-free combustion.  No modifications are made to the moving parts, including the piston.  Compared to operation on gasoline, SCS HFEs achieve equal to or reduced fuel consumption (18%-28% less at cruise rpm) and produce no visible smoke.

In SI two-stroke engines, the SCS enables the combustion of heavy fuels through design modification of the cylinder heads to achieve a chemically/thermally enhanced combustion process while still relying on the spark to initiate combustion.  The SCS embodies a unique cylinder head with combustion chamber insert housing the proprietary SCS technology and a glow plug starting system.  SCS design features in the cylinder head provide control of fuel vaporization late in the compression stroke such that a portion of the heavy fuel is then vaporized near the spark plug.  As the combustion event progresses it causes the fuel to fully vaporize and combust.

For two-stroke gasoline engines that have the cylinder head and cylinder in a single casting, the stock cylinder head portion is removed by machining and the remaining cylinder casting is decked.  The SCS head assembly and cylinder are reattached to the crankcase by bolts.  The following figure shows an example of a disassembled SCS two-stroke HFE cylinder head assembly.

Disassembled SCS Two-Stoke HFE Components, from left to right:

Combustion chamber insert containing SCS design embodiments,

Cylinder head, including glow plug heater, and

Cylinder body with stock head removed

As described below, extensive prototype HFE development for a small (1 - 2 horsepower) gasoline engine for Insitu, Inc. has moved to the pre-production phase.  Other Sonex HFE designs in various stages of development are available for immediate application to commercially available SI gasoline engines.  For engine models not previously converted, Sonex will undertake development to achieve an optimized design.  The gasoline performance of an engine to be converted is thoroughly assessed using a baseline procedure to document thrust, fuel consumption, cooling and overall engine durability. The deliverable prototype HFE is achieved by applying the SCS engine design modifications in a two-step, best efforts process based on (1) a feasibility demonstration and (2) optimization of a prototype through design trade-off iterations to match the baselined gasoline power and improve fuel efficiency of the engine.

In 2005 Sonex began working on an application of the SCS HFE technology with Insitu, Inc. of Bingen, Washington.  Insitu designs, develops and manufactures unmanned aircraft systems and provides associated services for commercial and military applications.  Insitu developed its long endurance, low cost ScanEagle UAV in partnership with The Boeing Company.  ScanEagle has been used to provide services for the U.S. Marine Corps, U.S. Navy and Australian Army in the Middle East since being deployed in 2004.  In 2008 Insitu was acquired by Boeing and now operates as a wholly owned independent subsidiary of Boeing’s Integrated Defense Systems’ Military Aircraft division.

Sonex developed a prototype combustion system to convert the 3W-28i, two-stroke, SI gasoline engine used in the ScanEagle to heavy fuel operation, and in 2006 signed a license with Insitu, which is now non-exclusive, for the SCS HFE^TM technology applicable to UAVs with HFEs that are twenty horsepower or less. 

In January 2007 Insitu announced that it set an endurance flight mark for the ScanEagle by running its engine modified with HFE technology licensed from Sonex, along with other enhancements developed by Insitu engineers, on JP-5 heavy fuel for 28 hours and 44 minutes.  The previous longest flight on a ScanEagle was 22 hours and 10 minutes using gasoline.  The endurance flight on JP-5 was conducted in temperatures ranging from -6^o C to -16^o C with no observed problems.  At cruise power levels, the ScanEagle using the SCS HFE^TM consumed up to 28% less fuel than with the stock gasoline engine.

The HFE version of the ScanEagle UAV began production in 2008.  Due to its use of SCS HFE^TM technology in its 2 hp engine, ScanEagle can be launched and recovered from ships.

Other Sonex HFE designs in various stages of development are available for immediate application to commercially available SI gasoline engines.  For engine models not previously converted, Sonex will undertake development to achieve an optimized design.  The gasoline performance of an engine to be converted is thoroughly assessed using a baseline procedure to document thrust, fuel consumption, cooling and overall engine durability. The deliverable prototype HFE is achieved by applying the SCS engine design modifications in a two-step, best efforts process based on (1) a feasibility demonstration and (2) optimization of a prototype through design trade-off iterations to match the baselined gasoline power and improve fuel efficiency of the engine.

Sonex is seeking partners to fully develop other applications of the SCS HFE technology.   Additional licensing opportunities are available for Sonex HFE technology for use in UAVs, all-terrain vehicles, pumps, outboard engines, small watercraft, and generator sets.