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Is There a Spark-Ignited Heavy Fuel Engine in Your Future?

By Anthony J. Liberatore, EAA 99484

A 100cc two-stroke-powered UAV converted to run on JP-5 for the U.S. Marines.

With a new war effort under way, a new military requirement has emerged that is a technology driver; it requires that all ground power units (GPUs), unmanned aerial vehicles (UAVs), and other units with small engines run on JP-5 (jet fuel) by 2010 in order to eliminate the logistics of carrying different fuel types into a theater. 

This requirement is driving a number of manufacturers and entrepreneurs to find creative solutions to fill this need. Some current, unique solutions have been found by Evan Guy Enterprises; Dan Gurney (of racing fame); Sonex Research Inc. (not to be confused with Sonex Ltd., the kit plane manufacturer); and Mercury Marine.

While there are some true diesels, such as the DeltaHawk, that are trying to fit in and fill given roles as UAV engines, the goal of many of these innovators is to modify existing engines to run on “heavy fuels.” That said, we may not see general aviation applications for a long time. In fact, with today’s emission regulations, any application outside the military might never come to fruition. Nevertheless, the concept is so tantalizing that we cannot help but dream of what an engine this would be for our aircraft.

What they have created with these “spark-ignited heavy fuel engines” are engines that can burn a diesel-type fuel without the high diesel compression ratios normally needed for the combustion process and without the associated high engine weight that is required for strength. It gets even more enticing; many of the engines they are converting are two-strokes, which are even lighter than the small certified engines we are accustomed too.

Two Entrants, Two Solutions
Sonex Research Inc., of Annapolis, Maryland, has a unique and patented approach using a design that can be applied to the cylinder head or the piston. This design utilizes a center chamber that has a series of smaller chambers surrounding it, as shown on the image. The outer chambers are connected to the center chamber via individual passageways. A current application of Sonex’s technology in the field is a number of two-stroke, 100cc-powered UAVs converted to run on JP-5 for the U.S. Marines. Sonex has two variations of its technology still in R&D mode. With these variations, one is touted to make diesels run cleaner, and the second could allow for the elimination of a spark source when direct fuel injection is used, while at the same time allowing heavy fuel use. This is a small and relatively new firm, and many of these projects may need to mature.
Another interesting entry in this arena is the OptiMax JP outboard engine by Mercury Marine. Mercury Marine is developing an outboard and a “jet” (both in the more-than-200-hp class) for Navy SEAL use. The OptiMax JP is a derivative of the company’s successful OptiMax Series of outboard motors, which range from 75 hp up to 250 hp and are direct-injected two-strokes.

Heavy Fuel

Direct injection applied to a two-stroke gives the engine its inherent light weight, with four-stroke fuel consumption and emissions as well as a more robust lower end, since the crank and rod bearings are not exposed to fuel-diluted oil. These direct-injected two-strokes utilize Orbital Engine’s air-assisted direct-injection technology, which atomizes the fuel droplets down to eight microns, which is the industry benchmark. The flexibility of this system allows Mercury Marine to convert the engine to heavy fuel while maintaining 95 percent part compatibility with its gasoline-burning brethren.

As you might gather, what’s going on in this arena is cutting edge. However, we aviators and experimenters have always been on that edge when it comes to applying technologies. Perhaps a two-stroke, direct-injected, spark-ignited heavy fuel engine with an excellent power-to-weight ratio and great fuel consumption specifics would drive a new generation of airframe designs. Not to mention that it would be neat to taxi to the Jet A fuel pump to fill ’er up!

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