Tuesday, January 28, 2014
A Natural Gas To Fly
A Husky takes the alternative route to the skies
Early this year, Horn assembled a team of about eight Aviat employees complemented by half a dozen CNG experts to develop the dual-fuel system. "It took approximately six months until the first flight," which utilized a different aircraft, Horn said. The fuel controller, only partially visible on N15NG with the cowl open, regulates the dual-fuel operation.
One major concern during development was the possibility of ice buildup caused by expansion cooling—interfering with the CNG flow at the tank valve; the system resolves the issue by using engine oil to heat the valve. With the current tank, the dual-fuel system weighs 135 pounds.
The CNG power is the real game changer: CNG is homegrown, plentiful and cheap (about $0.86 per gas gallon equivalent).Fuel mapping for the engine—the curve that defines changes to the fuel-air mixture as power goes from idle to maximum—can be set at the factory, with the controller automatically compensating for density altitude, engine timing variations and other factors. But as a proof-of-concept installation still undergoing development, N15NG has a programmable interface for the fuel controller in the cockpit, a round gauge to the left of the multifunction display (MFD) in the center panel. A small fuel gauge tucked between the fuel controller interface and the MFD indicates CNG fuel level in quarter-tank increments via four vertical lights.
Flying On CNG
The fuel source is selected by simultaneously flipping a pair of toggle switches mounted on the starboard sidewall just below the instrument panel. When off, in the down position, avgas feeds the engine; when in the on position, the forward switch shuts the avgas fuel valve, and the rear toggle activates the electronic fuel injectors for the CNG system. When changing from avgas to CNG, the mixture control is simultaneously retarded to idle cutoff. (A separate CNG mixture control is located on the left side of the instrument panel.) When changing from CNG to avgas, the mixture control is advanced. Should electric power be lost when operating on CNG, the system automatically reverts to avgas power.
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