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How Do They Do That? Up, Up and Away with CNG

Key Points
  • Compressed natural gas is a promising aviation fuel because of its lower cost and emissions.
  • Benefits include better engine performance at higher altitudes and improved engine life.
  • Limitations include a lack of fueling infrastructure and a higher aircraft purchase price.
CNG aircraft

In 2013, a single-engine Aviat Husky A1-C flew more than 1,000 miles from Aviat Aircraft's headquarters in Afton, Wyo. to Oshkosh, Wis. for an aviation show. If you saw the plane, you were watching the first bi-fuel, piston-powered aircraft to operate on both compressed natural gas (CNG) and aviation gasoline.

Like land-based vehicles, switching to CNG offers fuel cost savings (up to 80 percent less expensive than aviation gasoline), and lower emissions (particulates and carbon dioxide are reduced by 90 percent and 30 percent, respectively). CNG also contains no lead, which is a significant issue with aviation gasoline.

For the pilot, there are other potential benefits:

  • Improved engine life. Lubricating oils last three to four times longer because CNG does not contaminate and dilute crankcase oil.
  • Better engine performance. CNG has a higher octane: 138 versus the current 100 of aviation gasoline. A higher octane improves performance at higher altitudes.
  • Improved safety. There is less danger of carbon monoxide buildup in the cabin; burning CNG reduces CO by 90 percent.
  • Lighter weight. One gas gallon equivalent (GGE) weighs 5.66 pounds versus 6.01 pounds for one hundred low lead (100LL) fuel.
  • Lower exhaust gas and cylinder head temperatures. Because these are approximately 20°F lower, better engine operating temperatures are maintained for climbing.
  • Instantaneous engine starts. There is no risk of 'vapor lock' or 'hot start' issues.
  • Eliminates residual fuel in the cylinders after shutdown. With no fuel, ungrounded magnetos (which produce the electricity for ignition) can't cause a propeller to turn unexpectedly.
  • Reduces the cost of learning to fly. At 10 gallons per hour, the cost of fuel alone could be reduced $40 to $60 per hour, making it economical for a flight school to install CNG refueling equipment on-site.

With the bi-fuel plane, changing fuels is possible with just the flip of a switch; the CNG fuel control system uses electronic ignition. The all-composite CNG fuel tank holds around 9 gallons in addition to the standard aviation gasoline tanks totaling 50 gallons. Both types of tanks may be filled to capacity and still carry a 220-pound pilot and 70 pounds of gear. The CNG tank is removable for refilling at a personal or commercial location.

A 200 horsepower, four-cylinder Lycoming IO-360-A1 D6 engine provides a cruising speed of 143 mph. The only modification to the engine was the installation of new pistons to increase the compression ratio from 8.50:1 to 10:1. The demonstration aircraft can be flown approximately seven hours at a 65 percent power setting. Landing and shutdown procedures are the same as those of a conventional airplane.

Despite CNG's benefits, there are still a few caveats:

  • The engine must be redesigned to take advantage of CNG's higher octane.
  • CNG adds between $12,000 and $15,000 to the base price of a gasoline powered aircraft.
  • If using CNG immediately after starting, engine oil temperature must be warm enough to help prevent ice formation in the gas pressure regulator.
  • Landing gear may require extending and repositioning to ensure adequate ground clearance for the CNG tank.

Aviat believes CNG will work best as a single fuel source, which requires the development of a fueling infrastructure.

New design enables CNG

A unique aircraft under development by VX Aerospace Corporation (Morganton, North Carolina), the VX-1 KittyHawk, may accelerate the adoption of CNG because of its larger usable internal volume. This larger volume allows CNG tanks to fit inside the aircraft without affecting aerodynamics—made possible by the KittyHawk's novel design. Called a ‘lifting body’ or ‘blended wing’ aircraft, the fuselage and wings are combined into one shape, similar to the side profile of a dolphin. When based on payload capability, the footprint is smaller than other aircraft. Because the aircraft is made from carbon composites, it also has better structural efficiency and durability.