Two are small businesses replacing their aging turboprops or twin-engine piston planes.

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In the seven or so years that I've been flying, I've put in more than 2, 000 hours at the stick of single-engine piston airplanes, including a Beechcraft Bonanza, two Cessnas and, most recently, a sleek Cirrus SR22 that I've owned for 18 months.

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EcoMotors and Achates are developing two-stroke, opposed piston diesel engines while Pinnacle is working on a four-stroke opposed piston gasoline engine.

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