Give pilots and aircraft owners credit for one thing: It takes a strong constitution to remain functional and focused when the avgas truck creaks to a halt and you realize that what you thought was a 5 on the price placard at 100 yards was actually a 6. So you whimper softly and pay the man, perhaps salving yourself with the idea that the new Jet-A burning airplanes will make this nightmare go away. But will they really? Are diesels that much more efficient than gasoline engines? Yes, possibly. But it depends on the engines being compared. When it comes to efficiency, Lycoming and Continental are not created equal. But the larger issue- and whats driving the Jet A piston revolution-is that Jet A-burning pilots (piston or turbine) neednt get their pants snagged worrying about their fuel being phased out in 10 years. They can also count on it being available in every country on the planet and at ever more airports. And thats why we write stories about Jet A piston engines, not because theyre cheaper to operate-they can be-and not because theyre more efficient-they are, but not as much as you might think. To put some numbers on this, we recently collected some diesel fuel and operating cost data from a Diamond DA42 Twin Star and bored into the perfect diesel vs. gas comparison: Diamonds Lycoming-powered DA40 vs. its Thielert-diesel powered DA40 TDI.
Whats Efficient?
Fuel efficiency is a slippery concept when applied to airplanes, partly because our frame of reference is automotive mileage. In the U.S., average real world automotive fuel economy for cars is about 22 MPG, according to
Consumer Reports. So for most us, 12 to 15 MPG may be unimpressive, while anything north of 30 MPG is good.In airplanes, we think in gallons per hour related to some particular cruise speed, but with avgas in the $4.50 to $6.50 range, fuel flow looms largest. If placed on equal footing with cars and with speeds corrected to statute miles, airplanes get similarly disparate fuel economy, with the worst under 10 MPG and the best-usually experimentals or diesels-delivering 20 MPG or better.
Not to be discounted is the speed factor. Its one thing to cruise in an SUV getting 16 MPG at 75 MPH and quite another to be in a Cirrus doing 240 MPH at 12.5 MPG. While fuel mileage is a valid way of looking at efficiency, the better way is to examine fuel specifics or brake specific fuel consumption. This is a shorthand and accurate way of comparing engine efficiency because it considers only energy in and power out, without distracting variables such as drag and weight. BSFC is expressed in pounds of fuel burned per horsepower per hour. If an engine burns 10 gallons per hour and delivers 130 HP, its BSFC is .446-not bad, but no award winner, either.