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When will a diesel be available for my aircraft? |
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News, facts, and comments on the coming revolution for piston-engine aircraft.News of September 18, 2005Is diesel too heavy?The most frequent objection I read to aero diesel is: Not only the engine with all ancillaries is bound to be heavier than a conventional engine because of higher compression, but on top of that, fuel being denser, your full tanks are heavier than before and your payload is further reduced. I want to address the later criticism because it triggers my sense of humor. So, there are Cessna and other owners who spend thousands of dollars to have extra tanks installed (luggage compartment, wing tips…), but the fact that, thanks to diesel fuel, you can put 10 to 14% more fuel in your plane at no cost would be a disadvantage! My answer is: nothing prevents you not to top off if you don’t need the range, when comes the (rare) occasion when you do want to carry a full load of passengers… Meanwhile I call this an advantage, not a disadvantage. Your new engine is burning 0.32 Lbs/HP-hour instead of 0.46 (to be discussed soon), and you have 10 to 14% more fuel in the same tanks (depending on the nature of kerosene fuel you are using). Let’s address the serious objection. Yes, a 4-stroke diesel of same cubic capacity than a gasoline engine is heavier because of the higher compression. However, all diesels coming on the market are smaller engines in cubic capacity. If, like the Thielert, they are geared engines, they are even smaller, but with the extra weight of the gearbox. Since all are turbocharged, this also adds some weight. But the biggest weight addition may come (not always) from the need to add heat exchangers to a cowl design which wasn’t meant for diesels in the first place. We already have one superb example of a conversion to diesel which is actually lighter than the original: The Beech Duke now beginning its test flights in Germany with 2 Thielert V-8s weighs 4,660 Lbs. empty against 5,000 for the original, meaning a weight gain of 340 Lbs. And the plane is poised to fly 10 K faster, and to cruise at equal speed on 22 gallons/hour of Jetfuel instead of 45 of Avgas. Martin Hagensieker, who owns the FBO undertaking the conversion, talks about a conversion cost of $ 435,000, future TBOs (at least 2,400 h) costing then the same than with O-540 engines. However we are talking of 350 HP here. In the 30’s and 40’s, the Junkers Jumo diesel of 600 to 1,200HP and more was also showing a definite weight advantage per HP against conventional engines, but it was a 2-stroke engine. There is no definite example, in 4-stroke engines in the 100-230HP range, of an actual gain in weight by converting an existing aircraft model to diesel. On the contrary, one does observe some very moderate weight increases. Will this situation last? It won’t for an obvious reason: Existing conversions are in their early stages. They have been built with expediency in mind: Get FAA or JAA certified. The actual serious work on weight savings is only beginning and it shows: The 182 SMA already lost some 50 Lbs. Then, existing engines will increase in power for same cubic capacity. And then we haven’t seen yet the full impact of the 2-stroke technologies: DeltaHawk, D-Air and Wilksch, because the few examples are prototypes improvised by very small firms who have restricted resources. And yet, the Luscombe Silvaire flying in UK with a D-Air 2-stroke opposite-piston diesel of 110HP gives you a clear signal that small 2-stroke diesels will be light and competitive: you can’t put much weight on a Silvaire… posted at 7:13 AM |
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Thielert, world leader in aero diesels, reports 75...
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