Are Diesels Really More Economical?


This week’s news brought an emergency airworthiness directive for Thielert’s new Centurion 2.0. The AD requires a bracket to prevent cracking in a high-pressure line between the engine’s injection pump and the common rail injection manifold. A little over 500 airplanes are affected.This AD reflects one of several daunting challenges with aerodiesel engines: although they’re perceived as smooth running thanks to clever engine mounts, the engines themselves vibrate more intensely than most avgas engines do. The torsionals along the crankshaft are so intense, in fact, that no one has come up with a way to mount a metal prop on these engines and have it last long enough to satisfy even minimal service expectations. The Thielert engines currently have wood/composite props, which soak up vibes and deliver satisfactory service life.Although I’d be perfectly happy with the composite props, some would-be owners aren’t and they cite this as a reason for not being interested in diesels. But as avgas approaches a $5 average, will the economy of diesels sway the market? To answer this question, I spent the last couple of weeks exhaustively researching the diesel vs. gas economy question. (The final report will appear in the April issue of The Aviation Consumer.)In a nutshell, what I found was this: Best case, the Thielert diesels are running a fuel efficiency of about .38 BSFC, versus .39 for the very best gasoline engines-the Continental IO-520s and IO-550s. Lycoming engines aren’t quite as efficient, typically running at .40 to .42, but as much as .45 BSFC in some instances.If you run the numbers through a TBO cycle, the diesels beat the Lycomings on a cost-per-hour basis, but they run closer to equal with the Continentals. Diesels are definitely hobbled by their high overall costs. The Lycoming IO-360 used in Diamond’s Star costs about $25,000 to overhaul, while the four-cylinder Thielert costs $53,000 to replace. (This figure includes mandatory replacement of key parts at 600 hour intervals.) So even though the diesel costs twice as much to overhaul, its fuel efficiency makes up for it and it’s cheaper to operate.That equation falls apart when larger displacement engines are compared. Continental’s IO-550 costs about $32,000 to overhaul, versus a staggering $97,200 plus for Thielert’s 350-HP Centurion 4.0. But if that engine also runs at .38 against the Continental’s .39, it doesn’t save enough fuel to offset the higher overhaul costs.Of course, the real driver for diesel technology is availability of fuel first, cost of fuel second. Nonetheless, U.S. buyers are looking at diesel in both pure efficiency terms and with the hope of at least holding the cost on flying. The smaller diesels-despite their maintenance issues-seem to deliver on that promise. But the large displacement diesels aren’t there yet.