What‘s Up With Diesel?

Diesels fell from favor before World War II, but three-quarters of a century later, diesel power has once again been proffered as a solution for economical air travel, this time for fixed-wing, general aviation aircraft burning jet fuel. As nearly anyone who hasn't been living on the dark side of the moon has probably heard, avgas is becoming scarce and more expensive, though not so much inside the U.S.

Traditional wisdom is that refineries are backing away from high-octane avgas because of poor volume. Mobil is out of the avgas business altogether, and companies such as Chevron, Shell and Exxon are becoming concerned that the relatively low volume of avgas simply isn't an appropriate allocation of resources.

Similarly, many manufacturers have taken a wait-and-see attitude about certifying models to fly behind diesel power. Austria/Canada's Diamond Aircraft is the only company to embrace diesel wholeheartedly with a progression of engines: first, the flawed Thielerts, then the follow-on, improved Centurions and most recently, Diamond's own Austro diesels, produced in a new manufacturing facility next door to the main Diamond plant in Wiener-Neustadt, Austria.

Even Cessna Aircraft jumped in with a French engine, the SMA SR305-230 turbo diesel, mounted on the venerable 182. Cessna went so far as to cancel production of the standard, normally aspirated avgas-model Skylane, a leap of faith for one of the most trusted Cessna models in the company's 87-year history. That's all the more surprising, considering that prior to the Skylane JT-A's introduction, the standard 182 consistently outsold the turbocharged avgas model by two to one.

But now what? Interest in diesel technology hovers somewhere between stalled and jump-started. Though Dia-mond continues to produce the Star and Twin Star with their captive Austro diesel power plants, even they seem to be hedging their bets by offering either airplane with a conven­tional Lycoming avgas option. Still, Diamond has over 1,000 diesel-powered singles and twins flying, so there must be some significant demand, especially overseas.

For its part, Cessna isn't letting on what they've planned for the Skylane JT-A, especially considering that the model isn't certified yet. An engine failure last August cast a pall on certification efforts, though the pilot walked away, and the aircraft was undamaged in the emergency landing.

Cessna CEO Scott Ernest manifested little interest in the diesel-powered Skylane at last year's NBAA Convention, though neither he nor anyone else at Cessna has uttered a truly discouraging word. Cessna has built something like 22,000 model 182s since the 1950s, and most pilots can't imagine a sky without Skylanes.

A few other manufacturers have shown serious interest in jet-burning diesels, though the type is more popular overseas where avgas is already becoming scarce, and a gallon can cost $10. So far, none of the American airframe manufacturers have committed to development.

Cirrus Aircraft enjoyed a good year in 2013 with 276 aircraft sold, and for that very reason, they may be more determined to stay the current course rather than make a left turn toward diesels. The company is spending a ton on development of the Vision Jet project, and that may make them an unlikely candidate for diesel power, at least in the short term. The Vision is still scheduled for first delivery in late 2015.

Piper also had improved sales in 2013, and the company looked at the available diesel power plants, as well, but hasn't gotten beyond the looky-loo stage. Certainly, a major concern for any prospective aircraft company is the certification costs involved to produce what might be only 50-100 airplanes a year. The FAA has made it practically cost prohibitive to even consider certifying an airplane with diesel power.

Nevertheless, diesels are far from dead. Lycoming has a turbo-diesel engine that's a total mystery to general aviation, the DEL-120, a 205 hp engine that currently powers drones for the military. Lycoming gave us a firm, "No comment," when we called and asked for details about the DEL-120.

In addition, Continental Motors recently concluded negotiations to quietly acquire the revived Thielert diesels of Germany. Thielert now builds the Centurion diesels, variations on the start-up power source for the first Diamond twins mentioned above. Premier Aircraft of Fort Lauderdale is in the process of pursuing an STC to install Thielert diesels in reconditioned late-model Cessna 172s, producing what will be in essence a fully rebuilt Skyhawk.

Diesel aircraft engines do have several advantages over avgas mills. The two most important are fuel availability---jet fuel is rapidly becoming available practically everywhere---and efficiency. Diesels are far more efficient than avgas mills. Spontaneous combustion engines enjoy a specific fuel consumption of about .32 lbs./hp/hr., whereas avgas engines rarely score better than .42 lbs./hp/hr. That's 31% less fuel burn to generate the same power. Additionally, jet fuel is considerably denser than avgas, though it takes up the same space volumetrically. Diesel weighs 6.7 pounds/gallon compared to 6.0 pounds/gallon for avgas. While that may cut into payload, it also translates to an additional 12% efficiency.

In other words, diesel engines start off about 43% more efficient than avgas mills. In real terms on comparable 200 hp engines running at 75% power, an avgas power plant could be expected to burn 10.5 gph, roughly the book burn in a Piper Arrow, Mooney 201 or Cardinal RG (all of which used the popular Lycoming IO-360). A diesel using the same percentage of power will consume more like 6.0 gph to produce the same 150 hp.

That has a major impact on range, enough that the Diamond Twin Star was the first diesel-powered, fixed-wing aircraft to cross the North Atlantic nonstop. The Diamond made the trip in 12.5 hours, burning a mere 2.87 gallons per hour per engine.

Most diesels are turbocharged for smooth, quiet operation up high, and they're often computer controlled with FADEC (Fully Automatic Digital Engine Control). That means you can dispense with the prop and mixture controls, and there's no longer any need to worry about hot or cold starts.

The question still remains whether there's a sufficient market for diesels that transcends the need for a fuel to replace avgas. On one hand, it's hard to imagine that any 100LL replacement could be developed, certified and in place within the next five years. On the other, the road to certification can take nearly that long.

Diesels (which, ironically, aren't certified to run on diesel fuel) can burn a fuel that's here and now. We have only to wait and see if the major American manufacturers will adjust their thinking to embrace the new generation of an old technology.