Diesel Aircraft Engines Revolution

They’re more efficient, run on cheaper fuel, and the EPA isn’t threatening to regulate them out of existence. With so much going for them, are Jet fuel-burning diesels ready to take over?


What pilots in Europe, Africa, South America and Asia already know about aviation diesels — or, to use the more technically precise term, compression-ignition engines — we’re slowly learning for ourselves here in America. And the more we learn, the more we’re starting to think we could get used to the smell of jet fuel powering our piston airplanes.

As you may know, diesels offer some compelling economic advantages over gasoline engines. An important one is better specific fuel consumption — the measure of power output for each pound of fuel consumed. Another benefit of diesels, which run perfectly well on jet-A, centers on the rising price and uncertain future of avgas. In some parts of the world, 100 low lead can cost more than $20 a gallon — if it’s even available. Here in America, the Environmental Protection Agency has set a goal of outlawing leaded aviation gasoline before the decade is out. The FAA is preparing for tests of replacement fuels, but what it will cost to fill up our tanks is a big unknown. “More than today” is our best guess.

So has the aero diesel’s time finally arrived? Outside North America, turbocharged diesel aircraft engines are in high demand, and that trend will only accelerate. Here in the United States, there is a growing fascination with diesels, but it’s too soon to say we’ve embraced them as the answer to our woes. After all, diesels are heavier, they usually require liquid cooling, they’re often limited by certification in how high they can operate, and they cost more to build. The success of diesels in America, if it happens, will be rooted in the cost-benefit analysis — and right now the math doesn’t favor diesels over a good-old Lycoming or Continental piston gasoline engine.

Cessna is betting big on a four-cylinder engine from SMA for the new 182 JT-A.|

But that could change in a hurry depending on what happens with avgas. One thing is certain: The days of 100LL aviation gasoline powering the general aviation fleet are numbered. Maybe that’s OK. If you’ve been following diesel developments lately, you know today’s diesel-cycle engines are really nothing like the dirty and noisy ones many of us remember from diesel cars of the 1970s and 1980s. They no longer spew toxic soot or rattle at idle like a coffee can full of marbles. They also deliver impressive power, and as experience is gained with these newer technology engines, we’re seeing time between overhalls (TBOs) stretch to 1,500 or 2,000 hours and beyond. Eventually, TBOs for diesels could be longer than gasoline engines.

An Image Problem

Still, no matter how good the technology becomes, diesels won’t supplant gasoline engines in America anytime soon. One reason is the sheer number of gasoline piston engines out there, flying in everything from aging Cessna 150s to brand new Cirrus SR22s — around 225,000 of them in all. Another is the continuing dominance of gasoline engines in nearly every production piston single and twin sold today. And many smaller general aviation airports throughout the United States sell only 100LL avgas, limiting where you can fly a jet-A-burning airplane. Also, because they lack an ignition source to keep their fires lit, diesels are restricted in how high they can fly, even with twin turbochargers and hot glow plugs — although this too could be a temporary issue that goes away as diesel experience is gained.

The latest Diamond DA42-VI with its twin Austro diesels and supplemental oxygen is a prime example. It has a service ceiling of just 18,000 feet — which also happens to be the single-engine ceiling for the

airplane. Diamond says it has flown the DA42’s Mercedes-Benz-produced Austro engine to 30,000 feet without ill effect, but for now they are limited by certification from venturing into the rarefied air of the flight levels. Diesels can be prone to flaming out at high altitude where the air is less dense — if it’s cold enough and the air is thin enough, a successful restart could be in doubt.

Because they’re usually liquid-cooled, diesels like this one in a Diamond DA40 incorporate unusual geometry under the cowling to fit extra components.|

Diamond Aircraft is staking its future on diesel technology. After the messy insolvency of Thielert a few years back, Diamond decided to create Austro Engine GmbH, its own company in Germany that buys small and efficient Mercedes A-class diesel engines, pulls off the automotive parts that aren’t needed, bolts on the aviation parts that are and installs them in brand new DA40 and DA42 models built at the Diamond factory in Austria. Diamond’s North American production facility in London, Ontario, is still turning out a fair number of DA40s, but they’re all built with Lycoming gasoline engines. As of this writing, there’s just one diesel-powered DA40NG in the United States. Diamond’s Austrian owners aren’t convinced we’re ready for diesel singles.

Diesel twins are another matter. The latest DA42-VI with twin Austros is being promoted heavily here. Still, Thielert’s troubles probably set the diesel movement back by at least a decade in America. Company founder Frank Thielert is now in jail awaiting trial on fraud charges and could face many years behind bars after being accused of misrepresenting the value of the bankrupt company and its financial position. The problems started when Thielert ran low on cash and stopped honoring warranties for engines installed in Diamond DA42s while jacking up parts prices. Making matters worse was a recurring

300-hour gearbox inspection requirement. Cessna had just announced a new version of the Skyhawk to be certified with the Thielert engine when the company entered insolvency and the business unraveled. Diamond’s reputation also took a hit as customers blamed it for the Thielert debacle.

But the Thielert story, as bad as it was, looks as though it might have a happy ending after all. The company was successfully restructured in 2009 and renamed Centurion Aircraft Engines. Its first goal was to lower its parts prices and improve customer support, which it did — and which led to growing profits. The resurrection of the company and predicted demand for diesels in Asia prompted China’s AVIC to buy the Centurion line a few months ago and roll it into its Alabama-based Continental Motors subsidiary (which has been developing a diesel, the TD-300, as well) under the name Technify Motors. The plan, according to insiders, is to brand gasoline engines under the Continental name and diesels under Technify. The 135 hp Centurion is the heart and soul of RedHawk from Redbird Flight Simulations, an intriguing airplane based on a plan to develop remanufactured Cessna Skyhawks for the flight-training market.

This is actually a diesel engine from a Mercedes-Benz A-Class economy car. Diamond’s Austro engine in Germany modifies it for duty in the DA42 Twinstar.|

Growing Competition

The shakeout in the aero diesel market has created some sorely needed competition that should drive innovation and benefit consumers. The Austro AE300 has been receiving rave reviews as a car engine that has made the successful leap to aviation. It is approved to use a variety of fuels including jet-A, jet-A-1 and even straight diesel fuel. Of course, when it’s cold, diesel fuel can turn to Jell-O. Without a fuel preheater or recirculation system, that could pose problems at extremely low temperatures. If you’re going to be flying where it’s really cold with an aero diesel, it’s probably best to go with jet-A mixed with Prist anti-ice additive — as long as it’s approved for use in your engine. In the AE300, it is.

Besides Austro Engine and Continental/Technify, there are three other noteworthy players in the aviation diesel market. They are SMA, a division of France’s Safran that has partnered with Lycoming to bring diesel power to the latest Cessna 182 JT-A; Engineered Propulsion Systems, a Wisconsin startup that has begun testing a prototype 4.4-liter diesel with a target output of 350 hp; and DeltaHawk, another Wisconsin company that has developed a two-stroke, 200 hp diesel with firewall-forward kits available for a variety of homebuilt models including the Van’s RV-7 and RV-10.

One of the key features all of the engines from these manufacturers have in common is single-lever power control. At AirVenture Oshkosh, I had the chance to check out a newly delivered DA42-VI with Diamond distributor John Armstrong and the airplane’s new owner, Dave Passmore. As they explained, flying the latest generation DA42 is a simple affair thanks to its full-authority digital-controlled diesels and single-power levers, reminiscent of the condition levers in a jet. The start-up procedure couldn’t be easier: Hop in, flip the master, wait for the glow plug indicator to illuminate and turn the key. In flight, there are no mixture or prop controls and no need to adjust throttle or RPM settings. Simply select economy or high-speed settings and let the computers do the rest as they maintain all engine parameters automatically.

One diesel danger centers on ensuring the right fuel goes into the tanks. A handful of misfueling incidents have occurred with deadly consequences.|

When Diamond founder Christian Dries created the DA42, he promised to deliver a 200-knot airplane — and he failed. The original Lycoming-powered DA42 cruises at about 162 knots. Thanks to the 168 hp Austro diesels and a number of aerodynamic improvements in the latest VI version, the max cruise speed is right around 190 knots. That’s still short of the original target, but it will get you to your destination surprisingly fast while burning surprisingly little fuel. Even with the power levers shoved forward for high-speed cruise, fuel burn is an economical 18 gallons per hour total.

Power Struggle

So far, the choices for aero diesels are limited to engines producing less than 200 hp. But that’s about to change. Engineered Propulsion Systems of New Richmond, Wisconsin, is developing the Vision 350, a liquid-cooled diesel that will be compatible with a number of high-performance light GA airframes and will deliver 350 hp. The eight-cylinder flat-V engine weighs about 50 pounds more than a conventional gas-piston six-cylinder engine, such as the TSIO-550, but will provide far better specific fuel consumption, the company says. The design is EPS’s own, but is based on German automotive diesels.

At 65 percent power, the Vision 350 will burn around 11 gph, according to EPS. The company has been performing ground runs of the engine since November 2011 and says it’s now ready to move on to its second prototype as it prepares to enter production. Perhaps the biggest question mark at this point is funding for the project, although interest from investors is said to be high. If EPS can keep the 350 hp market to itself, it could be a runaway success.

Wisconsin startup engineered propulsion systems is testing a prototype 350 hp, eight-cylinder diesel engine.|

Of course, that will be tough. Technify and SMA Safran have already started discussing plans for larger engines. SMA is developing the 330 hp SRA460. For now, its lone product is the four-cylinder air-cooled SR305-230, a 227 hp version of which powers Cessna’s new 182 JT-A single. Designed to run on jet-A or jet-A-1 fuel but not straight diesel, the engine burns about 10 gallons an hour and has a certified ceiling of 20,000 feet. Best of all, the SMA engine’s TBO is 2,400 hours, better than many gasoline engines. Eventually, SMA plans to expand into the 400 hp market, the company says. Technify, meanwhile, is working on a 350 hp, 4.0-liter turbodiesel.

SMA, Technify and EPS likely will face plenty of competition in the upper-market echelon if U.S. pilots ever fully embrace diesel-cycle engines. When Cessna introduced the 182 JT-A at Oshkosh in July 2012, markets outside North America figured to account for the bulk of orders. Instead, Cessna now says the majority of 182 JT-A buyers are from the United States. Continental was so convinced that diesels have a place in its future that it rushed its TD-300 engine into the limelight by licensing technology from SMA.

The question then becomes whether today’s diesel euphoria will lead to hangovers tomorrow as over-competition takes a toll on producers. Diesel makers will also need to rehab the market’s image to convince customers to trust in the technology, all while providing top-notch customer support and solving problems, such as low TBOs for some engines and maintenance headaches like 300- or 600-hour gearbox inspections. Still, there are so many benefits to modern diesel engines that it seems only a matter of time before America decides the time has come to replace our avgas-burning engines with something different. When we do, of course, we’ll want to make sure the placard on the wing that reads “Jet Fuel Only” is big enough for even the most farsighted FBO lineman to clearly see — and even then, we’ll probably want to stand close by to verify the correct fuel goes into the tanks.

Still, gasoline engines aren’t going to disappear from U.S. airports no matter how high the price of avgas climbs. As jet fuel-burning diesels start appearing on the ramps beside airplanes that run on avgas, it will give us choices in what we can fly. Of course, the pilot of the jet-A sipping airplane will know the truth: He can fly a lot farther on the same amount of fuel or, if he chooses, fly the same trip carrying less fuel while bringing aboard more passengers and payload. Once you start crunching the numbers this way, diesels start looking more and more like the smart choice for the future.

Take a closer look at how diesels stack up against gasoline engines here.

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