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Diesels on the cusp

Various certifications expected in 2015

The Continental CD-135 installed on a Robin Aircraft. Photo courtesy of Continental Motors Group.

By even the most optimistic estimates, it will take at least two more decades before Jet A becomes a dominant fuel in the U.S. piston aircraft fleet—despite some significant advantages that have made compression-ignition (diesel) aircraft engines an appealing alternative to avgas, particularly for high-use operators that log hundreds of hours on a given airframe in a year. For starters, jet-fuel-burning engines are typically more efficient, both in terms of fuel consumption and the comparatively low cost of jet fuels—a price difference that grows extreme in remote and far-flung markets and drives the intense overseas interest in jet fuel power.

With avgas prices consistently higher in low-volume overseas markets, it is no accident that the development of compression-ignition engines to date has been geared primarily toward customers outside of the United States.

Aviation analyst Brian Foley of Brian Foley Associates predicts that the North American aviation market for both turbine and piston aircraft (which once represented nearly three-quarters of worldwide aviation) will, in future years, continue to represent half of the world market, or less. That shift has already happened, giving companies and investors more incentive than ever to invest in propulsion technology suited to places where avgas is in short supply, extremely expensive, or both.

“As a result that offshore market is probably more important than ever," said Foley. China, which is only beginning to develop general aviation infrastructure, has invested heavily in compression-ignition, and various jet fuel formulations are widely available there already. "That’s a pretty smart move they made, in my opinion."

The high-performance segment of the market for jet-fuel-burning engines remains a waiting game, though two major players are working toward certifications in the coming year: Continental Motors Group (owned by Aviation Industry Corp. of China) announced the first flight of its V-6, 310-horsepower CD-300 at EAA AirVenture in Oshkosh, Wisconsin, in July.  French rival SMA (Société de Motorisations Aéronautiques, a subsidiary of Snecma/Safran Group) used its AirVenture press time to detail plans to upgrade the power output of the SR305-230E (a 227-hp engine being flown for certification in the Cessna 182 JT-A), adding 260- and 285-hp versions to the lineup.

DeltaHawk Diesel Engines arrived at AirVenture in a Cirrus SR20 powered by the 180-hp, four-cylinder engine that the Florida firm hopes to certify in 2015—a hoped-for timeline shared by the aforementioned Continental and SMA teams.

SMA also hopes to certify in the coming year its own six-cylinder entrant to the jet-fuel-burning market, the SR460 that was first announced in 2013.

Aviation analyst Rolland Vincent of Rolland Vincent Associates expects Jet A will gradually supplant avgas (and avgas alternatives including unleaded avgas now in development, and automotive gasoline) in the piston fleet, but it will be a slow process. Vincent expects that pressure to eliminate lead emissions will drive the transition to alternatives, including diesel, but retrofits in the gasoline-powered fleet will constitute a relatively small portion of the U.S. market for such engines because of the comparatively high cost of a diesel retrofit, particularly when considered relative to the value of older airframes that comprise most of the general aviation fleet.

“I don’t think that we’re going to see a massive re-engine-ing,” Vincent said. “It will come from the factory new.”

In strictly economic terms, the business case for diesel retrofits in flight training and other commercial applications requires a large cost advantage for Jet A over avgas—a condition found exclusively, for now, outside of the United States. André Teissier-duCros, a former SMA consultant who publishes the DieselAir newsletter, published an analysis in August comparing various flight school business scenarios. Teissier-duCros calculated (assuming a price break of 59 cents per gallon on Jet A compared to avgas, and a two-year replacement cycle, among other things) that a used gasoline-powered Cessna 172 or 152 would break even at 150 hours, while a Cessna 172 Jet-A conversion (from either Premier Aircraft Sales of Florida or Redbird/RedHawk in Texas) would require 325 hours to break even. Beyond 600 hours, aircraft burning Jet A begin to generate more net profit for the operator than avgas alternatives, though Teissier-duCros expects demand for new or retrofit diesel to remain relatively small through 2025. That is in large part because the technology remains relatively new, and the cost of developing it—long since amortized by makers of avgas engines—remains to be recovered.

Growing pains associated with all new technologies have certainly not spared developers of diesel engines, and many programs have gone months or years past target dates for various milestones.

Yet many aircraft are actually flying, new certifications are pending, and 2015 could prove to be a turning point year for aviation diesel.

Continental Motors Group

Under Chinese ownership since 2010, Continental Motors Group remains the market leader in compression-ignition engines for aviation, with more than 4 million hours flown by more than 4,000 powerplants installed since the first certification of an aircraft compression-ignition piston engine in 2001. (The company in 2013 bought the German firm formerly known as Thielert Aircraft Engines—before Thielert’s bankruptcy—and later as Centurion.)

Continental rebranded three different engine lines under the Continental Diesel banner in 2014: the 100-series CD-135 and CD-155 (formerly the Centurion 2.0 and 2.0S), the CD-200 series engines (formerly the TD-300) in 230- and 245-hp versions, and the aforementioned CD-300. The CD-230 was certified by the FAA in 2012; the company announced certification of the CD-230 in China in July

The CD-135 and CD-155 are physically identical, differentiated in power output by the software of the full authority digital engine control (FADEC) system, along with required modifications to the turbocharger and cooling system to support the higher power output of the CD-155 (155 hp, compared to 135 hp from the CD-135).

The CD-100 series engines found new homes in 2014: Mooney announced a pair of new aircraft—the M10T and M10J—in November, with expected certification in 2017. Cessna Aircraft announced a Jet A-burning 172 JT-A in July that will use the CD-155, offering a factory option to compete with the CD-135 retrofit program announced in 2013 by Redbird, followed by a similar upgrade program announced  by Premier Aircraft Sales in February. (The Cessna 172 pairing with the Thielert-designed engine had previously been approved in 2007, just before Thielert’s bankruptcy, Teissier-duCros noted in a July blog post.)

Glasair Aviation USA LLC introduced a kit, the Sportsman Diesel, at AirVenture in July, powered by the CD-155. AOPA put the Sportsman Diesel (with a $249,000 base price, $295,000 as tested) through its paces for the December issue of AOPA Pilot.

Continental spokesman Sebastian Wentzler said the company is working to extend the life of the CD-100 series, which is short: The CD-135 has a time between replacement (TBR) of 1,500 hours, and the CD-155 has an even shorter replacement interval of 1,200 hours. Engines undergo destructive analysis at the factory to build the knowledge base (which will in turn help Continental make the case to regulators for a longer service life in the future). Furthermore, the cylinder surfaces have a coating which cannot be honed, and the design is “too complex for a simple workshop to overhaul,” Wentzler noted. The company has contemplated introducing a rebuild (overhaul) program in the future, “but increase of lifetime has priority.”

A CD-135 costs $55,000 to $60,000 in a gasoline engine replacement scenario, including all necessary parts (such as a new propeller, engine instruments, engine mounts, and other parts) depending on the aircraft, while the CD-155 costs $63,000 to $70,800 for the same initial installation kit (dollar amounts converted from Euros). A replacement CD-135 costs a little over $40,000 (excluding taxes, shipping, and labor), while the replacement CD-155 would cost just under $46,000 (with the same exclusions). Older Centurion engines can be upgraded for prices somewhere between the aforementioned scenarios, Wentzel said.

Piper Aircraft announced European certification of the Archer DX in April, also using the CD-155. The Archer DX retails for $399,495, with deliveries expected to begin in 2015. Piper also will offer a CD-155 retrofit option.

SMA (Safran Group)

The French firm SMA spent years—and $200 million or more on development, by some estimates—to develop the SR305-230E, which was certified by the FAA in 2011, and chosen by Cessna to power the 182 JT-A. The path to certification of that installation has not been a smooth one: Originally planned for 2013, Cessna is still working to secure certification that is now expected early in 2015.

Cessna has not recently explained the delayed appearance of that $515,000 aircraft. An engine failure in 2013 forced an off-airport landing of a Cessna test aircraft, and design modifications followed. Deliveries of gasoline-powered Cessna 182s ceased in late 2013, according to data reported by the General Aviation Manufacturers Association; it remains unclear if Cessna halted the line, if customers are simply waiting for the diesel version, or if pilots are opting for other aircraft.

SMA has also had some bad luck completely unrelated to its engine, or aviation: The other announced customer for its SR305 engine is in Ukraine: Odessa Aviation Plant, established in 1911 and known for making military aircraft during the two world wars of the 20th Century. It is unclear if they are still making aircraft during the civil war that has raged for months. Teissier-duCros posted a photo of the four-seater that will use the SMA diesel powerplant, sooner or later, one presumes.  


DeltaHawk Engines was founded in 1996 to develop a “clean sheet” aviation diesel engine, and ran the first prototype a year later. In July, the company flew to Oshkosh in a Cirrus SR20 fitted with a four-cylinder, 180-hp engine, the first of three variants the company hopes to certify in 2015. It was also the first time the pubilc got a chance to see one in action.

The Racine, Wisconsin, company has developed a variety of installation kits for experimental aircraft, and designed its engines to be simple, fuel efficient, and reliable, thanks in part to minimal reliance on electronics. DeltaHawk engines are also being made for the unmanned aircraft market.

The four-cylinder version (the company has plans to produce engines with as few as two cylinders and up to eight, covering the full power spectrum for piston aircraft—and other applications) pulled the SR20 from Florida to Oshkosh in six hours, with an hour of fuel still on board at landing. The company focuses its marketing on the cost advantages of its engines over similar avgas-powered models, and expects that the 180-hp version will rival the performance achieved with the 200-hp Continental IO-360-ES that runs on avgas and is standard on the SR20.

DeltaHawk has targeted a $100,000 price for the Cirrus conversion kit. 

Diamond (Austro Engine)

Founded in 2007 by Diamond AircraftAustro Engine has since produced compression-ignition power for a variety of Diamond models. (Diamond had previously purchased Thielert engines.) All of the four-seat DA40 series aircraft made in China are equipped with the Austro AE300 powerplant.

Diamond transitioned to the AE300 series engines beginning in 2009, a long story in its own right detailed by AOPA in 2010.

More recently, Diamond announced June 4 that the DA52 compression-ignition twin announced in 2012 will be renamed the DA62 to better differentiate the model from the DA42. The company is working toward European certification of the DA62 in 2015.  


Three years after the first proof-of-concept design was complete, Engineered Propulsion Systems (EPS) flew its Graflight V-8 diesel on a Cirrus SR22 piloted by Dick Rutan, who declared it “a new paradigm in aviation propulsion” following the May 5 test.

EPS President and CEO Micahel Fuchs and Vice President and Chief Technology Officer Steve Weinzierl said 2014 proved to be a successful year, though their clean-sheet aviation engine is still a few years from certification.

Weinzierl said more than 700 hours have been flown with the proof-of-concept versions, and “they told us all that we need to be told.” Flight tests have ceased, for now, as the company turns to developing the production model for certification testing that will likely begin in 2016.

Unlike many engine makers, particularly startups, EPS is not dependent on outside suppliers for components, Fuchs said.

“We are here in full control of any component we will put into this engine,” Fuchs said.

The company is working on the production tooling and preparing to build the first production version.

“Everything we do from now to end of 2016 is geared towards production and certification,” Weinzierl said.  


While manufacturers in the United States and Europe have struggled with setbacks and technical challenges posed by compression-ignition engine development, one of the aviation power giants has waited on the sidelines—for the most part.

Lycoming Engines has yet to develop a GA diesel engine, though it quietly made inroads in the unmanned market (also a target of other diesel engine makers) with the DEL-120, a four-cylinder turbodiesel made for unmanned military aircraft.

The project attracted media attention in February, apparently following years of development. It remains to be seen if Lycoming’s diesel will be adapted for GA use. Lycoming General Manager Michael Kraft told AVweb in March that development of a diesel engine for GA would be driven by OEM interest, which thus far has not materialized.

Jim Moore

Jim Moore

Managing Editor-Digital Media
Digital Media Managing Editor Jim Moore joined AOPA in 2011 and is an instrument-rated private pilot, as well as a certificated remote pilot, who enjoys competition aerobatics and flying drones.
Topics: Aviation Industry, Flight School, Technology

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