Throw out the textbooks, diesel airplanes are here

June 19, 2003
A diesel engine developed in Germany and installed in Austrian airframes could change all our notions about designing piston-powered aircraft.

Diesel engines are unheard of in modern airplanes. There's a new one, however, that gives every indication of breaking the rules for mating engines to airframes.

Ronald KholEditor

Diamond Star, or DA40 TDI, uses the Centurion 1.7 diesel engine built by TAE in Europe. The Centurion replaces a Lycoming powerplant and is now in production in Austria. Despite the fact that automotive engines adapted to aircraft have not enjoyed an illustrious track record, the Centurion appears headed for success.


The Twin Star, a diesel-powered twin-engine aircraft built by Diamond Aircraft Industries, flies 126.6 mph at maximum economy settings and gets the equivalent of an astounding 42 mpg.


It only takes one control to keep the diesel Centurion humming. There are no mixture controls or carburetor heat.


The Piper PA-28 is another common airframe which has been retrofitted with the Centurion diesel. A PRO/E CAD rendering shows how it fits beneath the cowling.


Cessna 172s have been retrofitted with the Centurion diesel. One of the test aircraft registered fuel economy of 27.4 mpg at 126.6 mph. The plane's maximum speed at 10,000 ft is 131 knots, or 150 mph. Range with standard tanks and 45-min reserve at 10,000 ft is 954 miles. With long-range tanks, it is 1,212 miles. Takeoff distance to clear a 50-ft obstacle is only 1,548 ft.


The engine was originally designed for automobiles, but already its performance is raising eyebrows in the aviation community, especially with regard to cutting the cost of flying.

Diesels have never been successful in modern airplanes. And although gasoline automobile engines have been adapted to aircraft, none has been a commercial success. But this new engine blends high performance with incredibly low fuel consumption, the legendary durability of diesels, and advances in turbocharging that make it lightweight and reliable. A light aircraft powered by two of these diesels can carry four passengers at speeds up to 231 mph while racking up 19.6 mpg. If you prefer throttling back to a leisurely 126 mph, you get 42 mpg.

Thielert Aircraft Engines (, also known as TAE, a division of the Thielert Group based in Liechenstein, Germany, took a four-cylinder engine first used by Mercedes and totally reengineered it for use in airplanes. All of the critical parts were redesigned by Thielert and are now manufactured by them.

It is the first modern diesel engine fitted to a certified airframe and scheduled for production with solid orders on the books. Thielert says the engine reduces the costs of flying by 70% compared to gas-powered aircraft (based on gas prices in Germany). And total operating costs for the engine over its lifetime are projected to be an order of magnitude less than that of gas engines.

One European aviation magazine says diesels are the wave of the future for light aircraft. Not only are they cheaper to operate, they use more widely available fuel, have substantially increased ranges, and are more durable as well as safer than gas engines.

From autos to airplanes

TAE was formed three years ago to adapt an automotive engine to aircraft. The program benefited from Thielert's experience in optimizing Formula 1 engines and helping auto companies develop new engines. Two major motivating factors behind the development of the aircraft diesel were the high cost of aviation gasoline in Europe and growing global concerns about that fuel's future availability as more and more aviation refining turns to jet fuel.

The Thielert aviation diesel was initially called the TAE 125, but was renamed the Centurion 1.7. Both engines are identical, but the company feels the Centurion name has more cachet. The engine operates on diesel and Jet A fuel, but not aviation gasoline. Nor are biodiesel fuels approved. So far, the engine has undergone 10,000 hr of testing, with 1,500 hr of that time in an aircraft.

The Centurion 1.7 is a turbocharged four-cylinder, four-valve, in-line engine with common-rail fuel injection. It is liquid cooled and uses off-the-shelf antifreeze. It puts out 135 hp and 302.4 lb-ft of torque. Displacement is 1.68 liters or 103 cu in., with a 3.15-in. bore, 3.31-in. stroke, and a compression ratio of 18:1.

The engine weighs 259 lb dry, which is 6 to 9 lb more than a Lycoming O320. With all its accessories, the engine tips the scales at 295.4 lb. And a complete installation from the firewall forward in a Cessna 172 weighs 430.2 lb. The engine measures 30.6 in. wide, 31.2 in. long, and 23.2 in. high.

Gear reduction between engine and prop lowers prop speed to a more efficient level and has the ancillary benefit of decoupling vibrations between the engine and prop. Special, soft engine mounts further reduce vibrations and noise. In retrofits, soft mounts must replace existing ones.

The engine runs at 3,900 rpm, which gets reduced 1.69:1 to turn the prop at 2,300 rpm. Only a constant-speed (i.e., variable-pitch) prop can be used. The only approved prop is the three-blade Muehlbauer.

The engine comes with 90-A alternator and belt, starter, turbocharger, vacuum pump, and prop governor. The engine uses a 12-V electrical system, but 12 and 24-V configurations are due out this year. The 24-V system will add approximately $1,782 to the $19,500 engine, which is delivered broken-in.

There are no special starting procedures for either hot or cold engines. This is in contrast to contemporary Cessna fuel-injected gas engines, which pilots sometimes find hard to start after brief, hot shut downs on warm days.

Aside from the engine's fuel economy, pilots also appreciate the fact power is controlled by a single lever. There are no fuel mixture or carburetor heat controls requiring pilot attention. And the engine's entire operating history is logged on a data-recording device as part of the standard electronic controls. It warns pilots of impending problems and provides data for servicing.

The Centurion 1.7 should only be overhauled at the Thielert factory, and it has to be replaced after 2,400 hr of operation. Engines are built in a one-man, one-engine approach instead of on assembly lines.

Down the road is a further development from Thierlert, the Centurion 4.0. It is a diesel slated to put out 310 hp.

Now flying in Europe

Currently, the only OEM aircraft manufacturer preparing to deliver diesel airplanes is Diamond Aircraft Industries (, with headquarters in Wiener Neustadt, Austria. Diamond is installing Centurion engines in its Diamond Star DA40, an airplane currently being delivered with 180-hp Lycoming gas engines under the designation DA40-180. As one aviation expert observes, the DA40 with the gasoline engine is a beautiful new airplane powered with 50-year-old technology.

Fitted with the diesel, the airplane is renamed the DA40 TDI Diamond Star. It has earned JAR 23 certification by Austro Control, the Austrian Airworthiness Authority, and is the first production aircraft powered by the Centurion 1.7.

Although Diamond has a plant in London, Ont., Canada, installing the first 100 Centurions will be limited to Europe so that the engines are close to the factory if customers need technical assistance. The company is evaluating possible sales of the Diamond Star TDI in North America, and says it will make a decision in the near future.

Another Diamond airplane under development and powered by the TAE Centurion is the twin-engine, retractable gear, DA42 Twin Star. It first flew last December 9. The first flight with gear retracted was February 13 of this year, and during that flight, the craft climbed to 18,000 ft. The Twin Star can carry 900 lb of passengers and cargo when completely fueled. The Twin Star and Diamond Star are four-place aircraft. The Twin Star should go on sale in mid-2004, and in North America, the price is projected to be $360,000. Thielert says it is working with other OEM airframe manufacturers, but confidentiality agreements prevent naming any other than Diamond.

A Piper PA-28 and various Cessna 172s have been retrofitted in Europe with TAE diesels. These retrofits require Supplemental Type Certification, or STC, and various models of 172s already have this certification or are near to it. STC conversions cost $40,000, including engine and labor. Certification has also been granted for TAE-powered kit planes and experimental models. It is expected that toward the end of this year, people building experimental craft will be able to install their own Centurion engines. The Centurion, however, is regarded as too heavy for ultralight aircraft.

The Cessna 172s and Piper PA-28 are the only two installations Thielert has in its immediate plans for retrofits. However, based on the Centurion's engineering features involving torque, gear reduction, and prop pitch, the engine can replace conventional aircraft powerplants up to 180 hp.

Interestingly, inquiries to Cessna on their Centurion retrofits provided only a cursory response devoid of meaningful specifics. An inquiry to Piper was not answered.

The apparent blackout on information from the firms could be meaningful. It might indicate that both companies see the Centurion as a threat. Is it possible Diamond will gobble up market share in the light aircraft market? What would massive public acceptance of the Centurion do to the overall business of dealers operating maintenance facilities? And have the firms, like Diamond, embraced the engine or at least included it in future plans?

SPEED AND ECONOMY TRUE AIRSPEED ECONOMY Power (%) Speed (knots) Speed (mph) Gallon/hr Mpg 80 181 208 10.7 19.5 60 166 191 8.0 24 50 147 169 6.7 25 Max Economy 110 127 3.0 42

Dynamic Diesel performance

The twin-engine Twin Star, at 80% power and 18,000 ft altitude, flew 201 knots true airspeed while consuming 11.8 gallons/hr. That means the diesel-powered plane got 19.6 mpg while traveling over 231 mph.

At 75% power and 10,000 ft, the airplane's absolute range is 1,012 miles with standard 52-gallon tanks. The 74-gallon long-range tanks boost that range to 1,440 miles. But with the standard tanks and minimum power settings, the theoretical maximum endurance is 19 hr and 2,532 miles, which is farther than from Chicago to Los Angeles.

Loaded to just 15% below maximum gross weight, the Twin Star climbs more than 2,000 fpm at 90 knots. At 110 knots, the climb rate is 1,700 fpm. Single-engine climb rates to 12,000 ft averaged 600 fpm.

The beauty of diesels and variable-pitch props

Aircraft engines are more heavily loaded than auto engines, but they tend to run at constant speeds, which reduces wear. Automobile engines suffer immense wear by running at a wide variety of speeds as they constantly accelerate and decelerate.

The Centurion 1.7, for example, has only three power levels: takeoff, cruise, and idle. Thus, mechanical components, as well as engine management and prop operation, can be optimized for these three regimes. This lets airplanes get more usable power from their engines, which tend to be more durable than automotive engines.

Although rated at only 135 hp, the Centurion 1.7 can replace more powerful gasoline engines because it provides more static thrust. Static thrust is what you feel when you push the throttle forward. It is a function of engine torque, gear reduction to the propeller, and pitch of the blades. The Centurion static thrust, for example, is greater than that of a Lycoming O320 and equal to that of the more powerful Lycoming 0360.

With a fixed-pitch prop aircraft, takeoff rolls are much like trying to speed off in a stationary car that's in high gear. The variable pitch (i.e, constant-speed) prop of the Centurion provides somewhat of an aerodynamic "gear reduction" and is more efficient than fixed-pitch props used with Lycoming engines. This leads to the Centurion 1.7's better takeoff performance as compared to a PA-28 or Cessna 172.

Under so-called standard conditions, the Centurion's climb rate will be slightly less than that of the Cessna or Piper, but it still meets FAA standards. And with turbocharging, the Centurion can be expected to perform well in hot weather and from high-altitude airports, both of which degrade engine performance.

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