Early this year, Thielert Aircraft Engines (TAE) earned both EASA (European Aviation Safety Agency) and Federal Aviation Administration approval of its 135-horsepower Centurion 1.7 turbodiesel engine retrofit for the Cessna 172K, -L, -M, -N, -R, and -S models. FAA approval of a kit for Piper Warrior IIs, IIIs, and Cadets should have been granted by the time you read this. This is a big step forward for general aviation's diesel "movement" — and for Thielert's ambitious designs. So far, TAE has shipped some 405 Centurion 1.7 engines — most of them destined for Diamond Aircraft.
The Centurion 1.7 starts off as an in-line, liquid-cooled and intercooled four-cylinder Mercedes-Benz turbodiesel engine. Mercedes ships these engines to Thielert's Lichtenstein, Germany, plant, where they are stripped of many original components. Crankshafts, camshafts, cylinders, and other high-dollar parts are tossed out and replaced with components designed and built to Thielert's specifications. TAE uses its own state-of-the-art, computer-controlled milling machines to make these parts to very precise tolerances.
The company also builds the engine's reduction gearbox, as well as its full authority digital engine control (FADEC) unit, engine instruments, engine mounts, exhaust components, and cooling shrouds. The three-blade, constant-speed, composite-construction propeller is from MT-Propeller. The engine, its shrouds, gearbox, and other components, by the way, fit within the standard Cessna 172 cowling.
TAE ships the 950-part, firewall-forward kit in three boxes, and its price is currently set at 45,500 euros, or about $55,055 at the current exchange rate — which was 1.21 dollars to the euro as of this writing in June 2005. The PA-28 kits should run about $59,220. TAE says that installation time should require approximately 110 hours of labor.
Flying the TAE Skyhawk
A flight in a TAE-powered Skyhawk best illustrates the engine's attributes, and this past April I was fortunate enough to fly D-EECC — a Thielert demonstrator — on a two-and-a-half-hour cross-country across Germany. With me was Sven Poga, one of TAE's staff pilots. The trip was from Friedrichshafen to Altenburg, a former Eastern Bloc fighter base located near the TAE factory.
Engine start is a real no-brainer. After switching on the glow plug (an electrically heated element used to initiate combustion during cold starts) and waiting for its annunciator light to extinguish — signaling that the air in the cylinders is hot enough for a start — you simply push the start button. After a few turns of the prop, the engine rumbles to life. There's no priming, no hot starts, and no worries about mixture control with this engine. The FADEC figures out the best mixture for ambient conditions.
It's a smooth-running engine, too. Its sound and throttle response remind me a lot of the Mooney PFM's, which is no surprise. The PFM uses an automobile-based engine, too — a Porsche 911's — and it also has a single-lever power control.
Many wonder about the TAE's power output compared to the 172's stock 160-horsepower Lycoming O-320 engine. How can a 135-horsepower engine yield the same performance as an engine with 160 horsepower? The answer lies in the efficiencies brought about by the Centurion's reduction gearbox and the constant-speed propeller. Static thrust is 13 percent higher than an O-320's, says TAE, because the propeller turns more slowly. At altitude, the turbocharger — something the O-320 doesn't have — helps maintain maximum power levels to higher altitudes.
Pretakeoff checks include a test of the dual-channel FADEC computers, but other than that everything else is fairly conventional. Two engine display gauges — a CED (compact engine display) and an AED (auxiliary engine display) — give you plenty of information about the engine's status. The CED shows propeller rpm, coolant temperature, gearbox temperature, and power output, expressed in percent of load. The AED shows fuel temperatures in both fuel tanks, bus voltage, and fuel flow. There's also a warning light for low coolant level.
Takeoff seemed very Skyhawk-like, with good acceleration considering our maximum-gross-weight condition. You just advance the single power lever to the stop, then wait for rotation speed; V-speeds are virtually the same as those of a stock 172. After liftoff we settled into a 500-fpm climb and wended our way through a Friedrichshafen IFR departure procedure that took us over Lake Constance. The power lever was floored the whole time, which meant the propeller turned at its maximum setting of 2,300 rpm. During the climb, the FADEC automatically adjusted the engine for maximum power, leaving me free to fly the instruments to an airway interception. The FADEC also keeps a data log of the engine's operation, which can be downloaded or marked by the pilot for later scrutiny.
Oh, and it's quiet, with a 71.3 dB(A) takeoff noise rating — well under the 80.3 dB(A) International Civil Aviation Organization (ICAO) limit — an important feature in noise-sensitive European countries, where a too-loud airplane can land you a whopper of a fine. This low rating makes it one of the quietest piston singles. Throw in the very low vibration levels, and you've got an admirably racket-free cockpit. We wore noise-attenuating headsets anyway, but when I took mine off I was surprised at how low the noise level was — compared to a stock 172.
Soon we were level at 6,000 feet. I left the power lever at the maximum setting, then trimmed the airplane and set the autopilot. We were in and out of clouds and rain showers the entire flight, so there was little in the way of sightseeing and a lot of time for recording performance. Here's how the numbers worked out for a maximum power cruise: 113 KIAS; plus-5 degrees Celsius; 100-percent power; 2,300 rpm; 7.6 gph; and 124 KTAS. A standard-issue Skyhawk would have flown a couple knots slower, and burned about a gallon more per hour.
More dramatic economies are realized at lower power settings, of course, and here's where the TAE engine shows a lot of promise. An 80-percent power setting produced a 2,140-rpm propeller speed and a mere 5.9-gph fuel burn. Indicated airspeed dropped to 102 knots.
The turbocharger's critical altitude (the altitude at which it can no longer help the engine produce sea-level power output) is about 7,000 feet, according to Thielert, although the engine will continue to make as much as 90 horsepower at altitudes as high as 18,000 feet.
The flight ended with a VOR approach to Altenburg in 500-and-one-mile conditions, made all the easier thanks to the single-lever power control. With one notch of flaps, a 30-percent power setting usually gives you a 500-fpm descent rate at 80 knots or so, according to TAE's Poga. Then it was 65 knots on short final, followed by a touchdown with the stall horn chirping. It was interesting to see the hardened hangars running the length of the runway, earthen domes lined with cement blocks. Once they held MiG-21s. Now they serve as garages for fuel trucks, an ultralight, and more conventional general aviation airplanes.
The 1.7's future
TAE is hoping that its diesel's fuel economy and simplicity of operation will prove as attractive to customers in the United States as they do to those in Europe. There, where avgas prices can hit $8 per gallon (and Jet-A goes for half that), general aviation diesels make a great deal of sense. Already, more than 400 Centurion 1.7-equipped Diamond DA40 TDI airplanes have been sold in Europe. And U.S. orders for the TAE-powered Diamond Twin Star have surpassed the 200 mark. A French single — Apex Aircraft's Robin 135 CDI — is also poised for its certification with a Centurion 1.7.
Will American flight schools and individual customers go for the Skyhawk and PA-28 retrofit kits? That's the big question. On the plus side, there is a big advantage to fuel savings, especially with surging fuel prices. The FADEC and the single-power lever is another big draw. On the other hand, there are maintenance issues. Currently, the Centurion 1.7 has a TBR (time between replacement) of 1,000 hours. By the end of this year, TAE anticipates that a favorable service history — more than 60,000 hours have been accumulated on Centurion 1.7s so far — will bump that figure up to 1,600 hours. Ultimately, the goal is a TBR of 2,400 hours, which should occur in 2006.
What happens at TBR? Simple. You turn in the old engine and buy a new one. There is no core credit for the value of the run-out engine. Instead, customers plunk down $25,602 for a brand-new one. Thielert opted to go with replacement rather than overhauls or remanufacturing out of a concern with maintaining high levels of safety. "Overhauls and rebuilds don't guarantee safe and reliable engines with calculable operating and maintenance costs over their lifetimes," a TAE spokesman said. So instead of reworking and recycling its engines at TBR, Thielert disassembles and analyzes them in order to learn how to best make improvements.
The Centurion 1.7's warranty is 12 years or 2,400 hours, which is based on the assumption of TAE's eventually earning that 2,400-hour TBR. If one of today's engines has a defect preventing it from making TBR, it will be replaced and coverage will then extend to the new engine's lifetime.
Some are concerned about the depth and expertise of Thielert's service network in the United States. But TAE is addressing this issue. Beginning in 2004, the company started establishing 31 service centers for the American market. And a training center for mechanics will be set up in Dallas.
By the way, TAE engines require little in the way of routine maintenance. For example, its 50-hour-interval inspections include:
- Changing the fuel filter.
- Performing a leak test on the coolant system.
- An inspection of the air filter, belts, oil, fuel and exhaust system, wiring harness, and electric fuel pump.
- Performing an engine test run, followed by an engine control unit data download.
Bigger aspirations
There's more to the TAE story. Last November, TAE received EASA type certification of a 350-horsepower, four-liter V-8 turbodiesel called the Centurion 4.0. So far, this engine has flown in a composite-construction, five-seater twin called the TT62, manufactured by High Performance Aircraft. This airplane uses the Centurion 4.0 in a twin-pusher configuration using drive shafts running through fuselage pylon mounts. FAA certification is expected this summer, and so is an extension of the Centurion 4.0's TBR from 1,000 to 2,000 hours.
TAE, part of the Thielert Group, has built its reputation on its manufacture of engine components for the mainstream- and racecar automobile markets. Within TAE, another business unit is developing small engines for unmanned air vehicles. It's a company on the move, and its first step on American shores is with two of the most popular general aviation airplanes ever built. It will be interesting to see the Thielert story further unfold.
E-mail the author at [email protected].
Links to additional information about Thielert Aircraft Engines may be found at on AOPA Online ( www.aopa.org/pilot/links.shtml).
