Airframe and Powerplant: The New Powerplants

Some 3,500 Thielert Jet A engines ply the skies

May 1, 2008

Many consumers and aviation experts have heralded the arrival of piston engines burning Jet-A fuel as the future of small-aircraft powerplants. The Thielert Group of Germany now has more than 3,500 of its small Jet A-fueled piston airplane engines in the field. Compared to avgas-fueled piston engines, these new diesels are quieter, more environmentally friendly, more efficient, and smoother running.

Every Thielert diesel engine that’s running on an airplane is closely monitored through mandatory parts changes and digital data gathering. In addition, Thielert has implemented a policy of inspecting 100 percent of what it terms the critical components before an engine leaves the factory.

This inspection schedule is extremely rigorous for mass-production parts, which bodes well for operators of these diesel engines. Thielert’s monitoring program also means that there will be a vast difference between maintenance and troubleshooting as it’s practiced on the average Continental or Lycoming engines, and maintenance as it’s practiced on the Thielert Centurion Jet A-fueled line of engines.

Thielert Centurion 1.7 and 2.0

Thielert Motors is a 16-year-old company based in Hamburg, Germany. Headed by Frank Thielert, the company initially honed its skills by applying engineering and manufacturing techniques to automotive engines for companies such as Toyota Racing, Volkswagen, Audi, and Daimler Chrysler. In 1999, the company started adapting an existing diesel-fueled automotive engine for aircraft use. The result was a 135-horsepower, 1.7-liter (103-cubic-inch) displacement four-cylinder, four-valve engine that is approved by both the European Aviation Safety Agency (EASA) and the FAA. The engine, tagged with the Centurion label, was introduced to the North American aviation world at AirVenture 2004.

All Centurion engines are full authority digital engine-controlled (FADEC), liquid-cooled, fuel-injected, burn Jet A, are turbocharged and intercooled, and feature a propeller-reduction gear assembly coupled to the engine through a torque pulse-smoothing clutch assembly.

The Centurion 1.7 engine is installed in Diamond Aircraft’s futuristic-looking DA42 Twin Star light twin. Installations are also approved in Europe for a number of models of the Piper PA-28 Cherokee line, the Diamond DA40, and the Cessna 172, among others. The Cessna 172 installation is approved under a U.S. STC. In late 2006, Thielert announced that the Centurion 2.0 was replacing the Centurion 1.7. According to a Thielert spokesman, a change in the automotive supply chain caused Thielert to outsource the manufacturing of a larger displacement aluminum block of its own design.

Despite the larger displacement, Thielert officials said the 2.0 has the same horsepower rating and is approved to replace the 1.7. The 2.0 is improved over the 1.7 in the cylinder-head design, the fuel-rail design, the fuel-injector design, the prop-control design, and a long list of other features. The 2.0 weighs 295 pounds, the same as the 1.7-liter engine.

At AirVenture 2007, Cessna announced that a 155-horsepower version of the 2.0-liter engine would be available as a factory-installed option in its 172s. It’s not clear at press time whether this higher-powered version of the 2.0 will be added to the existing Cessna 172 retrofit STC.

Other Jet A fuel aircraft engines in the Thielert stable include the 230-horsepower Centurion 3.2, and the 350-horsepower Thielert 4.0. The 4.0 engine is approved in Europe for installation in Cessna twin-engine models 340, 414, and 421, and the single-engine 206. A Thielert spokesman said that FAA approval of an STC to retrofit the 4.0 engine in Cessna 206s should occur in 2008.

“On condition” maintenance? Fagettttaboutit!

Private aircraft owners in the United States have an amazing amount of freedom when it comes to making decisions about maintenance of their airplane engines. Are owners required to comply with engine manufacturer’s service bulletins? Not unless the airplanes are flown for hire, or compliance is mandated by an airworthiness directive (AD).

When oil consumption increases to one quart every two hours, is the pilot required to fix its appetite for oil? No, the service bulletins related to oil consumption are only advisory for private pilots. When the engine gets long in the tooth and bumps up against the manufacturer’s published time between overhaul (TBO), must the pilot stop flying the second the engine hits TBO? No. This decision and many others are left to the discretion of the owner and the certificated technician who does his or her maintenance.

These decisions, and others related to engine maintenance, are made based on the condition of the part and are termed on-condition changes. Some refer to this as breakdown maintenance. On-condition maintenance is not an option on Thielert engines. Parts that are expected to wear are incorporated into what Thielert calls a life extension program (LEP). This preventive maintenance approach means that parts are changed and the engine is replaced when a Thielert-mandated number of engine operating hours—or calendar months—clicks off the clock.

Life extension parts (LEPs)

Thielert uses digital data gathering and the LEP to keep close track of the engines it already has in the field. Thielert provides a 2,400-hour or 12-year warranty on its 1.7 and 2.0 engines. To maintain the warranty, Thielert requires that engines be maintained in accordance with its maintenance schedule, and that all work be performed at a Thielert Aircraft Engine (TAE) service center. There are now more than 250 of these service centers worldwide. All that’s required to become a service center is the successful completion of a TAE school—and yes, there is a test—the purchase of specific maintenance tools or a signed affidavit attesting the tools are available, and a signed contract. Attendees are taught to accomplish field replacement of engine sub-assemblies such as the propeller reduction gear/clutch assembly, the fuel-feed pump, and others.

At 1,200 hours into the factory mandated time before replacement (TBR) of 2,400 hours, the engine must be removed and sent back to the factory for overhaul. Replacement engines are provided to reduce downtime. There is no provision for field overhaul.

Currently, the LEP requires maintainers to remove and replace certain assemblies such as the clutch and high-pressure fuel pump at fixed-hour intervals. Inspections are performed in conjunction with the replacements. These are then shipped back to Thielert for evaluation. Changes are incorporated as needed. The normal service interval is 100 hours. Oil is changed and samples of the engine and propeller reduction gearbox oil are taken.

The 100-hour service and inspection kit includes all filters, O-rings, and sample catch bottles needed for the inspection and service. Service parts costs are around $200 from Superior-Thielert. This cost may sound high, but it is offset by the fact that each 100-hour engine service and inspection only requires about three man-hours of labor. By comparison, the Cessna labor allowance manual allows 14 man-hours labor for a 100-hour inspection on a pre-1996 Cessna 172—field experience suggests that inspection and service of the 172’s avgas-fueled engine takes five or six hours.

Thielert—king of the FADEC

The Centurion 1.7 and 2.0 liter engines are controlled by a very sophisticated FADEC system. To quote the Thielert maintenance seminar manual: “In broadest terms, the pilot selects the desired load, and the (FADEC) system regulates all engine parameters to achieve the desired condition.” Desired load means percent of power. For instance, during a flight in a Thielert-engine Diamond DA42, desired load was indicated by a percent of power digital readout on the Garmin G1000 avionics suite. Set it and forget it. No gain or loss in altitude or change in aircraft pitch angle influenced it.

Based on the few hours I flew the Thielert engines, I predict that the Thielert system will be the standard by which all other FADEC systems are measured for years to come. It’s that good.

A total of 16 sensors and nine actuators control the engine. There are five main control loops. They are the boost (turbocharger) control; the fuel-injector control, which consists of the fuel-injection volume, the fuel-injection angle, and the idle-control circuit; the (fuel) rail-pressure control; the propeller (rpm) control; and the glow-plug control. Boost control, propeller control, and rail-pressure control are closed-loop systems, meaning that these control systems are constantly adjusted for differing atmospheric conditions to produce optimal control. Let’s take a look at one of these closed-loop systems.

The fuel-injection system has to deliver the appropriate amount of fuel at the appropriate time to the combustion chamber. The primary control sensors in this control loop are a potentiometer on the FADEC power lever that transmits the desired power setting and the engine rpm. Secondary sensors adjust for deviations in manifold pressure, barometric pressure, air temperature, coolant temperature, and fuel-injection rail pressure.

These secondary sensors fine-tune the time and duration of the fuel-injector cycles. Fine-tuning capabilities divide individual cylinder-fuel delivery operations into three injection cycles (during one compression stroke) when the engine is cold and thus relatively inefficient. As the engine warms and becomes more thermally efficient, the number of injection cycles is reduced to one.

Of the 14 input signal sensors feeding each ECU—the barometric pressure and manifold pressure sensors are integrated into each unit—on the throttle lever, the camshaft position, and the crankshaft position sensors are so critical that dual sensors are used. Should any one of the 11 secondary sensors fail, the system will still run well, but the performance of that system won’t be optimized. According to Markus Becker, Thielert sales and support engineer, the pilot won’t even notice the difference.

Diesel aficionados may say that Thielert has taken the simplest internal combustion engine ever designed and complicated it unnecessarily. But it’s now the twenty-first century and digital electronics improve the already dependable diesel technology. One benefit of incorporating digital electronics on a diesel engine is laptop computer fault finding and troubleshooting.

The FADEC Service Tool

Each FADEC system has two engine-control units (ECUs A and B). A diagnostic system is constantly monitoring the health of each ECU. In the event of a fault—such as the failure of a secondary sensor—the system automatically switches to the healthier of the two ECUs. If a failure occurs, the system switches to the healthier of the two ECUs, a “check engine” light on the instrument panel illuminates, and an entry in the FADEC event log records the nature of the problem.

Engine health parameters are re-corded on two memory devices. The engine event log maintains a record of all engine parameters for the life of the engine. In addition, each ECU also has an internal data logger (IDL), which automatically records the engine operating data for the last two hours, after which the data is overwritten with new data. In a case where a “check engine” warning light is illuminated, the fault must be diagnosed and repaired before the fault light is cleared.

In most cases, this requires technicians to conduct a ground run to capture a real time log file (RTLF). This file is imported to the FADEC service tool software—supplied to each qualified service center—to guide field technicians through the troubleshooting process.

Another tool to help in the diagnosis of the RTLF is the diagram/graph function of Microsoft Excel. If the fault can’t be replicated during a ground run, a RTLF can be captured in flight.

During normal scheduled maintenance operations, a standard data package consisting of the event log and an RTLF is compressed and sent to Thielert, where it’s stored. If a problem occurs that is beyond the scope of field maintenance, an extended data package consisting of everything in the standard data package and files from the IDL are sent to the support branch of Thielert.

The Thielert model is a glimpse into the future of light-airplane engines. AOPA members have written asking when they will be able to install a diesel-fueled engine in their airplane. Based on what Thielert had accomplished over the past 10 years, it may be sooner than many of us realize.

E-mail the author at steve.ells@aopa.org.