Diesel Dawn

Turbodiesel power for Cherokees and Skyhawks

BY THOMAS A. HORNE (From AOPA Pilot, November 2001.)

Bulletin from Europe to the American general aviation community: It's high time you guys think real hard about diesel power. The advantages are many, the logic inescapable. Let us list them:

• Refiners would rather not make 100LL — not enough sales volume or profit margin. In Europe plans are afoot to eliminate 100LL by 2003. In the United States there have been longstanding worries that avgas may be phased out because of its lead content. Some think that could happen as soon as five years from now.
• Diesel engines are sturdier than gasoline engines and promise better reliability. That's because they're built to operate at high compression ratios and withstand higher internal stresses. The upshot: 3,000-hour recommended TBOs (time between overhauls).
• Diesel engines are more economical because diesel fuel has a higher energy density than gasoline. A gallon of diesel can make about 147,000 BTUs; a gallon of gas is worth 125,000 BTUs. So a gallon of diesel goes farther than a gallon of gas.
• The new generation of diesel-powered aircraft engines incorporates single-lever power controls and full authority digital engine controls (FADECs) — computer-driven, electronic control units for optimum performance, fuel economy, and low pilot workload.
• New-generation diesels use direct injection, a method of injecting timed pulses of fuel directly into the cylinders. There's no mixing of air and fuel before the fuel enters the cylinders (as with carbureted engines) and no conventional, port fuel injection — where fuel nozzles located in intake ports deliver fuel to the cylinders. Direct injection means more complete combustion, fewer emis-sions, and better fuel economy than conventional engine-intake setups.
• Some new-generation diesels use reduction gearboxes, which decrease propeller rpm. This means less propeller noise and vibration, along with more thrust.

For the past few years, European firms have steadily increased their efforts to develop a marketable diesel aircraft engine for FAA-certified, Part 23, light general aviation airplanes. One company, Zoche Aerodiesel, is even developing eight-cylinder, compound-radial diesels for the experimental market. Today there are two diesels with JAA (Joint Aviation Authorities — the European certification agency) approval, and their manufacturers are actively seeking FAA certification in mainstream airplanes. These engines have been flying in test airplanes for many months, and the results are encouraging. Many details have yet to be worked out, but by next year these could be FAA-certified and flying in a number of popular airplanes. So far, French and German companies have taken the lead in GA diesel technology.

SMA's SR 305

First, in 1997, came a joint effort "between France's Aerospatiale and Renault Sport. This produced the prototype engines that would later evolve into today's turbocharged, four-cylinder, air- and oil-cooled, 230-horsepower, SR 305 engine. Built by a successor company that includes EADS (European Aeronautics Defense and Space Group, parent company of Socata Aircraft and manufacturer of the TB series of piston singles) as a partner — Société de Motorisations Aéronautiques (SMA) — the SR 305 is now certified in Europe to Joint Aviation Requirements standards. Tests in pursuit of FAA certification are now under way.

SMA officials are aiming for a 3,000-hour TBO and a retail price of approximately $80,000 for the SR 305. That price is for the single-lever engine control unit and all firewall-forward components, including a Hartzell two-blade propeller and its governor, engine mounts, a redesigned cowling, a new vacuum pump and alternator, and the cost of installation.

SMA's intention is to position the SR 305 as an OEM (original equipment manufacturer) or aftermarket engine. So far, agreements have been made to install the SR 305 in the Socata TB20 Trinidad, the Cirrus SR21 TDI, and the Maule M7. By this time next year SMA anticipates that the SR 305 will have earned FAA certification and be offered in those airplanes.

As for the engine's aftermarket progress, Embry-Riddle Aeronautical University (ERAU) is working with SMA toward certification in the Cessna 182. At this year's EAA AirVenture, a Cessna 182 with an SR 305 did flight demonstrations and was on display at the ERAU booth. The airplane had been flown to Oshkosh from ERAU's campus at Daytona Beach, Florida.

Talks are now being held with Riley J Superskyrocket LLC, a Carlsbad, California, mod shop, to install SR 305s in the twin-engine, centerline-thrust Cessna 337 SkyMaster.

So far, ERAU's test program has put about 60 hours on its 182's engine. Data indicate that at lower altitudes cruise speeds with the SR 305 are virtually the same as those obtained with the factory-standard Continental O-470 engine. The difference, ERAU says, is that the SR 305's turbodiesel will let the airplane cruise faster and farther — with lower fuel flows — at altitudes above 10,000 feet: 153 versus low-altitude maximum cruise speeds of 139 knots; 1,313 versus 932 nm; 6.9 versus 10.9 gph.

The plan is to establish a network of installation and service centers around the United States, beginning with a site near ERAU's campus in Daytona Beach. SMA thinks there's a market for 600 SR 305s in 2002; 1,200 engines in 2003; and 2,000 sales by 2004. By that time the company may well be in a position to offer a six-cylinder, 300-hp version of its turbodiesel.

Thielert Aircraft Engines

The newest player in the aero-diesel market is Germany's Thielert Aircraft Engines (TAE). TAE, a division of Thielert AG, was founded by Frank Thielert (tee-lert), an engineer and entrepreneur who recently caught the flying bug. Thielert began building auto engines 10 years ago when he formed Thielert Auto Technik and Thielert Motoren — Hamburg-based companies that earned reputations for manufacturing very high quality prototype crankshafts, camshafts, and cylinder heads for BMW, Porsche, Audi, and Volkswagen. From there, Thielert went on to design and custom-grind crankshafts and camshafts for Porsche racecars. Porsche 3.6-liter, 700-hp racing engines with Thielert components won both the Daytona and Sebring races in last year's American Le Mans/GT Road Racing Championships. We're talking concave-radii camshafts, splined shafts, involute gears, titanium valves, and much, much more — all computer ground out of single billets as big as a roll-aboard suitcase.

Using subsidies from the German government, Thielert built a brand-new, ultramodern aircraft engine facility in Liechtenstein, Saxony — in the southeast corner of former East Germany. There, supported by CADCAM (computer assisted design and manufacturing) technology and a multitude of the latest multiaxis milling, machining, grinding, and measuring equipment, TAE's 50-odd employees have been building and testing the first 30 TAE 110 and 125, four-cylinder, liquid-cooled, FADEC turbodiesel engines. The TAE 110's takeoff power is rated at 110 hp; maximum continuous power is 90 hp. This engine is JAA-certified for use in European recreational airplanes, and has been flying in several testbed airplanes based at the Altenburg, Germany, airport — a Soviet air base during the Cold War.

The TAE 125

The focus is on the 125-hp TAE 125, an engine that's soon to reach American shores. It's been designed as a replacement engine for the 160-hp Lycoming O-320 used in Cessna 172s, as well as the Lycoming O-320s and 180-hp O-360s in the Piper PA–28-161 and -181 Cherokee/Warrior/Archer series of airplanes.

Though the TAE 125 makes 125 hp, its thrust levels are great enough (540 lbs) to match those of the O-360 and to beat the O-320's by 77 lbs. TAE anticipates that its diesel engine will give the Warrior and Archer essentially the same cruise speeds as the stock engines. This, thanks to a reduction gearbox that takes the engine's power output level from 3,800 rpm and cuts it back by a 1.68-1 ratio to a tamer 2,300 rpm at the propeller.

The TAE 125 has spent 6,500 hours in the test stand, undergoing dynamometer trials using Jet-A and auto diesel fuels. More than 120 hours have been spent in flight testing. The test-bed airplane is a brand-new Piper PA–28-161 Warrior. During a recent visit with Thielert at TAE's factory in Liechtenstein, AOPA Pilot went for a brief flight in the diesel-powered Warrior. The airplane was noticeably quieter than a conventional avgas-powered Warrior, and the engine was much smoother — fewer vibrations — than you'd expect from a diesel. The company says that maximum interior noise levels are approximately 78 decibels; the standard airplane's is approximately 84 decibels. As for flyover noise, TAE's Warrior meets Germany's strict noise standards without any exhaust modifications — unlike a standard Warrior.

As with any diesel, starting the TAE 125 involves waiting for the engine's glow plugs to heat up the cylinders. This takes but a few seconds, and then it's time to turn the key. The engine leaps to life without any priming because the TAE's electronic control unit (ECU) and single-lever power control do all the work. The ECU is built by TAE at its Hamburg facility and meets the JAA's strict lightning protection standards.

Taxiing from the ramp, we passed by the fortified hangars — and a derelict MiG–21 — that dot the landscape around the Altenburg airport. Takeoff was nothing unusual, and performance at 3,000 feet seemed about the same as that of a standard Warrior. TAE will offer a cluster of color LCD engine gauges, and on this airplane a glance at the manifold pressure showed that we were pulling 64 inches of manifold pressure during takeoff. That's the turbocharger at work.

"The cylinder and fuel injection pressures of this engine are very high," said Thielert. "This ensures better fuel economy and more complete combustion. The internal cylinder pressures are about 1,600 psi, and the fuel injection pressures are 20,000 psi. That's several times higher than a gasoline engine, but it's what you need for this new generation of direct-injection diesels to operate more quietly and efficiently. It also lets the TAE 125 produce much more power at altitude than the O-320."

When asked about the stereotype of diesels as detonating, soot-belching, and underpowered, Thielert dismisses it. "That's in the past. That was when diesels used precombustion chambers to mix the fuel and air away from the cylinders, and under less pressure. With the turbocharger to increase pressures, with fuel injection directly into the cylinders, and by electronically timing the fuel pulses with our FADEC, the detonation is gone, the fuel is more completely burned, and the engine makes more power at higher altitudes," he said. "Look at today's diesel cars. Are they slow? Do they make a lot of black exhaust? No, because they're all using turbocharging and direct injection."

That conversation took place in a diesel-powered, A-class Mercedes-Benz (a boxy-looking mini-SUV that's not available in the United States) on the way to the airport. To make his point, Thielert stomped on the accelerator and the Benz quickly surged to 1Û0 kilometers per hour (about 110 mph) on a lonely stretch of road. "There. Does that seem like a diesel to you?" he intoned. Then he backed off on the throttle, adding, "I have to watch my speeding. Too many tickets."

Back to the TAE 125. One of the most interesting features was the airplane's fuel consumption. At takeoff power the airplane burned just 6 gph; in a 2,350-rpm cruise, fuel flow was 4 gph. "Half the fuel for half the price, and with almost twice the TBO," Thielert says of a TAE 125's direct operating costs. This assumes the use of auto diesel fuel, which Thielert has been using most recently in his Warrior.

Landings and all other procedures in the TAE 125-powered Warrior were no-brainers, even more so because of the single-lever power control, which automatically leans the mixture for optimal performance at all rpm settings. The test airplane uses a three-blade, composite-construction Mühlbauer MT propeller. A two-blade Hartzell will most likely be the propeller of choice in American installations.

The Superior Air Parts connection

Worldwide (except Europe) sales and servicing of the TAE 125 will be under exclusive rights to Dallas-based Superior Air Parts Inc., best known as one of the largest replacement parts and engine manufacturers, with its Millennium series of piston engines. Superior plans to offer the TAE 125 under the Superior name at $19,900 — a price that includes the engine, ECU, 12- or 24-volt alternator, and gearbox only. The propeller, engine mounts, exhaust system, intercoolers, and installation costs will be extra. Cowling modifications will be minimal, Superior says, so aftermarket customers can keep their old cowls. At this point, Superior has yet to select a propeller or set final prices, and it is in the process of selecting vendors for the engine's external components. Superior is currently planning to offer a newly designed Hartzell composite propeller with the TAE 125.

Installations and servicing of the new Superior/TAE engine will take place within Superior's network of 40 to 50 Millennium Certified Pre-Owned Service Centers. Superior expects FAA certification by the fourth quarter of 2002 and says it can begin installations immediately thereafter.

If projections are correct, this engine's future is promising. Thielert says he can build 200 engines in 2002, 765 in 2003, 1,180 in 2004, and 1,500 engines a year from 2004 on. The aging fleet of thousands of SkyHawks and Cherokees, the principals believe, will guarantee the engine's success and keep TAE building engines and replacement parts for decades to come. If all goes well, TAE says it will set in motion plans to offer a 300-hp diesel engine as early as 2003.

Does general aviation's future belong to diesel power? It's too early to say right now, but it's safe to say that the beachhead for a diesel alternative has already been made. We'll know a lot more about diesel's future in a few months, and it will be interesting to watch as more experience is gained. For those planning to attend this year's AOPA Expo in Fort Lauderdale, Florida, be advised: Both SMA and Superior will have their diesels on display. You can judge for yourself if the diesel path is for you.

E-mail the author at tom.horne@aopa.org.

How Diesels Work

Invented by Rudolf Diesel in 1892, the diesel engine differs from conventional gasoline-powered engines in several ways. First, there are no spark plugs in a diesel. Instead, diesels rely on the heat of compression to ignite the fuel-air mixture. To make sure the compressed air is hot enough, modern diesels use turbochargers and high compression ratios to raise the mixture to the burning level. For cold starts, glow plugs — battery-powered heating elements — in each cylinder start the heating process. Once the engine's running, the turbocharger keeps a constant, high volume of air moving through the engine. Electronic engine controls respond to power commands via computer calculations of a wide range of variables (engine speed, ambient temperature, turbocharger discharge pressure, propeller rpm, and throttle position, to name a few) by changing the timing of the fuel pulses, much the way a spark's timing is advanced or retarded in a modern gasoline-powered engine. In this way, the correct fuel-air mixture is always maintained — there's no running too rich or too lean. While fuel is injected directly into the cylinders under very high pressure, only the incoming air is compressed. This differs from a gasoline engine's compression cycle, where both fuel and air are compressed prior to spark ignition. Compress a gasoline engine's fuel-air mixture too much and it can spontaneously ignite, detonate, and cause knocking. A modern diesel's high compression ratio, on the other hand, lets the engine generate more power and burn fuel more completely. — TAH

Don't Say 'Diesel'

In Europe, where gasoline prices can easily run two to four times as expensive as in the United States, diesel acceptance — and availability — is high.

Here, diesel is a dirty word associated with huge, stinking, tailgating trucks bellowing columns of blackened soot containing carcinogens and who knows what else. Availability isn't as great either. All this and more has given diesel a bad name on these shores.

To help overcome this bad image, the Europeans have taken to giving their diesel aircraft engines clever alternative names: Compression ignition, Jet-A, and Kerosene burning are the more popular descriptors. But before too long, diesel engine durability and economic advantages may supercede any image-related concerns. As avgas becomes ever pricier and threats to discontinue its production become more real, diesel engines could easily come into their own. — TAH