Powerplant Revival, Re-Examined

October 1, 1998

New programs promise to bring general aviation powerplants into the future

Every aeronautical curmudgeon who says we haven't come very far since the days of the Wright Flyer doesn't have perfect historical recall. Though revolutionary in its day for lightness and power output, the 179-pound, 12-horsepower four banger that helped set the Wrights aloft is incredibly crude by today's standards. All due respect goes to Charles Taylor, an associate of the Wrights who advanced the science of powerplants; remember, this was the era of steam engines. At about the same time as the Wrights' first flight, William Harley and the Davidson brothers had just coaxed 3 hp from their motorcycle engine.

And while it's true that the development of light-aircraft piston engines peaked in the 1950s, a new era is dawning. Programs to push piston-engine technology have been announced from a number of companies. Engine manufacturers have come to realize that the status quo cannot be maintained indefinitely. The call for engines to run on lead-free fuel — sparking research into computer-controlled fuel-delivery and ignition systems — has pushed the issue, as has the now more-strident clamor from the airframe makers for more efficient, easier-to-use powerplants. As the gaze to global markets widens, alternative-fuel engines are being given more emphasis, hence the re-emergence of the diesel as a highly touted replacement for our gasoline burners; worldwide, jet fuel is less expensive and more readily available than 100LL.

Five years ago, we looked at the state of the art and few of these concepts were even on the horizon. Continental was touting its liquid-cooled Voyager engine — a powerplant that has yet to find the ideal home — but nothing like the GAP engine (above). Many industry watchers foresaw automotive-based engines coming over from the kitbuilt side and putting the old warhorses out to pasture. Didn't happen. The Rotax 912, then just an experimental engine, would eventually earn FAA certification, but the rest of the hopefuls remain little more than that — hopeful.

It's no secret that Toyota has refined and been granted certification for a version of its Lexus V-8 — a liquid-cooled, four-valve-per-cylinder bantamweight with full-authority digital engine controls, no less. But that program, after being proven on a Piper Malibu testbed, has been retired; according to knowledgeable sources, the benefits of a liquid-cooled V-8 shoehorned into a space designed for an air-cooled flat-six did not outweigh the disadvantages.

And therein lies the lesson. Save for the hyperenthusiastic Williams engineers, no one making a paycheck in aerospace seriously predicts a return to late-1970s production levels. So the market lies in retrofit. To exploit this avenue, any new-technology engine (or management system) must be made to work in existing airframes. New technology in new airframes is well and good, but those airplanes will be in the minority for a very long time. In the meantime, it's gratifying to see development by the big two — Continental and Lycoming — and it's also wise to note that the time may be ripe for heretofore unknowns, such as Orenda and Renault, to take a slice of the pie.

Future Fuels

Where do we go after 100LL?


It's no secret that 100-octane, leaded fuel is facing extinction. In this country, no other major form of transportation uses leaded fuel. The U.S. Environmental Protection Agency has been on the beat, trying to rid our homes and skies of what it believes is the scourge of lead. All the while, fuel producers are working overtime to manage what is arguably a rogue product in the lineup — aviation fuels require a level of isolation and inspection that's far beyond what is normal for a typical refinery.

And yet, here we are, with older-technology engines that need a fuel with antiknock properties most conveniently provided by adding lead. True, the 82-octane unleaded aviation fuel is finally a reality, after nearly a decade in development. It will provide a cleaner fuel for the part of the fleet that can use sub-100-octane fuel safely. Moreover, 82UL is significant in that it is, at heart, based on automotive fuel technology. Yes, the additive package is different from what you'll find at the local gas station, and there are no seasonal or permanent oxygenates. It also has a significantly tighter set of tolerances and inspection procedures. Essentially, 82UL will probably be, when it is put into production, auto fuel done right for aircraft.

Market demand will be the motivating factor for FBOs and distributors who decide to sell 82UL. Unfortunately, with the gradual phasing out of 80-octane leaded fuel — which most often does not need lead to make octane minimums, even though the American Society for Testing and Materials (ASTM) spec says that it may contain lead — many FBOs have just one set of storage and dispensing systems. As a result, the FBOs are taking the wait-and-see attitude about 82UL.

In the meantime, researchers are working diligently on an unleaded alternative to 100LL. Unfortunately, the solution is far more complicated than removing the lead and substituting various octane-boosting additives to get to 100 octane. (In fact, the researchers understand that they have to hit somewhere above the 100-octane mark to have sufficient margin in the final fuel to accommodate the normal variations. Typically, 100LL specs out at around 104 octane.) Aviation fuel has myriad specification requirements that include octane rating, vapor-pressure standards, water separation and freeze point, stability, and volatility.

Right now, the missing element is the octane rating. Depending upon who you talk to, the achievable octane rating with current unleaded fuel technologies is about 94. In truth, that's probably enough for some portion of the fleet certified on 100LL — understand that many engines were certified on 100LL because it was the convenient standard, not because they needed all of the antiknock qualities.

Almost everyone participating in the research programs agrees that the ideal long-term solution is a true 100-octane unleaded fuel, which would allow a basically seamless transition away from lead. It is also AOPA's position that any replacement for 100LL must satisfy the needs of the entire fleet — grounding certain high-performance turbocharged airplanes is not an acceptable tradeoff.

Many, though, think that the most likely solution will arrive somewhere between the ideal of 100-octane unleaded fuel and something just short of it. This will probably re-quire some form of electronic engine management — at the least variable ignition timing — for some of the critical engine models. Work on this front has already started.

Aurora Flight Sciences, under a NASA grant, developed an electronic ignition and fuel-injection system on a Cessna Skymaster (see " Under New Power Management," April Pilot). Continental has been working on electronic engine management for nearly a year with new Teledyne subsidiary Aerosance. Lycoming and Unison Industries (maker of Slick mags) announced at Oshkosh a program called EpiC — for electronic propulsion integrated control. Along with electronic management of fuel and spark, EPiC provides a single-lever power control. Both systems let the pilot retain control of the throttle assembly and force the electronics to match fuel flow and ignition timing to the conditions.

The powerplant engineers will probably have enough time to refine the electronic engine controls. It took a decade to turn 82UL from a bright idea to a usable specification from the ASTM. Industry observers predict that a transition to unleaded fuel for high-performance airplanes could take at least as long. The important point here is that the industry is working hard to find a solution to lead additives, including alternative fuels. The main down side to alternatives like methanol and ethanol is their lack of heat content, which means greater fuel burn to attain rated horsepower. You'll either need to carry more fuel or put up with dramatically reduced range in your airplane on these fuels.

So even though the crystal ball is a bit hazy, one image is clear — at some point we will have to abandon our cherished 100LL, because of regulatory pressures, or marketing issues stemming from the rising cost of production and distribution. The good news is that these changes are unlikely to come overnight.

E-mail the author at [email protected].

Progessive Powerplants

A look at two kerosene burners for future GA airplanes


It's been said that airplane designs can develop only as far as the engines that power them. With that in mind, it's no surprise to see that recent technological breakthroughs in certified airplanes have been made only in the instrument panel and not under the cowling. Sure, we now have electronic ignition systems and tailored fuel injectors, but by and large, we're still flying 1940s technology.

In 1994, the National Aeronautics and Space Administration (NASA) and a consortium of several companies within the general aviation industry launched the Advanced General Aviation Transport Experiment (AGATE) in an effort to revitalize the light aircraft industry in the United States. Companies such as Cirrus and Lancair are working on new airframes, while others, such as Arnav and Raytheon, are working on advanced cockpit-display technologies. Another important aspect of the joint ventures between NASA and participating companies is the General Aviation Propulsion (GAP) program. Two familiar names in the industry are working with NASA on new engines for light aircraft. Teledyne Continental Motors and Williams International are each pursuing certification of new engines that are expected to revolutionize the GA industry and allow it to progress beyond the stranglehold imposed by performance limitations of old-tech piston engines and the expense of existing turbines.

NASA has some pretty lofty goals for the AGATE programs — in fact, some sound too good to be true. First, the administration hopes to see production of light aircraft skyrocket to as many as 20,000 units per year in 20 years. (In GA's heyday in 1979, about 18,000 airplanes were produced.) By using high-volume production and low-cost, high-TBO engines, NASA believes, the cost of GA airplanes can fall within reach of a larger portion of the population. If the GAP plan comes together, the cost for the GAP piston engine will be half that of current-technology recips. The turbine version is expected to cost the same as today's piston engine/prop combination.

Continental's entry

Teledyne Continental Motors hasn't designed and produced an all-new engine for quite some time. Older readers may recall the Tiara engine that was certified in the early 1970s but failed miserably in the marketplace. Chalking up the Tiara program as experience, TCM plans to take reciprocating engine technology to the next level with its yet-unnamed entrant in the GAP program. This 200-horsepower, four-cylinder, compression-ignition engine (don't call it a diesel) will be a two-stroke design that to the uninformed will probably conjure thoughts of a smoke-belching behemoth that sounds like an outboard motor. Continental wants more than anything else to erase all that is popularly known about diesels and two-strokes.

Through the use of the diesel cycle, the engine will not need spark plugs and will run on jet fuel, which has a far more certain future than does 100LL avgas (see " Future Fuels," p. 59). The engine will also be liquid cooled to eliminate the potential damage caused by thermal shock and promote more even, controllable heat rejection. It is also hoped that controlled cooling will increase the time between overhaul (TBO) periods to intervals now seen only in turbines.

Several new technologies are to be used in the GAP engine. Among the most interesting are the directly opposed cylinders that cut down the size of the engine, reduce the parts count, and shorten the length of the crankshaft to reduce the stresses imposed on it — a very important consideration for a compression-ignition engine. Most engines have horizontally opposed cylinders that, when viewed from the top, are staggered. This design requires one crankshaft journal for each cylinder, resulting in a longer crankshaft, bigger engine, more weight, and more parts. TCM's four-cylinder GAP engine will have a crankshaft with no counterweights, only two journals, and a unique slipper connecting rod system that allows two connecting rods to ride on a common crank journal. TCM says that the slipper connecting rods eliminate some of the bending stresses that are imposed on current-technology crankshafts.

Like today's engines, the GAP engine is encased in aluminum and split vertically along the crankshaft line. Each bank of cylinders is cast monoblock with the respective case half. Replacement of individual cylinders — a top overhaul — is not an option — in fact, TCM is confident enough in its design that it intends for the GAP engine simply to be replaced instead of repaired. If design goals hold true, the engine should not need to be opened up prior to the proposed 3,000-hour initial TBO.

The power-generation process of the two-stroke GAP engine is quite different from today's four-stroke engines. The intake process can be compared to that of engines that power model aircraft. Inside the GAP engine's cylinders are cast-iron sleeves with intake ports milled into the bottom circumference. When the piston reaches the bottom of its travel, pressurized intake air is pushed into the cylinder by a conventional exhaust-driven turbocharger. Just before the piston reaches the top of its travel, a metered amount of fuel is misted into the cylinder. Under the influence of the high (18:1) compression ratio, the mixture instantly ignites, pushing the piston down on its power stroke.

The GAP engine differs from model aircraft engines in its method of exhausting combustion gases. The loop-scavenge system used by model aircraft engines and other two-strokes heats the rings too much, says Gil Hensien, director of TCM's GAP program. For this reason, TCM has adopted a four-valve-per-cylinder arrangement in which all valves are exhaust valves. After combustion, when the piston is on its way back down the cylinder, the four valves at the top open to scavenge the hot gases from the cylinder. Remaining cylinder pressures and the incoming charge of air push the rest of the exhaust out. When the piston reaches the bottom of the cylinder, the process starts over. Because the engine is a two-stroke, every cycle has a power event. According to Hensien, the engine should be as smooth as a V-8 engine, given that the four-cylinder, two-stroke design has the same number of combustion events per revolution and because the power pulses are directly aligned, eliminating some vibration.

Fuel will be delivered by a dedicated fuel pump and injector for each cylinder. The pumps will be activated by special lobes on the camshaft and will deliver a finely metered mist of fuel, unlike the continuous-flow injection used on today's Continental engines. Continental says that a fully electronic fuel-injection system will come later — for now, however, a mechanical linkage will have to do.

Starting the GAP engine will introduce somewhat of a problem. Since it has no glow plugs or other means to get the fire lit, this diesel will probably require a miniature electric air compressor (turbo) to get air moving through the cylinders to promote combustion. Of course, this air pump will add more weight and more expense. The engine is expected to weigh about 300 pounds, about the same as a 200-hp Lycoming IO-360. That seems like a lateral move technology-wise, but when you consider that most 200-hp diesels weigh more than 1,000 pounds, Continental is off to an admirable start.

Once the GAP engine is running, it's anyone's guess as to what the thing will sound like. It is expected to idle fast and easily rev to its 2,200-rpm redline just to break away from a parking spot. Instead of a throttle plate, the single power lever controls only how much fuel enters the engine, which is typical of diesel engines. The propeller is expected to be automatically controlled by a digital prop governor, to minimize pilot work load. In flight you can choose any rpm, as long as it is 2,200. This relatively low rpm should keep overflight and cabin noise levels to a minimum.

By the time you read this, the prototype engine should be running at TCM's Mobile, Alabama, headquarters, which is ahead of the original schedule. By March the engine should be installed and flying on an airplane.

For pilots, there's lots of promise in TCM's GAP engine. Reliability, simplicity, durability, and economy are its biggest attributes. If the cost and weight can be kept under control and 3,000-hour TBOs can be reached, then TCM may have a winner in its future lineup.

Williams chimes in

Jet engines are notoriously expensive. A set of two AlliedSignal (Garrett) TFE-731s on a Cessna Citation III cost more than $1 million. Williams International hopes to change that with the FJX-2 turbofan — its entrant in the GAP program. (Williams International has gained much respect in the industry, as evidenced by the popularity of the FJ44 series of turbofans that power the Cessna CitationJet, Swearingen SJ30-2, and Raytheon Premier I entry-level business jets.)

The FJX-2 is a lightweight, low-cost, 700-pound-thrust engine that is expected to weigh less than 100 pounds and burn about 100 pounds of fuel per hour — about the same as a 300-hp piston single. At high altitudes, however, a four-seat airplane powered by an FJX-2 could reach speeds of 250 knots or better, while burning the same amount of fuel as a Beech Bonanza. With that kind of speed versus fuel burn, the cost-per-mile figure could rival that of a four-place single. In Europe, where avgas prices can be double that of jet fuel, the jet's cost-per-mile advantage is even greater.

But can the average private pilot adapt to life in the flight levels at speeds that could be three times faster than he or she is used to? Dr. Sam Williams, chairman and CEO of Williams, believes that with minimal training and advancements in avionics and weather-avoidance equipment, the average pilot can make the transition. Attention usually devoted to engine management will be almost eliminated. A single lever with full authority digital engine control (FADEC) will make power changes a set-it-and-forget-it affair. Thermal shock and throttle-jockeying don't faze the FJX-2, either. In a sense, jet technology can cover up a pilot's miscalculation of descent rates, crossing restrictions, and other aspects of flight-level flying.

Other concerns revolve around runway performance. Turbofans in general don't perform as well on the runway as anything using a propeller. Williams believes, however, that the light weight of the FJX-2's components will allow the engine to spool up faster than typical jets, providing runway performance good enough to allow this new generation of light jets to use current general aviation airports with runways that are 3,000 feet or longer. Besides, the engines will be easily adapted to life as a turboprop for even better static thrust.

Part of the goal of the FJX-2 engine is to keep the cost down — way down. The only way that Williams can achieve this is through high-volume production. Plans are to adapt the engine for use in several aircraft: jet airplanes, turboprop airplanes, and turboshaft helicopters. The combined production of all of these may bring the price down to a more reasonable level.

Today, there are two flying airplanes ready and waiting for the FJX-2 engines. Chichester-Miles Consultants of England is flying the Leopard, a four-place jet currently powered by two FJX-1s. With the more efficient FJX-2s, the Leopard is expected to cruise at 0.76 Mach at Flight Level 450. The 4,000-pound airplane was displayed and demonstrated at this year's EAA AirVenture in Oshkosh. Last year the V-Jet II, a Burt Rutan-designed follow-on to Williams' V-Jet which debuted in 1985, flew at Oshkosh with FJX-1s. Other airplane designs, such as VisionAire's Spirit (see "Pilot Briefing," September Pilot), are awaiting the FJX-2.

Williams is currently on schedule with the FJX-2, and components are being tested now. It expects the assembled engine to be running by the end of the year. Testing will continue throughout 1999, and Williams says that it is on target to have the V-Jet II flying with the FJX-2s at EAA's AirVenture 2000. As more airframe manufacturers see on-airplane demonstrations of the engine, Williams hopes that more and more of them will design airframes around it. We expect October's National Business Aviation Association convention to bring an announcement or two from some popular GA manufacturers. Stay tuned.

E-mail the author at [email protected].

Piston Power

Orenda Aerospace breaks the piston-engine horsepower barrier


From early experimental-aircraft designers using castoff auto engines to Al Mooney's gambit of trying the two-cylinder Crosley in the first Mites, longing gazes have been cast toward automotive-engine technology. It hasn't stopped. With what many feel is an unacceptably high cost of aviation engines that lack the kind of technology found on the lowliest of rental cars, it's no wonder airplane creators continue to look at their four-wheeled counterparts for replacements.

Various auto engines have been converted for use in experimental/homebuilt airplanes — ranging from the Corvair flat six and various mass-produced V-6 and V-8 configurations to the Mazda rotary and opposed-cylinder Subaru models. A few, like the Thunder Mustang's rip-snorting, Chevy-small-block-based Falconer V-12, have gone right to the top of the horsepower figures that, at the dawn of the turbine era, neatly demarcated the choice of pistons or turbines.

None of these engines has ever been certified for production-aircraft use, and it's unlikely that they ever will be. But the Orenda has. With its roots way back in big-block racing engines, this Canadian V-8 has been certified and put into production. At press time, the engine has flown on a Beech King Air C90 and an Aero Commander 685. Orenda is working hard to nurture the retrofit market for the 600-horsepower V-8, and, once the plate of early retrofits is empty, will focus on such stalwart airframes as the de Havilland Beaver, Otter, and Twin Otter; Piper P-Navajo; and a variety of agplane platforms.

The King Air pitch is that you can remove high-time Pratt & Whitney PT6s and install the Orendas and still come out ahead; expected retrofit costs are about $400,000 for the pair. This is low by turbine standards. Orenda emphasizes the performance improvements, which makes sense. Turbines are like normally aspirated piston engines in that they make full power only at sea level or, depending upon the degree to which they are flat-rated, a comparatively low altitude. From that point on, power declines as the airplane climbs. Turbo-charged piston engines can be tailored to make full (or maximum continuous) power to very high altitudes; the best production turbos can maintain 75-percent power to 20,000 feet or more. As a result, the King Air conversion results in a substantially faster airplane at altitude. Predictions are for the C90 to cruise at better than 270 knots above 20,000 feet, an improvement of between 20 and 50 knots, depending upon the version of the C90's original engines. Moreover, climb performance improves substantially, particularly at high altitude. Finally, the Orenda's claimed specific fuel consumption of 0.42 pounds per horsepower per hour is a tremendous improvement over the typical PT6's 0.65 pph/hp figure.

Although the Orenda's roots are undeniably automotive, there's not much left from the big-block Chevy in the production version. An aluminum block and heads are used, as are iron cylinder sleeves; in typical race car fashion, these sleeves are run "wet"; that is, they are supported at the top and bottom of the water jacket only. These features help to keep the dry weight to 750 pounds with accessories, including the integral gear reduction drive. With a reduction of 0.4765:1, the Orenda turns the prop at a maximum of 2,057 rpm — a bit faster than the PT6 would do — at the engine's 4,400-rpm redline. It uses a whopping 52 inches of manifold pressure to make 600 horsepower; the turbo system employs an air-to-air intercooler.

Inside the Orenda, you'll find eight 4.43-inch-diameter pistons riding a four-inch stroke, for a total displacement of 495 cubic inches. That's the same stroke as a Continental IO-520, so the piston speeds are about 40 percent higher. Compression ratio is a moderate 8-to-1. Because of its comparatively high horsepower-to-displacement ratio, the Orenda's combustion pressures are also high — the brake mean effective pressure (BMEP) is approximately 218 psi at takeoff. Very high by typical opposed-piston powerplants, but still well under the records set by the last, most powerful turbo-compound radials of the 1950s. Even at this stress level, Orenda expects the engine to make the 1,500-hour TBO, because liquid cooling provides more uniform cylinder temperatures and helps to keep the bores from going out of round, as typically happens in an air-cooled engine operating at high temperatures and pressures.

One of the big selling points of the automotive powerplant is the high technology of its ancillaries, but you might be surprised to see that the Orenda has a great deal of traditional aviation gear. It uses a massive Bendix mechanical fuel injection system; a massive, single Garrett turbocharger developed just for this engine; hydraulic wastegate controllers; dual Bendix magnetos for ignition; and a hydraulic prop governor. Expect to see two sets of three levers — throttle, prop, and mixture — for each engine in the cockpit. Orenda says that a FADEC system will come to the engine soon, perhaps just a year and a half away. Now, though, the company is focusing on getting the production spooled up.

The big question for now is this: Will King Air owners willingly strip out what is regarded as the most reliable of turbines to install piston engines? Orenda and Stevens Aviation, the U.S. firm contracted to undertake the installation of the V-8, think so. They point to lower overall operating costs and improved performance as the triggering qualities. Perhaps of greater interest to the whole of general aviation is the potential for the Orenda to show up on a Piper Malibu-class airplane or, better yet, a design penned with the rorty V-8 in mind.

E-mail the author at [email protected].

Detroit Meets Williamsport

Lycoming announces a new partnership and diesel engine


Textron-Lycoming, of Williamsport, Pennsylvania, and Detroit Diesel Corporation, of Detroit, Michigan, have put their heads together and are actively pursuing the production of a new diesel powerplant for aviation applications. The announcement was made almost in passing at a press conference for the EPiC system at EAA's AirVenture '98 in Oshkosh. Under the terms of the recently penned partnership, Lycoming and Detroit Diesel "will share responsibility for the design, development, certification, and manufacture of aero-diesel engines, should performance, reliability, and market targets be met," according to a Lycoming press release.

This particular engine design was entered by Lycoming in the NASA General Aviation Propulsion competition but was edged out by Continental's more high-tech submission (see " Enginuity: Progressive Powerplants," p. 60). Lycoming's engine is more mainstream compared to Continental's diesel — it too is turbocharged, but it will have a four-stroke combustion cycle and removable cylinders like those of today's gasoline engines.

Lycoming started with an abandoned Italian-designed, liquid- and air-cooled, four-stroke, four-cylinder engine project. When Lycoming has completed its adjustments, this engine is expected to produce 205 to 210 horsepower at 2,400 rpm. Like today's Rotax engines, Lycoming's new diesel will have liquid-cooled cylinder heads and air-cooled crankcase and cylinder barrels.

The engine dates to the 1980s when VM Motori, the largest independent manufacturer of automotive diesels, designed, built, and bench-tested the same prototype engine that Lycoming is using. Big plans were made for the engine back then — the 1989-90 edition of Jane's All the World's Aircraft lists four-, six-, and eight-cylinder versions with horsepower figures of 206, 315, and 424, respectively. (On the down side, Jane's listed weights for those engines, at a beefy 408, 536, and 657 pounds, respectively. Lycoming's turbocharged four-cylinder TIO-360, six-cylinder TIO-540, and normally-aspirated eight-cylinder IO-720 weigh a maximum of 350, 500, and 570 pounds, respectively, depending on configuration.) The engine was also well received at the Paris Air Show in 1989. But when piston aircraft engine production bottomed out in 1991, the Cento, Italy-based company, then in the midst of a buyout, shelved the project. In January 1995, Detroit Diesel acquired VM Motori and made it a wholly owned subsidiary.

It is doubtful that Lycoming will consider the eight-cylinder engine for production, but there is definite interest in producing the six-cylinder variant. Among other tweaks, Lycoming and VM are aggressively working at trimming the weight and increasing the power output so that full power can be reached at a quieter 2,400 rpm instead of the engine's original 2,640 rpm.

Right now, Lycoming claims that the yet-unnamed diesel can boast fuel efficiency figures of 0.35 to 0.36 lb/hp/hr burning widely available jet fuel. Current-tech gasoline engines achieve efficiency ratings of 0.385 to 0.45 lb/hp/hr. For the four-cylinder diesel, that efficiency rating translates into a fuel burn of 8.6 gallons per hour at 75-percent power (157.5 hp) and best economy mixture. A comparable Lycoming TIO-360 would burn at least 10 gallons per hour at that setting.

After recently running the original prototype in a test cell, Lycoming sent the engine back to Italy to go on a diet and for other tweaking. Upon its return, Lycoming plans to put the engine through a 500-hour endurance test, which should be completed around June 1999. Initial time between overhaul will be determined after the 500-hr test. As for the sound of the engine, a Lycoming official wouldn't comment but said that there was "no doubt it was a diesel."

E-mail the author at [email protected].

The French Connection

A promising alternative from across the pond


The new aviation engine that has progressed the furthest is the one you've probably heard the least about, and it comes from a most unlikely source: Renault, the French automobile manufacturer. In early 1997, Renault Sport and Aerospatiale formed a partnership, called Societe de Motorisations Aeronautiqes, under which Renault would develop a diesel aviation engine for the general aviation market and Socata, Aerospatiale's general aviation division, would provide the testbed aircraft and do the flight testing and certification work.

While Renault is relatively unknown in the United States, the company is famous in France not only for its traditional automobiles, but also for its high-performance race cars, which often dominate European motorsports.

The objective of the partnership is the development of a family of general aviation engines with power of 180, 250, and 300 horsepower. The engines will be horizontally opposed and turbo-charged with direct fuel injection. Both oil and air will be utilized for cooling, providing the benefits of relatively light weight (air cooling) and protection against thermal stresses (oil cooling). The engines use diesel technology but are designed to take advantage of the ready supply of jet fuel at the world's airports. Computer controls simplify power management and allow for a single lever in the cockpit.

The 180-hp engine will be direct drive; the larger engines will employ a gear reduction to keep maximum propeller rpm at about 2,000 to reduce noise. The 300-hp engine may have an inverted oil system for aerobatic applications. Around the engines Socata plans to develop a family of airframes to be called Morane. The Moranes will be derivatives of Socat's Caribbean series of aircraft — the Tampico, Tobago, and Trinidad.

By mid-July 1997, Renault was running a 200-hp engine in a test cell. The Paris Air Show last year featured a Trinidad mockup with the Renault engine on the front. Test cell work continued through 1997 and early 1998. In early summer of this year, Socata test pilots made their first flight behind the new powerplant, derated from 250 hp to 200 hp during the testing because of a gearbox limitation.

When AOPA Pilot visited the Socata factory in Tarbes, France, in July, the pilots had made four flights on the first engine. It had been returned to Renault for additional test cell work, and a second engine had been put on the Trinidad. According to Jean-Francois Sochor, one of Socata's test pilots, the 200-hp turbocharged engine performed well on flights all the way up to 19,000 feet, where the climb rate was still 180 feet per minute.

Sochor was most surprised by the diesel's vibration levels, or lack thereof. He described the vibration level as being equivalent to that of the Lycoming IO-540, which normally resides in the Trinidad. The IO-540 is known to be a rough-running engine, but diesels have a reputation for being particularly cobbly, so for the first engine to achieve vibration levels equivalent to that of a gasoline engine is good, according to Sochor. He believes that with additional internal balancing and modifications to the mounts, the diesel can be made to operate much more smoothly than existing engines.

The engine was deliberately set to run richer than normal during the test flights, so Sochor could not predict production fuel specifics or temperatures.

The second engine was scheduled to fly in September. Meanwhile, Socata engineers had fabricated a modified cowl that reduces the size of the two vertical cooling inlets on the front of the cowl. In addition, an induction air inlet will be created just below the propeller spinner. On the original cowl, the induction air snaked into the engine through the lower half of the left vertical inlet. The dedicated induction inlet smoothes the air flow to the engine.

When certified in late 1999, Socata believes that the new engines will cost about the same as existing avgas-burning powerplants. However, operating costs are expected to be 30 to 40 percent lower than those for traditional engines, because of a 3,000-hour TBO, lower maintenance costs, and lower fuel burn, and also because jet fuel costs less than avgas, particularly in Europe. Indeed, even if it weren't for the lower operating costs, the new engine would be welcomed to the GA market, thanks to its ability to use jet fuel rather than the endangered avgas.

Besides the new Morane airplanes, Socata and Renault intend to make the powerplants available to other airframe manufacturers and also to aircraft modifiers. They will use the same engine mount as those used for the Lycomings in the Socata airplanes. However, retrofitting to existing aircraft will be challenging because of the many required changes to the fuel system — and to the cockpit itself, to accommodate the single-lever power control.

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The 300-pound, 12-cylinder Dyna-Cam engine hasn't been around forever, although it seems like it. Using six sets of double-headed pistons acting on a massive, sinusoidal cam, the Dyna-Cam's configuration is quite unusual. The 200-hp engine produces a stunning 650 foot-pounds of torque at just 1,200 rpm, making it a good candidate for direct-drive and low prop speeds. Dyna-Cam got Piper to sample the engine on an Arrow in the late 1980s, but since that time has been trying to lure sufficient investment to begin production. — MEC


Rotax's 912 has received a recent boost in power — from 81 horsepower to an even 100 — to create the 912S. The increase in power is a result of new pistons that up the compression ratio from 9:1 to 10.5:1. There is a five-minute limitation for the full 100 hp, which is reached at 5,800 rpm. Maximum continuous power is 95 hp at 5,500 rpm. Otherwise, the Rotax remains largely unchanged. It's still a geared four-cylinder, four-stroke, liquid/air-cooled engine with dual electronic ignition and altitude-compensating carburetors that eliminate the need for a mixture control. Weight of the engine without accessories is 125 pounds, and fuel burn is a low four gallons per hour, making it a very popular engine for kitbuilt aircraft. The 912S is an FAR Part 23-certified engine and was to be installed in Diamond's Katana before the company went with the Continental IO-240. — PAB