Exploring the Mazda Wankel rotary asan alternative aircraft engine

PAUL LAMAR, EAA 171194

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Aviation headline nine years from now:

Rotary-Powered Racer Bests Warbirds at RenoRENO, NV—Employing turbocharged, twin three-rotorengines, the Rotary Ranger demonstrated what a lowcross section, lightweight, reliable, 1,000-horsepower en-gine can do....

O kay, so it hasn't happenedyet. But let me convey whysuch a headline isn't thatoutrageous. When married

with the proper airframe, a rotaryengine could certainly give the oldwarbirds a run for their money. Sowhy do engine experts seldom in-clude the rotary engine when dis-cussing the merits of auto engineconversions for aircraft? Most likely,it's lack of knowledge.

If I could define the criteria for arealistically "perfect" aircraft engine,I th ink you'd agree that the follow-ing attributes are desirable in anyaviation powerplant: reliability; ro-bustness; extended time betweenoverhauls (TBO); damage tolerance;smoothness; no shock cooling; abil-

44 FEBRUARY 2002

Perry Mick's Mazda-powered Long- EZ features a ducted fan shroud giv- ing it an extra-futuristic profile.

ity to run low-cost fuel; low fuel con- sumpt ion ; low purchase cost; h igh h o r s e p o w e r - t o - w e i g h t ratio; com- pact size; and simplicity.

The Mazda Wankel rotary engine meets most of these criteria.

Wankel HistorY 1)r. Felix W a n k e l i n v e n t e d the e p o n y m o u s e n g i n e in the 1930s, and I use "Wanke l e n g i n e " in the generic sense to differentiate it from all o t h e r r o t a r y eng ines . A r o u n d 19S0, l)r. Wanke l built a running pro- totype, but it d idn ' t generate much com- mercial exci tement . Several c o m p a n i e s were i n t e r e s t e d in the engine, however.

One of t h e m was Toyo Kogyo, known to A m e r i c a n s as Mazda Motors, and it created a state-of- t h e - a r t c o m p u t e r - ized test facility ex- p r e s s ly for d e v e l o p i n g the en- gine. After a lengthy development period, Mazda s ta r ted sell- ing cars with rotary engines in 1967.

Over the years engineer ing chal- lenges continued to crop up, and the other rotary players gave up further development. But Mazda persevered.

Top, Tracy Cook, of Real World Solu- tions (see sidebar page 48), is help- ing aircraft owner Dave Garber get his dual Mazda turbo engine-powered Reno racer back in shape. Note the dual radiators aft of the cockpit.

Bottom, concept of an over-and-un- der dual Mazda Cosmo turbocharged three-rotor engines of 1000 hp each installed in a custom-designed Reno racer.

It believed in the rotary engine and solidly understood its superior kine- matics. Eventually, Mazda sold more than two million rotary engine-pow- ered cars.

Why Rotary? Kinematics is the science of mechan- ics that deals with motion, and poor k inema t i c s and rec ip roca t ing mo- t ion have always been the bane of the piston engine. Things stop then start up again in the opposite direc- tion twice per revolution.

This reciprocat ing act ion creates t r e m e n d o u s stress on c o n n e c t i n g rods, p is tons , p is ton pins, valves,

va lve spr ings , c a m s h a f t s , l i f ters , pushrods , and c r a n k s h a f t s - - e s p e - cially at high rpm. The crankshaft in a p i s ton engine , for example , is a s t ress a n a l y s t ' s w o r s t n i g h t m a r e with its twisting and turning every which way.

Wankel engines have no recipro- ca t ing m o t i o n because every par t moves in a circle or moves in nlore or less the same direction. A two-ro- tor Wanke l eng ine has on ly th ree moving parts: two rotors and the ec- centric shaft, which is analogous to the piston engine 's crankshaft. Un- like a crankshaf t , however , the ec- c e n t r i c sha f t is s t r a i g h t a n d ex-

A B C E

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Spo"t Aviation 4.5

tremely strong.The turbocharged,

800-hp, two-rotor, 10000rpm Mazda racing engine of

stock displacement has demon-strated this strength. Not includingthe monster turbo and other acces-sories, this powerplant weighs amere 200 pounds. This represents anunprecedented power-to-weight ra-tio for all but a turbine engine.

Nobody in his or her right mindwould think of mounting an 800-hp Mazda Wankel engine in a lightgeneral aviation airplane, but itmight make an ideal engine forUnlimited air racing. It does illus-trate the robustness of the MazdaWankel engine.

Because they do not reciprocate,the Wankel's rotors are made ofrugged cast iron, and unlike the pis-tons in a piston engine, they arenearly impossible to melt. A rotaryengine has no exhaust valves to meltor burn, a common problem in air-craft engines, and it has no camshaftsto gall and no lifters to score.

Like the rotary itself, the engine'scombustion cycle is unique.

The engine discharges its exhaustgases as the rotor moves over open-ings in either the rotor housing orthe end housings. This action is sim-ilar to that of a two-cycle piston en-gine, but the Wankel is a four-cycleengine, and like a four-cycle, four-cylinder engine, it fires twice peroutput shaft revolution.

Because nothing in the Wankelreciprocates, stops and starts movingin the opposite direction, it remainsin perfect balance, just like a turbineor an electric motor. It's impossibleto perfectly balance a piston enginebecause certain parts oscillate or re-ciprocate—by design—throwing offits center of gravity. Consequently,the rotary runs smoother than anypiston engine.

Wankel engines cannot fail cata-strophically in the "hand grenade"fashion, as sometimes occurs withpiston engines. One of the worstthings that can happen to a Wankelis an overheated engine caused byloss of coolant, but it will still runand usually will get you to a nearbyairport. Cold pressure checking thecoolant system before f l ight is im-portant, and using high quality AX

aircraft plumbing is a necessity.Another testament to the engine's

bulletproof design is Mazda's trackrecord at the 24-hour endurance racein LeMans, France. Mazda was theonly Japanese car manufacturer towin the race outright. In 1991 Mazdafielded a car with a four-rotor versionof the two-rotor RX-7 engine, and itbeat all comers, the who's who of ex-otic European carmakers.

Tearing down the engine af terthe race revealed l i t t le or no wear.The following year, LeMans bannedrotary engines from the race. Now,roughly 10 years later, Mazda rac-ing ro t a r i e s are e i the r severe lyhandicapped or limited to compet-ing against other Mazda rotaries inauto racing.

In homebuilt aircraft , however,Mazda Wankel engines are seeing arenaissance of sorts for several rea-sons, low cost not being the pri-mary driver. The m a i n reason isdurability. The stock engine will runfor many hours at high power withlittle or no wear.

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46 FEBRUARY 2002

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INTAKE

SIDE EXHAUST TYPE COOLING DIAGRAM FOR KIS TR-4 SHORT COWL VERSION

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gine Newsletter and others testing ro- taries have further improved on what Mazda accomplished by using more costly high-tech materials for certain engine parts. Dr. Francisco Inanette, for instance, has developed a ceramic apex seal (which is affixed to the ro- tor) that's so hard and smooth it has no noticeable wear rate. Nobody has worn one out vet.

High tempera ture , 500°F Teflon encapsu l a t ed si l icon water jacket seals and oil scraper O-rings have also been developed. These increase the engine's ability to withstand in- advertent overheating, which hap- pens occasionally as homebui lders r e l ea rn p re -Wor ld War II lessons about liquid-cooled aircraft engines.

These costly materials, a l though

Turbocharged Mazda Wankel RX7 en- gine mounted on the author's Pul- sar/KIS Cruiser with single radiator obliquely mounted under engine. Dia- gram is the author's installation draw- ings for his rotary powerplant.

i m p r o v e m e n t s , are n o t requ i red . Based on the wear rate of the stock apex seals and cruising at about 150 hp and 4500 rpm, the projected TBO of a stock Mazda two-rotor engine in aircraft use is about 3,000 hours.

Through real-world testing home- bu i lders have f o u n d t h a t m i x i n g two-cycle oil with the fuel reduces the apex seal wear and the carbon deposits inside the engine. The ratio used is about one ounce of two-cycle oil to one gal lon of gas. The stock engine injects oil from the crankcase i n to the c o m b u s t i o n c h a m b e r . Crankcase oil contains certain wear add i t ives t h a t rea l ly make it un- suited to this task, and users disable the oil injector when using the en- gine in a race car or airplane.

Rotary Specifics • Power-to-Weight Ratio The power-to-weight ratio of the

stock Mazda engine is excellent. De- spite its mos t ly cast i ron hous ing parts, the bare eng ine weighs 200 pounds. Expensive a luminum end housings cut this weight by about 50 pounds.

The f i rewal l f o r w a r d i n s t a l l ed weight of a normally aspirated two- rotor Mazda engine with propeller speed r e d u c t i o n u n i t (PSRU), coolant , and radiators runs abou t

the same as a Lycoming O- 320 or O - 3 6 0 - - r i g h t a round 300 pounds. With n o n - s t o c k pe r iphe ra l in- take ports, a normally aspi- rated ro tary can genera te 240 hp on takeoff with ex- cellent durability.

• PSRUs Severa l p o p u l a r pro-

pe l le r speed r e d u c t i o n units are available. Weigh- ing 40 pounds each, most

Spor t Av ia t ion 4 7

Rotary SuppliersSpeak Out

The homebuilt industry supports several aftermar-ket and accessory suppliers for rotary engines. Power-

Sport Inc. in Osceola, Wisconsin, has been diligently work-ing on a more highly engineered reduction drive. "Thisseems to be the main deterrent in using the rotary engine inaircraft," says PowerSport Manager Keith Holm. Since theMazda 13B engine is torsionally stiff, it produces a specificresonant frequency at certainpower settings that can rattle apoorly engineered reductiondrive. "Our RD-300 drive hasbeen available for more than ayear now," Keith says, "andsolves that problem."

PowerSport's drive is rated for300 hp continuous at 2,000hours at an 80 percent duty cy-cle. The 2.29-to-l reduction ratioturns the prop at 2620 rpm at itsengine's 6000-rpm limit. Power-Sport currently has its rotary en-gine conversion with its propri-etary electronic enginemanagement system and tunedintake and exhaust systems in-stalled in an RV-6A test aircraft(displayed at EAA AirVentureOshkosh 2001). PowerSport's engine produces 215 hp con-tinuous at 6000 rpm—far below the limits of the stockMazda 13B. You can find out more about PowerSport at itswebsite: www.powersportaviation.com.

PowerSport engines are all built from modified new partsdirectly from Mazda, but other engine builders—and homecraftspeople with an engineering bent—start at the salvageyard. Companies like Atkins Rotary Specialties Inc. producesome solid aircraft engines from the bulletproof Mazda 13B.But even a seasoned auto racing pro like Dave Atkins is im-pressed by the PowerSport reduction drive because of itsability to handle high-torque loads.

Dave typically uses a highly modified Ross drive, but hishigh-torque engines can be hard on them. Atkins has pre-sented the Rotary Engines in Aviation forum at EAA AirVen-ture and says, "A lot of people are still afraid of the rotaryfrom the bad press the engine received in the early '70s.But Mazda fixed most of those problems in 1974."

Dave's RV-6, which has flown from the Seattle area toOshkosh a half-dozen times, cruises at 160 knots and 8 gphof unleaded auto fuel. Drag racing in the early 1980s taughthim a lot about the Mazda, and Atkins Rotary Specialties(tvtvtv.atkinsrotary.com) uses custom manufactured parts,factory parts, and modified parts to build its enhanced avia-tion powerplants. One Atkins customer has more than2,500 hours on his engine, and Dave has never had a cus-tomer experience an in-flight failure.

"You install a rotary because you want one," Dave says."I'm saving $8 an hour in fuel, and my annual last year was$50. A total rebuild would cost me around $2,000. Overall,it's a cheaper way to go. But don't try to build the engineyourself, and don't just go to your local garage. They won'tdo it right."

Tracy Crook of Real World Solutions Inc. (www.rotaryavia-tion.com) says, "I've done a lot of work on the PSRU and theEFI/ignition controller during the past three years and re-

cently came out with a higherhorsepower version of the PSRUfor turbocharged 13B and 20Bthree-rotor engines (300-hp max)."

Tracy has flown his rotary-pow-ered RV-4 for more than 1,000hours, and he's building an RV-8that's about a year away from itsfirst flight. "There were six ro-taries at EAA AirVenture 2001,and there are at least 100 rotaryprojects currently underway andmore starting all the time," Tracysays. "The auto fuel capability isdefinitely a factor in many buildersdeciding to look at alternatives."

Keith Holm of PowerSportwarns that plenty of challengesexist for homebuilders consider-ing rotary power. "Do as much re-

search as possible to be well informed about the optionsavailable. Be honest with yourself about your mechanicalabilities and your project goals."

Homebuilders should also check with their insurance car-riers to see how a particular engine/airframe combinationwill affect their rates.

Next year Mazda is introducing the RX-8 to the U.S. mar-ket with an updated and more powerful rotary engine,known as the Renesis. Even though the engine won't beavailable to the aftermarket for quite some time, it does in-dicate a bright future for the rotary.

—Mike DiFrisco

48 FEBRUARY 2002

of them are based on theFord truck automatic trans-mission planetary gear set.Ford recently beefed up thistransmission by changing the num-ber of pinion gears from four to six,making it suitable for the three-rotorengines or the higher power tur-bocharged two-rotors.

• TurbochargersTurbochargers, which maintain

engine power at high al t i tudes ,weigh about 35 pounds . To myknowledge, the upper horsepowerlimits of the turbocharged versionsare unknown because nobody hasrun one in an airplane for more thanabout 100 hours.

• Engine SizeThe two-rotor engine is compara-

tively tiny. Without any accessoriesit measures about 12 inches wide, 14inches high, and 18 inches long. It'squi te possible to squeeze a tur -bocharged two-rotor engine, includ-ing radiators, inside a stock kit air-plane cowling. (I've done it with myKIS Cruiser . By comparison, theroom in a Van's RV cowling wouldbe considered voluminous.)

• Three-Rotor EnginesThree-rotor engines develop 50

percent more power than two-rotorengines, and you can import themfrom Japan. Baa- three-rotor enginesweigh less than 300 pounds, and afirewall forward installation wouldweigh around 400 pounds without aturbocharger. Racing Beat in Ana-heim, California, has a three-rotorturbocharged aircraft engine on thedyno generating 900 hp with about200 hours of dyno runs so far.

The eccentric shaft in the three-rotor is the same diameter as theone in the two-rotor, and it gives notrouble. With exactly the same ec-centric shaft diameter, the MazdaLeMans engine was a four-rotor ,normally aspirated unit developing690 hp at 9500 rpm. Many of theother three- and four- ro tor par tswere similar if not identical to thetwo-rotor RX-7 engine.

• Fuel ConsumptionThe Mazda will happily run 100

Rotary User GroupsRy Rotary Website (pilots and their projects)—www.members.home.net/flyrotary/Rotary Engine Website (Paul Lamar)—http://home.earthlink.net/~-rotaryeng/

degrees lean of peak, and it won'tburn the exhaust valves—becausethe engine doesn't have any. At thissetting, the EGT has dropped intothe piston engine range, and the en-gine is only slightly less economicalthan an air-cooled piston engine.

Tracy Crook, who has a Mazda ro-tary on his RV-4, estimates that his

engine burns 0.47 pounds of fuelper hour for each horsepower gener-ated; an air-cooled piston engineburns about 0.45. If you run the en-gine full rich at peak power, it willburn 0.65, just like an air-cooled pis-ton aircraft engine under the sameconditions. The new RX-8 engine isreputed to be much more economi-

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Sport Aviation 49

3.00

2.50

2.0O

ca). Th eMazda

happilyruns on autogas

that , r ight now, isabout half the cost ofavgas.

• CoolingThe total heat re-

jected by a Mazda ro-tary is only slightlyhigher than a pistonengine. All engineshave a heat balance,which is the distribu-tion of energy con-tained in the fuel dueto bu rn ing it. Someenergy is rejected tothe coolant as waste,heat, some goes outthe exhaust, and theremainder comes out the shaft asmechanical energy. One horsepoweris equal to 746 watts, and an enginegenerating about 200 hp would be

Power-to-Weight vs. HPVarious Engines

oQ.O)enoX

1.50

1.00

0.50

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Garrett TSE 231-1(gas turbine) ——— *. 4

Holler Rotary

Garrett TSE 36-1/(gas turbine)

Wankel LCR 814 1/2 Le Mans / V Mazda SideRotary/ Engine——^ p̂,̂ E||̂ ne

Midwest Harrier / /Midwest Hawk /maM|a 138 RotaryRotary \ S / Rotary if

^•'' / Lycoming 0-235 Lycoming IO-360-AIA

/ / /Continental 10-470-H

. / * t * *^1 \ "* * ̂

Wankel ^ *

R™"r?7 FMW-Z"»C

D 50 100 150 200 250 300 350 400 450 50Rated Horsepower

rejecting about the same amount ofhorsepower to the coolant, about150 kilowatts.

This is not unique to the Wankel!

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All internal combustion engines dothe same more or less by only a fewpercentage points. A Wankel en-gine's heat balance differs by onlysingle-digit percentages compared toall other forms of internal intermit-tent combustion engines. (A gas tur-bine is an internal continuous com-bust ion engine, in case you werewondering.)

So Why Not a Wankel?To be frank, you'll have a consider-able engineering challenge installinga rotary engine in your airplane. Theengine itself is not the problem;careful planning is absolutely neces-sary with any non-aircraft or autoengine conversion. You can ease thischallenge by mak ing mechanicaldrawings and mock-up models be-fore starting the building and instal-lation process.

You may have to design and buildcomponent par ts like the motormount, oil pan, alternator mount,and intake manifold and modify thewater pump inlets and outlets. Asthe engine becomes more popular,many of these unique aircraft-spe-cific parts should eventually becomeavailable to home craftspeople.

On the upside, overhauling therotary engine is trivial by Lycoming

or Continental standards.Strict attention to cooling details

is essential . Most rotary home-builders have trouble with cooling,and many have s imply given up.The exact shape of the ducting tothe radiator is critical, and sealingthe cowl properly to prevent backpressure on the radiator is essential.The design of the cowl flaps and theopening around them are other im-portant considerations. Wankel en-gines reject a third of their coolingheat through the oil, so a good oilcooler is mandatory, and the stockMazda RX-7 oil cooler works well.

The Wankel engine is ideal forturbocharging because of the shapeof the combustion chamber, the in-herent low compression ratio, andthe lack of exhaust valves. You'llneed an intercooler to keep the in-take temperature down. High intakemanifold temperatures lead to deto-nation, which can f rac ture apexseals with consequent scoring of therotor housing and rotors. The en-gine will not fail catastrophicallyunder this scenario, but it will runsomewhat low on power.

A good muffler is essential with aWankel because the exhaust portopens much more rapidly than in apiston engine and emits supersonicshock waves. This will not be a prob-lem on the new Mazda RX-8 motor,which has side housing exhaust portsthat open and close more gradually.

A turbocharger helps quiet theengine and adds no weight be-cause it replaces the muffler. In the1989 to 1992 R X - 7 model years,Mazda built a good turbochargerfor a ircraf t use. Later, dual tur-bochargers were too complicatedand less reliable, while earlier onesjust plain cracked.

When purchasing a Mazda en-gine for aircraft use, it's best to buy arunning car because it comes with alot of parts you can use, includingthe alternator, ignition coils, fuel in-jectors, starters, oil coolers, etc.Good Mazda vintages are 1989 andlater. 1992 and earlier second-gener-ation RX-7s can be had for less than

$3,000. The 1993 models and latersell for more than $15,000 becausethey were more expensive and so-phisticated cars to begin with.

With all it has going for it, theWankel rotary just may be the lightaircraft engine of the future. c£*.

A former aerospace and (Chaparralrace car engineer, Paul Lamar inventedand manufactured computerized auto-motive performance test eijidpment. Paidlias been an EAA member for 25 years,owns a Cessna J82, and has loggedabout 3,000 hours. Now retired, he'sbuilding a KIS Cruiser with a Mazda

turbocharged two-rotor rotary engine.Paid edits the 3-year-old e-mail Air-

c raf t Rotary Engine Newsletter,which has about 1,000 subscribers. Heis married to Robin Lamar, a full-timeUnited Airlines mechanic, sometimeA&P teacher, president of the Associa-tion of Women Aircraft Mechanics, andwinner of the prestigious Aviation Tech-nician of the Year award given byWomen in Aviation International.Robin has no time left over to help Paulbuild his airplane.

For more information visithttp://home.earthlink.net/~rotaryeng/.

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Sport Aviation 51