Incredible Mazda wankel rotary aircraft engine. RevisedCopyright 1988-2009 By Paul Lamar
Aviation head line nine years from now "Rotary Powered Racer Beats War-birds at Reno, 2010". Employing turbo charged twin 3 rotor engines in the Rotary Ranger demonstrated what a low cross section, lightweight, reliable, smooth 1000 HP engine will do ..... OK, so it has not happened yet, but let me convey why such a headline is not out of this world..... There is an engine already proven which when married with the right air frame could certainly give the old war-birds a run for their money......
Dave Garber built such an airplane in the 1980's and it is now being taken out of mothballs and refurbished. Dave had some cooling problems and fuel injection computer problems. He has recently made contact with members of the Aircraft Rotary Engine Newsletter with experience in solving these kinds of problems.
Mazda rotaries have been raced in the Sun & Fun, Sun 100 air race at Lakeland Florida. Allan Tolle averaged 238 MPH in an RV3 back in 1994 using special intake porting. Tracy Crook averaged 209 MPH in an RV4 with a near stock 1989 RX7 engine in 2003. In 2004 Tracy averaged 217 MPH in the same airplane using the same engine.
"Why do engine experts seldom? Never? include the rotary engine when discussing the merits of auto engines for aircraft use" The last two issues of a popular kit plane magazine had nicely done articles on assessing auto engine for such an application - but, they did not even mention the rotary. Are they afraid to? Is it lack of knowledge, or is it because if they did it the rotary would jump out as a far superior choice to any reciprocating auto engine?
Let me define the criteria for a "realistically perfect" aircraft engine.
C: Extended TBO
D: Damage Tolerance
F: No shock cooling
G: Ability to run low cost fuel
H: Low fuel consumption
I: Low purchase cost
J: High HP to weight ratio
K: Compact size
I think the Mazda wankel rotary engine meets most of these criteria.
The wankel engine was invented way back in the 1930's by Dr. Felix Wankel. I will use the term wankel engine in the generic sense to differentiate it from all other rotary engines. It was not before about 1950 that Dr. Wankel was able to build a running prototype. Many companies became interested in the engine and undertook development. Among them was Toyo Kogyo AKA Mazda Motors. Mazda installed a state of the art computerized test facility in 1963 for the express purpose of developing the engine. After an intensive development period Mazda started selling cars with rotary engines in 1967. The first of the six rotary engines cars that I have owned was a 1973 RX2 bought new. It was a very zippy little car. My current car is an RX8.
Many problems came to light over the years. Only Mazda persevered and perfected the engine. All but Mazda gave up. Mazda had a strong belief and good understanding of the superior kinematics of the engine and adopted the Churchillian attitude of never ever give up. Kinematics is the branch of theoretical mechanics that deals with motion. Eventually Mazda sold over two million rotary engine powered cars. Eight hundred thousand RX7s were sold alone. The RX8 is now selling at the rate of about 2,000 to 3,000 a month in North America.
Reciprocating motion is the Achilles Heel of the piston engine. Things stop and start up again in the other direction twice per revolution. This causes tremendous stresses on connecting rods, pistons, piston pins, valves, valve springs cam shafts, lifters, push rods and crankshafts at high RPM's. The crankshaft in a piston engine is a stress analyst worst nightmare with twists and turns every which way. The worse part about it is the stresses reverse. Metal fatigue occurs when stresses reverse. The wankel engine has no major internal parts that have reversing stresses. In theory the life of the major parts in the engine such as the e-shaft and rotor have unlimited life. The failure in the catastrophic hand grenade mode is highly unlikely with a wankel engine.
There are no reciprocating motions in the wankel engine. Absolutely nothing stops and starts up in the opposite direction in a wankel engine! Every part moves in a circle or keeps moving in more or less the same direction. Many engineers and mechanics that should know better still think the rotor translates, reciprocates or wobbles somehow in the rotor housing. Nothing could be further from the truth. Some think the radius of the rotor journal on the e-shaft is somehow equivalent to the stroke of a piston engine. Also not true as stroke is defined as a back and fourth motion. The key is to think only of the center of the rotor. That is the center of gravity of the rotor and what that does is all that counts. The rotor CG rotates around the center of the eccentric shaft pure and simple much like the earth rotates around the sun. In fact some call the wankel a planetary engine. Needless to say the wankel is a subtle engine and difficult to fathom.
There are only three moving parts in a two rotor wankel engine. The eccentric shaft and two rotors. The eccentric shaft is analogous to the crankshaft in a piston engine. Unlike a piston engine the eccentric shaft is straight through, extremely strong and has demonstrated the ability to live in a highly turbo-charged 800 HP two rotor 10,000 RPM Mazda racing engine of stock displacement. The bare engine weight of this engine is only 200 pounds not including the monster turbo charger and other accessories. That is an unprecedented power to weight ratio for all but a turbine engine. Nobody in their right mind would think of asking 800 HP from a two rotor Mazda wankel in a general use aircraft of course but it might be fine for air racing at Reno. I mention this as it illustrates the extreme robustness of the Mazda wankel engine.
The rotors are cast iron and unlike pistons in a piston engine are nearly impossible to melt. They can be made from heavy cast iron because they do not reciprocate. There are no exhaust valves to melt or burn. Burnt exhaust valves are a common problem in aircraft piston engines. The piston engine exhaust valves run at cherry red temperatures and are slammed open and closed 1200 times per minute. Is it any wonder they are highly problematic? No camshafts to gall. No lifters to score. The essential valving of intake charge and spent exhaust gases is done by the rotor moving over openings in either the rotor housing or the end housings. This is similar to a two cycle piston engine but the wankel is definitely not a two cycle engine. It is indeed a four cycle engine and like a four cycle, four cylinder engine it fires twice per output shaft revolution.
The engine is in perfect balance just like a turbine or an electric motor. It is impossible to perfectly balance a piston engine as the CG of certain parts necessarily oscillate or reciprocate. The rotary is always much smoother running than any equivalent piston engine with the same number of power pulses per engine revolution.
The engine never catastrophically fails in the hand grenade mode as is common with piston engines. The worst thing that can happen is an overheat due to loss of coolant but the engine will still run and usually get you to an airport. Cold pressure checking the coolant system before flight is very effective in detecting incipient leaks before they can cause problems. High quality AN aircraft plumbing is mandatory. Wiggins couplings and aluminum pipe are used in place of automotive hose clamps and rubber tubing. There is no possibility of the cast iron rotor expanding and seizing in the aluminum rotor housing. A common problem with two cycle piston engines.
Mazda was the only Japanese company to ever win the 24 hour race at Le Mans France outright over all contestants. They did it with a four rotor version of the two rotor RX7 engine in 1991. There was little or no wear on the four rotor engine after completing and winning the 24 hour race. The other contestants included the who's who of European exotic car makers fell by the wayside. Rotary engines were outlawed the next year at the Le Mans race. It would be embarrassing to let a rotary engine win the race year after year. Demand for the engine might have increased in Europe forcing car manufactures to scrap billions of dollars worth of piston engine manufacturing facilities and piston engine parts sales and support infrastructure. Currently Mazda rotaries are either severely handicapped or only race against other Mazda rotaries as the engine apparently has been deemed unfair to piston engines. Mostly for durability reasons as they tend to outlast piston engines ten to one in racing situations.
The increasing popularity of the Mazda wankel engine for use in aircraft has several reasons. Low cost is not the main reason. The main reason is durability. The stock engine will run for many many hours at high power with little or no wear. The FAA certification requirements are only 150 hours on the dyno with a prescribed power duty cycle test. Any Mazda rotary engine could easily pass that test. The problem is the FAA requires a quality control paper trail on the engine parts as they are manufactured and Mazda would never agree to that. Mazda rightly reserves the right to make changes and improvements at any time. Apparently Europe has different rules as I know of one converted German diesel auto engine that is sort of certified for use in aircraft. The FAA has a reciprocity agreement with most European countries so there appears to be a loop hole in this arrangement. Be that as it may there is a company in Switzerland called Mistral that is certifying a Mazda based turbo two rotor for aircraft use by making all new parts in house.
The subscribers of the Aircraft Rotary Engine News Letter and others have actually improved the durability on what Mazda accomplished by choosing more recent, somewhat more costly, high tech material for certain parts. A silicon carbide ceramic apex seal has been developed by Dr. Francisco Inanette that is so hard and smooth it has no noticeable wear rate. Nobody has worn one out yet. Projected engine life in gen set applications predict a TBO of 20,000 hours. If true no turbine can approach this life expectancy.
High temperature, 500 degree Fahrenheit, teflon encapsulated silicon water jacket seals and oil scraper O-rings have been developed. These increase the ability of the engine to withstand inadvertent overheating. A common occurrence as many home builders are relearning lessons learned back in WW II or before about liquid cooled aircraft engines. These costly materials are not required as the projected TBO of a stock Mazda two rotor engine in aircraft use is about 3000 hours based on the wear rate of the stock apex seals and cruising at about 150 HP and 4500 RPM.
It has been found that simply mixing 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 gallon of gas. The stock engine injects oil from the crankcase into the combustion chamber. This crankcase oil, due to certain wear additives, is really unsuited to the task of lubing the apex seals. It leaves's undesirable deposits on the rotor and rotor apex seal slots that sometimes cause the apex seals to stick. The stock oil injector pump is disabled when the engine is raced or used as an aircraft engine and two cycle oil is mixed with the fuel. Another reason for doing this is Mazda used cheap nylon, five cent tubes to feed oil to the apex lube injectors. These tubes are hidden and inaccessible under the intake manifold. After a few years these get old, get brittle and get cracked. Many if not most Mazda rotary engines in cars that have died after ten years and several hundred thousand miles have died for this ridiculous reason. Mazda should have used steel tubing for this critical function.
POWER TO WEIGHT RATIO
The power to weight ratio of the stock Mazda engine is excellent. As mentioned above the bare engine weighs 200 pounds despite the use of mostly cast iron housing parts. Expensive aluminum end housings are available to cut this weight by about 50 pounds. The fire wall forward installed weight of a non turbo charged Mazda two rotor engine with PSRU, coolant and radiators is about the same as an O-320 or O-360 Lycoming. Right around 300 pounds. With non stock peripheral intake ports and no turbo charger as much as 240 takeoff HP can be had with excellent durability using a 2.85:1 propeller speed reduction gear box ratio and 7500 RPM for takeoff.
PSRU'sSeveral popular propeller speed reduction units are available that weigh about 40 pound each. Most of these are based on the low cost Ford truck automatic transmission planetary gear set. Ford recently beefed up this transmission by changing the number of pinion gears from four to six making it suitable for the three rotor or higher power turbo charged two rotors.
Turbo chargers for high altitude power add another 35 pounds. We don't know the long term continuous HP upper limits of the turbo-charged versions because nobody has flown one so far for more than about 300 hours to my knowledge. I suspect the basic engine would outlast the turbo charger.
The two rotor engine is downright tiny compared to an air cooled aircraft piston engine at about 12 inches wide 14 inches high and 18 inches long less accessories. It is quite possible to squeeze a turbo charged two rotor engine, including the coolant radiators, inside a stock kit plane cowling. I know as I have done it with my Kis Cruiser. By comparison to a Kis Cruiser the room in a Vans RV cowling is voluminous.
THREE ROTOR ENGINES
Three rotor engines are available imported from Japan and will develop fifty percent more power. Bare engine weight for the three rotor is about 260 pounds. I would guess firewall forward would be about 400 pounds. This is comparable to the larger 500 cubic inch air cooled aircraft engines like the O-540 and O-550. More weight for a turbo version of course. Racing Beat in Anaheim CA has a three rotor turbo aircraft engine on the dyno generating 900 HP with about 300 hours of dyno runs so far. That of course is 300 HP per stock Mazda single rotor displacement. The eccentric shaft in the three rotor is the same diameter as in the two rotor and it gives no trouble. The Mazda Le Mans engine was a four rotor non turbo charged developing 690 HP at 9500 RPM with exactly the same eccentric shaft diameter. This is a testimony to the extreme strength of the shaft in the two rotor engine. Many of the other three and four rotor parts are similar if not identical to the two rotor RX7 engine.
FUEL CONSUMPTIONThere is a mis-perception in the world that the rotary engine is thirsty. This was very true in the early days. Particularly in automobiles primarily used in the city. Critics pointed at the surface to volume ratio of the combustion chamber as the main culprit. The shape of the combustion chamber has advantages and disadvantages. Since the combustion chamber actually rotates the heat is dissipated over a very large surface area. This allows extremely high HP per cubic inch of engine displacement with out melting the rotor housings. It is also the main reason the rotary runs well on hydrogen.
The fuel consumption of a rotary expressed in Brake Specific Fuels Consumption or BSFC for short is now very close to a typical air cooled air craft engine particularly at the high power levels needed in air craft engines. The engine is steadily being improved in this regard.
First and foremost the Mazda rotary will happily run on auto fuel which is half the cost of aviation fuel. Aircraft owners flying rotary engines plan their cross country trips around airports that sell auto fuel. Lycoming does not recommend auto fuel in their engines. The Mazda will happily run 200 degrees lean of peak with no worry about burning an exhaust valve. There are none of course!
Tracy Crook believes the reason it runs so well when leaned out is because the wankel is naturally stratified charge. What that means is the fuel droplets or fuel vapor is separated from the main air charge and is thrown outwards toward the spark plugs by centrifugal force due to the rotating combustion chamber. The intake charge rotates around the center of the engine just inside the rotor housing. This concentration of rich mixture makes it much easier for the spark plugs to ignite the overall lean mixture.
At this very lean setting the EGT has dropped down into the piston engine range and prior to the RX8 engine is only slightly less economical than an air cooled piston engine. Tracy Crook estimates the pounds of fuel burned per hour for each HP generated is 0.47 while an air cooled piston engine is about 0.45. If you run the engine at peak power full rich it will burn 0.65 BSFC just like an air cooled piston aircraft engine under the same conditions. The new Mazda RX8 side exhaust port engine is reported to be much more economical. We don't have the BSFC numbers yet but the RX8 car will yield 27 MPG at 55 MPH, 25 MPG at 70 MPH and 20 MPG at 80 MPH. That is equivalent to 2, 2.8 and 4 gallons an hour in aircraft terms. Cars have more drag than airplanes so it is hard to relate this to aircraft service. In the side exhaust port RX8 engine any residual un-burned fuel is re-circulated to the next combustion cycle. The exhaust port timing is also modified in the RX8 engine to allow a longer expansion cycle recovering more energy from the fuel. The RX8 engine has much lower exhaust temperature confirming the lower fuel consumption.
Another advantage of the Mazda wankel is it is ideally suited to turbo compounding which can result in a further drop of fuel consumption of 20%. this would result in an automatic 20% increase in range with the same fuel load. HP is also increased by 20% with no increase in the cooling system requirements. The reason the Mazda wankel is ideal for turbo compounding is it has no failure prone exhaust valves to break up and destroy the exhaust blow down turbine on their way out the exhaust pipe. This was a common problem with turbo compounded piston engines widely used in the 1950's.
The total heat rejected by a Mazda rotary engine is only a little higher than a piston engine. All engines have a heat balance. Heat balance is a term for the distribution of energy contained in the fuel by burning it. Some energy is rejected to the coolant as waste heat, some goes out the exhaust and the rest comes out the shaft as mechanical energy. One HP is equal to 746 watts. An engine generating about 200 HP would be rejecting about the same amount of HP to the coolant or or about 150 Kilo Watts. Half of the fuel energy goes out the exhaust making the recovery of this waste energy the most fruitful area to improve the efficiency. This is not unique to the wankel! All internal combustion engines do the same thing more or less. A wankel engine's heat balance only differs by single digit percentages compared to all other forms of internal intermittent combustion engines. A gas turbine is an internal continuous combustion engine in case you were wondering.
IS THE MAZDA ENGINE ALL PEACHES AND CREAM?
Absolutely not! You will have a considerable engineering challenge installing one in your airplane. Overhauling the rotary engine is trivial by comparison to any piston engine. The engine itself is not the problem. Careful planning ahead is absolutely necessary with any non aircraft engine and it helps if you can make mechanical drawings and mock-up models. You may have to design and build many parts such as, the motor mount, custom oil pan, alternator mount, intake manifold and modify the water pump inlets and outlets. As the engine becomes more popular many of these unique aircraft parts are becoming available to purchase off the shelf.
Strict attention to cooling details is also essential. Most installers have trouble with cooling. Many have given up. The exact shape of the ducting to the radiator is extremely important and sealing the cowl properly to prevent back pressure on the radiator is also essential. The design of the cowl flaps and the opening around the cowl flaps is also extremely important. Wankel engines reject one third of the waste cooling heat through the oil so a good oil cooler is necessary. The stock Mazda RX7 oil cooler works well. Cooling is so important that I wrote a book on the subject called "How to cool Your Wankel" available from Aircraft Spruce and Mazdatrix.
The largest problem we have had so far is inadequate quality of cooling plumbing. Automotive cooling plumbing parts are not reliable enough for aircraft use and high quality AN hose is called for. Silicone hose and Breeze constant torque hose clamps are also a considerable improvement over automotive plumbing.
The wankel engine is ideal for turbo charging because of the the shape of the combustion chamber, the inherent low compression ratio and the lack of exhaust valves to fail. When turbo charging the engine an inter-cooler is absolutely necessary to keep intake charge temperature down. High intake manifold temperatures lead to detonation and detonation can fracture stock three piece apex seals with consequent scouring of the rotor housing and rotors. The engine will not fail catastrophically but will still run somewhat down on power with leaking apex seals. Thicker and tougher apex seals are available for turbo charging. Newer apex seals starting with the RX8 engine are now only two piece and much tougher.
A good muffler is essential with a wankel as the exhaust port opens much more rapidly than a piston engine. Supersonic shock waves are emitted. This problem is about to go away as the new Mazda RX8 motor will have much more gradually opening and closing side housing exhaust ports of double the area.
A turbo charger helps quiet the engine and its additional weight is off set by the non existent weight of the mufflers that would normally be required. Mazda built a good stock turbo charger for aircraft use in the 1989 to 1992 RX7 model years. Later dual turbo chargers were too complicated, heavier and less reliable. Earlier ones just plain cracked due to poor design. Many builders use off the shelf after market Garret turbo-chargers.
When purchasing a Mazda engine for use in an aircraft it is best to buy a running car. Many parts from the car are usable such as alternator, ignition coils, fuel injectors, starters, oil coolers, etc., etc.. Good years are 1989 and later. 1992 and earlier second generation RX7's can be had for under $3000. The model years 1993 and later are selling for over $15,000 as they were initially much more expensive and sophisticated cars.
I am convinced the wankel rotary is the light aircraft engine of the future.