This is the last version positively :-)

BTW Mike a little background on the ACRE NL. I have financed the
advertising in the back of Sport Av out of my own pocket for the last
three and a half years. I make no money from this.
It is a labor of love. I do sell a CD for $40 but in three years
I have sold less than half a dozen.

Paul


The incredible Mazda wankel rotary aircraft engine.
By Paul Lamar

Aviation head line nine years from now "Rotary Powered Racer Beats Warbirds
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 airframe could certainly
give the old warbirds a run for their money......

 "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.  

A: Reliable
B: Robust
C: Extended TBO
D: Damage Tolerance
E: Smooth
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
L: Simple

I think the Mazda wankel rotary engine meets most of these criteria.

The wankel engine was invented way back in the 30's by 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 were interested in the
engine and under took development. Among them was Toyo Kogyo AKA Mazda
Motors. Mazda installed a state of the art computerized test facility for
the express purpose of developing the engine. After a lengthy development
period Mazda started selling cars with rotary engines in 1967. The first of
the five rotary engines cars that I have owned was a 1973 RX2 bought new. 
It was a very zippy little car.

Many problems came to light over the years. Only Mazda persevered and
developed the engine. All but Mazda gave up. They had a strong belief and
good understanding of the superior kinematics of the engine and adopted the
attitude of never ever give up. Eventually Mazda sold over two million
rotary engine powered cars.

Poor kinematics and reciprocating motion are the bane 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. Did I forget anything? The crankshaft in a piston engine is a stress
analyst worst nightmare with twists and turns every which way.

There is no reciprocating motion in the wankel engine. Every part moves in a
circle or keeps moving in more or less the same direction. 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 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. 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 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.

Absolutely nothing stops and starts up in the opposite direction ever in a
wankel engine! 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 piston engine.

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
important and has been found to work well. High quality AN aircraft plumbing
is also called for.

Mazda was the only Japanese company to ever win the 24 hour race at Leman
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 outright the 24 hour race.

The other contestants included the who's who of European exotic car makers.
The next year, rotary engines were outlawed at the Leman 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.

Now a days, generally speaking in auto racing, 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 from a
durability point of view I suspect.

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 wants to see a quality
control paper trail on the engine parts as they are manufactured and Mazda
would never agree to that.


RECENT IMPROVEMENTS 
The subscribers of the Aircraft Rotary Engine Newsletters 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 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.

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 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. The oil injector is disabled
when the engine is raced or used as an aircraft engine.


POWER TO WEIGHT RATIO 
The power to weight ratio of the stock Mazda engine is excellent. 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 runs 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.


PSRU's
Several popular propeller speed reduction units are available that weigh
about 40 pound each. Most of these are based on the 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
Turbo chargers for high altitude power weigh another 35 pounds.
We don't know the HP upper limits of the turbocharged versions because
nobody has run one in an airplane for more than about 100 hours to my
knowledge. I suspect the basic engine would outlast the turbo charger.

ENGINE SIZE
The two rotor engine is downright tiny 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 the room in a Vans RV cowling is voluminous.


THREE ROTOR ENGINES
Three rotor engines are available imported from Japan and will develop one
third more power. Bare engine weight for the three rotor is well under 300
pounds. I would guess firewall forward would be about 400 pounds. More for a
turbo version. Racing Beat in Anahiem CA has a three rotor turbo aircraft
engine on the dyno generating 900 HP with about 200 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 Leman engine was a four rotor non
turbo charged developing 690 HP at 9500 RPM with exactly the same eccentric
shaft diameter. Many of the other three and four rotor parts were similar if
not identical to the two rotor RX7 engine.


FUEL CONSUMPTION
What about fuel consumption? The Mazda will happily run 100 degrees lean of
peak with no worry about burning an exhaust valve. There are none! At this
setting the EGT has dropped down into the piston engine range and the engine
is only slightly less economical than and air cooled piston engine. Tracy
Crook estimates the pounds of fuel burned per hour for each HP generated is
.47 while an air cooled piston engine is about .45. If you run the engine
at peak power full rich it will burn .65 BSFC just like an air cooled piston
aircraft engine under the same conditions. The new RX8 engine is reputed to
be much more economical. The Mazda will happily run on auto gas which
is about half the cost of aviation fuel.

COOLING
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. This
is not unique to the wankel! All internal combustion engines do the same
more or less by only a few percentage points. 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 engine is trivial by
comparison. 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 mockup models. You may have to design and build
many parts like 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 should eventually become
available to purchase.

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 cooling heat through the oil so a
good oil cooler is necessary. The stock Mazda RX7 oil cooler works well.

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. When turbo charging the engine an intercooler is absolutely
necessary to keep intake charge temperature down. High intake manifold
temperatures lead to detonation and detonation can fracture 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.

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.

A turbo charger helps quiet the engine and its additional weight is off set
by the non existent weight of the mufflers that would be normally required.
Mazda built a good turbo charger for aircraft use in the 1989 to 1992 RX7
model years. Later dual turbo chargers were too complicated and less
reliable. Earlier ones just plain cracked.

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 going for over
$15,000 as they were much more expensive and sophisticated 
cars to begin with.

I am convinced the wankel rotary is the light aircraft
engine of the future.

Brief Biography: 
Paul Lamar is a former aerospace and Chaparral race car
engineer. He became an entrepreneur and invented and manufactured
computerized automotive performance test equipment. He also simultaneously
started the second personal computer store in the world back in 1975
initially selling Apple II computers which he owned for 20 years.

Paul has been an EAA member off and on for at least 25 years. He owns a
Cessna 182 and has about 3000 flying hours. He is retired and is currently
building a Kis Cruiser with a Mazda turbo charged two rotor rotary engine.

Paul edits the three year old internet Aircraft Rotary Engine Newsletter
with about 1000 subscribers from around the world.

Paul is married to Robin Lamar, full time United Airline mechanic, sometime
A & P teacher, president of the Association of Women Aircraft Mechanics and
winner of the prestigious Aviation Technician Of The Year award given by the
Women in Aviation organization. Robin has no time left over to help Paul
build his airplane.