Power by Mazda
Peter Garrison
For decades the rotary engine has hovered like the Grail before a certain
cadre of aeronautical innovators. I don't mean the World War One rotary -- a
radial in which the entire engine spun while the crankshaft remained still.
I mean the engine generically called the Wankel, in which a triangular rotor
whirls inside an oval housing, subdividing it into three combustion chambers
of continually varying volume. Wankels are basically quite simple; they have
no valves, for one thing. A single-rotor Wankel can, in fact, be compared to
a three-cylinder two-stroke, because it has three combustion chambers each
of which delivers one power stroke per revolution of the rotor.
Beginning in 1958, when Curtiss-Wright acquired a license from the inventor,
Felix Wankel, to exploit the technology, a series of prominent corporations,
including AVCO Lycoming and John Deere, worked on developing an airworthy
Wankel. (On the automotive front, General Motors licensed the technology in
1970, but let the license lapse in 1974.) NASA contributed a good deal of
money to development of a 340-hp engine called SCORE, for "Stratified Charge
Omnivorous Rotary Engine."
The "stratified charge" part of SCORE refers to a method of timing and
locating the fuel injection and the spark to ensure complete combustion of a
lean mixture in the rotary's awkwardly thin and wide combustion chamber.
Incomplete combustion results in emissions problems and also in
comparatively high fuel consumption. The "omnivorous" refers to the valuable
ability to run on all sorts of fuels, including diesel or jet fuel,
regardless of octane rating.
By the completion of NASA's study of several competing engine types, the
makers of the stratified-charge rotary were reporting fuel specifics
superior to those of current recips, and the engine was judged the most
promising in a field that included a small turbine and a diesel. But
despite all this, the Grail seems no closer. In 1984 John Deere sold off its
rotary engine division to Rotary Power International of Wood Ridge, New
Jersey, which continues to advertise a line of industrial SCOREs ranging in
yield from 15 to 3,000 horsepower, but has announced no further
aviation-related developments.
The Wankel has attractions beside its indifference to fuel octane. It puts
out a lot of power for its size and weight, is virtually vibrationless, and,
unlike a horizontally-opposed recip, is nicely shaped to fit in a slender,
streamlined nacelle.
Several automobile manufacturers, even including a Russian one, have used
rotary engines in production vehicles. Mazda of Japan has built millions of
rotary-powered cars. Their admirers characterize them as compact, light,
powerful, smooth, and highly reliable. Several amateur builders have
installed Mazda engines in homebuilts. Running around 6,000 rpm, they
require reduction gearing but still come out competitive in weight with, and
superior in size and shape to, conventional airplane engines.
Russian, German and British firms produce small rotaries for the aircraft
market, but the model of choice for American homebuilders seems to be
Mazda's two-rotor 13B, nominally an 80-cubic-inch engine rated at 143 hp.
Some random cases in point:
Since 1994 Tracy Crook of Clearwater, Florida, has been flying an RV-4
powered by a carbureted 13B that he originally bought from a junkyard. He
estimates the cost of the entire engine installation, including gearbox and
cooling system, at $5,000 - less than a third the cost of an equivalent
Lycoming or Continental. He runs it on 87 octane auto gas (or 100LL when
auto gas isn't available). Performance is equivalent to that achieved with
the more usual 160-hp Lyc: 177-knot top speed, 135-knot economy cruise on
5.5 gallons an hour. Crooks says that a teardown at 600 hours revealed no
discernible wear. "I'm convinced," he states, "that it's as reliable as
anything Lycoming or Continental ever built, or more so." He anticipates a
2,000-hour TBO, but with overhaul costing only $600 it really doesn't much
matter if it turns out to be less.
A Canadian builder, Neil Kruiswyk, has a 13B in a Lancair 235. He guesses it
would cost $10,000 to duplicate his installation, which weighs 300 pounds
complete - about the same as an equivalent Lycoming. Both Kruiswyk and Crook
use electronic ignition (as do an increasing number of homebuilders) and
have recently converted their engines to fuel injection.
Brice Daunay, a Tulsa architect, installed a modified 3-rotor, twin-turbo
Mazda 20B engine in a Velocity. (The basic engine is used in a Japanese
domestic-market sedan called the Cosmo.) Daunay estimates the cost of the
engine at $17,000, and its output at more than 300 hp. It burns about 14
gallons of regular auto gas per hour in cruise. He is very enthusiastic
about the engine, which is quiet (it's because of the turbos; rotaries
usually aren't), smooth, and, after 70 hours of operation - not a
significant duration, of course -- trouble-free.
Perhaps the most systematic development work on modifying the Mazda for
aircraft was done by Oregon-based Powersport, Inc. Partners Everett Hatch
and Steve Beckham eventually arrived at a 167-pound engine delivering 200
hp, but after Hatch's death in a 1997 airplane accident (unrelated to their
rotary engine), Beckham sold the company to Ratech Machine of Osceola,
Wisconsin. Ratech intends to manufacture engines, and eventually perhaps
complete powerplant installations including cowlings, for Van's, GlaStar,
and Velocity builders.
People willing to dwell on the negative side of rotaries are hard to find.
Neither doting homebuilders nor people with something to sell are expected
to emphasize the down side. But I did locate one. In his view rotaries, as a
class, have not replaced recips because they require comparatively complex
and costly processes to manufacture; because they do not fare well running
continuously at high power; and because once the "trochoid" - the outer
housing - is worn, it cannot be economically reconditioned, and must be
discarded. In other words, even though recips have many more parts, they are
simpler and cheaper to manufacture and overhaul.
None of these objections will deter homebuilders, who are happy to let Mazda
bear the cost of tooling; find rotaries to be in fact cheaper to acquire
than recips; generally do not run their engines through several overhauls;
and find, with Tracy Crook, that wear is in reality not a significant
factor.
At present, many rotaries in aircraft are prototype installations by
individual experimenters. We seem to be at the threshold of a new phase, in
which increasing numbers of users will buy ready-to-use engines from what
might be termed "amateur manufacturers." This point is similar to the one at
which your adorable but obstreperous three-year-old starts nursery school,
and you're finally exposed to someone else's opinions of his or her charms.
We will soon find out the aviation rotary is really a miracle or a monster.
Recycled Heat
A couple of readers have written to ask why waste heat from a reciprocating
engine cannot be used to deice the leading edges of wings. One suggested
ducting exhaust gas through the leading edges; another, a propos of a
liquid-cooled engine, proposed that coolant might do the same service.
As you may imagine, people didn't wait until 1998 to have this idea for the
first time, and so if it hasn't been isn't done, there has to be a reason.
The reason is simply that there isn't enough heat available.
Not that internal combustion engines don't produce a lot of waste heat. As I
have often dolefully remarked in this column, only between a quarter and a
third of the energy that is potentially available from fuel actually turns
up as useful work at the end of the crankshaft. The rest, except for a small
percentage that flies off in the form of noise and uncombusted chemicals, is
dissipated as various sorts of heat. Two-thirds of this waste heat blows out
the exhaust pipe; the rest is conducted or radiated to the ambient air.
Let's say, then, that an engine cruising at 200 hp is burning 650 hp worth
of fuel. It must be putting 450 hp worth of heat into the environment, and
about 320 hp of that into the exhaust alone. That sounds like enough to
deice a wing, doesn't it?
Strangely enough, it isn't, and it's fairly easy to see why. For a sample
case I'll use a 285-hp engine running at 70% of power, consuming 14 gallons
of fuel an hour. I'll use English units, for consistency with aviation's
fidelity to the antediluvian knot. (I'm indebted to Gregory R. Travis, one
of those mysterious Internet presences of unknown geographical location from
which one learns so much, for working out the following numbers for me.)
Heat, which is equivalent to energy, is measured in something quaintly
called the British thermal unit, or Btu. One Btu is the amount of energy
needed to raise the temperature of one pound of water by one degree
Fahrenheit. Burning one gallon of gasoline releases about 112,000 Btus; so
if an hour's worth of exhaust contains about the half the heat yielded up by
14 gallons of gasoline, it must contain about 784,000 Btus.
So now we know how much heat is available. The next question is, how hot
does the leading edge have to be? You might be tempted to reply, "Just above
freezing," but you not only don't want ice forming on the leading edge; you
also don't want meltwater flowing back from the leading edge and then
freezing farther aft on the wing surface. I'm not sure how hot a leading
edge would have to be to prevent both icing and refreezing, but I'm going to
guess 200 deg. F above ambient.
The tricky part is figuring out how rapidly air is going to carry heat away
from the leading edge skin. Travis came up with about 10 Btu per second per
square foot of heated wing surface. So if we decide to heat 15 square feet
of the wing skin, we'll need 150 Btu/sec, or about 540,000 Btu/hr.
Well, we have 784,000. What's the problem?
The problem is that we can't get all that heat energy back from the exhaust
gas. The rapidly flowing gas will heat the wing skins effectively only when
it's several hundred degrees hotter than they are. We want the skins to stay
above 230 deg F, not just near the engines but all the way out to the tips.
So the exhaust gas is going to have to still be quite hot when we dump it
overboard. And there are other inefficiencies; for example, some of the heat
is going to get lost on the way to the leading edges, and some of it is
going to heat parts of the wing other than the leading edge skins. And the
harder we try to recover as much of the heat as possible, the heavier and
more complicated the system becomes.
The final result is that while there's a lot of heat energy in the exhaust,
the amount we can actually extract from it is not enough to deice the wings.
Since a liquid-cooled engine's coolant would carry much less heat than the
exhaust gas, there's no chance at all that it could do the job.
This is an ideal opportunity to misuse the expression "begs the question,"
but I won't. The fact that half the energy of the fuel is turning into
nothing but chemical and thermal pollution might move us to ask the
question: "Can't that energy be put to some use?"
That was the question the designers of aero engines were asking half a
century ago, and the answer they found was something called
"turbocompounding." The idea was to send the exhaust gas out through a
turbine - basically, a windmill - and to connect the turbine shaft, through
appropriate gearing, to the crankshaft. Wright aeronautical used three such
turbines on the turbocompound version of its R-3350 Cyclone, and claimed to
recover 550 hp (out of a maximum takeoff rating of 3500) from the exhaust.
The proof of the pudding would be in the specific fuel consumption: If
you're really getting back power that would otherwise be going to waste, you
ought to get the same performance for less fuel. The turbocompound Cyclone
did; its fuel consumption - about .38 pounds of fuel per hour per horsepower
-- was about 10% lower than that of conventional engines of the same type.
The British engine-building firm of Napier, which seems to have been in love
with complexity, did even better with its turbocompound diesel Nomad, whose
fuel consumption was down around .32 lb/hr/hp. But they failed to get the
engine into service before simple, light gas turbines rendered their big
complicated hybrid recips obsolete.
