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I have enjoyed the engine controversy, and since it seems to be quieting
down, I thought I would pour some Avgas on the coals and see if I can
get the controversy raging again, this time from another perspective.
Here's my thesis:
If you are going to burn gasoline (diesels excluded from this
discussion) and you are going to do it in a high power engine at high
altitudes at high power settings, you are theoretically better doing it
in a liquid cooled V-8 rather than an air cooled flat 6. And I believe
actual practice will show this to be the case as time wears on.
Supporting Argument
1) Current aircraft engines work fine, and generally make TBO if they
are aspirated, but are troublesome when turbocharged. Most aspirated
engines are operated at fairly moderate power settings (65%) and in
thick air where air cooling is adequate if the installation is well done
and the baffling in good repair. Generally you can not get more than
75% power above 7000 feet, and power is falling continuously during
climb.
With modern turbocharged engines, heat management becomes the dominant
problem. To get good numbers for the spec sheets, manufacturers
authorize lean to peak or lean past peak operation. Allowable maximum
turbine inlet temperatures have risen from 1550F to 1650F and now 1750F
in some engines, but the alloys of construction have not changed
appreciably. As a result exhaust valves burn, aluminum cylinder heads
crack, top cylinder wear accelerates, and exhaust manifolds crisp and
crack. The problem is worsened when you consider that to provide the
necessary detonation margin, the compression ratios have been lowered on
turbo engines which further increases exhaust temperatures. (Remember,
to be certified, the engine has to operate at redline temperatures and
full power without detonation.)
One needn't operate a 350 HP TSIO-550 to see the deleterious effects of
turbo charging. I owned a TurboSkylane RG for 19 years and ran it to
2000 hours before overhaul. It is a carburated Lycoming engine, maximum
manifold pressure of 31 inches, 540 cubic inch engine putting out a
modest 235 horsepower at 2400 RPM. Lean to peak operation is approved,
as is power settings up to 79% (25 inches, 2400 RPM) since the engine
would seem to be loafing. I flew leaned to peak in cruise at maximum
cruise typically at 11-12,000 feet with EGT's around 1450F, and head
temperatures of 400F. At 800 hours I had to overhaul three cylinders
due to burned exhaust valves. I started running 50F+ rich of peak and
pulled back 1 inch of manifold pressure and 100 RPM (about 72% power) to
get the fuel economy I previously enjoyed from leaning, but with a
modest speed penalty. At the end of the period I found the cost was a
wash: you pay in cylinders or you pay in Avgas, but either way you pay a
lot more than you would with an aspirated engine.
Flying TSIO-550's in the flight levels at high power settings and
pressurizing the cabin puts an even greater load on the engine top end.
And the engines do not appear to be making it anywhere close to TBO, at
least as far as the hot end of the engine is concerned. And no
surprise: temperatures around exhaust ports and the tops of cylinders
are high even if the cylinder head temperature seems OK because the
cooling is simply not as effective at high altitude. The pressure drop
required across the engine to get adequate cooling air is 2.5 to 3 times
greater at 25,000 feet than it is at sea level. Especially during
climb, cooling is marginal.
2) Liquid cooled engines offer numerous benefits. In Vol. 2 of the
classic "The Internal Combustion Engine in Theory and Practice" Taylor
compares the inherent heat transfer coefficients of air versus water and
shows that water cooling has an inherent advantage as a coolant 175
times greater than air (pg. 364). He goes on to write "In practice this
advantage can be largely offset by using air at much higher velocity
and at lower temperature than in the usual water-cooling system,
together with the addition of finning to increase the area of the
outside surface of cylinders. Typically, the finned area of an air
cooled cylinder is from 10 to 125 times larger than the area of the
unfinned cylinder, and the air velocities are four to eight times as
great as the corresponding water velocities. ... However, with
comparable quality of design, air cooled cylinders generally show higher
temperatures at the critical areas (exhaust valves, seats, and ports,
and spark plug bosses) than water cooled cylinders under similar
circumstances." He later notes that successful air cooling only works
with cylinders no larger than 6 inches in bore size. Our engines are
typically 5 inches or a bit more, pushing the upper limit.
The Continental Voyager engine converted an air cooled design to liquid
cooled by adding cooling passages to the top of the cylinders and the
heads, and uses oil cooling to the cool the bottom portion of the
cylinder. The ASME paper reporting on the Voyager engine notes that the
metal temperatures adjacent to the combustion chamber were typically
100F cooler than with air cooled heads of the same configuration.
Ultimately compression ratio was raised which resulted in improved
specific fuel consumption that made the Voyager engine preferred for the
around the world Voyager aircraft.
But the Continental Voyager engines did not take full advantage of the
benefits of liquid cooling. To do so leads you to V-8's as I shall note
below.
Air cooling requires that cylinders be widely spaced so that fins and
cooling air can flow between cylinders. Go to liquid cooling, and the
spacing can be substantially reduced. Go to a ninety degree V-8
configuration and you have an inherently balanced configuration with
opposite cylinders sharing a common crank throw. Thus the V-8 has four
crank throws and a comparatively short crankshaft, while the flat six
has six crank throws (one for each cylinder) and a long crankshaft
results. The secondary twisting moment arising from offset cylinders is
dramatically reduced when cylinders share a common crank pin, and much
smoother operation results. Whereas the large opposed aircraft engine
requires vibration dampers on crankshaft counterweights, the V-8 can be
dampened adequately with a single damper on the the nose of the
crankshaft.
The compact configuration of the V-8 also permits one to use an integral
cylinder block and crankcase which is much more rigid than is possible
with the flat 6. Moreover, mating joints between cylinder and crankcase
and between crankcase halves are eliminated along with the stress
concentrations associated with the use of bolts and holes needed to
fasten all the parts together. As a result, the basic structure of the
V-8 is more rigid and much less subject to flexing than with the flat
six.
Numerous other benefits accrue in addition to compact size and improved
smoothness.
1) Uniform temperatures throughout engine. The liquid cooled engine
does not need "choke" in the cylinders, the taper at the top of aircraft
cylinders that is ground in to offset the differential thermal expansion
that arises because the top of the cylinder is so much hotter than the
bottom. (Note that a given amount of choke is perfect only for one
operating temperature.) On liquid cooled engines, the top and bottom of
cylinders are virtually all the same temperature in comparison. Ring
movement and wear are reduced. Further, tighter clearances are possible
since the engine does not have to be certified to operate up to 475-500F
head temperatures. Lower cylinder leakage and lower wear are the
benefits, along with reduced oil consumption.
2) Valves, valve seats, and valve guides all operate at much lower
temperatures improving fit, lubrication, and life.
3) Cylinder head temperatures are substantially lowered. Aluminum loses
considerable strength at the temperatures routinely encountered in
aircraft engine heads. Lower temperatures mean higher strength and
greater resistance to head cracking, a major bugaboo for turbo
air-cooled engines.
4) Lubrication in the hot end of the engine is improved because oil film
temperatures in valve guides and on cylinder walls are much lower.
None of this is to suggest that one should go out and bolt an auto
engine in your airplane. As noted previously, auto engines spend most
of their life at low power settings and are optimized for low cost. For
aircraft applications, strength must be considerably greater. Thus
pistons, piston pins, connecting rods, and crankshafts all need to be
beefier and made of higher strength alloys. Cast crankshafts, widely
used in Detroit, simply will not do. Nor will standard automotive
pistons and rods. But it is clear that given adequate design of these
components as well as care in design of supporting structures and
adequate cooling passages for higher power settings (particularly
between intake and exhaust ports), one can design a liquid cooled V-8
with as much toughness and durability as a modern air cooled aircraft
engine. And the liquid cooled engine will be much more durable where
the turbocharged air cooled engines are weakest - anywhere it gets hot.
Liquid cooled engines charge a price: the cooling system. There are
already two liquid systems in an air cooled engines: the fuel system and
the oil system. Cooling systems add a third, but modern fittings and
practices make cooling systems as reliable (perhaps more so) than fuel
systems, for example. But cooling systems charge a cost in weight as
well.
Offsetting the weight penalty are two factors to complete the
discussion. The first is that eliminating the need for ultra rich
operation during take off and climb and better specific fuel consumption
during cruise all combine to reduce the fuel weight required for a
specific mission compared to an air cooled engine of comparable power
output. For a cruise power of 262 HP (75% of 350 HP), and assuming you
chose to operate 50F rich of peak to keep you air cooled engine happy,
the reduced fuel consumption is at least 2 gallons per hour (probably
more), or nearly 40 pounds for a three hour trip, more when you consider
the additional fuel during take off and climb. So the weight
differences become minor in the larger context of the entire mission.
The second factor to keep in mind with the liquid cooled engine is
cooling drag. Cooling drag for air cooled engines becomes a major
portion of total drag when you fly above 20,000 feet. For an aspirated
aircraft at 6,500 feet, drag is typically 6-7% of the total for a
Bonanza, for example. But when the airframe gets slick (Lancair IV drag
typically being half that of the Bonanza at the same speed) and when you
go high where the air gets thin, the cooling drag penalty can become as
much as 25% of the total for an air cooled Lancair IV at 25,000 feet.
Most drag arises from the high pressure drop required to achieve the
necessary volumetric air flow rate across the engine.
With liquid cooling, one can design the radiator for much reduced
pressure drop while rejecting the same heat load. Radiator pressure
drop can be as little as one fourth that of the air cooled engine while
keeping the radiator at reasonable size. The challenge is to design the
entire cooling system (inlets, diffusers, radiators, and exits) for
minimum drag to obtain the benefits. I believe that the Lancairs have
very little drag reduction left in the airframe. Any further drag
reductions (speed increases) will have to come in front of the firewall
in the form of reduced engine cooling. It represents the last frontier
for MORE SPEED.
Your comments welcome.
Fred Moreno
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