Message-ID: <009601c18add$3acfffc0$b46c8a90@direcpc.com>
From: "Fred Moreno" <FredMoreno@bigpond.com>
To: "Lancair list" <Lancair.list@olsusa.com>
Subject: Efficiency: Auto engine versus aircraft
Date: Sat, 22 Dec 2001 19:38:45 +0800
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Gary Casey wrote:

"While I am an auto engine enthusiast, I have to say that NO WAY can any
spark-ignition engine burn 25% less than a standard Lycoming or
Continental.
While these engines may not represent the latest in technology they are
quite efficient and reliable."

I can understand why Doug Pohl's comment sparked such a response.  An
elaboration is necessary. It is a question of what the engines do on
paper, and what they do in the real world.

To a first approximation, Gary is right.  Modern piston engines operate
at their best efficiency (high manifold pressure, high load) with a
specific fuel consumption of about 0.4 pounds of fuel per horsepower if
they are operating on or slightly lean of peak near the maximum
efficiency mixture ratio.

Once you get the fuel mixture right, the ignition right, assuming you
have not screwed up on the intake and exhaust systems to badly, and get
all the other things more or less right, the biggest effect on gasoline
engine efficiency is compression.  And the maximum allowable compression
ratio depends on the onset of detonation.  And that depends on a number
of issues including combustion chamber design, fuel octane, and
temperature.  (Manifold pressure is also an important ingredient, but
let's set that aside for a moment, and assume they are more or less the
same for both engines.)  In reading text books on the subject, I found
that a rough rule of thumb is that for aircraft engine detonation, every
20 degrees F decrease in cylinder head temperature can allow a 1 point
decrease in octane rating.  (So a drop from 400F to 200F can drop the
allowable octane rating from about 100 to about 92, keeping in mind that
Avgas octane and Mogas octane are not quite the same.)

Also, our air cooled, magneto-fired engines operate with fixed timing,
and this setting is based in large part on avoiding detonation at
redline cylinder head temperature (475F) while operating full rich (a
certification requirement) as well as operation at cruise power at more
economical power settings.

So taking all this together, with a liquid cooled engine operating with
a cylinder head temperature of 200F instead of 400F, and with variable
ignition timing (even knock sensors, if necessary) one should be able to
operate with lower octane and/or higher compression ratio than the
comparable air cooled engine.

So the liquid cooled engine, should in theory, be able to operate with a
higher compression ratio than the air cooled engine and get a bit of an
efficiency boost.  Recall that the TSIO-550 Continental operates with a
7 to 1 compression ratio (if memory serves correctly), lower even than
the aspirated version.

So will a higher compression ratio get you a 25% fuel savings?  I agree
that if both engines are operating at the SAME, "optimal" mixture ratio
(lean of peak, rich of peak, on peak, you can define "optimal" for
yourself), the answer is No Way.

Aye, but there's the rub.

The TSIO-550 evolved from the TSIO-520 engine designed and built for the
original Piper Malibu.  Continental did a lot of work improving the
intake manifold system to improve the uniformity of fuel mixture.  With
this engine they introduced "lean of peak" operation into the pilot's
world.  On paper it was great.  In practice, the engine had problems
(perhaps unrelated to this mode of operation) and Piper switched to
Lycoming.  Continental stroked the 520 to make it a 550 and with some
other minor changes, arrived at the TSIO-550 that sits in most Lancair
IVs today.

The Lancair IV experience has been that operated as suggested by
Continental, a lot of cylinders gave up and died.  Top overhauls at
400-800 hours were not uncommon.  I do not have the details of the
failures, nor am I privy to the exchanges with the Continental factory.
But in the real world at higher (approved) power settings at high
altitudes, many of the engines were not making the grade.

As a result of this general trend of bad experience, the Lancair factory
came out with its own recommendation in their newsletter over a year
ago.  I would guess that the factory has burned more avgas in the
TSIO-550 that many of us combined, so their advice is worth listening
to.  Orin Riddell wrote that they now recommend operating well rich of
peak to get the engine lifetime a customer should be able to expect.
Here are the recommended figures from their newsletter:

100%, 350 HP, take off, Full rich 41-43 gallons per hour
75%, 262 HP, cruise climb, Full rich 27-28 gallons per hour
65%, 227 HP, economy cruise, 100F rich of peak 20-21 gallons per hour
55%, 192 HP, economy cruise, 100F rich of peak 16-17 12.5-13.5 gallons
per hour

Note that 75% power is no longer recommended as a cruise power setting,
except full rich during climb.

Using the 65% figure of 227 horsepower and 20 gallons per hour, my
calculator comes up with 0.528 pounds of fuel per horsepower hour.
Knock 25% off this and you get about 0.40, the figure mentioned above.

So, in the REAL WORLD the Lancair Factory recommends that real pilots
with real Lancair IV's should fly at these power and fuel settings, but
any "reasonable" engine should be able (if it can last) to operate with
fuel consumption that is 25% less.  What the experience of the Lancair
IV community tells us is that in the real world, you will pay for extra
gasoline (an extra 4 or more gallons per hour, or $10 per hour) or you
will pay for cylinders and exhaust valves, but you are going to pay.
It's up to the pilot to decide.

Liquid cooled engines SHOULD be able to avoid expensive top end problems
because cooling with liquid is about 20-50 times more effective (in
terms of heat transfer coefficient) than cooling with air.  Cylinder
heads are much cooler, exhaust valves, exhaust valve seats, valve
guides, oil around valve guides and rocker arms etc. are also much
cooler.  And a bit higher compression gives a bit more efficiency which
results in a small decrease in exhaust gas temperature.

Now, the question is:  Will a fleet of new (but "inexperienced") liquid
cooled engines provide the promised cost benefits compared to the old
(but "experienced") air cooled engines?  Only time will tell.  The
technical details suggest that the benefits SHOULD be achievable, and I
am betting they will.  So I am putting my money where my mouth is.  Wish
me (and all the other pioneers, regardless of engine preference) luck.
If it works, aviation will be better off.  If it does not, pray for
aerodiesels, but don't hold your breath.  Or spend LOTS of money on
turbine fuel. (Here, too, the real world differs significantly from the
claims on paper.  But that is another topic for another day.)

So, a summary.  In theory, all gasoline piston engines should be able to
achieve more or less the same efficiency.  But in the real world, it
looks like the Continental TSIO-550 can't cut the mustard, and must be
operated rich and at reduced power settings to survive at the hands of
the average Lancair IV pilot.  The liquid cooled engine should be able
to permit the same power with 25% less fuel flow than the TSIO-550
operated as recommended by Lancair.  Flight testing to date confirms it.
But until we get a LOT of airplanes with liquid cooled engines with a
LOTS of hours, the jury must withhold judgment.

The comments, contributions, and corrections of others are always
welcome.

Fred Moreno

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