Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Thu, 30 Jan 2003 08:44:03 -0500
Message-ID: <redirect-2004029@logan.com>
From: StarAerospace@aol.com
Subject: Re: diesels
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<< ... a TIO-540 has a BSFC min of around 0.415, NOT .59. 

...a BSFC as low as 0.385 which is typical of a 300 HP IO-550. >>

Well, we have to separate flying fact from engine company fiction here.

The TIO-540U2A (note the dash number please) is used in the Aerostar 700 and 
has a cruise fuel flow of over 25 GPH/side at FL250 and 75% power, running 
~100F ROP for a BSFC of .59.  If you try to lean any farther at FL250, they 
overheat.  The TIO-540A2 in the Mirage is about the same;  we have one in the 
shop right now for a new engine.  The TIO-540S1AD updraft engine runs 50F ROP 
at 75% at a fuel flow of 19 to 22 depending highly on the age of the engine.  
The updraft engine runs a lower cooling load because it is not exchanging as 
much exhaust heat from the pipes to the intake manifold since they are on 
opposite sides of the cylinders, as is the arrangement of the downdraft 
cooled TSIO-550N.  The factory recommended engine for the Lancair IV 
(TSIO-550N) runs at 18.5 to 20 GPH at factory recommended ROP settings for a 
BSFC of between .42 and .46.  BSFC min is irrelevant, since all these numbers 
represent max cruise where most people tend to fly their airplanes.  

BSFC min is measured at the engine's most efficient point, which is typically 
40 to 55% power and LOP for the above engines.  Cruise altitude, 
turbochargers and specific configurations change the fuel burn numbers quite 
a bit within an engine family.  We cannot take the BSFC min of one dash 
number and expect that every other dash number in the family has that same 
BSFC at all altitudes, power and mixture settings.

Yes, people do in fact run these bathtub-chambered, constant-flow-injection, 
wet-fuel-unburned-out-the-pipe engines LOP at max cruise MAP for fuel burns 
in the case of the Lycoming of 20 to 22 GPH and as low as 17 GPH for the 
Continental.  Yes, there are effects of LOP that are relevant to the issue of 
BSFC and cooling load.

First, power is not a constant at a given MAP and RPM.  Higher engine 
temperatures, specifically inlet temperature (at the cylinder port) has a 
large negative effect on volumetric efficiency, and therefore, power.  Power 
also falls off by several percent when the engine is leaned beyond peak EGT.  
So calculating BSFC based on a fuel flow running LOP and a MAP/RPM 
combination that the factory lists as a given power ROP is cheating.

LOP does reduce the amount of fuel that burns late in the power stroke, 
during the exhaust stroke, and in the exhaust pipe.  This reduces EGT and 
with it all of the heat in that radiates and conducts to the rest of the 
engine.  However, since power is also reduced, the net gain of reduced 
cooling requirements is less than we would hope for.  Dyno testing here is 
not as applicable as we would hope, since the cooling and heat transfer 
dynamics change dramatically when we get into the rarefied atmosphere of 
pressurized cruise altitudes.

Stick the dyno in an altitude chamber and things start to get interesting...

Of course LOP may lead to a visit from Captain Detano, so weigh the cost of 
your top end carefully against your fuel bill before you lean too far!  
Obviously, an injector set (like GAMI) that gets all of the cylinders to peak 
at the same point is preferable to a set which allows one cylinder to go LOP 
while others are ROP.  Better still would be a better engine that doesn't 
have unbalanced cylinders to begin with!

Most engines run best if they are run quite lean at combinations of high RPM 
and low MAP;  around stoichiometric where RPM and MAP lead to slightly higher 
cylinder pressures;  and rich at high MAP vs. RPM which lead to the highest 
pressures and temperatures.  Takeoff and maximum continuos power settings of 
most aircraft engines fall in the latter category.  Timing should be pushed 
to the point of detonation and retarded as little as reliability allows.

As far as heat rejection of diesels and the higher efficiency, I have to ask 
what myth allows such high efficiency to exist at compression ratios of up to 
20:1, yet in the same breath talks about 6:1 open chamber gas engines being 
nearly as efficient?

The fact is that beyond 12 to 14:1, friction and thermodynamic losses 
increase in about the same proportion as extraction efficiency.  The primary 
reason higher compression ratios are used in many auto diesels is operability 
and cold starting.  Compression ignition engines that are not slaved to a 
wide operating envelope are far more applicable to aircraft than auto 
diesels.  These engines typically run 20 to 30% under stoichiometric to 
prevent soot formation.  This leads to lower specific output and lower power 
to weight just as the higher cylinder pressures require more metal.  This 
lower power to weight is the real barrier.

Caveat Emptor on those fuel burn vs. power ratings.  Unless we mate a real 
time torque sensor to the engine in flight at altitude, power cannot be 
measured accurately.

Eric Ahlstrom