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Date: Wed, 29 Jan 2003 19:38:04 -0500
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From: George Braly <gwbraly@gami.com>
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Subject: RE: [LML] Re: diesels
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Ah.... there is a lot here.



>>Heat rejection is far more of a function of specific fuel consumption,
EGT, 
and the exhaust placement vs. the cooling flow than any comparison of engine

type.<<

That is sort of correct.  How about a function of the BSFC(min) - - rather
than the selected BSFC.

>>  I can definitely assure you that a TIO-540U2A at 263 HP rejects far 
more heat and requires far more cooling than a modern 300 HP compression 
ignition engine at the same power simply because of the difference between a

.59 SFC and the C-I's .36.  For given amount of output HP, the lower the SFC

the lower the rejected heat.<<

The problem with that statement is that a TIO-540 has a BSFC min of around
0.415, NOT .59.

Thus, the difference is not anywhere near what you postulate.  

Further,  the are turbocharged 300 Hp aircraft S-I engines that operate with
BSFC(min) down at 0.38 with fixed timing magnetos and they will do better
with variable timing. At this point, there is really not enough SFC
difference between them to make a heat exchange issue about.   The lower
crankshaft torsional loading of the S-I engine compared to the C-I engine is
much more of an issue.
 

>>Since C-I requires high compression ratios to work, this leads to higher 
expansion ratios and lower EGT's.  Lower EGT also leads to lower cooling 
requirements so the nod goes away from the S-I engine in that area.<<

Higher CR does lead to lower EGTs... but it leads to HIGHER peak cylinder
pressures which leads to much higher BTU transfer to the cylinder heads.
Lower EGTs are not correlated with lower CHT cooling requirements.  It can
be much higher.

For example, if you advance the spark advance, you get much cooler EGTs.
But hotter CHTs.  

>>However, modern S-I engines are so much more efficient than 
our current Lycoming and Continental offerings that the cooling load would 
likely be less for a properly designed aircraft C-I engine with an SFC of
.36 
or less.<<

I don't think that  statement stands up to close inspection of the data.  
I know of no modern  S-I car engine that has a BSFC as low as 0.385 which is
typical of a 300 HP IO-550.

The fabled Porsche Mooney engine struggled to get a BSFC(min) of 0.425.

>>... and the GA aircraft industry is rife with bad 
system design.<<

Yes it is.


Regards,  George