Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Thu, 30 Jan 2003 10:34:57 -0500
Message-ID: <redirect-2004143@logan.com>
From: "Gary Casey" <glcasey@adelphia.net>
Subject: diesels again
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<<Thanks for the info Gary but if I understand you correctly, you are
implying
that the diesel doesn't need as much cooling.  So why does the SMA air
cooled
diesel have double the intake area compared to the standard C182?>>

I didn't mean to imply that the diesel needs less cooling.  There are
significant differences in the heat distribution of the engines.  These are
very rough numbers, but if my memory serves correctly, compared to SI
engines, about 5% more of the heat is converted to crankshaft horsepower, 5%
more into the coolant (including oil), with 10% less into the exhaust.  This
gives a BSFC of maybe 20% better, 20% more coolant load, and of course lower
exhaust temperatures.  Why would they double the inlet area on the Cessna
testbed?  Maybe they figured they would start too large and then reduce it.
Also, to get the same heat out of a liquid coolant you need more air - the
differential temperature between the coolant and air is less than between
the cylinders and the air.  That means you can tolerate a higher temperature
rise with the air-cooled engine than with the liquid-cooled.  But we're
mixing cooling methods and engine types here.

I also take exception to the notion that since aircraft engines are an "old
design" they are necessarily inefficient.  The first overhead valve Chevy
V-8 was sold before 1920, so that makes them old, too.  The efficiency of an
engine depends on a lot of things, compression ratio being one of the big
ones.  Next comes the friction losses and heat losses.  These last two can
be reduced by using fewer, large cylinders and quiescent combustion
chambers.  Dual spark plugs can overcome the effect of slow flame travel.
High combustion chamber surface temperatures reduce heat transfer even
though they limit the compression ratio possible.  Then when you look at the
whole picture a large, air cooled cylinder with an open combustion chamber
and dual plugs comes out looking very good, even though the maximum
compression ratio is limited by the octane requirement.  A "modern" car
engine, even with its higher compression ratio, would be lucky to break
even.  In summary, the efficiency of the engine depends more on the design
than it does on when it was designed.  I do agree with the notion that there
are a lot of marginal designs in the aircraft business.  Too many variations
and not enough time spent on any given design to keep all the bugs out.  I
would bet that less than 1% of the resources used to design an automotive
component is spent on the typical equivalent aircraft component.

And finally, George's response probably summarized it better than I could
anyway.

Gary Casey