<<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.  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.

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.>>

The BSFC's are not as different as you imply with the spark ignition
aircraft engine running lean being more like 0.44 than .59.  We have to keep
track of which are the causes and which are the results of the combustion
process.  Diesels have a better BSFC primarily because of the higher
compression ratios they can tolerate - 14 to 20 compared to 7 to 10.  Since
more of the heat input goes to usable work, less needs to be rejected, just
as you imply.  The only place the heat can go is the exhaust, coolant and
oil.  Much of heat rejection to the coolant is during the high pressure
portion of the combustion.  Diesel combustion pressures are typically over
1,000 psi while spark ignition pressures are more like 800 psi.  Also,
diesel combustion chambers are designed for maximum turbulence and that
velocity increases heat rejection.  The result is that compared to a spark
ignition engine more of the heat goes into the coolant and oil and therefore
less into the exhaust.  Sure, the aerodynamic cooling DRAG of the air-cooled
engine can be high because it takes significant power to push enough air
across the cylinders.  Whether the air is going up or down has little effect
except that if you put the exhaust pipes on the cold air side you will
pre-heat the air and therefore need more of it.  Most of our engines have
the exhaust on the bottom so a down-flow system makes the most sense.  And
then there is the extra weight of the large battery required to crank the
diesel.  All in all, diesels have an uphill battle trying to make their way
into light aircraft.

Gary Casey