Hi Rob,
I have been watching your dialogue with George on this topic
with great interest. I was quite surprised, however, when you responded to George
on intercoolers.
George said:
>>4)To get the air temperature back
down to tolerable (detonation margins) ranges requires an intercooler
Rob said:
All sorts of engines run without intercoolers, without
damage. Detonation is a function of temperature AND pressure in the
cylinder at, or near, full compression, not the temperature in the intake. Besides, if you are flying up high, where an add on
system makes sence, the air starts out colder so it isn't all that much warmer
than it would have been at ground level.[emphasis mine]
The principle reason for adding an intercooler is the same
as for adding a supercharger, to make the intake air denser and increase the
mass flow through the engine.
In particular, I take issue with your suggestion that
intercoolers are not needed at altitude. Let’s take a look at a supercharger
(mechanical or exhaust driven) installation on a 7.5:1 compression ratio engine
providing 30” MAP at 20’000 pressure altitude and 20C above ISA. While
a perfect supercharger (100% efficient) might only raise the adiabatic (no heat
added or lost in the process) motored (no spark or fuel, just air) compression
temperature 9%, a real world supercharger (76% efficient) would raise the
motored compression temperature over 31% (using the Rankine temperature scale).
I think that is a significant number.
By way of example, this Christmas, I was flying in Texas at 17,000’
with a pressure altitude of nearly 20,000’ (13.8”) and an OAT +22C
over ISA (my Density Altitude was 23,500’). My Compressor Discharge
Temperature (“CDT”) was 212F at 33” MAP with a pressure ratio
of 2.4 for a compressor efficiency of 76% (TSIO-520R). This is an 180F
temperature increase over ambient! I think that is a significant number.
An intercooler with only 60% efficiency would have reduced
my inlet and turbine temperatures by over 100F ! That would have moved my TIT from
1622 to 1522, well below my max TIT. Similarly, it would decrease the heat
rejection requirements of the engine and improve my CHTs a bit. Furthermore, it
would result in decreased peak combustion pressures and a slight delay in the
pressure peak, resulting in less stress on the engine. All of these factors
significantly increase the detonation margin between safe operation and engine
damage. More importantly, the lower temperatures substantially reduce the probability
of preignition, and this is the real killer.
Having said all this, I must conclude by commenting that my
aircraft does not have an intercooler. As far as I know, I have not damaged the
engine by operating without an intercooler. On the other hand, I also realize
that I am close to a number of the thermal limits for my engine and that I must
limit the HP output in cruise so as not to exceed these limits and cause damage
to the engine or turbo. I really wish I had an intercooler for my engine.
Best Regards,
Mike Hutchins
Forever struggling to find a loophole around the Laws of Thermodynamics