# 3
Marc

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From: George Braly
Subject: Turbocharging Temperature Relationships

>> George, could you post a graph that shows the relationships between
OAT, CDT, and IAT as altitude varies from SL to FL250. In other words,
how much temperature increase is produced by the turbocompressor and how
much temperature decrease is produced by the intercooler as the aircraft
climbs up to the Flight Levels?

So, Mike, your wish is my command!

In this context, CDT is the compressor discharge air temperature (after
it has been compressed and experiences the resulting temperature rise
from that process.)

The IAT (Induction Air Temperature) is the air temperature that actually
enters the throttle of the engine. In the absence of an intercooler, the
compressor discharge air flows directly to the throttle and the CDT will
be equal to the IAT, within a very few degrees.

Frankly, doing this for Mike takes a fair bit of work. But, Mike is a
good friend! So I decided to try to spend a couple of hours and model it
up. The results are consistent with my actual data to a remarkable degree.

As mentioned, when looking at the graph, keep in mind that when one does
not have an intercooler, then the CDT = IAT within a very few degrees,
by definition.

As a result, I did not plot CDT as an extra curve, but the CDT is
evident from the graphs, by reference to the non-intercooled compressor
output lines (RED).

Allowing for the various issues described below, the results are
consistent with actual data to a remarkable degree.

In the graph below, the assumptions are these:

1) Departure from a 5,000 foot MSL airport on a hot summer day ( OAT~
100 deg F) and then a direct climb to altitude. The further assumption
is made that the atmosphere (away from the surface) is an ISA + 40d F
day. By making that assumption, anybody can figure the OAT at any
altitude without having to plot it out. For easy of reference, at:

Altitude OAT (dF) Pres (in Hg)

10,000 63 20.6

15,000 45.5 16.9

20,000 28 13.75

25,000 10 11.1

2) Further assumptions are that there is an inherent gain in compressor
inlet temperature (CIT ) of about 10d F (empirically verified) above the
measured ambient (OAT) temperature, just due to the air flow going from
the outside to the inside of the engine compartment. I have never seen
an installation that did not have more than 10 degrees of rise from OAT
to CIT. Further, it assumes some real world pressure losses from the
compressor to the throttle of about 0.5" Hg for the non-intercooled
configuration and about 1.5" Hg for the intercooled configuration. Those
are also based on empirical experience.

3) The mis-matched compressor efficiency number is also from real world
experience with older compressor designs or even more recent designs
where the compressor is adapted from one design point and used in
another application where the speed (RPM) or the mass flow is "off" of
the normal compressor map sweet spot.

4) There are multiple well matched and well designed compressors that
can operate with efficiencies well into the mid 70's percent range. An
Efficiency of 0.74 was chosen as being consistent with real world
experience.

5) The intercooler efficiency of 60% is actually "OK" for a single
engine aircraft, but low for a twin engine aircraft (higher indicated
speeds results in better intercooler efficiencies). It is realistic for
a range of aircraft and climb airspeeds and it would be low for cruise
for a well designed intercooler.

6) These plots assume the compressor efficiency is constant across the
climb profile. That is an over simplification. They are not. One will
move out of the sweet spot on the compressor at map at one end or the
other of the climb.

7) I did this in a bit of a hurry, so it is possible I made a mistake
somewhere!

_http://www.secure4host.net/upload/files/braly1..bmp_

_http://www.secure4host.net/upload/files/Braly2.bmp_

Some observations:

1) The most significant observation is that, with a well designed turbo
& intercooler combination, one can operate a piston aircraft under very
hot ambient conditions and the engine will continue to see relatively
modest induction air temperatures - - even all the way to 25,000 feet.

2) And, conversely, without the intercooler, one sees some difficult
temperatures up into the mid or high 200 degree F range at 25,000 feet.
Better compressor efficiencies mitigate this somewhat.

If the compressor is limited in its ability to deliver manifold pressure
because of speed or drive shaft torque limitations, then below, is a
further example of how that compressor, without an intercooler would
perform. Note, the example assumes a compressor with a maximum pressure
ratio of about 1.4:1.

So, Mike, is this the data that you wanted to see?

Regards, George


Michael D. Busch wrote:
That's extremely interesting data, George. Thanks for developing it. As
you know, I fly a Cessna T310R powered by a pair of TCM TSIO-520-BB
engines. These are 285 HP, very modestly boosted 7.5-to-1
compression-ratio engines (32" MP redline up to 16,000', then
pressure-ratio limited above that) and non-intercooled. Although I do
not have instrumentation that lets me see CDT=IAT, I do know that
keeping CHTs well-controlled at Flight Level altitudes is difficult
(despite cold OATs) and often requires operating profoundly LOP. I
suspect that the situation would be very different if these engines were
intercooled. Unfortunately, nobody offers an intercooler STC for the
T310R, and engineering one would be challenging because the geometry of
the induction system on the Cessna T310 and 402 mount the throttle body
physically right at the turbocompressor discharge port.

I've looked at a lot of turbocharged engine installations in a lot of
airplane makes and models, and I've never seen one that even comes close
in efficiency and maintainability as the TATI TN in the Cirrus SR22.
Ever since I saw the prototype TN SR22 with its cowling off in Ada years
ago, I've been suffering from a severe case of turbo-envy!

George Braly wrote:
Mike,

I've got to confess that I'm glad you teased me into taking the time to
do those graphs.

I had forgotten how nearly "perfect" the IAT curve is for the TN SR 22
as one goes to altitude. I checked the mathematical model in the graph
against the certification data and they agree pretty closely.

The pressure ratio in the TN SR 22 at 25,000 feet is nearly 3:1. As you
know, the turbos in the Cessna T 3xx and T4xx series aircraft tend to be
pressure ratio limited above 16,000 feet (about 1.9 : 1 on the pressure
ratio.)

If the pressure ratio is not increasing with further altitude, then one
can start to mitigate the rise in IAT (as demonstrated in the 1.4:1
pressure ratio limited example in the second graph.)

But doing that when the pressure ratio is continuing to rise in
proportion to further altitude changes above 16,000 feet - - is another
trick, entirely.

With that in mind, it is all the more "nice" that the efficient turbo's
in the TN SR 22 coupled with the non-standard design of the intercoolers
(rather different than the TCM approach to their configuration) results
in the IAT that essentially remains constant in the face of the rapidly
rising pressure ratio and the corresponding rapid rise in CDT. This is
largely due to the efficiency of the intercoolers that make good use of
the steadily dropping OAT as one goes to altitude.

Regards, George

Kevin G Boles wrote:
>> the efficient turbos in the TN SR 22 coupled with the non-standard
design of the intercoolers (rather different than the TCM approach to
their configuration)

George, can you please comment on that a bit further? Thanks in advance!

George Braly wrote:
The turbo's on the TN SR 22 are among the two most efficient models
(both nearly identical) ever put in aircraft engines. Further, the
design point for the engine results in their operation in the typical
climb and cruise at a mass airflow and pressure ratios that keep them
very near the center of the sweet spot in the compressor maps. We spent
a lot of time and effort to ensure that as part of the preliminary
design trade offs and early flight testing.

The intercoolers - - are another story. Take a look at a Columbia
(TSIO-550C) some time or at a Lancair IVP (same engine).

Look at the intercoolers. They are roughly the same length as the TN SR
22. But they are WIDER and they are SHORTER (vertically) than the TN
intercoolers.

The result is that they have much larger cross section for cooling air
to flow through them - - and the cooling air has a much lower
restriction for air flow. The result is that those intercoolers "enjoy"
a much larger air flow of cooling air than do the intercoolers on the TN
SR 22.

At this point, I probably have you shaking your head, right? Your
thoughts are, so lets see, if intercoolers are so wonderful, wouldn't it
be "more wonderful" if you gave them more and more cooling air flow ?

And the answer is "yes" - - but only if the extra cooling air flow is
"free" - - like it would be in, for example, a liquid cooled automotive
application.

In the airplane, any of that extra airflow inside the upper side of the
cowling is airflow that liberally and easily goes through the
intercoolers - - and NOT... where?

Answer: NOT through the cylinder heads.

We had a fairly extensive "discussion" with the people that do
intercooler core designs about our desire to NOT design these the way
they had been traditionally designed - - which was a design which had no
constraint on the amount of cooling air that was available.

So we optimized the intercooler physical dimensions to force a smaller
amount of cooling air flow to "interact" with the compressor discharge
hot airstream for a "longer" period of time.

By using this configuration, we actually need LESS cooling mass air flow
than is used by the TSIO-550C in order to get equal or better reduction
in the compressor discharge temperature before the air hits the throttle
plate.

We pay a very small price for that in the weight of the intercooler
"core" - - but we more than made up for that by engineering much lighter
weight "tanks" that are welded on to each end of the intercooler to
connect up the plumbing.

The result is a "system" that has a larger fraction of the total cooling
airflow that is available to cool the cylinder heads, than is the case
with the typical TSIO-550 installation.

Does that answer your question?

George

Kevin G Boles wrote:
Actually it was a wonderful explanation! And it actually makes good
sense. Thank you very much! One hopefully short follow-up question: why
didn't you/Cirrus simply increase the allowable airflow into the upper
section of the cowling? i.e. why not have slightly bigger holes behind
the prop for air to enter - thus providing sufficient airflow for both
cylinder AND intercooler to use? I am presuming it has something to do
with drag (or perhaps aesthetics :)) but am not certain.

George Braly wrote:
Aesthetics? Maybe elegance. We didn't have to increase the air inlet
size on the Bonanzas to get them cool. I was pretty sure we could do it
without changing the Cirrus cowling.

Besides, Dale and the Cirrus folks were already pretty skeptical about
the whole turbo concept back in February of 2006 when we first met and
talked about it. Everybody said we couldn't keep it cool - - - without
major changes to the cowling. So we hunted around on the computer and
put up on the screen the climb cooling test results from the G1
prototype with the standard cowling. It cooled better than the normally
aspirated SR 22.

We promised to finish the STC without changing the cowl inlet design
---- didn't want it to look like a Columbia with soccer ball sized
inlets on each side of the spinner!

George