Making the rounds on the internet boards and of great interest. Second
in installment on turbochargers.
Marc Wiese

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From: George Braly [GAMI]
Subject: INTERCOOLERS ARE WONDERFUL DEVICES!

As is indicated in the previous blog on Turbochargers vs
Turbonormalizers - - intercoolers are one of the "secret ingredients" in
the whole engineering effort that protects the high powered aircraft
piston engine from detonation. If you get the CHTs cooled off and you
get the induction air temperature (IAT) down close to that experienced
by the similar normally aspirated engine, then you really don't threaten
the engine with adverse combustion events any more than they already are
with a normally aspirated engine.

The value of the intercooler is often misunderstood. Few pilots and
mechanics have any notion of the magnitude of protection from detonation
that is afforded by a good intercooler installation. The data shown in
the graph below helps to quantify the magnitude of protection from
detonation that one obtains when good intercoolers are installed on an
engine with a supercharger.

This data in this graph is adapted from some rather excellent WWII era
data - - all of which was originally obtained with gear driven
superchargers.

_http://www.secure4host.net/upload/files/braly3.bmp_

But the same issues apply exactly the same way to any piston engine with
a compressor - - whether it is driven from the crankshaft by a gear or
belt drive or whether the compressor is driven by a spinning turbine
wheel sitting in the exhaust stream.

The left hand vertical axis is in units of horsepower. The scale shows
how much LESS horsepower is available to a detonation limited 300 Hp air
cooled aircraft engine when one increases the induction air temperature,
which is shown across the bottom horizontal axis.

In many text books and papers, it is common to characterize an engine's
margin of safety from detonation by establishing how much additional
horsepower in excess of its rated horsepower that the engine could make
- before it encounters detonation - - assuming all of the other relevant
parameters are held constant.

In this case, we have presented the data in a slightly different manner.
This graph first establishes a reference point for a maximum detonation
free horsepower at an induction air temperature of 120 F, and then
incrementally increases the IAT above the 120 F starting point. This
process establishes how much less horsepower the engine is able to make
before encountering detonation as the induction air temperature is
increased. In order to have a real world example in mind, think of this
example engine as a certified engine with something like 470 cubic
inches and a normal rated maximum horsepower of 260 Hp at 2700 RPM. For
our example, we have tested this engine and found that when boosted up
to 300 Hp and with the induction air temperature measured at 120 d F,
the engine is just barely free of detonation. This engine would have a
"detonation margin" of 40 HP (300 - 260) at an IAT of 120 F.

The blue line shows the loss in detonation free horsepower for the
engine operating at 2700 RPM. The red line shows the further reduction
in detonation margin from the original power that is present if the RPM
is reduced to 2500 RPM.

It is clear from the data, that an engine that is capable of making 300
detonation free horsepower at 2700 RPM with the induction air
temperature at 120 F - - would only be able to make about 215 to 230 HP
free of detonation if the induction air temperature is allowed to
increase into the 165 F to 185 F range, and the engine is operated at
2500 RPM. This represents a loss of some 68 to 85 horsepower in the
capability of the engine to operate free of detonation. That is a loss
in detonation margin of about 22 to 28% of the power of the engine.

That 22% to 28% loss in maximum detonation free horsepower can be
recovered with a good intercooler installation.

The industry, the FAA, and the pilot community all consider it an
acceptable design configuration for an aircraft piston engine to be able
to detonate under certain power and environmental conditions. That is
why pilots are trained in engine operation and that is why we have POH
limitations that need to be followed. Thus, if those types of engines
are inappropriately set up by the pilot with the wrong combination of
manifold pressure, RPM & mixture, they can operate outside of the
established detonation margins. We operate these engines with these
design constraints because it is sometimes essential in order for the
aircraft to perform as they are intended. The broad range of pilot
selected operating conditions is necessary in order for the pilot to be
able to extract the maximum horsepower from the engine during certain
critical phases of flight.

This widespread and historically successful aircraft engine operating
paradigm has been acceptable because the engines came with manifold
pressure, tachometer, temperature & fuel flow gages and the means to
manipulate the values, which, if done in accordance with the POH, would
allow the pilot to avoid engine operation in areas that would otherwise
cause detonation.

Since harmful detonation is most likely to occur at full power during
takeoff and climb. Because, under those conditions, the only "tool" at
the disposal of the pilot is the mixture control, the FAA defines
requirements for a "margin" on the fuel flow so that even if the fuel
flow were improperly set up by the mechanic by some "margin" the engine
would not detonate on the unsuspecting pilot during a full power takeoff
and climb. The certification standards require a demonstration that the
engine can be operated free of detonation with the mixture lever
positioned so that the fuel flow is as much as 12% below the specified
full power set point.

As an example, if one operates a stock 350 HP Lycoming TIO-540J2BD
(Navajo Chieftain) at full power and then foolishly brings the mixture
control back to around 13 to 15% below the specified full rich mixture
fuel flow - - then that engine is likely to begin to detonate under many
environmental conditions.

On the other hand, when operating that engine, if one first reduces the
manifold pressure by 8 to 10 inches, then one can lean the mixture to
almost any mixture and it will not detonate. The cylinders will get hot
if you lean it near peak TIT and leave it there for a while, but it will
not detonate.

There are available after-market intercoolers for the Navajo Chieftain.
If they are properly installed on those engines, then the detonation
margin improves so dramatically, that one can "foolishly" set the
mixture in that range from 13 to 15% below full rich and the chances of
any detonation are dramatically reduced under almost all environmental
conditions, even when the engines are operating at full power.

When we did certification testing on the TN SR 22 - - we did that in
flight. We did that during a very hot period in June and July of 2006.
The routine daily temperatures were in excess of 100 F. The purpose of
the testing was to insure there were adequate margins to allow operation
of the engine at full throttle with the mixture set lean of peak. I was
the PIC for the flight tests.

We conducted the tests with half of the intercooler cooling air inlet
area blocked off. At various times, we set the manifold pressure to
still higher values than the normal 29.6". We exercised the RPM from
2700 down to much lower RPM values while maintaining manifold pressure
well in excess of 30". While doing that, and with the intercoolers
partially blocked, we were able to force the induction air temperature
to values much higher than is possible to obtain with the intercoolers
"unblocked" and functioning normally.

The results were fully consistent with the predictions in the old data
in the graph above: When the induction air temperature began to rise to
values in excess of 160 d F, we were able to measure the onset of
detonation at reduced mixture settings.

However, with the intercoolers functioning properly we could not force
the induction air temperature above 130dF even on a hot day in a slow
climb at 24,000 feet at full power, and detonation was never observed at
any mixture setting.

The result of the careful design and thorough testing - - all born out
of a lifetime of real world turbocharger engine operating experience and
many years of highly instrumented test cell engine operation - - has
been a robust Cirrus "Smart Turbo" system that is widely recognized as
the most efficient and the easiest turbo system to operate of any
general aviation aircraft. Ever.

Michael D. Busch wrote:
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?

Dave Brown wrote:
As a mechanical engineer flying a Cirrus I love this discourse between
two smart guys. I'm hanging by my thumbs waiting for the rest of this
exchange. Thanks to you both.


Marc Wiese