P337 Intercooler Flight Test Comparison

[Walter Atkinson]

From George Braly:

INTERCOOLER EXPLANATION:

For a year, I spent a good portion of my time re-engineering an intercooler installation and obtaining FAA approval for the changes. Very careful instrumentation and measurement. I have more recently done more of that with a different engine on the engine test stand (see www.engineteststand.com).

I learned a lot. I am a strong supporter of intercoolers, so some of what I am going to say may sound strange at first.

So, here goes: Frankly most of the conventional wisdom on this subject - - like a lot of the now famous LOP/ROP discussions - - is simply not true.

The practical application results in an outcome that is rather different than everybody anticipates. It is largely a myth that 30" of MP AFTER the intercooler gets you more HP than 30" of MP BEFORE adding the intercooler. In fact, a poorly executed intercooler installation can - - and often does -- result in LESS horsepower at sea level than the same engine without the intercooler.

I don't know who started the whole theory of de-rating an engine after an intercooler, but it sure as the devil was not somebody who was measuring the actual engine torque - accurately - - rather, they were just following the theory as you outlined it and making the calculations, etc. (And, yes, I have heard a story about one after market intercooler outfit that claims to have put a torque meter on before issuing the reduced MP instructions based on that result... but until I see the data, I will have a very hard time with that notion.)

The big problem is that in the euphoria over the large and very beneficial drop in Induction Air Temperatures (IAT) that one gets with an intercooler, the "engineers" forget all about something else: How much of the "good stuff" (ie, good, cold, high density air) you can get into the cylinder on each intake stroke - - also depends on how much of the BAD STUFF (ie,
exhaust products) you got out of the cylinder on the previous exhaust stroke.

The ratio of the new "good stuff" to the theoretical maximum "good stuff" is called the cylinder volumetric efficiency Ve. Normally aspirated engine values are up around 85 to 92% of the cylinder's displacement - - although with turbocharged engines, the number will substantially exceed 100%.

When you add an intercooler to a turbocharged engine - - and leave the MP constant - -, you place a restriction in the intake plumbing. That means that the compressor discharge pressure is now two or three (or 4 or 5 or 6 in one case) inches of Hg higher than the wide open throttle MP!!!

In order to generate that extra pressure, the compressor has to work harder. And that means that the turbo has to work harder. And that means that the wastegate is closed a bit more. And THAT means the exhaust back pressure increases and that reduces the cylinder volumetric efficiency.

Result? The improved number of molecules you get into the cylinder due to the denser air is just about perfectly offset by the reduced Ve. Thus, no net increase in useful airflow through the cylinder. And the result of that is no net increase in horsepower at the same MP.

Now... having said THAT - - in my view, it is almost criminal for anybody to operate a turbocharged engine without an intercooler.

There are excellent old SAE research papers that show a HUGE improvement in the detonation tolerance of these engines by use of even a modestly efficient intercooler. These improvements are not trivial. They are substantial.

Installing an intercooler means that you are very very much less likely to inadvertently cause detonation during a moment of inattention during a busy high power climb in an IFR environment - - when you accidentally fail to have the mixture rich enough. Lots of other benefits.

For the same reasons, it lowers peak cylinder pressures substantially and that seriously promotes reduced exhaust valve temperatures and greater valve and cylinder longevity.

Last, and this is something almost nobody appreciates, adding an intercooler results in much better fuel atomization by your fuel injectors during high power operation.

Like I said, the "conventional" wisdom on this subject is seriously flawed.

The benefits of intercoolers are sort of over promoted for the wrong reasons and vastly under promoted for the right reasons all at the same time.

They are generally a good investment.


Regards, George Braly - Tornado Alley Turbo & GAMI

[Walter Atkinson]

HP calculation ROP (mass airflow):

(100+(((RPM-maxRPM)/100*2.5)+((MP-maxMP)*3.5)))/100*maxHP.


HP calculation LOP (FF):

NA engines = 14.9 * FF = HP

TC engines = 13.75 * FF = HP

[CaptainRod]

Excellent posts, Walter. Thank you.

[JamesC]

Walter I really appreciate your input - LOP ops certainly become easier given the simpler relationship between fuel flow and HP in that regime. Much safer than choosing CHT targets particularly at cold altitudes.
For ROP your formula leads to 100 % power at max RPM and MP.
However AA placards 35.3 " MP for takeoff power - was this based on HP dyno measurements or math ?
Even if there is exhaust back pressure, could the peak cylinder PSI's still not be too high at 37 "MP full rich with the extra O2 from the colder denser intercooler air?
Thanks very much.

[Walter Atkinson]

Originally Posted by JamesC

For ROP your formula leads to 100 % power at max RPM and MP.
However AA placards 35.3 " MP for takeoff power - was this based on HP dyno measurements or math ?

The engine test stand data confirmed the math! This is a very good "rule of thumb" calculation that can be off a very small amount as other factors do affect the mass airflow. Those differences are insignificant operationally.

Originally Posted by JamesC

Even if there is exhaust back pressure, could the peak cylinder PSI's still not be too high at 37 "MP full rich with the extra O2 from the colder denser intercooler air?
Thanks very much.

Very good question. The answer is "no." Peak ICPs are controlled nicely. The additional FF provided by the fuel controller covers that problem in conjunction with the decreased volumetric efficiency. Mass airflow-type systems like Lycoming's make it a non-issue. The TCM fuel controller handles the increased O2 a different manner, but it works just as well.

[JamesC]

So then if AA placards t/o power MP = 35.3" then that is what we use in the equation for MaxMP when calculating cruise HPs, correct?
The AA data from their table however correspond more accurately to the formula when you plug in 37" as MaxMP.

[Walter Atkinson]

Originally Posted by JamesC

So then if AA placards t/o power MP = 35.3" then that is what we use in the equation for MaxMP when calculating cruise HPs, correct?

Yes.

Originally Posted by JamesC

SThe AA data from their table however correspond more accurately to the formula when you plug in 37" as MaxMP.

Sounds like they may have forgotten to include the exhaust back pressure effects in their calculations. Not sure how they got their numbers.