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Turbo Cirrus Blog By Braly » New Induction Air Temp probe in the Cirrus Perspective

New Induction Air Temp probe in the Cirrus Perspective

This is a new "blog"  and a new "experience" for me.

Rather than talk about "blogs" and "blogging"  - - I prefer to just BEGIN:

The  Cirrus Perspective has something new under the cowl.   It is a direct reading measurement of the induction air temperature immediately upstream of the throttle plate.

This air temperature,  commonly referred to as the  "Induction Air Temperature"  (or IAT) is often confused with either OAT or less often with  compressor discharge temperatures (CDT).  The CDT is the temperature of the air coming out of the turbocharger's  compressor section - - measured before the air has gone through the intercooler.  But the CDT is not measured in the Cirrus.  Really no need to do that. 

But if you pay even close attention to your Garmin engine display,  you will never find any displayed indication of the IAT.

So what is it for ?

The short answer is that the IAT provides the data that works the magic to drive the  LOP  target fuel flow indicator in the Garmin engine display.

As any of you who may have happened to spend a weekend with the APS engine management class know,  manifold pressure and RPM control the mass air flow through the engine.

That is well and good - - but to get it really "right"  you need to also know the temperature of the air flowing in the induction system at the point where the MP is measured.

But if you know the rate of mass air flow through the throttle and you know the fuel flow - - then it is just a few short steps away to get the GARMIN to calculate and display a "target"  air/fuel mixture ratio.

This is the magic that has enabled the ultra simple "set it and forget it"  mixture magic in the Cirrus  Perspective.  

 

 

 

 

 

 

Posted 26 Aug 2008 20:47 by George Braly
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Comments

Curtis Sanford wrote re: New Induction Air Temp probe in the Cirrus Perspective
on 26 Aug 2008 19:04

"But if you know the rate of mass air flow through the throttle and you know the fuel flow - - then it is just a few short steps away to get the GARMIN to calculate and display a "target"  air/fuel mixture ratio."

And only a servo away from losing the mixture knob completely?

Trip Taylor wrote re: New Induction Air Temp probe in the Cirrus Perspective
on 6 Sep 2008 22:32

With about 50 hours of Perspective time, I am in awe of how this plane is incrementally better in so many ways from the '07 model.  The little magic triangle and the better reaction of the digital gauges are a good example of this.  Thanks for the hard work.  
George Braly wrote re: New Induction Air Temp probe in the Cirrus Perspective
on 10 Sep 2008 20:59

Curtis,

"... a servo away" from losing the mixture control  completely.

Well... that and a half million dollar DO-178 software certification effort!!!   And a lot of redundant hardware.

That's all.
Marcus Adolfsson wrote re: New Induction Air Temp probe in the Cirrus Perspective
on 28 Feb 2009 14:02

George,

Let's say the blueline shows an optimum fuel flow of 16.9gph for the given conditions, which results in a power less than 85%. If you increase the mixture to 17.6gph to gain some power, is that detrimental to the engine? (How does that compare the pre-perspective turbo's that are usually operated at 17.6gph when LOP)
George Braly wrote re: New Induction Air Temp probe in the Cirrus Perspective
on 15 Oct 2009 22:28

Marcus,

Sorry for the inexcusable delay in seeing your post.

The Cirus/GARMIN implementation of our algorithm (the intellectual property for this belongs to TAT) for estimating the correct target fuel flow is pretty well done.  So typically,  if the blue line is indicating  16.9 gph  when the MAP is 29.3,4,5,6+ and the RPM is near 25xx - - that will mean it is a rather hot day and the mass air flow from the turbo is decreased due to the temperature (ie, you will have a higher density altitude than your pressure altitude by several thousand feet.)

(As an aside,  the only reason this "blue line"  target fuel flow indicator was not implemented on the AVIDYNE was a general perception at the time that the original Avidyne CPU was cycle challenged.  R9 is very  much more capable and I would expect to see some nice upgrades in future software releases from Avidyne for the R 9 systems.)

The bottom line  - - - if the  blue line is indicating  16.9 and you try to ignore that by more than small amounts - - I would predict that you will end up with CHTs rising past the 380dF working target.

It is useful to take a look at the actual language in the  AFMS for the turbo leaning procedures to see how those instructions nicely "integrate" with the concepts in this response.

So, if you put a conforming  G3 with Avidyne in formation with a conforming G3 with G1000   I would expect,  when the  G3 Avidyne person finally finished pushing the fuel flow as far up as he could and still stay under the 380dF CHT target - - that the fuel flow on that airplane would likely be reasonably close  ( +- 0.2  gph)  to the fuel flow on the formation aircraft  G3 G1000 blue line.

Cut me some slack on the numbers because there are a lot of calibration errors and tolerance build ups that will make that exercise very difficult to re-produce in the real world.

Does that answer your question ?

Regards,  George

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