Intercoolers on a piston PA46

One of the really cool aspects of my work is the ability to work alongside the super-neat people that own/operate a PA46.  Those that own PA46’s tend to be interesting and successful people with a special business niche in the American marketplace.  Recently I got to train someone that I found particularly interesting…MMOPA past president and current Ombudsman with Piper, Jon Sisk.  He’s been a part of many STC’s, upgrades, and enhancements to the PA46 family of airplanes, and many of us have benefitted from his creativity and leadership.  His airplane has several modifications that I found interesting (AOA indicator, avionics upgrades, LED lighting, etc,.), but the one I found most intriguing was his engine monitor.  He set up his EDM900 to display the induction air temperature just downstream of the turbo charger and just after the intercoolers.  These temps told me much that I was not aware about the Lycoming engine.

All of the included pics in this Article are of the EDM 900 Engine monitor on Jon’s Mirage.  A bit of explanation…
* All temperatures illustrated are Fahrenheit (degrees F), except for the OAT, which is listed in Celsius (C).  I converted Celsius to Fahrenheit in the discussions below.
* CDT = Compressor Discharge Temperature (Temperature of the air leaving the Turbocharger)
* IAT = Induction Air Temperature (Temperature of the air leaving the Intercooler)
* OAT = Outside Air Temperature

Look at the arrow in the picture below to see where CDT, IAT, and OAT are presented:

Look for the RED arrow...

Look for the RED arrow…

In a Mirage, Matrix, or Malibu (Continental-powered), outside air enters into the airbox and then gets diverted to one of two turbochargers.  The turbos suck in the air and increase the velocity, pressure, and temperature of the air, jamming the air into the induction system.  We desire the increase in pressure (as it provides pressure for the turbo-charged engine, cabin pressurization, and a few other items), but the increase in temperature and velocity are generally unwelcome.  The sonic nozzles amply handle the excessive velocity delivered to the cabin pressurization, and the intercoolers (if allowed) do a fair job of decreasing the temperature of the air entering the intake manifold.  How well do the turbochargers intercoolers work?  Most PA46-drivers have no idea, and I did not until my experience with Jon.  But, with his monitoring of those temps, a lot became clear.

Low Power, at idle on the ground

Low Power, at idle on the ground

The picture above shows the engine at idle on the ground.  It was a rather hot day at JSO (25C), and the CHT’s began to heat with the minimal air movement and long ground-run (we were training).  Notice the spread of temperatures: OAT = 25C (79F); CDT = 101F; IAT = 92F.  With the engine loafing at idle, the turbochargers were not turning fast, so the temperature increase from the turbos was only 22F (101F-79F), and the intercoolers (with the limited airflow) decreased the temperature only 9F (101F-92F).  Not much happening….until takeoff and climb…

Climb Power, early in the climb

Climb Power, early in the climb

In the climb, the turbos really ramped up the air temperature.  It was early in the climb when this picture was taken, and the climb speed was just starting to enter the “cruise climb” phase.  Notice the CDT = 156F; the IAT is 119F; and the OAT is still 26C (79F).  The turbos are starting to produce a lot of additional heat (an increase in 77F), and the intercoolers are also starting to do their job by cooling the air 37F total degrees.

Climb at FL180

Climb at FL180

As the climb continued, we approached FL180 and the numbers started to get interesting.  Notice the Compressor Discharge Temperature (CDT) rose to 256F, which is a 224F increase in temperature (CDT 256F – OAT 32F).  I find it super-interesting that the turbochargers are capable of ramping the temperature up 224F so quickly.  I checked several trusted sources, and all told me that the turbos turn upwards of 90,000 RPM at high altitude (That’s a BUNCH!). The intercooler is working better too…it dropped the temperature 123F total degrees (CDT 256F – 133 IAT) in the climb.

Cruise at FL210

Cruise at FL210

In cruise at FL210, the compressor is able to increase the temperature a total of 209F from OAT -7C (19F) to 228F CDT.  And, look at the effect of the cold air (OAT 19F) on the intercooler…it was still able to drop the temperature of the pressurized air a total of 122F.

What did I learn from this new-to-me information concerning the turbochargers and intercoolers?  Here’s a list:

  • The Turbochargers are VERY capable of developing super-high temperatures:  The turbos are an integral part of the engine system and do an admirable job of producing pressure and (consequently) temperature.  Ensuring you have strong turbos is critical.  Be sure to have the turbos checked at every annual and have every “change in behavior” associated with the turbos checked by a quality mechanic.
  • The intercoolers work MUCH better with lots of airflow.  At idle on the ground the intercoolers do very little.  In cruise (with lots of airflow) they do a good of cooling the air before it enters the intake manifold.  Cool air entering the engine translates into a cooler running engine.  But…look at the temperature of the air entering the engine during the climb at high altitude…it was the highest of all.  I’m a big proponent of keeping the IAS in the climb above 135 KIAS for prolonged climbs.  This good air movement helps keep the cylinders cool, but it also allows the intercoolers to do their job better too.
  • Instrumentation is available: The EDM 900 is a VERY capable display, adaptable for some pilot-desired information.  I really DON’T like the from-the-factory instrumentation in most Mirage airframes as they only present CHT and TIT.  I’m a HUGE fan of engine monitors in the PA46 airframes as they provide information to the owner/operator that cannot be found anywhere else.  And…information is critical when operating these big pistons.
  • Engine baffling is super-important.  All of the air that enters the front opening of the cowl should be used for cooling…either the cylinders themselves, the oil cooler, accessory items, or the intercoolers.  If the baffling has gaps or holes or is poorly fitted, air will leak through those openings and NOT go through the intercoolers.  To have poorly fitted baffling would be akin to leaving the door to your house open on a hot day, and then complaining about high energy bill and uncomfortable temperature in your house.  I see a wide variance in engine temperatures in the different PA46’s that come to me for training, and I suspect that baffling is the greatest variable.  Here’s a LINK to baffling that Jon recommends and uses on his Mirage.

I hope this helps your understanding!  I found it fascinating.  Thanks Jon!

About Joe Casey

ATP, CFI, CFII, MEI, CFI-G Commercial Pilot - SE, ME, Rotorcraft, Glider US Army AH-64 Pilot and UH-60 Instructor Pilot and Instrument Flight Examiner
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