Engine Cooling for the Jabiru engine

The Jabiru Engine which is used in many other aircraft.

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lessersg
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Re: Engine Cooling for the Jabiru engine

Unread post by lessersg » Thu Apr 19, 2018 3:28 pm

heisan wrote:
lessersg wrote:I have a question for the boffins.

Truth be told please------

Speaking to an instrument technician a few weeks ago, he indicated that it is the norm for the RDAC or any other make to measure the ambient temperature inside the cowl and add this to the CHT and EGT temp reading. Is anyone familiar with this equipment or can confirm this statement.

I do know that this happens with the MGL RDAC unless I misunderstood I did ask three times for an explanation ......
A thermocouple (as used to measure CHT and EGT) measures the difference in temperature between the probe and the 'cold junction' (the point where the thermocouple wire connects to the display)*.

Old temperature gages just estimate environmental temp and display the result (so if the cold junction is 10 degrees hotter than normal, it will show 10 degrees *colder* than expected).

Modern EFIS systems though a re 'cold junction compensated' - there is a separate thermometer that measures that absolute temperature of the cold junction, adds the thermocouple relative temperature, and displays the real temperature (irrespective of environment temperature).

The MGL RDAC *is* cold junction compensated, so should be accurate in all conditions.

* NOTE: Some people extend thermocouple wires with copper wire. This moves the 'cold junction' to the join in the wire, and the resulting reading will be off by the difference between the temperature at the junction and the temperature at the instrument.

Ok if it adds the thermocouple relative temp how can it be a true temp reading of the cylinder head...
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Re: Engine Cooling for the Jabiru engine

Unread post by heisan » Thu Apr 19, 2018 3:35 pm

lessersg wrote:Ok if it adds the thermocouple relative temp how can it be a true temp reading of the cylinder head...
The thermocouple measures the temperature difference between the RDAC and the cylinder head.

Say the RDAC is under the cowl at ~ 70C and the CHT is 170C - the thermocouple will measure the difference as 100C.

The RDAC has an internal thermometer, which measures the cold junction (in this example 70C). So it adds the cold junction temperature (70C) to the thermocouple measurement (100C) to get the compensated temperature of 170C (which is now correct).
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lessersg
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Re: Engine Cooling for the Jabiru engine

Unread post by lessersg » Thu Apr 19, 2018 3:42 pm

Thanks understand now.

This from the man himself.....

Thermocouples do not give you an absolute temperature reading but only a relative reading. The RDAC converts this to an absolute reading at the probe by adding the temperature at the "cold" side of the probe which is where it screws into the RDAC.

So to answer your question: Yes the RDAC adds the ambient temperature at the terminals of the RDAC itself. If it would not do that the temperature reported would either under or over read depending on your ambient temperature. This is in fact the case with all mechanical EGT and CHT gauges - these WILL give you a wrong reading as they cannot perform this operation. The RDAC however can.
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Re: Engine Cooling for the Jabiru engine

Unread post by T4flyer » Fri Apr 20, 2018 8:45 am

Interesting discussion. I installed 4 new CHT probe recently as I wanted to move them from the "under the plug" location to the newer position between the plugs. Anyway I wired in the new probes, boiled some water popped in a very accurate mercury thermometer, stuck in the probes, turned on the iEFIS and low and behold the reading were all low :shock: I phoned Rainier at MGL and he told to check the readings with the probes disconnected against ambient and let him know. I disconnected the probes and switched on the iEFIS and all the reading were lower than ambient by the same amounts :?

I pulled the RADAC and sent it down to MGL for testing, MGL returned it after testing confirming it was operating 100% correctly, however I was told that the CHT reading is compensated for the temperature that builds up in the RADAC due to heat generated withing the unit which if I remember correctly is due to current flow (my term not theirs, I'm no electronics expert, actually an electronics dumbo :oops: ) When I was testing the probes I was turning on the iEFIS checking the readings (all low!) and then switching the iEFIS off again (didn't want to flat the battery) Rainier explained I needed to let the unit warm up for 5 minutes or so before checking the temperature reading of the probes. So I put everything back switched on the iEFIS and the probe temperatures were less than ambient 8-[ So I left the iEFIS on for 5 or so minutes as told to and the temperatures moved up to match ambient :) now I did the hot water test, all probes were withing +-1 degree at 95 degrees =D> =D>

Hope this helps, Don.
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Re: Engine Cooling for the Jabiru engine

Unread post by lessersg » Fri Apr 20, 2018 8:59 am

Ok thanks guys, this just verifies that all the reading I'm getting are correct and that my instrumentation is good. Have a better understanding now of the system.

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