Coolant System

Sean Griffin wrote:

"The Circulation of the coolant is fast enough that there is almost no significant temperature drop."

Sorry Bill - this does not make sense to me - if there is no temperature drop across the radiator, there can not be any cooling effect ergo the radiator is not doing the job that I expected of it.

"I would have expected the Radiator to be very close to the head temp."

So would I - so why would the CHT be around 115C & the radiator be 70C?

"Or possibly you are measuring the surface temps of the radiator header tanks (Plastic?)"

Yes I taped the thermocouples (one each side) to the  radiator tanks (fairly sure they are metal but will check again). Used an insulated tape to contain the temp around the thermocouple & reduce any effect of air movement.

"By Ground-Ops...", 

At the moment the fuselage (wings/tail yet to be installed) is just secured (static ie not moving) cowling facing into wind. Yes fully expect any forward movement (taxying) to improve cooling BUT as I have said, day ambient is about 15C at this time of year (winter/spring) and will rise to mid 40C's mid summer. If the cooling system can not keep temps below 100C x 15min run time, at this time of year, how quickly will temps rise when ambient is say 30C?

Bill is correct. The fluid is moving so quickly through the radiator, the temperature of the fluid going in is going to be very close to the temperature going out. But that doesn't mean that it is not doing its job. It is still radiating a lot of heat. All of that heat that is radiated is lost from the engine.

The disparity between CHT temp and radiator temp seems unusual. I would suspect one of two things. Either you are not measuring temperatures accurately, or there is very little circulation of fluid in the radiator. If only a little bit of hot fluid is entering the radiator, it would stabilize at a temperature well below the CHT temperature. Still, the quantity of fluid within the radiator is so small, it would have to be a very low amount of circulation to notice any significant difference between inlet and outlet temps.

Then again, I am just speculating. The temperature of the radiator is going to stabilize at something below the CHT. I guess it is possible that your numbers are normal and do not indicate a circulation problem.

Just to put this in perspective;  Assuming a few arbitrary but reasonable numbers.

Ref. 14,000 BTU/Hr = 5.5 HP of heat.

Assume we need to get rid of 14,400 BTUs per hour. (4 BTUs per second) and we are moving 30 gallons of coolant through the radiator each minute. (4 lbs per second.)
That would mean the radiator would need to cool the passing coolant by 1°F continuously.

The coolant might exit the engine and enter the radiator at 200°F and then exit the Radiator and re-enter the engine at 199°F and get reheated to 200° and then back to the radiator to get re-cooled. Around it goes.
There will only be a 1° gradient across the whole circulation path but 14,000 BTUs are being created and dissipated continuously.

The engine loses most of its heat out of the exhaust and some additional heat off the Cylinder fins.

A 45°C Temperature difference from the CHT to the Radiator is suspicious.  I suspect is a measurement issue.

*** Do you have a Coolant thermostat in the System? ***

"*** Do you have a Coolant thermostat in the System? ***"

No and pipework to/ from radiator has no sharp bends or any other external sign of possible restriction.

Coolant system, as per standard Rotax, with the exception of CozyCarb carburettor heating collars (ice minimisation)

Your explanation regarding  "BTU/Hr" loss is very well put - I do understand in, a dim sort of, fashion.

Radiator/CHT measurements -

Rotax /Dynon CHT sensors X2 are registering within a degree or two of each other.

Rotax radiator/ Off the shelf (cheap) fridge style digital temperature sensors. Starting from ambient,have a consistent .5C difference between the two. Ambient temps as per Dynon (OAT) read out,  so I assume that all are reading actual temp..

Conducted further tests today.

Ambient 15C. All temperature sensing devises/read outs within 1C of each other, assume all good.

First did a brief run with upper owling removed and "radiator" cap off. Tried to observe coolant movement . No luck, to much vibration/wind too make any sense of coolant activity. My scepticism of this test for coolant circulation remains.

Installed a temporary cowl flap on trailing edge of cowling, just behind radiator.

Ran engine at 2200RPM for 15 minutes.

This time all temperatures remained below 100C, aft cowl flap would seem to have achieved sufficient drop in pressure to "suck" more air through radiator/cowling. Radiator now showing a 6-10C difference between in & out tank temperatures - makes more sense.

Also did a "tuft" test around main cowling air inlets (nostrils) and lower oil cooler air inlet. Interesting results - Smooth air at all times entering oil cooler inlet. Larger nostrils would seem to have relatively smooth air entering over lower lip but extremely turbulent air in the area from about half way to top lip ??????

Next step is to design make and install an airworthy cowl flap. The flap will be hinged, have the ability (down the track) to be fitted with a linier  actuator, for in flight opening/closing but initially will be fixed.

Thank you all for your help.

When you opened the Expansion Tank/Radiator Cap, Did you find that the radiator was completely brimming full right to the Cap?  

This would be the correct fill level.  No Air whatsoever! 
I am sure that would make it pretty hard to judge what is going on below.

That is why I recommend a Clear/Transparent expansion hose.

It sounds like you have it under control.