Coolant/oil heat exchangers

Albert Yowell wrote:

My friend recently bought a Sirius TL-3000 with the 912IS. The previous owner installed a small radiator fan that we use on the ground. This keeps the coolant temp below 200. Airborne temps get up to 240; rarely more.

"would be uncomfortable with this, preferring to see cruse temperatures, both oil & coolant, in the low to mid 90C (194-203F) range" 

Under 100C (212F) Oil Temp on the ground is fine, although lower will give you less anxiety on a hot day.

115C (240F) Oil Temp on Climb Out is also fine. Its important that Oil Temp gets over 100C, for a sustained period, so as to drive off volatile factions.

In my opinion, 115C (240F) Oil Temp in cruise is not fine. The oil is having to "work" too hard at these hotter temperatures and is in danger of starting to break down before its service life is reached.

My preference is to see Cruise Oil temperatures 90-95C (194-203F). 

I also like to see 90-95C (194-203F) for my Cruise Cylinder Head (Coolant) temperature

Perhaps an unnecessary point, but a vital one: make sure your cooling system is really full. Rotax uses a cooling scheme that was (briefly) common on cars, but resulted in destroyed engines. (Hence modern cars all have two lines to the 'overflow' bottle and the system fills from there). The plastic coolant overflow tank is just that; filling it will *not* fill the system. (It can be full while the system is desperately not full0. Filling is only to be done through the "radiator" cap on the expansion tank - that spider on top of the engine. Fill, cold of course, right to the tippy top, even with the flat brass lip where the inner gasket of the cap presses against. Ideally, the overflow tank is located above the cap so any trapped air is decisively purged. If that isn't the case, air may not purge itself. A test: note the fluid level in the overflow bottle while cold. Run/fly the engine until fully warm. Now check the level; it should be clearly higher, line an inch. Wait until cold and check again. The level should now be back to the original position. If it fails this test, there's air in the coolant system. If the expansion tank is still full, as defined above, there's air trapped in the hose connecting that with the overflow bottle.

I noticed that the coolant level in the overflow bottle is quite low. Is there a reason for that? The previous owner of my ship told me to keep it low so it doesn't blow out. Not only did he not know how to fill the system properly, the cap was old and leaking pressure, causing coolant ejection.

Finally, make every effort to protect oil and water lines from exhaust heat. Not only were the exhaust pipes factory-wrapped, there's a metal plate separating the muffler from the engine. Net result is my 912 is wildly over-cooled and I have to block off at least 1/3 of the oil cooler, even at 70 deg F outside temperature.

I hope this helps.

Steven, I once come across the comment that Rotax does not declare an oil cooler mandatory, which I noticed as ‘surprising’. Since I have never seen a set-up without an oil cooler I got a little struck when I overlooked the oil cooler on your pics and thought ‘finally there is one plane without oil cooler’ - oops.

I have fiddled about with different mixtures and found no relevant effect. The anti-freeze argument only plays a role at low temps I was prepared to change the mixture to a winter set-mixture when needed. But as I said, changing the mixture only brought marginal effects.

To improve the baffling is by far the better bet. On a C42A we achieved real improvement by closing the gaps around the water cooler. Little effort, significant effect. 

Flushing the coolers may or may not help, it’s worth a try and no big effort. Cleaning the fins is a strong recommendation. Done that, made a difference.

Go for the simple measures first to improve the cooling system, like flushing, cleaning fins, checking tubes for too tight bends, etc.

Personally, I would invest some time to take out the xchanger for a while to get a clear indication which of the two cooling circuits needs help and then go for larger actions (like repositions of a cooler and directed air flow). Since you are up to cleaning and flushing anyway you are close to taking the xchanger out of the equation anyway. A little step for you, a great step for the analysis ;-)

Hi Peter,

ALL systems for dissipating heat are "heat exchangers". In the context of Rotax engines I would suggest that you use the term Radiator when when talking about the Coolant/Air system and Oil Cooler when referring to the Oil/Air system. 

"Rotax does not declare an oil cooler mandatory,"

All internal combustion engines (ICE) (alternative name heat engine) in production today, must get rid of waste heat or self destruct fairly quickly. All ICE's are air cooled (there is also a fuel cooling  factor) - this is via the lubricating oil (Oil/Air or Oil/Coolant), liquid (Coolant/Air) or fins (Metal/Air)  combination of 2 or all 3 (Rotax 9's) systems. If satisfactory cooling can be achieved, without a supplementary Oil Cooler, then one is not fitted - most, non performance, ground based engines do not have crankcase oil coolers fitted, relying instead on oil volume to handle the heat load  .

"I have fiddled about with different mixtures and found no relevant effect"

I stand to be corrected, however I believe that modern engine coolants are no longer referred to as "anti freeze" this being just one of its functions. Modern coolants, reduce the freezing level of the water, raise the boiling point of water, lubricate seals and importantly inhibit electrolysis within the system. Every coolant concentrate that I have seen in recent times recommends a 50:50 concentration with pure H2O, so that you get the optimum performance from the mixture. "Fiddling" with this recommendation, is likely to diminish one or more of the mixtures properties. When the anti freeze component was what we were all looking for, some coolant concentrates used to give lower ratios, where the anti freeze qualities where not so important (warmer winters) - I doubt this is common today.

Of course using the definitions Sean has brought in (thanx) are the best way to describe the cooling system(s) precisely. I hope the use of “xchanger” used to refer to the MOCAL gadget (shown on one of the pics provided) to share cooling capacity between the oil- and water cooling circuits did not cause too deep misunderstandings.

By writing “I have fiddled about with different mixtures and found no relevant effect.” I was trying to share two things. Firstly that I found no relevant effect on the temperature level since there was no significant temperature reduction (which I had been hoping for). Secondly, “fiddling about” was meant as an indication for a quick and dirty approach to get quick indicative results on the chase to reduce the temperature level leaving other important aspects (like long term corrosion protection or the anti-freezing function (since I was tying that out for a limited time span during summer times)) out of the equation at this first experimental step trying out other mixtures than the recommended 50:50 (which I went back to once my tests showed only marginal results).

I hope that the definitions Sean kindly provided in conjunction with my extended comments here put my earlier message in a clearer perspective.

In a nutshell I am now tempted to say “skip the ‘fiddling’ with mixture ratios to save your time to work on more promising options”. Good luck!