Well, Jeff, that IS a bit more delta than I’ve hear of before.  Assuming the data is factual and not some factor of sensor/gauge/location then there are a couple of things that could be bearing on it.

 

The first that comes to mind is that perhaps the coolant is not overly hot, but perhaps the oil is overly cool.  I generally don’t even take off until my oil temps reach 120F.  You do seem to have a lot of fin area poking through the open for the oil cooler – is it stock? Of course the OATS are mightily in your favor at the moment, so perhaps not the best time to determine the extent of any problem (if any).

 

Given how cool it’s been while you’ve made our test flights, I would guess that your oil is getting too much air flow and your radiators too little.  But, that is a guess. 

 

The two major factors are heat transfer liquid (water or oil) flow and air flow through your heat exchangers.  IF either one is deficient, then your temps will tend to rise until equilibrium is reached – which may be at too high a point for the good of the engine.

 

So if the coolant is flowing less than it should (for whatever reason) then the temperature of the coolant will, of course, increase as it fails to remove sufficient heat from the block.  On the other hand, if your air flow through your radiator is insufficient to remove required amount of heat, then the coolant temps will also increase.

 

Since your have fixed your slipping belt problem, I assume that the coolant flow rate is adequate.  So that would tend to suggest that your may need to divert (if possible) some of the air flow going to the oil core over to the radiator core.  That’s about all I can think of at the moment.

 

Normally, it’s the oil temps that are elevated, as oil is not as good a heat transfer fluid as water is, the oil is picking up heat from the hot iron rotors (as well as other parts of the engine).  It takes a lot more heat to raise a quart of water 1 degree than it takes to raise a quart of oil 1 degree.  So that means that if your coolant and oil temps were the same (assuming the same volume of flow for each) – the coolant is carrying much more heat away per unit volume than the oil. However, the flow rate of each will be the dominant factor in the amount of heat transferred from the engine to the cores.  The amount of air through your cores would then determine which one got rid of its heat better.  It appears that your oil cooler is either NOT carrying its share of the heat load OR it has excessive cooling capacity. 

 

But, then you are testing under rather benign temperature conditions – with that considerably in your favor at this point.  Higher OAT may reveal new trends {:>). 

 

The oil cooling is basically unpressurized in that the rotor cooling portion is basically oil sprayed from the Eshaft to the underside of the rotors and they spin on the shaft which then drains by gravity back to the sump (more hot oil) and then finally out to the oil heat exchanger to get rid of the heat.

 

The coolant on the other hand is constantly under pressure through the system thereby insuring better contact with the hot surface (than the sprayed oil).  For that reason (and the area it covers) and the better heat transfer characteristics of water,  the coolant is responsible for getting rid of approx 2/3 of the waste heat (discounting the exhaust heat of course) with the oil responsible for about 1/3 of the waste heat.

 

I would first try diverting some of the air flow to the radiators.  You might try partially blocking air flow to your oil cooler to see what effect it has on your oil temps – they should increase of course. IF not then something else might be at play.

 

 

Thanks Ed, any comments on the extreme delta between my water 190F and my oil at 110F?

Jeff

 

 

Making progress, Jeff!  Good work!

 

When I first flew with high oil temps the water temps were elevated as well.  When I solved my oil temp problem, the coolant temps also dropped.  So there is no question there is some “leakage” of heat between the oil and coolant system – not too surprising as both circulate in the block where all that hot metal is.

 

In the winter time, I ,like Mark, see coolant and oil temps at cruise down in the lower 140F range – cooler than I would like.  But, in the summer they are in the 160-170F range (both oil and water)

IF I flew a lot in the cold/hot weather, I would probably put some time of hand operated baffle in the oil duct to divert some of the air flow causing its temp to rise and the coolant would tend to follow it up.

 

I have tried using a thermostat on two occasions and almost fired my engine on the ground.  I am not certain but because I am plugs up, I think I may trap some air around the thermostat stopping it from heating up and opening – even though I drilled some 1/8” dia holes around the lip of the thermostat to get rid of any air.  In any case, I never tried flying with one because the engine got too hot just on the ground.  You engine is installed in the “normal” orientation so should be able to operate with a thermostat.

 

Just remember the engine’s cooling system was originally designed for relative low average power being produced (as in touring down the free way at 2500 rpm) where as we are asking for “more power! Scotty!” from take off to level off.  High power = high heat with low air speed = low air mass flow = High temp problems on take off.

 

If you optimize your engine cooling for cruise, it’s likely you will have a cooling deficit during the take off phase.  If you optimize if for take off then you will likely have more drag at cruise than necessarily.

 

While, I am not advocating it, I have accepted the cooling deficit during take off  in order to have my “high speed” aircraft have minimum cooling drag at higher speeds.  I typically will hit 220F with the coolant and 200F with the oil for the 2-3 minutes it takes me to get up to a comfortable altitude and power back a bit, once the airspeed hits 120 mph IAS then my cooling deficit is overtaken and all it good.

At 8000 ft MSL my max possible power (WOT) and rich air/fuel mixture will push my coolant and oil temps right up between 195 and 200F.  So if I had any less cooling capacity (or any more power), I would face a cooling problem, but as it is I am satisfied that my system is very close to “optimum” at cruise.

 

Sounds to me like you are zeroing in on what needs to be done. 

 

Enjoy your success!!

 

Ed

 

 

 

Mark,

I’m not sure what the cruise temps are as we have been flying short, tight circuits only. I think cruise temperatures need to be determined before doing too much else, though re-installing the thermostat may happen.

What is the relationship between the water and oil temp? I guess you have stock Mazda thermostat 195F?

Most of the rotary world has cheek radiators up front, with the oil cooler going in whatever space is left. My oil cooler is up front on top of the PSRU with inlets/outlets on both ends and the water radiator (19x5.5x5.5) is below/behind the oil pan … two independent systems one working too well the other not well enough … see attached photo.

Jeff

 

 

 

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