General liquid cooled engine questions

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Eugene

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I have Rotax 912 - 100 hp engine with water and oil thermostats installed. But I have some questions about all liquid cooled engines that we have today.

- If water temperature stays at 180°F during any flight conditions than my cooling air duct oversized?

- If water temperature occasionally go above 180°, how high can it go safely before I should start getting nervous and increase my ductwork size? What is normal operating temperature? What is absolute limit?

- Same questions for oil temperatures. I was told by Rotax guys that they prefer oil temperature to be a little higher than water. What is normal oil temperatures and absolute not to exceed?

- I learned this morning that GM engines timing chain very sensitive to oil change intervals. They recommend maximum 6000 miles on synthetic oil. But they tell me if you really care about your engine you should do every 4000 miles. How often would we change oil on automotive engine if we use it in the Aircraft? Rotax have recommended 50 hour oil change intervals. Will it be a good idea to change it more often?

- When I lived in Germany, we were told to buy used vehicles from older people. Because old people not driving on autobahn in the left lane. They believed back then that high rpm kills engines. Is it true that we should run our engines at lowest RPM as possible to prolong their lives?

- Some guys believe that you supposed to operate your 912 engine between 5000-5500 RPM. I talked to her a tax representative at Oshkosh the summer and they tell me that it's OK to go down to 4300 RPM.

- They also tell me that my 1 inch balancing tube between intake manifold is absolute nonsense. But I do feel the difference at low RPM. To have two separate engines with two separate carburetors spinning one crankshaft was not a good idea. Right or wrong?
 

wsimpso1

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Hmm. I am only going to talk about cooling systems.

If we are on the thermostats, we have excess cooling. How much excess, we do not know. Here is the critical concept to carry along. The amount of heat being moved from heat source to a the cooling medium is proportional to:
  • The delta T between the hot surface and the cooling medium;
  • The rate at which cooling medium flows (mdot) over the hot surface.
Turn up our mdot and we can live with smaller deltaT. So to figure out how much excess cooling we have, we can raise the cooling airflow temperature, or we can turn down the airflow, knowing the deltaT. Let's say ambient is 60F and coolant is 180F. Delta T is 120F, and our air inlet is wide open. We are on the thermostat. To figure out how much excess you have to handle a 100F day, you could wait for a 100F day. Or you could reduce the airflow (mdot) by taping over part of the inlet or part of the radiator. If you reduce inlet area by some amount and still the coolant temp is 180F, you need to tape over some more of the inlet. Small adjustments and go fly watching how fast the temps rise on takeoff and climb. Terminate and RTB immediately if the temps take off.

Once the coolant temp runs a little above thermostat temperature, we are now at thermal capacity of the system. Lets say we are at 55% of base inlet area and 190F, still at 60F ambient. Now we have 55%*130F = 71.5 worth of cooling capacity. If we want to know how big the inlet has to be when ambient is 100F (the common target in the car companies), the deltaT for 190F coolant is 90F. Hmm 71.5/90 is 79.44%, so 80% of the base opening is what we shoot for.

Now is 190F our target? Usually not. 50-50 water-EG coolant has a boiling point of about 229F at sea level pressure, so a really good target Max Temp will likely be less than that but more than 190F. If we are willing to flirt with over heat , you can go for 229F coolant, delta T of 129F on a 100F day, and use about 55% of your original opening. Any additional thermal load and we will start boil away our coolant. Most of us will pad those limits and use a little more opening.

We do know that if we have a pressure cap on the radiator, we can get some more coolant temp before we boil coolant, but let's remember that:
  • We can go up in altitude, reducing ambient pressure;
  • The cap holds the cooling system at ambient pressure plus whatever the cap pressure setting is;
  • Ambient air temp drops as we rise, but air density drops too. Net effect is that at same true airspeed, mdot decreases as we climb;
  • So to cover altitudes, we need to calculate air inlet opening as a percent of some base value, deltaT, and mdot at different altitudes, and multiply all three together to get capacity;
  • Then compare coolant temp to max temp for boiling at each altitude;
  • Oil does not have these boiling issues, but most motor oils should not go above 250F ever, and probably lower limits are wise for intended operations.
I will suggest that we want to figure out what our cooling is while flying at best climb rate airspeed. mdot is small, and heat rejection is likely near maximum.

So now we can figure out how small the inlet (or radiator face exposed) can be for one condition of altitude and temperature, then adjust for some Hot day, and know that we will be fine in climb up to that condition. Hotter may require lowering the nose and climbing at higher airspeed but lower climb rate.

All this stuff applies to water-glycol radiators and oil coolers, etc. Given Eugene's installation is probably fixed on oil and water heat exchangers, most likely one will start to rise in temp while the other stays on the thermostat. If we really want to figure things out, let's say the oil is still on the thermostat when the water-glycol starts up. Hmm. Leave the inlet restriction where you had it, but tape over a bit of the oil cooler until it too starts up. You may eventually cover part of one or the other heat exchangers and reduce air inlet size for general use.

Notice, I have not suggested closing the outlet. Outlet restrictions work too, but for Eugene's modified Skyboy, that really won't be much of an option, but the inlet can be reduced or enlarged.

Billski
 

TFF

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You will need to look up the Rotax engine limits. Hotter than they recommended is always bad. There is built in tolerance but you can use it up if it gets hot all the time. Same with too cool.

Rotax will have oil temp limits too. Oil temp is usually higher but somewhat close to water temp.

Clean oil is always good. Can’t hurt to change oil sooner. How much more life with 25 hour change interval, tough to say but reliability of clean oil is always good. Usually if oil consumption goes up a good bit, time to change oil.

Rotax engines need to run at a power setting that doesn’t let the gearbox gears float. 4300 is high enough if that’s the speed you want to fly at. Make it last longer? too close to matter if the rest of the maintenance is kept up.

Your 1” balance tube is your secret. Most airplane people are not engine people. They are interested in flying. Car people tend to be engine people. Car racing is more prevalent than airplane racing, so just not as many care to experiment on their engines.
 

Eugene

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Hmm. I am only going to talk about cooling systems.

If we are on the thermostats, we have excess cooling. How much excess, we do not know. Here is the critical concept to carry along. The amount of heat being moved from heat source to a the cooling medium is proportional to:
  • The delta T between the hot surface and the cooling medium;
  • The rate at which cooling medium flows (mdot) over the hot surface.
Turn up our mdot and we can live with smaller deltaT. So to figure out how much excess cooling we have, we can raise the cooling airflow temperature, or we can turn down the airflow, knowing the deltaT. Let's say ambient is 60F and coolant is 180F. Delta T is 120F, and our air inlet is wide open. We are on the thermostat. To figure out how much excess you have to handle a 100F day, you could wait for a 100F day. Or you could reduce the airflow (mdot) by taping over part of the inlet or part of the radiator. If you reduce inlet area by some amount and still the coolant temp is 180F, you need to tape over some more of the inlet. Small adjustments and go fly watching how fast the temps rise on takeoff and climb. Terminate and RTB immediately if the temps take off.

Once the coolant temp runs a little above thermostat temperature, we are now at thermal capacity of the system. Lets say we are at 55% of base inlet area and 190F, still at 60F ambient. Now we have 55%*130F = 71.5 worth of cooling capacity. If we want to know how big the inlet has to be when ambient is 100F (the common target in the car companies), the deltaT for 190F coolant is 90F. Hmm 71.5/90 is 79.44%, so 80% of the base opening is what we shoot for.

Now is 190F our target? Usually not. 50-50 water-EG coolant has a boiling point of about 229F at sea level pressure, so a really good target Max Temp will likely be less than that but more than 190F. If we are willing to flirt with over heat , you can go for 229F coolant, delta T of 129F on a 100F day, and use about 55% of your original opening. Any additional thermal load and we will start boil away our coolant. Most of us will pad those limits and use a little more opening.

We do know that if we have a pressure cap on the radiator, we can get some more coolant temp before we boil coolant, but let's remember that:
  • We can go up in altitude, reducing ambient pressure;
  • The cap holds the cooling system at ambient pressure plus whatever the cap pressure setting is;
  • Ambient air temp drops as we rise, but air density drops too. Net effect is that at same true airspeed, mdot decreases as we climb;
  • So to cover altitudes, we need to calculate air inlet opening as a percent of some base value, deltaT, and mdot at different altitudes, and multiply all three together to get capacity;
  • Then compare coolant temp to max temp for boiling at each altitude;
  • Oil does not have these boiling issues, but most motor oils should not go above 250F ever, and probably lower limits are wise for intended operations.
I will suggest that we want to figure out what our cooling is while flying at best climb rate airspeed. mdot is small, and heat rejection is likely near maximum.

So now we can figure out how small the inlet (or radiator face exposed) can be for one condition of altitude and temperature, then adjust for some Hot day, and know that we will be fine in climb up to that condition. Hotter may require lowering the nose and climbing at higher airspeed but lower climb rate.

All this stuff applies to water-glycol radiators and oil coolers, etc. Given Eugene's installation is probably fixed on oil and water heat exchangers, most likely one will start to rise in temp while the other stays on the thermostat. If we really want to figure things out, let's say the oil is still on the thermostat when the water-glycol starts up. Hmm. Leave the inlet restriction where you had it, but tape over a bit of the oil cooler until it too starts up. You may eventually cover part of one or the other heat exchangers and reduce air inlet size for general use.

Notice, I have not suggested closing the outlet. Outlet restrictions work too, but for Eugene's modified Skyboy, that really won't be much of an option, but the inlet can be reduced or enlarged.

Billski
Thank you for the detailed explanation! I will print this message and save as my little engine cooling Bible.

In my case oil and coolant temperatures very much related to engine RPM. I remember going around Green Lake and trying to keep up with everybody I kept my RPM at 5500. Water temperature was 187°F but oil temperature got to 230°F and red alarm started flushing right into my face and making me nervous.

Rotax guys explained to me that there's nothing wrong with 230°F oil temp and alarm limit was probably set wrong by previous owner.

But good to know that engine is working hardest during climbing.

I am usually cruising at 5200 RPM, which is I was told approximately 75% power for 912 engine.

Sounds like I will have to work on harmony between oil and water temperatures and tweak them by blocking one or another little bit.
 

Eugene

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Your 1” balance tube is your secret. Most airplane people are not engine people. They are interested in flying. Car people tend to be engine people. Car racing is more prevalent than airplane racing, so just not as many care to experiment on their engines.

Rotax recommends to install thermostats if you flying in colder climates. Yes, too cold is not good.

I agree with the statement that most normal pilots like to fly versus working on airplane. I can see it at my local airport.

But I was for sure thinking that experimental aviation is about experiments. From what I know today I was wrong. It is about 51% rule.

And walking around Oshkosh and looking at all different engines, I was under the impression that experimental engines are a big part of experimental aviation.
 

TiPi

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Thank you for the detailed explanation! I will print this message and save as my little engine cooling Bible.

In my case oil and coolant temperatures very much related to engine RPM. I remember going around Green Lake and trying to keep up with everybody I kept my RPM at 5500. Water temperature was 187°F but oil temperature got to 230°F and red alarm started flushing right into my face and making me nervous.

Rotax guys explained to me that there's nothing wrong with 230°F oil temp and alarm limit was probably set wrong by previous owner.

But good to know that engine is working hardest during climbing.

I am usually cruising at 5200 RPM, which is I was told approximately 75% power for 912 engine.

Sounds like I will have to work on harmony between oil and water temperatures and tweak them by blocking one or another little bit.
5,200rpm is 75% power IF your WOT and level rpm is around 5,700-5,750 (@ sea level). To set 75% power, run your engine at WOT, then throttle back to 91% of that rpm (only valid for fixed-pitch prop).

Rotax recommends 90-110deg C (190-230deg F) as the normal temp range and 140deg C (285deg F) max. It is also recommended to reach the upper range of normal once per flight to burn of condensation and fuel.
 
Last edited:

BoeveP51

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Hmm. I am only going to talk about cooling systems.

If we are on the thermostats, we have excess cooling. How much excess, we do not know. Here is the critical concept to carry along. The amount of heat being moved from heat source to a the cooling medium is proportional to:
  • The delta T between the hot surface and the cooling medium;
  • The rate at which cooling medium flows (mdot) over the hot surface.
Turn up our mdot and we can live with smaller deltaT. So to figure out how much excess cooling we have, we can raise the cooling airflow temperature, or we can turn down the airflow, knowing the deltaT. Let's say ambient is 60F and coolant is 180F. Delta T is 120F, and our air inlet is wide open. We are on the thermostat. To figure out how much excess you have to handle a 100F day, you could wait for a 100F day. Or you could reduce the airflow (mdot) by taping over part of the inlet or part of the radiator. If you reduce inlet area by some amount and still the coolant temp is 180F, you need to tape over some more of the inlet. Small adjustments and go fly watching how fast the temps rise on takeoff and climb. Terminate and RTB immediately if the temps take off.

Once the coolant temp runs a little above thermostat temperature, we are now at thermal capacity of the system. Lets say we are at 55% of base inlet area and 190F, still at 60F ambient. Now we have 55%*130F = 71.5 worth of cooling capacity. If we want to know how big the inlet has to be when ambient is 100F (the common target in the car companies), the deltaT for 190F coolant is 90F. Hmm 71.5/90 is 79.44%, so 80% of the base opening is what we shoot for.

Now is 190F our target? Usually not. 50-50 water-EG coolant has a boiling point of about 229F at sea level pressure, so a really good target Max Temp will likely be less than that but more than 190F. If we are willing to flirt with over heat , you can go for 229F coolant, delta T of 129F on a 100F day, and use about 55% of your original opening. Any additional thermal load and we will start boil away our coolant. Most of us will pad those limits and use a little more opening.

We do know that if we have a pressure cap on the radiator, we can get some more coolant temp before we boil coolant, but let's remember that:
  • We can go up in altitude, reducing ambient pressure;
  • The cap holds the cooling system at ambient pressure plus whatever the cap pressure setting is;
  • Ambient air temp drops as we rise, but air density drops too. Net effect is that at same true airspeed, mdot decreases as we climb;
  • So to cover altitudes, we need to calculate air inlet opening as a percent of some base value, deltaT, and mdot at different altitudes, and multiply all three together to get capacity;
  • Then compare coolant temp to max temp for boiling at each altitude;
  • Oil does not have these boiling issues, but most motor oils should not go above 250F ever, and probably lower limits are wise for intended operations.
I will suggest that we want to figure out what our cooling is while flying at best climb rate airspeed. mdot is small, and heat rejection is likely near maximum.

So now we can figure out how small the inlet (or radiator face exposed) can be for one condition of altitude and temperature, then adjust for some Hot day, and know that we will be fine in climb up to that condition. Hotter may require lowering the nose and climbing at higher airspeed but lower climb rate.

All this stuff applies to water-glycol radiators and oil coolers, etc. Given Eugene's installation is probably fixed on oil and water heat exchangers, most likely one will start to rise in temp while the other stays on the thermostat. If we really want to figure things out, let's say the oil is still on the thermostat when the water-glycol starts up. Hmm. Leave the inlet restriction where you had it, but tape over a bit of the oil cooler until it too starts up. You may eventually cover part of one or the other heat exchangers and reduce air inlet size for general use.

Notice, I have not suggested closing the outlet. Outlet restrictions work too, but for Eugene's modified Skyboy, that really won't be much of an option, but the inlet can be reduced or enlarged.

Billski
Very interesting info Billski. I will analyze my radiator setup and report back.

It may be some time though. I just had a nose wheel collapse on landing and have to go through the repair cycle and get this thing flying again.

And, Billski, it was not an engine failure............ So you can't count this one.

Merle
 

wsimpso1

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Very interesting info Billski. I will analyze my radiator setup and report back.

It may be some time though. I just had a nose wheel collapse on landing and have to go through the repair cycle and get this thing flying again.

And, Billski, it was not an engine failure............ So you can't count this one.

Merle
Bummer about the nose gear problem. I hope that the repair is quick and low pain.
 

Eugene

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I think Eugene was asking if the temperatures being referenced were for oil or water.
I think we settled already for water temperature to be between 190° and 210°F about.

The way I understood that 180° thermostat is absolute minimum to ensure safe engine operation. Engine protection so to speak. 228° is boiling point and we need to stay below that. I think I got it at this point. Thank you!
 

TiPi

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That is how I have set my prop. WOT at 2,500' is just over 5,600rpm, WOT on take-off and climb is ~5,350, which means cruising at 2,500' and 5,200rpm is roughly 75% power. At higher altitudes, that power reduces with density if 5,200rpm is maintained.
 
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