Cool Oil is Cool
Oil temperature of the plane's engine plays a much more important role than often assumed. The manufacturer typically gives a range of acceptable values. As long as oil temperature is within that range, the pilot assumes that all is well. So did we, until recently.
The Rotax manual specifies a normal oil temperature range of 190-230 deg F. For short periods, the temperature may go as high as 285 deg F with synthetic oil.
Let's evaluate these numbers. Oil oxidation doubles for every 18 deg F temperature increase, and oil life decreases by 50% for every 15 deg F above 140 deg F.
So the rate of oxidation approximately increases by a factor of 4 over the range 190-230 deg F. At the same time, oil life is reduced to less than 20%.
Over the range 190-285 deg F, the oxidation rate increases approximately by a factor of 32, and oil life is reduced to less than 2%.
These are substantial factors and reductions.
In the past we wondered why the oil drained during an oil change after 25 hrs of flying had a bad smell when oil temperature had been around 230 deg F for much of those 25 hrs. Also, the oil looked dirty, as if it contained sludge. Significant oxidation produces these results.
Another indicator of oil deterioration was oil consumption of 0.5 qt during the 25 hours.
The above rate increases for oxidation partially explain why flying at 230 deg F has these effects. A more complete explanation also considers that the Rotax 912 engine requires just 3 qts of oil. This is a small amount considering that the engine must produce 50 hp just for a low-power cruise.
If we translate that performance to the automotive world, then this is roughly equivalent to propelling a 4,500 lbs SUV at 70 mph with an engine requiring just 3 qts of oil. Automotive designers specify much larger amounts of oil for such an engine, with good reason.
Another indication of the importance of oil temperature is that, for some cars, the mileage interval between oil changes is now based on oil temperatures reached by the engine.
For example, the manual for the 2012 Nissan GT-R specifies oil changes every 6,000 miles if the oil temperature stays below 230 deg F. In the range 230-266 deg F, the interval drops down to 3,000 miles. And if oil temperature goes beyond 266 deg F, the oil must be changed immediately after stopping.
These considerations motivated us to install an auxiliary fan for the oil cooler of our Zenith 601HDS with Rotax 912 engine. The fan allows tight control of the oil temperature during all phases of flight and for almost any ambient air temperature. The goal is to maintain an oil temperature in the range 190-210 deg F at all times.
With use of the oil cooler fan as needed, the oil after 25 hours of use has no smell, indeed looks almost like new oil. Moreover, there is no oil consumption at all. Surely such oil is conducive to long engine life. As we write this, the engine has completed 1,400 hours, which is already 200 hrs beyond the projected TBO of 1,200 hrs at the time of installation.
Acknowledgment: We thank M. Truemper for help with the research on oil performance.
The Rotax manual specifies a normal oil temperature range of 190-230 deg F. For short periods, the temperature may go as high as 285 deg F with synthetic oil.
Let's evaluate these numbers. Oil oxidation doubles for every 18 deg F temperature increase, and oil life decreases by 50% for every 15 deg F above 140 deg F.
So the rate of oxidation approximately increases by a factor of 4 over the range 190-230 deg F. At the same time, oil life is reduced to less than 20%.
Over the range 190-285 deg F, the oxidation rate increases approximately by a factor of 32, and oil life is reduced to less than 2%.
These are substantial factors and reductions.
In the past we wondered why the oil drained during an oil change after 25 hrs of flying had a bad smell when oil temperature had been around 230 deg F for much of those 25 hrs. Also, the oil looked dirty, as if it contained sludge. Significant oxidation produces these results.
Another indicator of oil deterioration was oil consumption of 0.5 qt during the 25 hours.
The above rate increases for oxidation partially explain why flying at 230 deg F has these effects. A more complete explanation also considers that the Rotax 912 engine requires just 3 qts of oil. This is a small amount considering that the engine must produce 50 hp just for a low-power cruise.
If we translate that performance to the automotive world, then this is roughly equivalent to propelling a 4,500 lbs SUV at 70 mph with an engine requiring just 3 qts of oil. Automotive designers specify much larger amounts of oil for such an engine, with good reason.
Another indication of the importance of oil temperature is that, for some cars, the mileage interval between oil changes is now based on oil temperatures reached by the engine.
For example, the manual for the 2012 Nissan GT-R specifies oil changes every 6,000 miles if the oil temperature stays below 230 deg F. In the range 230-266 deg F, the interval drops down to 3,000 miles. And if oil temperature goes beyond 266 deg F, the oil must be changed immediately after stopping.
These considerations motivated us to install an auxiliary fan for the oil cooler of our Zenith 601HDS with Rotax 912 engine. The fan allows tight control of the oil temperature during all phases of flight and for almost any ambient air temperature. The goal is to maintain an oil temperature in the range 190-210 deg F at all times.
With use of the oil cooler fan as needed, the oil after 25 hours of use has no smell, indeed looks almost like new oil. Moreover, there is no oil consumption at all. Surely such oil is conducive to long engine life. As we write this, the engine has completed 1,400 hours, which is already 200 hrs beyond the projected TBO of 1,200 hrs at the time of installation.
Acknowledgment: We thank M. Truemper for help with the research on oil performance.
Thanks for share info. Very useful
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