Oil temp threshhold for engine breakage? (Mainly Merlin engines)

[*Buzzsaw*]

Originally Posted by Dirkan

Hi!

Has anyone tested the exact oil temperature at which the engine will break?
Or perhaps too much boost in combo with RPM, regardless of oil temp?
The radiator always breaks at 120 Celsius, 10/10 times, however, the oil temp is a bit more elusive.
(I don't give a rats ass about the pilot notes)

Thanks in advance

Taking all aircraft but the Spit IIA over 98 degrees is a mistake, the Merlin III might not break immediately, but there will be consequences.

The Spit IIA Merlin XII can run at temps below 105 degrees below 5000 ft, but as you get higher, the temperature limitation threshold drops. Over 10,000 ft, not a good idea to go over 95 degrees.

[Dirkan]

I imagine the temp threshold drops for the other engines as well. That explains why the engine broke after a while at 91 degrees oil temp at 14k feet.

[*Buzzsaw*]

Originally Posted by Dirkan

I imagine the temp threshold drops for the other engines as well. That explains why the engine broke after a while at 91 degrees oil temp at 14k feet.

Yes, temp max. for the Merlin III, (all aircraft but the Spit IIA) is 98 degrees below approx. 5,000, but as you go over that, temp limit gradually drops. And it is different for different aircraft, ie. the Spit IA 100 octane is different from the standard Spit IA.

This is major bug in the game. The higher you go, the cooler the engines should run, and less likely the chance of damage.

If you want to do comprehensive testing that would be nice... Ivan and I have already done testing for the engine misfiring bug for the British aircraft, but not on the overheating yet.

109 engines do not have any oil temp problems as long as you keep the oil at approx. 25% and radiator open to 3/4 mark. AFAIK!

[Crumpp]

he higher you go, the cooler the engines should run, and less likely the chance of damage.

That is wrong.

The higher you go, the less effective the cooling due to pressure and density reduction despite the cooler temperatures.

So while the temperature does get cooler IAW the lapse rate, the density is reduced so we end up with less molecules to transfer heat away from our engine.

[*Buzzsaw*]

Originally Posted by Crumpp

That is wrong.

The higher you go, the less effective the cooling due to pressure and density reduction despite the cooler temperatures.

So while the temperature does get cooler IAW the lapse rate, the density is reduced so we end up with less molecules to transfer heat away from our engine.

I can't say I am an expert on this matter. I made the assumption there is enough flow with the higher speeds at altitude and the cooler temps to cool the aircraft faster. Ie. IAS at sea level and 6000 meters may read the same, but TAS is much higher at altitude, and therefore airflow is much higher.

Anyone else want to comment?

[Crumpp]

IAS at sea level and 6000 meters may read the same, but TAS is much higher at altitude, and therefore airflow is much higher.

Airflow is not much higher because of True Airspeed.

[bongodriver]

Indicated airspeed is what matters, if indicated is high enough then cooling will be sufficient.

[NZtyphoon]

Originally Posted by *Buzzsaw*

Taking all aircraft but the Spit IIA over 98 degrees is a mistake, the Merlin III might not break immediately, but there will be consequences.

The Spit IIA Merlin XII can run at temps below 105 degrees below 5000 ft, but as you get higher, the temperature limitation threshold drops. Over 10,000 ft, not a good idea to go over 95 degrees.

The idea in CLOD was to recognise the differences between the Merlin II & III, which were cooled by 100% glycol, and the Merlin XII which was cooled by a 70-30% water-glycol mix. As a rule the earlier Merlins ran at higher temperatures and were more likely to have oil seals fail - unfortunately the CLOD engines are still too sensitive and too prone to failure but, as JTD said, there will be no more development so that's the best that we can expect

[Crumpp]

- basic principles of heat exchangers don't fully explain general aircraft engine oil temperatures
- general aircraft engine oil temperature characteristics don't fully explain Spitfire specific oil temperatures

The basic physics and principles of heat exchangers explain ALL aircraft heat exchanger behavior.

[JtD]

Yes. I totally agree with that, heat exchanger physics explain heat exchange.