Air cooled vs Water cooled engine Oil coolers

[fastmongrel]

This isnt the very well beaten to death Air cooled Radial engine vs Water cooled V engine question.

I was wondering about the differences between the 2 types of engines oil cooling systems. Did a radial engine have a larger oil radiator and therefore a larger oil capacity or was it the other way round. Or was it simply a function of capacity, power output and how much of the oil was used for cooling and how much for lubricating.

What got me thinking was Spitfires with 2 stage engines had equal sized water and oil radiators under the wings.

Also which was more vulnerable the water cooling pipes and radiator or the oil cooling pipes and radiator. Oil pipes on cars are smaller but when they are damaged because of the much higher pressures the oil is lost about as fast as a water pipe damaged in a similar way.

 

[cherry blossom]

I seem to remember (sorry no reference) that when the loss rates of Hellcats and Corsairs undertaking similar ground attack missions were calculated, more Corsairs were lost. It is possible that this relates to the oil coolers of Corsairs being in the wing root whilst the oil cooler of the Hellcat was near the engine. However, we could equally well imagine that it was easier to land a damaged Hellcat.

 

[pbehn]

The later equal sized radiators housed radiators for the engine, oil, and intercooler.

 

[tomo pauk]

The 'close coupled' oil system on the P-40 weighted ~60 lbs, plus ~90 lbs of oil in the tank, plus 28 lbs of trapped oil. Similar system, but on the F6F, weighted between 144 and 148 lbs, trapped fuel and oil is at 89.5 lbs, the oil tank carrying 97.5 lbs of oil when just internal fuel was caried (1500 lbs), while 142.5 is when fuel totalled 2600 lbs.
All in all - looks to me that rule of the thumb is that size of oil system volume of oil carried is closely related to engine power and to the endurance (or, the time the engine is expected to run during one mission). We can observe that PR Spitfires have had increased oil tankage to cater for great endurance, and similar increase was for the Spitfires that were self-deployed via usage of increased amount of both internal and external fuel.

 

[wuzak]

Note that the oil tank capacity had to allow for loss of oil in the engine. Thus, the PR Spitfires needing to fly 2-3 times as far as the fighter version needed bigger oil tanks.

The size of the oil cooler relates to how much heat has to be removed from the oil after the engine. Air cooled engines tend to have more heat rejected through the oil system than liquid cooled systems.

During the US Hyper engine project it was thought that running higher coolant temperatures would lead to smaller radiators. This was true, but they also discovered that the higher temperatures (they were aiming for 300F/149C) that more heat was rejected through the oil, and so a larger oil cooler was needed, negating the gains made by the smaller radiator.

 

[pbehn]

During the US Hyper engine project it was thought that running higher coolant temperatures would lead to smaller radiators. This was true, but they also discovered that the higher temperatures (they were aiming for 300F/149C) that more heat was rejected through the oil, and so a larger oil cooler was needed, negating the gains made by the smaller radiator.

So, no free lunch then? A bit of a surprise!

 

[rinkol]

Note that the oil tank capacity had to allow for loss of oil in the engine. Thus, the PR Spitfires needing to fly 2-3 times as far as the fighter version needed bigger oil tanks.

The size of the oil cooler relates to how much heat has to be removed from the oil after the engine. Air cooled engines tend to have more heat rejected through the oil system than liquid cooled systems.

During the US Hyper engine project it was thought that running higher coolant temperatures would lead to smaller radiators. This was true, but they also discovered that the higher temperatures (they were aiming for 300F/149C) that more heat was rejected through the oil, and so a larger oil cooler was needed, negating the gains made by the smaller radiator.

I recall reading that there were thermal issues with the DB V12s resulting from the use of dry cylinder liners, a construction feature which reduced the ability of the coolant to remove heat from the cylinder liner. This was at least partly compensated for by relying more on the oil cooler to remove heat. Some of the later engines may have sprayed oil on the underside of the pistons to help remove heat.

In general, to some extent, the oil system can be used to compensate for limitations elsewhere in the cooling system. I know that auxiliary oil coolers used to (maybe still are) available for automobiles subject to heavy usage, such as in pulling trailers.

 

[tomo pauk]

Even on the similar engines, like the Merlin and Allison were, there was a significant difference between oil colling loads. Eg. for the Merlin, between ~11 and 14% of cooling load was taken by oil system, while on the Allison it was between 20 and 25%. Having a hydraulic drive for a compressor stage/stages also increases the demand on the oil cooling system, from additional 1/3rd to 1/2 in case of 2-stage Allison for max power and max slip - basically, at sea level. We might expect similar for the Daimler Benz engines that usually employed the hydraulical drive for their compressors.

 

[GregP]

Allisons need more oil cooler and less radiator than Merlins, just as Tomo says. We see that in every Allison installation. Most people don't "get it," but I see Tomo does. Also, the Merlin squirts oil onto the ingoing side of the reduction gears while the Allison squirts it on the outgoing side. So the Allison gets longer gear life and less stress on the reduction gears as a result.

Nice that someone other than an Allison overhauler knows it about the Allison's extra oil requirements ...

 

[tomo pauk]

Truth to be told, I've just posted the data from 'Vee's for victory' book, a great book that I can't recommend enough. Data from post #4 is of course from 'America's hundred thousand', another great book.

 

[Koopernic]

The barrel shaped radiator of the Junkers Jumo 213EB provided cooling for the three sources of heat in an aviation engine: the water-glycol circuit, the oil and the intercooler. This no doubt allowed an optimisation of the radiator cross sectional area against its temperature and the different requirements of the oil, coolant and intercooler. This required an interposing heat exchanger (warmaustaucher) but this didn't add frontal area. The higher the temperature of the radiator the smaller the area the radiator can be and the easier it is to recover ram effect jet thrust. If done properly it might be able to protect the oil cooler from battle damage and by having cut-off valves in a partitioned radiator.

 

[tomo pauk]

The barrel-shaped radiator was already present in the Jumo 213E, where indeed the heat exchanger was used to provide cooling for oil and intercooling systems. For the 213EB, that should be the high compression version of the 213E (8.5:1, C3 fuel only), the radiator installation was to feature the 'barrel' with longer side(s), cowled in the long-chord cowling in order to resuce the drag. The radiator itself was not an unitary unit, bur featured 4 segments - should help when one of the segments is damaged/pierced.
The Jumo 213A already featured the heat exchagner for the oil system, and it might be that French were the 1st to incorporate such a system in a combat aircraft with D.520 Ameiloree (sp? means 'Improved') and it's HS 12Y engine. The Mikulin AM 37 used the main cooling system to cool the intercooler, but the engine was often overheating.
As for drag reduction - TASTAAFL applies, the intercooled 213E was making greater cooling drag than non-intercooled 213F.