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View Full Version : Engine overheating/overcooling: what's changed between WW1 and WW2?



Manu-6S
08-28-2009, 03:23 AM
The Subject say it all.

In my opinion both were sensitive to overheating (easy) but also to overcooling.

What do you think?

Wasn't really overcooling a problem in the WW2?

Viper2005_
08-28-2009, 04:32 AM
http://forums.ubi.com/eve/foru...001071587#2001071587 (http://forums.ubi.com/eve/forums/a/tpc/f/23110283/m/3081078387?r=2001071587#2001071587)

During WWI, engines operated at much lower temperatures.

Water-cooled engines didn't have pressurised cooling systems, so were forced to operate at less than 100ºC. In fact, because the boiling temperature of water decreases with decreasing ambient pressure, the maximum operating temperature of the cooling system would decrease with increasing altitude.

This wasn't much of a problem in most cases because the engine power output also decreased with altitude.

Maximum metal temperatures were lower in WWI because of the less advanced metallurgy of the period.

Because machining technology was less advanced, air cooled engines had fewer cooling fins, and water cooled engines had less elaborate radiators.

This meant that heat rejection was often limiting, despite the fact that there was far more "cooling frontal area" per horsepower in WWI than in WWII.

Cooling drag was taken to just be a fact of life, and unfaired radiators were just dumped into the airflow like hot airbrakes.

Because fuel quality was much lower (c. 30 octane), detonation and pre-ignition were major problems. Since both could be initiated by hot-spots, there was a tendency for overheating to "run away" to failure quite rapidly.

Instrumentation was less advanced, so pilots had far less idea of what was actually happening to their engine(s).

Production tolerances were wider, so some aircraft were under-cooled, whilst others were over-cooled. Because one has to design around the worst case, this meant that many aircraft were lumbered with unnecessary mass and drag, which ironically required them to run at higher power, generating more waste heat.

Because WWI aircraft were slower, they didn't ram as much air through their cooling systems as WWII aircraft, and because of this and their generally smaller size, they operated at lower Reynolds number, which didn't help them.

Despite all of this, temperature was not the main direct cause of engine trouble during WWI, because it's pretty easy to detect and fix at the design stage.

If a water-cooled engine tends to overheat in the prototype, you just fit a bigger radiator.

If a rotary engine overheats on the bench then it simply isn't going to be allowed to fly.

If it overheats when installed then you just take the cowling off, and accept that the pilot will suffer the effects of ingesting large quantities of castor oil, not to mention the extra drag.

Instead, WWI fighter design was arguably dominated by inefficient but effective cooling system designs. Indeed, the rotary engine itself was nothing more or less than an elaborate cooling system for a radial engine.

As a result of this vast expenditure of effort to attack the cooling problem, engine failures were mostly caused by mechanical design problems such as fatigue (erroneously known at the time as "crystallisation").

Towards the end of the War, progress in the field of engine cooling was driving the Rotary engine towards obsolescence. This can be seen from the move towards radial engines in 1918/19, such as the Cosmos (later Bristol) Jupiter, which went on to enjoy great success in the inter-war period, and the ABC Dragonfly, which would have caused the RAF great distress had the war lasted into 1919.

The Dragonfly failed for several reasons, though the main problem was that it ran at what turned out to be a resonant dynamic mode of its crankshaft. This resulted in rapid engine failure due to fatigue and overheating; it seems likely that the overheating was due to the engine performing mechanical work destroying itself rather than due to a direct failure of the cooling system design itself, since it had been designed based upon the assumption that the mechanical work would leave the engine via the propshaft rather than being turned into crankshaft deformation...

Unfortunately for the RAF, the Dragonfly's defects only became apparent when most other engine programmes had been cancelled in its favour...

This sort of problem was endemic to engine design during WWI as the computational firepower necessary to avoid trouble with dynamic modes at the design stage simply wasn't available, and so designers had to either be cautious or brave. Sometimes they didn't know which, and not all of them were lucky!

Indeed, crankshaft dynamic modes were still troublesome in certain contexts during the WWII period; the difference was that trouble was expected based upon experience, and the necessary resources were deployed early in order to keep things under control. An idea of the design problems involved is given by:

http://www.enginehistory.org/N...800%20Crankshaft.pdf (http://www.enginehistory.org/NoShortDays/Development%20of%20the%20R-2800%20Crankshaft.pdf)

Note that these problems are as old as the piston engine; designers during WWI simply faced them without the benefit of quite as much experience...

Manu-6S
08-28-2009, 04:42 AM
Very interesting!!!

Thanks Viper!

AndyJWest
08-28-2009, 04:51 AM
The Dragonfly failed for several reasons, though the main problem was that it ran at what turned out to be a resonant dynamic mode of its crankshaft. This resulted in rapid engine failure due to fatigue and overheating; it seems likely that the overheating was due to the engine performing mechanical work destroying itself rather than due to a direct failure of the cooling system design itself, since it had been designed based upon the assumption that the mechanical work would leave the engine via the propshaft rather than being turned into crankshaft deformation...

Ouch! Didn't know that. An engineering classic. Thank god we have computers now so things can't go wrong any more...

BillSwagger
08-28-2009, 08:30 AM
I think overheating the engine was really only an issue for ww2 planes while they were at idle or when at lower air speeds.
There can be other components that take heat from the engine, like turbos, as well as superchargers that produce their own heat, that can would be a heat concern in ww2 aircraft.
WEP, and boost systems could push higher manifold pressures that could be a concern to the pilot for prolonged periods depending on the system.
The WEP of the P-38, P-47, and P-51 is really only a system that keeps the engine cooler under higher compression ratios and output. There is a R-2800 test that ran the engine for over 7 hours at WEP with no mechanical failure, and remained in running condition afterward. This doesn't exclude other heat problems in the aircraft system, such as the turbo or exhaust lines where most of the heat is sent, if the pilot doesn't divert the exhaust through waste gates.

Over cooling was of increased concern as planes started flying really high, but i think that was less a function of the engine heat, and more of a function of the oil cooler. If the oil gets too cold it can lose its viscosity and cause problems inside the engine, or oil lines. So the pilot would need to manage the oil temp.

I'm less familiar with WW1 aircraft but it seems they were less complex in many ways. I'm not sure any had turbo/supercharging systems or anything that resembled a boost system. I think the propellers were still largely fixed or only had two settings, which would also limit efficiency, considering that higher RPMs generally produce more heat. I actually think it would still be easier to overheat a WW1 plane because the advancement of ww2 aircraft aloud for high sustained outputs with much less chance of overheat.
I'm not sure if this is moot, but the WW2 planes also flew at much higher speeds pushing more air through there cooling systems.

Viper2005_
08-28-2009, 09:26 AM
WEP = War Emergency Power.

It is an American nomenclature for the combat rating. Unfortunately in IL2 it has become almost synonymous with ADI (Anti Detonation Injection). This is misleading.

The vast majority of P-51s, and all of those which were operational in WWII had no ADI. War Emergency Power just meant pulling more inches of Mercury. In fact, it was basically the same as Combat Power in the Spitfire, the main difference that the American ratings were chosen to give a nice number in "Hg whilst the British ratings were chosen to give a nice number in psi boost. ADI for the Mustang was in the works, but the war ended before it arrived. The P-51H had it, but the ultimate P-51 would have been the L model with the mighty -11 engine.

Equally, not all R-2800s had water injection systems, but that didn't preclude them from having a WEP rating.

If you want to talk about endurance running, there were Merlins run at combat power for 100 hours solid without maintenance as part of the development programme. As I have said, the limit to running most WWII engines at high power was getting a reasonable overhaul life (c.150 hours) reliably.

Overcooling is generally not a problem for liquid cooled engines because you can not only shut the radiator, but if needs must you can shut down the water pump.

Air cooled engines can have real trouble during extended descents, especially if they are fitted with turbochargers, because you have to be careful to avoid shock-cooling the turbo as well as to avoid shock cooling the main engine itself. The need to keep power on during the descent can push some aeroplanes quite close to VNE during descent, and can be a design driver.

The big problem for a WWI pilot was engine management. Many if not most aeroplanes lacked even the most basic automation. Instead of a throttle, the pilot would have separate controls for air and fuel, and possibly a fine adjustment or two. The controls would have to be tweaked to maintain optimum output (indeed to maintain any output) as airspeed and altitude were changed. One of the main reasons for "blipping" to descend rather than reducing the steady-state engine power was to avoid "loosing" the engine settings for high power. It would be very easy to forget about the engine and let it get out of its operating limits due to a change of airspeed or altitude in combat.

Another problem was that because fuel wasn't especially well standardised in those days, different batches of fuel could have very different characteristics, especially with regard to octane number and volatility. So a new fuel delivery could considerably change the behaviour of the engine, and might do so in a non-linear way with respect to altitude.

Throw in relatively poor instrumentation, fatigue, hypoxia, hypothermia and a high workload dealing with questionable aircraft handling characteristics, an absence of navigational aids, combat stress and so on, and the potential for inadvertent mishandling is obvious. I believe that such inadvertent mishandling was probably at least partially responsible for many failures, though that isn't quite the same as crying "pilot error", because the avoidance of error would often have been somewhere between unreasonably difficult and impossible...

Manu-6S
08-28-2009, 10:16 AM
Originally posted by Viper2005_:
Overcooling is generally not a problem for liquid cooled engines because you can not only shut the radiator, but if needs must you can shut down the water pump.

Air cooled engines can have real trouble during extended descents, especially if they are fitted with turbochargers, because you have to be careful to avoid shock-cooling the turbo as well as to avoid shock cooling the main engine itself. The need to keep power on during the descent can push some aeroplanes quite close to VNE during descent, and can be a design driver.


Infact these are the parts that IL2 lacks.

I've bolded some words.. in real life "you can" ("you should" IMO) but in IL2 "you don't care"

Also for the aircooled engine... it was more complex that our game. In that aspect I rate RoF EM more realistic than the IL2's one (but again IL2 is 8 years old)

Kettenhunde
08-29-2009, 04:59 AM
avoid shock-cooling

This is a huge debate that is still rages on Pilot discussion boards.

If "shock cooling" was true, what happens when you start and stop your engine on a cold winters day?

Lycoming recommends avoiding a CHT temperature change of 50 degrees in one minute. I have never seen such a large change even pulling to idle and diving on descent from a properly set up engine compartment. The most I have ever seen is ~30 degrees.


If shock cooling were a definite hazard, your engine should fall apart when you bring the mixture into idle cutoff at the end of a flight. CHTs fall at a rate of 100°F/min or more in the first seconds of shutdown—triple the rate that starts the typical "shock cooling" annunciator blinking. Does anyone complain that repeated shutdowns are causing head cracking? Of course not.

http://www.avweb.com/news/maint/182883-1.html


Manufacturers have long recommended keeping CHTs under about 400 deg. F in cruise because doing so has a positive effect on durability. Collected data highly support that idea; many are more conservative and adopt 380 deg. F as a better "red" line. There is compelling evidence that if one has CHTs under about 420 deg. F that it's just not possible to shock cool anything.


http://goliath.ecnext.com/coms...ngine-myths-the.html (http://goliath.ecnext.com/coms2/gi_0199-6732264/Top-five-engine-myths-the.html)


While the subject is controversial and hotly debated, many believe shock cooling, as commonly explained, is nothing but a myth. This position is supported by the fact twin engine planes commonly experience ideal conditions for shock cooling during simulated, single engine failures, yet statistically show no difference in wear or damage distribution between engines. Equally, it has been pointed out the rate cylinder head temperatures drop off after a normal engine shutdown is often much faster than the usual rates deemed to present a shock cooling risk. Furthermore, others believe damage usually associated with shock cooling is actually caused by rapid throttle changes where fuel, which has been supercooled during high altitude flight, is introduced into a very hot engine cylinder during descent, where rich of peak (ROP as opposed to lean of peak LOP[2]) operation is considered the norm, thus causing higher operating temperatures. In fact, it is well established, high operating temperatures in of themselves, can contribute to excessive component wear and damage, which is typically associated with "shock cooling". Given the available data, it strongly suggests "shock cooling" is nothing but a myth, at least in the context as commonly explained. Nonetheless, the topic will remain highly controversial and surely continue to be hotly debated well into the future.

http://en.wikipedia.org/wiki/Shock_cooling_(engines)

I think you might be thinking of a cooling imbalance. This is serious issue in aircooled engines. Torn baffle material, damaged baffles, and improper baffling can all cause a temperature imbalance. When one bank or even individual cylinders is cooler than the others, it creates power robbing friction.

Extending the engine compartment ~6 inches in the FW-190 series introduced a cooling imbalance which robbed the motor of ~150hp. The answer was the fitting of an attenuating ring which restored the balance increasing performance.

If our engine compartment is correct, we will not have this issue. It is not a normal condition and once identified will be rectified by the design team.

All the best,

Crumpp

Kettenhunde
08-30-2009, 10:16 AM
Torn baffle material, damaged baffles, and improper baffling

"Torn baffle seals, damaged baffles, and improper baffling design" I think is clearer.

Baffles are the hardn structure and the seals are attached too. Seals are typically silicon or composite rubber compound. In WWII it was not uncommon for them to be asbestos impregnated vulcanized rubber.

Blutarski2004
09-01-2009, 06:14 AM
Excellent thread!!!!!


+100

PanzerAce
09-01-2009, 11:42 PM
I think you might be thinking of a cooling imbalance. This is serious issue in aircooled engines. Torn baffle material, damaged baffles, and improper baffling can all cause a temperature imbalance. When one bank or even individual cylinders is cooler than the others, it creates power robbing friction.

Not just aircooled engines, and not just ancient engines either. With liquid cooled engines, the interior coolant passage design can also lead to problems. Datsun/Nissan L-6 motors for instance have problems with the cylinder head developing a hotspot on the #5 cyilnder, which becomes the limiting factor on many of the high horsepower builds.

Kettenhunde
09-02-2009, 09:14 AM
With liquid cooled engines, the interior coolant passage design can also lead to problems.

Certainly!

DrHerb
09-02-2009, 09:51 AM
My opinion on this topic is this. Pilots in WW2 would not be too concerned in overcooling their engines. Overheating would be a much larger concern for them. There were engines aplenty in the war anyway. The only cooling effect I would think would be a concern, is carb icing.

danjama
09-02-2009, 10:26 AM
I am shocked. I've actually found an interesting topic on the ubi forum. First in a while. Good stuff here.

Dash_8
09-02-2009, 11:37 AM
As a pilot in RL, I too have heard the debate over 'shock cooling'.

Nine years ago I was flying a Piper Navajo Chieftian for an engineering firm. The Chief Pilot was a believer in the shock cooling myth. One day when I flew with him, I pulled the throttles back and dropped about 20 inches of Manifold pressure in less than a second. I thought he was going to have a heart attack he was so mad! It never did hurt the engine.

Our mechanic said what was posted above, that it's no worse than shutting down the engine and shock cooling is a myth.

On a side note, there was one mistake I made when I was new to the Navajo and could have damaged the engine (well, the Turbo to be exact.)

Our mechanic had just done some work on the magnito of the left engine and needed me to do a run-up. So I started the engine and ran the power up to do the magnito check. It passed. He then wanted to see it at full power. So, I pushed the throttle all the way up. The Navajo pulls 49 inches of Manifold Pressure at full power (if I remember correctly, its been awhile.) The mechanic was pleased with run-up and started to head for the door. I figured thats it, idled the throttle, and pulled the mixture to shut down.

I have never seen a mechanic get so angry! His concern was for the turbo charger. As he put it, I just had that turbo spinning at over 30,000 RPMs at full power and just took the oil pressure away by shutting down the engine without letting the turbo spin down. Now its out there spinning with out any lubrication and could damage the bearings.

I don't know in game where this would be applicable, but it is something that a pilot of a turbocharged warbird would have to be aware of.

PanzerAce
09-03-2009, 01:08 AM
Dash, it was probably spinning a good deal faster than 30,000rpm if it was anything like auto turbos http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

While an *immediate* shutdown like you describe is bad, it isn't the worst that can happen. I don't know if the condition would ever happen in aircraft, but in cars, running the turbo alot (say, highway), pulling off in a rest stop and shutting down the engine for awhile. Say hello to coked oil, and a turbo that probably just had it's useable life cut in half. That's why water cooled BB turbos (rather than oil cooled plane bearing turbos) are the hot thing for cars these days (gassers especially. Diesel burners get to cheat a fair bit with far lower EGTs)

BillSwagger
09-03-2009, 01:21 AM
im no aircraft mechanic, but don't some airplane turbos also have their own separate oil system, including a cooler, and pump?
If the engine is shut off, the momentum of the turbine is still going to pump oil through its own system, right?

I guess it depends on the bird.

Bill

PanzerAce
09-03-2009, 02:56 AM
Considering that turbos are suppose to be spinning really fast while balanced, I doubt that any of them are going to be hooked up to an oil pump. The only thing that oil is going to do in the turbo when the engine is shut off is drain, and if the engine is hot enough, heat soak until it starts coking.

BillSwagger
09-03-2009, 04:30 AM
There are separate oil systems for turbo chargers and i can see easily how a turbine can remain balanced while the center axle is also geared to turn a an oil pump. Even with the engine shut down, the momentum of the axle would keep oil pumping through the system.
I think it has more to do with components like turbochargers being placed away from the engine, and not actually being attached to it other than through some ducting. That very premise means it needs its own oil system.

Cars turbo systems are much different in this way.

Certainly not the case for every plane, but when you have your engine in the front and the turbo behind the pilot it is very necessary.

Dash_8
09-03-2009, 06:28 AM
Say hello to coked oil, and a turbo that probably just had it's useable life cut in half.

This is probably what our mechanic was concerned about when I shut it down immediately. Like I said, it was 9 years ago when I flew piston engine airplanes and don't remember all the details of the systems. I know some aircraft have a separate oil system such as the P47 since the turbo is in the tail and ducted up to the engine up front, but the Navajo isn't like that. If I remember correctly, the turbo is on the back left part of the engine.

I've been flying DeHavilland Dash 8's for the past 8 1/2 years so I'm more up to date and familiar with turboprops now than turbochargers.

Outlaw---
09-03-2009, 08:48 AM
IIRC, P-38s had issues with over cooling but it was related to the speed control on the turbo chargers. When the oil in the governor was too cold the turbos could over speed. I think the max RPM of the turbos was 26,400 RPM at full WEP.

WW-II aircraft turbo chargers turned much slower than auto turbo chargers which can easily surpass 100,000 RPM. The aircraft turbos were HUGE compared to auto turbos and needed to be as reliable as possible which, for the day, meant running slower.

--Outlaw.

yuuppers
09-03-2009, 12:16 PM
I have the max rpm of the GE turbochargers at 22,000rpm. Not sure if this was for all models though. Overspeeding was usually caused by a stuck waste gate. Overhaul times were typically 400hrs.

PanzerAce
09-03-2009, 06:07 PM
Originally posted by Dash_8:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Say hello to coked oil, and a turbo that probably just had it's useable life cut in half.

This is probably what our mechanic was concerned about when I shut it down immediately. Like I said, it was 9 years ago when I flew piston engine airplanes and don't remember all the details of the systems. I know some aircraft have a separate oil system such as the P47 since the turbo is in the tail and ducted up to the engine up front, but the Navajo isn't like that. If I remember correctly, the turbo is on the back left part of the engine.

I've been flying DeHavilland Dash 8's for the past 8 1/2 years so I'm more up to date and familiar with turboprops now than turbochargers. </div></BLOCKQUOTE>

yah, oil coking sucks in a car, but it's probably downright dangerous in a plane.

And thanks for the info guys, didn't know that even modern aero turbos were going to be spinning that slow.

Kettenhunde
09-03-2009, 07:29 PM
The P47 series had a carburetor, a supercharger, and a turbocharger.

While nothing restricts the use of the turbine at sea level, it is not normal procedure except for a short field take off. As part of the pre-flight run up, the pilot sets the throttle to maximum take off manifold pressure with the supercharger on. This is because of density effects of the atmosphere. He sets the point at sea level on the throttle as his maximum throttle position. Above 7000 feet he can use the throttle to the stop gate.

On landing, the turbine is shut down as part of the approach procedures. The aircraft is flown on the supercharger or carburetor depending on conditions. When operating with the supercharger, the throttle has to be moved slowly to prevent over boosting.

Personally, I would use the carburetor so I could just firewall it if I needed power quickly to go around.

The P-38 series had an automatic regulator that turned on the turbo-supercharger as needed. On approach the pilot is instructed to slowly ****** the throttles keeping manifold pressure above 15" InHg. If the pilot has to make a go around, the throttles must be advanced smoothly and not slammed forward instantly.

All the best,

Crumpp.

PanzerAce
09-03-2009, 08:27 PM
Ah, the days before they had turbo surge figured out. But hey, better to slowly move the throttles then to start snapping turbine blades.