Engine (shock) cooling what's the deal
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Marine engines see use profiles much closer to aircraft engines than cars. Until relatively recently this wasn't too relevant as weight concerns weren't a major factor in the marine business so the engines didn't have much in common with aviation. For good safety reasons marine engines are almost all diesel except where power to weight was a major concern. Cooling can simply be managed through using a thermostat to dump the heat in the primary cooling circuit into the sea.
However this is changing: modern high performance, turbo and/or supercharged, diesels have appeared over the last decade or two. I've some experience with the steyr straight six monoblock engine that's got variants in the 2-300hp range. Our use profile is very demanding, flat out for ten mins or so then to idle and worst case to cold before starting again, and our problem was with shock heating (presumably 'cos the thermostat and liquid cooling smoothed out the cooling). We needed a steady supply of spare engines as we'd kill them regularly with only a couple of hundred hours on them, if that. Happily the boats are twins... Anyway the problem was massively reduced by running shore powered electrical block heaters. The primary cooling circuit is now kept at running temp and things are much happier. I read somewhere that the new six pot diesels being played with by Austro are based on a steyr engine.
However this is changing: modern high performance, turbo and/or supercharged, diesels have appeared over the last decade or two. I've some experience with the steyr straight six monoblock engine that's got variants in the 2-300hp range. Our use profile is very demanding, flat out for ten mins or so then to idle and worst case to cold before starting again, and our problem was with shock heating (presumably 'cos the thermostat and liquid cooling smoothed out the cooling). We needed a steady supply of spare engines as we'd kill them regularly with only a couple of hundred hours on them, if that. Happily the boats are twins... Anyway the problem was massively reduced by running shore powered electrical block heaters. The primary cooling circuit is now kept at running temp and things are much happier. I read somewhere that the new six pot diesels being played with by Austro are based on a steyr engine.
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I don't think that there is much of a basis to compare liquid cooled marine engines to air cooled aircraft engines in terms of cooling. The massive cylinder head of a marine engine has the thermal mass to resist/absorb temperature changes which result from changed operating conditions, so they cannot occur rapidly.
Air cooled engine cylinders are thin and light by comparison, and when operated carelessly, can change temperature hundreds of degrees in a few seconds. Worse, this temperature change will occur in some places in the cylinder, but not so much in others. This caused stresses within the cylinder structure, and is when the damage can occur.
Air cooled engine cylinders are thin and light by comparison, and when operated carelessly, can change temperature hundreds of degrees in a few seconds. Worse, this temperature change will occur in some places in the cylinder, but not so much in others. This caused stresses within the cylinder structure, and is when the damage can occur.
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Hi All, the mechanism of the shock-cooling is a little complicated to decide.
At first, you would think it was due entirely to the cold high altitude air cooling the cylinders fins from the outside.... However that has remained constant even whilst under power.
The only real change (dT/dt.) has been to the internal surfaces of the cylinders having their source of heat (fire) removed and replaced by a cooling flow of non-combusted gas.
Also the metal mass of each cylinder / cylinder head contains areas that have different temperatures when in normal ops.... The cylinder walls are subject to the heat of combustion; The Inlet Valve area has the very cool inlet fuel/air mixture, with the Exhaust Valve in the hot exhaust gas flow.
So the temperature gradients are quite high in these areas. Any sudden attempt to bring them back to a uniform temperature is likely to result in shock-cooling, and not necessarily in the areas of the fins cooled by the airstream.
Just as an aside,,, In the motor-cycle world, Racers tend to run their fuel mixtures for max power, whereas owners of Concours machines run richer mixtures to prevent heat discolouration or rusting of the exhaust pipes.!
Pete
At first, you would think it was due entirely to the cold high altitude air cooling the cylinders fins from the outside.... However that has remained constant even whilst under power.
The only real change (dT/dt.) has been to the internal surfaces of the cylinders having their source of heat (fire) removed and replaced by a cooling flow of non-combusted gas.
Also the metal mass of each cylinder / cylinder head contains areas that have different temperatures when in normal ops.... The cylinder walls are subject to the heat of combustion; The Inlet Valve area has the very cool inlet fuel/air mixture, with the Exhaust Valve in the hot exhaust gas flow.
So the temperature gradients are quite high in these areas. Any sudden attempt to bring them back to a uniform temperature is likely to result in shock-cooling, and not necessarily in the areas of the fins cooled by the airstream.
Just as an aside,,, In the motor-cycle world, Racers tend to run their fuel mixtures for max power, whereas owners of Concours machines run richer mixtures to prevent heat discolouration or rusting of the exhaust pipes.!
Pete
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I guess all I was trying to point out is that they are converging. The steyr 'marine' block will be on airframes soon and it certainly can have some thermal management issues - even if they are heating rather than cooling, in my experience.
I terms of the original question: even relatively heavy liquid cooled engines can suffer thermal management issues so I'm certain that light, air cooled, aero engine can.
I terms of the original question: even relatively heavy liquid cooled engines can suffer thermal management issues so I'm certain that light, air cooled, aero engine can.
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P.s. how do you quote in a reply?
Click on that , it will put [quote] [quote] in the box. Copy & paste what you want to quote between these.
Marine engines see use profiles much closer to aircraft engines than cars.
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The hangar queen point is fair, especially since the demise of red deisel, but most small high performance marine engines get used for a few hours at a time: mainly at very high power settings. My point was that unlike cars they are routinely at 70%+ for several hours at a time.
I think I'm going to give in now!
I think I'm going to give in now!
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Do air cooled engines also have a problem with the airflow coming from the front causing differential cooling from the front of the cylinder to the back side? Presumably that could cause oval bores and stresses.
Would it be more serious on radials than flat or inline air cooled engines? I seem to recall reading something about more even cooling being one of the reasons for the early flirtation with rotary engines (as opposed to radials)
Would it be more serious on radials than flat or inline air cooled engines? I seem to recall reading something about more even cooling being one of the reasons for the early flirtation with rotary engines (as opposed to radials)
A system of baffles dirrcts air to ensure cooling on the backside of the cylinders. One thing that has not been mentioned is the Importance of baffle condition in controlling CHT's. I have seen many privately owned aircraft with cracked and bent baffles and/or tattered and torn flexible rub strips. Poor baffling will allow localized cylinder hot spots and increase inter cylinder CHT difference . This is one area where saving money on maintenence is a false economy.
In particular if you do not have the red silicone baffle rub strips, you should IMO fit them at the next annual.
In particular if you do not have the red silicone baffle rub strips, you should IMO fit them at the next annual.
Last edited by Big Pistons Forever; 28th Aug 2012 at 15:45.
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Baffles...
I have just been re-doing mine, using this material. Interesting claims about less vibration transmission than silicone, and I quite believe it.
The gaps visible in the pics do actually close up perfectly when the top cowling is on - this was verified with photos.
The CHTs dropped a good 10F, so that was well worthwhile.
I have thus far re-made just the top baffles; the bottom ones will need a hangar.
The gaps visible in the pics do actually close up perfectly when the top cowling is on - this was verified with photos.
The CHTs dropped a good 10F, so that was well worthwhile.
I have thus far re-made just the top baffles; the bottom ones will need a hangar.
Last edited by peterh337; 28th Aug 2012 at 17:38.
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Hmm, that looks quite interesting! my upper cowling visibly vibrates about 2mm in flight...
Do you reckon this could be installed on a G-reg TB10?
Do you reckon this could be installed on a G-reg TB10?
Last edited by ppl_fresher; 28th Aug 2012 at 20:10.
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Definitely. It's a "cosmetic" repair only.
BTW if you really have 2mm of vibration, something is wrong. Perhaps a part of the engine assembly is touching the cowling.
BTW if you really have 2mm of vibration, something is wrong. Perhaps a part of the engine assembly is touching the cowling.
Last edited by peterh337; 28th Aug 2012 at 21:37.
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Back to PiperBoy's original question
Big Pistons Forever's summary on page 1 dated 22nd June is about the most accurate and simple summary of engine care required I've come accross in decades.
From those I hear from that do this kind of thing for a living (engine development sorts) spam can engines can endure more abuse than it's possible to throw at them during normal (and accidentally abusive) use.
From those I hear from that do this kind of thing for a living (engine development sorts) spam can engines can endure more abuse than it's possible to throw at them during normal (and accidentally abusive) use.
