Shock Cooling

[Lurch]

I saw this term used in a post recently and thought it might be a interesting discussion.

What is "Shock Cooling" and is it really a problem?

Has anyone ever seen a cylinder crack due to shock cooling?

If shock Cooling is such a threat why don't we worry about shock heating? Your engine goes through a more drastic increase in temperatures during take off then it ever would by reducing the powers quickly.

Is this all just a flight schoolism or an old wise tale that has no true bearing on how we should fly then airplane?

P.S lets keep this to fuel injected, Normally aspirated engines, the others just complicate things.

Lurch

[trey kule]

There are two ways in which you can shock cool and engine. The first involves simply pulling the power back while flying in cold air. Is it true. According to some yes. According to some people I consider experts, only marginally so. In any event it does not hurt to reduce the throttle in small increments rather than just bringing it to idle. Takes a bit more planning.
The other way one can shock cool which, according to my sources, is much more critical,, is by enriching the mixture to fast. Introducing a whole bunch of extra fuel that is very cold. Students are taught generally that prior to a descent they should put the mixtures to rich. I think the GEM people even have a video on it. Certain engines also have limitations on how low you should let the EGT go on a descent, and recommend that the mixture (in most cases) be at full rich. In the interest of "safety" many FTU's simply say mixture to rich..and in fairness to them many do not have an egt.

The whole issue is generally mute if you fly with finess and professionalism, as you wont find many instances where it is necessary to bring the throttle back from cruise to idle quickly or enrich the mixtures instantly to full rich.

I know you asked to limit the engine type, but particularily on geared engines and some of the larger horizontally oposed engines reducing the throttle to fast can result in the engine acutally becoming a pump.

so the (not anymore) short answer is I really dont know but generally this is onlywould be an issue if you are hamfisted with the mix and throttle controls.

[iflyforpie]

Well, I have seen jugs crack and can attribute it to shock cooling (front jugs). The culprits are usually Continental engines, probably because of the large number of fins they have compared to Lycomings.


The problem is you have a jug made out of two different materials. An aluminum head that is screwed and shrunk onto a steel barrel (all barrels are steel regardless of whether they are nitrided, chrome, ceramic, or other on the inside). Aluminum has a higher thermal conductivity than steel and a higher coefficient of expansion. So, putting the coals to the engine on takeoff, the head expands at a greater rate than barrel, no big deal because the head is on the outside.

The aluminum piston in the steel choke bore doesn't matter either because the clearances are so ridiculously loose. But because the clearances are loose you can get piston slap if the engine it too cool when you apply takeoff power (a ROT I use is two minutes and 200F CHT before going above 1000 RPM).

Now lets take the power off. Now the aluminum cools and shinks at a faster rate than the barrel. The heads are also generally where most of the cooling surface area is (largest fins) and aren't heated by another source (there is still friction and oil splash on most of the barrel, plus residual heat from the crankcase that will prevent it from cooling as fast). The head barrel interface can then become too tight. Since the head is a casting of a complex shape, it can be very prone to cracking.


We go 3000 to 10000 back down to 3000 with Continental IO-520s and IO-360s and we have not had a cracked jug in four years by stage cooling (inch a minute) and leaving mixtures lean to keep EGTs up. We target 380F for CHTs and keep them above 300F in descent when possible.

[Lurch]

Ok, let's follow your train of thought and of course ignore the bad pilotage apsect, that would defeat the purpose of this thread.

So I'm at cruise power and pull the throttles to idle, Let's take 350 Degree CHTs and 1445 EGTS, How much of a drop in CHTs do you believe will occur?

Next question, what in the design of the piston engine causes the damage?

I didn't want to bring in other engines because I wanted to avoid the difficulties that the turbo, geared engines and carbs can create and confuse the issue. Once we've beaten this horse we can move on to engine torques and turbo over heating.

Lurch

[trey kule]

Lurch, it is not so much about how much the temps drop, but the shock of being hot and then suddenly being shocked by cold air or cold fuel.
If I recall, egts should be kept around 1300-1400 degrees on most engines Do that, and slowly reduce the throttle, as Iflyput it, (though for a number of reasons which are not germain to this thread I dont like his formula), and you should have no issues.
No egt....then you must do things slowly in increments as much as possible.
It also, when flying in the winter, helps to have a proper winter kit installed.(not some home made thing...an yah I have used duct tape ...but it is not a good idea.)

[iflyforpie]

Trey, you probably don't like my formula because you probably aren't flying NA engines at 10,000. WOT for me is 20", and I actually do most of my descent at 18".

[Lurch]

Pie was a little too quick on the technical, I wanted to save this for a bit but I'll throw it out there now.

P.S I'm not stating I agree or disagree yet, just stirring the pot.

Shock Cooling: Myth or Reality? Email this article |Print this article

Powerplant management guru Kas Thomas of TBO ADVISOR examines the physics and metallurgy of "shock cooling" and concludes that, contrary to the conventional wisdom, it is not a major contributor to cylinder head cracking.
February 19, 1997
by Kas Thomas

This article first appeared in the March 1996 issue of TBO ADVISOR and appears here by permission. The article is Copyright 1996 © Kas Thomas.

Not long ago, a writer for a major aviation publication called to ask my opinion(s) on the subject of shock cooling. It turns out the caller had already written his article, but he wanted to run some ideas by me to make sure he wasn't missing something. Since I get a lot of calls on this subject, I had some ready answers for him. Not necessarily correct answers—just ready answers.

I don't think anybody has provably correct answers to questions involving shock cooling of aircraft engines. To my way of thinking, there is no scientific proof that shock cooling plays a significant role in cylinder damage in aviation. "Scientific proof" is perhaps a poor choice of words. What I'm simply trying to say is, the hard evidence is scanty. I know of no fleet studies on this subject. I know of no pilot who can say "I went up and did this and this and this to the engine, and then when I landed I found these cracks that weren't there before."

Still, it's hard to argue with common sense, and common sense says that if you thermal-cycle a piece of cast aluminum (especially while beating on it!) you just might induce it to crack. Pilots can perhaps be forgiven for harboring a strong gut feeling that yanking the throttle back is a good way to bring on cylinder cracking. Certainly, many millions of dollars' worth of spoiler kits and CHT systems have been sold to pilots on this basis over the years.

My own gut tells me that shock cooling—while bound to induce dimensional changes in the engine—is not a great contributor to cylinder cracking. We know it induces dimensional changes, because (for example) valve sticking has been induced in some engines by sudden power reductions. (A Lycoming Flyer article once stated: "Engineering tests have demonstrated that valves will stick when a large amount of very cold air is directed over an engine which has been quickly throttled back after operating at normal running temperatures." See 101 Ways to Extend the Life of Your Engine, page 96.) But it's a big jump to go from that to saying you can make a cylinder head crack just by pulling the throttle back too quickly.

To my knowledge, Bob Hoover has not experienced any problem with cylinder-head cracking on his Shrike, despite his rather odd predisposition to feather both engines while in a redline dive. (Maybe this is what FAA meant by "cognitive defect"? Just kidding.)

Besides which, I think any careful examination of the concept of "cooling" (as it applies to current aircraft engines) will leave one virtually empty-handed, because I think it could be argued that cooling fins on aircraft cylinders are of mainly ornamental value. I suspect that you could hacksaw much of the finnage off, say, a TSIO-520's cylinders and not affect inflight CHT readings by very much. As it happens, this is exactly what Continental did when it created the "lightweight" Crusader engine—the TSIO-520-AE used in the Cessna T303. The cooling fins on this engine are fewer in number, and about half the size of, those on a standard TSIO-520. And yet, CHTs in the T303 are remarkably cool. (One of our readers, in fact, reports a problem in getting CHTs to stay in the green; see this month's "Questions and Answers," page 26.)

Various investigators have done "energy balance sheets" on aircraft engines, and the result is always the same: Only about 12% of the heat energy generated in combustion goes out an "air-cooled" engine's cooling fins. The biggest fraction (around 44%) goes right out the exhaust pipe, of course. Another 8% or so finds its way into the oil—which is quite interesting, because it means the oil plays almost as big a role in cooling your engine as air does. The remaining energy shows up as work at the crankshaft.

Throttle placement doesn't have nearly as direct an effect on CHT as you might think. Back in 1983, there was an SAE paper (No. 830718) by three Texas A&M researchers who tried to correlate OAT (outside air temp), CHT (cylinder head temp), EGT (exhaust gas temp), power settings, air density, and cowl pressure drop in Lycoming TIO-540 engines. Their work was partly based on the NACA Cooling Correlation (NACA Report No. 683, published in 1940), which in turn was based on pioneering work done by Fred Weick in the late 1920s. The Texas A&M group merely extended NACA's approach, verifying their results with inflight measurements taken on a Piper Turbo Aztec and a Rockwell 700. One of their key findings was that the difference between CHT and OAT is proportional to the difference between EGT and CHT, which is (if you dwell on it long enough) intuitive, since the difference between the average exhaust temperature and CHT is what "drives" CHT changes to begin with. (If this isn't intuitive to you, you may want to go back and re-read Fourier's classic Analytic Theory of Heat.) This portion of the group's findings might be summarized by saying that the stored heat of the cylinder head is proportional to the input heat, represented by EGT minus CHT.

But there are two aspects to cylinder cooling. One is the "supply side" aspect (which we have just been taking about—all this business about EGT minus CHT), while the other is the taking-away of heat, or "cooling" aspect. The Texas A&M group accounted for this too. They found that the stored heat is proportional to the input heat—proportional, that is, by a factor y. The factor y, in turn, is made up of engine power raised to a certain exponent, divided by cooling airflow delta-p raised to a certain exponent. The engine-power exponent is fractional; for the Rockwell 700 it turns out to be 0.33. (It varies from plane to plane depending, apparently, on peculiarities of engine installation and operating envelope.) The air-cooling delta-p exponent is also fractional (0.29). In plain English: CHT depends on the cube root of engine power, divided by the cube root (roughly) of the cooling-airflow pressure drop.

After a few rough scratchpad calculations, you find that cutting an engine's power by half (but leaving airspeed constant, such as in a descent) results in a CHT drop of only 10% or so, or about 80¡ F. (Recall that in calculations of this sort, you want to use a Rankine temperature scale, which begins at absolute zero, or minus-460°F.) Most of the time, a 50% power cut is accompanied by some loss of indicated airspeed, which would tend to offset the CHT drop, making it less than 80° F. The numbers are within reason, evidently. But is this kind of CHT drop capable of trashing a set of cylinders? I doubt it.

Of course, the rate of the drop is plainly an important factor here (not just the magnitude of the drop). In this connection, I am reminded of an experiment once done by John Schwaner (of Sacramento Sky Ranch). It seems Schwaner, curious as to whether he could "crack" a cylinder at will, in a shop environment, one time took a cylinder that was heated to several hundred degrees in an oven (I believe it was an O-320 jug, although here I'm going from memory) and dunked it in a bucket of cold acetone. The abruptly cooled cylinder was later examined, and no abnormalities could be found in it.

And then there's ordinary rain. Every pilot flies through rain at one time or another, and rain should be a very effective coolant (more so than mere air, certainly)—yet no one, as far as I can determine, ascribes cylinder damage to flying through too much rain. In fact, most pilots (I think) consider just the opposite to be true; namely, that flying through rain is good for an engine, because of the extra cooling.

Let us assume that a moderate downpour contains one cubic centimeter (one gram) of water per cubic meter, and let us further assume a cooling airflow of 100 cubic meters per minute for a high-performance engine. (David Thurston's Design for Flying suggests 77 cubic meters per minute as typical for many engines.) We might reasonably expect, therefore, that 100 grams of water might enter the cowling per minute while flying in rain. Considering that water has a heat of vaporization of about 540 cal/g, it's not impossible for 100 g/min of rain influx to give about 54,000 cal/min of cooling, which is about 200 British Thermal Units per minute.

The question is, how does this compare with the heat of combustion? We can do a rough calculation this way: We know that (by ASTM spec) avgas contains a minimum 18,720 BTU per pound or about 112,320 BTU per gallon. If an O-470 burns 13 gal/hr in cruise (or 78 lb/hr, roughly), the engine is capable of producing 24,336 BTU per minute of combustion heat—if combustion is 100% efficient. In the real world of mixture maldistribution, rich mixtures, and incomplete combustion, we can safely say that probably no more than 21,000 BTU/min of heat is actually liberated, of which 12%, or 2,520 BTU/min goes to the outside world via the cylinder cooling fins. If rainwater cooling was 100% efficient (no droplets escaping between cooling fins; all of the water 100% evaporated in contact with fins), we might expect rain to reduce the cylinder fins' burden by about 8% (200 divided by 2,520). If you could somehow translate this into a direct CHT reduction, it might mean a reduction of 64°F (assuming your CHT started out at 800° Rankine). That's a pretty sizable reduction of CHT. In fact, it should qualify as shock cooling.

I think the fact that Navajos and 421s aren't raining engine parts down on unsuspecting civilians while flying through precip (I was going to say while penetrating virga—but decided against it) is pretty good evidence that "sudden cooling" of an air-cooled engine does not contribute in any dramatic way to cylinder-head cracking.

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.

Then why are we worried about pulling the throttle back?

[Lurch]

We go 3000 to 10000 back down to 3000 with Continental IO-520s and IO-360s and we have not had a cracked jug in four years by stage cooling (inch a minute) and leaving mixtures lean to keep EGTs up. We target 380F for CHTs and keep them above 300F in descent when possible.

How long does it take for you to descend the 7000'? also how long does it take for your engines to cool from the 300 degrees to 100 upon landing/shutdown?

Lurch

[iflyforpie]

Interesting read.

I never noticed for CHTs on shut down, but it goes down to 300F pretty quickly on initial power reduction since we also speed up quite a bit coming down. Every decent I do is a fight against excessive cooling (saving the engine), excessive speed (saving the airframe), excessive vertical speeds (saving the ear drums), and excessive track miles (saving the company).

You won't ever get below 200F on a running engine, but I don't buy the 'shock cooling on shut down' story. 100F per minute sounds very fishy. Jugs are still very hot to the touch even a half hour later (ask me how I know, a great way to keep guitar calluses up is removing hot plugs with bare fingers).

There is no cooling air going over the cylinders so the aluminum by virtue of its excellent thermal conductivity might retain more heat. Also the engine isn't getting pounded by explosions after shutdown, which was one of the things alluded to in the introduction (heating and cooling and hammering aluminum). The Comet passed all of its pressurization tests, but they were not done with flight loads on the test subject, a fatal omission.

I am curious if you could actually see the difference in CHT when flying through rain (I've never, but then I'm usually looking outside). Or whether by virtue of mass (think PT6 particle separator) if most rain would wind up on the baffles or cowling, especially on an upflow cowl like a Navajo, Commander, or old Aztec. Of course a West Coast Beaver is going to get a ton of rain and maybe even some spray on the engine.

I don't think that every cylinder will crack by shock cooling (as many forced approaches and in-flight shutdowns prove) and I do believe a certain number are pre-destined to crack. But I think that being nice to your engine might close one of those swiss cheese holes. ~$1500+ for a jug plus at least a half day to re and re is nothing to scoff at, and having one engine keeping you safe encourages me to treat it nice even more.

[Hedley]

I never bought into the "reduce an inch of MP a minute" nonsense. You don't limit yourself to an inch a minute increase on takeoff, do you?

You can do horrible things to a Lycoming. Doing aerobatics you sure don't limit yourself to an inch a minute, and also the airflow through the engine varies wildly (eg hammerhead) which is certainly going to contribute to shock cooling/heating, at least as much as throttle position. Aerobatic airplanes rarely have cracked jugs - it's always other stuff that goes first on the engines (camshaft, other internal stuff).

You want to shock cool your engine? You don't need to do aerobatics. Just level off. Seriously. Your engine will be cooking in the climb - full power, limited air through the cowling at slow climb speed. Just nose over and drastically increase your airspeed and the mass of cold air aloft through the cowling. And, no one worries about that, ever.

[RenegadeAV8R]

Shock Cooling: Myth or Reality? Email this article |Print this article

Powerplant management guru Kas Thomas of TBO ADVISOR examines the physics and metallurgy of "shock cooling" and concludes that, contrary to the conventional wisdom, it is not a major contributor to cylinder head cracking.
February 19, 1997
by Kas Thomas

This article first appeared in the March 1996 issue of TBO ADVISOR and appears here by permission. The article is Copyright 1996 © Kas Thomas.

This article, posted by Lurch, is also part of the book titled Fly the Engine, also by Kas Thomas.

[Big Pistons Forever]

I only have anecdotal evidence but I once flew a C 421 that went 1400 hrs with no cylinder, exhaust,or turbo work. It was only flown by 3 pilots over that period and all carefully managed the engine temperatures in all stages of flight by judicious use of staged power reduction, careful use of the mixture control and airspeed management.
At the same time another 421 operator on the field who did not believe in stage cooling, favored high speed descents, and aggressively leaned the mixture in cruise but went to full rich at the top of descent, went through 2 rounds of cylinders on both engines ........

I see a lot of pilots of turbocharged piston twins who think as long as they are doing the 2in of MP a minute thing they are taking care of the engine. But to ensure trouble free operation you have to manage the temps from start up to shut down and intrinsic to this is proper use of the mixture control, something that IMO is often poorly understood by a lot of the folks flying Ho's and the like.

[Hedley]

carefully managed the engine temperatures in all stages of flight by judicious use of staged power reduction, careful use of the mixture control and airspeed management

Sure, but you don't know which techniques were important, and which ones didn't make any difference. All pilots could have worn purple hats, but that probably didn't affect the cylinder cracking.

I have forty years of experience with the same cylinders in my Maule, which has the TCM IO-360, which has an absolutely horrible reputation for fragile cylinders (see Mooney, Seneca twin) much like the GTSIO-520 in the C421.

I still have all the original 43 year old cylinders on the Maule engine. Not one jug has cracked, and as I said above, I really don't believe in many of the shock cooling myths that everyone else here does.

Also, you simply wouldn't believe what we do to Lycoming AEIO-540's, and those jugs never crack, either. All aerobatic pilots break all the shock-cooling rules, and we don't crack jugs.

TCM engineers say the inch a minute myth is complete hooey. They say the rich mixture, dumping cold gas in the GTSIO-520 cylinders, is what cracks them. I believe the TCM engineers, even if no one else here does. I land the 421 with the props at 1800 rpm (cruise) and mixtures leaned, unlike everyone else here. No cracked jugs on the 421 so far.

[MichaelP]

There's nothing on here that would make me change my mind about how carefully one must treat an engine.
I watch CHTs and I'm very careful how I treat an engine in cold air and somewhat less so in warm air.
Bob Hoover's displays were seldom or never done in the winter...

There's thermal shock... that might be one thing, but then like when one takes a bit of metal and bends it until it breaks thermal induced fatigue is a real possibility.

Regardless of what is real and what is false, treating one's engine with care and respect is the best way to assure a long life for both yourself and the aeroplane.

[fish4life]

on this topic I saw one with liquid cooled engines which model was that?

[MichaelP]

Probably the Rotax 912 series of engines.
These have liquid cooled heads and air cooled cylinders but we're not writing about modern engines on this forum.

The Rotax certified V6 engines in the 200 - 300hp range could have revolutionised aviation by providing modern efficient powerplants to replace the Briggs and Stratton lawnmower motor derivatives most of us fly behind these days...
Liquid cooling ends almost all thermal problems associated with old air cooled engines.

[trey kule]

TCM engineers say the inch a minute myth is complete hooey. They say the rich

mixture, dumping cold gas in the GTSIO-520 cylinders, is what cracks them. I believe the TCM engineers, even if no one else here does. I land the 421 with the props at 1800 rpm (cruise) and mixtures leaned, unlike everyone else here. No cracked jugs on the 421 so far.

"no one else here does"????? Interesting conclusion on what I posted.

[Hedley]

Sorry - do you fly a 421, and land it with the props at cruise RPM, and the mixtures leaned?

I do it, but I have never heard of anyone else doing it. TCM says they have great difficulty getting part 135 operators to get such a procedure approved by the FAA. I am sure Transport would be similiarly hostile.

[costermonger]

Sorry - do you fly a 421, and land it with the props at cruise RPM, and the mixtures leaned?

AFAIK, that's how it's done on the 421 here. How I do it in the PA-31 too.

[trey kule]

Not on the 421 to answer your question, but your comment sounded a bit more general than that. My error if I misunderstood.

I do have a few hunderd hours on geared engiines similar to,, I believe those in the 421 ( mine in 541's..), and as someone else mentioned the technique is also used for navajos and several other planes, both single and twin. It is taught as a technique at flight safety, so its not big secret.