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View Full Version : Richest cylinder hottest? How can that be?


stepwilk
25th Apr 2010, 18:08
I'd ask him myself if I knew how to get in touch with him, but acknowledged engine expert Kevin Cameron, writing about the Wright R-3350 in the Aircraft Engine Historical Society's journal "Torque Meter," said that--I'm paraphrasing--"Everybody knows that the cylinder of an engine that is running richest is doing the most work and therefore is the hottest." (He's discussing the classic mixture/fuel maldistribution of the R-3350.)

When I fly a GA airplane with a multi-probe EGT, I can see that the hottest cylinder is the the one that reaches the _leanest_ permissible fuel-flow setting first, and I always thought that rich cylinders ran cooler because of the extra fuel being flowed through them.

Cameron is such an experienced engine guy--not just a writer but a hands-on builder and racer--that I'm having a hard time assuming he made an error. Is there something about big radials that I'm missing?

PLovett
25th Apr 2010, 23:03
SW, surely the hottest cylinder is not the one running at the leanest permissible fuel-flow because that would be running lean of peak and therefore running cooler than the others.

My limited understanding of radials was that they were run lean of peak and consequently were producing less power which was compensated for by increasing manifold pressure. The hottest cylinder was by definition closest to peak and therefore was the hardest working cylinder.

There have been a number of threads on PPRuNe about running lean of peak which have included contributions from people far more learned than I on the subject. However, the one thing that everyone has said on those threads is that there is no difference between operating radial and horizontally opposed engines when it comes to mixture control.

stepwilk
26th Apr 2010, 00:23
As a pilot, by "leanest permissible fuel flow," I don't mean lean of peak, since that is on most GA engines _not_ permissible, at least by typical standards. I would adjust mixture by leaning to peak on the hottest cylinder, then enrichening 50 degrees F by an EGT reading.

But my question remains: why would an accepted recip-engine expert say that the hottest cylinder would be the richest one?

Brian Abraham
26th Apr 2010, 01:16
The graph of temp/mixture ratio is an inverted "U". The suggestion the author is making would seem to be that the richest cylinder is operating on the lean side of the peak of that "U". In airline operations the engine was run LOP in order to get max range (see graph for HP/lb fuel/hour), in which case the statement holds. As a stand alone statement it makes little sense, without that sort of qualification.

http://www.avweb.com/newspics/194452_piston_combustion_graph.jpg

Stephan, as an aside you will find that many GA engines permit LOP operation, the problem is often you can't because of the terrible mixture distribution and the resulting rough running if you try.

stepwilk
26th Apr 2010, 15:33
I don't know of "many" GA engines that allow LOP operation. Last one I had anything to do with was the Continental in the Malibu, but as I remember, Piper decided that was a bad idea, assumedly because of the maldistribution you refer to. I believe engines that use George Braly's fuel-injection mods maybe can run LOP.

barit1
26th Apr 2010, 16:12
In small/medium size radials, the CHT thermocouple is always on the top cylinder(s) because gravity stratification in the intake manifold makes them the leanest, ergo the hottest.

stepwilk
26th Apr 2010, 16:57
That's very nice, but what about my original question? See post #1.

Brian Abraham
28th Apr 2010, 00:01
Last one I had anything to do with was the Continental in the Malibu, but as I remember, Piper decided that was a bad idea, assumedly because of the maldistribution you refer to
If the aviation industry ever needed a practical demonstration of the lack of knowledge among the community about leaning the Malibu was it. And the inability to read the POH and/or follow its instructions.

The TSIO-520BE (Malibu) Engine

That engine was designed for lean-of-peak (LOP) operations, and this led many pilots not familiar with LOP to run the engine rich of peak (ROP), outside the parameters recommended by the manufacturer. Early in the life of the Malibu, the TCM engine proved troublesome for many reasons beyond the leaning issue. Piston pins and crankshaft bearings, not to mention main bearings, turned the engine into a nightmare for many pilots, and led to an ugly battle between Piper and TCM.

The following is from John Deakin.

Let's look at the TCM "Maintenance and Operator's Manual" for the TSIO-520-BE engine, from February 1990.

This was the engine introduced in the Piper PA-46 Malibu, with some interesting results. ("Interesting" in the Chinese sense.)

On page 13-2 we find:

3. The maximum recommended cruise setting is 235 HP at 2400 RPM and 31.0" Hg. MAP with the mixture set at 25 F to 50 F lean of peak T.I.T. At cruise settings below 65% engine may be operated at peak T.I.T. or below if obtainable.

They actually got this one reasonably right, if you read it carefully. The engine is rated at 310 HP, so 235 HP is about 75%, and this is clearly lean of peak (LOP). By dropping the HP to 65%, it's OK to operate it slightly richer, at peak TIT.

Some of the blowhards who shout, "I wouldn't tell my worst enemy to run LOP!" and "Running LOP will burn up your valves!" just hate this engine, because they cannot explain it. There's nothing special about the basic parts, or the valves, or the metallurgy.

TCM probably used up the talents of the last true engineers in the development of this engine during heavy competition with Lycoming for Piper's powerplant selection for this new airplane (according to Carl Goulet, the long-time VP of engineering at TCM). Unfortunately, they missed one little detail. They thought they were getting uniform fuel flows to all intake ports (by the famous "coke bottle test" -- see sidebar above at right), but ultimately they did not realize that their stock nozzle configuration wasnt quite good enough to get balanced fuel to the combustion chambers. In missing this key point, they went at the problem backwards, and spent the better part of two years tweaking the induction system, adjusting the airflows to match the fuel flows and trying to bring the "fuel/air balance" into line. This worked fairly well, but it did leave some "unbalanced" power between the cylinders, which meant the engine wasn't as smooth as it might have been.

Having properly set the engine up to operate LOP, TCM decided they could squeeze one more bit of performance out of the engine by moving the spark timing away from their normal 20 or 22 degrees before top-dead-center (TDC) for turbocharged engines. They reset the timing out to 24 degrees TDC. No other TCM big-bore engine has timing set this far before TDC. But it was the right thing to do, because they had the engine set up for LOP operations, and they required LOP operation, where the fuel/air charge burn-times are relatively slow, which placed the peak combustion pressure far enough after TDC to be effective and safe.

This advanced configuration -- together with some of the old wives tales in GA -- led to a thought process in the pilot population that killed some people, and -- in some uninformed circles -- gave the engine a bad reputation. The thinking went something like this:

"Well, gee, the (holy) book says I can run 75% at 25F to 50F LOP. But I don't like the slight roughness I find there, and this LOP stuff scares the daylights out of me. I don't need to save every last gallon, and the wife wants to make a pit stop, anyway."

Sounds reasonable, right?

Our new Malibu owner muses on ...

"Everyone knows leaner is hotter, so I'm going to enrich just a bit, and cool things down."

When they did that, they got more power, the engine smoothed out nicely, and they were happy. What they didn't know -- due to lack of instrumentation -- was that by enriching, the burn-time of the combustion event speeded up and moved closer to TDC, the CHTs went much higher, the internal combustion pressures went much higher, and parts began failing. That early timing, so clever for LOP operations, now became devastating.

In this case, had they followed the TCM advice on page 13-2 of the manual quoted above, the engines probably would have made TBO pretty routinely. In fact, if you go to a Malibu Mirage Owners meeting and ask around among the pilots who fly with the TSIO-520 BE engines -- and religiously operate them LOP per the book -- they usually do make it to TBO or close to it.

Years ago, the editor of a major flying magazine flew one. Apparently the engine was not set up properly and it did not run very smooth LOP. The editor correctly noted the problems, and by experimentation, found that by running 25-75F ROP, the engine ran nice and smooth, and produced a lot more power and airspeed. He then related his experience to a few hundred thousand pilot/readers and effectively put his reputation behind the ROP operation of the engine, regardless of the fact that the engine manual does not authorize ROP operation of the engine at high-power cruise settings. Word spread, with many "gurus" making comments like "fuel is cheap, engines are expensive." Many -- perhaps most -- Malibu pilots followed that terrible advice, and ended up operating their engines at much higher actual power outputs than 75%, at the hottest possible mixture setting! Its no wonder that the pilots who followed that advice ended up with poor results.

The very next line in the engine manual, right after it states pretty clearly that 25 to 50F LOP is required, says:

Caution: Do not operate with E.G.T. settings of 25F on the lean side of peak at any power setting.

How on Earth did this line sneak into this manual? The Malibu, as delivered, didn't even have an EGT gauge (it did have an analog TIT gauge), and all-cylinder monitors were not then in common use.

Why do they speak of TIT in one paragraph, and EGT in another? TIT and EGT both peak at the same point on the mixture curve, so 25F LOP TIT is the same mixture setting as 25F LOP EGT. In any event, this line is a complete and clear contradiction with the preceding paragraph.

A little poking around reveals one possible answer. That line is probably nothing more than "boilerplate" left over from some previous manual! That same line is found in every TCM manual I could get my hands on. Whoever wrote the manual simply typed in the "new stuff," and left the old alone. Or, the lawyers proofed it, and said, "Hey, they forgot this standard line," and stuffed it in. Certainly no real engineer read it at the factory, because a real engineer would have spotted the inconsistency. Inconsistency my foot -- that's a direct contradiction!

But what about that line, on its own? What is the evil magic with exactly 25F? Does that mean that 24 and 26 LOP are OK, and only exactly 25 is "bad," and prohibited? If you get that needle right on exactly 25F LOP, the engine will just explode? Wanna buy a bridge?
I don't know of "many" GA engines that allow LOP operation
Perhaps I overused the word "many" but the Chieftain, for example, permits 50 to 100 LOP operation.

stepwilk
28th Apr 2010, 00:15
"Years ago, the editor of a major flying magazine flew one."

That would be my old boss Dick Collins, as I remember. Thirty-five years ago, he fired me, but not because I flew LOP.

Best thing that ever happened to me. turned me into a successful, happy freelancer. Thank you, Dick.

Brian Abraham
28th Apr 2010, 03:32
Every dark cloud has a silver lining Stephan. Well, perhaps not those going by the name of Cb. :p