What Detonation Looks Like

Last time, I showed what preignition looks like, so this time I thought it might be good to take a look at detonation. In this case, we're seeing the result of mild, occasional detonation (such as you would get by overleaning in cruise-climb). The mildness is evident by the fact that only the outer edges of the piston are involved. (Notice the characteristic "sandblasted" look.) The somewhat peened or smoothed-over appearance of the pitted areas, particularly on the left side of the piston (as viewed in this photo), suggest that detonation was encountered only now and then, but over an extended period of engine service. (Over a sufficient period of time, roughness gets "polished out" by the action of lead-containing combustion particulates.)

The positioning of the detonation zones at the far outermost edges of the piston is consistent with operation right at the beginning of detonation onset (in other words, mixture just lean enough to cause incipient detonation, but not lean enough to cause full-on explosive detonation). The mechanism for this is not hard to understand: The abnormal combustion process associated with detonation occurs when unburned fuel and air (far away from the flame front) are heated and compressed past the point of "thermal cracking." There's a short period of time when the fuel molecules actually begin to decompose (split into radicals) on their own, unassisted by combustion with oxygen. Unfortunately, they all tend to do this at once. The radicals so produced then instantly grab onto the nearest oxygen molecules. Instead of steady, slow conflagration, you get an instant bang.

In normal combustion, you don't see fuel molecules crack apart en masse into radicals (detonation precursors) prior to oxidation. Instead, "cracking" happens only inside a very thin flame front. The flame front progresses steadily across the charge volume, creating radicals (in that thin flame zone) as it goes.

So in the engine from which the above piston came, you can imagine that normal combustion happened until the flame front got pretty far away from the spark plug(s). Then the remaining compressed fuel and air at the very edges of the piston went ka-bang!

The engine from which the above piston was taken was not an aircraft engine, but I've seen piston edges that look like this in torn-down Lycomings and Continentals countless times. The next time you visit an engine shop that keeps a garbage bin of old pistons, take a look and you'll see what I mean.

What Preignition Looks Like

Preignition doesn't always burn a hole straight through the center of the piston, but sometimes it does, and this is what it looks like. This is what might be called a deposit burn-through (or for want of a better term, an ash-hole). The entire top of the piston is overlaid with crusty deposit buildup. What likely happened here is that a an amalgam of very-high-melting-point calcium (or other) deposits started to accumulate on the very top of the piston, in the center. After a period of high BMEP (perhaps runaway turbo boost), the deposits, heated to 1500 degrees or more, simply caused the aluminum underneath to soften. (Aluminum melts at 1220 degrees Fahrenheit, or 660 Celsius.) This is clearly a melt-hole rather than a detonation failure. In the latter case, there would be sharp cleavage planes under the lip of the hole, on the back side (the "underneath" side), sort of like what you see on the back side of a bullet hole in a glass window. This piston happens to be from a marine racing engine. But the same thing can happen (has happened!) to aircraft engines. Depending on the source of the preignition (spark plug vs. deposits) and the flame pattern in the cylinder, the piston can either melt through the center, or melt at the edges, or both. More often than not, the entire piston overheats and over-expands in the cylinder bore and starts to make rubbing contact with the barrel wall, giving rise to smearing/scoring or vertical streak-marks on the sides of the piston. You can see some of that at 7 o'clock on this piston as viewed here.

Thankfully, this sort of failure is rare in aviation.

Lycoming High-Squish Piston

Somehow I stumbled across a 2004 patent by Lycoming and Toyota containing some ideas I wouldn't have expected to see from Lycoming.

The patent in question, "Cylinder assembly for an aircraft engine" (U.S. Patent No. 6832589), describes an elegant combustion-chamber design in which a domed piston with special cutouts squishes fuel into a small pocket for rapid-swirl combustion. This general notion has been patented to death in the automotive world, of course, and wasn't new in 2004. But somehow Lycoming and Toyota got the USPTO to sign off on it one more time.

In the above diagram (taken from the patent), I've colored the piston to make it easier to see. Note the close proximity of spark plugs to piston. The "squish zone" (C) calls for a piston-to-cylinder-head clearance of just .047 +/- .016 in., or about enough for five hours of deposit buildup in an O-235, but this wouldn't be for an O-235. This is a high-compression-ratio design for ultra-lean operation on low-octane unleaded fuel, and I guess that's the real punchline.

Combustion Chamber Video

Stop what you're doing right now and go see this video taken from inside an operating engine. Except for the exhaust stroke (which the video skips over for some reason), this clip gives a remarkably informative view into the workings of the Otto cycle.

I was surprised by a couple of things. First, the intake valves (this is a 4-valve engine) spin so fast that they continue to rotate long after they've closed. In fact you can see the intake valve nearest the camera still spinning as the exhaust valve opens.

Another thing that caught my attention is the cyclonic flow pattern of the combusting fuel and air. If you look closely, the flames swirl in a counterclockwise fashion as seen by the camera. The coolest portion of burning charge (bright yellow-orange flame) seems always to be on the intake side of the cylinder (no surprise, I suppose).

Combustion is still underway as the exhaust valve opens. (You can see yellow flames still burning.) The rapid pressure drop as the valve opens makes the fire die down quickly.

This particular engine (I don't know what kind of engine it is) seems to have no detectable valve overlap. In fact, the intake valve doesn't even begin to open until the piston is already traveling downward on the intake stroke. This is extremely late valve opening, indicative of a turbocharged engine.

As they say, a picture is worth a thousand words.

CHT Voodoo

Advice about cylinder head temperatures seems to me to be an area rife with voodoo. On page 143 of the new edition of Fly the Engine, I quote from a recent Lycoming service publication (SSP-400, for Piper Mirage owners) that says this about CHT and cylinder life:

No matter what approved power setting is used, cylinder head
temperatures should not exceed 435 degrees F in level flight
cruise. For optimum service life, cruise cylinder head temperatures
should be maintained below 400 degrees. [emphasis added]

This bit of advice can be traced to statements made in the O-290 manual going back 50 years. I've never seen or heard of any substantiating evidence for it. It's hard to imagine Lycoming running engines for the thousands of test-cell hours of testing that would be needed to gather statistically meaningful data to back such a claim up. If they're relying on "data" from engines in the field, how good could that possibly be? (Think about it. How accurate is the average CHT system? How much CHT data could owners be giving Lycoming?) And why 435 degrees F (224 Celsius), in particular? What's magical about that number? And why, also, 400 F? The only thing magical about 400 is that it's a conveniently round number. It's the same as saying you should keep your cylinders below 859.7 degrees Rankine.

I strongly suspect this decades-old advice is based either on no data, or poor data. I intend to look into it further. Meanwhile, if anybody knows the real story on this, please write to me at fly.the.engine@gmail.com. It's time aviation got past the voodoo stage.

Fly the Engine on Amazon

FTE is on sale at Amazon now, not as an in-stock item but as an item that can be ordered through a private seller (namely me).

It turns out Amazon will let you sell items privately (like eBay, sort of) and take "only" a 15% commission for themselves. I can live with that.

But if you're a publisher and you want Amazon to actually stock your books and fulfill orders directly, they keep 55% of every gross order dollar, plus miscellaneous merchant fees (and a month's float on balances due you). That's fair, right?

Let me see . . . 15%, or 55% . . . I wonder which is better . . .

BTW, Rita and I will be happy to give a quantity discount to anybody who buys 5 or more copies of Fly the Engine (for UPS-ground shipment to a single address). The discount won't be 55%, but it'll be worthwhile. Write to us for details: fly.the.engine@gmail.com.

Cirrus Jet Fat-Wing Design Revisited

Yesterday I heard from Mike Van Staagen, VP of Advanced Development at Cirrus, who saw my AOPA blog of a week or so ago, in which I made light of the Cirrus Jet's short, stumpy wings. He gave me some insight into why the wings are that way. "We're limiting the span to 38.5 feet," he pointed out, "so that the current Cirrus-owner population, which is approaching 4000, can use their existing hangars." This has been a common request, apparently. And it makes good business sense to listen to your customers, so I can see his point there.

He also noted that a thick airfoil cross-section allows for the "lightest possible structure," which I suppose might be true. I don't know. I'm not a structural engineer.

Cirrus is also trying to meet a 61-knot stall speed requirement. This can be approached in different ways, obviously, but starting with a wing that has inherently high induced drag probably doesn't hurt.

Cirrus has prioritized things in such a way as to put aerodynamic efficiency well down the list (which they've admitted all along). Van Staagen notes that even with a short/fat wing, the Cirrus Jet has sufficient power to reach its 25K-foot cruising altitude quickly, certainly quicker than most piston pilots are used to.

But there's a lot of competition in the nano-jet arena (see Philip Greenspun's excellent overview), and not all customers will be basing their decisions on purchase price alone. Some will want a practical long-range cross-country machine that can be justified on the numbers. It remains to be seen whether the Cirrus design will stand up to the competition in a total-ROI sense. Let's face it: if all you want is a snazzy pocket rocket that can get to FL 250 quickly and fly 400 miles before refueling (at a cost well under $1 million), there are any number of L-39s (and other jet trainers) out there right now that can fill the need for $400K. They won't carry 7 people, but the snazz factor is there and you're cruising quite a bit faster than the Cirrus Jet will.

Personally, I think Cirrus is falling into a pattern often repeated in aviation, of designers designing a fine airplane that later needs longer wings. Ted Smith went down this road with the Aero Commander. He did it again with the Aerostar. The original Cessna 421 had short wings; the 421B got longer ones. The Citation I had short wings; they were lengthened in the Citation II. In the Piper PA-28 series, Piper went from a short/fat wing to long/tapered. Peter Garrison eventually lengthened the wings of his Melmoth homebuilt. (The list is a long one if you count military aircraft, airliners, and helicopters that got longer blades.)

I can think of no successful production aircraft that later underwent wingspan reduction.

I'm no aeronautical engineer, but I think history is clear on the fact that adding a bit of wingspan is one of the cheapest ways to increase a plane's utility and efficiency. Payload, range, rate of climb, all increase. Stall speed goes down. Cruise speed, practically unaffected. In a jet, overall fuel economy improves dramatically because you get to high altitude faster.

But if you've got to fit a jet into a piston-aircraft hangar, all bets are off.

I wish Mike and the Cirrus folks well. I can't wait to see their little jet fly. It may be the Grumman Yankee of jets, but you know what? I liked the Yankee.

Shameless Plug

Did I mention that the new 2008 edition of Fly the Engine is 278 pages long and contains 20% new material? Or that it's in stock now and available for immediate purchase? Or that it makes a terrific Christmas gift for your flying friends? Check out this free sample of the book! Order online, it's quick and easy. Go to our order page.

Largest Lycoming

You think you have oil consumption problems? Meet the Lycoming R-7755, a 36-cylinder, 5000-hp, turbosupercharged monster displacing 7,755 cubic inches (bore/stroke 6.375 X 6.75 in.) and weighing a mere three tons, give or take a beer keg.

Two of these babies were built in 1946 (one carbureted, one fuel-injected), for the Convair B-36. Pratt & Whitney won the engine contract, ultimately, with its 28-cylinder R-4360 after the Lycoming proved too unreliable. (Think about that; the R-4360 won on reliability.) Had Lycoming gotten the contract, the B-36 would have gone into the air with 216 cylinders and 432 spark plugs. Imagine trying to keep 432 spark plugs clean, operating on postwar 115/145 avgas.

The R-7755 was innovative in a number of ways. It was liquid-cooled, which is why the cylinders line up in a perfect line (in 9 rows of 4). Each bank of cylinders had an overhead camshaft. (I don't know of another radial with an overhead cam, do you?) Each cam, in turn, had two sets of lobes: one for high power, the other for long-distance economy cruise. When the pilot chose a different setting, the entire cam would slide lengthwise a couple inches to engage the other set of lobes.

The Air Force spent 10 years battling engine problems in the B-36, many of them related to poor cylinder cooling, others involving carb ice and carburetor fires. None of which would have been a problem with the Lycoming R-7755.

Fly the Engine on Google

Today I put some ads up on Google to spread the word about Fly the Engine. Try a search on "EGT systems" or "TBO busting" or just "Fly the Engine," and you should see a Google AdWords ad in the right-hand vertical column of ads.

It took about 30 minutes to set up the Google account and specify the ad's wording, choose the keywords (search terms) it should be linked to, and so forth. The ads went live within an hour. Unreal.

Fly the Engine is Back in Print!

This is an exciting day for me. Through the miracle of print-on-demand, Fly the Engine is back in print once again (for the first time since 1995)! I just "published" it today on Lulu.com; haven't actually seen a hard copy of it yet, but I trust Lulu to do a good job. I've seen their books. They look pretty darn good.

It took a good while to update the original manuscript. All in all, I'm surprised how well the material has stood up over time. But I made quite a few changes, additions, corrections, and deletions; and I added a good bit of new material (resulting in an increase in the page count, to the tune of around 45 pages). This is a true page-by-page revision, not a quickie once-over fluff job.

The book is available for purchase now at http://www.lulu.com/content/1291090 (check out the free online sample, too).

AOPA 2007 in Hartford

I managed to make it to Hartford last weekend and took a few photos while cruising the aisles of the AOPA show. The most popular exhibits seemed to be those of Cessna, Cirrus, and Piper. The latter two brought mockups of their single-engine jets. Piper's was the more convincing by far. It's obvious Piper has thought this one through and can pull it off. The Piper jet's thin, high-aspect-ratio wing and long cabin say it all: This baby is meant to go long distances at high altitudes. This is your upgrade path if you're a Mirage owner.


The Cirrus design doesn't seem particularly well-considered. The wing is almost laughably short and fat, for example, which will cut the plane's rate-of-climb needlessly and severely limit its ceiling. The short fuselage, stubby wings, and V tail spell "Dutch roll" to me. All in all, it looks a bit like a one-off proof-of-concept homebuilt rather than something that's supposed to go into production. The short, fat wings tell me all I need to know, frankly.


Cessna will be using a Thielert turbo-diesel in the 172 next year. They didn't bring a 172 to the exhibit hall, but they did bring a copy of the engine.



The particular Thielert model that will be used in the 172 will produce 155 horsepower and (according to the chief engineer on the project; the guy in the blue shirt) will actually weigh a bit more than a Lycoming O-360. The engine displaces something like 121 cubic inches. It gets much of its horsepower from turbocharging and sheer rpm (1.6:1 gear reduction at the prop). In fact, maybe it gets a little too much horsepower that way.

Time (between overhauls) will tell.

Inaugural Blog

Not much to say in this first post except: Here I am. Back from the undead.

And also: Stay tuned for progress reports on the upcoming new edition of my book, Fly the Engine, soon to be back in print. Hopefully in time for Christmas.