Operation Of ABC OR ABN Engines

I have been following the thread on sagging of the TT 25 during break-in. I understand the importance of not running rich during break-in of ABC or ABN engines. I understand why this should be.

What I don't understand is why after intial break -in it's OK to run rich! The difference in thermal expansion of the brass sleeve vs the aluminum piston is still there, so why wouldn't wear be accelerated when we start using the engine in a plane and trying to achieve a 4-2-4 run? Seems to me that this expansion difference is intrinsic to the materials.

Taking it a bit farther. After the engine is broken-in in the richer operating mode, would there be a drop in power as the engine is leaned out and the temp goes up and clearances open up?

>I have been following the thread on sagging of the TT 25
>during break-in. I understand the importance of not running
>rich during break-in of ABC or ABN engines. I understand why
>this should be.
>
>What I don't understand is why after intial break -in it's
>OK to run rich! The difference in thermal expansion of the
>brass sleeve vs the aluminum piston is still there, so why
>wouldn't wear be accelerated when we start using the engine
>in a plane and trying to achieve a 4-2-4 run? Seems to me
>that this expansion difference is intrinsic to the
>materials.
>
>Taking it a bit farther. After the engine is broken-in in
>the richer operating mode, would there be a drop in power as
>the engine is leaned out and the temp goes up and clearances
>open up?

I think the real point here is, when we break in our iron piston or ringed engines, we tend to run them very rich--too rich compared to how we would be flying them. That is OK for the iron piston versions (in fact, good for them) which tend to tighten up as they get hot. The parts get to wear in and get a better fit while not being too tight. It is never good for these to run them screaming lean. We run these rich for a while, and then when the parts begin to fit we basically take them up to the rpms where we will normally be running them. Even then, there will be more wear in before they are really at their peak.

On the ABC (AAC and ABN) type engines, they are tightest when cold. It is never good to run them slobbery rich, because then they never do heat up. We don't want them screaming lean, either, but we want to break them in with smaller props (reducing the load) and higher rpms so they are still a bit rich, but running fast and hotter so the cylinder expands at the top. It is true, the rate of expansion should be the same (if designed properly) but the cylinder gets hotter at the top under normal operation, and that is where it will expand more. Thus we need to break them in the same way as we will run them so the cylinder is in its expanded mode.

They are designed with a slight taper (narrower at the top), so when at operating temperature the cylinder will be straight. We break them in that way while the parts get their fit, and then run them at the "normal" operating rpm when on our planes. That "normal" rpm in some cases will still be faster than with some of the older engines, but it will be in the operating range, and the cylinder will be hot enough to be straight as well.

But, in response to your last question, yes, if we run them in a slobbery 4-2 mode, even after proper break in, we can destroy the "pinch" at the top. But with the constant cooling/heating cycles of starting a cold engine and bringing it up to temperature, this is where the biggest wear point is going to be. Matt has worn out 2 1/2 PA engines in the short time that he has been running the pipe system (No knock on the engines at all, but after a thousand flights, something has got to give) and you can see the wear point at the top of the cylinder. Because there is a greater pressure there when the engine is cold, that is where it is going to wear while it is warming up. If we ran our engines continually instead of these 6 1/2 minute runs where we allow the engine to cool before starting it again, we would probably get a lot more mileage out of our engines.

Hi Dick,
I believe that the most important factor is to stabilise or grow the high Si content piston material, if it doesent get hot enough it wont stabilise at all. My experience is with racing & speed engines mostly, but they are more critical, especially the .15's, than the average stunt engine using these materials.

The higher the Si content of the piston, the hotter the engine needs to be to achieve the stabilisation. ferinstance, some of the Ukrainian engines use 31% Si pistons (typical stunt engine 15-20%)- the makers specifically warn against any bench running! and this includes one make of stunt engine.

It is important too, I think, to get the engines into the ballpark in operating temperature - if the engine is to be closely cowled, maybe it should always be run in a similar cowl.

With the F2A engines, they get 3 short runs (60 sec total) in the model on the ground, leaned out to the first pipe stage, then they get flown - mind you this is with a shutoff that gets used at the first sign of too lean.

These days, I dont bother ever bench running a ABC type engine, I've had to refit pistons too often for other guys.
Cheers, Dave

The main thing to remember with an ABC/ABN engine, is that when you break it in, you should run it NEAR to where you plan to actually use it. That way you won't remove so much of the pinch during the break-in that the clearance is gone when you get around to using it.

Also remember, that if you normally run the engine rich, don't expect it to have lots of oompf when you lean it out. That's because when it goes lean, the temperature goes up, and now the cylinder expands away from the lower-expansion silicon piston. The engine sags, since the compression has just gone south. This may be the reason that you hear people say that the Fox 35 ABC has a little less power than the standard Fox 35. With the iron piston steel cylinder setup, the expansion coefficients are much closer, so when the engine leans out, the fit stays (actually you can get the standard Fox to seize if you lean it too much).

So the moral of the story is that the ABC/ABN setup tends to have a narrower temperature range of operation. The sagging at high temperature is actually a "feature", since it prevents the engine from seizing, saving it for another day!

LAST EDITED ON Jan-03-04 AT 07:49 PM (CDT)
 
One of the major reasons for this is not only is the cylinder tapered, smaller at the top, but the cylinder head is fit snuggly into the top of the liner. The head is typically now fit about like a piston fit...push fit. As much of the heat goes out through the head, the aluminum head (and not a high silicon/low expansion alloy) expands and stretches the top of the liner to loosen up the "pinch". This is done to keep the cylinder round, as I understand it. GMA's website had a good, short article on breaking in ABx engines, and I had it copied to my computer at one time, until it crashed. I'll see if I printed it out and if so, post it. By the way, GMA did THE best engine rework article ever, in one of the AeroModeller Annuals, about '66. I think I finally got a copy of it. George wrote me that the first ABC setup he got, he honed or lapped so it fit great, just like an iron piston. And of course, it was ruined. At least he could hone out the old chrome and rechrome in his own garage. The first one I got, 2nd hand, fitted that way, and it was slow as an old dog on a hot day. While the typical R/C engine is not fit with a lot of "pinch", an engine with a really good fit (lots of pinch), will wear or snap rods if run too rich, too soon. Steve

>GMA's website had a good, short article on breaking in ABx
>engines, and I had it copied to my computer at one time,
>until it crashed. I'll see if I printed it out and if so,
>post it. By the way, GMA did THE best engine rework article
>ever, in one of the AeroModeller Annuals, about '66.

Try here, I don't think it's the entire article you are refering to but, it has the main points. http://www.tulsacl.com/WorkshopEngine.html

Ron B.
F4Fguy

Ok,I can't stand it any longer.I've gotta jump in.

The basic principle of all the "Alphabet" engines;ABC,ABN,AAC,or,in one case AA AL2O3,is the same.That is to use materials of different expansion coefficients in the piston and cylinder,shaped and fitted at ambient temp in such a way that they will have the best possible fit and geometry at operating temp.This usually means a low expansion hypereutectic silicon alloy aluminum piston running in a brass, bronze,or low silicon aluminum cylinder with a bore coating of some anti-wear/anti-scuff material.

Hypereutectic silicon alloys are alloys with more than ~12.5%silicon.They all have low expansion coefficients vs "normal" aluminum(.00001"/in./degree F. vs .000013"/in./degree F.).If you consider the fact that the piston crown normally runs about 150degF.hotter than the top of the sleeve,you can see that they follow each other very well.It doesn't stop there however,as both components have fairly steep,and different,temp gradients top to bottom.This is why the piston has a different taper than the sleeve.Since it is too expensive in high production engines to lap the components to the actual profile required,most engines have tapers rather than the parabolic shape corresponding to the temperature gradient in the respective components in operation.(An interesting factoid:Most full scale aircraft recips have bores that are "barreled" at the top for the same reason.)Also keep in mind,we're talking thermal expansion,a totally reversible condition,not to be confused with growth,which is not reversible.
This is where proper "break in"comes in.By carefully running at or near the condition you intend to run,the parts will wear to the desired shape and fit.Running too rich(or lean) or at too much(or little)load changes the thermal conditions and as a result the clearance and shape of the components.The reason the "pinch"is necessary is only to accomodate the warm-up period.When the engine is running at temp everything is(if it's done right)straight and at running clearance.As you can see,a break-in at another load or speed will not give the proper fits.

To go back to the silicon alloys,there is some permanent growth in these(and other )Al alloys,but it is not,or is only minimally,related to the amount of silicon above the eutectic.There is some small effect on expansion coefficient,but not on permanent growth.From the as cast to the fully degrowthed condition ,there is about 800 Microinches/inch of potential growth.All of the silicon above ~12.5%whether 13%or40%*precipitates above the solidus and doesn't contribute to growth.The growth is due to precipitation of silicon from solid solution as only about 2.5 %can stay in solution.This is a time/temp process which can be accellerated by heating.For example,450degF.for eight hours will yield full growth.This is a non reversible condition,except by remelting.
*This can change slightly with casting chill rate but only to a small extent.

One final point,the care with which the head is torqued can certainly effect the concentricity of the bore,but the head does not control the bore taper in any way.

I'm flying a Tower 40 glowplug with a piston and liner that ran diesel for a while. I am really pleased with how it runs. What does it all mean?

It means be happy!