About Engines...

Recently I've read about iron/steel being less than ideal because the piston expands when too lean, and ABC being less than ideal if run too rich because the cylinder shrinks and puts excessive wear on the piston. I wonder if anyone has tried a high silicon aluminum piston in a chromed steel cylinder? With or without a ring.
Perhaps there is already something with this combo. We seem to say, "Have you tried the latest ***** engine?" and not "Have you tried this latest metalurgical marvel?".

Or, perhaps it wouldn't work.

Thoughts?

> Recently I've read about iron/steel being less than ideal
>because the piston expands when too lean, and ABC being less
>than ideal if run too rich because the cylinder shrinks and
>puts excessive wear on the piston. I wonder if anyone has
>tried a high silicon aluminum piston in a chromed steel
>cylinder? With or without a ring.
> Perhaps there is already something with this combo. We seem
>to say, "Have you tried the latest ***** engine?" and not
>"Have you tried this latest metalurgical marvel?".
>
> Or, perhaps it wouldn't work. :)
>
> Thoughts?
>
> George

Ron B.
F4Fguy

George :
As far as I know,all of the ABC,ABN,AAC,ETC.engines use high silicon AL pistons.The problem is that the temp differential between the piston crown and cylinder is so great that,even with a low expansion coefficient piston,its only going to fit properly in a narrow range of running conditions.The temp difference is on the order of 1:1.2-1.3. The expansion coefficient difference is in the 1:0.9-0.8 range,depending on silicon content and other alloying ingredients.

The basic premise that you shouldn't run an ABC etc.rich,is not necessarily true.If you break it in rich that's where it should be run,as that's where the P/C fit has been established.After a rich breakin,running it LEAN will destroy the fit.

Ron B.
F4Fguy
>
> George :
> As far as I know,all of the ABC,ABN,AAC,ETC.engines use high silicon AL pistons.The problem is that the temp differential between the piston crown and cylinder is so great that,even with a low expansion coefficient piston,its only going to fit properly in a narrow range of running conditions.The temp difference is on the order of 1:1.2-1.3. The expansion coefficient difference is in the 1:0.9-0.8 range,depending on silicon content and other alloying ingredients.

Ron,
Thanks for answering. The information you provided above is why I was suggesting a light AL piston in a steel sleeve...to more closely match the expansion coefficients. My thoughts are that it may help prolong engine life if abused with overlean or over rich runs while trying to dial in the needle.

The basic premise that you shouldn't run an ABC etc.rich,is not necessarily true.If you break it in rich that's where it should be run,as that's where the P/C fit has been established.After a rich breakin,running it LEAN will destroy the fit. >
> Ron B.

Hmmm, I think you intended to express it the other way around? Once you have broken it in fast (lean) you will destroy that fit when you run it rich?
Agree that running it (after initial break-in) where you intend to run it all the time is the way to go. I would assume thousands of stunt engines would attest to that.
I wonder how well an engine (ABC, ABN, AAC, etc.) that has been run 4-2-4 will run in a fast 2 setup? Would it be "over the hill" for a fast 2 run?

Ron B.
F4Fguy

George:

I meant it as stated;break it in as you intend to run it.If you run breakin at 4/2/4 it will not run well at a fast 2,and if you run it there you'll ruin the fit for 4/2/4.The reason is that heat balance and the fits required are very different at different running conditions.Consequently the piston(and to some extent the top of the cylinder)wear to different shapes during breakin.

In our lab,we ran literally hundreds of material compatibility tests againstAl.Aluminum/steel is a very poor combination,unless the steel is hardened to at least 45rc.Aluminum/cast iron is excellent,as is Al/Cr,and Al/Ni(hard).

Personally I think way too much is made of the "pinch" in ABx, running rich or lean and so on. The idea sounds good in that the liner is supposed to expand at roughly the same rate as the piston but what do they do with the top of the liner? They clamp it tightly against a cylinder head made from normal high expansion rate alloy. Kinda defeats the purpose it seems to me.

But all of this ignores what gives the actual seal in any engine and that's a continuous oil film. This film can still be present even where there appears to be an interference fit (pinch) so long as the engine is turning over fast enough that the film hasn't enough time to get squeezed out. The trapped oil then generates hydraulic pressures that both compresses the piston and expands the liner. In this case obviously the lower limit is a film one oil molecule thick. The other extreme is where the clearances are so large that compression pressures can drive the oil film down giving a clear path for the mixture to escape past the piston.

The ideal film will be somewhere between these two extremes. It takes power to generate hydraulic pressures but you're assured of an excellent seal and any loss of film from too much clearance is undesireable.

So it seems it all comes down to what's considered to be the proper fit. Most likely an ABx runs usually somewhere between generating hydraulic film pressure (cold starts) and adequate (but far from excessive) clearance when hot. Given that there's really not all that much difference in running temps between 4 stroking rich and peaked 2 stroking I suspect that clearances for both still fall into the range of adequate and that this clearance will be attained no matter how it's run in.

The RC guys in particular make a big deal about how you should never run an ABx 4 stroking rich but they forget about their cold starts, idling, flying at part throttle, winter and summer air temps. Eventually all engines run in to suit the worst possible conditions. I think what I'm trying to say is that if an ABx is run in rich and then run lean it won't so much destroy the fit as alter it..but still in that desireable range of adequate. And vice versa of course, run in lean then run rich.

Downunder, you're forgetting about the fact that ABC/ABN/AAC engines have the cylinder head fit very tight into the top of the liner...a fit much like the piston/cyl. fit in an iron/steel setup. Much of the heat is expelled through the head, so it expands much more than the high-silicon piston, stretching the top of the liner and reducing the pinch.

It looks to me like the R/C dominated engine market has found that old habits are hard to break, and a lot of older guys won't run an AXX type as requested in the directions. Who reads directions? They are all bad translations into English anyway, right? The result has been loose fitting pistons with only slight pinch at the top. The engine makers didn't like the warranty problems or the bad rep. Steve

>Personally I think way too much is made of the "pinch" in
>ABx, running rich or lean and so on. The idea sounds good in
>that the liner is supposed to expand at roughly the same
>rate as the piston but what do they do with the top of the
>liner? They clamp it tightly against a cylinder head made
>from normal high expansion rate alloy. Kinda defeats the
>purpose it seems to me.
>
>But all of this ignores what gives the actual seal in any
>engine and that's a continuous oil film. This film can still
>be present even where there appears to be an interference
>fit (pinch) so long as the engine is turning over fast
>enough that the film hasn't enough time to get squeezed out.
>The trapped oil then generates hydraulic pressures that both
>compresses the piston and expands the liner. In this case
>obviously the lower limit is a film one oil molecule thick.
>The other extreme is where the clearances are so large that
>compression pressures can drive the oil film down giving a
>clear path for the mixture to escape past the piston.
>
>The ideal film will be somewhere between these two extremes.
>It takes power to generate hydraulic pressures but you're
>assured of an excellent seal and any loss of film from too
>much clearance is undesireable.
>
>So it seems it all comes down to what's considered to be the
>proper fit. Most likely an ABx runs usually somewhere
>between generating hydraulic film pressure (cold starts) and
>adequate (but far from excessive) clearance when hot. Given
>that there's really not all that much difference in running
>temps between 4 stroking rich and peaked 2 stroking I
>suspect that clearances for both still fall into the range
>of adequate and that this clearance will be attained no
>matter how it's run in.
>
>The RC guys in particular make a big deal about how you
>should never run an ABx 4 stroking rich but they forget
>about their cold starts, idling, flying at part throttle,
>winter and summer air temps. Eventually all engines run in
>to suit the worst possible conditions. I think what I'm
>trying to say is that if an ABx is run in rich and then run
>lean it won't so much destroy the fit as alter it..but still
>in that desireable range of adequate. And vice versa of
>course, run in lean then run rich.

Ron B.
F4Fguy

Downunder:

What you say would be correct if there were no thermal gradients in the piston or sleeve,and if the differential temp remained the same for all conditions.As presently practiced,the factory fits are approximations of the average required shapes for the design condition.The ideal shapes are going to be(approximately)parabolic,and are established during breakin.The head configuration has little to do with the final shape and fit,other than how well the localized distortions from the bolt loads are distributed.The head has no higher coefficient of expansion than the sleeve and case material.Only the piston uses the lo-ex hi-sil alloy.It has been stated elswhere that the head gets hotter than the top of the sleeve.Actually Top of the sleeve is usually somewhat hotter than the head because it has a thermal barrier at the flange and at the side wall.The hottest part in the system is the crown of the piston,and the piston also has the highest gradient top to bottom.These thermally induced shapes are only constant at one running condition.

They are also affected by more than just load and throttle condition.The 4-2-4 requires mixture rich enough to prevent firing on every other revolution.This provides an enormous amount of cooling through fuel evaporation,especially on the piston crown.This heat has to be "made up"before the piston crown temp begins to rise above system temp.Consequently the piston shape and fit are very much different than at a best power two stroke setting.

Running in at either condition wears the components to configurations and fits detrimental to the other.

The forces generated by thermal expansion,in the absence of proper fits,are far greater than the hydraulic pressures in the oil film.In fact the film will also have both temp and pressure gradients.The pressure will be,approximately,the combustion pressure at the top and essentially 0 at the bottom of the piston,and varies throughout the stroke.These conditions also change at the bottom of the stroke where outof round due to intake vs exhaust comes into play.

The viscosity of the oil also varies with temperature and is lowest at the top of the stroke.As to the oil film thickness,while a one molecule thickness may inhibit bonding and consequent scuffing on a perfect surface like a Jo-block,it certainly will not provide a low friction film on a working engine.Once again,if the fits and shapes aren't right,you get line to line contact which pushes the lube away resulting in high wear.

All the above are also dependent on design.The side forces on the piston obviously have considerable influence.These are determined by; the connecting rod length/stroke ratio,the cylinder offset(DeSaxe)if any,and piston offset if any.

In sum,for long life,and optimum performance,break it in as you intend to run it.

> Downunder:
>
> What you say would be correct if there were no thermal
>gradients in the piston or sleeve,and if the differential
>temp remained the same for all conditions.
But this is exactly my point. These conditions vary continuously from day to day and even during a flight. There's also the differential temp required between the engine and the ambient air for heat rejection. This difference is on the order of 100C regardless of ambient temp (ignoring any density change) so the head temp for instance can vary considerably from winter to summer. Imagine what this does to the clearances in your carefully run in engine? If the clearances close up in winter it then runs in to suit but is it then ruined for summer useage?

Even more extreme would be the piston crown temps especially running a 4 stroke the whole flight except for a few seconds of leaned out 2 stroke at the end of the tank. Say it runs a peaked 2 stroke the last 6 seconds or about 1 lap. In 100 flights you've given the engine 10 minutes of a solid 2 stroke heating the piston crown but not enough time for the extra heat to transfer through the greater mass of the liner/head/crankcase.

This is why I said any engine will run in to suit the worst conditions it faces giving optimum clearances for that condition (so long as the conditions don't change too rapidly for the engine to accomodate them). At anything other than these worst conditions the clearances will then be larger than the optimum. I suspect we'd be talking 10ths of a thou here or less but we're saved by the oil film so as far as we're concerned the engine is running 100% OK.

LAST EDITED ON Feb-14-04 AT 01:15 PM (CDT)
 
>
>> Downunder:
>>
>> What you say would be correct if there were no thermal
>>gradients in the piston or sleeve,and if the differential
>>temp remained the same for all conditions.
>But this is exactly my point. These conditions vary
>continuously from day to day and even during a flight.
>There's also the differential temp required between the
>engine and the ambient air for heat rejection. This
>difference is on the order of 100C regardless of ambient
>temp (ignoring any density change) so the head temp for
>instance can vary considerably from winter to summer.
>Imagine what this does to the clearances in your carefully
>run in engine? If the clearances close up in winter it then
>runs in to suit but is it then ruined for summer useage?
>
>Even more extreme would be the piston crown temps especially
>running a 4 stroke the whole flight except for a few seconds
>of leaned out 2 stroke at the end of the tank. Say it runs a
>peaked 2 stroke the last 6 seconds or about 1 lap. In 100
>flights you've given the engine 10 minutes of a solid 2
>stroke heating the piston crown but not enough time for the
>extra heat to transfer through the greater mass of the
>liner/head/crankcase.
>
>This is why I said any engine will run in to suit the worst
>conditions it faces giving optimum clearances for that
>condition (so long as the conditions don't change too
>rapidly for the engine to accomodate them). At anything
>other than these worst conditions the clearances will then
>be larger than the optimum. I suspect we'd be talking 10ths
>of a thou here or less but we're saved by the oil film so as
>far as we're concerned the engine is running 100% OK.

Ron B.
F4Fguy

I think we may be at cross purposes here.What you said is true,there are variations throughout every run.I'm not talking about catastrophic failure,but accumulated wear over time.My point is that for optimum life,you need to minimise the time at any condition other than your desired norm.As you indicate,running at full lean will induce wear-in to suit that condition,and subsequent running at 4-2-4 will have larger clearances.That's precisely what I'm getting at.The more you run at one condition,the worse the fits for the other.Over time,this means hard starting for either condition and over heating or low power,or both, depending the direction of change.Ironically,increased clearance at 4-2-4 may cause piston over heating due to reduced heat transfer,and even more rapid wear at the top of the piston and sleeve.

Re the oiling,there's an optimum film thickness.Too thin and you get localised contact and excessive wear,too thick and heat transfer suffers.Excessive clearance gives an opportunity to "blow out"the film giving excess blowby and accellerated wear,overheat,and upset thermal distribution in both sleeve and piston.

It still comes to the useful life span of the engine.For maximum life breakin should be at ,as near as possible,the proposed running condition.The reason RC engines,even with the best of care,have lower life is that they're running all over the map,all the time.

As the article in the original post above said,the best of all is AXX+ring.This avoids almost all of these problems.