Forged pistons & silica content 1

[ICman]

I (think) I just read that forged alloy (hypo eutectic) pistons are bad for air-cooled engines. I don't understand it all, but the author indicated that aluminum with > 12% silica were considered hyper eutectic, less ductile, stronger, and had less heat transfer (a good thing?) than those with less than 12% silica. He said that all auto and motorcycle manufacturers use hyper eutectic pistons.

The down side of hyper alloy is increased machining costs, and more difficult to tune.

I just wanted to see if any piston designers out there agree or disagree.

 

[ShaunT]

These pistons allow much tighter clearances say .002.
They also help in transferring heat from the piston into the cylinder wall. They have considerable less swell rate than an aluminum piston with little or no silicon. The tighter clearance aides in ring seal because of the reduction in piston rock.
On another front, A lighter weight oil also helps in removing heat because it flows at a higher gpm. The pressure may be less than with a thick oil, but you are putting much more volume through the crank. A cold start engine knock that disapears when the oil pressure raises, can usually be eliminated by switching to a lighter oil and a filter with an anti-drainback valve.

Shaun TiedeULTRADYNE Arl,TX(stiede@ev1.net)

 

[Retracnic]

ICman,

Aluminum alloys used for pistons come in a wide variety, with varying degrees of copper, magnesium and silicon content. Though each element adds its own special properties to the alloy, silicon content is a prime driver in the overall characteristics of the alloy.

Hypoeutectic – 11% or less silicon content.
Eutectic – 12% silicon content.
Hypereutectic – Greater than 12% silicon content.

In the piston business hypoeutectic alloys are referred to as low silica alloys, while eutectic and hypereutectic are referred to as high silica alloys.

In short, hypoeutectic alloys posses a thermal expansion rate greater than typical engine block materials. This requires running higher clearances, which affect engine noise, wear and emissions. On the up side, these alloys are strong and well suited to extreme forced induction applications.

Hypereutectics have thermal expansion rates equal to or less than the surrounding cylinder. Allowing, as ShaunT stated, tighter clearances to be run. Unfortunately they are brittle, and less tolerant to extreme cylinder pressures.

Either can be used in an air-cooled engine, as long as correct clearances are insured and the right silica content is chosen based on your requirements. Keep in mind that the terms hypo/hypereutectic does not automatically imply that the piston was forged. Hypo/hypereutectic is only a description of the alloy used. Most OEM hypereutectic pistons are cast units.

Good Luck

Bryan Carter

 

[strokersix]

Bryan,

Hypereutectics have thermal expansion rates equal to or less than the surrounding cylinder.

Is this true that hypereutectic aluminum piston alloys have lower thermal expansion coefficients than cast iron? I don't have my reference materials handy (they are at work and I'm on vacation this week!) or I would look it up myself.

Thanks, Mike

 

[Retracnic]

Strokersix,

In general the Hypereutectic alloys used for forged pistons, do not have a thermal expansion rate lower than ductile, gray, or pig iron. In OEM applications hypereutectic pistons are often installed in aluminum blocks making use of some form of low carbon steel alloy cylinder liners. In these cases, the liner's alloy is usually tuned to closely match the thermal expansion rate of the block material.

Another thing to keep in mind is that overall thermal expansion is a combination of the alloyed used and the method of manufacture. Varying casting and forging techniques can yield significantly different expansion rates from the same alloy.

The sentence should have read:

Depending on the method of manufacture, hyperteutectics may posses thermal expansion rates equal to (or less than) the surrounding cylinder.

Thanks for reminding me. If you dig up some better info. Post it for us all to learn from.

Good Luck

Bryan Carter

 

[strokersix]

Bryan,

Information I have found is rather generic but:

CTE grey cast iron = 6 in/in/F
CTE high nickel cast iron = 6.5-6.8 in/in/F
CTE various cast aluminum alloys = 9-13.7 in/in/F

CTE of 9 is for aluminum alloy 393 Si 22 Cu 9 Mg 1 Al bal.

Source: Materials Selector 1986

I suppose an iron liner shrunk into an aluminum block could end up with an expansion of 10 in/in/F. A high silicon piston with 9 in/in/F at higher temperature than the block could then expand exactly as the cylinder. Pistons probably don't expand uniformly, I expect their geometry is tweaked to control skirt expansion. Pistons certainly do not have the same temperature throughout either.

I look forward to comments from others more knowledgeable than as the above is conjecture on my part.

Also, thermal expansion may not be linear. Invar, for example, has quite low expansion coefficients near room temperature but makes up for it at higher temps. Could a non-linear expansion effect be utilized for piston clearance design?


Mike

 

[Azmio]

Guys,
I have designed a hypereutectic Aluminum block (now they have it in high pressure die as well) before but I have never designed and developed a cast iron block. However, I do know that piston profile for Aluminum and Cast Iron blocks are different.
Before engine operation, piston is oval due to its machined geometry profile. Once the engine is in operation, the piston will become circular and matches the cylinder bore.
I understand strokersix has a point by saying that the thermal expansion of the cast iron block is less than an aluminum piston. It is a simple fact.
For this reason, it is very important to have the right piston to bore clearance. This is also the reason the bore diameter will determine what sort of piston diameter we should use. At the assembly floor, pistons are graded to match block grades.

AO

 

[Warpspeed]

But surely you cannot just specify the thermal expansion of the piston to be the same as that of the cylinder bore ?

The temperature of the block is going to be fairly constant, at least in a water cooled engine. But piston crown temperature is going to vary considerably with engine speed and load.

Also, heat flow is always going to be from piston to block, so the piston will run hotter than the bore.

Forged pistons always seem to run with higher recommended clearances than cast pistons, can anyone shed any light on this ?

I have also been assured that forged pistons with larger running clearances in no way effect engine life or ring sealing, but I find this difficult to believe.

 

[Azmio]

Theroretically, if we use the hypereutectic block, the expansion of the cylinder bore is roughly the same as the piston provided that the material specification is the same. But of course, the head bolt, uneven cooling by the water jacket, structural integrity, etc. will still cause some distortion. That's why we still need the recommended clearance, piston profile and ring to take care of the distortion.

So far, when we tested the two combinations, it works very well. Even the oil consumption is excellent at 7600rpm.

As for the forged piston, the density of the hypereutectic aluminum is more if compared to cast piston. The higher density means it can store or accept more energy during engine operation whether we like it or not. With constant supply of energy to it, the energy content is higher thus promoting to higher thermal expansion. Perhaps that is the reason why it needs more clearance. This is just my guess, appreciate comment from others.

AO

 

[Tmoose]

getting most aluminum's density to vary very far from 2.7 gram/cc is a pretty good trick, best done by alloying, not forging. I think the common Generic Description of forging's "density" advantage may have origins in the "grain density" = small grains compared to a basic cast part.

Tighter (less) clearance is made possible by any of many features of piston design. Rarely is a feature strictly connected to the manufacturing process, so saying "forged" does not really "define" everything about how a piston is made. Some Examples are -
- Flexibility - might be from curvy skirt profiles, unsupported tails, or slits in the skirts like olden days.
- Different expansion rates for different materials (old iron pistons were set up pretty tight). The shinier California forged pistons are higher expansion alloys than the dark gray TRW/Speed Pro/Wiseco/Mahle/Kolbenscmidt forged pistons.
- oil return slots rather rather than oil return holes in or near a 4 stroke piston's oil ring groove. Slots make the piston weaker, but keep the skirt cooler and let it operate more independently from the hot swollen piston head. Probably provides flexibility at the skirt top too.
-Including steel expansion fighting struts in the piston. This strut trick is pretty much limited to cast piston technology. See >>one<< version of it here on old Harley pistons.

http://www.aa-ok.com/20420f.jpg

The forging process kind of precludes including anti-expansion struts. If the application is real Heavy Duty or High performance then the strength enhancing/expansion provoking features of oil groove holes rather slots are likely to be included. The result would be forged piston that would probably need greater cold clearance.

 

[Warpspeed]

My understanding of the forging process, is that the forging dies work under extreme pressure, and the dies themselves are solid metal slugs that come together. For this reason, the shape, particularly the internal shape must be such that the internal die can be removed after the forging process(obviously).

Casting is a low pressure process, and the internal part of the mold can be in several interlocking parts that can be collapsed and removed after casting for reuse. So the inside of the piston can be shaped in such a way that it can be thinner and lighter, with reinforcing ribs and other complex re entrant shapes possible.

So the forging process itself DOES limit you to certain more crude shapes which might of necessity end up being heavier. So forged pistons are mainly heavier because of the extra mass of material, not because the same alloy when cast is lighter.

 

[patprimmer]

As I understand it, forged pistons are manufactured by hammering whatever material into the approximate shape, the machine finishing, whereas cast pistons are made by pouring or injecting liquid material into a mould allowing to cool, the demoulding and maching.

hypereutectic is not a manufacturing process, but a material class, being high silicone content aluminium alloy.

I think (but please correct me if I am wrong) that high silicon content in aluminium alloys adds strength and reduces friction and improves dimensional stability after suitable heat treating.

Even if pistons are of identical material, they still need some clearance to allow for the fact that they run hotter than the bores, and vary by a larger margin in temperature during operation, and accross the piston (inlet side runs cooler than exhaust side).

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[SMOKEY44211]

The final skirt shape is significantly different with the high silicon alloy. I'm sure that's part of the reason for the seemingly tight clearances recomended by Federal-Mogal on some of thier aftermarket pistons. (.0015) Looks like GM is throwing a few bucks away on the cold operation noise complaints in thier new inline 6. Maybe in the end it will prove to be better. To me it seems like change for the sake of change with little if any improvement.-----Phil

 

[turbododge]

One thing that has not been mentioned is that the high silicon pistons are considerably more brittle and do not handle impact well. Detonation in any engine, and particularly boosted engines, can break high silicon pistons much more easily than the low silicon, high ductility versions. If you are going to run high silicon pistons to get the benefits of the tighter fits (I do in our twin turbo V8) you have to be very certain you have the correct state of tune and control so that you don't get detonation.

 

[Tmoose]

Drop a TRW/Sealed power piston on the floor and the tail of the skirt will bend pretty far and not break. Never tried it with a Hyper piston. 70s stock Ford/Chevy/Harley pistons would crack I think.

Always best to avoid detonation.

It takes a pretty serious plasma Moly face to stay put when subjected to detonation. Seen a few with the milled channel in the face empty in big patches.

Modern mapped ignitions and fuel management are amazing.