Billet pistons shape optimization

[Mauroc2r]

Hello,

In my Company C2R Engineering we made a set of billet pistons out of 2618 Alloy. The pistons worked great, and actually won their first race. Nonetheless looking into the cylinders with a Bororscope we see some scratching on the clinder liner. Our hypothesis is that when the engine is cold, due to the clearances needed in 2618 the piston thrust is rotating the piston in the liner and scratching it we suspect this because of the noise the engine makes when cold. Once the engine gets hot, it sounds normal and works perfectly. We have barrel and cam profiles in the piston skirt. One option for reducing it would be to lengthen the skirt, but we obviously don't want to this for a racing application. Any tips on cam and barrel profiles that can be useful? We have FE capacity, any insight on the thermal loads of the piston in order to calculate expansion at different points during engine warm-up?

Thanks in advance

Mauricio Toro

 

[Tmoose]

Way more info needed, mostly pictures.

A leak-down test of all the cylinders before engine disassembly would be a good first step.

"scratching" in the traditional sense generally "sounds" more like dirt or other contamination.
If Galling, scuffing, or scoring better describe the damage the piston skirt profile //might// be on the list of suspects.

Examination of the piston rings, the piston skirts, and the entire cylinder wall needs to be made, with bright light and magnification, and the damage quantified.

http://engineaction.com/FailureManual.pdf

https://www.ms-motorservice.com/fil...n-damage-Recognising-and-rectifying_52014.pdf

https://www1.gardnerinc.com/dealers/kohler/kohler_study_guide/10 - Failure Analysis Guidebook.pdf

 

[TugboatEng]

I feel that the piston shouldn't be able to scratch the cylinder. It can, however, transfer aluminum to the cylinder surface by galling if contact pressures are excessive. Aluminum transfer may look like scratches through a boreoscope.

Are the scratches in line with the skirt or are they in the 2, 5, 8, 10 o'clock positions?

 

[gruntguru]

It should be a simple geometric calculation to assess how far the piston can "rock" with cold clearances. A longer skirt (on the thrust faces only) does not necessarily increase friction. Of course the rocking may not reduce if the extended skirt protrudes below the bottom of the bore.

je suis charlie

 

[EHudson]

Some check the fit by applying a coating that ablates so that the piston shows you the preferred shape after some run-in. Are you coating the skirts with anything?

 

[TugboatEng]

I mentioned the scuffing at 4 points because pistons tend to expand a lot more around wrist pin bosses. Perhaps it's getting tight in that area.

 

[enginesrus]

And what material is the cylinder made of? All liners scratched? Is if very slight or very deep scratches? What is the ring material? What is the oil used?

 

[SwinnyGG]

I feel that the piston shouldn't be able to scratch the cylinder

Don't forget that any aluminum piston is actually an aluminum piston coated with a micron-scale layer of aluminum oxide, which is very hard.

Aluminum pistons can and frequently do scratch/score/scuff steel liners if there are problems with the geometry that determines how they interact.

 

[Mauroc2r]

Hi guys, thank you all for your help and sorry for the late reply. Apparently the team manager doesn't want to take the engine appart so the galling we see is just in the boroscope. We heve done all the CAD simulations is cold temperature and in hot and we suspect the problem is in the skirt (it's too stiff) and thats what's causing the cold galling. We got an order for a second set of pistons for the team's second car. We will make them with lower skirt stiffness (Very small reduction). I will reportback to you when they are working.

 

[TugboatEng]

Is te scuffing perpendicular to the wrist pin, 15 ish degrees to the pin, or random?

 

[Tmoose]

I don't think I'd dare to infer much about piston skirt condition just from cylinder bore cope evaluation.

I would still like to see pictures of what you saw, how many cylinders have "it", if a decent dyno break-in and track warm up procedure are in use, and a bunch more.

 

[enginesrus]

So again bore material? The piston is not the problem. And how do we help when no answer is given to questions?
And sorry to sound so gruff, if our questions aren't answered there is no reason to even post the problem on the site.

 

[Pablo_C2R]

Hi my name is Pablo Gaviria and I work with Mauricio at C2R. As he told you before, the engine builder didn't want to open the engine for inspection...(it is working well when operation temperature is achived). The engine is a Renault f4r we assume the cilinder material is cast iron (we don't have enough information about it).

I hope the boroscope image might help in the understanding of the problem. the main scratches appear perpendicular to the pin within a 15 degree range.

 

[gruntguru]

Are the scratches full length of the bore or just at certain points? Rocking pistons will only impact at certain points.

je suis charlie

 

[enginesrus]

Are you talking about the wide looking vertical streaks? I would be way more worried about the horrible finish on the cylinder if that is what I am looking at in the photo. It looks like a horrible case of cutter chatter, and an unfinished NOT honed cylinder wall. That is the problem and its acting like a file on the piston.

 

[Pablo_C2R]

Yes is the full length of the bore. regarding the finished surface of the cylinders... those are properly honed and rectified within +/-0.005mm. the image was taken with an old optic boroscope. That might be the reason you are confused with the image.

 

[gruntguru]

Full length scratches are probably not a result of pistons "rocking".

Are you sure they are "scratches" and not aluminium deposits on the cylinder?

je suis charlie

 

[enginesrus]

I too think it looks more like deposited Al on the cylinder wall caused by the finish. As far as the finish you mention, how do you know? You said no one wants to take it apart? And maybe someone forgot to finish them to what you say and just assembled it. How many hours were spent in assembly?

 

[Pablo_C2R]

Ok guys, here is an update, the owner is the engine builder and is one of the largest and more successful racing team in colombia (Renault backed factory team). so i don´t think they didn't check the cylinder finish before the assembly. They have removed the cylinder head (image attached) and is definitely scratches and not Aluminum deposits, those are only present at the admission side.

Our theory is that the piston is rocking when the engine is cold in the compression stroke, due to too much clearance between the piston and the cylinder, causing the rings to scratch the cylinder. when the engine is warm enough the rocking sound disappears.

We just finish a new set of pistons with tighter tolerances at the skirt, i will let you know how those perform.

Let me know if you agree with our theory or any other suggestion.

 

[EHudson]

Looks like a bit of scuffing - micro welding of the piston material to the bore. From the location, I would look at the top or second ring land clearance. The scuffing appears to extend above the oil ring. May we see a picture of the piston?