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Billet pistons shape optimization

Billet pistons shape optimization

Billet pistons shape optimization


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

RE: Billet pistons shape optimization

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.




RE: Billet pistons shape optimization

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?

RE: Billet pistons shape optimization

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

RE: Billet pistons shape optimization

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?

RE: Billet pistons shape optimization

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.

RE: Billet pistons shape optimization

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?

RE: Billet pistons shape optimization

Quote (TugBoatEng)

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.

RE: Billet pistons shape optimization

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.

RE: Billet pistons shape optimization

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

RE: Billet pistons shape optimization

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.

RE: Billet pistons shape optimization

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.

RE: Billet pistons shape optimization

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.

RE: Billet pistons shape optimization

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

je suis charlie

RE: Billet pistons shape optimization

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.

RE: Billet pistons shape optimization

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.

RE: Billet pistons shape optimization

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

RE: Billet pistons shape optimization

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?

RE: Billet pistons shape optimization

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.

RE: Billet pistons shape optimization

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?

RE: Billet pistons shape optimization

The scuffing looks like it extends above the top compression ring.

RE: Billet pistons shape optimization

Scuffing definitely extends above the ring pack. That helps, since you know it can't be flutter.

Is the pattern the same on the other side? (1800 around the same cylinder liner)

RE: Billet pistons shape optimization

At last, they took the engine a part, as i told you before, the scratches are only present in the admission side of the piston.

As for the ring land clearance the cylinder is bored to 82.73 mm

RE: Billet pistons shape optimization

How does the other side of the skirt look?
Heavily scuffed/scratched at the very bottom of the skirt, 180 from the top land heavily scuffed?

I'd expect the top land ( above the too ring ) to have the most clearance, and it looks like it could use more.
It is cylindrical most likely, close to a solid disk of aluminum, and the hottest part of the piston, so clearance is it's only defense mechanism to keep off the cylinder wall.

if they are not running air cleaners, they should.
If they are, the air cleaners are not very effective.

Closeups of the ring faces would be informative.

Oil analysis looking for chemistry related to the region's soil or just sand would be interesting too.

Running a single compression ring ?

Formula Renault ?

RE: Billet pistons shape optimization

So then that is opposite the major thrust side? And looks like washing and oil film break down.

RE: Billet pistons shape optimization

You can't have oil film breakdown in an area where there's no oil film.

I think THE MOOSE is on the right track re: clearances.

RE: Billet pistons shape optimization

I think if the piston were rocking you would see similar damage on the bottom of the skirt on the opposite side.

I have seen this kind of damage on diesel engines. I wasn't involved in the teardown or prior operation so I never really got to determine a cause. I know the engine had some bad fuel injectors. I thing jgKRI is on to something. Fuel wash is a possibility.

RE: Billet pistons shape optimization

jgKRI (Mechanical) 1 Nov 17 12:48
You can't have oil film breakdown in an area where there's no oil film.

I think THE MOOSE is on the right track re: clearances.

? And what makes you think there is no oil film there? If there was none then the engine or piston ring / cylinder life would be very short lived. 100 hours maybe? And what does fuel washing do then? The ring pack is constantly carrying oil / lube to that area.

RE: Billet pistons shape optimization

The area is above the oil control ring, there is very little oil up there to start with. The rest of this comment is purely speculation. A poorly designed intake port or some combustion chamber protuberance can knock fuel droplets out of suspension and cause them to collect on things like cylinder walls. This can wash away the miniscule amount of lubrication that is there.

RE: Billet pistons shape optimization

All sliding parts in an IC engine are separated by an oil film. (Unless something is wrong)

We need the answer to Tmoose's question.

Is the bottom edge of the opposite skirt showing signs of loading? It may not be scuffed thanks to the superior lubrication further down the bore.

Reducing the skirt clearance is definitely your best move. Next is increasing top land clearance.

Pretty sure this is not happening during warm-up.

je suis charlie

RE: Billet pistons shape optimization

There are some smart experienced engine building/designing folks, including at least one piston manufacturer over on http://www.speedtalk.com.

I value most of their opinions.

Dan T

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