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

 

[TugboatEng]

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

 

[SwinnyGG]

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? (180[sup]0[/sup] around the same cylinder liner)

 

[Pablo_C2R]

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

 

[Tmoose]

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 ?

 

[enginesrus]

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

 

[SwinnyGG]

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.

 

[TugboatEng]

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.

 

[enginesrus]

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.

 

[TugboatEng]

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.

 

[gruntguru]

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

 

[Tmoose]

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