Automotive What parameters make a long lasting engine?

[Ranger Mike]

the piston is oval in a cold state. When the piston iswarmed to operating temperature the shape is ROUND to provide maximum sealing of combution chamber and this is most effective shape to transfer the linear combustion power to the crankshaft and covert to rotary power.

 

[Stormer]

the piston is oval in a cold state. When the piston iswarmed to operating temperature the shape is ROUND to provide maximum sealing of combution chamber and this is most effective shape to transfer the linear combustion power to the crankshaft and covert to rotary power.

It's the piston rings that does the sealing, not the piston shape. And the piston rings float in the piston ring groves. So the rings take the cylinder shape, not the piston shape.

And my question is still how do you come to the conclusion that the shape of the piston when it is cold is what causes the oval wear to the cylinder?

 

[Ranger Mike]

i never said the oval shape of the piston causes oval wear to the cylinder.

 

[Stormer]

i never said the oval shape of the piston causes oval wear to the cylinder.

This is a quote of you and what i replied to:

Piston wear occurs most during start up. The engine is cold and oil has drained off the cylinder wall surface. When you cold start you do in fact have aluminum to iron rubbing. This causes wear on the es surfaces. Pistons are not round but oval. This is because when at operating temperature the top of the piston where the piston rings are located is now Round due to thermal expansion. That time period from cold start to proper operating temperature is where all the wear happens.

Over time the piston will wear the cylinder into an oval shape.

How can i interpret this in any other way than that you are implying that the oval shape of the pistons when they are cold is what causes the oval wear of the cylinder?
If not why are you even mentoning the pistons cold shape in a comment about cylinder wear?

 

[Ranger Mike]

you can interpert this any way you want. Not my problem! as i said, i never stated oval shape of the pistons when they are cold is the cause of oval wear. You mistakenly assumed this for some reason.
Piston thrust causes the cylinder wear during warm up.
It is coincidence that ovality is common wear pattern and is the cold state of the piston. I mention this because the orgional post asked for parameters to be considered for max life engine design. If you designed and engine with a round piston it would not last ten minutes.

 

[Stormer]

If you designed and engine with a round piston it would not last ten minutes.

No it would last a lot longer than 10 minutes...
In fact on a lot of homemade engines the pistons is turned on a simple manual lathe.
The only thing that means is that you need to have a larger tolerance between the piston size and the cylinder size to account for the uneven expansion of the piston. And that gives a larger unsupported portion of the piston rings because of the larger difference between the piston and the cylinder size. And On smaller engines this is less of an issue than on larger ones.

 

[Ranger Mike]

Go ahead and make um round. wish other racers had your ideas.

 

[Stormer]

Go ahead and make um round. wish other racers had your ideas.

Racers? Where did that suddenly come from?

Yes modern engines use oval pistons to account for the uneven expansion of the piston because of the uneven material thickness/geometry because of the design of a piston that is connected directly to a crankshaft with a connecting rod. That allows them to run tighter tolerances getting less piston slap and so on. Or on some big crosshead diesel ship engines the connecting rod to the piston runs in a straight line that allows you to pretty much eliminate the piston skirts and design the piston completely round making it expand evenly all the way around.

And no that does not mean that a round piston in a traditional engine could not run more than 10 minutes. As i said you just have to make the tolerances between the piston and the cylinder looser to account for the uneven expansion. Or you can even taper the skirt section of the piston to make it narrower.

The two are not mutually exclusive. Do you really think otto and diesel engines in the early 1900's had oval pistons?

 

[Ranger Mike]

glad you finally got it! good job!