Piston to wall clearance, during operation. 4

[Ibrahim M]

Hello,

On car engines it is common practice to run about 0.05mm clearance betwern piston and cylinder.

But what does this clearance become when at operatimg temps, like full load ???

Some person says that the pistons becomes a slight press fit but i dont believe it.

Thanks

 

[TugboatEng]

If the out of round were meant to compensate for thermal expansion, wouldn't it be less expensive to simply machine the piston hot vs using expensive multi-axis machines?

 

[SwinnyGG]

An aluminum piston being machined 'hot' would need to be sprayed with coolant to deliver the required surface finish, IE it would remain 'hot' for maybe the first 5 seconds of the operation.

 

[TugboatEng]

The coolant would be heated as well.

 

[SwinnyGG]

Even so, it would be impossible to produce the temperature gradient across the piston which is present in operating conditions, and is the reason why a piston would be produced with a complex envelope shape. The difference in temperature between the skirts/pin bores and the crown can be 150 degrees or more. In order for 'hot' machining to produce the right shape, you would need to apply this gradient and maintain it continuously for the entire machining operation.

I also suspect that at scale, operating a machine in a cloud of oil that's being maintained at 300 deg C is not cheaper/safer/more efficient than having a dedicated machine which does the operation at room temp.