ok i got into a little arguement with some other kids last night. Now, i remember in autotech my instructor mentioned the piston rings expanded when the engine was warm and they expand to what size the are supposed to be ran at when the motor is at operational tempurature. Obviously it's like .000x inches or w/e so its not like its expanding to the point where the piston can't move and not contracting where it wasn't holding compression.....

any comments?

 

There's a zillion different things going on that are different for cold vs. warm engines. They all contribute in their own way...

Thermal expansion is EVERYWHERE. Not just rings, but valve clearances, bearings, etc.
Oil heats up & it's viscosity gets lower.
Fuel get easier to vaporize so you don't need to inject extra just to get the right mixture.
O2 sensor heats up & starts working, then your ECU can control mixture better.
Cold cylinder walls condense fuel which gets past rings into oil.
Cold combustion chamber quenches flame-fronts.

 

thank you. i just wanted to kno about the piston ring expansion part. See, i do pay attention in class lol. If the ring's aren't expanded (right op temp) you get blow by into the oil.

thanx.

 

The rings own 'spring'-tension holds them out against the cylinder wall. Plus they're a little self-energizing by the pressure getting behind them. Thermal expansion would be really REALLY tiny for a part that size; and more related to ring end-gap then ring/cylinder pressure.

Blowby when the engine's cold might actually be more from oil viscosity & dilution with fuel.