Why does a car sputter when cold?

[GarthC]

I have read about temp sensors, fuel mixes etc but I am wondering why does it actually happen, surely once the pistons and valves etc have turned over and have run for a few seconds they should run smooth, what is it about cold weather that makes the engine run differently? Is it a question of contracting metal? A 20 degree Celsius change in engine temp surely cant affect the fiction of the metal parts?

 

[dgallup]

It's almost completely due to poor air/fuel mixture formation. Cold fuel has a low vapor pressure and a lot of it remains liquid and does not mix with the air or burn. There is a huge cold start enrichment to make up for it whether it's a mechanical choke in carburetors or simply programed in delivery adjustments in EFI. Many of the advances in cold start emissions reductions have to do with improved mixture formation, the others have to do with faster warm up. There are some additional friction losses due to pumping cold oil but that is just a relatively small parasitic loss.

Most modern fuel injected cars run very smoothly right from the start although they will have an elevated idle speed and they are running richer than normal.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[GarthC]

Thank you dgallup, that does help me.

Garth