Originally Posted by OilUzer
Originally Posted by SonofJoe
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3) Burnt oil. This could be due to worn valve seals or worn rings. However nowadays, the most likely source of burnt oil is light base stock, stripped out of the crankcase by hot blow-by & recycled through the PCV system. If you want to minimise burnt oil related deposits, use a low Noack oil. ...
Hope I'm not derailing the thread ...
I've read suggestions/opinions that the recycled oil through the pcv is oil mist and independent of noack number.
Question:
is oil mist or vapour like water evaporating without being boiled? You would think the higher the noack number, the higher the evaporation in general regardless of "evaporation temperature".
Mist is also liquid phase of vapor so again it can't be independent of noack number! No?
Can you explain this please?
Thanks.
I'll try and give you the honest answer...
In the world of engine oil volatility/oil loss, there are three ways of looking at things.
- There's real-life where a oil might spend say 250 hours fluctuating between ambient & 100°C
- There's the Noack test which whacks up the oil to an unrealistically high 250°C for just one hour
- Finally there are the industry standard engines like the Sequence IIIG which keeps the oil at 150°C for 100 hours on a cyclically operated engine.
I can say absolutely categorically that with the industry standard tests (not just the US Sequence IIIG but also the European Peugeot TU5), oil consumption IS very Noack dependant. The correlation isn't perfect but nor would I expect it to be.
In both tests, you can see from the used oil analyses, carried out during the test, that the oil is, for want of a better word, 'distilling', with simple, additive-free light base oil exiting the crankcase with the blow-by & leaving all of the heavy stuff in the oil in the sump. This implies true 'evaporation' of oil into the vapour phase; not just physically 'misted' fully formulated oil (although it's quite possible the two look alike to the naked eye).
Just to highlight how big a deal this is, on the TU5, the oil contaminated blow-by is not returned to the intake system but cooled, condensed & vented. Put a 15% Noack oil on the TU5 and in 72 hours you might completely empty the sump of oil! However you usually get very clean pistons at end-of-test because no oil has been burnt.
On the IIIG, oil contaminated blow-by is (just like on a real car) routed through the PCV/intake system and subsequently burnt. It's impossible to empty the IIIG sump of oil with a 15% Noack oil but I guarantee that the pistons will look diabolical at the end of the test!
The big question, one that people like me can't honestly answer, is how does this all relate to real life, where the oil is generally cooler but exposed to hot blow-by for far longer? Instinctively I'd say this is less severe but some oil will ALWAYS get evaporated & burnt because that's how multi-component vapour-liquid equilibrium works. Also real life involves stuff like fuel dilution (something neither the Noack nor industry tests addresses) and the cyclic re-evaporation of condensed gasoline & water from the sump will act to exacerbate the distillation of base oil from the sump.
So IMO, if you want clean pistons, think low Noack.