Oil flow vs. viscosity

[glennc]

I've been re-reading some of the AEHaas articles where he makes what I feel to be convincing arguments that thinner oils have many benefits and few drawbacks in the vast majority of driving conditions. One argument I haven't found convincing is that there is an inverse relationship between viscosity and flow: less viscosity, more flow; more viscosity, less flow.

It seems like a simple question: aren't most oil pumps of a positive-displacement variety, and if so wouldn't the flow be based entirely on rpm and completely independent of viscosity?

Or am I mistaken about oil pump design, or otherwise missing something?

 

[Mudcat]

Yes, you are correct regarding positive displacement pumps.

 

[Gary Allan]

Yep. What most use as "faster" is really "easier". That's the whole basis for using lighter fluids.

That said, there are some pump designs that fall out of the conventional view. They appear to have more wiggle room on the volume/pressure inter-relationship and have an appearance that would not be totally incorrect if stated as "volume on demand".

I don't fully understand how they work ..but I think that the Titan V8 has one such pump. It appears to maintain a uniform pressure until you hit engine speed outside of the internal compensating mechanism. Indications on this engine appear to be altered by what oil filter that you use (cold indications), which would normally not be possible outside of a mechanical defect...much like having a uniform pressure would not be (typically) possible at varied engine speeds over the full spectrum of oil temp/visc.

 

[jpr]

I don't have the answer, but I think I can contribute a couple of good questions -

I think you need to add "pressure" to the question, that is, "what is the relationship between oil flow, oil pressure, and oil viscosity?"

The leads to the secons question - flow to where? Somewhere in every system is an over-pressure relief valve. The fluid will always take the path of least resistance.

 

[Gary Allan]

Oil flow is harder for most to integrate. It's a flow dictated equation that defies our normal observed pressure over resistance experience. Viscosity adds an impedance factor that can also alter more advanced models. That is, if the relief is not employed, then it's a flow dependent equation. If the relief is active, it reverts to the pressure over resistance model that we all know best (garden hoses, wall outlets, etc.).

It requires an adaptive view.

 

[TallPaul]

Need enough pressure that the oil will flow through the top end in sufficient quantity. Generally anything from a 5w20 through a 20w50 will do it for a warm engine, but the optimal is towards the thinner end of this range. Personally I like 10w30 as an excellent compromise. For a racer, thinner is better so long as thick enough for the engine to survive the critical race. Of course racers use high flow pumps so it's a different ballgame. One safety factor in going thinner is to use Redline which generally has the HTHS of the next higher grade (i.e., their 5w20 has HTHS of a 30 weight).

 

[1sttruck]

Pump pressure will depend upon the pump, the range and number of clearances the oil is expected to flow thru, temperature, the oil, and the relief valve. Flow will vary depending upon clearances encountered, temperature, the oil, and the pressure, where it's important to flow in all of the smallest clearances as designed as well as the primary oil channels. People like to state that single grade 20 wt was often used in ye olden days and it worked well, but it was merely the best compromise between enough protection when warm and the ability to flow when cold.

If the oil is too thin there is too much 'leakage' in the distribution of clearances and pressure will drop to lower than desired levels, something that can be common in older engines on hot days. In general though in spite of the claims about 'tighter clearances' and the need to use thinner oils, mots of the clearances are dictated by the thermal properties of the materials being used and the temperature range they're expected to operate under, and haven't changed much at all. Better manufacturing capability has enabled better control of clearances, so tighter distributions have meant that there are fewer larger than desired clearances.

US driving conditions, oil change intervals, and vehicle ownership patterns enable dino 20 wt multigrade oils to be used with almost no problems when recommended.

 

[Gary in Sandy Eggo]

Oil pumps are gear pumps. Volume of oil pumped per unit time is rpm dependent, and constant regardless of viscosity. Any quantity of pumped oil that cannot make it through system as a result of viscosity will be vented past the oil pump pressure relief valve, a simple ball and spring check valve. They're typically set anywhere from 60 to 90 psi.