Originally Posted By: Shannow
* in fact I don't run 50s, full stop (unless you want to pick on the 40 blend in my sig as having 50 in it)
0W20 vs 5W30 in 1999 E46 323i
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Your sig is gone as of this writing; time to 'fess up to the SAE 50 in the sump.
Originally Posted By: turtlevette
Originally Posted By: Shannow
The oil film thickness is massive compared to the length of the lubricant molecules.
l/d is the length on diameter of the bearing...IC engines are somewhere between the 1/4 and 1/2, but for the sake of discussion, just use the 1/4 line.
Sommerfeld number is the bottom line, and using your noggin, you can see that making the Sommerfeld Number bigger makes the oil film bigger.
Things on the numerator side that make oil film bigger are, in order that you come across them in the formula:
* radius - more shaft diameter, more film thickness;
* viscosity - more viscosity, more film thickness
* Speed - more RPM, more film thickness.
On the denominator side (make these smaller to make Sommerfeld number bigger)
* radial clearance (smaller radial clearances, more film thickness)
* Applied pressure (smaller loads, more film thickness).
So a thin lubricant, e.g. for formula 1 use (tight clearances, very high RPM) is of zero suitability for a diesel with high BMEP, and low RPM potential.
Stribeck curve uses the last portion of Sommerfeld to demonstrate hydrodynamic, mixed, and boundary regimes.
Note...more viscosity, more likely to be hydrodynamic. More speed, more likely to be hydrodynamic. Less load, more likely to be hydrodynamic.
Note that designers are increasingly allowing MOFT to fall into the contact zone, and are using friction modifiers to prevent failure, and lower friction.
These additives are NOT increasing the film thicknessOriginally Posted By: CATERHAM
Your lack of practical experience is showing here as you really are over thinking this.
We need not be concerned at all about MOFT. All one need know is what's the minimum safe operational viscosity (a proxy for which is minimum oil pressure) under extreme WOT conditions and the engine manufacturer/tuner will have that info acquired through experience. Getting this info is not always easy so the fall back info is the manufacturer's test oil pressure spec's.
Consider the L/D of about 1/4 as the discussion point, and 0.1 to 0.4 as being reasonable design conditions at design point.
To get the oil pump out of relief, you are running on a fairly flat part of the curve, which requires a fairly large change in Sommerfeld number (viscosity all things being equal) to effect a small change in leakage flow, the leakage flow that is required to close the bypass.
Consider that change in the MOFT chart below, where going from 0.4 to 0.1 is more than a 50% reduction in MOFT.
Chasing your oil minimum oil pressure, bypass closed target, particularly when min oil pressure is by definition on the SPECIFIED grade of oil) reduces the margin of safety in the bearings.
You've stated that it will always remain adequate, and bring in your belts and braces drivel, but you have no idea (other than lack of catastrophic failures) where you are on any of this.
MOFT further depends on load and RPM...your "spirited" driving is different to the average Astra driver, with considerably more weight, and much more moving from a standing start.
Shannow help me out here. When the terms "bypass" and "relief" are being use what are we talking about oil pressure relief valve and filter bypass?
The oil pump pressure relief valve is usually always in operation at speed to prevent filter blow out and over pressure.
AFAIK its not an on/off valve but more of a varied opening to control pressure at the upper end.
Are oil filter in bypass at their rated bypass pressure of say 23 psi like on the Subaru? Does this mean that the filter is basically in bypass anyway at anything above idle regardless of the oil used?
TIA for your insight.
Originally Posted By: Shannow
Your premise is to get the oil viscosity low enough to stay out of bypass. (while still maintaining the minimum oil pressure that the manufacturer considers low enough...on their recommended viscosity to be worthy of a rebuild or remedial action).
What you are doing is dropping the viscosity (all things being equal, load, same engine) to increase the side leakage out of the bearings to get your magical OP result.
Chart below shows the side leakage to the total circulating oil volume in the bearing..ratio of 1 is all oil supplied runs out the clearance. ratio of 0 is all oil is contained within the bearing and recirculates...It's actually why operation on bypass is harmless to the engine (although wasteful of power, the higher viscosity leaks less (less backpressure in your parlance), but spends more time recirculating in the bearings...as I've stated before, self compensating to an agree)
Consider the L/D of about 1/4 as the discussion point, and 0.1 to 0.4 as being reasonable design conditions at design point.
To get the oil pump out of relief, you are running on a fairly flat part of the curve, which requires a fairly large change in Sommerfeld number (viscosity all things being equal) to effect a small change in leakage flow, the leakage flow that is required to close the bypass.
Consider that change in the MOFT chart below, where going from 0.4 to 0.1 is more than a 50% reduction in MOFT.
Chasing your oil minimum oil pressure, bypass closed target, particularly when min oil pressure is by definition on the SPECIFIED grade of oil) reduces the margin of safety in the bearings.
You've stated that it will always remain adequate, and bring in your belts and braces drivel, but you have no idea (other than lack of catastrophic failures) where you are on any of this.
MOFT further depends on load and RPM...your "spirited" driving is different to the average Astra driver, with considerably more weight, and much more moving from a standing start.
Consider the L/D of about 1/4 as the discussion point, and 0.1 to 0.4 as being reasonable design conditions at design point.
To get the oil pump out of relief, you are running on a fairly flat part of the curve, which requires a fairly large change in Sommerfeld number (viscosity all things being equal) to effect a small change in leakage flow, the leakage flow that is required to close the bypass.
Consider that change in the MOFT chart below, where going from 0.4 to 0.1 is more than a 50% reduction in MOFT.
Chasing your oil minimum oil pressure, bypass closed target, particularly when min oil pressure is by definition on the SPECIFIED grade of oil) reduces the margin of safety in the bearings.
You've stated that it will always remain adequate, and bring in your belts and braces drivel, but you have no idea (other than lack of catastrophic failures) where you are on any of this.
MOFT further depends on load and RPM...your "spirited" driving is different to the average Astra driver, with considerably more weight, and much more moving from a standing start.Originally Posted By: Shannow
Originally Posted By: Shannow
Originally Posted By: CATERHAM
Originally Posted By: Garak
Well, if I was driving it from a cold start for about a mile, and then shutting it down for hours, and repeating, I'm sure a 20 grade would be harmless. Outside of that, stick with something of specified viscosity.
Wow.
It's like having two radios on, each tuned to a different station.
Originally Posted By: Caterham
As for your claim that running a heavier oil grade with the oil pump in by-pass or relief mode does no harm is downright foolhardy. No engine is designed to run once up to operating temp' under maximum load with the oil pump in by-pass mode thereby reducing oil flow through the entire engine. Not only will the "recycled oil" in the bearings allow them to run hotter than they otherwise would but ever part of the engine will be supplied with less oil and the oil that it does get through will be hotter. Just a bad scenario all round.
Quote:
Positive displacement pumps, unlike centrifugal or roto-dynamic pumps, theoretically can produce the same flow at a given speed (RPM) no matter what the discharge pressure. Thus, positive displacement pumps are constant flow machines. However, a slight increase in internal leakage as the pressure increases prevents a truly constant flow rate.
A positive displacement pump must not operate against a closed valve on the discharge side of the pump, because it has no shutoff head like centrifugal pumps. A positive displacement pump operating against a closed discharge valve continues to produce flow and the pressure in the discharge line increases until the line bursts, the pump is severely damaged, or both.
A relief or safety valve on the discharge side of the positive displacement pump is therefore necessary. The relief valve can be internal or external. The pump manufacturer normally has the option to supply internal relief or safety valves. The internal valve is usually only used as a safety precaution. An external relief valve in the discharge line, with a return line back to the suction line or supply tank provides increased safety.
Originally Posted By: Trav
You are advocating keeping pressure lower than the normal operating relief valve settings which is almost impossible regardless of the oils viscosity anyway.
One car comes to mind that is fitted with a mechanical OP gauge and a very strong pump from the factory, the MB diesels.
Anything above idle and the gauge is pegged! It is in relief no matter what viscosity oil you use in it.
Originally Posted By: Trav
Shannow help me out here. When the terms "bypass" and "relief" are being use what are we talking about oil pressure relief valve and filter bypass?
The oil pump pressure relief valve is usually always in operation at speed to prevent filter blow out and over pressure.
AFAIK its not an on/off valve but more of a varied opening to control pressure at the upper end.
Are oil filter in bypass at their rated bypass pressure of say 23 psi like on the Subaru? Does this mean that the filter is basically in bypass anyway at anything above idle regardless of the oil used?
TIA for your insight.
Originally Posted By: kschachn
Wow.
It's like having two radios on, each tuned to a different station.
Originally Posted By: CATERHAM
Originally Posted By: Shannow
Originally Posted By: Shannow
Originally Posted By: CATERHAM
Better than "harmless", it would provide superior lubrication.
Oil filter bypass is 3.5 bar, I have had gauges on them the oil pump relief pressure is about 6 bar and they can make that above 3000 RPM in a tight engine with a properly operating pump.
MB even installed a damper on the valve to quiet them, in the very old ones you knew the relief valve was in operation because you could hear it.
In either case the oil is still being bypassed somewhere, either at the pump or the filter at anything above idle.
The pressure relief is an absolute necessity on engines with a positive displacement pump otherwise the pump will just make pressure until it either blows the filter or breaks.
It is a more a control mechanism than just a safety valve.
Your theory seems to work in a closed dead head hydraulic system but a engine is not a closed system but an open one.
Meaning the gauge may show acceptable oil pressure but all the bearings may not have the indicated pressure depending on wear, 001 of an inch wear on a bearing can have a 20% reduction in pressure at that particular bearing.
I would think that bearing(s) is depending more on film strength to stay alive more than the others. From what i understand from this board is that generally lower viscosity oils have a lower film strength.
That being the case your oil pressure gauge argument goes right out the window as it only monitors the main oil galley pressure not the pressure at the individual bearings.
what do i know i am just an old wrench turner but i think there is a lot more to this than you are presenting or taking into account.
Originally Posted By: Trav
That being the case your oil pressure gauge argument goes right out the window as it only monitors the main oil galley pressure not the pressure at the individual bearings.
what do i know i am just an old wrench turner but i think there is a lot more to this than you are presenting or taking into account.
Originally Posted By: Shannow
Only design that I've done that I'm embarrassed about (serves as a good example when mentoring 'though) was a turbine jacking system, where I matched pump flow to system resistance perfectly. Problem was that once the oil thinned at temperature, the internal leakage of the pump increased (thinner oil pumps less in Positive Displacement pumps), I lost flow through the releif/pressure controller, and the system couldn't meet minimum pressure to lift the bearings.
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Originally Posted By: Shannow
Saves energy, the heating effect that CATERHAM talks about...to the tune of about 50-70 watts.
Originally Posted By: Trav
From what i understand from this board is that generally lower viscosity oils have a lower film strength.
Originally Posted By: Trav
That being the case your oil pressure gauge argument goes right out the window as it only monitors the main oil galley pressure not the pressure at the individual bearings.
what do i know i am just an old wrench turner but i think there is a lot more to this than you are presenting or taking into account.
Originally Posted By: CATERHAM
What, you drive like a granny already (hi Garak)? Well then you don't have a whole lot to be concerned about.
Originally Posted By: turtlevette
It"s a lot more than that. Why would the trend be for variable displacement pumps to save that little. Its probably more like a couple of horsepower. 1500 watts or so.
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