Thicker oil better for engine start up?

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I see and read lots of people talk about how thinner oil is better for engine start up because it "gets to the bearings faster". I have not really put any thought effort into it, and generally considered it an acceptable argument. After some thought, I am not so certain this is a valid argument.

Assumptions:
the viscosity of the oil is not so high that it would cause oil pump cavitation
oil follows the same path for

Some quick points:
in general, Volumetric efficiency of a fixed displacement pump INCREASES as fluid viscosity increases

so, when an engine is first started, the pump starts pumping the exact same theoretical volume of oil (displacement * speed), but the volume of oil being pumped is actually a function of its volumetric efficiency (displacement * speed * Ve), where the efficiency decreases the volume of oil being pumped. If the volumetric efficiency for two identical pumps is the same, and the only difference is the viscosity of the fluid, then the pump pumping the fluid with the higher viscosity will be pumping more volume.

If the pump pumping MORE fluid due to the higher viscosity has the same oil passages to fill to get to all the parts of the engine, then how does it take longer for the oil to get to all the components when the oil has a higher viscosity? Not to mention that the volumetric losses through bearings/etc. are lower due to the higher viscosity, further increasing the volume of oil available to fill the oil passages, and get to the extents of the oil system even more quickly.

Another question...does bulk modulus also increase with increased viscosity? If so, that would also help this argument.

Looking forward to the discussion.
 
Interesting topic. As Trav has stated earlier today, and several others have stated over the years, bearings already have enough residual oil to protect them from a cold start or from sitting for a long period of time. If I were inclined to worry, I'd worry more about cylinder walls than bearings during a cold start.
 
During a cold start, thin oil is good for about 30 seconds. After that it is a liability. 30 seconds is about how long it took for the thrust bearing in BIL's Datsun 510 to quiet down in a cold start.
grin2.gif
 
This has been pointed out numerous times and there is a thread or two that explains the difference between fixed volume/variable pressure and fixed pressure/variable volume pumps. However the most enthusiastic thin oils fans flatly ignore all these inconvenient facts and keep on harping flow=lubricating arguments.
 
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Just a few quick thoughts: The oil pump is a positive flow pump...but the oil pressure relief valve dumps it all back into the crankcase. The multifaceted report....funded by GM and several major oil companies and published years ago in the SAE journal (sadly, you need to be a member to read) noted that:

The pistons/cylinders are only lubricated by oil that flys offs the rod bearings and the tiny squirter hole on the rods. The crankshaft oil is fed from the mains, though the crank, then into the rod bearings....by then under almost no pressure and no flow when thick and cold....the end result is only one drop every few seconds flying up to the cylinder/piston interface. Later, when the oil is hot and thin this is a continuous shower of oil. The conclusion of this study was related to the switch from 10W30 to 5W30.....and that many typical driving profiles that this alone would increase engine life by 30 percent. Data was reinforced by multiple testing procedures, including pistons, rings and sleeves using various radioactive isotopes and running instant real time wear measurements. The first few minutes of operation cause more wear then the next hour!

Wish I could provide a link to this published study (likely 30 years ago) but years ago while using 10W40 and 20W50 I often rebuilt engines after 100K miles. Now using 0W20 in sometimes identical engines and with no substantial design changes for 300K miles and seeming no wear from a users viewpoint.

Yeah, no reason to doubt that thinner oil at startup is important for those of us in the north.

fsskier
 
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Originally Posted By: DriveHard
I see and read lots of people talk about how thinner oil is better for engine start up because it "gets to the bearings faster". I have not really put any thought effort into it, and generally considered it an acceptable argument. After some thought, I am not so certain this is a valid argument.

Assumptions:
the viscosity of the oil is not so high that it would cause oil pump cavitation
oil follows the same path for

Some quick points:
in general, Volumetric efficiency of a fixed displacement pump INCREASES as fluid viscosity increases

so, when an engine is first started, the pump starts pumping the exact same theoretical volume of oil (displacement * speed), but the volume of oil being pumped is actually a function of its volumetric efficiency (displacement * speed * Ve), where the efficiency decreases the volume of oil being pumped. If the volumetric efficiency for two identical pumps is the same, and the only difference is the viscosity of the fluid, then the pump pumping the fluid with the higher viscosity will be pumping more volume.

If the pump pumping MORE fluid due to the higher viscosity has the same oil passages to fill to get to all the parts of the engine, then how does it take longer for the oil to get to all the components when the oil has a higher viscosity? Not to mention that the volumetric losses through bearings/etc. are lower due to the higher viscosity, further increasing the volume of oil available to fill the oil passages, and get to the extents of the oil system even more quickly.

Another question...does bulk modulus also increase with increased viscosity? If so, that would also help this argument.

Looking forward to the discussion.


Good discussion. I suspect if the oil was very cold and thick the pump might cavitate because it is simply atmospheric pressure (approx. 14.7 psi) that feeds the inlet to the pump. The pump is bolted to the block but there is a pickup tube extending down into the sump. That would be one issue.

Sf
 
What most miss is depending on the temps at start up is how long the oil takes to go from the oil pan to the oil pump. At room temps a straight 30 or 40 will get to the pump plenty fast. At -5*f it may never reach the pump
 
Originally Posted By: fsskier
Just a few quick thoughts: The oil pump is a positive flow pump...but the oil pressure relief valve dumps it all back into the crankcase.


It definitely does not dump it all back into the crankcase. This is easily demonstrated with a valve cover off and watching the oil come out the pushrods. One can also overwhelm the bypass, which is observed by indicated oil pressure exceeding the relief pressure. I've seen both on a cold start with heavy oil in an SBC.
 
Originally Posted By: Snagglefoot
Originally Posted By: DriveHard
I see and read lots of people talk about how thinner oil is better for engine start up because it "gets to the bearings faster". I have not really put any thought effort into it, and generally considered it an acceptable argument. After some thought, I am not so certain this is a valid argument.

Assumptions:
the viscosity of the oil is not so high that it would cause oil pump cavitation
oil follows the same path for

Some quick points:
in general, Volumetric efficiency of a fixed displacement pump INCREASES as fluid viscosity increases

so, when an engine is first started, the pump starts pumping the exact same theoretical volume of oil (displacement * speed), but the volume of oil being pumped is actually a function of its volumetric efficiency (displacement * speed * Ve), where the efficiency decreases the volume of oil being pumped. If the volumetric efficiency for two identical pumps is the same, and the only difference is the viscosity of the fluid, then the pump pumping the fluid with the higher viscosity will be pumping more volume.

If the pump pumping MORE fluid due to the higher viscosity has the same oil passages to fill to get to all the parts of the engine, then how does it take longer for the oil to get to all the components when the oil has a higher viscosity? Not to mention that the volumetric losses through bearings/etc. are lower due to the higher viscosity, further increasing the volume of oil available to fill the oil passages, and get to the extents of the oil system even more quickly.

Another question...does bulk modulus also increase with increased viscosity? If so, that would also help this argument.

Looking forward to the discussion.


Good discussion. I suspect if the oil was very cold and thick the pump might cavitate because it is simply atmospheric pressure (approx. 14.7 psi) that feeds the inlet to the pump. The pump is bolted to the block but there is a pickup tube extending down into the sump. That would be one issue.

Sf


That's what the MRV test is designed to replicate, the ability for the oil to be drawn into the pump.
 
Originally Posted By: Garak
Shannow has posted extensively about timed oil flow. If he stumbles across the thread, he might be able to post his charts again.


This chart would be interesting with very cold oil:

 
Originally Posted By: Garak
Shannow has posted extensively about timed oil flow. If he stumbles across the thread, he might be able to post his charts again.


This is the usual counter-post, though its from a rather old study and you'd think there'd be a bit more data on this rather basic question. IIRC rocker oiling time was judged on the basis of sound.

 
Originally Posted By: NGRhodes
http://www.emeraldinsight.com/doi/pdfplus/10.1108/00368790010352691

"This study has demonstrated a correlation between relative average cylinder liner wear rates at low engine start-up temperatures, base oil composition and oil viscosity"

Emphasis mine.


So I have to pay 32 USD to read that and then if I'm convinced I have to buy "full synthetic SAE 5W40 grade oils based on Polyalphaolefin"", and a diesel truck.

Ooer! Just as well cold starts mean 10C here.
 
Originally Posted By: 4WD
This chart would be interesting with very cold oil:

The Esso video on YouTube does show what happens when one tries to pump an absolutely unsuitable oil for the ambient temperature.

Ducked: Imperial Oil likely did more in their cold weather studies. Finding the data is another matter.
 
OK, you hop in and turn the key.

The first thing that has to happen is that the engine cranks...to simulate this behaviour, the standard test is the "Cold Cranking Simulator" (CCS on the J300 viscosity charts)...it's important, as no crank equals no start.

Here, thick is not your friend.

It also tells you something else...

That the oil wetted components, as many of us have been saying forever, are still full of oil. Those of us who have pulled stuff apart, have never pulled apart a dry engine. Bearings, pistons, and the ring belt are full of the stuff.

That's what provides the drag to the starter motor.

If the oil all drained back, then there would be no difference in cranking speed at any temperature.

The fact that the engine starts wet, and with viscous drag says that lubricant is there.

(the tribofilms that exist are also there, and they take a while to wear away anyway).
 
Engine cranks, and hopefully starts.

At that point it reaches a self sustaining speed, and the oil pump starts pushing oil into the galleries...same volume every revolution.

First thing that needs to happen is that the oil that's pushed towards teh engine needs to be replenished with oil coming in behind it, through the friction of the pickup tube...and then to replenish THAT the oil in the sump has to "fall" into the suction screen.

Failure at this end is "air binding"....not enough oil makes it to the pump, and the pickup tube acts like a straw on an overly thick thickshake, that sudden apparent release of suction pressure, and no flow.

The metric for this is the MRV, Mini Rotating Viscometer.

Two metrics within that. Firstly the viscosity itself at the temperature...too much drag on the pump inlet increases the chance of air binding. Then there's the "yield stress" component, in that the oil cannot stand up like a Jello block, it has to be liquid enough to fall into the pump pickup.
 
The oil has to flow through the galleries.

That's another low shear viscosity component.

Positive displacement pump, provided that the oil is pumpable in the last example pushes the same volume ahead of it and into the galleries. Galleries are a finite volume, and take the same number of pump rotations to fill.

In theory, with no viscosity, no pressure is developed until the galleries are full.

In practice, the lines TO the relief valve, and FROM the relief valve to the engine are pretty small, so viscosity has an effect, and pressure can build due to viscous effects, and before the galleries are full.

The relief can be pushed open before the galleries fill, and when it's cold they will...but it's not all the flow dumping to the sump...the relief set at 60PSI say has 60psi pushing the oil to the remote points of the engine...and it's only pushing air out agead of it.
 
Galleries full, squirters start flowing.

They are pressure/density, and quite immune to viscosity.

Traditional style engines just used the oil slinging out of the big ends, and they too are full.

Just the more viscous the oil is, the less flow there is from a big end...but the more viscous the oil is on the walls and pistons too.

However, even at running temperature, the oil in the piston/ring area has a residence time of 10 seconds or thereabouts. At mid stroke, when the piston speed is highest, the oil is ALWAYS too thick, and the reversal points top and bottom are pretty well additive dependent
 
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