Mikes reply:
All of this is from what you asked me to read, people can make up their own minds, I have given my opinion and I am sticking to it.
(1) No engine has 100% cooling provided by the circulation of engine oil. Every engine has either a liquid or air-cooling design. Liquid cooled engines usually have some air cooling as well.
I have had this discussion/argument over decades with mechanics and car enthusiasts.
What I learned, many years ago, in Cat Engine diesel school was an oils properties (in order):
1. Cool engine parts
2. Lubricate
3. Keep the engine clean and free of contaminates
We were taught the same when I was trained on the F-16 in the Air Force.
(2) Respectfully they have it backwards then
The first and most distinguishing characteristic of any oil in any tribology regime is to reduce the COF of bodies in motion (lubricate)
Then comes heat transfer (cooling)
Then washing/cleaning
Sadly, there is a large body of trade literature out there that teaches this also because the term "cooling" is often misleading and misused.
(3) My 2011 BMR R1200R MC was described as air/oil cooled. Much oil is directed to the area of the head around the exhaust valve to remove heat. Works I guess, but a water-cooled engine soon followed.
(4) So, everyone here on this board should believe that the thinner oil flows and cools better theory is a total fallacy?
I think the theory needs some rephrasing.
First, 100 gpm is 100 gpm regardless of if I am pumping warm water, slurry at a mine, caramel at a food plant or tar at Oil Sands Canada so "Z"s initial statement on that is correct.
"Better" is a word that's meaningless unless qualified and in this case, it isn't so we will remove it.
In heat transfer, viscosity is an important factor but other critical factors are, total heat holding capability of the fluid (retention and removal), total volume of fluid working to remove the heat, total surface contact area, temp of the fluid entering, how fast heat is removed from the fluid.
This is also not counting the endothermic properties where the fluid itself creates heat during the working process.
These are all the things the design engineer has to figure out when devising things like what type oil, volume, flow, mass of heat sinks, air flow, fins etc. They all have to work together in a circuit to keep a machine operating at design temperature.
So, changing one part of that very long equation "can" add a benefit to a point but it "can" also create a problem and reduce thermal transfer too. (usually by creating a bottleneck somewhere else)
(5) Since on a hot engine the oil pump isn't going to be running in bypass mode until high rpm, the oil volume flowing through the engine is not dependent on viscosity. The oil pump displaces a fixed volume per rpm. Also, the majority of the engines operate in full hydrodynamic lubrication.
Moreover, the heat carried away by the oil is largely generated by the oil. In fact, the oil heats the bearings. Lower viscosity oil will generate less heat through shear friction and result in cooler engine parts and less fuel consumption. You can lower viscosity only so much before the risk of extended metal to metal contact during periods of low oil flow rates becomes excessive though.
(6) Bearings are a unique pressure and temp regime … but both the GM and Mopar in my driveway have piston cooling jets/coolers … one more job for our favorite motor oils …
(7) Finally Found the paper again... Interesting in the viscosity and piston temperature, and how much (little) effect was gained by cutting off access to oil spray under the pistons, indicating that viscous shear on the cylinder walls played more of a part than "cooling" from the oil. (Yes, it's old)
http://naca.central.cranfield.ac.uk/...report-698.pdf
(8) Isn't oil flow to be considered in areas other than those which are being pushed by the oil pump?
Yes, no and maybe and in every case its more of a calculated "guestimate"- here's why.
As stated, many times here, any person can calculate flow from a PD pump going through a tube and a void area equation and deduce thermal transfer. (basic heat exchange)- you just got to know the dimensions, expected heat and flow.
Let's call the other "splash and fall" (or pool and fall) to cover everything with spray, cavities or whatever.
We might reasonably know the total internal surface area (since we designed it and someone has to cast/machine it) and have a good idea of the overall regional (s) skin temp from the material analysis. (There would be more than one heat region)
What we won't know (with any degree of accuracy or consistency)
The total volume (in terms of thickness and "hang time") of the coverage of that area since all non "pipe flows" are random and subject to various influences.
We really wont fully know the absorbed heat/temp of the oil as it hits the area to start transferring heat (that's critical because a given volume of anything has a peak absorption until either it can't take any more or a change of state starts)
So, those 2 big agglomerates of variables exist in the engine, affect the overall transfer and are real and need to be factored in- nobody disputes that.
Capturing and calculating that data is a little more difficult in reality so we do the best we can then usually add a buffer.
(9) Moreover, the heat carried away by the oil is largely generated by the oil. In fact, the oil heats the bearings.
To add as well, the convective heat transfer rate is mostly the same regardless of oil viscosity. It's also largely the same between oil groups I-IV. Some group V glycols, naphthalene's, and esters can have better heat transfer rates largely due to their higher density. Also note that air entrainment must be considered as an oil that's aerated will not transfer heat well regardless of composition or viscosity. Shearing should also be considered.
The heat generated in the bearings is coming from the friction in the bearings mostly. Very little combustion heat makes it to the bearings. NASCAR cup engines ride a fine line in this area as they typically see ~75°F temperature rise in oil temp at bearing exit. Sump temps at ~280°F with bearing temps at ~355°F with a 0w-20 oil.
Most of the engines operate in mixed or boundary lubrication. The bearings are the exception as well as the rings and pistons during peak piston speeds. Given there's sufficient MOFT to maintain full hydrodynamic lubrication in those areas, of course.
We'll all be fine and dandy but not considering heat and absorption of heat in oil not within the galleries but let's consider the already heated oil returning to the sump. I would think a thinner oil would return to the sump quicker and, as such, contribute to cooling the engine.
That's your next problem
The heat that's "absorbed" let's say on round 1 (for simplicity's sake) has to be removed from the oil to cool it down so it can absorb more otherwise the circulating oil will make it hotter.
It's overly simplistic (and wrong) to believe velocity alone will achieve some level of heat removal.
The key to heat exchange be it in an engine or a shell/tube or any other configuration is balancing to achieve optimum results in the soak time for the fluid to absorb heat to its potential then the time it takes to remove said heat from the fluid so it can do it again.
In general terms, a sump (in and of itself) does little to assist in removing heat from the fluid so there is a combination of things working together.
Ok but then where does the oil get cooled? The sump is supposed to contain the coolest oil in the engine and that is where the returning oil dissipates its heat (I probably have that all wrong too).
Sure, I will be happy to help explain this but you have to look at it as a whole for it to make sense.
A good thermal imaging scan of a machine shows this very clearly for the eye to see.
For discussion purposes, let's say an engine generates 1000 units of heat (just to pick a number that's round)
700 of those units come from the heat of combustion
300 come from the total area where friction is happening ( cylinder walls, pivots on piston pins, valve stems and everything else realizing that the journal bearings should not be contacting but fluid shear is generating some heat too)
Out of all of that heat- some goes out through the exhaust, some radiates from the mass to the skin of the engine, some is removed from the coolant. (the key here is physically removed from the engine)
That's going to account for say 600-700 units (lot of variables affecting that)- that's going to be the bulk of generated heat.
Now the oil absorbs its portion (still in the engine so the engine has yet to be 'cooled)- heat is removed from the skin of the pan and into the air (while it's in aerosol form, thin film and even streams so that's much more surface area and a variable)
Whatever "remains" becomes the "normal operating temperature" of a machine.
It's at that point where the way it's operated determines the additional properties of the oil, coolant or other transfer means (external coolers etc.) that need to be added.
Granted this is a simplistic breakdown at 1000 ft. but that's essentially the process and how oil absorbs and removes heat in a machine.
This is why things like varnish (just like scale in a steam system) affect the heat removal from the oil.
Ok but then where does the oil get cooled? The sump is supposed to contain the coolest oil in the engine and that is where the returning oil dissipates its heat (I probably have that all wrong too).
The oil sump is where oil will accumulate and where the heat in the oil is removed to the outside air. And it may be the view of the "engineers" that bearing heat from friction, but most of their rise in temp comes from the heat generated by combustion, about 98% of all the heat comes from combustion. And to boot, nearly 80% of all friction in engine is from piston rings.
Read all of post #1 again. And beyond "engineer" words on a forum, proof that very little % heat is by friction, simply attach the motor to an electric drive on the crank and remove the spark plugs, spin it at 6k rpm for an hour, measure the oil temp. Repeat the same test with plugs but without spark. Lab data tells all most of the time.
Sure, I will be happy to help explain this but you have to look at it as a whole for it to make sense.
A good thermal imaging scan of a machine shows this very clearly for the eye to see.
For discussion purposes, let's say an engine generates 1000 units of heat (just to pick a number that's round)
700 of those units come from the heat of combustion
300 come from the total area where friction is happening ( cylinder walls, pivots on piston pins, valve stems and everything else realizing that the journal bearings should not be contacting but fluid shear is generating some heat too)
Out of all of that heat- some goes out through the exhaust, some radiates from the mass to the skin of the engine, some is removed from the coolant. (the key here is physically removed from the engine)
That's going to account for say 600-700 units (lot of variables affecting that)- that's going to be the bulk of generated heat.
Now the oil absorbs its portion (still in the engine so the engine has yet to be 'cooled)- heat is removed from the skin of the pan and into the air (while it's in aerosol form, thin film and even streams so that's much more surface area and a variable)
Whatever "remains" becomes the "normal operating temperature" of a machine.
It's at that point where the way it's operated determines the additional properties of the oil, coolant or other transfer means (external coolers etc.) that need to be added.
Granted this is a simplistic breakdown at 1000 ft. but that's essentially the process and how oil absorbs and removes heat in a machine.
This is why things like varnish (just like scale in a steam system) affect the heat removal from the oil.
Where is this "lab data"?
Lab data is everywhere. As a tenet of lubes, you should know where the data is at, yes?
Just 3% of input energy is converted to heat via friction. 20-25x that is heat from combustion process.
Where the Energy Goes: Gasoline Vehicles
Only about 12%–30% of the energy from the fuel you put in a conventional vehicle is used to move it down the road, depending on the drive cycle. The rest of the energy is lost to engine and driveline inefficiencies or used to power accessories. Therefore, the potential to improve fuel efficiency with advanced technologies is enormous. Note: Energy use and losses vary from vehicle to vehicle. These estimates are provided to illustrate the general differences in energy flow in different vehicle types during different drive cycles.
The bearing heat delta in a NASCAR engine is found using a Spintron, a machine that uses an AC motor to spin an engine at a set rpm. The bearing temp rise was 75°F at 8000 rpm using oil that's already heated to 280°F in the sump. Note there is no combustion happening, just friction.
I'm surprised it's that high for a journal bearing but I guess at 8 grand it's really working the fluid.
I use a similar rig but usually I am artificially radially loading the bearing and most of mine are rolling element types.
You stated that the oil was a coolant...I countered that with what ACTUALLY happens. - this is the 2nd time you've attributed something to me that I didn't say. What I said was, "oil acts like a coolant" and not, "oil is a coolant". It's weird because while the coolant and radiator fulfill the principal role of keeping my engine cool, the oil in my engine also picks up heat as it passes through the engine and lets it go into the atmosphere via the oil cooler. Your oil doesn't function similarly? Oh, and it's not the "little more throttle", as the heat is related to RPM, not power output. - weird. In my car, whenever I give the engine gas the rpms go up...how does it work in your car?
The majority of the heat in the oil is heat generated by the oil itself undergoing shear...not the number of (non)explosions taking place in the cylinder...yes, the (non)explosions make the motive power, but the friction generates most of the heat...
If you mean the information in his post as written and in context with the discussion he was in, he is correct and I referenced the same thing up thread about the shear inside the bearings generating heat.
I think the point of confusion is that the scenario here is not the same one as there and the context is misleading.
It's important when doing thermal transfer (and most things engineering) to remember that there are specific things that have bookends in themselves and can't be mixed together like in a conversation.
Use the above as an example to illustrate the point...
The heat IN the oil GENERATED by the oil.......- That's a measurement of endothermic heat by the fluid itself as its worked. You need to know that to determine the mass, metallurgy, fluid displacement and add it to the overall cooling need. That's best to test a cold, spinning an engine with a motor to see exactly what it is isolated.
The oil in the sump is that measurement PLUS (whatever absorbed heat the oil got by conduction via contact - whatever heat it lost to air via convection) + whatever latent heat is still transferring into the oil via the mass of the engine - whatever else is removing heat= "the heat' remaining.
There's a lot of detail, specificity and differential calculus and ranges of these equations. They are not static or linear and then different designs and different lubrication regimes have different inputs and outputs.
Hope that helped a bit.
All automotive engines are air cooled. Coolant and oil are just mediums for the transfer. Water is a better medium for heat transfer because it is a better conductor of heat than oil. if oil was a great medium for cooling our radiators would be filled with oil.
The engine oils function in the engine is to keep the moving parts apart. It does a better job of this when the oil remains cool enough to maintain MOFT. That is why many engines use coolant to oil exchangers upstream of the bearings in the oil circuit. It provides a greater level of assurance for oil temperature control than the conduction of heat through the oil pan, which can vary widely due to ambient temperature, while a thermostat-controlled coolant system is much more predictable and reliable.
And it may be the view of the "engineers" that bearings heat from friction, but most of their rise in temp comes from the heat generated by combustion ...
Not the big end rod and crankshaft journal bearings. The rods are too long to transfer much of any heat from combustion into the rod bearings. All the heat generated in the rod big end and crankshaft journal bearings is simply from the oil shearing in the hydrodynamic wedge.
No engine has 100% cooling provided by the circulation of engine oil. Every engine has either a liquid or air-cooling design. Liquid cooled engines usually have some air cooling as well.
The oil is generally cooled by indirect air cooling. The oil pan is a big oil cooler. In the olden days, oil pans were often finned to accommodate better cooling, and in line with airflow under the car.
Wouldn't thicker motor oil get hotter more from pumping losses (the oil pump has to work harder to push it through the filter & bearings), more from friction in the bearings, and make the engine work harder as it does so? That's the whole concept behind CAFE driven viscosity decreases, to try to minimize friction (for some incremental MPG gains). I'm sure straight 40 or 50 takes a lot more energy than 0W-8...
Yes, a thicker oil will heat more from shearing in the same exact use conditions. The difference in shearing friction becomes smaller the hotter the oil becomes, so part of saving fuel is also in the warm-up stage of the engine running when the oil goes from very thick to very thin, relatively speaking.
Yes, oil flow is critical to the internal oil cooling process. So thinner oils *can* cool better than thicker oils, which is the main reason why I am not a fan of going too thick (a little is ok IMO). But oil flow is not determined by viscosity alone... the type of oil pump, deposits & sludge, etc. all have an effect on oil flow.
No engine has 100% cooling provided by the circulation of engine oil. Every engine has either a liquid or air-cooling design. Liquid cooled engines usually have some air cooling as well.
They all have a lot of air cooling. The air is what cools the coolant.
Coolant will heat up quicker. Oil will lag 5-10min.
But then at steady state oil should be hotter than coolant.
Coolant should usually be <200F, Oil should be 200-210F
This of course is weather, altitude, and load dependent.
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