Engines designed around oil?

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Hi all Frequenter here at BITOG. It's been said many times that the ENGINE is designed around an OIL weight. They don't build engines then decide on an oil. My question is, what exactly do they design around an oil? Stroke? Bore? Clearances? etc. Throt
 
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When you are designing say a bearing, viscosity is the first assumption that you have to make. Do the heat balance, calculate Minimum Oil Film Thickness (MOFT), and heat generation within the bearing...then use that heat rise to calculate viscosity, and do the math over again until the design works (then have to get into bearing stability etc.) I'm not sure that's what you are asking 'though, and I doubt that they would be designing any of the major architecture with an oil choice in mind. Clean slate, they would design ring packs, bearing sizes and clearances, piston skirts etc. so that they are robust with the oil they want to use.
 
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Welcome to our nightmare Throt!! I don't think an engine is designed with a specific oil in mind. We could make a list of engine design features that allow minimum tolerances in bearing and piston clearances. Controlling flex, harmonics and peak cylinder pressure that reduce operating clearances would be three of them. The difference between the hot and cold shape of engine parts would be another.
 
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I'm only aware in terms of modern engine design sets the standards and the requirements for lubricant (thus the various SAE and JASO valve train wear tests, Sequence IV tests, etc.), and not the other way around, which pushes it's (lubricant) performance envelope and lubricant developments. (*simply look at the past 20 yrs where API progressed from SL to now SN due to ever demanding engine requirements (developments), and/or ILSAC from GF2 to currently GF5 requirements and then you'll see who is the driver in the lubricant development*) Who knows? Maybe back in ford model Tee days, it's the oil that drives the engine development around? Q.
 
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So is it a bad idea to start going off piste with the oil viscosity that we use in our engines?
 
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Sorry, here's another one. Do you/is it possible to design wider cold start clearances to accommodate the thicken oil?
 
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Riggaz, the bearings will only draw from the pressurised galleries the oil that they need to replace side leakage. The artifact of this is increased gallery pressure (or oil pressure) as viscosity goes up. They largely self compensate, and thicker in an engine "designed for" thin can sap power,and heat the oil more, it's largely a non-event. Putting thin in an engine designed and clearanced for "thick" will reduce MOFT...will self compensate until excess available volume is gone, then you are out of any sort of self compensating range.
 
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Ok, so you can go thinner as long as you stay within the manufacturers oil supply pressure range?
 
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Another feature that makes a lighter grade of engine oil possible is direct injection. The fuel is injected during the compression stroke instead of the intake stroke with DI, reducing the exposure of fuel to the cylinder wall. The fuel can be injected anytime before the spark plug fires during a period of maximum turbulence. The more complete the burn and reduced exposure of fuel to the cylinder wall, results in less engine oil fuel dilution.
 
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The topic of variable discharge oil pumps was raised but never discussed in detail as to exactly what they do, and why they allow for a lighter grade of oil to be used in an engine. As I understand it, the engine oil is heated by the friction of the oil, and not so much from the absorption of combustion temperatures that we previously thought to be the case. It is also believed that the higher the oil pressure is, the better lubricated the bearings are. That makes sense doesn't it? When the pressure is high, more oil is being pushed through the bearings. When the oil heats up and thins out the pressure drops, eventually until the bearings run out of oil, and Shannow you owe me for this.
 
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I don't believe engines were designed around oil viscosity at all. You can look at engine clearances from engines produced in the 30's and 40's, and the bearing clearances are pretty much the same as engines made today. However, the Accuracy of those clearances are better in todays engines than back in the day. Many of those older engines were spec'd for 20 and 30wt oils, much as they are today. They lasted pretty much as long as they could considering the quality of oils back then. I am willing to bet, if you took a brand new, out-of-the-crate engine, made in the 30's or 40's, and ran it with even early 90's fuel delivery technology, and with todays thinner oils (keeping in mind something added for flat tappet lubrication) that it would last well over 100-150K miles. Many of us on this site have commented that some engines simply run and sound better on a 30Wt over a 20wt, or that many engines, with NO internal changes, were suddenly spec'd from a 10w-30 to 5w-20wt oils with no apparent issues. With these two examples alone, I feel the oil was manufactured around the engine, not the engine around the oil.
 

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Originally Posted By: used_0il
Another feature that makes a lighter grade of engine oil possible is direct injection. The fuel is injected during the compression stroke instead of the intake stroke with DI, reducing the exposure of fuel to the cylinder wall. The fuel can be injected anytime before the spark plug fires during a period of maximum turbulence. The more complete the burn and reduced exposure of fuel to the cylinder wall, results in less engine oil fuel dilution.
While that is fine in theory, in application we've had a lot more DI engines fuel dilute on here than we have port injected ones. The reason for this would be that in a port injected application, the air is flowing past the injector as the fuel is sprayed into it. In DI, the fuel is sprayed directly into the cylinder, preventing the same quality of blending from occurring.
 
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I think weve seen bearing width and diameter changes for lower viscosity oils, as well as oil jet cooling of the lower mass piston crown
 
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Originally Posted By: Shannow
I never said that...did you read that in what I posted ?
I presume you were replaying to me rather than the OP. Putting thin in an engine designed and clearanced for "thick" will reduce MOFT...will self compensate until excess available volume is gone, then you are out of any sort of self compensating range. I thought that when you said it will self compensate until excess volume is gone meant that the thinner oil is not a problem until the pump cannot keep up with the supply demand...
 
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I'll give you an example My Briggs and Stratton mower, I ran This 5W20 over winter (never got cool enough that the grass really died off), and wasn't happy with the hot knock that it had on restarts. Thought I'd do an experiment, and take the oil temperature (type K thermocouple down the dipstick), and after mowing, 10 mins free revving against the Governor, and at 4C, I got 87C oil temperature. Using the Widman Calculator gives 11.4cst at 87C. To bookend the experiment, I put Thsi 20W60 in it. If I could have got a 25W70 on the day I would have. Same experiment and at 4C it ran to 97C...the oil was taking more work from the engine, and in order to clear that to the atmosphere, ran 10C hotter. According to the viscosity calculator, that's 26.24cst. The "self compensation" isn't linear, it doesn't self adjust to make the viscosity in the bearing, and therefore the MOFT, the same under all conditions. The OEMs supply their minimum oil pressure recommendations to establish the mechanical integrity of the engine on their recommended lubricants, not as a viscosity selection tool...Oil pressure never has, and never will tell anyone about the MOFT in an operating engine.
 
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