mono-grade oil requirement DD two stroke diesels? 1

[Bevans6]

I'm trying to find out the root reason why Detroit Diesel two stroke engines require, to this day, mono-grade engine oil, normally straight 40 weight. Empirical evidience, supported by recommendations from DD, is that the engines last longer between rebuilds using straight weight oil in normal service. I just don't understand why. what is there about a two stroke application that would make a multi-grade oil, of completely modern and high quality manufacture, less protective in this application?

The "common knowledge" answer seems to be that multi-grades revert to their cold viscosity under the pressure of two stroke compression cycles, that the hydrodynamic layer of oil in the bearings can't recover between power strokes, or that the long-chain molecules used in viscosity modifiers shear and break down. I just have a hard time lend credence to how a two stroke diesel, with 18:1 compression, running at 2100 rpm is that different from some other high duty engine requirement.

Any ideas?

Thanks for looking, Brian

 

[JSteve2]

Is it possible that this is a requirement developed a long time ago that no one has felt the need to re-visit? If there is little development going on with these engines, there will be little incentive to spend the money testing updated specifications.

 

[rmw]

I don't mess with any DD's any more, but when I last did about 4 years ago, mono grades and especially 40 wt which was the grade recommended for my climate were getting harder and harder to find. This was for a bus and you had to take the oil with you because you just couldn't find it in truck stops any more. And, that meant taking a lot because the beast drank (or blubbered) a gallon every 500 miles (or 10 hrs) which was exactly per the rating in the book.

I did a lot of research and I don't exactly remember what the actual reason was, but the final analysis was that mono grade was the only way to go.

As a personality, I am one that doesn't take no as an answer very well, and I finally threw in the towel on this one.

I remember one website called "bobistheoilguy" dot com or something like that and there was a Shell site or a site by a Shell technologist, and it was dealt with in both places but alas, mono-grade was the only way to go.

Wish I could offer you some encouragement but I can't. Sorry.


rmw

 

[drwebb]

A footnote in their "Lubricating Oil, Fuel and Filters" manual allows 15W-40 under certain conditions if it has a HTHS viscosity of at least 3.70 cP. Sounds like they are concerned about viscosity loss due to temporary shear thinning of the polymeric ingredients, since that's what HTHS vis indicates.

 

[Bevans6]

Thanks guys. the consensus seems indeed to be shear thinning of the viscosity improvers. What would cause this? The DD has all accessory drives with gears, no chains, and there are a lot of them. it's also a two stroke - there are five piston rings and oil squirters for the bottom of the piston. What is the primary mechanical cause of oil shear?

Brian

 

[drwebb]

Well, generally geared drives are more shear severe than roller chain drives. Any place there is a high load high speed sliding contact will cause higher shear stress in the fluid. If there are long chain polymers adding to viscosity they will stretch & compress, reducing their efffective hydrodynamic volume and viscosity contribution (temporary shear loss). If they get very stressed it will break bonds- statistically somewhere in the middle- leading to shorter, less viscous polymer chains as evidenced by permanent shear viscosity loss of the oil.

For VI improvers, generally both types of shear stability are related to cost. There are a host of performance & cost levels available today, meaning that a performance-suitable VI improver could probably be incorporated at some additional cost. Alternatively, multi-vis oils may be prepared without polymeric VI-improvers by utilizing high VI base oils. If you Google 15W-40 with 6V92TA or MIL-PRF-2104G you will find some multi-vis products claiming to meet both 2-and 4-stroke requirements.

 

[dicer]

Old aircraft engines are users of mongrade as well.
I better stop there.

 

[turbomotor]

Hi Bevans6,

I'm not an expert on this subject, but I have conversed with those who are. One that comes to mind readily is a John Martin. He lives in Phoenix and still (I think) consults. John is an MIT grad that worked years for EMD and then worked the DARPA funded program at Garrett (Honeywell) in Phoenix to build and test a very high performance two stroke diesel. If you can find him, I think he will have an answer.

My limited understanding is that the boundary lubrication issues at the piston pin slipper bearing demand different lube characteristics than typical hydrodynamic lubes, but mine is only a cursory understanding.

Dick