why - not 15W-40 oil for DD two strokes?

[bevans6]

i'm not questioning this advice, I'm just wondering what the reason behind it is, beyond simple experience or DD recommending it.  The technical reason behind the recommendation.  The reason I'm asking is I just read the invoice for the last oil change the previous owner had done, and it says they used 15W-40 oil.  So now I have to change it, with 800 miles on the oil.  Because I have no way of knowing if the mistake was in writing up the invoice, or in what they put in the engine.  Which irritates me no end. 

The only reason I can think of why a multigrade oil is not superior in the application is the  viscosity modifiers.  Modern oils, particularly synthetics, don't even always use viscosity modifiers, they meet the cold/hot viscosity tests without them.  So I have no idea why straight weight oil is so much better for two strokes.  The oils will have similar additive loads, presumably have similar ash contents, have similar abilities to support bearing loads, the multi-grades are just less viscous when cold.  And why is that a bad thing.

Looking for engineering analyis, not "It's the way we always done it and if it was good enough for my grandpappy, it's good enough for me"   

As an aside, I learned far more than I ever wanted to about engine oil when I lost a very expensive race motor due to oil manufacturers reducing the zinc content in their automotive oils without particularly telling anyone.  Zinc is an extreme pressure modifier, and without it, flat tappet pushrod motors tend to wipe cam lobes.  so I now know just enough about engine oil to be dangerous...

Brian

[Melbo]

As I understand it multi vis oils are not consistently thick like straight weight oils

A two stroke has power on every stroke and the oil does not have time to recover before another power stroke

A VERY simple explanation but that is the short one given to me

Melbo

[HighTechRedneck]

Multi viscosity oils use a long chain molecule polymer to prevent the oil from thinning as it heats.  That can sheer/breakdown under the high stresses in a DD 2 stroke.

http://www.bobistheoilguy.com/motoroil.html#MULTI-VISCOSITY

[RTS/Daytona]

see---> http://www.tejascoach.com/ddcoil.html

[Sean]

The simple answer is that the viscosity of 15W-40 is too low at engine operating temperature to protect against shear.  (There is, of course, a more complex answer, too, and tomes have been written, including several lengthy posts right here on this board -- check the archives).

This is actually detailed in the official Detroit lube oil recommendations, publication 7SE270.  (Pretty much everyone on this board should have a copy.)  Find it here:
http://www.detroitdiesel.com/support/on-highway/manuals/lubricants_fuels_coolants/

There you will find that the actual kinematic viscosity of 15W is less than half that of straight 40-weight, making it unsuitable for the high shear environment at the piston wall.

Detroit once tried to loosen this recommendation, back when they started selling four-strokes into previously exclusively two-stroke fleets (operators complained loudly about having to stock two different lube oils), with disastrous results.

-Sean
http://OurOdyssey.BlogSpot.com

[buswarrior]

Multi-grade gets squeezed out from the bearing surfaces, as noted by Melbo. Won't protect properly.

Ash is bad, can't remember why, tejas site tells you? IIRC?

Up until the newest oils for the 2007 DPF engines, the rest of engine oils had way too high ash for DD.

Not just any straight 40 wt oil will do, it has to have low ash/ "approved for DDC" stuff on it.

No literature with the oil, it doesn't get bought.

And it is frequently reported that the engine burns more multi-grade oil than straight wt oil.

Harper's in Etobicoke, just off the QEW, carries the appropriate Exxon XD3, last time I picked some up.

Dumping the PO's oil change means you KNOW what is in it. How do you know it isn't 5W20 and the bill was written up wrong?

The engine won't mind a short change!

happy coaching!
buswarrior

happy coaching!
buswarrior

[luvrbus]

If you guys believe that oil was the problem on the 6v92 DD on Charlie's (Tejas) site I have some ocean front lots here in Az for sale. Oh were art thou Don, Cole,Gary and others that know.
I have a photo of pistons from a 8v92 that never had anything but Delo 100 I will email it to guy and let him post it for me looks the same as the ones on Tejas.



good luck

[akroyaleagle]

I offer this post for information only. I do not wish to start a flame war. I have logged near 400,000 miles in my Eagle with no breakdowns. I feel I am doing my maintenance right.

First let me say that I use 40wt (or 30wt if forced or in extremely low temps) in my 8V92TA and 71N series engines. That's what my manuals say, the way I was taught and have always done it.

There are many unmanned remote sites in Alaska that use two stroke DD power. Many of them use 15-40wt.

The Army has a lot of equipment that uses two stroke motors. They are still using them, especially the 8v71 and 8v92 twin turbo. They use 15-40, stating it is 40wt at operating temps and makes the equipment easier to start in cold weather.

When I managed the Transportation Motor Pool at Ft Richardson, I tried every argument I could find including those stated in the posts below and the manuals to show them the equipment required 40wt, especially the buses.

I discussed this in detail with the oil analysis lab. They convinced me over time that no harm was being done and the newer multi grade oil was superior.

I know a lot of folks that use 15-40 in their Allisons with appartently no adverse results. That bothers me.

IMO the manuals for the two strokes were written long before the multi-grades hit the shelves and manufacturers are reluctant to change data in them. Liability?

There are many retired employees that argue for procedures that may be outdated now.

I use Lucas products in everything. My oilers and differential have only Lucas in them. The engine gets one gallon at oil changes. Lots of folks have argued against that. Visit any store that has the glass demonstration case for Lucas and form your own opinion.

To the original poster: You are doing this part right. Get all the info you can from the boards. Just keep in mind that many in the bus community know only what they have read on these boards and what they've been told. They have limited experience beyond that but someone they look up to has stated it.

40wt is becoming harder to find for some of you apparently. You are sometimes required to make a decision. Gather your info and decide.

[bevans6]

I've been thinking about this  all day, since I posted the original question.  So many of the answers make no sense to me, if I am just considering the lubrication requirement of the engine.  40 weight oil has the same viscosity at high temp, regardless of if it starts as a 15W 40, a 5w-40, or a straight 40, so any arguments that say a multi-viscosity oil doesn't hold up at high temp leave me cold, so to speak, particularly when the Motul 300v 15W-50 that I use in my race engines is good up to 365 degrees F!  It retains it's viscosity to about 260 degrees, which is why I use it in my race engines that are running that hot at the end of a race.  Of course it costs $20 a can, too. They use it in Le Mans cars.

The issue with viscosity modifiers/enhancers also leaves me thinking.  Many modern synthetic oils just natually meet the viscosity ratings for a 5W-40 or a 10W-40 with no additives, it's just the way they are.  So they are kind of straight weight multi-viscosity.  There is a note in the documents that Sean linked to that specifically says synthetics are OK for two strokes IF they have no viscosity modifiers.  So I got thinking about what else, other than actual lubricating and bearing cooling and such at operating temperature, does an oil do?

One thing it does is cling to surfaces and provide protection at startup.  I can imagine that two stroke pistons may have some wacky thermal expansion characteristics that would make them react differenty than 4 stroke pistons when cold.  They see twice the operating cycle heat, obviously.  Maybe there is something to do with piston size/fit when cold that likes an ultra-thick oil to prevent cold scuffing or something.  In my car engines I run some pistons that need .005" cold clearance because the alloy they are made of expands a lot.  the pistons basically don't fit until they are hot, and they smoke a lot when cold.  Hmm, what other engine do I have that smokes a lot when cold?

Another thing oil does, especially in a two stroke, is get into the combustion chamber and get burnt.  That's the reason, I expect, that we specify a low ash oil.  Not only is the exhaust stroke happening twice as often as in a four stroke, the combustion process is less efficient, there is less complete combustion of all fuel, and there is more lubricating oil burnt along with the fuel than in a four stroke.  That explains the ash content, but unless there is something I just totally don't get, it doesn't really speak to viscosity modifiers.

The last thing I thought of was viscosity reduction through fuel dilution or other contamination of the oil.  A two stroke suffers from fuel dilution a lot more than a four stroke does.  Fuel dilution will reduce the viscosity of the oil, and probably do all sorts of other bad things, so maybe that is why the recommendation is for mono-grade oil.

While I really wish I knew the real reason, I'll be happy just putting some straight 40 in and leaving it at that.  I found it interesting that the Detroit Diesel document (which is a 2007 version, fwiw) doesn't list an monograde oil for two strokes that they approve of for operation below freezing, though.  You're supposed to stop and change they oil when the temp gets up above freezing.  Oh well....

Brian

[akroyaleagle]

Here's another point I came across in my files. Maybe the reason I decided to stay with 40wt:

A surprise find in the world of tribology    By: Christopher W. Ferrone

After many years of premature engine failures my frustrations turned to further investigation. For more than 25 years beginning with the Detroit Diesel (DDC) 92 series in 1981, I have researched ways to use quality lubricants and creative methods and intervals to extend the life of engine components.

In 1995, I joined forces with Detroit Diesel to assist with a failure modes and effects study using my empirical fleet data on Series 92 engines. That project lead to a technical paper, single source part number (23514770) repair kit, and a national ad campaign to assist operators with the care and feeding of their Series 92 engines.

As a result of emissions compliance the newer engines are 4-stroke cycle unlike the 2-stroke cycle of the 71 and 92 series. Other than the fact one is a 2-stroke and the other a 4-stroke, from a purely mechanical standpoint, these two engine types are similar in function and lubrication intervals.

Since the crank journal is constantly under load as compared to the 4-stroke engines, the 2-stroke engines require a single-grade oil with a higher viscosity. The boundary lubrication never has a chance to recover for 2-stroke engines.

In 4-stroke engines, the boundary lubrication does have a chance to recover during the non-loading turn of the crankshaft, which allows the use of multi-viscosity oils.

The use of single viscosity oil in 4- stroke engines can actually extend the life cycle with really no downside.

As our fleet became a mix of 2-stroke and 4-stroke engines, I was concerned with the accidental introduction of multi-viscosity oil into a 2-stroke engine, which, of course, would have led to a catastrophic failure. With this in mind, I called Detroit Diesel to discuss the use of single grade oil in a 4-stroke Series 60 engine. Told this would not be a problem, we began to replace engine oil in the Series 60 with single viscosity oil. Thanks to indoor parking, we can use XHD 40 weight oil in both the summer and winter. Where starting the engine in cold weather is a concern, simply use XHD 30 during the winter months. For fleets that still incorporate 2-stroke engines, this will not be an issue because they use a heavier single-grade oil anyway.

With any one cylinder/piston and main bearing journal, the 2-stroke journal is under load with every turn of the crankshaft. In a 4-stoke engine, the journal is under load every other turn.

A 2-stroke engine only uses single viscosity 30- or 40-weight engine oil. Oils for 4-stroke engines are generally multi-viscosity 15W-40. In some instances, the engine is acceptable to synthetic oils, which are outstanding for a number of reasons, but are very high priced compared to traditional engine oils.

Having used this oil in DDC Series 60, Cummins 5.9L and PowerStroke 4-cycle engines for more than 10 years, the engines have never experienced a lubrication failure or shown any negative effects. In fact, the PowerStroke engine currently has 483,000 original miles all to the credit of single grade oil — well beyond its projected life cycle of 250,000 miles. The Cummins 5.9Ls also have lasted well beyond the expected life cycle. My original Series 60 test engine has 375,000 city miles, in addition to thousands of idle hours, which equates to almost 1.2 million highway miles. I attribute the additional success of these engines and their extended life cycle to the use of single grade (XHD 40) motor oil.

Single grade oils are much less expensive than the synthetic oils recommended for 4-stroke engines. It is worth mentioning that in addition to extended engine life, specifically crankshafts and bearings, the peripherals such as turbochargers, air compressors and gear trains also benefit from the single grade oils. There is no need to keep two types of oils in the shop and run the risk of a mechanic accidentally pouring a multi-viscosity oil into a 2-stroke engine.

Though this has been an ongoing process in our operations for more than 10 years with the utmost success and simplicity, it is still advisable to contact the engine dealer to confirm single grade oils are acceptable before making the switch. 

People may think I am crazy, but I use XHD 40 motor oil in all of my automatic transmissions and power steering as well.

[bevans6]

That's an interesting article, for sure.  The problem I have with it is that crankshaft journals are not lubricated with boundary layer lubrication, which is where the load is carried by the surface asperities (the actual bearing material and crank shaft material rather than by the lubricant), they are lubricated by hydrodynamic lubrication whereby the oil is pumped in at relatively low pressure, and the rotating action of the journal relative to the bearing creates the pressure that actually supports and floats the journal.  That occurs constantly, as long as the engine is turning, and is not affected by what is happening at the piston.  A two stroke engine like ours is actually a four half-stroke engine, and there is lots of time for the oil volume inside the bearing to be replenished between when the exhaust valves open and the intake ports get closed.

Boundary layer lubrication, in a crankshaft, is what happens just before the rod sticks a hole in the block...

Brian

BTW, I'm not trying to be an @$# or create a meaningless debate, I'm going to run straight weight oil.  I just want to know, for sure, the actual reason why.  I bet even DD doesn't know any longer.  I can hear my Dad now "Why? because I said so is why!  Now get out there and change the oil!"  whap upside head.   

[luvrbus]

I love these oil debates but always wondered how many the of people here follow DD guide lines when the outside temp is a 100 or your engine is running at 200 stop and change to 50w oil per DD.
I run 50w in the summer here in Az in fact I am changing the oil today back to 40w.


good luck

[Charles in SC]

Just use what da book says to and move on with your life.

[TomC]

In simplicity form-think of 15w-40 multi viscosity oil as a rubber band.  When it is squished in between the bearings during compression or power stroke, it starts to think as a 15 weight oil (which it is). But when the engine relaxes on either the intake or exhaust stroke, the rubber band springs back to make the engine think it is a 40w oil.  The problem in a 2 stroke, it the engine never relaxes.  It is always under stress during power and that rubber band squishes down to its' original 15w size and never comes back.  So while the oil says 15w-40, with the pressures in a 2 stroke the oil compresses to only cover the engine with 15w protection.  That is why you want straight 40 weight (149 series engines in mining operation uses straight 50 weight).  As simple of an explanation as I can get.  Good Luck, TomC

[HighTechRedneck]

And for the not so simple form:

http://www.zddplus.com/TechBrief13%20-%20Oil%20Viscosity.pdf

The whole thing is informative for those that want to know the nitty gritty of how oil works down to the atomic level and all the scientific details of oil viscosity.  But a key portion of it that pertains to this discussion is at the bottom of page 10 under the heading "Viscosity Index (VI) Improvers".

Then to take that and apply it here.  "Heavy Duty" single grade oils have minimal "VI Improvers".  Multi grade oils have more.  The higher the span of the multi grade, the more "VI Improver" polymers it contains.  The more it has, the less stable it is under high temperature and sheer. 

By high temperature I am not so much referring to the overall engine temperature or even the overall oil temperature, but rather the temperatures it encounters while cooling the piston head and lubricating cylinder walls.