Sulfated ash and carbon deposits...

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Had a long convo with Amsoil tech support earlier today about using a low-ash engine oil in my RX-8. BIG kudos to them for spending as much time with me as they did.

They gave me a lot of good advice, but they also said one thing that I'm struggling with: they told me there's no reason to expect a lower-ash oil to leave this engine any cleaner than a higher-ash oil. The reason they gave is that most of the carbon deposits will be coming from fuel, and even at 1 qt/1000 miles, not much oil is getting burned.

This seems to make sense in isolation. But everything I've read elsewhere -- including in one of Amsoil's own TSBs on the RX-8 -- suggests that sulfated ash correlates significantly with deposit formation. I've also seen many suggestions that low ash content is extremely helpful if not mandatory for rotary engines.

Any thoughts? Am I missing something that'd reconcile all of this?
 
Covering their fundament? I thought sulfate ash was more about catalytic converters than carbon deposits.

When in doubt I'd listen to the TSB which was probably written by an engineer
 
For what it's worth, from my experience, Sulphated Ash categorically does NOT correlate with carbon deposit formation. In fact these days, particularly in the US, it's probably incorrect to even refer to Ash as being 'sulphated' as there's bugger all sulphur in engine oil and even less in gasoline.
 
I thought that sulfated ash was the residue that was left after burning all the oil off it, after reacting the metallic additive pack with sulfuric acid (making a sulfate solid ash after heating).
 
Originally Posted By: SilverFusion2010
Covering their fundament? I thought sulfate ash was more about catalytic converters than carbon deposits.

When in doubt I'd listen to the TSB which was probably written by an engineer

Originally Posted By: Koz1
TSBs over phone Techs every day of the week.

To be fair, the TSB doesn't literally say "you should use low-ash oils in the RX-8 to prevent carbon deposits". Here's the relevant passage:

Quote:
Carbon deposit formation may not be due to the type of base oil, but can also be related to the ash content of the oil additive package. Higher ash content oils can form more carbon deposits than lower ash content oils.

...but then it goes on to say this, which (as discussed in another thread) seems to indicate the document wasn't written solely by engineers:
Quote:
In recent years, API and ILSAC have reduced the ash content in engine oil to below 0.80% (max) by weight to meet API SM, ILSAC GF-4 specifications.

Ash is literally nowhere in the API SM or ILSAC GF-4 specs. What those specs limit to 0.80% is the phosphorus level -- related, but not the same (someone correct me if I'm wrong).
 
Originally Posted By: SonofJoe
For what it's worth, from my experience, Sulphated Ash categorically does NOT correlate with carbon deposit formation. In fact these days, particularly in the US, it's probably incorrect to even refer to Ash as being 'sulphated' as there's bugger all sulphur in engine oil and even less in gasoline.

Originally Posted By: Shannow
I thought that sulfated ash was the residue that was left after burning all the oil off it, after reacting the metallic additive pack with sulfuric acid (making a sulfate solid ash after heating).

My understanding is that the sulfated ash test uses sulfuric acid as part of a process to approximate the ash that might be produced in normal engine operation. So, yes, the resulting ash is sulfated whereas the ash from an engine wouldn't necessarily be -- but the result of the test is a mass % figure that is still considered useful as a rough indication of an oil's ash-forming tendencies in an engine. I.e., an oil with a lower sulfated ash number will tend to form less ash in an engine than an oil with a higher sulfated ash number, even though the ash formed in the engine wouldn't be sulfated. And ash, in turn, is one cause of carbon deposits (albeit usually not the main one).

Is that not correct?
 
Amsoil's interpretation of a TSB is undoubtedly biased.

I don't see S.A. creating "carbon" deposits...but certainly "deposits"....which was why S.A. became an issue in the first place.

It's pretty funny that Amsoil use the fact that their flash point "isn't that much different to dino, so therefore isn't a problem", when their other advertorials include statements that their "superior" flashpoint" differentiates synthetics from regular oils.

Early wankel issues from NSU days demonstrate that deposits building up on apex seal leading to seals jumping and chatter marks...S.A. can do that.
 
Originally Posted By: Shannow
I don't see S.A. creating "carbon" deposits...but certainly "deposits"....which was why S.A. became an issue in the first place.

Got it. Could you explain, or point me to where I could read about, the difference between "carbon deposits" as you used the term and the kinds of deposits that would form from ash?


Originally Posted By: Shannow
It's pretty funny that Amsoil use the fact that their flash point "isn't that much different to dino, so therefore isn't a problem", when their other advertorials include statements that their "superior" flashpoint" differentiates synthetics from regular oils.

Yep, noticed that as well.


So assuming I understand correctly, a lower-ash oil WILL reduce deposit formation -- BUT, if ash isn't the most troublesome kind of deposit in the engine, I might be better off with an oil that maybe has a slightly higher SA number BUT has better detergency to control other kinds of deposits. Does that sound about right?
 
Back in the day, when it wasn't uncommon for Group I base oils to contain 1% sulphur and for Diesel fuels to contain maybe 0.8% sulphur, there was a prevailing view that metallic ash going down the exhaust pipe was in the 'sulphated' form. The Sulphated Ash test sort of reflected this bygone reality.

The bulk of the ash in any engine oil originates from the overbased metallic detergent (eg 300 TBN Calcium Sulphonate) with a lesser amount coming from ZDDP. On the basis that both of these additives improve the oxidative stability of oil, you would expect both to reduce deposits.
 
Makes sense! Thanks, SonofJoe.

More not-even-half-informed ramblings to follow:

One of the practical questions in all of this is whether I'm barking up the wrong tree by pursuing high-end low-ash synthetic oils. My thought had been that an oil meeting a spec like MB 229.51 or 229.52 would offer the best chance at keeping the engine clean while retaining performance at-or-above the SN/GF-5 level in all other respects (except mpg).

If I'm understanding better now, using a low ash oil would mean the oil itself would contribute minimally to ash formation. Such an oil might also contribute minimally to other kinds of deposit formation IF the finished oil were sufficiently oxidatively stable despite the light additive package (e.g. thanks to highly stable base stocks). Is that correct?

If so, I think I understand where the fellow at Amsoil was coming from. He kept stressing that the vast majority of the deposits will be coming from the fuel. Might a lower-ash oil be less well equipped to control those deposits, all else equal? If so, might I be able to compensate by just keeping my OCI short?

Tangential: I've also read that low-ash oils can reduce intake valve deposits in DI engines quite significantly. Is that mechanism sufficiently unique to DI engines that it's meaningless here? If not, how does it fit?

Really appreciate you guys wading through all this and helping me understand!
 
Originally Posted By: SonofJoe
Back in the day, when it wasn't uncommon for Group I base oils to contain 1% sulphur and for Diesel fuels to contain maybe 0.8% sulphur, there was a prevailing view that metallic ash going down the exhaust pipe was in the 'sulphated' form. The Sulphated Ash test sort of reflected this bygone reality.

The bulk of the ash in any engine oil originates from the overbased metallic detergent (eg 300 TBN Calcium Sulphonate) with a lesser amount coming from ZDDP. On the basis that both of these additives improve the oxidative stability of oil, you would expect both to reduce deposits.


aaaaaahhhhhh...penny dropped moment here.

Thanks
 
Originally Posted By: SonofJoe
Back in the day, when it wasn't uncommon for Group I base oils to contain 1% sulphur and for Diesel fuels to contain maybe 0.8% sulphur, there was a prevailing view that metallic ash going down the exhaust pipe was in the 'sulphated' form. The Sulphated Ash test sort of reflected this bygone reality.

The bulk of the ash in any engine oil originates from the overbased metallic detergent (eg 300 TBN Calcium Sulphonate) with a lesser amount coming from ZDDP. On the basis that both of these additives improve the oxidative stability of oil, you would expect both to reduce deposits.


Forgive me if I'm misunderstanding what you're saying here. ZDDP carries four S atoms for every one Zn. Calcium Sulfonate carries one SO3 for each Ca, just itching to make CaSO4. Add heat and water in the combustion chamber and there's an excess of S needed to convert all of the metals to sulfated ash.

I can't for the life of me draw any connection between these additives' anti-oxidation properties in the oil vs what happens in the combustion chamber. There's a reason ZDDP and sulfonate (TBN) levels are reduced in oils intended for vehicles with particulate filters. It's not because their presence reduces deposits.

Ed
 
Originally Posted By: d00df00d
Makes sense! Thanks, SonofJoe.

More not-even-half-informed ramblings to follow:

One of the practical questions in all of this is whether I'm barking up the wrong tree by pursuing high-end low-ash synthetic oils. My thought had been that an oil meeting a spec like MB 229.51 or 229.52 would offer the best chance at keeping the engine clean while retaining performance at-or-above the SN/GF-5 level in all other respects (except mpg).

If I'm understanding better now, using a low ash oil would mean the oil itself would contribute minimally to ash formation. Such an oil might also contribute minimally to other kinds of deposit formation IF the finished oil were sufficiently oxidatively stable despite the light additive package (e.g. thanks to highly stable base stocks). Is that correct?

If so, I think I understand where the fellow at Amsoil was coming from. He kept stressing that the vast majority of the deposits will be coming from the fuel. Might a lower-ash oil be less well equipped to control those deposits, all else equal? If so, might I be able to compensate by just keeping my OCI short?

Tangential: I've also read that low-ash oils can reduce intake valve deposits in DI engines quite significantly. Is that mechanism sufficiently unique to DI engines that it's meaningless here? If not, how does it fit?

Really appreciate you guys wading through all this and helping me understand!



Okay, first off, you're looking at deposits in a Wankel engine. I, and probably every single senior oil formulator in the entire world, knows precisely naff all about deposit formation in rotary engines because they're so rare & no sane person would every expend the kind of money you need to properly understand them. These engines are very likely a law unto themselves and I'd guess that the only people that have any kind of fix on what impacts on deposits are Mazda themselves.

Taking the more general points you made in no specific order...

Regarding the 'deposits come from fuel' thing. What can I say? It's lifted straight out of the Big Bumper Book Of Business Excuses! When in doubt, blame something you're not responsible for. Does gasoline quality impact on engine deposits? Quite possibly yes but that's not why the excuse is being deployed which is to portray the oil as non-complicit.

Regarding low ash oils and Inlet Valve Deposits, there's an old Lubrizol presentation written by a guy called Mike McCabe that infers that low SAP oils reduce IVDs. Our own Brother Gokhan, in his uncritical naivety, posted this presentation up a few months ago like he'd discovered something of massive importance! Sadly what BG failed to pick up on is the author works in Marketing and the presentation was a 'marketing' presentation in support of Low SAPs oil as a new, and potentially profitable, market segment. The lubes industry absolutely loves market segmentation! It's a way of sucking more money out of the poor saps that buy engine oil. Personally I would totally disregard the 'low SAP oil means less IVDs' conclusion this presentation draws. It's standard practice in these matters to organise test work such that it draws you to a preordained, sleight-of-hand conclusion. Nuff said...

Regarding MB229.51/52 oils, it's been several years since I looked at these in detail but these will be top flight oils made from excellent base oils and loaded up with additives. As such, they will be better than your average GF-5 oil. You won't hurt your RX-8 by using them.

Finally, it maybe stating the obvious but low ash oils DO indeed produce less ash and have a reduced tendency to plug Diesel Particulate Filters (DPFs). I would add my normal health warning here which is that low Ash does not mean NO ash (or even that much lower ash than 'normal' oils!) and that even with low SAPs oil you can typically expect a DPF to be knackered after 80,000 miles. However it's wrong to conflate DPF plugging ash with engine deposits. I'll say why in my next post...
 
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I do believe that you “might” get oil introduced to the CC via the PCV ? Just a thought tho, maybe put a catch can on like I did and solve the whole issue ...... plus it might be better for HP due to not lowering the octane in the fuel at the same time. I don't think the RX engine has a PCV but all other engines do.
 
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The Ring-Dings purposely inject oil into the combustion chamber to lubricate the Apex seals.

Mercedes used a special porous floor to the intake manifold so that there was a thin "sheen" of oil that was introduced only to the apex seal area, but they had peripheral ports, so it worked with tiny quantities.
 
Ed,

In answer to the points you raised...

First off, the Sulphated Ash test is a really old test. It harks back to a time when there was a EXCESS of sulphur in fuel and oil. I have a vague recollection that SA was originally limited to 1.5% max because of spark plug 'whiskering' (I might be wrong on this because I heard this story when I was still working in Fuels).

TBH, when I first started out formulating oils, no-one paid much attention to SA other than as a tick-box exercise. There were lots of tests where oil related deposits needed to be considered but none related directly to ash. Gasoline sludge, piston ring/groove deposits and Teost deposits all figured prominently. Interestingly, combustion chamber deposits (which I think is what this thread might be focussed on) do not, as far as I know, figure in any engine oil specification.

Regarding the issue of whether there is enough 'bonded' sulphur in Detergent & ZDDP to ensure that ash is naturally 'sulphated' (even when both the base oil and gasoline are essentially sulphur-free), I am not sure this as clear cut as you suggest.

For starters, the great bulk of Calcium in overbased Calcium Sulphonate exists as Calcium Carbonate; not Calcium Sulphate. There is a bit of so-called 'neutral' Calcium Alkyl Sulphonate present but this is primarily there to ensure the Calcium Carbonate is kept in colloidal suspension. You should also bear in mind that some Detergents (Salicylates and Calixerates) are entirely sulphur-free.

Ash from burnt ZDDP could be sulphated but could just as easily exist as Zinc Sulphide or even Zinc Oxide (remember that ZnO is what's used to make ZDDP in the first place so at high temperatures, it might just 'revert').

To be clear, I'm not saying you're wrong. I'm saying that in the olden days, when you had a massive excess of sulphur swirling around the combustion chambers, it was easy to think of ash being in the Sulphated form. Take away that excess sulphur, and things become less certain. TBH, I suspect no-one probably knows the answer as all of the analysis will be of ash laid down in the DPF itself, which of course could very likely change the chemical nature of said ash.
 
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