Sulfated ash and carbon deposits...

[chrisri]

Originally Posted By: d00df00d
Originally Posted By: chrisri
RX8 is a special animal.

What you want is non PAO, high spec C3/229.51-52, LL04. Preferably GTL based oil.

Because?

(not disagreeing; just want to hear your reasoning)

BTW, Mazda's own synthetic engine oil for this engine is largely PAO per this page: http://www.mazda.co.jp/carlife/service/parts/detail/oil/lineup/synthe-renesis/


Good friend of my was a main Mazda tech, and that was what he told me years back. Then it had me thinking, I had similar experiences with normal piston engines that used oil. RX8 use oil by default.
His (and my) reasoning was that PAO based oil in engines that use lot of oil leave much harder deposits in chamber and cause all sorts of problems. It seems those are much harder to burn of( if possible) than group 2 or 3. GTL burns very clean. IMO.
Mazda specific oil is PAO , as you posted. How good did it do in these?

 

[d00df00d]

No idea, unfortunately. I just have to imagine Mazda knows better than anyone else on this.

I think I remember reading a while back that not all PAO is so foul when burned. And of course the total formulation is what really counts in the end. Hopefully someone better informed can chime in here.

FWIW, my car burns at most 1qt in 1000 miles. I'm pretty sure PAOs can be found in oils for piston engines that can burn that much and more.

 

[SonofJoe]

For the last couple of hours I've been trying to give myself a YouTube education about rotary engines, apex seals, its PCV system and basic lubrication. Yuk!! It's easy to see why so much of the automotive world turned its back on the Wankel engine! It all gets too complicated, especially if you throw a turbo into the equation!

Until I looked, I honestly didn't realise that the Wankel lubrication system is part closed (for bearing lubrication) and part 'dead loss' (for seal lubrication). It seems that some Mazda owners even turn off the Oil Metering Pump and pre-mix oil with their gasoline. TBH, I'm frankly amazed that Mazda can meet statutory emissions legislation with such a design given that the industry turned its back on two-stroke car engines several years ago.

Okay, my much revised oil perspective for what it's worth (which is not a lot I fear!)...

If you're going down the conventional two-stroke (2T) oil route, generally you're talking about very low ash oils. 2T oils tend to be high in Ashless Dispersant and contain a heavy solvent neutral oil (eg Group I 500SN). I presume this is because when the gasoline evaporates off, you want to leave behind tiny droplets of 'stuff' to coat the rotor/housing surfaces and lubricate the surface face of the apex seals. I suspect its important that the oil in the combustion chambers is as combustible as it can be to minimise deposits. Unlike conventional piston engines, you might need an oil that has very low resistance to oxidation (which is what combustion is). It's conceivable that PAO is not your friend in a rotary engine as it could resist burning and create more carbon deposits.

The other thing I picked up on was how many rotary engines reported oil in their intake systems. Do Wankel engines have a PCV system? I'm assuming they must do as apex seals must suffer from proportionately more blow-by that conventional piston rings? If that's the case, then oil must follow the blow-by gas to be burnt in the combustion cycle, further adding to the formation of deposits. I might assume that the flow of recycled oil is especially high at high RPMs.

Very tricky to say what oil is best for these engines. My advise is stick as closely as possible to what Mazda say.

 

[alarmguy]

Im no expert on this but Piston Aircraft Engine oil contain ZERO metallic anti wear additives such as zinc and moly.
The reason for this is to prevent piston and ring deposits.

Here you go, right from Shells Website - 15w50, no metallic anti ware, as with all avaition oils

Its why I say sometimes almost laughable that we call oils low in zinc etc as a poor add pack. Here we are flying in airplanes with engines that contain ZERO zinc ... etc, etc ...

 

[DWC28]

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).


Plus one.

We do not put ash, either sulfated or non sulfated in our oil. Sulfated Ash is a test that measures the amount of metallic detergents in oil. Detergents help to reduce deposits by both metal surface action and acid neutralization. If you are going to burn high bio fuels you better have high detergent levels. If you are burning high sulfur fuel you better have high detergent levels.

 

[chrisri]

I had to call my friend to refresh my memory. It goes like this.

Under warranty, or before failing, oil used was Mazda Dexelia Ultra 5w30. (After bit of searching I've found it is A5/B5 oil).

Then there was people who got new engines and tried all sorts of things to help with oil consumption and, inevitably, at some point, compression lost. PAO oils like Castrol 0w40 did not help, it would only exaggerate deposits on spark plugs. He remembers that Shell did well in those Mazdas-.for some reason (that's where my GTL theory came).
He said to ask you how fast does your car cranks? Fest cranking is a certain sign of incoming disaster. Brilliant cars otherwise, swap would be a real possibility for me.

 

[edhackett]

Originally Posted By: SonofJoe
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).

Thanks for the well thought out response. Reading it this morning along with rereading the rest of your replies helped me pull some thoughts together.

I think we're all getting hung up on the "sulfated" part of the sulfated ash test. I'll explain further as we go along. It's true that in the past that more of the ash was probably found in the sulfated form than today. It used to be that high sulfated ash(up to a point) was generally looked on as a good thing for automobile engines. It was an indicator of high additive levels and high TBN.

As a chemist I see the test using the sulfated form not so much as that was a common, maybe the most common form of ash back in the day, but as a way of getting consistent results. All of the metal is combined with a common anion. That makes measurement consistent. Just burning the oil without the sulfuric acid treatment would result in an ash measurement, but it would be inconsistent from run to run due to the random formation of various species; sulfates, oxides, carbonates, etc. as you point out later. CaO is much lighter than CaSO4.

Originally Posted By: SonofJoe

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.

Yes, this thread is focused on combustion chamber deposits. API and GF specifications don't have any sulfated ash standard. The ACEA standards do. I think the sulfated ash contribution would part of the results obtained from the TEOST and similar tests. Too high a SA could cause failure on those.
Originally Posted By: SonofJoe

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').

From a purely chemical point of view there is excess S to fully react all the Zn in ZDDP and the reactive metals in the sulfonates. We know that does not happen with 100% efficiency in the uncontrolled conditions of a combustion chamber. I was going to mention that in my previous post, but didn't get that thought in writing. As you point out all sorts of species can be formed from that Zn, or any other metal atom. They are all ash. The CaCO3 used to overbase sulfonates is ash. Sulfates, oxides, sulfides, and carbonates are all non combustible, solid, combustion byproducts that form or contribute to hard deposits.
Originally Posted By: SonofJoe

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.


We need to stop thinking of sulfated ash itself as being the problem. The sulfated ash number is just a measure of the ash forming potential of an oil. Sulfated ash is just one form of ash. Metallic ash, is the problem, regardless of the species.

I agree that the DPF may finish a bunch of reactions. There may be all sorts of "soft" metallo organic compounds in the deposits that get converted to ash during the regeneration cycle.

I hope that make some sense. It's the best my cold fogged mind could come up with at the moment.

Ed

 

[d00df00d]

So one thing as a preface: I have a "Series 2" RX-8, which is MY2009+. Essentially the same, but with a raft of changes to the engine and some other bits. Breathing, fueling, ignition, oiling, cooling, all changed a little or a lot.


Originally Posted By: SonofJoe
If you're going down the conventional two-stroke (2T) oil route...

Wouldn't be an option for me even if I wanted to do it. The Series 2 oil metering system is different and no one has made the relevant kit for it yet.


Originally Posted By: SonofJoe
I suspect its important that the oil in the combustion chambers is as combustible as it can be to minimise deposits. Unlike conventional piston engines, you might need an oil that has very low resistance to oxidation (which is what combustion is). It's conceivable that PAO is not your friend in a rotary engine as it could resist burning and create more carbon deposits.

These are all common notions that make eminent sense.

And then there's this: Mazda-branded fully synthetic oil labeled for this specific engine, in 0w-30, described by Mazda as having "PAO as a main component and an ester" (per Google translation of that page).

There's also this: a rotary-specific (though not RX-8 specific) fully synthetic engine oil, made by Mazda's partner for the 787B Le Mans car, with a flash point of 260º C (!).

Doesn't that suggest the opposite, i.e. that highly burn-resistant oils can be okay?

These are the two main sides of a debate that has been raging for years, and I have yet to find anyone who has offered a way to reconcile them. Everyone just seems to shrug their shoulders and repeat what they just said.


Originally Posted By: SonofJoe
The other thing I picked up on was how many rotary engines reported oil in their intake systems. Do Wankel engines have a PCV system? I'm assuming they must do as apex seals must suffer from proportionately more blow-by that conventional piston rings? If that's the case, then oil must follow the blow-by gas to be burnt in the combustion cycle, further adding to the formation of deposits. I might assume that the flow of recycled oil is especially high at high RPMs.

The oil-in-the-intake problem was largely solved in the Series 2 updates AFAIK. Yes, the RX-8 does have a PCV system.

The Renesis engine in the RX-8, with its ports on the sides of the combustion chambers, ends up naturally recirculating some fraction of the exhaust gases it produces. It's kind of like a built-in EGR. I'd have to imagine there are all kinds of oil-derived vapors in that mix.


Originally Posted By: SonofJoe
Very tricky to say what oil is best for these engines. My advise is stick as closely as possible to what Mazda say.

That'd be my instinct as well. The problem is that Mazda seems to contradict itself in different markets.

In North America, the line is "API/ILSAC certified non-synthetic 5w-20, period, end of story."

In Europe, they strongly recommend their own Dexelia Ultra 5w-30, which chrisri mentioned in this thread; being an ACEA A5 oil, doesn't that suggest at least a synthetic blend?

In Japan, they sometimes recommend 0w-20 but also sell that RX-8 specific fully synthetic 0w-30 I linked above.

In Australia, they again demand API/ILSAC and non-synthetic, but have a full chart recommending different viscosities for different temps.

I'm also hearing that their in-house racing support division recommends synthetics for race use in rotary engines; not entirely relevant, but perhaps suggestive WRT how well these engines burn the oil they inject, no?

 

[d00df00d]

Originally Posted By: chrisri
I had to call my friend to refresh my memory. It goes like this.

Under warranty, or before failing, oil used was Mazda Dexelia Ultra 5w30. (After bit of searching I've found it is A5/B5 oil).

Then there was people who got new engines and tried all sorts of things to help with oil consumption and, inevitably, at some point, compression lost. PAO oils like Castrol 0w40 did not help, it would only exaggerate deposits on spark plugs. He remembers that Shell did well in those Mazdas-.for some reason (that's where my GTL theory came).
He said to ask you how fast does your car cranks? Fest cranking is a certain sign of incoming disaster. Brilliant cars otherwise, swap would be a real possibility for me.

Not sure about that Castrol 0w-40, but Mobil 1 0w-40 (similar oil made to similar specs, right?) has a sulfated ash number above 1% -- way higher than the oils Mazda recommends. If that Castrol 0w-40 you mentioned is similar, then maybe that's why.

My car has no problems right now. Just had a rebuilt engine installed under warranty. I'm interested in oil selection because I'm planning for when the warranty ends, and hoping to contribute to the larger discussion in the meantime. :]

 

[bigj_16]

Originally Posted By: d00df00d
So one thing as a preface: I have a "Series 2" RX-8, which is MY2009+. Essentially the same, but with a raft of changes to the engine and some other bits. Breathing, fueling, ignition, oiling, cooling, all changed a little or a lot.


Originally Posted By: SonofJoe
If you're going down the conventional two-stroke (2T) oil route...

Wouldn't be an option for me even if I wanted to do it. The Series 2 oil metering system is different and no one has made the relevant kit for it yet.


Originally Posted By: SonofJoe
I suspect its important that the oil in the combustion chambers is as combustible as it can be to minimise deposits. Unlike conventional piston engines, you might need an oil that has very low resistance to oxidation (which is what combustion is). It's conceivable that PAO is not your friend in a rotary engine as it could resist burning and create more carbon deposits.

These are all common notions that make eminent sense.

And then there's this: Mazda-branded fully synthetic oil labeled for this specific engine, in 0w-30, described by Mazda as having "PAO as a main component and an ester" (per Google translation of that page).

There's also this: a rotary-specific (though not RX-8 specific) fully synthetic engine oil, made by Mazda's partner for the 787B Le Mans car, with a flash point of 260º C (!).

Doesn't that suggest the opposite, i.e. that highly burn-resistant oils can be okay?

These are the two main sides of a debate that has been raging for years, and I have yet to find anyone who has offered a way to reconcile them. Everyone just seems to shrug their shoulders and repeat what they just said.


Originally Posted By: SonofJoe
The other thing I picked up on was how many rotary engines reported oil in their intake systems. Do Wankel engines have a PCV system? I'm assuming they must do as apex seals must suffer from proportionately more blow-by that conventional piston rings? If that's the case, then oil must follow the blow-by gas to be burnt in the combustion cycle, further adding to the formation of deposits. I might assume that the flow of recycled oil is especially high at high RPMs.

The oil-in-the-intake problem was largely solved in the Series 2 updates AFAIK. Yes, the RX-8 does have a PCV system.

The Renesis engine in the RX-8, with its ports on the sides of the combustion chambers, ends up naturally recirculating some fraction of the exhaust gases it produces. It's kind of like a built-in EGR. I'd have to imagine there are all kinds of oil-derived vapors in that mix.


Originally Posted By: SonofJoe
Very tricky to say what oil is best for these engines. My advise is stick as closely as possible to what Mazda say.

That'd be my instinct as well. The problem is that Mazda seems to contradict itself in different markets.

In North America, the line is "API/ILSAC certified non-synthetic 5w-20, period, end of story."

In Europe, they strongly recommend their own Dexelia Ultra 5w-30, which chrisri mentioned in this thread; being an ACEA A5 oil, doesn't that suggest at least a synthetic blend?

In Japan, they sometimes recommend 0w-20 but also sell that RX-8 specific fully synthetic 0w-30 I linked above.

In Australia, they again demand API/ILSAC and non-synthetic, but have a full chart recommending different viscosities for different temps.

I'm also hearing that their in-house racing support division recommends synthetics for race use in rotary engines; not entirely relevant, but perhaps suggestive WRT how well these engines burn the oil they inject, no?

How about these?:

http://www.idemitsu-usa.com/h/35/products.htm

 

[d00df00d]

I linked one of those in the post you quoted.

 

[d00df00d]

As an aside: With Shannow, SonofJoe, and edhackett in here, this thread is so refreshing! It's so nice to see real experts posting here. I'm personally grateful, especially in view of the time you guys have spent on this. I'm sure the community appreciates it as well. This is what BITOG is supposed to be!

 

[Garak]

Originally Posted By: edhackett
Yes, this thread is focused on combustion chamber deposits. API and GF specifications don't have any sulfated ash standard. The ACEA standards do. I think the sulfated ash contribution would part of the results obtained from the TEOST and similar tests. Too high a SA could cause failure on those.

Yes. Even in the ACEA regime, the SA limits are fairly liberal, aside from the C sequences and the tighter variants of the E sequences. When it gets to HDEOs these days, I would suggest it's pretty hard to find one with a level above 1.0 unless it's an oil with a legacy spec.