Dexos 1 and dexos 2 clarification

[SonofJoe]

I was thinking about this a bit more in the bath...

Today, almost all gasoline will contain additives to prevent inlet valve deposits (IVDs). It's been a very long time since I played around with gasoline additives but generally speaking, the main constituent is a polymeric detergent. Chemically speaking, these are not too dissimilar to the polymeric ashless dispersants you have in engine oil. The entire point of these additives is that they are non-volatile. On an MPI engine, when the gasoline is injected into air, the bulk of the gasoline is evaporated but the polymer is not. Minute droplets of polymer have momentum and physically 'hit' the back of the inlet valve to impart a degree of 'physical' cleaning. At the same time active nitogen sites on the polymer backbone do a bit of 'chemical' cleaning.

Now these polymers are very thick and viscous and to make them usable, they are usually diluted in a 'carrier oil'. As often as not, this would be a heavy Group I 500SN base oil. This functions like a poor man's detergent. It's essentially non-volatile so, like the polymeric detergent, will do some physical scouring of the valve surface (but won't do any chemical cleaning).

Now while this is all fine and dandy in an MPI engine, I do wonder if in a TGDI engine, these very additives might be contributing in a big way to LSPI. In a GDI engine, fuel is invariably sprayed DOWNWARDS. Now the bulk of the gasoline will evaporate but anything that doesn't evaporate will have momentum and carry on it's downward path until it hits something like the top of the piston or the side of the cylinder. It doesn't take a whole lot of imagination to see how this mix of very heavy fuel/polymeric detergent/heavy base oil might accumulate in the top ring land space.

So to all of you guys who think the answer to LSPI us using top-tier gas, then you might want to reconsider, as it might be that that's causing the problem!

 

[userfriendly]

If engine designers played with the piston and combustion chamber shape to promote swirl, injected gasoline into the turbulence late into the compression stroke, the fuel might not reach the cylinder wall and piston top in a liquid state.
With turbo engines, the intake valve should not be closed until the pressure in the cylinder has equalized with the boosted manifold pressure. It's the boost that reaches the cylinder that counts, not the manifold pressure. Valve overlap is a non-issue with DI engines if the fuel is injected during the compression stroke after the intake valve is closed. At worst during valve overlap with DI engines, cylinder A might steal from cylinder B, but no fuel is exchanged. Idle quality is not affected by overlap and fuel is not lost past the exhaust valve, as with old school carbureted engines with big camshafts.

 

[ChrisD46]

Back in the day we called it : "Pinging" !
Originally Posted By: kenpoed
What are lspi events?

 

[zeng]

Originally Posted By: ChrisD46
Back in the day we called it : "Pinging" !
Originally Posted By: kenpoed
What are lspi events?


.... or knocking.

 

[SonofJoe]

Originally Posted By: userfriendly
If engine designers played with the piston and combustion chamber shape to promote swirl, injected gasoline into the turbulence late into the compression stroke, the fuel might not reach the cylinder wall and piston top in a liquid state.
With turbo engines, the intake valve should not be closed until the pressure in the cylinder has equalized with the boosted manifold pressure. It's the boost that reaches the cylinder that counts, not the manifold pressure. Valve overlap is a non-issue with DI engines if the fuel is injected during the compression stroke after the intake valve is closed. At worst during valve overlap with DI engines, cylinder A might steal from cylinder B, but no fuel is exchanged. Idle quality is not affected by overlap and fuel is not lost past the exhaust valve, as with old school carbureted engines with big camshafts.


Yes, I agree with all of this. I too an thinking that 99% of the time, swirl & turbulence will keep things such that heavy 'stuff', partially burnt or otherwise, exits the cylinder with the exhaust gas.

However the way I understand things, LSPI is more of a random event than a repeatable condition. With classical knock (please let's get into semantics again as I can do without another mad rant from Mr Grumpy) you just drive the engine to a given set of conditions and the engine knocks again, again and again. It wouldn't stop until you put a hole in the top of the piston!

With LSPI, yes you can drive the engine to the area where LSPI occurs but it doesn't 'repeat' in the same way. I seem to recall a lady from one of the OEMs (big red hair) taking about an LSPI test where ten separate measurements were required just to see one LSPI 'event'. If the root cause of LSPI is the top land space filling up and then randomly disgorging its contents, then it's logical that it takes a finite time to 'refill' the space. It may well be that you only need a small portion of the heaviest bits of the fuel to create accumulation over a long time to create one such 'event'.

So yes, notwithstanding the effects of swirl, I would still be rather concerned about the heavy stuff in gasoline and its negative impact on LSPI.

 

[dirty_projection]

Originally Posted By: SonofJoe
Yeah. All of the above!

One other thing to bear in mind is that all things being equal, gasoline Final Boiling Points (FBPs), and back-ends in general, tend to be heavier in winter than in summer. Gasoline volatility (as measured by Reid Vapour Pressure) tends to be less restricted in winter. This allows you to stick more light stuff (like Butane) into the blend. A lighter front-end allows you to get more heavy stuff into the back-end. There are many other factors that might impact on this (like season demand) but in general, the principle is true.

The thing is LSPI, if my understanding of the basic mechanism is right, is more likely to be a 'winter thing', as opposed to a 'summer' thing'. That's because direct injection of cold gasoline into a cold cylinder to mix with cold air is more likely to leave the heaviest bit of the gasoline in the liquid phase. This non-evaporated fuel could potentially accumulate in the top land space be later 'thrown-off' at low engine speed. Now if you add in on top of this that winter gasoline might contain MORE heavy stuff, well, you risk getting caught with a double whammy effect which can't be a good thing.


To clarify Dexos 1 Gen 2 difference is in the additive package mainly due to the studies being done by oil/auto companies and the discovery that a reduction in calcium also correlates to a reduction in LSPI events. LSPI events are not winter/summer fuel dependent, they are dependent due to the differences in combustion styles using direct injection as opposed to port injection. The issue is that trace carbon particles are still super hot and ignite the fuel before the spark causing a super-knock event when the spark ignites the fuel (for the second time) that exceeds 4X the highest pressure seen during normal combustion.

Source: LSPI

Also LSPI is counteracted by certain procedures an actively being studied and controlled to eliminate the instances. I would not worry about LSPI events even in most recent model years using Gen 1 due to the ECU software implemented in most recent DIT cars, however, Gen 2 is superior in controlling LSPI events.

It should be noted that this is first hand knowledge, but available to the public. Dexos certification does not apply to anything but synthetics as those are not used anymore due to the obvious advantages of using synthetic oils.

 

[userfriendly]

Maybe the piston crowns are not running hot enough, possibly from evaporation cooling with E-Blends? There are ways of keeping more heat in the combustion chamber without elevating the coolant temperature. Piston and chamber coatings would be one. Delaying the exhaust valve opening point at low RPM and load conditions would be another. Blaming the engine oil is lunacy gone wild. Would LSPI exist with LPG or CNG fueled engines?

 

[dirty_projection]

Couldn't state anything on those as my knowledge is mostly with gasoline fueled engines. LSPI is not a cause of engine oil at all, but oils with high calcium in the additive package has been shown to increase the odds of LSPI events. This is strictly a direct injection engine problem as port injection engines do not experience this phenomenon.

 

[SonofJoe]

Originally Posted By: userfriendly
Maybe the piston crowns are not running hot enough, possibly from evaporation cooling with E-Blends? There are ways of keeping more heat in the combustion chamber without elevating the coolant temperature. Piston and chamber coatings would be one. Delaying the exhaust valve opening point at low RPM and load conditions would be another. Blaming the engine oil is lunacy gone wild. Would LSPI exist with LPG or CNG fueled engines?


You raise some good points. I picked up on the gasoline end-point thing from early some work done by Shell. It struck a chord with me. If it was established that LSPI wasn't a thing on LPG/CNG engines, then this might be significant because (a) LPG/CNG contains no additives and (b) because it volatilises much more readily than gasoline and leaves behind no heavy-ends.

Blaming the engine oil for LSPI is sort of understandable. Plenty of studies point to the difference between Calcium & Magnesium on LSPI. If you trawl the patent data you can also find claims that Calcium Sulphonate can act as a combustion catalyst (albeit in diesel, not gasoline) which suggests it's got form. There's also a excellent 2004 SAE paper on oil loss written by MIT which talks about oil 'throw off' as being the dominant mode of oil migration at low engine speeds (although in absolute terms, rates of oil loss at low speed are very low).

One thing however that has always bugged me about this 'oil causes LSPI' thing is what's engine oil doing above the top ring so close to TDC??? Between the combined action of the top compression ring, the scraper ring and the primary oil control ring, there really shouldn't be ANY oil sitting in the top land space to interact with the process of fuel combustion. It made me wonder if what was accumulating in the top land space might be primarily derived from fuel and whether this was acting as a 'bridge' to ease the passage of oil from under the compression ring?

Anyways, it's always interesting to speculate on these things...

 

[userfriendly]

What happens to crank case pressure under boost, and how do Indy engines deal with it?
I'm thinking a fix might be higher tension rings, different cylinder wall finish, thicker engine oil.
A bunch of little things that all add up.

 

[SilverFusion2010]

Crank case pressure wouldn't change much if pcv system is working correctly. I speculate that engines that were run hard and sealed the rings really well probably have less potential for lspi. I have no evidence, just logical speculation if we posit that oil getting past the rings is a problem

 

[dirty_projection]

Crankcase will always be negative underboost or not. Typically values of -4 to -7 kPa would be normal for a turbo, but every engine is different so it's a bit subjective with not concrete number especially with different engine designs, etc.