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Review article on low-speed preignition (LSPI) and super knock #5164030 07/18/19 06:35 AM
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Gokhan Offline OP
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I just ran into a great 35-page-long review article on the LSPI subject, which has open access under the Creative Commons license, for those who are interested and have time. Enjoy!

https://www.sciencedirect.com/science/article/pii/S0360128516300764
Knocking combustion in spark-ignition engines
Zhi Wang (a), (b) Hui Liu (a) Rolf D Reitz (c)
(a) State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing, 100084, China
(b) Center for Combustion Energy, Tsinghua University, Beijing 100084, China
(c) Engine Research Center, University of Wisconsin-Madison, Madison, WI 53706-1687, USA
Received 17 July 2016, revised 29 March 2017, accepted 29 March 2017, available online 3 May 2017


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Re: Review article on low-speed preignition (LSPI) and super knock [Re: Gokhan] #5164130 07/18/19 09:01 AM
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Good read.


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Re: Review article on low-speed preignition (LSPI) and super knock [Re: Gokhan] #5164269 07/18/19 12:15 PM
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Thanks!


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Re: Review article on low-speed preignition (LSPI) and super knock [Re: Gokhan] #5164305 07/18/19 12:47 PM
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Thanks, bud!


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Re: Review article on low-speed preignition (LSPI) and super knock [Re: Gokhan] #5164330 07/18/19 01:19 PM
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Here are some of the paper's comments about oil:

3.3.1.2. Oil additives

Many researchers have noted that oil additives correlated directly with the frequency of super-knock due to possible catalytic reactions. The frequency of LSPI (super-knock) increased with the content of additives [151], [153]. The detergent Ca has a contributory effect, and MoDTC or ZnDTP have a preventative effect on LSPI [148], [152], [229], [230]. A new engine oil formulation has been developed [147], which reduced LSPI frequency to less than 10% of that of conventional ILSAC certified gasoline engine oils. High quality base oils and optimized additive components were formulated in which the amount of calcium-based detergent was reduced to levels lower than that in general ILSAC oils, and anti-oxidants were added. Based on these findings, a correlation to estimate LSPI frequency (relative) was obtained:
(6) LSPIfrequency=6.59·Ca[wt%]−26.6·P[wt%]−5.12·Mo[wt%]+1.69


In an assessment of engine oil degradation effects, the influence of wear metals and engine oil degradation was also investigated. It was found that addition of Fe and Cu compounds clearly showed contributory effects on LSPI frequency [148]. In addition, the observed auto-ignition frequency decreased with increasing viscosity and density, which is related to the physical property of the additives [228].

However, so far the reported work does not provide an explicit conclusion about the effect of oil additives. For example, there is no consensus on whether Zn and Mo have effects on pre-ignition [152], [231]. This is because pre-ignition in engine production is random and oil additive effects are only one of the possible pre-ignition sources. To isolate the effect of oil additive on pre-ignition, a single factor experimental method needs to be developed to reproduce pre-ignition and super-knock in a research engine [149]. This represents a promising area for future research work, as developing inhibited additives could provide solutions for suppressing pre-ignition and super-knock.


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Re: Review article on low-speed preignition (LSPI) and super knock [Re: A_Harman] #5164672 07/18/19 08:36 PM
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I as interested in the same part of the study. My take:

Ca, Fe, & Cu contributed to LSPI(superknock). So, choose a D1G2/GF-6 type oil with lower Ca levels and change your oil frequently to keep wear metals in check.

Pick an oil with decent ZDDP and moly levels.

Sounds like an A3/B4 oil to me. Add A40, 502.00, LL-01, and 229.5.

Zee Germans may have been 20 years ahead.

Discuss...


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Re: Review article on low-speed preignition (LSPI) and super knock [Re: Gokhan] #5165010 07/19/19 07:49 AM
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I want to look at this a lot more carefully over the weekend.
But, I think the info presented furthers the idea that LSPI problems have not been apparent with high calcium A3/B4 oils because they also have high ZDDP levels...the multiplier for the P subtractor in the simple equation quoted above is 26.6 while it is 6.59 for the Ca adder. A little ZDDP can make up for a lot of calcium (roughly 4 times as much).
I am still planning to stick mostly with d1G2/SN+ oils since they have been actually tested for LSPI mitigation, but I can see using A3/B4 oils in DIT engines without worrying (too much).


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Re: Review article on low-speed preignition (LSPI) and super knock [Re: LotI] #5165943 07/20/19 12:56 AM
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Gokhan Offline OP
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Originally Posted by A_Harman
LSPI frequency = 6.59 × Ca[wt%] − 26.6 × P[wt%] − 5.12 × Mo[wt%] + 1.69

Originally Posted by LotI
Pick an oil with decent ZDDP and moly levels.

Sounds like an A3/B4 oil to me. Add A40, 502.00, LL-01, and 229.5.

Zee Germans may have been 20 years ahead.

Discuss...

Good analysis, guys, thanks!

PQIA analyzed the average Ca and Mg in SN vs. SN PLUS oils:

http://www.pqiamerica.com/FSmagup2019.html

Their finding for average Ca is 2,060 ppm vs. 1,217 ppm in SN vs. SN PLUS oils.

Plugging this into the formula, this difference can also be made up with 209 ppm phosphorus.

Alternatively, if an A3/B4 oil has 1000 ppm phosphorus and 2,226 ppm Ca, this is equivalent in LSPI suppression to an SN PLUS oil with 750 ppm phosphorus and 1,217 ppm Ca. However, Mobil 1 FS 0W-40 has 3645 ppm Ca and 882 ppm phosphorus (P):

https://www.oil-club.ru/forum/topic/30271-mobil1-fs-0w-40-svezhee/

1,000 ppm = 0.1%

LSPI frequency for average SN PLUS oil = 6.59 × 0.1217 − 26.6 × 0.0750 − 5.12 × 0.0050 + 1.69 = 0.471
LSPI frequency for Mobil 1 FS 0W-40 = 6.59 × 0.3645 − 26.6 × 0.0882 − 5.12 × 0.0082 + 1.69 = 1.70

So, if everything else was equal and the additives were the only factor, an average SN PLUS oil would protect against LSPI 3.6 times better than Mobil 1 FS 0W-40 (would have 1/3.6 of the LSPI events of Mobil 1 FS 0W-40) according to this formula.

Of course, there are other factors in LSPI frequency in addition to the additives.


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Re: Review article on low-speed preignition (LSPI) and super knock [Re: Gokhan] #5166031 07/20/19 06:56 AM
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So how does this not occur in (for example) Mercedes engines running full SAPS oils (both my 2018 cars do)? This seems to be more of a design/engine management issue versus that of oil composition.


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Re: Review article on low-speed preignition (LSPI) and super knock [Re: 2015_PSD] #5166078 07/20/19 08:10 AM
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Gokhan Offline OP
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Originally Posted by 2015_PSD
So how does this not occur in (for example) Mercedes engines running full SAPS oils (both my 2018 cars do)? This seems to be more of a design/engine management issue versus that of oil composition.

The article explains this and more. It has to do with the power density -- torque per liter of displacement -- which is more conventionally expressed as the brake mean effective pressure (BMEP). LSPI followed by super knock happens in engines where BMEP is around 25 bar or higher. 27 bar is the current limit for production engines. For your MB engines, BMEP is probably about 20 bar or less.


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Re: Review article on low-speed preignition (LSPI) and super knock [Re: Gokhan] #5166105 07/20/19 08:34 AM
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Originally Posted by Gokhan
Originally Posted by 2015_PSD
So how does this not occur in (for example) Mercedes engines running full SAPS oils (both my 2018 cars do)? This seems to be more of a design/engine management issue versus that of oil composition.
The article explains this and more. It has to do with the power density -- torque per liter of displacement -- which is more conventionally expressed as the brake mean effective pressure (BMEP). LSPI followed by super knock happens in engines where BMEP is around 25 bar or higher. 27 bar is the current limit for production engines. For your MB engines, BMEP is probably about 20 bar or less.
My AMG is 21.8170805 bar so some clarity there. Thanks for the explanation.I used the following:

BMEP (psi) = 150.8 x TORQUE (lb-ft) / DISPLACEMENT (ci)
150.8 x 384 / 183 = 316.4

1 PSI = 0.0689476 Bar



Last edited by 2015_PSD; 07/20/19 08:39 AM. Reason: Added context

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Re: Review article on low-speed preignition (LSPI) and super knock [Re: 2015_PSD] #5166268 07/20/19 11:14 AM
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Originally Posted by 2015_PSD
Originally Posted by Gokhan
Originally Posted by 2015_PSD
So how does this not occur in (for example) Mercedes engines running full SAPS oils (both my 2018 cars do)? This seems to be more of a design/engine management issue versus that of oil composition.
The article explains this and more. It has to do with the power density -- torque per liter of displacement -- which is more conventionally expressed as the brake mean effective pressure (BMEP). LSPI followed by super knock happens in engines where BMEP is around 25 bar or higher. 27 bar is the current limit for production engines. For your MB engines, BMEP is probably about 20 bar or less.
My AMG is 21.8170805 bar so some clarity there. Thanks for the explanation.I used the following:

BMEP (psi) = 150.8 x TORQUE (lb-ft) / DISPLACEMENT (ci)
150.8 x 384 / 183 = 316.4

1 PSI = 0.0689476 Bar
In thinking about this a bit more, which engines are typically higher than 22 bar? My AMG is producing more than 2HP per cubic inch, so is BMEP higher than 22 bar the only reason for LSPI? I have not looked into what OEM/engine family suffers the most from LSPI, but it would mean they are producing much more than 2HP/2LB-FT per cubic inch if that is the root cause.


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Re: Review article on low-speed preignition (LSPI) and super knock [Re: 2015_PSD] #5166667 07/20/19 08:20 PM
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Gokhan Offline OP
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Originally Posted by 2015_PSD
In thinking about this a bit more, which engines are typically higher than 22 bar? My AMG is producing more than 2HP per cubic inch, so is BMEP higher than 22 bar the only reason for LSPI? I have not looked into what OEM/engine family suffers the most from LSPI, but it would mean they are producing much more than 2HP/2LB-FT per cubic inch if that is the root cause.

BMEP = 22 bar is actually already in the danger zone and you may be having occasional moderate LSPI + super knock events depending on the oil you are using.

The article mentions BMEP = 25 - 27 bar as challenging as far as controlling LSPI and super knock is concerned but it doesn't say that below these BMEP values, you wouldn't observe LSPI and super knock. Other research papers seem to suggest LSPI can happen above 20 bar and even below. Nevertheless, BMEP is certainly the main driving force behind LSPI and super knock.

Moreover, the oil you use in your AMG would probably qualify as SN PLUS. Plugging in the values into the formula above, you get:

LSPI frequency for Castrol 0W-40 A3/B4 = 6.59 × 0.1878 − 26.6 × 0.0960 − 5.12 × 0.0000 + 1.69 = 0.37

This is actually below the LSPI frequency for an average SN PLUS oil, which is 0.47. (See my post above.) Therefore, you seem to be running an oil that suppresses LSPI + super knock, even though it's not labeled so. In comparison the VOA values for Mobil 1 FS 0W-40 result in LSPI frequency = 1.70.

Here is a list of the 0W-40 VOAs:

https://www.oil-club.ru/forum/topic/18413-0w-40-svezhie/

There are definitely engines out there that push the BMEP to very high levels to meet Europe's latest fuel-economy rules. Take the Peugeot 1.0i 115 PS engine for example. It has 147.5 lbf⋅ft torque and 0.999 L = 60.96 cu in displacement. Using your formula above gives BMEP = 365 psi = 25.2 bar.

People are reporting LSPI in this engine over YouTube:



PSA Peugeot Citroen is certainly complaining about lack of adequate action on LSPI-suppressing oils:

https://www.infineuminsight.com/articles/passenger-cars/psa-looks-for-the-right-balance/


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Re: Review article on low-speed preignition (LSPI) and super knock [Re: Gokhan] #5166913 07/21/19 06:08 AM
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It appears that BMEP = 200 N⋅m per liter = 25.1 bar = 365 psi is the current state-of-the-art for most engines but there are some that have higher.

The LSPI test for API SN PLUS is Sequence IX and it seems to be using a 2.0 L Ford Ecoboost engine with 366 N⋅m. So, it has BMEP = 23.0 bar, somewhat on the low side for the latest maximally fuel-efficient TGDI engines.

I am guessing that the GMW17244 GM dexos1 Gen 2 LSPI test is using a 2.0 L GM Ecotec engine with 400 N⋅m but I don't know. This would have BMEP = 25.2 bar. The dexos1 Gen 2 test also seems to be far more strict in the maximum number of LSPI events allowed.

They also have 2.0 L Ford Ecoboost and GM Ecotec engines that produce 420 N⋅m, which is BMEP = 26.4 bar. So, they are certainly pushing toward 27.0 bar, which was said in the article to be the current BMEP challenge for production engines.

The formula is:

BMEP (bar) = 0.125789 × torque (N⋅m) / displacement (liter)
BMEP (psi) = 1.82442 × torque (N⋅m) / displacement (liter)


2020 Toyota Prius Prime XLE plug-in hybrid, 2ZR-FXE engine, ~ 0,000 M
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Re: Review article on low-speed preignition (LSPI) and super knock [Re: Gokhan] #5166977 07/21/19 08:12 AM
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Slight correction to the multiplication factors in the formulas -- only roundoff error for most purposes:

BMEP (bar) = 0.125664 × torque (N⋅m) / displacement (liter)
BMEP (psi) = 1.82260 × torque (N⋅m) / displacement (liter)


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