Hi,
Jim - This is quite an interesting subject and should be of interest to operators of all diesel engines. In your case the Neuss is likely to be a "soot producer" and I suspect a limit of around 2% using a modern lubricant is near its limit
The acceptable soot level is usually provided by the engine's manufacturer (as you know not always easy to obtain) and typically in modern high speed heavy diesel (HSHD) engines this will be around 3.5% when using the latest specification lubricant
Until UOAs became readily available a simple "blotter test" was the parameter used by many - including me! It was and still is a great indicator of soot content!
Always try and obtain the engine Manufacturer's acceptable maximum and use it as yours! The Oil Company will also have a limit for the lubricant you are using and that can be used!
FACTORS - Engine design
We have moved along way from pre-chamber mechanically injected diesels to direct injection via real time electronic management. We have low sulphur fuels and much better lubricants now than ever before
A number of things affect soot uptake - of these the greatest factor is engine design, Some engines are designed to dump soot into the lubricant - others expel it via the exhaust (when that was acceptable it was clearly visible)! Now exhaust filters capture it!
Up until the 1990s Euro HSHD engines typically had smaller sumps than their NA competitors but they had specific oil type requirements much earlier too (Volvo's VDS being a leader). Euro HSHD engine manufacturer have moved to larger sumps, synthetic lubricants and supplementary oil cleaners over the last decade or so - and prescribed OCIs out to >100kkms
Other significant factors are engine (and operating acilliaries) condition and servicability along with application and use (conditional on design and intended use)
As designs became more complex the formation of ACEA (and the Euro emission's legacy) and the API's formation of the HDEO engine Manufacturer's consultative Committee, lubricant technologies firstly had to play a "catch up" and then a move on game. The Cummins L10 engine had much to do with this!
Typical ACEA soot related tests are the Cummins ISM (see L10 history) Mack T8E, T11 tests which have sequences ranging from 3.9 to 6.7% soot levels. These were originally OEM Tests and would never have happened via the API alone a decade or two ago. ACEA has been moving the cursor!
FACTORS - Lubricant
IME all things being equal it is the overall composition of the lubricant that determines its soot loading/loaded performance. Some Brands and types do this better than others - and have done for a very long time. It is all about the dispersion qualities of the package leading to the prevention (or minimisation) of agglomeration of the soot particles.
It is the agglomeration of the soot (not necessarily the volume) that promotes excessive wear
Aligned with this is the depletion of the dispersion additives which leads to an ever increasing speed and level of agglomeration in some cases
Keeping the lubricant clean is the answer and a centrifuge oil cleaner may lead to up to double the life of the lubricant (or more) before the soot limit is reached - if the Fe level is not reached beforehand of course!! Centrifuge cleaners tend to do this better than "barrier" by-pass filters. Increases in viscosity (due to soot uptake) in older lubricant formulations were a reality and limits of around a 40% at 40C were allowed
As internal engine technologies progressed such increases became unacceptable!
I have seen (and often argued "goodwill" Warranty on behalf of Clients) where a non prescribed lubricant has been used and the engine is full of deposits. In some cases this requires an engine tear down! The deposits have seriously affected the engine's performance!!
FACTORS - Experience
I experimented with synthetic PCMOs (Castrol's Formula R 15W-50 - later 10W-60) in Japanese light diesels (Isuzu, Kubota etc) commencing in 1979-80. These pre-chamber diesels were notoriously "dirty". By using UOAs we managed to obtain 3500hr OCIs in these engines with the synthetic PCMO. The normal OCI was around 500hrs using an HD oil of that era
In my own case (Class 8 vehicles) I experimented from the early 1990s with high grade mineral (15W-40), semi synthetic (15W-40) and fully synthetic (5W-40) HDEOs over extensive time and distances. There is no doubt that the synthetic HDEO maintained it's viscosity much longer and this is a good measure of the dispersant activity. As well it was always a "see-saw" between which condemnation factor was reached first - Fe (at 150ppm) or soot (at 3.5%)! I used a general limit of 3.5% (averaged) although the lubricant's additive Supplier said that up to 6-7% was acceptable without excessive wear worries
On a "tear down-measure up" inspection of a typical and randomly selected engine of mine by the engine's Manufacturer at 1.2m kms (750k miles) it was found that all parts were within useable tolerances!
FUTURE
It Europe earlier this year I found that at least two Oil Companies were monitoring fuel dilution in small high speed diesels at unprecedented levels and that soot levels were somewhat assisted by this!
We are likely to see sealed lubrication systems and lubricants with dedicated formulations. Centrifuge cleaners and OLMs may lead to no OCIs at all in the longer term
UOA
As for using UOAs - well much has been said about their accuracy, replication and etc. Blotter testing and UOAs (used properly) and along with a PQ Index, TAN and TBN (and PC if cost effective) to determine an acceptable soot level in your engine/application becomes more of a reality via trending the UOA and other results
IMO and depending on the modern quality rated lubricant being used,a limit of around 1.5-3.5% is probably acceptable
As I commented earlier, always try and obtain the engine Manufacturer's acceptable maximum (and the Oil Company's too) and use it as yours!
