Lube-Tech
Following this stage, a number of marine diesel cylinder lubricants (MDCLs) were tested in a 2-stroke engine to evaluate performance in terms of deposit control and corrosion protection, the results are discussed below.
Engine testing with Very Low Sulphur Fuel Oil (VLSFO)
Base number (BN), a measure of acid neutralisation capacity, has become a defining performance characteristic for MDCLs, forming the basis of OEM lubricant selection guidelines in relation to the sulphur content of the fuel being used. During combustion the sulphur undergoes oxidation reactions which leads to the formation of sulphuric acid (H2
SO4 ). Where temperatures in the combustion
space drop below the acid dew point condensation occurs, which drives corrosive wear of critical engine parts such as the cylinder liner. Corrosive wear has been shown to be effectively controlled by selecting a MDCL with sufficient acid neutralisation capacity.
When operating on high sulphur HFO, which currently averages globally around 2.7% w/w sulphur, the use of 70 BN and above MDCLs is recommended. The move to <0.5% sulphur fuels will reduce the total amount of acid that can be generated and thus the potential for corrosive wear. Therefore, a lower BN lubricant is more appropriate for operating on low sulphur fuel as excessive base can accumulate as hard abrasive deposits on piston crown lands with the potential to cause bore polishing or scuffing. 40 BN lubricants emerged as the preferred choice when operating on up to 1.5% w/w sulphur fuel and since 2015 when the fuel sulphur content in emission control areas was capped at 0.1% w/w, 15-25 BN lubricants have been utilised with satisfactory protection of engine hardware from corrosion.
Additives used to deliver base in lubricants are over-based detergents that also help to keep the
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.124 page 4
engine clean of deposits. One of the benefits of 70 BN and higher lubricants is they inherently contain a substantial amount of detergent soap and these generally cope very well with the variation in HSFOs today in terms of deposit control.
The type of detergents selected (e.g. sulphonates or phenates) also impacts this aspect of performance. Reducing the BN by simply adjusting the over-based detergents without rebalancing the formulation with additional deposit control additives will severely affect the lubricant’s ability to keep the engine clean, even without the added variability anticipated with fuels post 2020.
Variability is inevitable when considering the range of blend stocks and refining processes that could be used to produce <0.5% sulphur fuels in order to meet demand. This variability will present a variety of challenges to ship operators from determining compatibility of different batches, to engine deposit formation where having a lubricant with the right performance characteristics will be key.
In order to determine the appropriate BN and deposit control requirements of MDCLs for use with <0.5% sulphur fuels, Lubrizol formulated a series of 25 BN and 40 BN MDCLs and tested these with commercially sourced <0.5% sulphur fuels in a stationary two stroke marine diesel engine.
Measuring residual BN in scrape down samples every 25 hours during the engine test is used to indicate if the BN of the lubricant is delivering sufficient protection from corrosive wear. To maintain satisfactory corrosion protection, the residual BN of the scrape down oil should be around 15 BN according to OEM guidance. The average residual BN for the 25 BN MDCLs across all tests was 12.5 BN compared to an average of 24.2 BN for the 40 BN MDCLs, as shown in Figure 2.
LUBE MAGAZINE NO.153 OCTOBER 2019
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