These tests were chosen specifically by the technical team in order to identify and help treat the cause of the problems experienced by these engines.
Chevron field investigation overview
Following the identification of issues with piston rings, the Chevron Marine Lubricants Field Technical Specialist team identified seventeen vessels requiring further investigation. The age of the ships varied between four and twenty years, across a wide range of ship types including bulker, oil tanker and container vessels. All engines were two-stroke marine main engines from the major OEMs. It should be noted that in the majority of cases the piston rings fitted in these engines were not of the latest design — hard coated and gas sealing rings.
Cylinder condition observations
The issue of red deposits did not impact all of the cylinders, but it was found in some cylinders with preexisting poor cylinder liner or piston ring pack condition. In most cases the scuffing appeared only on individual liners. Some scuffed units showed red deposits on piston crowns with other units showing no signs of either scuffing or deposits.
It became clear that this issue wasn’t limited to a single lubricant supplier or linked to the BN level of the lubricant used in the application. In fact, it was found in vessels using 40BN to 100BN lubricant grades. Therefore, the problem was unlikely to be the result of cylinder lubricant performance or engine brand.
Analysis of red deposits in the samples
To establish the cause of the deposit formation and to be able to advise on how to deal with the issue, identification of the chemical composition of the samples, as well as a detailed analysis of the used oil, was carried out. Details on the analytical methods employed can be found in Figure 2.
Summary of findings
Following this program of analysis, the results indicate that the deposits contain very little organic material. This suggests that the source of the calcium sulphate — not a cause for concern in itself — is probably as a result of harmless detergent additives from the lubricants. The iron oxide, however, is the result of harmful abrasive wear of the piston and liner.
The key element that changed prior to the observation of red deposits is the introduction of lower sulphur fuel due to IMO 2020 implementation. Therefore, it is important to look in detail at the characteristics of the fuels in use.
We have calculated the CCAI (Calculated Carbon Aromaticity Index) and determined the combustion characteristics ECN (Estimated Cetane Number) via Fuel Combustion Analysis (FIA/FCA-IP 541) for the various fuel types most commonly bunkered post IMO-2020 enforcement, and compared them with HSFO, which would have been the predominant fuel used in these engines pre-2020.
Key characteristics of VLSFOs
• VLSFOs analysed showed excellent combustion properties — typically faster burning, higher energy release than traditional HFSO.
• The time between injection and reaching maximum pressure is shorter, hence peak combustion is reached earlier in the combustion cycle.
• Overall, more energy released — typically higher max ROHR level.
VLSFO from liner scuffing cases showed typically low CCAI and high ECN. The key observation is that most VLSFOs typically have a lower CCAI in comparison with compared to HSFOs.