Combating cylinder liner lacquer formation 1

Lacquer formation on the cylinder liners and the resulting excessive rates of lube oil consumption is a continuing problem for some highly-rated medium speed engines operating on distillate fuel

Cylinder liner lacquering has been known for over 15 years, the phenomenon affecting medium speed engines in a number of shipping sectors. Research into the causes by Shell Global Solutions led to the development of a solution via the lubricant for those engines which, from a combination of factors, are susceptible to the problem.

Lacquer build-up fills honing grooves and creates a smooth glazed cylinder liner surface, undermining lubricant adhesion to the liner. Such a surface layer makes it difficult for the lube oil film to adhere to and lubricate the piston ring/liner surfaces, resulting in a significant increase in the oil consumption rate.

Once the problem has started it normally requires the engine to be overhauled and the liners rehoned. Rehoning is the only 100 per cent effective way of returning a lacquered liner to its original condition, although cleaning off lacquer with an acid (such as acetic acid) can sometimes work. There is a risk, however, of some deposit remaining and becoming sites for renewed formation of lacquer.

Surface roughness measurement of the liner is a key technique in determining whether lacquering (with filled honing grooves) has actually occurred since visual examination in field conditions can be subjective, says Shell.

Four main factors contribute to liner lacquer formation, which is thought to occur when more than one of these combine to create a severe situation:

• engine design

• engine load pattern

• fuel

• lubricant.

Engine maintenance – reflected in fuel injector condition and liner and air intake temperatures – can also affect the chances of liner lacquering taking place. Too low temperatures may promote condensation of heavier and partly-combusted fuel components on the liner surface, an initial step in the lacquer formation process.

More highly-rated engines are found to be prone to lacquer as well as engines operating under a mixed load regime (for example, periods of very high loads, part loads and long periods of idling). Lacquer formation was seen particularly in fishing vessels deployed in the North Sea. Lower sulphur fuels were also found to be associated with the phenomenon, and it is thought that heavier fuel ends were another contributory factor. Lubricant formulation and quality was another influence.

Liner lacquer is believed to form when partially combusted and ‘cracked’ fuel components condense on the liner surface. Here, they undergo oxidation and polymerisation and mix with calcium and zinc salts from the lubricant to form a layer. This layer is thought to spread to fill the honing grooves by piston ring action and become ‘baked’ to form a hard glaze through the heat of combustion (see schematic).

It has been postulated that quinones are major components of liner lacquer, formed by the partial oxidation of heavier aromatic fuel constituents such as benzantrene (four condensed aromatic rings). The presence of quinones would explain why lacquer dissolves readily in a weak organic acid, such as acetic acid (vinegar), this behaviour not being expected of a resin material. The proposal has yet to be confirmed, however, by detailed chemical analysis of lacquer.

Lubricant formulation can prevent lacquer forming in susceptible engines. Special test conditions were developed at Shell Amsterdam in a single-cylinder Deutz 628 engine to discriminate between different lubricants in terms of lacquering tendency. The test was used to formulate a lube oil that can virtually eliminate lacquer, giving a result well beyond that of the best commercial oil at the time. This lubricant was tested in a range of other laboratory and rig tests before being released for field trials.

Field testing of the anti-lacquer lube oil showed it formed no appreciable liner lacquer and that oil consumption was controlled at normal levels in two different engines known to give lacquer problems. Over 30,000 engine-hours had been accumulated with the new anti-lacquer lubricant chemistry when the new oil, Shell Gadinia AL, was introduced commercially with approvals from major enginebuilders. MP