Advertising for modern lubricants promises a great deal, such as:
- “Industry-leading, fully synthetic oil providing ultimate protection”
- “Uncompromising durability and long life”
- “Improved to deliver even better wear protection”
- “Maximum protection against piston deposits”
Little wonder that many yacht owners are tempted to pay premium prices in return for the confidence that they believe they will gain from putting the best product available into their auxiliary engines. But does 'best' mean what you think?
Engine lubricants may be considered as having two main components: the base oil and the additive package.
Base oil
Mineral oil is a hydrocarbon, extracted from crude oil by distillation. It comprises a mixture of hydrocarbon molecules, of varying lengths.
When this oil is exposed to high temperatures, such as on piston crowns and piston rings in high-performance engines, the shorter molecules are liable to 'crack', losing their hydrogen by burning and leaving their carbon behind as gums and deposits.
This problem has become increasingly prevalent as the power output and internal temperature of engines has increased in the past 15 years or so.
The response of the oil companies has been to develop synthetic oils. Although still hydrocarbons, they are not distilled from crude oil but instead are synthesized from liquids, such as hexene.
The result is a fluid whose molecular length is closely controlled, giving it excellent properties at both high and low temperatures. The problem of gum deposition on high temperature surfaces has almost been eradicated by this development.
Additive packages
Additives in engine oils are a complex package, containing up to 20 components intended to reduce wear and friction, control oxidation, reduce foaming, remove deposits, etc.
As with base oils, some are designed to perform in high-performance engines and others are not. Logically, the high-performance additive packages go into the synthetic base oils whereas the lower-performance packages can be found in the mineral oils.
When fuel burns in an engine, the combustion product is acidic. Engines that run at high power generate a more acidic product than do cooler ones, so the additive package is tailored to neutralise these acids.
This property of lubricants is known as Total Base Number (TBN). The TBN of a synthetic lubricant intended for a turbo-charged engine running for extended periods at the top of its rev range might be 12 - 15. Conversely, the TBN of a lubricant designed for cool-running, low power duties such as yacht auxiliary engines will be about 3-4.
API classification
It is not necessary for the user to understand all this technical stuff because it has all been summarised for us in the API classification system. The full details of this system can be seen at www.apicj-4.org/EngineOilGuide2006.pdf
The operating manual for your engine will advise the type of oil it needs according to this classification. Very many small yacht engines, such as Yanmar, Volvo, Bukh, etc., call for oils to API CD which, as can be seen in the link, is obsolete. However, oils to this classification, or to the European equivalent ASEA E1, are still obtainable after a search.
Over-specification
Yacht auxiliary engines generally run for short periods, at lower temperatures than any other duty, and typically in the lower half of their rev range. From a lubricant point of view this is a very undemanding duty, accounting for their low API requirement.
Combustion temperatures never reach levels at which a high-performance lubricant can be effective, so the overall chemistry in this area is not neutral but alkaline (basic).
There is a great deal of evidence to show that use of an oil with a TBN that is too high for the duty can lead to several problems, particularly high wear rates of cams, tappets and cylinder bores.
Bore wear results in a condition similar to bore glazing, producing a highly polished surface that leads to poor lubrication and high oil consumption.
What to do?
First and foremost, as far as possible use the grade of oil that the engine manufacturer recommends. In many cases this will be API CD, which can be difficult to obtain. Some of the main manufacturers still make oil to the older specifications, for example Shell Rimula C, see www.epc.shell.com
and Total Rubia H and Rubia B, see www.fcdawes.co.uk/pdf/COMMERCIAL.pdf
.
Otherwise, almost all car parts shops, Halfords, etc stock oils to API CD by less well-known manufacturers. Most French hypermarkets sell API CD oils by Exxon, Total, Elf and Shell, so it's worth stocking up if you find yourself across the Channel. Yanmar's own oil is to API CD, although this is not the cheapest.
Otherwise, use a quality oil to API CF-4, which is the official replacement for API CD.
But use a synthetic? I wouldn't - although I do in my car.
