21265543 Volvo.Penta Gear

engines. Some uncertainty may arise in what factors should be considered in selecting the appropriate lubricant. The Operation and Maintenance Guide gives general guidelines for lubricant selection which should be a dedicated Natural Gas Engine Oil. DiscussionThe selection of lubricant also depends on the characteristics of the fuel going to the engine. The fuel can be "sweet" (contains little or no H2S or acid forming compounds), or "sour" (contains H2S, or may contain halogens or chlorofluorocarbons or other acid forming compounds).The Operation and Maintenance Manual for the particular engine model, and / or Application and Installation Guide for Gas Engines, can be referred to for additional information and details relating to fuels."Sour" gas fuel can result in acid forming products that require alkalinity in the lubricant to neutralize these acids. Alkalinity in the lubricant comes from additives that also contribute to the sulphated ash level of the lubricant. Some sulphated ash additives are essential for adequate lubricant performance, and to prevent excessive valve seat wear (valve seat recession)."Sweet" fuels would require a lubricant with 0.3% - 0.5% sulphated ash, Refer to Lube and Maintenance Guide for the particular engine model. "Sour" fuels would require lubricants with higher sulphated ash levels, depending of the fuel characteristics. Refer to the Lube and Maintenance Guide for the particular engine model.However sulphated ash components in the lubricant are a contributor (along with combustion byproducts) to deposits forming on the exhaust valve seats. If these deposits are too thick they can insulate the valve (raising the operating temperature to an unacceptable level) by preventing heat transfer to the head insert, or be unstable (allowing cracking or flaking). Both of these results can be initiators contributing to valve guttering.Ideally there should be a balance between valve seat wear (measured by valve recession) and cylinder head overhaul interval, ie the valve recession limit would be reached at the cylinder head life. Where there is minimal valve recession occurring (which would indicate cylinder head overhaul intervals of more than 1 1/2 times expected intervals, Refer to Operation and maintenece Guide for the engine model) it is likely there is too much deposit on the valve seat preventing wear. Some wear is desirable to minimize valve problems mentioned above.Solution:
Where the gas analysis shows the fuel to be "sweet", and valve recession measurements indicate negligible wear, or projected life greater than 12,000 hours, it is probable that a reduced sulphated ash lubricant can be of benefit in reducing the potential of valve failures from excessive deposits. Select a lubricant based on the amount of "sour" components in the gas analysis, that provides an adequate oil drain interval. It can result that a (lower sulphated ash) lubricant that has less valve seat deposit forming tendency may result in a lower oil drain interval than that experienced with a (higher sulphated ash) lubricant that has a higher valve seat deposit forming tendency. In this case the cost of a valve failure repair and downtime must be considered against the cost of reduced oil drain interval. Some customers may elect to trade a longer oil drain interval against the risk of valve failure.COPYRIGHT 1995 CATERPILLAR OF AUSTRALIA LTD. ALL RIGHTS RESERVED