Oil Consumption of Medium Speed Diesel Engines

Tuesday, November 02, 1999
Reliability and availability are of paramount importance to all users of internal combustion engines. Inherent engine design reliability must be complemented by a suitable periphery and the appropriate choice and treatment of working media such as lubricants and fuel. CIMAC Working Groups have a long tradition of preparing recommendations and guidelines for the internal combustion engine industry and its users. In performing this work, CIMAC takes advantage of its wide and competent international membership; Working Group -"Lubricant" is truly worldwide, with active members from Japan, U.S. and all round Europe/Scandinavia. Specialists from all relevant background - Equipment/Engine Manufacturers, Oil Companies, Additive Companies, Classification Society, Shipyard etc - meets twice a year and additionally in various sub-group to work on matters/guidelines/recommendations to help better serve the industry. The work is done on a voluntary basis. The proof of its effort is shown by CIMAC Documents No. 13 and 15, which are Lubrication Guidelines for Trunk Piston Medium Speed Diesel Engines and two-Stroke cross-head diesel engines respectively. Success from these effort, has given impetus to the group in scoping other key industry issues, relating to lubrication, to work on. The WG has now completed another document entitled "Oil Consumption of Medium Speed Diesel Engines" and are also into the advance stages of finalizing four further documents, entitled as below, which are to be phased into the industry over 1999 to year 2000: •The Impact of Fuels on Lubrication: 4 & 2-Stroke •Lubrication of Large High Speed Engines •Oil Degradation •Gas Engine Lubrication Oil Consumption of Medium Speed Diesel Engines Today's medium speed engines vary in oil consumption from as low as ~0.15g/kW-hr to several times this figure. Added to the severity of low oil consumption, engine oil sump sizing is not getting any bigger. Against these complexity, there are a variety of engines in service — varying from medium/high speed engines (typically on distillate fuel) down to large medium speed engines of lower rpm traditionally on heavy fuel — which may have different practices with regard to oil sump changes, oil treatment, oil type etc. The requirement/impact of one engine on distillate fuel and with regular oil change interval (stipulated by the engine builder) is different compared to another on heavy fuel, and which works its sump to the oil condemning limit (based on used oil analysis) and changes its sump on that basis. All these gave good reason for a document to thoroughly discuss the issues and bring it to the attention of the industry. In examining the parameters impacting on oil consumption in medium speed engines a range of factors were identified. Examples of these included engine design & operating conditions, the fuel quality and the lubricant quality. With respect to the engine design the power density is expressed by size and cycle parameters, lubricant charge expressed as kg/kW and lubricating oil consumption g/kWh are some of the parameters that affect the degradation of the lubricant. For many large medium speed engines, ship owners have come to expect that periodic renewal of the lubricant charge is not necessary. Under normal operating conditions this view has been correct as the lubricant received periodic replenishment by the addition of make-up oil great enough to stabilize the oil at levels within the designers recommendations. A low oil consumption/make-up invariably leads to partial or complete renewal of the charge when the increased stress due to reduced oil make-up causes oil parameters to fall below engine-builder recommended limit. This poses the question whether a low make-up rate is economical. The answer lies in the complex debate/interaction of make-up and renewal charge volumes. In comparing the whole life cost of different lubrication regimes account must be taken of the possible differential lubricant cost and also the cost of disposal of the used oil. The document addresses oil consumption (defined as amounts of oil make up plus system charge renewal) required i) to keep the engine operating satisfactorily and ii) to control the oil quality within limits prescribed by the engine builder or suggested by the oil supplier. Major factors impacting on oil consumption are summarized together with the consequences of inadequate quantity/quality. It also offers some advice to cope with such situation.
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