The Cost Of Lubricating Trunk Piston Engines

Over the past 25 years, the specific fuel consumption for marine trunk piston engines has reduced and an increasing number of such engines are designed for residual fuel operation whether they are for main propulsion or auxiliary purposes.

The cost of fuel varies widely as the spot market price of fuel oil fluctuates. Also the cost of lubricant is an expense, which while it is not generally subject to such price variations, may be converted to a fuel equivalent value in order to better understand the running costs of a trunk piston engine.

The reduction in specific fuel consumption has taken place through evolutionary design stages and has been achieved by a general increase in pressure and temperature in the operating cycle, so making greater demands upon the ring pack. The function of the lubricant in such engines is multipurpose, it has to lubricate the cylinder to minimize wear, lubricate the bearings and running gear and cool the underpiston crown. As a result of the first function, debris from the ring pack is transported to the circulating oil and this carbonaceous matter is seen as insolubles. Treatment of the oil is carried periodic makeup, filtration and also often by suitable centrifuging of the charge.

Even after all this, there is sometimes a definite trend for the insolubles burden to continue to increase so that partial or total renewal of the charge is necessary to enable the lubricant to satisfactorily perform the functions defined above. In addition, other trends have taken place which also have an effect upon the lubricant. These are the reduction of the oil charge in some designs, when expressed on a kg/kw basis and also a reduction of the lubricating oil consumption by attention to the ring pack design.

These trends have a significant effect on the oil in circulation with respect to alkalinity reserve, expressed as Total Base Number TBN mgKOH/g, and insoluble burden.

After a period of time the alkalinity reserve will stabilize at a value which is a function of the engine design, sulphur level of the fuel burnt and the oil consumption rate.

The total oil consumption related to oil change interval is shown in Fig. 1. It should be noted that the total consumption is the addition of the make up oil used and the oil used to replenish the charge when expressed on a g/kwh basis. By way of illustration two conditions are shown, condition A, which is for a design charge of 1.2 kg/kw and consumption of 1.3 g/kwh, and condition B, which is for a charge of 0.5 kg/kw and consumption of 0.6 g/ kwh. Experience has shown that for condition A oil changes under normal operating parameters are infrequent and hence the total consump- tion can be approximated to that of the make up consumption. While for condition B, oil changes are required.

The lubricating oil used, being a consumable item, can be related to fuel and this associated cost or penalty can be determined from Fig. 2 or Fig. 3, depending on whether the engine has a high or low consumption.

Usually marine lubricating oil is purchased under term contract and hence is not subject to severe price fluctuations, its price being determined by the component cost and market forces. On the other hand, fuel oil is purchased on the spot market and is subject to wide price variation. In this decade the fuel price has fluctuated in the range of $35-$220/ton. Such a variation has a significant effect on the lubricating oil fuel equivalent penalty (LOFEP) for trunk piston engines.

Determination of LOFEP is illustrated by the plotted example in Fig. 2 where the following values have been used: Total oil consumption 1.5 g/kwh; lubricating oil price, $l,600/ton; and fuel price, $90/ton.

Using these values, the procedure for determining LOFEP from Fig. 2 is as follows: (1) construct a vertical line representing 1.5 g/kwh to intersect the lubricating oil price ($l,600/ton); (2) from this intersection, construct a horizontal line to intersect the fuel price line ($90/ ton); drop a vertical line to the horizontal axis to determine LOFEP.

In Fig. 3 which is for a low consumption condition the procedure is similar to that for Fig. 2. This is illustrated with the values 0/625 g/ kwh, lubricating oil price $1,600/ ton, fuel price, $90/ton, and a cost factor of 1.0. This procedure is identical for the first two steps as described for Fig. 2. At the third step the vertical line is taken to the intersection of the cost factor line (in this case 1.0). Then from this intersection construct a horiziontal line to the vertical axis for determining the LOFEP under the plotted conditions.

The factor line, third quadrant (bottom left) in Fig. 3, allows the cost of alternative lubricants to be examined. This may be due to a change in cost because of a variation in TBN or consideration of another lubricant. The LOFEP value obtained, whether it be from Fig. 2 or Fig. 3, should be added to the specific fuel consumption in order to take account in fuel terms the effect of a particular lubrication regime.

For further information on marine lubricant products from BP Marine, Circle 96 on Reader Service Card

Maritime Reporter Magazine, page 16,  Oct 1989 Nicola Hindle

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