Lubrication Of Large Bore Diesel Engines

Considerations for Emissions By Louis Lemos

Commendable strides in the pursuit of alternative fuels and energy sources are being made, thanks mainly to private enterprise initiative and governmental endeavor to share in the potential publicity. Meanwhile, public awareness of this progress is assured by recent articles published by leading maritime-related magazines on the topic of atmospheric pollution by exhaust emissions from ships.

Proposed counter measures within the realm of feasibility, and currently in use include legislation mandating that ships use ‘Low Sulfur Fuel’ within coastal waters; switching to "Shore Power" while moored alongside the pier; increased use of Selective Catalytic Reduction (SCR) systems and electronically-controlled fuel injection systems. The overall effect of such measures has produced a significant drop in nitrous oxides, sulfur dioxides, hydrocarbons and particulate matter.

The trend toward Diesel-electric main propulsion plants involving medium speed engines of the trunk piston type, mainly for large passenger cruise liners, coastal ferries, research ships and some offshore support vessels, has also contributed to the reduction of pollutant bearing exhaust emissions. The difference in NOx emissions produced by Diesel-electric plants can be as much as 20 percent lower than their straight-Diesel powered counter parts. This is attributable to the fact that their Diesel engines operate at a fairly constant speed, conducive to fairly constant combustion, maximum efficiency, optimum fuel economy and resultant uniformity of exhaust emissions with reduced pollutant content.

CYLINDER LINER LUBRICATION

Currently, most large container ships, bulk carriers and supertankers are propelled by large slow speed, large bore engines of the cross head type. Many are now equipped with electronically controlled common rail fuel injection systems in addition to Selective Catalytic Reduction (SCR) systems of which there are several variations. Because of their immense size, these engines with cylinder bores ranging from 500 millimeters to 980 millimeters (19.68 inches to 38.58 inches) cannot rely on splash lubrication of the cylinder walls exclusively. For this purpose a separate cylinder liner lubrication method is employed using Mechanical Lubricators similar to the early 1950's model shown in Figure 1, independent of the main engine-driven lubrication system. The purposes of independently lubricating the cylinder liners of slow speed, large bore engines are to neutralize acids formed during combustion and thereby protect the cylinder liner from cold corrosion attack, to establish a reasonably stable oleous film between the cylinder liner and the piston rings and to preserve a degree of cleanliness of the cylinder liner surface and piston ring pack.

MECHANICAL LUBRICATORS

One of the most common mechanical lubricators is the Liquid-Filled Sight Glass Lubricator. In this type there may be six or eight pumping elements to an assembly, with a separate assembly for each cylinder. Each pumping element has its own suction and discharge ball type check valves and delivery of oil is metered by the plunger. The discharge nozzle of each pumping element is covered by a glass tube (sight glass), filled with either distilled water or a solution of distilled water and glycerin and capped with a delivery valve assembly. The liquid minimizes emulsification but occasionally, when clouding occurs, the liquid has to be changed. Oil globules discharged by the plunger are guided up through the center of the liquid column by a strand of fine wire extending the full length of the sight glass. After several plunger strokes the delivery valve is unseated and the oil flows into small bore tubing for delivery to the point of lubrication.

The delivery tube leading to the engine cylinder is connected to a quill that penetrates the water jacket and cylinder wall and provided with a final check valve to prevent blowback of cylinder gas pressure into the oil system. When cylinder pressure falls below that of the oil in the delivery tube, the final check valve opens and a small quantity of oil is discharged to lubricate the cylinder wall. Since the number of drops per stroke varies as the plunger stroke setting, the rate of feed for any pumping element can be increased or decreased by adjusting the plunger stroke. This engine driven cylinder wall lubrication system is used on most large bore slow speed main propulsion Diesel engines.

LOW SULFUR FUEL COMPATIBILITY

It has been established that the requirement for continued reduction in allowable sulfur content of fuel, such as Low Sulfur Fuel (LSF), and Ultra-Low Sulfur Fuel (ULSF), adversely affects cylinder lubrication, particularly of the large-bore, slow speed main propulsion Diesel engines. This is attributable to the use of cylinder oil having a rather high Total Base Number, (TBN), such as 70TBN, that may result in excessive deposits on the pistons and scuffing of the cylinder liners. The severity of such adverse factors will vary in accordance with the degree of usage of LSF. For ships operating on trans-oceanic routes, wherein the majority of their running time is outside of the above-mentioned coastal areas, the use of conventional diesel fuels with a relatively higher sulfur content will be permissible. However, upon approaching such regulated areas it will become mandatory to switch to LSF for the duration of passage and/or presence therein.

Conversely, Ships engaged mainly in coastal trade on a full-time basis, and burning LSF, will require the use of a cylinder oil of a correspondingly lower Total Base Number such as 40 TBN. Given that the acidity of Diesel fuel is proportional to the level of sulfur content, the Total Base number of the lubricant is relative to the oil’s ability to neutralize the acid. This is why the Total Base Number of an oil is also considered to be its Neutralization Value, and can be expressed as a measure of the acidity or alkalinity of the oil, whichever characteristic it possesses, and is also called the Acidity Number. A significant advantage of these mechanical lubricators is that based on the known sulfur value of the fuel, the corresponding feed rate of the cylinder lubricant can be adjusted accordingly, for maximum effect. Should the need arise to change from the 70TBN cylinder oil, commonly used for high sulfur fuels, to the lower 40 TBN cylinder oil, for use with low sulfur fuel, it is advisable to contact the lubricant supplier to determine the recommended feed rate of the lubricant, in order to ensure that the appropriate degree of cylinder lubrication is maintained.

While the enforcement of restricted exhaust emissions is of prime concern to ship-owners, so too is the difference in cost between conventional heavy fuel (380 centistokes) averaging 2.5% Sulfur Oxide (SOx), currently available for about $480.00 per ton, versus Marine Diesel Oil (MDO) rated at less than 0.1% SOx, reportedly, around $695.00 per ton, depending upon the location of the bunkering port. Hence, compliance with the law becomes a rather expensive proposition. However, there is still hope. A viable alternative to the Ultra-Low Sulfur Fuel, lies in the potential efficacy of “Exhaust Gas Scrubbers”, (prevalent among many European vessels), depending upon the degree to which such exhaust gas scrubbing systems can reduce the levels of emission effluents (expected to be enforced within the next five years) by a factor of not less than 95% Sulfur Oxide, (SOx); 78% for Nitrogen Oxide, and about 83% for Particulate Matter (PM).

Yet another possible alternative lies in the feasibility of converting contemporary main propulsion Diesel engines, whereby they become capable of burning “Dual Fuel”. This would involve retaining the conventional Marine Diesel Oil (MDO) capability and modifying them to be capable of also burning Liquefied Natural Gas (LNG). Several recently built European tankers, designed for the LNG trade with dual fuel capability plus a fleet of Norwegian ferries currently in operation are also burning LNG, from which the exhaust emissions are reportedly extremely low.