TECHNICAL FIELD The disclosure relates to a method for preparing an oil to be supplied to the cylinders of a two- stroke crosshead engine, in particular to a method for preparing such an oil on board of the marine vessel where the internal combustion engine is installed or on site at a power plant where the engine is used as a prime mover and to an apparatus for preparing an oil to be supplied to the cylinders of a two-stroke crosshead engine.

BACKGROUND

The two oils used in the lubrication of a large two-stroke compression ignited internal combustion engine are very different in chemical composition and physical properties from one another. The oil used in the crankcase lubrication system is referred to as system oil, and lubricates and cools the main, bottom end and crosshead bearings, the crosshead slippers, camshaft, bearings and followers, and the chain or gear drive. On modern engines the oil is also used to cool the piston undersides. The system oil is in general never fully replaced, but topped up over time to maintain its condition The oil used to lubricate the piston rings as they reciprocate in the cylinder liner and to neutralise the acids formed in the combustion chamber by the combustion of sulfur in the fuel is referred to as cylinder oil. The generic purposes of a cylinder lubricant (oil) are to protect the cylinder liners, pistons and piston rings from the harmful effects of combustion by-products and provide an oil film between piston rings and cylinder liners.

To achieve this, the cylinder lubricant is required to:

Spread uniformly over the cylinder liner surface and form a stable oil film

Provide a gas seal between the liner and the piston rings

Neutralise acids formed from the by-products of the combustion process

- Minimise deposit formation on piston surfaces and ring grooves

Flush out particles formed during combustion from the combustion chamber as well as wear particles

Prevent a build-up of deposits in the piston ring grooves which can lead to ring sticking or breakage Prevent corrosion of the cylinder liner and other combustion chamber components while the engine is idle.

A cylinder lubricant usually comprises a base oil or a blends of base fluids a number of different additives that are designed help improve the lubricating ability of the base fluid dependent on the quality of the base fluid and its intended use.

The different additives are formulated from various over-based and neutral additive metallic alkaline salts, colloquially referred to as detergents. These chemicals are designed to be soluble in oil, and insoluble in water. Detergent additives provide a means to dissolve otherwise insoluble metallic salts such as calcium and magnesium carbonate into the lubricating oil. Marine detergent additives are classed as multi-functional additives due to the numerous different and beneficial performance features attributed to the type of additives used, i.e. they provide basicity to neutralize the acids (neutralization), and contribute to the anti-oxidant and dispersant properties of the lubricant to keep the piston and piston ring-land surfaces free from detrimental thermal oil degradation and fuel deposits. Moreover, certain detergents are known to have good anti-wear properties. Certain detergent additives are known to be very effective with high sulfur cylinder oils, e.g. over-based detergents, whilst neutral detergent additive technology tends to be used with low or very low sulfur fuel types.

The concentration of over-based additives needs to be matched to the fuel sulfur content to ensure that the lubricant has sufficient alkalinity reserves to neutralise the acids produced by the combustion process before it can have a damaging impact on the engine components, i.e. the higher the sulfur content of the fuel the higher the lubricant basicity reserve requirement. However, the basicity needs to be reduced when used in conjunction with low sulfur fuel types since the unreacted additives can form hard abrasive deposits which can accumulate on the piston crown- land and piston ring groves leading to a potential damage or unplanned shutdowns.

Acid neutralisation is a critical property to protect the inner surface of the cylinder liner from the acids generated during combustion. The cylinder lubrication oil is a total-loss oil since it is consumed through combustion and scrapedown in the lubrication process. The cylinder oil is depleted with each revolution of the main engine and replaced by fresh oil through intermittent injections. The oil that is supplied to the cylinders usually has an SAE (society of automotive engineering) viscosity equivalent grade of 50 and can have any total base number (TBN) between 5 and 150 for the neutralization of acid products generated during the combustion process. Typically system oil has an SAE viscosity grade of 30 with a relatively low TBN that is typically below 10. These values are though merely by way of example, and may vary depending on the actual application- specific design of the systems that the oils are used in.

In recent years, there has been a trend towards blending the oil for delivering to the cylinders of the main engine on board the marine vessel where the main engine is installed. Hereto, used system oil is withdrawn from the crankcase lubrication system and blended (cut back) with a TBN agent to create a cylinder oil with the appropriate lubrication characteristics.

In recent years two-stroke crosshead engines have been operated for a large portion of their operation time at an engine load that is significantly below their maximum continuous rating. This is due to the fact that many freight ship companies choose to sail slower (slow steaming) than before, which results in the main engines being operated well below the maximum continuous rating, since these marine vessels were originally constructed to sail at significantly higher speed.

The viscosity of the cylinder oil specified for a particular engine is generally selected such that the actual viscosity (dynamic (shear) viscosity) at the temperature of the cylinder liner where the cylinder oil is applied is optimal for the cylinder liner temperature at maximum engine load (100% maximum continuous rating), i.e. sufficiently high for providing a proper lubrication of the piston rings against the inner surface of the cylinder liner at maximum engine load. The viscosity of a cylinder oil in operation is affected by a number of factors. As temperature increases, the viscosity of a fluid decreases, and vice-versa. During the operation of a two-stroke crosshead engine the temperature varies according to engine load and RPM (revolutions per minute). As engine load and RPM increase, the temperature also increases and hence the viscosity of the lubricating oil decreases.

Cylinder oil prepared in accordance with the above blend on board method typically (like any typical conventional cylinder oil) has a relatively low viscosity index (mono grade type oil)i.e. they have a relative large change in viscosity with temperature. Figs. 3 and 4 illustrate the relation between temperature and viscosity in centistokes (cSt) by way of example for a SAE 30 oil and an SAE 50 oil, both having a Viscosity Index (VI) of 97.

For a typical two-stroke crosshead engine the oil delivered to the cylinders should have a viscosity between approximately 1 .5 and 3 cSt at the temperature of the cylinder liner. In such an engine, the cylinder liner temperature at the maximum continuous rating of the engine (maximum load) will typically be well above 200°C, e.g. 240 to 250°C. As can be seen in Fig. 4, this results in an effective viscosity for the SAE 50 oil at the temperature of the cylinder liner of just below 2 cSt, i.e. within the specified range of 1.5 to 3 cSt.

The inventors have realized that the resulting effective viscosity of this cylinder oil when it is applied in the cylinders during slow steaming (engine load well below maximum continues rating) will be much higher than required, e.g. close to 5 cSt higher than required, because the cylinder liner temperature can be as low as 170°C during slow steaming. This results in the operational viscosity of the cylinder oil being much higher than the optimum range, and thus the slow steaming operation involves against expectations an aspect that is not energy-efficient. The inventors thus realized that it is desirable to adjust the cylinder oil viscosity to the actual engine operating conditions, e.g. by lowering the viscosity of the cylinder oil when the cylinder liner temperature is low, for example at lower engine loads.

The inventors have realized that adjusting the amount of detergent additives relative to the amount of system oil or other base fluids in a blend on board process to e.g. reduce the alkalinity of the resolving blended product can also potentially cause a shortfall in other performance factors such as thermal and oxidation performance of the resulting blended product due to insufficient levels of overall detergent content in the oil supplied to the cylinders, which is something that needs to be carefully balanced with the engine's design and its operational duty cycle and its impact on the lubricant.

EP 276-7578 discloses a process for the production of a cylinder oil comprising the steps: providing a used oil, providing a fresh cylinder oil, and blending the used oil with the fresh cylinder oil, wherein the used oil has a lower TBN value and a lower viscosity than the fresh cylinder oil. The fresh cylinder oil is added to the used oil in order to increase the BN value of the used oil, i.e. as a ABN additive, and to increase the viscosity of the used cylinder oil simultaneously, to thereby render it suitable for use as a cylinder oil. SUMMARY

Based upon this background it is an object to overcome or at least reduce the drawbacks indicated above.

In particular, it is an object to avoid operating the two-stroke crosshead engine with a cylinder oil that is delivered to the cylinders with a viscosity that is higher than required at the actual temperature of the cylinder liner.

This object is achieved in accordance with a first aspect by providing a method for preparing an oil to be supplied to the cylinders of a two-stroke crosshead engine with crossheads, pistons in cylinder liners and with a crankcase lubrication system operating with system oil, said method comprising:

- determining or estimating the actual cylinder liner temperature,

determining an actual desired viscosity for the oil that is supplied to the cylinders at the determined or estimated actual cylinder liner temperature,

withdrawing system oil from the crankcase lubrication system,

preparing an oil for supplying to the cylinders by blending said withdrawn system oil or base oil with a TBN agent, and

adjusting the viscosity of the prepared oil to the actual desired viscosity by blending the prepared oil with a viscosity agent to obtain a viscosity adjusted oil.

With this method the viscosity can be freely adjusted to an optimized combination to match engine design, engine performance and engine load. The system can either be blending directly (in line blending) to each unit or blending to a day tank as batch blending.

The TBN value is in a possible implementation adjusted to an actual desired value, based on measurements or estimates of actual engine conditions. Thus, both the TBN and viscosity of the cylinder oil can be adapted to high load and low load, with high sulfur fuel and with low sulfur fuel for any values in between and combinations thereof.

In a first possible implementation form of the first aspect blending said system oil with a viscosity agent or with a high viscosity oil is a batch wise process. In a second possible implementation form of the first aspect blending the prepared oil with the viscosity agent is an in-line blending process. The advantage of in-line blending is that the blends can be quickly adjusted to changes of engine load by adjusting the viscosity accordingly.

In a third possible implementation form of the first aspect blending said system oil with a TBN agent or with a high TBN oil is a batch wise process.

In a fourth possible implementation form of the first aspect determining the amount of viscosity agent to be added comprises calculating manually using a defined methodology or determining using computer software programmed with appropriate algorithms or equations.

In a fifth possible implementation form of the first aspect said TBN agent comprises fresh cylinder oil, preferably a fresh cylinder oil with a high TBN, such as for example a TBN above 60.

In a sixth possible implementation form of the first aspect the viscosity modifying agent comprises a lubricating oil such as a trunk piston engine oil, gear, hydraulic or turbine oil, base oil, recycled oil, a high viscosity additive, or a fresh cylinder oil with a low TBN, such as for example a TBN below 30.

In an seventh possible implementation form of the first aspect determining a desired viscosity for the cylinder oil comprises calculating manually using a defined methodology or determining using computer software programmed with appropriate algorithms or equations a desired viscosity for the cylinder oil for use in the cylinders at the determined actual cylinder liner temperature.

In an eighth possible implementation form of the first aspect determining the actual cylinder liner temperature comprises measuring the actual cylinder liner temperature at a defined point or averaged across the surface of the liner with a sensor in or at the cylinder liner surface. In a ninth possible implementation form of the first aspect determining the actual cylinder liner temperature comprises obtaining the actual engine load and RPM (revolutions per minute) and deriving an estimated cylinder liner temperature from the obtained engine load and RPM. In a tenth possible implementation form of the first aspect deriving an estimated cylinder liner temperature comprises calculating manually using a defined methodology or determining using computer software programmed with appropriate algorithms or equations using the engine load as the basis for calculating the estimated cylinder liner temperature.

In an eleventh possible implementation form of the first aspect determining the actual cylinder liner temperature comprises obtaining the actual cylinder jacket cooling water temperature and deriving an estimated cylinder liner temperature from the obtained cylinder jacket cooling water temperature.

In a twelfth possible implementation form of the first aspect the method further comprises determining or obtaining a desired TBN value for the oil that is supplied to the cylinders, and applying a TBN modifying agent in said prepared oil to adjust the TBN of said oil supplied to the cylinders to the desired TBN value. With this feature the alkalinity (TBN) of the oil can be freely adjusted to match the fuel sulfur level, engine design, engine performance and engine load.

In a thirteenth possible implementation form of the first aspect the method is carried out aboard the marine vessel in which the two-stroke crosshead engine is installed. In a fourteenth possible implementation form of the first aspect the TBN agent comprises over- based detergents, the method further comprising blending a neutral detergent with the prepared oil.

In a fifteenth possible implementation form of the first aspect the amount of neutral detergent blended with the prepared oil is increased when less TBN agent has been added to the blended product and wherein the amount of neutral detergent blended with the prepared oil is decreased when more TBN agent has been added to the blended product in order to adjust the overall amount of detergent in the oil to be supplied to the cylinders to ensure thermal and oxidation performance.

In a sixteenth possible implementation form of the first aspect the viscosity agent and the neutral detergent that are blended with the prepared oil are added together in the form of a fresh cylinder oil with a relatively low TBN, preferably a TBN below 30. In a seventeenth possible implementation form of the first aspect the method further comprises supplying said viscosity adjusted oil to said cylinders.

The object above is also achieved according to a second aspect by providing an apparatus for preparing oil for supplying to the cylinders of a two-stroke engine with crossheads, pistons in cylinder liners and with a crankcase lubrication system operating with system oil, the apparatus comprising:

- a first source of system oil withdrawn from the crankcase lubrication system,

- a second source of a TBN increasing agent,

- a third source of viscosity agent,

- a controllable blender configured to blend the withdrawn system oil, the high TBN oil or TBN increasing agent and the viscosity agent in individually controllable ratios to prepare the oil for supplying to the cylinders with an individually adjusted TBN value and in individually adjusted viscosity.

In a first possible implementation form of the second aspect the apparatus comprises an electronic control unit connected to the controllable blender, the electronic control unit being in receipt of the cylinder liner temperature or a parameter indicative of the cylinder liner temperature, the electronic control unit being configured to determine a desired viscosity of the oil supplied to the cylinders taking into account the actual cylinder liner temperature, the electronic control unit further being configured to determine the amount or proportion of viscosity agent to be blended into the oil for supplying to the cylinders in order to obtain an oil with the desired viscosity, and the electronic control unit being configured to control the controllable blender accordingly. In a second possible implementation form of the second aspect the apparatus comprises an electronic control unit connected to the controllable blender, the electronic control unit being in receipt of a desired TBN value for the oil for supplying to the cylinders, the electronic control unit further being configured to determine the amount or proportion of high TBN oil or of TBN increasing agent to be blended into the oil for supplying to the cylinders in order to obtain an oil with the desired TBN value.

In a third possible implementation form of the second aspect the oil for supplying to the cylinders has a controlled TBN and a controlled viscosity. In a fourth possible implementation form of the second aspect the viscosity agent is a fluid or oil with a high viscosity and is a viscosity increasing additive, such as for example a fresh cylinder oil with a relatively low TBN. In a fifth possible implementation form of the second aspect the viscosity agent is a fluid or oil with a low viscosity and is a viscosity decreasing additive.

In a sixth possible implementation form of the second aspect the blender is an integral blender that is configured for blending the withdrawn system oil, the high TBN oil or the high TBN agent and the viscosity agent, preferably in a batch wise blending process.

In a seventh possible implementation form of the second aspect the blender comprises a TBN blender for blending the withdrawn system oil with the high TBN oil or the high TBN agent and a viscosity blender for blending the blend prepared with the TBN blender with a viscosity agent.

In an eight possible implementation form of the second aspect the TBN agent from the source of TBN agent comprises over-based detergent, the apparatus further comprising a fourth source of neutral detergent. In a ninth possible implementation form of the second aspect the source of viscosity agent and the source of neutral detergent is formed by a single source of fresh cylinder oil with a relatively low TBN, preferably a TBN below 30.

Further implementation forms are apparent from the dependent claims, the description and the figures. These and other aspects of the invention will be apparent from the embodiment(s) described below.

BRIEF DESCRIPTION OF THE DRAWINGS In the following detailed portion of the present disclosure, the invention will be explained in more detail with reference to the example embodiments shown in the drawings, in which:

Fig. 1 is a diagrammatic sectional view of a lubrication system for a two-stroke crosshead engine according to an example embodiment, Fig. 2 is a diagrammatic sectional view of a lubrication system for a large two-stroke crosshead engine according to another example embodiment,

Fig. 3 is a diagram showing the relation between temperature and viscosity of two types of oil, Fig. 4 is an enlarged detail of the diagram of Fig.3, and

Fig. 5 is a diagrammatic sectional view of a lubrication system for a two-stroke crosshead engine according to an another example embodiment, and

Fig. 6 is a diagrammatic sectional view of a lubrication system for a large two-stroke crosshead engine according to another example embodiment.

DETAILED DESCRIPTION

In the following detailed description, a lubrication system for a two-stroke engine crosshead engine will be described by the example embodiments. Fig. 1 diagrammatically shows a large low speed turbocharged two-stroke diesel engine 1 with a crankshaft 7 and crossheads 5 sectional view. Two-stroke crosshead engines typically have between four and sixteen cylinders in line, carried by an engine frame 10. The engine 1 may e.g. be used as the main engine in an ocean going vessel, or as prime mover in a stationary power plant. The total output of the engine may, for example, range from 5,000 to 110,000 kW.

The engine is a diesel (pressure ignited internal combustion) engine of the two-stroke uniflow type with scavenge ports 14 at the lower region of the cylinders 1 and an exhaust valve 12 at the top of the cylinders 1. The engine can be operated on various types of fuel, such as e.g. marine diesel, heavy fuel, or gas. The scavenge air is passed from the scavenge air receiver 1 1 to the scavenge ports 14 of the individual cylinders 2. A piston 3 in the cylinder liner 2 compresses the scavenge air, fuel is injected and combustion follows and exhaust gas is generated. When an exhaust valve 12 is opened, the exhaust gas flows through an exhaust duct associated with the cylinder concerned into the exhaust gas receiver 13 and onwards through a first exhaust conduit to a turbocharger (not shown), from which the exhaust gas flows away from the atmosphere. The turbocharger delivers pressurized scavenge air to a scavenge air conduit leading to the scavenge air receiver 1 1.

A piston rod 4 extends from the bottom of the piston to the crosshead 5. A connecting rod 6 connects the crosshead 5 to one of the throws of the crankshaft 7. The crankshaft 7 is rotation suspended in the engine frame and bedplate by the main bearings 8. A thrust bearing (not shown) is provided at the aft of the engine to accommodate the thrust created by a propeller (not shown) driven by the engine 1. The thrust bearing is supplied with lubrication oil by the same conduit that supplies the main bearings 8. In the main bearings 8 an oil film between the bearing surface and the journal surface carries the journal and prevents substantially any direct contact between the journal surface and the inside surface of the shells and provides lubrication. A flow of lubrication oil is supplied to the bearing surface. The lubrication oil film assists in cooling the main bearing.

Two-stroke crosshead engines include many components that are for lubrication and/or cooling purposes supplied with lubrication oil. All these components are provided with lubrication oil via the crankcase lubrication system except for the cylinders and piston rings which receive another type of oil from the cylinder oil system.

The crankcase lubrication system is essentially a closed loop lubrication system in which the system oil is recirculated. The crankcase lubrication system provides lubrication for cooling a range of components of the engine. For example, the crankshaft 7 is placed in an oil sump 9 that is provided in the lower part of the engine 1 and supplied with lubrication oil under pressure that is circulated through the oil sump 9. Other lubrication positions, such as bearings, etc. are separately provided with lubrication oil, as will be described in greater detail further below. The surplus leakage oil is collected in the oil pan 9. A lubrication oil supply loop is provided for supplying lubrication oil to all lubrication oil consumers. The lubrication oil supply loop includes a supply conduit 15 that starts at the oil sump 9. The supply conduit 15 includes two low-pressure pumps 16 arranged in parallel with respective electric drive motors for arranging the oil transport (although it is understood that there could be any other number of supply pumps). The supply conduit 15 also includes a cooler 19 for cooling the lubrication oil and a filter 17 for filtering out contamination. This can in one embodiment be a ΒσΚ 50μ filter.

The supply conduit 15 splits downstream of the filter 17 into an oil sump supply conduit 52 and a bearing supply conduit 20. The oil sump supply conduit 52 delivers filtered and cooled lubrication oil to the oil sump 9. The bearing supply conduit 20 branches into a main bearing supply conduit 23 and a crosshead bearing supply conduit 26. The main bearing supply conduit 23 also provides the thrust bearing the aft of the engine 1 with lubrication oil. The main bearing supply conduit 23 includes an electronically controlled valve 21 for controlling the flow. The main bearing supply conduit 23 also includes a feed pump 22. In the present embodiment a pair of parallel feed pumps is shown, but it is understood that any number of pumps could be used, although a plurality of pumps is preferred for redundancy reasons. The feed pump 22 is driven by one or more electric drive motors. The main bearing supply conduit 23 delivers a substantially constant flow of lubrication oil to the main bearings 8 during engine operation.

The crosshead bearing supply conduit 26 includes an electronically controlled valve 24 for controlling the flow. The crosshead bearing supply conduit 26 also includes a feed pump 25. In the present embodiment a pair of parallel feed pumps are shown, but it is understood that any number of pumps could be used, although a plurality of pumps is preferred for redundancy reasons. The feed pump 25 is driven by one or more electric drive motors. The crosshead bearing supply conduit 26 delivers a substantially constant flow of lubrication oil to the crosshead bearings during engine operation.

A portion of the system oil is withdrawn from the crankcase lubrication system. Hereto, a feed pump transports an amount of system oil from the oil pan 9 to a used system oil tank 60. A cylinder oil system supply conduit 18 connects to the used system oil tank 60. The cylinder oil system supply conduit 18 includes a feed pump 30 for providing the cylinder oil system with used system oil that has been withdrawn from the crankcase lubrication system.

In order to maintain a substantially constant amount of system oil while in the crankcase lubrication system, an approximately equal amount of fresh system oil as withdrawn therefrom is added to the crankcase lubrication system from a fresh oil system tank 17 using a feed pump that connects to the oil sump supply conduit 52. Thus, the system oil is continually replenished with fresh system oil, thereby rendering it practically unnecessary to completely replace the system oil in the crankcase lubrication system with fresh system oil.

The cylinder oil system includes a blender 66 that receives used system oil, preferably a controlled amount, from the used system oil tank 60. In an embodiment (not shown) the blender receives a preferably controlled amount of base oil instead or together with the system oil. The base oil can be any suitable oil such as recycled oils or auxiliary engine oil. In an embodiment of Fig. 1 the withdrawn system oil is supplied to the blender 66 continuously at a steady rate for in-line preparation of the oil to be delivered to the cylinders 2. In another embodiment the system oil is supplied to the blender intermittently for batch wise preparation of oil to be delivered to the cylinders 2; this embodiment is explained here below in more detail with respect to Fig. 2.

In the blender 66 the withdrawn system oil is mixed or blended with the TBN agent and with the viscosity agent to prepare the oil to be delivered to the cylinders. The details of the blending process are described in detail further below. From the blender 66 the prepared oil, "cylinder oil" is transported by a feed pump to a cylinder oil dosage pump 55. The cylinder dosage pump 55 ensures precise and correctly timed dosage of the cylinder oil to the individual cylinders 2.

The blender 66 receives a preferably controlled amount of TBN agent from a source of TBN agent 61 via a supply conduit that includes a supply pump. The source of TBN agent can be a tank with a dedicated TBN agent with a TBN for example up to 400, or a tank with an oil with a high TBN such as a fresh commercially available "cylinder oil" with a high TBN such as e.g. a TBN equal to or above 80 or equal or above 100.

The blender 66 also receives a preferably controlled amount of viscosity agent from a source of viscosity agent 62 via a supply conduit that includes a supply pump. The source of viscosity agent can be a tank with a dedicated viscosity agent or a tank with an oil with a high viscosity, such as an oil with a viscosity equal or above SAE 50 or equal to above SAE 60. Oils with a viscosity ranging between 18 and 24 cSt at 100°C such as fresh cylinder oil with a relatively low TBN, such as a TBN below 30 is suitable for use as viscosity agent since cylinder oils typically have a viscosity between 19 and 23 cSt. The oil used as viscosity agent is in an embodiment a single grade SAE.

The system oil for the crankcase lubrication system is a high quality paraffinic base oil containing a number of performance additives. The alkalinity of the system oil (defined by its TBN number) must be sufficient to neutralize any strong acids formed from combustion of the fuel which may find their way into the crankcase.

The characteristics of an example of system oil for use in the crankcase lubrication system of a two-stroke crosshead engine are as follows: SAE No 30

Specific Gravity (15/15C) 0.894

Flash Point min C 229

Pour Point max C -15

Viscosity: cSt at 40C 108

at 100C 1 1.8

Viscosity Index 97

Total Base Number 5.3

Sulphated Ash, % wt. 0.73

The total base number is an indication of the alkalinity of an oil in the milligrams of acid, expressed in equivalent milligrams of potassium hydroxide (KOH), required to neutralize all basic constituents. The oil delivered to the cylinders by the cylinder oil system must be thermally stable. The oil needs be able to retain an oil film at the high surface temperatures of e.g. the piston rings and the cylinder liner 2. The oil delivered to the cylinders must have anti wear characteristics and detergents to minimize deposits on the pistons 3 and in the ring grooves. The oil delivered to the cylinders typically has a high TBN between 30 and 100 to neutralize the acids formed by the combustion of the sulfur in the fuel. However, the required TBN value depends on the sulfur content in the fuel (which may vary) and may have a value anywhere between e.g. 30 and 100. Alkaline additives can make up about a significant portion of the oil. The viscosity of the oil delivered to the cylinders is relatively high (e.g. 21 cSt at 100°C for an SAE 50 oil) in order to lubricate effectively at the higher temperatures (e.g. 190°C) of the cylinder liner 2 resulting in a viscosity of approximately 3,2 cSt where the oil is applied.

The oil delivered to the cylinders is a "use once consumable". The oil is injected into the cylinder at a feed rate to give optimum protection against acid corrosion and microseizures (scuffing).

The characteristics of an example of oil for supplying to the cylinders of a two-stroke crosshead engine are as follows: SAE No

Specific Gravity (15°C) 0.942

Flash Point °C 241

Pour Point °C -9

Viscosity: cSt at 40°C 247

cSt at 100°C 21

Viscosity Index 100

Total Base Number (mg. KOH/g) 70 The list of viscosity agents includes but is not limited to:

Used and fresh finished lubricants such as hydraulic oils, turbine oils, monograde and multigrade engine oils, gear oils, base oils including naphthenic and paraffinic mineral oils (of Group I, II and III), synthetic polyalphaolefins (PAO) polymeric entities such as polymethylmethacrylate (PMMA) and olefin copolymers (OCP) or fresh cylinder lubrication oils with a relatively low TBN and mixtures thereof.

The source of viscosity agent may comprise a source of high viscosity agents for increasing the resulting viscosity and a source of low viscosity agents for decreasing the resulting viscosity.

Adjusting the TBN to actual (present) engine operating conditions preferably comprises adjusting at least one additive level or adding one or more additives, where the additives comprise at least one base comprising basic salts of alkaline or earth alkaline elements, and/or detergents and/or dispersants. Alternatively, adjusting the TBN comprises blending with a high TBN oil, such as a commercial cylinder oil with a high TBN value, e.g. with a TBN above 100.

The alkaline/earth alkaline elements may be e.g. K, Na, Ca, Ba, Mg or the like. The basic salts may belong to the Inorganic chemical families of e.g. oxides, hydroxides, carbonates, sulfates or the like. The detergents may belong to the organic chemical families of e.g. sulfonates, salicylates, phenates, sulfophenates, Mannich-bases and the like. The dispersants may belong to the organic chemical families of succinimides or the like.

Example of blending proportions: Example 1.

The TBN agent is a TBN additive package in oil with a TBN value of 140 and a viscosity of St@ 100 °C.

The viscosity agent is a fresh cylinder oil with a TBN of 25 and a viscosity of 19.5 cSt@ 100 deg.

°C

The system oil has a TBN of 6 and a viscosity of 1 1.5 cSt@ 100 °C.

The system oil is blended with various portions of TBN agent and viscosity agent in accordance with the table below in order to achieve the target BN for the blended product.

Table 1 :

Target % % % Resulting

Blended Proportion Proportion Proportion (estimated)

product Viscosity TBN agent System viscosity of

BN agent High BN Oil(6BN) the blended

Low BN (140BN) product

(25BN) Additive (cSt @ 100

fresh package in °C)

cylinder oil oil

140 - 100 - 23.0

120 - 85 15 20.5

1 10 - 77 23 19.5

100 - 70 30 18.5

90 - 63 37 17.5

80 - 55 45 16.5

70 10 55 35 16.8

60 22 43 35 17

50 45 30 25 18

40 67 18 15 18.5 30 89 6 5 19

25 100% - - 19.5

The use of the viscosity agent, in this example low BN (25 BN) fresh cylinder oil allows the blended viscosity to be controlled to a desired viscosity level, in this example using 16.5 cSt as a minimum level.

An electronic control unit 50 receives signals that contain actual (present, substantially real time) information about the engine, such as specific temperatures and pressures, such as e.g. the cylinder liner temperature, and operating conditions, such as the engine load and speed. The electronic control unit 50 is also connected, e.g. via signal cables to the feed pumps 22 for the main bearings, the control valve 21 in the main bearing supply conduit 23, the feed pumps 25 for the crosshead banks, control valve 24 in the crosshead bearing supply conduit 26 and to the control valve 24 in the crosshead bearing supply conduit 26.

The electronic control unit 50 is also connected, e.g. via signal cables to the feed pumps that deliver the viscosity agent and the TBN agent to the mixer 66, both to the mixer 66 itself and to the cylinder oil dosage pump 55. Alternatively, the mixer 66 may be provided with its own electronic control unit (not shown), that carries out the functions that are described below that relates to the mixer 66. The electronic control unit 50 is configured to determine the amount (flow rate) of lubrication oil that needs to be delivered to the main bearings 8. The electronic control unit 50 is also configured to determine the amount (flow rate) of lubrication oil that needs to be delivered to the crosshead bearings. The electronic control unit 50 is also configured to obtain or determine the actual required TBN for the oil that is delivered to the cylinders 2. Hereto, the electronic control unit 50 is in receipt of actual information on the fuel quality and determines the actual required TBN from a lookup table stored in the electronic control unit 50 or by using an algorithm or equation stored in the electronic control unit 50.

Further, the electronic control unit 50 is configured to determine the actual required viscosity for the oil that is delivered to the cylinders 2. Hereto, the electronic control unit 50 is in receipt of a signal representative of the actual engine load. The engine load is a parameter that is indicative of the actual cylinder liner temperature. Alternatively, the electronic control unit is in receipt of a signal from the temperature sensor (not shown) that measures the jacket cooling water temperature. The jacket cooling water temperature is also indicative of the temperature of the cylinder liner and can be used as an alternative parameter by the electronic control unit 50. Alternatively, the electronic control unit is in receipt of a signal from a temperature sensor 57 in the cylinder liner. Based on the actual temperature of the cylinder liner, or on a parameter representative thereof the electronic control unit 50 is configured to determine the actual optimal viscosity for the oil that is delivered to the cylinders.

Alternatively, the electronic control unit 50 determines directly from the parameter that is indicative of the actual cylinder liner temperature the actual required proportions of withdrawn system oil, TBN agent and viscosity agent. The electronic control unit 50 is provided with an algorithm that determines the required proportions of withdrawn system oil, viscosity agent and TBN agent that results in a prepared oil in the blender 66 that has the determined actual optimal TBN value and the determined optimal viscosity. The electronic control unit 50 is configured to control the blender 66 and optionally the feed pumps that deliver the withdrawn system oil, the viscosity agent and the TBN agent respectively to deliver the appropriate amounts of respective fluid to the blender 66 and to blend or mix the appropriate amount of the respective fluids in the blender 66, preferably in a continuous process.

The electronic control unit 50 is configured to recalculate the required proportions of system oil, viscosity agent and TBN agent when the temperature of the cylinder liner changes and/or when the sulfur content of the fuel changes and to control the blender 66 and the feed pumps of the withdrawn system oil, of the TBN agent and of the viscosity agent accordingly.

Fig. 2 illustrates another embodiment that is essentially identical to the embodiment that is described with reference to Fig. 1 , except that there are two blenders: a TBN blender 67 and a viscosity blender 68. In Fig. 2 the viscosity blender 68 is downstream of the TBN blender 67 with a tank 63 in between. This allows batch wise production of an oil blend with an adjusted TBN value and storing this oil blend with the adjusted TBN value in the day tank 63 and allows online adaptation of the batch wise produced oil blend to the required viscosity value in the online viscosity blender 68.

Changes to the sulfur content of the fuel are not frequent and can normally be foreseen well in advance. Thus, it is normally not necessary to adjust the TBN value of the oil to be delivered to the cylinders quickly or unexpectedly and therefore a batch wise production is unproblematic. However, changes to the engine load could be caused by e.g. an unexpected change in the weather or other conditions that are beyond control of the operator of the marine vessel. Therefore, it is advantageous that the adjustment of the viscosity is performed in an online blending process and allows for a quick and instantaneous adjustment of the viscosity to actual operating conditions.

Fig. 5 illustrates another embodiment that is essentially identical to the embodiment that is described with reference to Fig. 1 , except for the following differences.

In the blender 66 the withdrawn system oil is mixed or blended with an a controlled amount of over-based determent acting as the TBN agent from a source of over-based detergent 64. The source of over-based detergent 64 can be a tank or container containing over-based detergent connected to the blender by a conduit including a supply pump controlled by the electronic control unit 50. However it is understood that the over-based detergent can be added in a controlled manner to the blender 66 in other ways.

A controlled amount of viscosity agent is added to withdrawn cylinder oil in the blender 66 in the same way as in the example embodiment of Fig. 1 from a source of viscosity agent 62.

A controlled amount of neutral detergent is added to the withdrawn system oil or base oil in the blender 66 from a source of neutral detergent 67. The source of neutral detergent 67 can be a tank or container containing neutral detergent connected to the blender by a conduit including a supply pump controlled by the electronic control unit 50. However it is understood that the neutral detergent can be added in a controlled manner to the blender 66 in other ways.

The control algorithm in the electronic control unit 50 is configured similar to the embodiment of Fig. 1 to control the amounts of withdrawn system oil or base oil, of over-based detergent, of neutral detergent and of viscosity agent. The electronic control unit can be configured to do this in response to instructions from a human operator in the form of desired viscosity, TBN and total detergent content for the oil prepared un the blender and supplied to the cylinders of the engine 1. Alternatively the electronic control unit 50 receives the actual temperature of the cylinder liners and the actual sulfur content of the fuel and determines the required TBN and viscosity on the basis of these two measure parameters. The system can be used to maintain a required overall amount of detergent in the oil supplied to the cylinders by increasing the amount of neutral detergent added to the withdrawn system oil or base oil in the blender 66 when the amount of over-based detergent added to the withdrawn system oil or base oil in the blender 66 is lowered to lower the TBN of the oil supplied to the cylinders. The electronic control unit 50 is in an embodiment configured to automatically maintain the required overall amount of detergent in the oil supplied to the cylinders.

The embodiment of Fig. 6 is essentially identical to the embodiment of Fig. 5, except that the source of viscosity agent and the source of neutral detergent are combined in one source of low TBN fresh cylinder oil 72 in the form of a tank with low TBN cylinder oil connected to the blender 66 via a conduit including a supply pump controlled by the electronic control unit 50. This embodiment provides fewer degrees of freedom than the embodiment of Fig. 5. But it is a less complicated solution (since fresh cylinder oil is readily commercially available), more practical in terms of tank space on board, and less complicated to implement. However, the system still enables blending of a cylinder oil with optimum properties for a large range of engine operating conditions.

The invention has been described in conjunction with various embodiments herein. However, other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. The reference signs used in the claims shall not be construed as limiting the scope.