Advanced cylinder Liner Lubrication System

Introduction

Some of the most arduous conditions for lubrication exist inside the cylinder liner where the lubricating agent would be subject to highly corrosive environment. This situation is tackled in large marine diesel engines by means of an advanced lubricating system.

Cylinder Lubrication

We know that a piston moves to and fro within a cylinder and there are several piston rings inserted in the grooves on the piston which perform multiple tasks including sealing of pressure inside the combustion chamber lest it leaks from below. The speed of rubbing between these piston rings and the cylinder liner is quite high and apart from that there are extreme conditions of temperature, pressure and corrosive gases inside the combustion chamber to which any lubricating fluid between the liner and the piston would be subject to. The type of lubrication found inside the cylinder is of the thin film or boundary type of lubrication for most parts of the piston motion except for the upper and lower extremes of motion where this changes to imperfect lubrication as the speed reduces to zero at these points and direction of motion is reversed.

It does not require much reflection to imagine that lubricating oil used inside a cylinder liner must be able to withstand conditions of high temperature, pressure and should have good corrosion resistance as well. Just to give you a broad idea, typical lubricating oil used inside the liner should have viscosity in the range of 115-150 cst at temperatures of 50 degrees Celsius and this should not vary too much even at higher temperatures. Special additives could be added to the oil in order to increase its oiliness or stickiness. Apart from that the amount of oil supplied should be ideal because if too little oil will cause the surfaces to tear apart rapidly, too much of oil will also be a problem as it would interfere with normal combustion inside the chamber and also cause damage to various parts such as valves, valve seats and the exhaust pipe.

 

Lubrication System

In case of large two stroke marine diesel engines used for the main propulsion of the ship an independent lubricating system is used for the huge cylinder liners. Basically the system consists of a number of lubricating quills located around the circumference of the cylinder to which oil under pressure is supplied twice during each revolution of the crankshaft. I might devote a separate article to explain this system in detail but for the moment a simple diagram would suffice to explain its working. The circular section shows the top view of a cylinder liner with 8 lubricating quills fitted along its circumference which contained lubricated oil under pressure. Of course the entire system is much more elaborate and there is provision for indicating actual oil flow, not to mention the manual and automatic adjustments to vary the quantity of oil being supplied to the cylinder liner.

Cylinder liner lubrication was difficult under any way, but now the problem has increased markedly by the use of lower quality, high sulphur content, viscous, and residual fuel in place of distillate and semi-distillate fuels.

IVO CYLINDER LINER LUBRICATOR

Cylinder Liner Lubrication of Marine Two Stroke Crosshead Diesel Engines; Cylinder lubricators supply oil to the accumulators and the lubricating quills present on the cylinder liner as we have seen earlier, but this paragraph explains the detailed construction and working of an IVO cylinder lubricator of a specific type of such cylinder lubricators in details used in modern Sulzer engines and is the IVO lubricators.

IVO Cylinder Lubricator

1. Pump shaft

2. Gear shaft

3. upper and lower outlet pipes

4. control plunger

5. main plunger

6. pump element

7. twisted disc for control plunger

8. twisted disc for main plunger

9. set-screw for adjustment

10. inlet pipe

 HOW IT WORKS

After you have taken a look at the IVO lubricator diagram it is time to understand how things actually work.

The gear shaft is rotated through the pump shaft using rack and pinion gear arrangement. This gear shaft has got two twisted discs or plates which move through appropriate grooves in two plungers so that when the gear shaft moves in a rotary fashion, the discs make the two plungers move in to and fro motion depending on the disc geometry. The suction or inlet pipe draws lubricating oil from a reservoir which is taken up into the pump chamber when the control plunger connects the same to the inlet pipe. The main plunger pumps the oil into the two outlet pipe one at a time. The arrangement is such for every to and fro motion of the control plunger, the main plunger does this motion twice.

The set screw shown in the diagram can be used to control the delivery amount of the lubricating oil to the quills fitted in the liner about which we already studied in the previous article. Also it must be noted that the oil from the lubricating pump going to the quill gets discharged twice inside the liner during each revolution of the main engine crankshaft – once during the beginning of the expansion stroke as the piston is moving downwards and secondly during the end of the compression stroke.

Among the several steps that have been taken lately to convert a conventional marine engine in to a modern, smarter 2-stroke marine engine, the development of intelligent cylinder lubrication plays a vital role.

 

The main reason for developing hi tech cylinder lubrication system is to reduce the operational costs of the engine. Moreover, the most expensive lubricating oil is generally used for the engine’s combustion chamber as cylinder lube oil. The development of intelligent cylinder lubrication thus makes perfect sense.

PULSE LUBRICATION SYSTEM

In the shipping industry, two giants – MAN Diesel and Wartsila have introduced a remarkable technology for modern electronically controlled marine engines. Known as Alpha and Pulse lubrication systems, this new technology is one-of-its-kind. In this article we will understand what does pulse lubrication means and how it helps to reduce the cylinder oil feed rate and eventually the operating costs of the ship.

Wartsila- A major player in the marine engine manufacturing industry has introduced an intelligent cylinder lubrication system in its electronically controlled engine. This system is popularly known as the pulse lubrication system.

WHAT IS PULSE LUBRICATION SYSTEM  

                  

     

 A pulse lubrication system is an electronically controlled cylinder oil lubrication system for Wartsila engines, wherein metered quantity of cylinder oil is injected in to the liner, depending on the engine load. This ensures that accurate amount of cylinder oil is delivered inside the liner at the correct set-time for that particular engine load.

Construction and Working of Pulse Lubrication System

• There are normally eight quills attached to the cylinder liner in a single row, which gets the oil suppl from the electronically controlled dosage pump
•  The oil is supplied to the dosage pump from daily tank via fine filter of 40 microns
•      The quills consist of a duct passage to store metered quantity of oil. The area of this duct passage and the quantity of oil can be altered by changing the position of the central piston
•   There are crank angle sensors attached to the engine which give signals to the control unit in order to  inject oil at the correct position of piston movement
• 200 bar high pressure servo oil reduced to 50 bars are supplied to the lubricator unit, which pressurises the centre piston in the quills. This injects oil inside the liner at adequate pressure for even distribution
•    WECS (Wartsila Engine Control System) which is the master controller of the Pulse lubrication system controls the solenoid valve opening and the oil injection
•     Each unit is provided with 8 lubricating quills, 2 piping systems of Cylinder oil and servo oil, and A 4/2 solenoid valve to servo oil flow.
•      After receiving signal from the crank angle sensor, at the correct position i.e. between the pack of piston rings, WECS allows the solenoid valve to open and pass the servo oil. This in turn presses the central piston and delivers the oil stored in the duct passage of the quills.

As soon as the injection is over, there is a small orifice which fills the duct passage again with the cylinder oil as the central piston moves backward. This ensures that the oil is always present in the chamber in metered quantity as decided by the WECS after calculating load and sulphur content of the fuel.

Advantages of Pulse Lubrication System

1.      Reduction in Cylinder oil Feed rate –up to 0.7 gm/kwh and thus reduction in operating cost.
2.  Well précised delivery of metered cylinder oil giving better lubrication to piston ring and liner
3. Better distribution of oil within the liner
4.   Less fouling of combustion space
5.   Less fouling of scavenge space

SPLASH LUBRICATION

This method of cylinder liner lubrication is used mainly in small trunk engines

The cylinder liner must be lubricated as well. This is so there will be a film of oil between the piston rings and the liner and also so that any acid produced by combustion of the fuel is neutralised by the oil and does not cause corrosion. Some of this lubrication will be supplied by so called "splash lubrication" which is the oil splashed up into the liner by the rotating crankshaft. However larger medium speed marine diesel engines also use separate pumps to supply oil under pressure to the cylinder liner. The oil is led through drillings onto the liner surface where grooves distribute it circumferentially around the liner, and the piston rings spread it up and down the surface of the liner. A pre lub pump is sometimes fitted especially to engines where the main pump is engine driven. This pump is electrically driven and circulates oil around the engine prior to starting.

 

Trends over the years

The piston rings on the liner surface mostly operate under boundary or thin film lubrication conditions. The high operating temperature, pressure, and the presence of a corrosive environment make the working condition very harsh. The two stroke crosshead type o Trends over the years

The piston rings on the liner surface mostly operate under boundary or thin film lubrication conditions. The high operating temperature, pressure, and the presence of a corrosive environment make the working condition very harsh. The two stroke crosshead type of marine diesel engines burning residual fuel provide the largest percentage of propulsive power for ships globally. Over eighty percent of total lubricant consumption is on cylinder oil alone and it represents a major expense in the daily operation of the engine. Also over the years, engine developments aimed at giving higher output per cylinder and adapting the engine to burn ever deteriorating fuel grades have worsened the problem of cylinder liner lubrication. The liner average surface temperature has increased from 200 degrees C to 275 degrees C from the 1970s to the 2000s, thus making lubrication more difficult. Similarly the maximum pressure and mean effective pressure also have increased drastically.

What makes Cylinder Lubrication Difficult?

1.      Low quality Fuel

The use of low quality high sulphur content, viscous and residual fuel to cut down running cost has either created some new problems or aggravated the existing ones.

The high sulphur content of fuels up to 4.5 % by weight has greatly increased the risk of cylinder liner and piston ring wear.

Ash, either soluble or insoluble, in fuel can increase the abrasive wear of piston rings and liners.

Mineral acids formed as a result of condensation of sulphur oxides produced during combustion create a serious hazard if they find their way into the crankcase.

Incompletely burnt heavy ends of the fuel can cause excessive deposits in the piston ring zone.

Short exhaust valve life due to deposit formation or corrosive attack by vanadium, sodium, sulphur, and other chemical compounds present either in the fuel or in the lube oil.

2.      Boundary lubrication conditions

From the mechanical point of view, the working conditions are about the worst possible for the establishment of hydrodynamic lubrication. The piston slows to rest before reversing direction on the return stroke. Thus at top dead center where the temperature is at maximum and also the radial pressure of the rings on the walls is highest, the piston slows down. Under such extreme conditions it is impossible for hydrodynamic conditions to exist except perhaps about mid-stroke.

3.      High operating temperature

Gas temperature exceeding 1667 degrees Celsius are encountered at the beginning of the firing stroke and the local temperature may be appreciably higher if the combustion is poor resulting in flame impingement on cylinder walls. The temperature of water cooled liner surface varies from 230 degrees C to 120 degrees C depending upon the design. Engine developments from the 1960s to the 2000s have led to higher average temperatures of liners in the upper zone (about 270 degrees C), which has also made lubrication difficult.

4.      Two strokes and crosshead operation of marine engines

All modern marine engines are of two stroke type. So there is no non-working stroke during which the oil film on the wall can be reformed at moderate temperature and pressure. Again, all modern engines are fitted with diaphragm plates and piston rod glands, which effectively separate the cylinder from the crankcase. There is therefore no cooling effect of crankcase oil on the piston, cylinder lubrication being entirely dependent upon the oil supply by mechanical lubricators. This makes cylinder lubrication critical, and any deficiency in it may lead to failure of the engine.

5.      Engine development to reduce cost of running

Engine development is basically aimed towards reducing cost of running, i.e. increasing the thermal efficiency, lowering the specific fuel oil combustion, and reducing the size and weight. This resulted in higher engine pressure, thinner oil film formation, high liner temperature, and increased corrosion. So cylinder liner lubricant needs to be competent enough to fight against these aggravated conditions.

In spite of these difficulties, the cylinder lubricants have to perform their duties. It must be remembered that the cylinder lubricants are used only as a once-through oil. That means that they have to accomplish their work in one go, and then they are discharged. Thus for this reason they are heavily loaded and are very expensive.

6.      Critical Working Conditions

What are the requirements and properties of a good cylinder lubricant, the odds the cylinder lubricant faces, and how they overcome them.

Requirements of a cylinder lubricant

The essential properties which a good cylinder lubricant must have are as follows:

1.          It must reduce sliding friction between the rings and the liner to a minimum, thereby minimizing metal to metal contact and frictional wear.
2.      It must possess adequate viscosity at high working temperatures and still be sufficiently fluid to spread over the entire working surfaces to form a good adsorbed oil film.
3.       It must form an effective seal in conjunction with the piston rings, preventing gas blow by and burning away of the oil film and lack of compression.
4.       It must burn cleanly, leaving as little and as soft a deposit as possible. This is especially true of high additive content oils as unsuitable types can form objectionable ash deposits.
5.       It must effectively prevent the buildup of deposits in the ring zone and in ports of port exhausted two stroke engines.
6.       It must effectively neutralize the corrosive effects of mineral acids formed during combustion of the fuel.

The odds against cylinder lubricating oil

At first glance it would appear that no lubricant, neither mineral nor synthetic, could withstand all above difficulties to fulfill the above requirements, but significant developments in the lubricating oil field have made it possible

·         Formation of Sulphuric Acid

This is taken care of by lubricating oil of high TBN values (60-80 TBN). Further it is ensured that the cylinder liner temperature doesn’t go below the dew point temperature of sulphuric acid in the upper zone and thus the corrosion is reduced to a minimum value. A typical large engine developing 20000 Bhp with a daily fuel consumption of 76103 Kg burning a 3000 second Redwood No 1 fuel at 100 F and a sulphur content of 4%, the total weight of sulphur burnt would be 3048 Kg or 3 tons. If this were all converted into SO3 and then into sulphuric acid, this would produce 10 tons of highly corrosive acid. But reliable analysis indicates that only 10 to 15 % is converted into sulphuric acid, and that amounts to 450 Kg of sulphuric acid per day which may lead to highly corrosive wear.

·         
      Abrasive wear

Additives to improve dispersancy and detergency have been developed to minimize abrasive wear and ring zone deposits.

·         Boundary Lubrication Conditions

Multilevel lubrication systems have increased oil film thickness considerably. The inclusion of additives such as polymethacrylates and ethylene-propylene co-polymers (OCP) to improve the oiliness helps in maintaining the requisite oil film thickness due to its molecular bending with porous cylinder liner.

·         
      Low spreadability of cylinder lubricating oil

Cylinder lubricating oil is highly viscous, which hampers its spreadibility thereby resulting in uneven wear on the cylinder liner. It will ultimately lead to blow down which further aggravates lubrication problems.

References

• Reference and Image Credit: wartsila
• Arab academy for science, technology and maritime transport, Alexandria, Egypt.
• Faculty of engineering
• video references - https://www.youtube.com/watch?v=XSTRzrJtCWw