Cylinder lubrication developments and Lubricants drain analysis

Lubricants are used to reduce the friction between the two moving surfaces at the point of relative motion. The process of using lubricant to ease the motion and reduce the wear down of contact surfaces is known as lubrication. In other words, we can say, Lubrication is the process or technique of using a lubricant to reduce friction and/or wear in the contact area of two surfaces. In earlier days animal fats such as tallow or mixed animal fat with calcium soap obtaining grease were used as lubricants on primitive machinery like the wooden wheel. Over history, there have been many lubricants made from different materials and always replaced by better ones. This has been going on and on until the lubricants used nowadays, which are made from crude petroleum or new generation synthetic oil.

In modern diesel engine lubrication of the moving parts is reducing the friction between the surfaces to prevent the loss of power generated within the engine cylinders, which would directly mean a cost reduction along with increases the life of the components and the engine efficiency.

The lubrication In modern diesel engine is separated into two parts, cylinder, and crankcase or system lubrication. In this blog, we are going to discuss in detail about the cylinder lubrication of marine slow-speed two-stroke crosshead engine.

For properties of lube oil and types of lubrication, kindly refer our following links:  Basics of Hydrodynamic, Hydrostatic, Elasto-Hydrodynamic and Boundary Lubrication

Cylinder Oil  Lubrication

Cylinder oil is used for the lubrication of piston ring and liner. Piston rings play a very important role in the performance and endurance of the engine hence proper lubrication should be ensured in all operating conditions. The functions of cylinder oil are:

  • To assist in providing an optimum gas seal between the cylinder and liner. The lube oil consumption and blow-by in an engine are directly related to the sealing function of the piston rings.
  • To eliminate or lower wear and friction by minimizing metal to metal contact between piston rings and liner.
  • To act as a carrier medium for the functional alkaline and additive system.
  • To provide a medium by which combustion deposits can be transported away from the piston ring and keep the groves clean for rings free movement.

In order to achieve an effective lubrication of the cylinders, it is required an oil that is able to create a tenacious and constant film on the cylinder walls to protect the parts from wear and make a seal to prevent gases from leaking between the piston rings and liner. The cylinder oil not only has to be thick enough to adhere to the moving surfaces at high temperature but it also has to be thin enough to spread evenly over the whole surfaces of the long stroke engine. In this case, the special additives for cylinder oils are needed to improve detergency and dispercency properties, acid-neutralizing properties, oxidation stability, corrosion resistance, and load-carrying properties.

Cylinder Lubrication- particle size in Lube oil
Size of particles in Lube Oil

The piston-ring-liner system is the major source of lube oil consumption. For that reason, it needs to be designed to minimize the cylinder bore distortion and lube oil consumption by increasing the piston ring stability and conformability, optimizing the liner surface finish, etc.

Cylinder Lubrication- surface asperities
Surface asperities

Even if a surface has been carefully polished and smoothed, irregularities are present. They form peaks and valleys at a microscopic level. These peaks are called asperities. For optimum work conditions, the lubricating film must be thicker than the length of the asperities. If the layer can’t fulfill this condition serious problems can occur such, micro-seizure for example.

While an engine is new or has been fixed, the oil consumption will be even higher because the asperities need to set for letting the oil create a good seal. It exists the myth that a brand new engine does not consume oil, but is not true.

Cylinder Lubrication- Running in condition
Surface Condition before & after Running-in

The cylinder liners are one of the most exposed to heat and extreme situations of the engine. They are just a cylindrical piece that separates the piston block and the combustion chamber and piston. This makes it easier to cool them and to repair the engine just by substituting them. The liner is manufactured using superior materials than the cylinder block. While the cylinder block is made from a grey cast iron the liner is made from a nodular cast iron alloyed with chromium, vanadium, and molybdenum. The alloying elements increase the resistance against corrosion andimprove also the wear resistance at high temperatures.

Four-stroke engines cylinder liner lubrication

For four-stroke trunk piston engines different ways for lubricating the cylinder liners are provided by the makers, depending on the size and type of the engine:

Cylinder Lubrication- oil scrapper ring
Excess oil on liner is scrapped by scrapper ring
  • Splash lubrication for small engines where lubrication oil is splashed by the revolving crankshaft.
  • Inner lubrication where the oil is supplied from the piston side.

In four-stroke engines system oil used for both cylinder and other lubrication and cooling requirements. For cylinder lubrication, a small amount of lube oil is left behind on the liner surface by the help of oil control ring/Scrapper ring, ending up in the combustion chamber and being consumed

The splash system is no longer used in automotive engines because of its low lubricating efficiency. It is widely used in small four-cycle engines for lawn mowers, outboard marine operation, etc.

Two-stroke crosshead engine cylinder lubrication

Most of this kind of engines are fitted with an independent system just for cylinder and piston lubrication. These systems use separate oil pumps to supply pressurized oil to the liner. The cylinder oil is stored in tanks and transferred daily to a small capacity tank by gravity from which will pass to the cylinder lubrication system.

In two-stroke crosshead engine, the piston scraper ring does not exist means that the cylinder oil is not recycled and reused. It can be said that once the oil has left the lubricating system is virtually “lost”. That shows the importance of the oil dosage. The lubrication is made regardless of the engine size and supplied from an external lubricating device via quills in the cylinder liner. These lubricators are single-acting reciprocating high-pressure pumps which pump lube oil at each stroke, on the liner surface.

The aim is to ensure a proper distribution of the oil on the cylinder liner running surface. The challenge is not only to ensure that the oil film on the cylinder liner is well maintained but also continuously refreshed in order to provide enough additives for the acid neutralization and cleaning processes.

Cylinder lubrication basic layout   

In earlier days, there has been two cylinder lubrication system, based on Timed Lubrication (MAN B&W &MHI) and on Accumulator Lubrication (Wärtsilä[Sulzer]) respectively. Common for these systems has been the injection of CLO in drops to be distributed over the surface of the liner through a circumferential zigzag groove provided on the liner. The pressure difference over the piston rings presses the oil to move in the groove and vertically distributed by the movement of the piston rings.

Timed Lubrication

Cylinder Lubrication- timed lubrication

The purpose of timed lubrication is to deliver LO to the sliding surfaces in the cylinder at a specific time in relation to the position of the diesel engine piston during its upward movement. The oil is pumped into the cylinder when the piston rings pass the lubricating orifices, during the upward stroke.

Timed lubrication as described above is in practice carried out by means of lubricators driven mechanically from and synchronously with the diesel engine. Lubricators are connected by means of couplings and intermediate shafts to the diesel engine drive, so that CLO is supplied at every diesel engine piston stroke and at the specific time in the diesel engine cycle.

Accumulator Lubrication (Wärtsilä)

Cylinder Lubrication- Accumulator lubrication

This system consists of a multi-element pump unit driven by an electric motor, and a progressive distributor for each cylinder with a number of quills with a spring membrane accumulator. The unit supplies the cylinder lube oil to the distributors, ensuring an equal distribution of the oil to each individual quill. When the pressure inside the cylinder at the quill level, which is normally located in the upper third of the liner, is sufficiently low, the oil is released by the spring force of the accumulator. The pressure inside the cylinder is lower than in the accumulator twice for every revolution. When the piston is moving down during the expansion stroke and when the piston is moving up as the piston rings pass the lubricating grooves.  The system releases a small amount of oil to the cylinder liner in each cycle, but this release is not timed. The feed rate is controlled by disc settings in the pump unit, and along with the variation of the rotational speed of the driving electric motor. This system is simple, robust and very reliable but it requires oil feed rate in the range of1.0 to 1.6 g/kWh.

RPM dependent lubrication system

In this system cylinder lubrication system is connected with the crankshaft or camshaft for correct timing of injection. The quantity of cylinder lube oil and dosing is done at every piston up going stroke as a function of engine RPM.

During operation even at a constant RPM, the load may vary. This variation in load will cause thermal instability and hence affect the cylinder lubrication. Thermal instability in the cylinder due to load changes may cause an exceptionally high risk of corrosive and frictional wear. Increased injection of oil is desired at these times to neutralize acid build-up and reduce friction.

Load-dependent lubrication

Load-dependent lubrication denotes an adjustment of the stroke in the lubricator oil pumps to correspond to the mean effective pressure or the actual engine load. The system is a natural consequence of specifying the required Cyl lube oil consumption as a function of engine load defined as g/kWh. Now days load dependent cylinder lubrication in conjugation with the sulphur content of fuel oil, the system is the most effective.

Lubricating Accumulator
Cylinder Lubrication- accumulator
Diagram & arrangement of LO accumulator

A hydraulic accumulator is a device that stores potential energy by compressing a gas, spring or a raised weigh to exert force against a relatively incompressible fluid.  They are used in fluid power systems to accumulate energy and release it in pulsations. A hydraulic system that uses an accumulator can use a smaller fluid pump because it stores energy in periods of low power demand. The energy is available for an instantaneous use and released at a rate many times greater than a pump alone could supply. If the supply tube is kept under sufficient pressure by the counter pressure in the non-return valve, the desired injection timing can be achieved.

Lubricating Quills

Quills are non-return valves screwed into the liner at the liner oil grooves. They help to moisten the pressure pulsations in the supply line preventing the gases made by the combustion to enter back into the oil line and also provide storage to pressurized oil in the accumulator. Each cylinder has usually two or more quills, creating a circumference as shown in fig

Usually, each quill is connected by a separate pipe. The cylinder of the quill needs a non-return valve in order to eliminate pressure pulsation in the delivery pipe and to prevent the gases to go in. Because of that, the delivery pipe keeps full of oil even when the engine is stopped. It has to be placed as close as possible to the liner, otherwise, the exhaust gasses will enter the line and deteriorate the oil before it can get to the cylinder. The quill is isolated from cooling water by a sealing pipe which allows an easy removal of the quill.

Alpha Adaptive Cylinder-oil Control (Alpha ACC)

The Alpha Adaptive Cylinder-oil Control (Alpha ACC) is introduced my MAN B&W and having following two criteria to determine the dosing of cylinder oil-

  • The cylinder oil dosage shall be proportional to the sulphur percentage in the fuel
  • The cylinder oil dosage shall be proportional to the engine load (i.e. the amount of fuel entering the cylinders).
Cylinder Lubrication-Alpha Adaptive Cylinder Oil Control (ACC)
Block diagram of Alpha Adaptive Cylinder Oil Control (ACC)

The implementation of the above two criteria will lead to an optimal cylinder oil dosage, proportional to the amount of sulphur entering the cylinders. The above principle is founded on the observation that the main part of the cylinder liner wear is of a corrosive nature, and the amount of neutralizing alkaline components needed in the cylinder should, therefore, be proportional to the amount of sulphur(generating sulphurous acids) entering the cylinders. A minimum cylinder oil dosage is set in order to account for other duties of the cylinder oil. The results showed really good results especially with respect to Cylinder oil consumption, particle emissions, and combustion chamber wear, showing substantial annual savings. This system can be implemented on all MC engines and their compact version MC-C.

Main Components of Alpha Adaptive Cylinder-oil Control

Pump Station and Starter Panels

The pump station consists of two individually operating pumps, heating coil, filters, and a suction tank. The power supply to the pump station starter panels is taken from two separate circuit breakers, one supplying each pump.

Lubrication Units

Each cylinder has its own lubrication unit, and each of them comprises two lubricators for 98-70 bore engines and one lubricator for medium or small bore engines. Each lubricator unit is equipped with one accumulator with pre-pressured nitrogen at 25-30 baron the inlet side, and one accumulator on the outlet side (for each lubricator), pre pressured at 1,5 bar. Depending on the engine type and design each lubricator could have in 3, 4, 5 or 6 lubricating pistons, one feedback pickup, and one solenoid valve. The lubricator itself has a small piston for each lubricator quill in the cylinder and the power for injecting the oil comes from the system pressure, supplied by the pump station. A common rail system isused on the driving side, but the injection side has a high-pressure positive displacement system, thus giving equal amount to each quill and providing the best possible safety, margin against clogging of single lubricator quill.

Alpha Lubricator Control Unit (ALCU)

It is composed by three main electronic components that are comprised in one steel cabinet and the so-called ALCU. These three components are:

  • Master Control Unit (MCU)
  • Backup Control Unit (BCU)
  • Switch Board Unit (SBU)

The 24 V DC power is supplied from two individual power sources, from different breakers in the UPS units.

Load transmitter
Cylinder Lubrication- Load transmitter
ACC Load Transmitter

The load transmitter is connected to the fuel rack, thereby continuously transmitting the fuel index % to the MCU, which calculates the engine load from this information and the detected engine rpm.

Trigger system (Shaft encoder)

The shaft encoder is connected to the fore end of the crankshaft, and the signals are transmitted to the computer panels via a terminal box. For engines on which the crankshaft fore end is not available for angle encoder installation, a trigger ring and tachograph pickups are installed at the turning wheel.

Cylinder Lubrication- Angle Encoder
ACC Angle Encoder
Backup trigger system

The backup trigger system comprises two tachograph pickups in a box at the turning wheel, there by transmitting the engine rpm to the BCU. The backup pickups are also connected to the MCU for surveillance purposes.

Cylinder Lubrication- tachograph
ACC Tachograph
Human Machine Interface (HMI) panel

On the HMI panel, individual cylinder lubrication adjustment is possible, various values and alarms are displayed, control buttons for the pump station are available, and manual execution of pre-lubrication is possible. As standard the HMI-panel is mounted in the engine control room.

It has a three-position mode switch that enables de selection between:

Auto-mode – BCU takes over automatically, if lubrication cannot be maintained by the MCU. If the BCU has taken over the control, this mode can only be cleared by manually switching to MCU-mode, and back to Auto position.

Working of Alpha Adaptive Cylinder-oil Control

Cylinder lubricating oil is fed to the engine at a pressure of 40-50 bar by means of a pump station usually mounted on the engine or sometimes placed in the engine room. The oil fed to the injectors has been pressurized by one or two Alpha Lubricators located on each cylinder and equipped with small multi-piston pumps.

The MCU controls the oil injection by activating a solenoid valve situated on the relevant lubricator. A feedback signal from each lubricator indicates that the injection has taken place. This is shown be Light Emitting Diodes (LEDs) on intermediate boxes for each cylinder.

Timing is based on two signals from the angle encoder, a Top Dead Center (TDC) cylinder marker and a crankshaft position trigger. The alpha lubricator System is normally timed to injector the cylinder oil into the piston ring pack during the compression stroke.

The cylinder lubrication is based on a constant amount of oil being supplied via injection. The specific feed rate is controlled by variation of the injection frequency. This frequency is calculated from index and speed, and is usually proportional to the engine Mean Effective Power) MEP. However, a power Mode or RPM Mode is possible. 

The basic cylinder oil feed rate at a Maximum Continuous Rating (MCR) of 100% is calculated as a correlation between a number of injections/rpm and the stroke of the lubricators. On the HMI panel, adjustment of lubrication feed rate for individual cylinders is possible between 60% and 200% although default mode is 100%.

During normal operation of the system it is controlled by the MCU. If any failures are detected in the system a common alarm is activated in the control room. The detailed alarm reference is displayed on the HMI panel. If a critical failure in the MCU is detected, then the BCU automatically takes over control (only if the control switch is set in “auto” position). An indication lamp “BCU in control” is lit on the HMI panel in newer installations and elsewhere in older ones.

The BCU is based on a random timing and RPM mode. The injection frequency is adjustable on the BCU and is normally, as minimum, set to the basic cylinder oil feed rate for the engine, plus 50%. Prior to start-up, the cylinders can be pre-lubricated and, during the running period, the operator can choose to increase the lube oil feed rate by 25%, 50% or 100%.

With the development of the latest control electronics, Pulse Lubricating System and Alpha LubricatingSystem provided timed lubrication with reduced cylinder lubricating oil consumption. By installing these modern systems, feed rate can be adjusted automatically with respect to engine operating condition, so cylinder oil will not be wasted and cylinder liners are in good condition for a long time.

Safety system of Alpha ACC

  • Any malfunctioning in the solenoid valve or transducer, the oil dosage will automatically increased to the maximum volume.
  • If the oil pressure falls, the control unit will start stand-by pump, close down the faulty pump and sets on the alarm.
  • In large bore engine where two lubricators are installed, if one lubricator malfunctions the oil dosage from the other lubricator will be automatically doubled and an alarm will be given whereas for small bore engines, where only one lubricator is installed, alarm and slowdown is initiated.
  • As shown is above figure an inductive sensor in each lubricator monitors the movement of the lubricator piston, it gives a local LED indication and a signal is sent to the control computer system which has a backup for safety.
What will happen if oil cylinder lube oil feed rate is high??…

For many years there has been a perception that the more oil used in the cylinder lubrication, the better. But this is not true, in fact it has been proved that the main reason for cylinder wear, broken piston rings and overall poorer cylinder condition is over lubrication. Over lubrication of a two-stroke engine means excess of additives(calcium salts for alkalinity) which burns during the combustion and produces carbonates, which keep on depositing over piston. Accumulation of these hard deposits may cause liner scoring or with add on of some soft deposits it can touch the liner wall and wipe out the lubricating oil film and lead to frictional wear of the cylinder liner and piston rings.

Oil feed rates should always also be adjusted according to the Sulphur content in the fuel oil. Especially when are taken into use low Sulphur fuels. With the expansion of low Sulphur Emission Control Areas on the global trade routes and future limits on the amounts of Sulphur in the fuel oil, normal practices are changing. There is a greater use of dual fuel handling, which requires switching between high and intermediate TBN lubricants according to the fuel on use.  This means that modifying cylinder lubrication oil feed rates becomes a daily task. The optimization requires daily monitoring of the residual drain oil

Cylinder lubricant drain analysis

The need for regular piston inspection is recognized by most operators. However, the incompetent educational standard of some ships’ staff may mean that piston inspection cannot always be relied upon to provide as much valid information about piston and liner conditions as would be the case with inspection by an experienced superintendent or specialist engineer. Regular basis Cylinder lubricant drain analysis onboard and ashore has opened a new source of information to help determine engine performance conditions, providing the operator with additional information to make engine maintenance decisions.

Advantages of cylinder oil drain analysis-

  • Visual inspection does, nevertheless, have its limitations for example drain analysis can pick up a wear problem which has not been noted during visual inspection of pistons, in other words, before the wear became a serious problem.
  • It is important to note is that Cylinder Lubricant drain analysis can detect a problem, such as ring and liner wear, water contamination of the cylinder lubricant, and piston rod gland leakage, before it would be picked up by conventional methods of inspection. Furthermore, by combining cylinder lubricant drain analysis with the physical inspection of piston condition through the scavenge ports, more precise maintenance decisions can be made.
  • Analysis of the drain oil, and comparison of the drain oil analyses with new oil analysis, enables conclusions to be drawn about the combustion and lubrication conditions of each cylinder unit.
  • A further comparison, of the waste oil from one cylinder unit with the waste oil from other cylinder units of the same engine, enables comparison of the performance and maintenance condition of each unit against the other units.
  • As a further step towards a more comprehensive engine performance monitoring service, engine manufacturers have been collecting engine performance data from selected ships. The combination of regular monthly engine performance data, photographic records of piston and liner condition, together with the data derived from cylinder lubricant drain analysis has enabled a more detailed and specific interpretation of engine performance condition to be provided to engine operators.
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Anurag Singh

He is working as an engineer in Synergy Marine Group. He is alumni of Marine Engineering & Research Institute(MERI) Mumbai. With his versatile talents, he loves to play cricket and write blogs. Speciality: Tanker operations

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