Four stroke engine lubrication systems
All machines with moving parts where friction can be created require lubrication but in marine engines – particularly two-stroke engines – the subject is more complex than merely ensuring that a level of lubricant is maintained.
Tribology – the science of friction, wear and lubrication – has particular application in marine engines where conditions and parameters vary probably more than in any other internal combustion engine.
In any engine operating on fuels containing sulphur, cylinder lubricants have three main purposes: to provide a barrier to metal to metal contact between piston rings and the cylinder liner, neutralising any sulphuric acid to control corrosion and to clean the cylinder liner and piston rings preventing damage from combustion and neutralisation residues.
In four-stroke trunk piston engines the same oil is used for cylinder lubrication and cooling but some of the cylinder oil by-passes the piston rings and ends up in the combustion space, where it is consumed. However, the piston in a four-stroke trunk piston engine has an oil scraper ring that scrapes most of the oil supplied to the cylinder liner back to the engine’s oil pan, from where it is drained, cleaned and recycled. In general, four-stroke engines are less complicated as regards lubrication than two-strokes.Two-stroke engine lubrication systems
Two-stroke engine lubrication systems
The two-stroke crosshead engine is different from the four-stroke in that it has no connection between the piston underside space and the oil pan. The cylinder lubricant is not recycled and will be consumed in the combustion process making precise dosing imperative and allowing for different oil types to be used in the two zones of the engine – cylinder lubricant in the combustion chamber and system oil in other parts of the engine including the turbocharger and valves.
Two-stroke cylinder lubricating oil has a number of important parameters including viscosity, base number (BN) and detergent additives. The viscosity is typically SAE 50 across most suppliers although other grades might be specified under certain operating conditions. The BN can vary and is normally in the range of 40 to 70 if running on a typical HFO with 3.5% sulphur content.
The BN indicates the content of alkaline additives used to neutralise the sulphuric acid produced during combustion. Higher sulphur content fuels require a lube with a higher BN and low-sulphur fuels a lube with a low BN. The BN could be higher if a higher sulphur content is contemplated and lower from 15 or 25 – 40 in low, ultra-low and gaseous fuels such as LNG.
Before the advent of SECAs and controls on SOx emissions, most operators would choose a lube with a BN that could deal with the 4.5% global sulphur cap. This would normally require a lube of BN70. As low-sulphur fuels became more common there was a great deal of debate over what strategy to adopt.
In order to avoid problems with deposit build-up on piston crowns, it was accepted wisdom to reduce the BN of lubes when operating on low-sulphur fuels. Short periods of operating on low-sulphur fuels normally present no problem and would not require a switch away from the high BN lube normally used. In order to avoid problems connected with the use of lubes with the wrong composition, any lube suppliers experimented with producing universal lubricants
which were typically of BN40 or 50 and these are now readily available and favoured by some operators. The use of these universal products is not without a degree of controversy and some engine makers have retracted previous approval of their use under some conditions and in specific engines.
Clearly it is unwise to ignore OEM recommendations but the benefits of using a single lubricant are attractive and relieve crew of having to cope with the problems of changing lubes as well as fuel oils during approaches to ECAs. Owners should therefore communicate with their engine and lube suppliers to determine the best and safest operational strategy.
Alternative fuels and two-stroke engine lubrication
In recent years the number of alternative fuels to oil that are being used, or may be used in the future, by ships has grown to include LNG, LPG, methanol, ethanol and ethane with hydrogen and ammonia being other recent newcomers. That all of these fuels could, with some modifications, be used in an engine that can also burn mineral oil fuels and biofuel variations is a testament to the versatility of the diesel engine and a good reason why its demise is not seen as being something that will happen anytime soon.
Operations with LNG in four-stroke dual-fuel engines are nothing particularly new but there is much less experience gained on dual-fuel two-stroke engines. While quite a number of these have now been fitted, many are only considered as gas-ready with no gas fuel system in place so are presently running as typical fuel oil burning engines.
The dual-fuel engines that can run on LNG are not the only two-stroke power units using alternative fuels. There are for example several ships in service that are using methanol as a fuel. Methanol is a liquid fuel that is considered a ‘drop in’ replacement for oil fuels but which obviously has very different characteristics and lubrication needs.
All of the fuel types mentioned have little or no sulphur content and no other obvious components that would increase the acidity of the fuel so a very low BN lubricant would normally be needed. Another positive benefit is that they would also be very unlikely to contain any cat fines unless contaminated at some point during storage and delivery.
From the point of view of CO2 emissions, LNG performs best with ethane, propane and butane, each having one more carbon atom than its predecessor. The latter two are collectively referred to as LPG. As regards lubrication all have similar properties as LNG and can be considered as essentially the same.
Under most circumstances, if operating on one or more of the mentioned fuel types and even current ECA-compliant fuels, the lubricant should be either a 25 or 40BN product – although WinGD suggests that for LNG the BN could even be as low as 15. However, it should be borne in mind that the ship could also be running on other fuels at different times especially in the early days after the new global sulphur cap becomes effective.
By their very nature dual-fuel methanol engines are subject to an even wider range of sulphur conditions. As things stand, they could be burning 95% methanol, which has zero sulphur, along with either a low sulphur or high sulphur pilot fuel; they could be burning a 0.10% sulphur distillate fuel for ECA compliance; they could be burning up to a 3.5% maximum sulphur heavy fuel oil; or they could be burning a mix of 70% methanol along with either a high or low sulphur oil product.
Since none of the ships presently burning methanol has a scrubber fitted, they will be obliged to use another compliant alternative when not running on methanol, although it is entirely possible that if no compliant fuel is available then something else may have to be used instead. If that is the case, the cylinder lubricant will need to be changed for the duration of the voyage on non-compliant fuel and the again when reverting to compliant fuels.
It is entirely possible that the operators of some of the increasing number of ships now being fitted with scrubbers may in the future also decide that methanol is a good choice as a fuel. That would further complicate matters as to determining the best lubricant to use.
The range of potential fuel options for ships now able to use methanol already means that decisions are needed as to which level to limit the cylinder oil to the liners, and what the BN of the cylinder lubricant has to be.
Use of alternative fuels has so far been extremely limited and consequently kept under careful observation by the owners, engine makers and lubricant suppliers. Crews have therefore been well supported in managing lubrication changes and monitoring the effects of new fuels on the engine. If there is a wider take-up of alternative fuels, the crew involved may have less support and care should be taken to investigate requirements in conjunction with lubricant suppliers and manufacturers and whatever other sources of assistance are available.
Blend on board systems
Almost all engine lubes are sourced from specialist manufacturers but the ability to produce a useful lube on board is now possible on some vessels. Maersk Fluid Technology (MFT) – an A P Møller subsidiary – has been experimenting and developing blending on board (BOB) as an alternative solution for some years. The system has been implemented on some Maersk vessels and is now being commercialised as the SEA-Mate Blending- On-Board system.
The concept is based on proprietary technology designed to enable the operator to custom blend a fit-for-purpose cylinder lubricant from recycled in-use two-stroke system oil and a high BN cylinder oil concentrate. Some ship engineers also do something similar but without the use of sophisticated equipment and manually blend the stem oil with an additive.Synthetic lubricants
Lubricants are usually produced using a mineral oil base with additives but even with the best quality controls in place, natural variability in the base oils means that performance can sometimes be less than expected and furthermore the base oils themselves are sometimes not ideally suited to the environment they are used in. In attempting to gain more uniform and improved performance, most leading manufacturers now offer synthetic products.
Most will claim that synthetic lubricants offer superior protection and performance compared to conventional mineral oils and that they have a longer service life in the engine, decaying at a known rate and therefore giving predictable performance. Other claimed advantages include high-temperature capability, low-temperature flow properties and a resistance to oxidation.
Tests do tend to show an improved performance that agrees with the makers’ performance claims but for the operator the downside is that synthetic lubes come with a premium price tag.
It will be for the operator to decide if the benefits outweigh the price premium or if the extra cost can indeed be clawed back through longer life. It is however important to investigate miscibility with other products if synthetic lubes are chosen. Most operators chose to use lubricants from a single maker whenever possible but even the best laid plans can go awry, and it is sometimes necessary to use products from different suppliers.
Except for a few specific lubricants, most manufacturers have equivalent products that can safely be used alongside competing products. Ensuring the compatibility of products before mixing is relatively simple as most makers produce charts listing compatible rival offerings.
The reduction of the IMO’s global cap on fuel sulphur levels in 2020 will lead to demand for a new generation of lubes and if refiners choose to produce HFO with the 0.5% limit rather than continue with the current 3.5%, some of today’s products will become obsolete except for those ships with scrubbers that can continue to use the higher sulphur fuels.
It might be thought that the difference between 0.5% and 0.1% is small enough to make the need to change lubricants when entering ECAs in future unnecessary. However, many experts believe that the need to match cylinder lubricants to the fuel will continue after 2020 and, for ships operating with high sulphur fuels and scrubbers, lubes with higher BNs will be needed because the scrubbing takes place after combustion.2020-compliant fuel oils bring their own problems
2020-compliant fuel oils bring their own problems
For the ships that are going to run on compliant fuels, cylinder lubrication has not necessarily become simpler just because only two grades of fuels (0.5% and 0.1% sulphur content) are being used. Many of the new fuels will be blended and as ships move between ports, the available fuels may have very different qualities and characteristics. Some of the fuels may contain biofuels in varying quantities and a very small number of vessels may be run wholly on biofuel.
Most common biofuel types include fatty acid methyl esters (FAME), ethanol and hydrotreated vegetable oil (HVO). These fuels can exhibit very different properties than traditional oil fuels and may even have a higher acid content with no sulphur present at all.
Blended fuels and biofuels may also have very different viscosities and combustion characteristics which will make the choice of an appropriate lubricant an important factor in operation and maintenance.
Most of the leading lubricant suppliers have spent much time and effort in developing lubricants suited to the new range of fuels. may currently have limited availability and so gathering experience with them in conjunction with new fuels will not be the easiest of tasks.
For ships that have regular routes or areas of operation, some experimentation of new fuels and new lubes may be possible but involving OEMs, lubricant suppliers and testing services would be a prudent precautionCold Corrosion
It is not only controlling SOx emissions that has created problems in the tribology of modern engines. Addressing the twin effects of controlling NOx and meeting the EEDI requirements has led to a new phenomenon – cold corrosion.
Cold corrosion is when sulphuric acid forms on the liner walls in an engine cylinder and corrodes the liner surface. This abnormal corrosion then creates excessive wear of the liner material. In order to comply with NOx and EEDI regulations, engine makers have needed to increase pressure and reduce operating temperatures, which has been done by way of lower rotations per minute, longer strokes and increased scavenge and combustion pressures.
This creates conditions below the dew point that allows water to condense on the cylinder liner walls. This then combines with sulphur from the combustion process to form sulphuric acid, which leads to cylinder liner wear, termed cold corrosion. The NOx and EEDI rules only apply to newer engines but older engines can also suffer from the problem if they are modified, as many have been, to allow for slow steaming when engine loads below the original design parameters become routine rather than exceptions.
Among the modifications are turbocharger cut-out, retrofit of variable turbo charger nozzle rings, exhaust gas by-pass valve fitted and engine tuning changes. Some modified engines become mildly corrosive whereas others may be more seriously affected. The problem can be exacerbated because, in order to avoid cold corrosion occurring, a simple solution is to keep coolant temperatures elevated.
However, this is precisely what engineers on board have tried to avoid in the past and it is counterintuitive to years of training and experience. Another mistake is over-lubricating in order to reduce acidity within the cylinder liner. To meet the ideal acidity level, it is not unknown for lubricant use to quadruple, which is expensive and creates other problems. It is therefore advisable to monitor feed rate and seek advice.
Electronic lubrication systems can reduce cylinder oil consumption but, as an open loop system, it does not provide feedback on the impact of such reduction and, sensibly, a safety buffer is often applied. Without a reliable feedback system to accurately monitor the effect on the engine, changing feed rates based solely on OEM’s recommendations could increase associated wear caused by under-lubrication and seriously harm the engine.
To penetrate the lubrication safety buffer, safely achieve the true optimum feed rate and realise maximum savings, offline or online tools are available to closely monitor lubrication conditions. In order to overcome the problem, most engine makers have issued advice on measures to identify the severity of the problem and what can be done to counteract it. MAN Energy Solutions recommends the use of a ‘Sweep Test’ and Wartsila has a similar ‘Quick Test’. In addition, most lubricant manufacturers have developed test kits and can offer more advice and assistance on conducting tests.Lubricant Analysis
Ensuring that engine lubrication is being properly managed is essential for both safe operation and economic reasons. Mostly this is the province of the chief engineer but at a time when many fuels and lubes are produced in quite different ways than used to be the case, some help for the engineer is essential.
There are regular reports of poor quality and even dangerous fuels being supplied to ships and this is the first area where checking is needed. Analyses of engine lubricants can reveal the condition of components within the engine and help predict developing problems inside the engine itself.
The cylinder lubricant in two-strokes is not retained in the engine as it is consumed in the combustion process but system oils from all marine engines and cooling and lubricating moving components can also be used as a means of ascertaining the health of an engine. Proper analysis of the in-service lubricant can give early warning of possible failures long before an incident occurs, allowing preventative measures to be taken.
Several companies and classification societies offer analysis services and it is possible for crew to make tests themselves using proprietary test kits. Some of the test kits for use on board are quite basic but others are much more advanced and include equipment usually found only in laboratories. As well as chemical tests to identify the presence of elements and compounds in the samples, some of the equipment now available makes use of techniques such as X-Ray fluorescence (XRF) to identify them. Some of the onboard systems will also be able to advise on feed rates for lubricants to improve their performance.
Used and waste lubricating oils also pose a problem with regards to their disposal and unless some means of reducing their volume is available on board there will inevitably be a large cost involved for disposal ashore.
A 2015 report from The Swedish Club shows that incorrect maintenance and repair continues to be the most frequent cause of main engine damage – a trend which has continued unabated since the club began monitoring the issue over a decade ago. With an average cost per claim of $926,000, lubrication failure is still the costliest cause of damage to the main engine, due to consequential damage to expensive parts such as crankshafts, pistons and the like.
According to the club, the root causes include crew with insufficient experience and training; experts not in attendance at major overhauls; contaminated lubrication oil and contaminated bunkers.
Implementing a condition monitoring programme to monitor the performance of the engine and cylinder oil is crucial to good ship operation. This programme should monitor the parameters of iron wear and the residual Total Base Number (TBN) in scrape-down oil.
Different components of an engine are made from different metals and minute particles find their way into the oil as they wear. The scrape-down oil from the cylinders shows up wear in rings and liners as well as giving clues to the combustion process, while circulating oil can carry traces of bearing wear from the crankshaft and connecting rods.
Analysis of scrape-down oil can sometimes show evidence of over-lubrication and suggest adjustments can be made to the feed rate to reduce consumption of lubricant as well as preventing fouling of turbochargers, valves etc. As well as showing evidence of engine wear, analysing lube oils also gives information on the quality of the oil itself.
Lubricants break down gradually and their effectiveness reduces as it does so. Topping up is necessary at times but if continued for very long periods can actually result in increased consumption because all of the work of the oil is being done by just a small amount of fresh oil while the remainder may be practically useless. Changing the circulating oil in any marine engine is an expensive task because of the quantities involved so is undertaken only when strictly necessary.
As with machinery components, changing at set intervals is normal practice but this could result in product that has a significant amount of ‘life’ remaining being sent to the waste tank. Regular analysis will reveal what the true quality of the oil is and can mean changes can be postponed for significant time periods. Almost every major lubricant producer runs a testing and analysis service and there are also several independent service providers. Engine makers are also becoming more involved and providing varying degrees of assistance from monitoring through to complete engine management.