Correct lubrication is an essential skill with more challenges ahead
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.
Marine four-stroke engines require just a single system lubricant, but lubrication of two-stroke marine diesel crosshead engines is achieved using two quite different lubricants.
In these engines, the combustion chamber and scavenging air spaces is separated from the crankcase by the piston rod stuffing box and a different lubricant is used for each area. Furthermore, the cylinder lubricant must be matched to the fuel being used so as to neutralise the acidity of the fuel that is determined by the sulphur content and to the load conditions under which the engine is operating. This means that the type of cylinder lubricant for such an engine may need to be changed several times in the course of a day.
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.
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.
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, many 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 particular 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.
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 Wärtsilä has a similar ‘Quick Test’. In addition, most lubricant manufacturers have developed test kits and can offer more advice and assistance on conducting tests.
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 attempt 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
For the ships that are going to run on compliant fuels, cylinder lubrication will not necessarily become simpler just because only two grades of fuels (0.5% and 0.1% sulphur content) will be 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 that will be used after 1 January 2020 and are gradually introducing these to the market. The full range of new products 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 precaution.
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.