Cylinder oil feed rate optimisation

Our industry is sometimes known to be slow to accept change and cautious in the implementation of new concepts and practices. There is nothing wrong with this approach in a mature industry, however, sometimes factors such as new regulations, true step-out technology advances and striving for economic advantage, can outweigh the normal cautions.

In the past several years, engine manufacturers, shipping companies and oil suppliers have taken significant steps in the attempt to lower the cylinder oil feed rates in two stroke applications. These efforts, driven primarily by economics, have in some cases been successful, but have also had disastrous results.

It is the position of this author that feed rate optimisation, not necessarily feed rate reduction, should be the goal. This is not to say that feed rate reduction is not an obtainable objective, but it must be achieved through a combination of small incremental steps, proper and timely analytical techniques and a full understanding of the cause and effect relationships of two stroke engine operation.

The bottom line is: feed rate optimisation is a balance between the cost of a specific oil feed rate vs the expense generated by wear.

Feed rate optimisation is both simple, and at the same time, very complex. In its simplest state, it is merely having the capability to determine if excessive wear is occurring, on a cylinder specific basis, at a set feed rate. The complexity comes into the picture when you consider the number of ever changing variables in the engine operation and the operational environment that can, and will, influence the wear dynamics.

For many years, Mobil, and now ExxonMobil, has been engaged in a comprehensive study of feed rate optimisation using “Scrapedown Analysis” as the principle analytical vehicle. This study, headed by Dr P Thomas Reischman at ExxonMobil?s Research and Engineering facility in Paulsboro, New Jersey, has provided some very valuable scientific data which has given us the ability to better understand and forecast what the optimised feed rate should be.

The study has gained experience with:

1) MAN B&W and Wärtsilä two stroke engines of various bore sizes

2) Fuel sulphur levels from 0.5% to over 4.0%

3) Various forms of lubrication delivery to include SIP (swirl injection principle) lubricators

4) Varying engine load profiles

5) A wide range of cylinder liner temperatures

6) Humid and dry intake air environments

7) Liners with various honing techniques employed

8) Engines in both marine and stationary power applications

9) Engines running on both standard (70 BN) and low BN (40 BN) products

10) The use of liner collection trays vs scavenge air space samples

The goal in employing the scrapedown project across so many different engine and operational conditions was to gain an understanding of the degree of influence that each variable had on the wear dynamics both independently and in conjunction with other variables. The methods employed to evaluate the scrapedown oil consisted of:

1) Iron measurements both onboard analysis and laboratory confirmation

2) TBN measurements

3) Particle Quantifier (PQ) analysis

4) Analytical Ferrography

5) Other elemental measurements

6) Actual wear measurements

An early concern was determining the best way to collect the scrapedown sample. We felt that it was imperative that any sample collected was representative of the true condition of the liner.

The initial belief was that it was necessary to collect the scrapedown sample directly off the bottom of the liner. To accomplish this, we worked with the major OEMs to design a collection tray that was mounted directly to the liner. Over time, we compared the scrapedown oil values derived from the collection trays vs those from samples taken directly from the scavenge air space.

The findings proved conclusively that although there were some differences in the amount of measured iron, it was determined to be insignificant. Based on this finding, it was decided that, because of simplicity, the scavenge air space was a preferable sample location.

Another early concern was gaining an understanding of the relative size of the wear particle generated in the scrapedown sample. The reason for this concern was to ensure that laboratory based Fe analysis techniques using ICP ? AES (Inductively Coupled Plasma ? Atomic Emission Spectroscopy) is unable to measure particles > 5 – 10 microns. If liner wear particles are typically 5 microns and larger, it is necessary to utilise a different method for determining the cylinder wear.

Fortunately, from an analytical standpoint, the dominant wear mechanism in crosshead diesel engines burning residual fuel is acid corrosion. This is demonstrated in Graph 1 which is a comparison of iron results determined by a conventional ICP method (ASTM D5185) and that same method preceded by a sample preparation step designed to chemically breakdown large particles. With few exceptions the results of the two analyses were identical, indicating that the wear debris is generally small and that conventional ICP method of analysis is sufficient to accurately measure iron in most scrapedown samples. For those few samples with large wear particles, Ferrographic methods of analysis can be used to monitor them.

The measurement of the BN of the scrapedown oil is also critical to the understanding of the liner condition. When the scrapedown BN level remains high, say greater than 50 BN (when a 70 BN is in use) it can be surmised that one of two things is occurring: (1) the amount of acid being created is relatively low and is being effectively neutralised or (2) the feed rate is substantially high thus introducing excess oil onto the liner surface over and above what is necessary to handle the amount of acid being produced. It is appropriate to mention at this point that we have seen cases where overfeeding the cylinders has increased the likelihood of scuffing.

Given the situation where the scrapedown BN is high, and provided that the iron measurement is sufficiently low, it is reasonable to assume that an incremental reduction in feed rate is possible. But, in making that leap of faith it is important to check at least one other factor, specifically, the possibility of system oil being carried up and contaminating the scrapedown sample.

With typical system oil BN being in the range of 6, it could easily dilute the scrapedown BN to the point where the wrong assumption could be made.

It is ExxonMobil?s practice to run a simple test to confirm or disprove the presence of system oil contamination. A standard metals analysis on the sample can indicate if any of the elements contained in the system oil (and not in the cylinder oil) are or are not present. On the positive side, new advances in stuffing box design have greatly reduced the event of system oil carry up.

Conversely, you can also see scrapedown results with very low BN values. Over the years, we have analysed scrapedown samples with final BN as low as 5 to 6. The normal expectation would be that corrosive wear would be taking place, and in most cases it clearly is. In a few other cases however, the measured iron values are sustained well within acceptable limits.

The conclusion drawn from these examples is that the amount of acid produced is completely using up the BN additive and that the cylinder is operating under a condition of virtually zero alkalinity reserve. Any change in operation such as increased moisture in the intake air, change in fuel sulphur level or load profile resulting in cooler liner temperatures, could move the wear situation from, “under control” to “excessive” in a very short period of time

Based on the thousands of scrapedown samples evaluated over the years, we have determined BN and iron limits which, if maintained, will normally result in acceptable ring and liner wear. These findings are priority but are represented on a relative scale in Graph 2.

Now here comes the difficult part. Care must be taken in determining the optimum lub oil feed rate. Account must be made for changes in operational and/or environmental conditions, which will impact the BN and iron levels in scrapedown oils, and hence the amount of ring and liner wear being experienced.

For example, take a large bore two stroke running at 80% MCR, using 2.0% sulphur fuel and a low feed rate of 0.7g/kWh in the temperate conditions of northern Europe, which then travels to the more humid conditions of Singapore and picks up 3.5% sulphur fuel, then operates at only 50% – 60% MCR. The question then becomes, how effective is the water separator operating and what effect will the lower MCR have on the liner temperature?

In the worst case, the intake air in Singapore will be more moisture laden and will produce a higher amount of water. When you combine this situation with the propensity of more acid being formed due to the higher sulphur fuel, and then aggravate everything even further by the fact that cooler liner temperatures will produce more condensate, it is likely that the engine will be operating in a much more corrosive environment than it was while in northern Europe. It is very possible that while in Singapore the “optimum” feed rate should be perhaps, 1.0g/kWh while in Rotterdam, going down to 0.6g/kWh is fully acceptable.

Using the above hypothetical, as an example, there is a likelihood that the chief engineer would not know that he had entered into an unsatisfactory wear regime while in Singapore, until it was too late. Why?

Well, let?s assume the vessel is taking scrapedown samples and sending them to a designated laboratory. What often happens is that not until two weeks later will the vessel personnel actually see the results. When you consider that the agent in Singapore has to mail the samples to the lab, the lab has to generate the results and send them to the superintendent engineer, and then he has to review the data and transmit the results back to the vessel, it is easy to see how a couple of weeks could pass by.

By the time the vessel does see the results from the Singapore samples, it is back in Rotterdam. Now the chief engineer sees the high iron values from the corrosive attack that occurred in Singapore, and makes the obvious decision to increase the feed rate from the 0.7g/kWh (which is more than adequate for the Rotterdam area) to, let?s say 1.0g/kWh. Now the engine is in an overfeed situation for that particular operational environment which could cause other problems and is generating undue cost because of the additional cylinder usage.

It is ExxonMobil?s contention that effective feed rate optimisation is dependant on fast, reliable laboratory results getting into the hands of the appropriate onboard personnel as quickly as possible. Although the company uses land based laboratory methods, our preference is to employ onboard analysis of the scrapedown sample using an exclusive portable scrapedown analyser that has been validated against the exacting results of laboratory test equipment. We feel that even when using onboard analyser, periodic confirmation of the onboard results by full laboratory analysis is always advised.

Over the past five years, ExxonMobil has worked with a strategic partner to develop a low cost onboard scrapedown analyser to give the chief engineer the pertinent oil analysis data that is representative of what is currently happening in the engine. It is our belief that the ability to interpret liner condition, virtually as it is occurring, and to be able to factor in numerous other oil and operational conditions, is essential to conducting a feed rate optimisation program.

Feed rate optimisation does not mean setting a feed rate that is lower than what is in place today and then just running on that feed for the life of the engine. More exactly, it means determining the feed rate based on where you are, how you are operating and what you just bunkered. Sometimes you may be able to run at a very low feed and at other times it may be necessary to increase the feed. But in the long run, the yearly average consumption should be lower, with little if any increase in wear rate, if you can monitor all the parameters as they are occurring.

ExxonMobil is actively working with one major OEM in the development of the next generation scrapedown system. The mutual goal is to produce a system that is fully automatic and comprehends all the necessary parameters in selecting the optimum feed rate. The ultimate performance target for this project is to regulate both feed and composition of the cylinder oil based on algorithms developed from actual field experience. Initial full scale testing of the prototype system is scheduled for later this year.

In our industry, lowering cost is a necessity, especially as the market goes through its inevitable cycles. Feed rate optimisation is one way to achieve some of these savings. The key however, is to optimise based on reliable data and to continue to monitor the situation and be prepared to make adjustments when required.