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Breakthrough in understanding oil stress in two-stroke marine diesel engines
Shell has been active in developing marine fuels and lubricants since the beginning of the last century, having opened its first dedicated marine testing facility in the 1920s.
Through the years, its commitment to innovation in the marine industry has been supported by scientists at the Shell’s Marine and Power Innovation Centre (MPIC) in Hamburg, Germany.
The pace of change in the marine environment is faster now than at any time in the past – with serious implications for engines manufacturers, lubricant developers and ship operators alike. Indeed, understanding the impact of these changes is critical to anyone developing products for the marine industry.
The global recession has put pressure on the shipping industry to move goods and raw materials around the world reliably and at a lower cost. At the same time, growing environmental concerns are setting in motion new legislations resulting in different fuel specifications, changes in engine hardware and tougher emissions targets for shipping operators.
To reduce costs, shipping operators today are employing slow steaming or even ultra slow steaming, and are looking to lower cylinder oil feed rates and extend maintenance periods without impacting their operational reliability.
Slow steaming is one of the factors that place increased stress on the engines, which are typically designed to run at higher service speeds. A new generation of better designed marine lubricants is clearly needed to address the changing operational environment.
Oil stress is the single most important factor that governs the lifetime and performance of an engine lubricant. While oil stress in four-stroke marine diesel engines has been well documented, oil stress in two-stroke engines has been much less researched and understood.
In contrast to the two-stroke engine, the four-stroke engine has an oil system where oil is supplied to the piston / cylinder running surfaces and then returned to a sump. As the sump volume in a four-stroke engine is quite large and the oil is homogeneous in its composition, this allows meaningful monitoring of the used oil condition over an extended period of time and the development of the oil stress model in these engines.
Because the two-stroke engine does not have a sump, it has been a challenge to adapt the existing four-stroke model to the two-stroke case. The two-stroke engine has a total loss lubrication system where a small amount of oil is injected, used and then drained away.
This cylinder drain oil from two-stroke engines has successfully been monitored for many years in terms of providing wear data, but due to the possibility of contamination of this oil by the system oil – and potentially cross contamination of drain oil within the engine – it has been difficult to get reliable information from this source to understand the actual oil condition on the liner surface of individual cylinders.
The key step is to first visualise all the lubricating oil in the cylinder as “the sump” (called a “mini-sump”) and then to understand how much oil is actually involved in the lubrication process before applying oil stress calculations. However, to do this requires an understanding of the condition of the used oil on the cylinder liner surface.
“Ring spray” sampling at two positions in the Bolnes 3 (1) DNL 170/600 research engine.
The recent Research & Development collaboration between Shell and Wärtsilä has provided new insights that significantly improve the understanding of oil stress in two-stroke diesel engines. This comprises an innovative sampling system and an oil stress theory which the authors believe defines the stress factors that the cylinder oil experiences, namely Humidity Stress, Acid Stress, Insoluble Stress and Thermal Stress. This pioneering work will enable more effective lubrication strategies to be developed in the future for low speed two-stroke diesel engines.
It all started with the observation of an unusual white “mist”, which appeared every time the piston rings went past the charge air inlet ports in the cylinder liner. A special sampling system, designed and developed by Wärtsilä, enabled this “mist” to be captured for analysis. The system consists of a specially placed pipe connected to the outside of the engine via a liquid/air separator.
Due to the differential pressure between the under-piston space and the engine room, the “mist” – now known as “ring spray” – was drawn along the pipe and through the separator, which collected any liquid (oil) present in the sight glass, resulting in the collection of a sample of oil called a “ring spray sample”. This new sampling method was also fitted to Shell’s Bolnes two-stroke research engine in the MPIC, Hamburg.
When the oil was analysed, it was found to be very different in characteristics compared with oil collected through normal drain analysis. The oil from the ring spray oil sample was much more stressed than oil collected from existing drain oil sources. It was also found that the characteristics of the used oil collected in this way varied greatly under different ambient conditions – even in the same engine, running on the same fuel.
This new insight was very important from the perspective of understanding not only how stressed the oil was, but also the types of stress factors that come into play. Data was collected from ten different vessels and results showed that properties of the used oil collected were highly dependent on whether the vessel was operating in a dry or humid area.
Used oil samples which were collected using the ring spray system showed significant differences when compared to samples collected using piston underside drain sampling, especially in terms of water content, Base Number (BN) depletion and iron content, confirming the importance and efficiency of the new ring spray oil sampling system for understanding and quantifying oil stress.
While oil stress theory in a four-stroke engine typically comprises three stresses – Thermal / Oxidative Stress, Asphaltene Stress and Acid Stress – oil stress in a two-stroke engine is a much more complex and rapid process. This study revealed that there are four types of stress in two-stroke engines to consider – Humidity Stress, Insoluble Stress, Acid Stress and Thermal Stress.
In the case of the four-stroke engine, the lubricant is recycled – scraped down by the oil control ring back into the sump – and the resulting Effective Lubricant Refreshment Rate (ELRR) of the oil film on the cylinder liner is extremely high when compared to that in the two-stroke engine. This physical difference in ELRR rates is the single most important reason for the much higher level of oil stress in the two-stroke engine build-up in a very short space of time.
Since the amount of BN throughput for the oil film in a two-stroke engine is significantly lower than in a four-stroke engine, Acid Stress builds up at a much faster rate. There is a rapid depletion of BN in the cylinder oil film as it races to neutralise the acid from the combustion process.
However, the neutralisation process creates a significant amount of insoluble calcium sulphate – more so than in the four-stroke engine which has a higher ELRR. To prevent the insoluble material from affecting the lubricity of the oil, the detergent system has to provide enough dispersancy to ensure that insoluble stress is well controlled to prevent deposits forming.
Building on Shell’s previous work around four-stroke engine oil stress, this latest research done jointly with Wärtsilä has revealed that Humidity Stress has a significant interaction with Acid Stress and Insoluble Stress to further challenge lubrication of the two-stroke engine. It has been found that there is a correlation between BN depletion and intake air humidity, in relation to the sulphur value of the fuel. Differences in BN depletion values have been attributed to changes in ambient conditions, and it can be seen that operations in areas of high absolute humidity can lead to much higher oil stress levels overall.
This study lays the foundation for the development of more effective cylinder lubrication strategies in the future. Shell’s research here has significantly added to the marine industry’s understanding of how to optimize lubrication in the two-stroke engine and highlights the importance of understanding and dealing with all aspects of oil stress in order to enjoy trouble free operations, now and in the future.
As shipping operators continue to adopt slow steaming to reduce fuel costs and CO2 emissions, further optimisation of the lubrication process will surely take place to maintain and ensure reliability of operation. The performance capability of marine lubricants will become increasingly important to shipping operators as they seek to balance economic, environmental and technical requirements. This provides an opportunity for development of a “next generation” cylinder lubricant to meet the emerging challenges of the industry in the 21st century.
In 2013, Shell also launched two new products - Shell Alexia S5 and Shell Alexia S6, high performance cylinder oils designed to help protect the latest engine designs from increased acid stress and corrosive wear.