TS-149
The Role of Temperature and Pressure in Wear Processes in Low Speed Diesel Engines
Cees SCHENK*, Jan HENGEVELD* and Kjeld AABO**
ABSTRACT
Good piston ring and cylinder liner condition in modern low speed two-stroke diesel engines is of ever increasing concern for today's operator. With the continuing advance of engine design over the past 20 years combustion chamber temperatures and pressures are moving up. The effect this has on the wear processes at the ring-liner interface is discussed. Focus is on two wear mechanisms in particular. One is corrosive wear as a result of condensing sulphuric acid and the other is adhesive or scuffing wear. The risk of corrosive wear is put in perspective by applying newly calculated dew point curves. Scuffing wear is discussed on basis of results from a novel boundary lubrication test in relation to actual field trial data. The conclusions summarise the engine conditions and oil requirements that are needed to allow for a safe operating window.
Key Words: Low Speed Engine Design, Component Temperature, Combustion Pressure, Corrosive Wear, Acid Dew Point Calculation, Scuffing Wear, Film Failure Testing Method
1. INTRODUCTION
Low speed two-stroke diesel engines have since long maintained a dominant position in the market for ship propulsion. Since 1970 their share in power installed in new-buildings has risen from a level of around 50% to 80% in 1998. The upward trend in the size and service speed of general cargo ships and in particular of container vessels [1]has favoured the selection of the low speed engine as the main propulsion unit of choice. With global containerisation further increasing, combined with a continued race amongst competing shipping lines to transport as many teu's as possible with lowest possible unit freight cost, average power requirements for such ships is expected to increase even further.
The rapid developments in recent years in low speed engine design have therefore focused on increased power output, low manufacturing and installation costs (simplicity in design and installation procedures) and reduced operating costs (reliability and fuel and lube oil efficiency).
This has resulted in operating conditions creating ever increasing stress levels for the lubricating oil. Thus, the actual lubricant's viscosity at operating conditions is reduced and the loads it must withstand are increased, while longer piston strokes have also greatly increased the time the lubricant must cope with extreme temperatures and corrosive acids originating from combustion of sulphur rich fuels.
The increased component temperatures are generally perceived to have had a beneficial effect on the control of corrosive wear. On the other hand, the higher temperatures and loads have also increased the possibility that the lube oil film cannot be maintained under all circumstances and in such a case metal-to-metal contact between piston ring and liner will occur resulting in uncontrolled adhesive wear, or scuffing. It is the challenge of the engine designer to create conditions that provide a good balance such that the negative effects of both wear processes are minimised. Although many of today's engines operate seemingly trouble free, there are nevertheless occasional reports of corrosion and of scuffing problems with certain engine series under certain operating conditions.
It is the purpose of this paper to highlight the developments in low speed engines in the last decade with particular attention for component temperatures and combustion pressures. Subsequently, the effects of the more severe operating conditions on the two main wear mechanisms is discussed in more detail. It is also illustrated that developments in lubricating oil technology are keeping pace with engine developments.
*Shell Global Solutions International B. V., Shell Research and Technology Centre, Amsterdam, P. O. Box 38000, 1030 BN Amsterdam, The Netherlands. Fax: +31 20 630 33 54, Email: cees.c.schenk@opc.shell.com
**MAN B&W Diesel A/S, Denmark
