2. CHANGES IN ENGINE DESIGN
Developments in material sciences and manufacturing processes have allowed improvements in engine cycle efficiency to be gained. Such developments result in lower fuel consumption and for the same engine bore up to twice the power output. One way to visualise the rate of change is to make a comparison between the performance of engines of different ages by the use of two factors that embody the major parameters of performance. These factors are the power factor defined as the power in kW developed in a piston divided by the area of the piston (in units of kW/cm squared) and the load factor defined as the mean pressure in bars during the combustion and expansion stroke multiplied by the mean piston speed in metres per sec (in units of bar x m/s). These are plotted at Fig 1 along with dates at which the powers and pressures were achieved.
Controlling the combustion space conditions, has a direct impact on lubricant performance and the component life of pistons and liners. An accurate assessment of conditions inside the combustion space to which detail engine design and lubricant development can be applied is fundamental and in the future the lubricant performance will have an even more critical role.
There are four major areas where recent engine design changes have taken place and their impact on lubricant performance is described below:
・ Improvements in combustion to reduce carbonaceous deposits on pistons crowns and crown lands and to a lesser extent in piston ring grooves. This has been achieved primarily by fuel pump design and variable injection timing. Additionally some benefits can be obtained through improved atomisation using water injection. Such improvements reduce the likelihood of carbon scraping the cylinder oil from the surface of the liner.
・ Introduction of carbon control or anti polish rings now being fitted to all large bore engines. Such rings aim to control the amount of carbon on piston crown lands. If the carbon builds up to sufficient thickness it will rub the oil off the liner surface during the stroke.
・ Control of surface temperatures of liners and pistons surfaces and piston ring grooves. As a consequence of the increase in the energy density in the cylinder from the combustion of more fuel the heat flow through the liners and thus the liner surface temperature are higher. Current engines have liner surface temperatures of up to 250 C and this could be set to increase to an estimated 280 C or even higher. Localised hot spots could well lead to thermally overloading the lubricant, causing oxidation and carbon deposits. If such deposits occur in piston ring grooves the subsequent effect is rapid adhesive wear due to restricted ring movement giving rise to high contact pressures.
・ The tribology of the running surfaces of piston rings and liners is critical. The surface geometry, metallurgy of surface coatings and the common use of gas tight rings are all part of piston ring development whilst fully honed liners are now the preferred standard. These developments indicate the importance now placed on the tribology of rings / liners interface and lubrication.
The trends to higher engine pressures and temperatures and the increased liner surface area have been reflected in the generally upward trend in the amount of cylinder oil required to provide adequate lubrication. For example if with a stroke to bore ratio of 2:1 the minimum recommended cylinder oil feed rate was 0.6 g/bhp hr then it is quite reasonable to expect the feed rate to be increased to 1.2 g/bhp hr. when the stroke to bore ratio is over 4:1. Engine designers are now looking at ways to reduce cylinder oil consumption because for large bore high power engines it represents a not insignificant ship operating cost. Oil consumptions as low as 0.6 g/bhp at MCR are envisaged which puts even more stress on the cylinder lubricant
The demand from container ship operators for even more powerful engines for vessels typically up to 12000 teu and 27 knots, but without large increases in the space required, will necessitate higher outputs. The two-stroke marine diesel engine has already been developed to be the most thermally efficient internal combustion engine so performance increases will be difficult to obtain. The likely power output for such large container ships would be in the region of 140,000 bhp a 50% increase on todays most powerful engine. At Fig 2 some scenarios are given to illustrate the difficulty in achieving such power outputs with engine designs not dissimilar to those currently being produced. However one simple conclusion is that combustion space pressures and surface temperatures are very likely to be increased which will place greater emphasis on the performance of the cylinder lubricant.
3. LUBRICANT PERFORMANCE AND LIMITATIONS
3.1. Oil Film Thickness
The thickness of an oil film generated between two moving surfaces is a function of the oil viscosity, the relative speed and the load. In the case of the lubrication of cylinder liners and piston rings the factors that most influence oil film thickness are viscosity, relative speed and load. Viscosity, and consequently oil film thickness, has a major influence on the load at which adhesive wear starts and the higher the viscosity the greater the load that can be carried. Fig. 3 shows the temperature viscosity characteristic of a typical cylinder oil whilst Fig. 4 illustrates the general relationship between the load carrying ability of cylinder oils and the cylinder lubricant viscosity at different liner surface temperatures. The relationship is not linear but shows a significant reduction in load carrying ability for lower viscosities (thinner oil films).
Around top dead centre the piston speed is low and the liner temperature is high which means that full hydrodynamic lubrication is not developed. These are the conditions for boundary lubrication and the consequence mean that adhesive wear is more likely to take place in this area of the liner. Adhesive wear is the condition in which two surfaces normally separated by an oil film come into contact and friction and wear occur. In Modern high temperature two-stroke engines the oil viscosity on the liner surface can be below 2 cSts near top dead centre with the resultant film thickness being less than one micron. Even under the conditions of thin oil films adhesive wear will be minimised provided the surfaces contact loads are not excessive and the surfaces are not rough.
