| Abstract: | Future diesel engines will be operated at higher cycle temperatures. These higher cycle temperatures will invariably pose tribological problems of the top ring, piston, valve seats, and valve guides. Current lubricating oil with a thermal oxidative stability up to 204°C will be inadequate: polyol ester-base formulated synthetic oil, such as Stauffer Chemical SDL-1 or US TACOM MRI-1, with 310°C thermal oxidative stability could possibly be the next generation of lubricating oil. However, as we get nearer to the time when diesel engines will be constructed in advanced structural ceramics or composites for ‘adiabatic’operation, the thermal stability of the lubricating oil will have to approach 445°C. To withstand a top ring reversal temperature beyond 310°C, polyphenyl ether base oil and other high-temperature liquid lubricants are sought, but the highly aromatic polyphenyl ether type oil has thus far shown little promise for this application. The hybrid piston with solid lubricated top ring has potential. Densified Cr2O3 on Cr2O3 has demonstrated the possibility of operating at 380°C top ring reversal temperature, with acceptable wear and life. NASA PS212 with Stellite 6B has also been tested without a liquid lubricant. The tribological needs of the next generation of high-output, high-temperature diesel engines can therefore be met with the synthetic polyol ester base, cost-effective, formulated lubricant. The adiabatic ceramic engine will require much higher temperature capability. To this end, tribologists will have to direct their efforts to other higher temperature liquid lubricants and to solid lubricants. A laboratory coefficient of friction of < 0.06 must be achieved in order to maintain acceptable brake-specific fuel consumption. This target is based on cast iron on cast iron with lubricating oil, and it must be achieved with acceptable wear and cost. |
