An important quality feature of a crankcase engine oil is its ability to control oil consumption. To the average motorist low oil consumption is synonymous with the oil maintaining good engine performance, low levels of engine wear, low running costs and a clean driveway. In the current environment it has assumed major importance because of the possible adverse effect of oil consumption on exhaust emissions.
An oil controls oil consumption by both its chemical and physical properties. Additive chemistry helps by minimizing piston ring and cylinder bore wear including bore polishing, the prevention of piston ring sticking and ensuring oil seals continue to work effectively. Tests to ensure control of these hardware variables were developed during the early 1980's. These test methods are briefly reviewed. Of the physical properties of an oil, viscosity and volatility are the two key characteristics known to affect oil consumption. In the past viscosity was held to be all important, but now with the trend to lower viscosity oils for improved fuel economy and low temperature operation, control of volatility is recognized as one key factor in achieving acceptable oil consumption in the hotter running engines of today and tomorrow.
The paper reviews the factors affecting oil consumption and tests and their requirements aimed at producing acceptable levels in service. It also reports on a program of work looking at the relative influence of viscosity and volatility of multigrade oils in a number of European and North American cars. The principal findings are that volatility is a more important parameter influencing oil consumption today than viscosity and that this applies to most passenger car engine types not just air-cooled engines. Equations relating oil consumption to fresh oil volatility at 375°F and fresh oil kinematic viscosity at 100°C for each engine type were developed. A precise gas chromatographic method (ASTM D 2887) for measuring fresh oil volatility was used successfully to develop these relationships. The program also demonstrated a virtually identical oil consumption rate for oils formulated with