ABSTRACT
The key performance feature of an engine oil readily visible to the motorist or truck driver is its control of its own consumption. If oil consumption is high it is soon noticed by dip stick checks, oil deposits on parking places, and high levels of exhaust emissions such as blue smoke. If as a result the oil level in the engine sump gets too low, engine damage will result. To most people low oil consumption is synonymous with the oil maintaining good engine performance with low levels of engine wear and low running costs.
Control of oil consumption has always been important, but today it is even more so because of its possible adverse effect on exhaust emissions and the life of exhaust catalysts and particulate traps.
An oil helps control oil consumption by both its additive chemistry and its physical properties. The additive chemistry helps by minimising piston ring and cylinder liner wear, the prevention of bore polishing and piston ring sticking, and ensuring oil seals on the valve stems and crankshafts continue to work effectively. Tests to ensure good oil performance in these areas were developed over many years and they are briefly reviewed in the paper. Of the physical properties of an oil, viscosity and volatility are the two key characteristics known to be important in the control of oil consumption. Over many years viscosity per se was held to be the all important factor, but with the continued trend to lower viscosity oils for improved fuel economy and lubrication at engine start, control of volatility is recognised as the key factor in achieved low oil consumption in today’s and tomorrow’s hotter running engines.
The paper reviews the key factors affecting oil consumption and the various tests developed to ensure acceptable levels in service in the field under arduous operating conditions. It also reports on a programme of work investigating the relative influence of viscosity and volatility of multigrade oils in different passenger car engine types. The principal findings were that volatility was a more important factor influencing oil consumption than viscosity and that this applied to most passenger car engine types, not just to air-cooled engines. Linear equations were developed for each engine type relating oil consumption to fresh oil volatility at 375°C (ASTM-D 2887) and fresh oil kinematic viscosity at 100°C. These gave an idea of the relevant importance of volatility and viscosity in the different engine types tested.
