Electrical techniques for monitoring the condition of lubrication oil

摘要

The lubricating oil used in engines for vehicle and other applications is renewed according to a schedule specified by the manufacturer. This timetable is, naturally, very conservative, and makes no allowance for the way in which the engine is operated. Constant-speed operation (such as motorway use) is much less harmful to the lubricant than variable-speed operation, such as urban driving, during which the oil experiences extreme variations of temperature and engine speed. The net result of the conservative lubricant replacement schedule is that most engine oil is discarded well before it has reached the end of its useful life. This paper reports a study in which changes to the dielectric and magnetic properties of the oil are assessed as methods of measuring the degradation of lubricating oil. The relationship between oil use (measured by the distance a vehicle has travelled) and oil viscosity is also measured. The conclusions from this work are that simple distance travelled (miles/kilometres) is not a good indicator of the state of an oil, as estimated by measuring its viscosity. The magnetic characteristics of lubricating oil (i.e. its magnetic permeability) do change as the oil degrades, but the measurements were poorly correlated with viscosity and do not seem to offer much promise as the basis of an oil monitoring system. The dielectric properties of lubricating oil are reasonably well correlated with viscosity, and it is proposed that this could form the basis of a useful sensing technique.