The Environmental and Economic Importance of Mixed and Boundary Lubrication

Abstract

One route to reducing CO2 emissions is to improve the energy efficiency of machines. For example, conventional combustion engines are being downsized (and also down-speeded), and are now running on lower viscosity lubricants (such as 0W-20 or lower viscosity grade lubricants) and often also have stop-start systems fitted.
Some of these changes may result in higher levels of mixed and boundary friction, and so estimating the friction losses due to mixed/boundary friction, and the corresponding wear levels, is becoming of increasing importance. There is recent experimental evidence that traditional approaches (such as the Greenwood & Tripp model [1]) to predicting friction in mixed and boundary friction tend to underestimate these losses [2-5].
A new model is described, based on experimental data, that estimates the proportion of mixed/boundary lubrication, X, as a function of the lambda value (where lambda is the ratio of the oil film thickness separating the surfaces to the combined root mean square roughness of the surfaces). The precise equation that describes the way in which X varies with lambda takes the form of a “reverse S-curve” which makes sense physically since S-curves arise naturally in growth processes and the real area of contact of rough lubricated surfaces grows as 1/lambda increases.
Numerical estimates of the amount of mixed/boundary lubrication losses in internal combustion engines are made and compared with recently published experimental data [2, 6]. In addition, these improved calculations are used to estimate both the financial cost of mixed/boundary lubrication for today’s vehicle fleet, and the CO2 emissions associated with these losses.