A Discussion of Yu, M., Zhang, J., Joedicke, A., & Reddyhoff, T. (2021). Experimental Investigation into the Effects of Diesel Dilution on Engine Lubrication. Tribol. Int., 156, 106828

The recent rapid progress in the understanding and prediction of film thickness in elastohydrodynamic lubrication (EHL) has been the result of a shift in method from the classical treatment of viscosity as an adjustable parameter to measuring viscosity in viscometers. See for example, Hooke et al. (2021) [1]. For researchers outside of EHL the latter method would appear to be the obvious best approach. However, measuring shear-dependent viscosity in lubricating oils is fraught with great difficulty because there are other phenomena which appear to be constitutive behavior. For example, viscous heating appears in viscometers as the decrease in viscosity with increasing shear rate and in many viscometers this was accurately described by the sinh-law model. The lack of understanding of thermal feedback in viscometers resulted in much wasted effort in EHL friction.