Shear rates in engines and implications for lubricant design

Abstract

By combining shear rate range data in engine components with measured viscosity shear rate curves on lubricants (at different temperatures), useful insights have been obtained on how the viscosity shear rate curve of a lubricant should be “designed” to give low friction (and hence improved fuel economy). A brief review is carried out of typical shear rates in key engine components, which is backed up by the authors’ own calculations (using in-house lubrication software that includes realistic viscosity/temperature/shear rate data). It is found that shear rates in journal bearings are typically in the range of 105 to 5 × 106 s−1, whilst peak shear rates for the piston rings can be as high as 2 × 107 s−1, and for the valve train, peak shear rates can reach 2 × 108 s−1. Accurate viscosity shear rate curves have been measured for different temperatures using a range of viscometers, including a novel mid-shear capillary viscometer that is capable of measuring viscosities in the shear rate range of 104 to 106 s−1. The use of such a viscometer is crucial to obtain good fits to the measured data (since usually, viscosity data are only usually available at low shear rates, 102–103 s−1, and extremely high shear rates, > 106 s−1, and such data are difficult to use for accurate viscosity/temperature/shear rate fits). The implications of the above data are then discussed for the design of low friction lubricants which give improved fuel economy. It is highlighted that the traditional high temperature high shear viscosity, HTHS150, measured at 150 ℃ and a shear rate of 106 s−1, although adequate for bearing durability purposes, is probably not the ideal parameter to use for estimating the fuel economy potential of an oil (since shear rates in engines are usually much greater than 106 s−1, and also because oil temperatures in fuel economy engine tests are usually much lower than 150 ℃). Other possible high shear viscosity parameters are discussed which may be an improvement on HTHS150. The work also highlights that the choice of viscosity modifier, and the amount used, can have a substantial impact on the fuel economy performance of a lubricant.

Shear heating, flow, and friction of confined molecular fluids at high pressure

“…The applied shear rate,γ = v s /h so hereγ ≈ 10 7 -10 10 s −1 . Although lower values ofγ are desirable to overlap with those used in tribology experiments and real components, 58 they are not computationally feasible for extensive parameter studies. 23,32 The T was controlled using a Langevin thermostat 59 set to 350 K, which is representative of the EHL regime.…”

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…TTCF involves averaging correlation products from many independent NEMD trajectories, making it rather computationally expensive. However, the main advantage of TTCF is that it allows very low shear rates to be simulated compared to NEMD, allowing direct overlap with those used in experiments and real components [42]. Indeed, it has been successfully applied to study the viscosity of short alkanes (n-decane) at shear rates as low as 10 5 s −1 [81].…”

“…The ability to simulate lower shear rates has been a long-term goal of NEMD simulations to facilitate direct overlap with experiments and real components [32]. For comparison, the high pressure viscosity of lubricants can generally only be measured up to shear rates of approximately 10 4 s −1 [37,38], tribology experiments can reach up to around 10 6 s −1 [41], and high-performance engine components can extend up to 10 8 s −1 [42].…”

Shear Thinning and Hydrodynamic Friction of Viscosity Modifier-Containing Oils. Part II: Impact of Shear Thinning on Journal Bearing Friction

“…However, oil #10 shows low friction because it has a very high viscosity index, which means that, since all the oils have the similar viscosity at 150 °C, it has a relatively low viscosity at lower temperatures. These two different methods by which VMs may reduce friction have been previously noted by Taylor [19]. Figure 15 shows an attempt to quantify these two different friction reduction contributions for oils #1 to #10 at 3000 rpm, 2 kN load and 60 °C.…”