Lube-Tech
Boundary lubrication is where the applied load is entirely supported by the rough metal asperities and is generally accepted to occur if λ < 1. In mixed lubrication, some of the applied load is supported by the rough metal asperities and some is supported by the fluid film. Mixed lubrication tends to occur when 1 < λ < 3. When the rough metal surfaces are completely separated by a fluid film (which is thought to occur when λ > 3) then all the applied load is carried by the fluid film.
Most machines are designed to have full fluid lubrication, so as to ensure a long machine lifetime. In an effort to improve energy efficiency, the viscosity of lubricants has been decreasing over the last 20-30 years (see Table 1), since a reduction in lubricant viscosity results in a decrease in friction losses. However, a decrease in lubricant viscosity also results in a thinner oil film separating the moving surfaces, and so there is an increased risk of mixed and boundary lubrication.
There is therefore increasing interest in being able to accurately estimate mixed/boundary friction losses, and in methods/materials that can help decrease mixed/boundary friction, as described in the following sections of the paper, and the costs of mixed/ boundary friction are estimated, together with the expected CO2
emissions that may arise from mixed/ boundary friction. s-1 .
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.152 page 2
Table 1: Table showing how viscosities (for passenger car lubricants) have been decreasing over the last three decades. The High Temperature High Shear Viscosity is measured at a temperature of 150°C and a shear rate of 106
Mixed/Boundary Lubrication in Real Machines Although Figure 2 shows a useful schematic of how the friction coefficient should vary with λ, it is also useful to measure friction losses in real machines. Figure 3 shows measured total engine friction losses (measured as a Friction Mean Effective Pressure – FMEP – in bars) versus engine speed (revs/min) for a 1990’s 2.0 litre gasoline engine, when lubricated with a 0W-8 lubricant (which does not contain a friction modifier), with a sump oil temperature of 93°C (representing a fully warmed up engine) [6]. Figure 3 also shows, separately, the fluid film friction and the mixed/boundary friction (in an engine, most of the mixed/boundary friction will come from the valve train and piston assembly).
Figure 3: Measured Friction Mean Effective Pressure (FMEP), versus engine speed (revs/min) of a 1990’s 2.0 litre gasoline engine when lubricated with a 0W-8 oil, with a sump temperature of 93°C [6]. The effect of mixed/boundary lubrication can clearly be seen for engine speeds lower than 1000 rpm.
Figure 2: Schematic curve showing how friction coefficient varies with λ.
Friction coefficient can also be measured in laboratory tribological test equipment. A typical example is shown in Figure 4, where the friction coefficient of
32 LUBE MAGAZINE NO.181 JUNE 2024
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