Lube-Tech Figure 3: Simulated ΔBSFC and bearing health maps for a modern passenger car engine [11]
lubricant viscosity is increased engine wear [8-15]. The hydrodynamic lubricant film thickness is directly proportional to lubricant viscosity. Therefore, to maintain hydrodynamic lubrication, substantial modifications in the engine hardware are often required including surface finish specifications, bearings, filtration systems, and oil pump, galleries and squirters. Without that the risk of excessive wear is real and cannot be ignored.
Figure 3 shows the simulated brake-specific fuel consumption (ΔBSFC) map for a modern passenger car engine and simulated bearing health map for the same. In the middle, the change in BSFC on changing from SAE 0W-20 (left hand side graph) to SAE 0W-8 (middle graph) is shown, the green area corresponding to improved fuel economy, the red area to degraded fuel economy. Up to 20% reduction in BSFC is feasible. Unfortunately, the maximum effect is restricted to medium-to-high engine speeds and low load. Such conditions apply if the engine is revved in neutral. Close to the engine “sweet spot” – the area around 3000 rpm and 60% load where the engine reaches the lowest specific fuel consumption – the effect is reduced significantly. However, the most troublesome observation is the red area at low rpm and high engine load, since this does not only signify a degraded fuel economy but also an elevated risk of wear as confirmed by the main bearing health simulation (right hand side graph).
The above example shows that it is under the low 26 LUBE MAGAZINE NO.162 APRIL 2021
speed – high load conditions that lubricant film may fail. Problems at high speed are mostly associated with inadequate oil pump capacity and can be addressed by using variable speed pumps. At high engine speeds, inertial forces acting on the reciprocating piston assembly and connecting rod and cavitation effects also play an increased role in wear and this may cause problems with the small end of the connecting rod/wrist pin interface and bearings. However, in general, lower viscosity lubricants tend to be less prone to cavitation.
Since the hydrodynamic film collapses when there is no relative motion between the rubbing surfaces, wear problems associated with the introduction of low viscosity lubricants are further aggravated due to automatic stop-start technology. Use of electric oil pumps and roller bearings for camshaft and balancer shaft helps mitigate the issue. Roller bearing supported crankshafts were also tried but found to be impractical.
Crankcase lubricants are formulated to balance a large number of different properties, a conscious and unavoidable paradigm shift from “being best at something” to “being good enough at everything”. Since fuel efficiency is viewed as an extremely important performance aspect – in fact, many OEM approvals explicitly demand it – the transition to lower viscosities will continue. It should be recognised, however, that there comes a point where fuel economy oils do not make much economic sense for the end
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.133 page 4
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