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Lube-Tech


Effect of motor oil on fuel economy A significant portion of the energy losses in internal combustion engines comes from the shearing of the engine oil. Hence a continuing trend toward lower viscosity oils [4]. However, whereas the use of low viscosity oil helps reduce friction losses, it increases the tribological stresses on engine components. This necessitates wider use of friction modifiers (FM) and antiwear (AW) additives in lubricant formulations to help protect the engine from wear [5]. The development of balanced formulations is not as straightforward as it appears, and numerous pitfalls may be encountered due to additive interactions. Figure 1 explains how “fuel economy” motor oil works: on the left-hand side is shown the actual torque curve of a typical production 1.6L gasoline direct injection (GDI) engine, and on the right-hand side is shown the friction torque loss for the same engine measured using a motored rig (an engine run by an external motor, with no fuel supplied to the engine during the motored test). Friction “eats up” around 1/10th of the useful torque the engine can produce. Below 2000 rpm, where the contribution of mixed and boundary lubrication is significant, friction can be reduced by using FMs or low friction coatings. Above 2000 rpm, when the hydrodynamic lubrication


PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE


No.133 page 2


prevails, friction can be reduced by using lower viscosity lubricants.


Various engine drive cycles have been developed and are used to compare fuel economy between different vehicles. In Europe there is the New European Driving Cycle (NEDC), in the US the EPA has several cycles for city and highway and in Japan the JC08 is used. In an attempt to harmonise the cycles, the Worldwide Light Vehicle harmonised Testing Procedure (WLTP) has also been developed.


For passenger cars, a change from legacy SAE 15W-40 grade to SAE 0W-20 brings on average 3-4% improvement in fuel economy under the NEDC or EPA conditions, and the subsequent migration to 0W-8 can bring an additional 2-3%, provided that engine hardware can safely handle such low viscosity. Under more gentle driving in the JC08 cycle, lower viscosity oils may produce even larger effect, up to 5%. On the contrary, for the more aggressive WLTP cycle, the effect is usually reduced by 0.3 to 0.6% compared to the NEDC.


Since the fuel economy performance of an oil depends largely on the engine design, vehicle type,


Figure 1: The torque curve and the friction torque for a production 1.6L i4 GDI engine. The primary engineering strategies for friction reduction are also shown [6]


24


LUBE MAGAZINE NO.162 APRIL 2021


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