Correlating Viscosity to Fuel Efficiency in the Daimler OM501 Heavy-Duty Diesel Engine Fuel Efficiency Test and the Influence of VII 2021-28-0015

Durability remains a primary concern when formulating heavy-duty (HD) diesel engine oils, but in future there will be increased attention to fuel efficiency, particularly in Europe where the European Commission is proposing the first ever CO₂ emission targets for heavy-duty vehicles.

Although there are no internationally recognised fuel efficiency tests for HD diesel engines, there have been some regional and OEM developments pushing in the direction of improved fuel efficiency. In Japan the relatively new JASO DH-2F standard adds a fuel efficiency requirement, measuring fuel efficiency using the Hino N04C engine which is also used within the standard for other performance testing. In North America API have introduced the FA-4 performance standard to allow users to specify an xW-30 oil of lower HTHS150 to help achieve fuel efficiency, but with no accompanying test to quantify. In Europe ACEA are planning a HD fuel economy “F” classification which may be something like API FA-4. Volvo and Daimler have developed their own fired engine tests to determine the fuel efficiency effect of the oil. Against this background, we selected the Daimler OM501 fuel economy test as a relevant and important option and used it for the work reported here.

A range of multigrade engine oils were formulated for evaluation in the OM501 fuel economy test. In addition to targeting the usual HTHS150°C value of ≥3.5 mPas. normally associated with xW-40 oils used in HD diesel engines, HTHS150°C of ≥2.9 mPas. as mentioned for xW-30 in API FA-4, and also lighter oils of HTHS150°C ≥2.6 mPas were included. In addition, we used a range of different VII (Viscosity Index Improver) types to get a variation in lower temperature viscosity values even when the HTHS150°C values were set as mentioned above. We used group III type base-oil and a suitable DI package throughout and the VIIs used included those recommended for such oils, plus fuel efficient PAMA comb type VIIs.

Results broadly show a strong dependence of fuel economy to viscosity in this test. This also applies to the lighter HTHS150°C ≥2.6 mPas. oils which also looked to be providing predominantly hydrodynamic lubrication and so were not improved in fuel economy by the application of organic friction modifier.

Furthermore, the viscosity when measured by HTHS80°C and HTHS100°C provided an excellent correlation to fuel economy (R² ~ 0.98) when viewed as a simple linear plot. All other viscosity values such as HTHS150°C (R² ~ 0.68) and KV100C (R² ~ 0.72) did not correlate as well. VI has no correlation to fuel economy. It was demonstrated that oils with the same viscosity protection in terms of HTHS150°C could have a significantly different fuel economy performance due to the differences in HTHS80°C and HTHS100°C brought about by the use of different VII types or different formulation strategies. Clearly, reducing the HTHS80°C and HTHS100°C gives an improvement in fuel economy for this engine.

The relationships established in this paper will be of use within the industry in predicting the likely OM501 fuel economy performance in advance of, or instead of actual OM501 testing. The HTHS80°C or HTHS100°C value of the formulation can instead be used to predict the likely OM501 fuel economy result.

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