Fuel Economy Engine Oils: Scientific Rationale and Controversies

Since a significant part of energy losses in the internal combustion engine comes from viscous dissipation, the trend has shifted toward low-viscosity oils from SAE 40 and 50 in the 1960s-1980s to current SAE 20 and lower viscosity grades. Use of low viscosity engine oils significantly reduces energy losses in the main bearing and piston/bore systems, while tribological stresses on the valvetrain - especially in flat-tappet cammed engines - may increase. This makes a strong argument for deploying new classes of friction modifiers and antiwear additives. However, development of a balanced formulation is not as straightforward as it appears, and numerous pitfalls may be encountered due to additive interactions. Another serious problem is that the definition of “fuel-economy engine oil” is rather vague, as it depends on choice of reference oil. Nowadays, the assessment of fuel economy is based on the Sequence VIE or VIF tests using a 2012 3.6L GM V6 gasoline engine. It is not unexpected that the results of this test turn to be largely misleading when extrapolated to modern heavily boosted low-displacement engines. Hence, many OEM-specific fuel economy tests also exist and different engine designs often produce controversial results. Furthermore, the “fuel economy” performance of the same oil in the same engine may change dramatically depending on the driving cycle. For instance, low viscosity oil may boost fuel economy at cruising speeds (high speed / low load limit) and degrade fuel economy during aggressive city driving (low speed / high load).

All the aforesaid circumstances are to be taken into account when trying to harmonize normative performance claims with customer expectations.

Introduction

New fuel economy standards for automobiles erected by governments in the G20 major economies and change in customer preferences driven by high fuel prices together with carbon taxes put increased pressure on car makers. In the USA, National Highway Traffic Safety Administration (NHTSA) and Environmental Protection Agency (EPA) have recently issued the Safer Affordable Fuel-Efficient (SAFE) Vehicles Rule that sets tough fuel economy and carbon dioxide standards. These standards apply to passenger cars and light trucks and set a moving target that is going to increase 1.5% in stringency each year from model years 2021 through 2026. Noteworthy, recognizing the realities of the marketplace, the expectations bar has been lowered to 40.4 mpg projected overall industry average required fuel economy in MY 2026, compared to 46.7 mpg projected requirement under the 2012 standards. The latter was again lower than the initial 2025 EPA targets of 62 mpg announced a decade ago – which was soon afterwards reduced to 56 mpg.

This shows that the progress is rather painful and the overambitious targets may not be achieved without a solid technological foundation and powerful financial incentives to drive the change.

Other markets follow the same trend, see Fig. 1.

Fig.1 Comparison of fuel economy standards in key vehicle markets (Source: ICCT, September 2019)

In Europe, the European Parliament and the Council adopted Regulation (EU) 2019/631 that sets CO2 emission performance standards for new passenger cars and new vans for 2025 and 2030. From 2021, the EU fleet-wide average emission target for new cars is set at 95 g CO2/km. This corresponds to a fuel consumption of around 4.1 l/100 km (57.4 mpg) of petrol or 3.6 l/100 km (65.3 mpg) of diesel. Today’s average CO2 emissions for new cars sold in EU is around 120 g CO2/km. Car manufacturers pay the penalty is €95 for each g/km in excess of the target.

Japan’s new fuel economy standards issued a year ago set a target for average fleet gasoline-equivalent fuel economy at 25.4 kilometers per liter (59.8 mpg) by 2030, some 30% improvement over today’s fleet average.

These political and economic factors intensify research and development efforts taken by major OEMs in their pursuit for better fuel efficiency. Apart from concerted efforts on powertrain electrification and the use of alternative energy sources to reduce greenhouse gas (GHG) emissions, a big emphasis is made on understanding tribological aspects of energy losses in powertrain and utilizing current advancements in lubrication engineering and coatings to minimize those losses. To encourage such eco-innovation, manufacturers are granted “emission credits” for deployment of innovative technologies that should - based on independently verified data - result in reduced CO2 emissions even though the test procedure used for vehicle type approval fails to demonstrate any effect. Besides that, manufacturers are granted “super credits” for putting on the market zero- and low-emission cars (BEV, PHEV) emitting less than 50 g CO2/km.

Development costs, material costs and production costs are always important factors when market potential of one or another approach is to be assessed.

Approximately one third of fuel consumption in cars is due to friction losses [1] with powertrain friction being one of the chief culprits, see Fig. 2.