Engine efficiency leaps forward with advanced lubricants

January 9, 2024

By Boris Zhmud, Tribonex AB, and Urban Morawitz, Ford Research Center, Cologne, Germany

In the automotive world, the quest for enhanced fuel efficiency is a prominent narrative. For decades, this aspect remained stagnant, but the onset of the 21st century and escalating climate concerns have breathed new life into innovations in this domain.

The Paris Agreement which was adopted in 2015 marked a significant milestone, galvanising global efforts to optimise engine technology. A silent yet potent player in this evolution has been the advancement of lubricants, particularly synthetic lubricants. These lubricants, characterised by their enhanced consistency, superior oxidation stability, and optimal low-temperature flow properties, have facilitated the transition from 10W to 0W grades.

The impact of these advancements is monumental. To put it in perspective, the enhancement in fuel economy, marked by the evolution from conventional engines to more efficient models, has contributed significantly to reducing CO2 emissions globally, surpassing the effects of total global sales of electric vehicles.

Engine friction and fuel economy are intrinsically linked, with lubricant type and engine design playing pivotal roles. Newer engine designs, especially those utilising roller finger follower (RFF) valvetrains and thermally sprayed bores, are optimally suited for ultra-low viscosity oils. These oils are instrumental in minimising engine friction, particularly at high revolutions per minute (rpm), leading to enhanced fuel efficiency.

Engine tribology, the study of interacting surfaces in relative motion, is a complex yet crucial aspect of this narrative. It involves a delicate balance between various factors, including engine design, lubricant properties, and operating conditions. The Ford Focus exemplifies these advancements in real-world terms. Over a decade, its fuel consumption figures have improved by 10%, with engine tribology contributing 2-3% to this enhancement.

The evolution of lubricants, marked by the shift towards lower viscosities and the incorporation of friction modifiers, is central to ongoing enhancements in fuel efficiency. The measurement of engine friction, though complex, is integral to understanding and optimising these dynamics for real-world applications.

Both base oil viscosity and additive package have a significant impact on fuel economy. Ultra-low viscosity motor oil formulations invariably deploy special types of additives known as friction modifiers. Friction modifiers are needed to control asperity friction in the boundary lubrication regime.

The theoretical limit of fuel economy that can be achieved by zeroing engine friction (by using a hypothetical inviscid lubricant and zero-friction surfaces) is estimated to be around 25% compared to production engines manufactured over the past two decades (see Figure 1).

Figure 1. Estimated friction reduction potential for a V6 gasoline engine under partial load operation (1400 rpm, BMEP = 2.1 bar)

Measuring engine friction is always a compromise between the ability to achieve sufficient accuracy and realistic operating conditions. Whilst the well-established tear-down method offers very precise measurement results, boundary conditions can differ considerably from real engine use—depending on the investigated system—especially in terms of loads. In contrast, testing under realistic operating conditions in a firing engine usually means a loss in accuracy of the results.

Depending on the load, absolute friction levels, as well as relationships between friction and the properties of lubricants and surfaces can change. For instance, when simpler non-pressurised engine test rigs are used, the impact of oil viscosity on engine friction tends to be overestimated. Indeed, a non-pressurised engine friction test remotely mimics the engine dynamics at zero load. As a result, the asperity load ratio tends to be zero and the engine effectively functions like a high-shear viscometer. When you increase the engine load, the asperity load ratio also increases.

Therefore, other factors, such as surface texture of mating parts, the presence of friction modifiers in the lubricant, and the presence of friction-reducing coatings, play an increasingly important role. For the same reason, motored tests tend to overestimate the fuel-saving potential of ultra-low viscosity oils. Firing engine tests provide more realistic fuel economy figures, but suffer from higher measurement errors. This is clearly reflected in the JASO GLV-1 specification that sets different fuel economy limits for ultra-low viscosity grades (compared to an SAE 0W-16 reference oil) using motored and firing engine rigs: > 1.1% for the firing Toyota 2ZR-FXE engine and > 2% for the motored Nissan MR20DD engine.

At the recent ICE2023 conference organised by SAE Naples in Capri, Naples, Italy, we presented a comprehensive account of the influence of various factors on engine tribology—from engine design to the properties of the motor oil. Two Ford production engines were compared: an older generation Ford EcoBoost 1.6L i4 GTDI from the Sigma family, and a newer generation Ford Dragon 1.5L i3 GTDI. (Refer to Table 1).

Friction torque data for the 1.6L i4 EcoBoost engine are presented in Figure 2 (a,b). The friction mean effective pressure (FMEP) contribution was evaluated as the difference between the brake mean effective pressure (BMEP) evaluated from the cranking torque and the indicated mean effective pressure (IMEP) evaluated from in-cylinder pressure readings. The squares represent the experimental points obtained for SAE 5W-30 motor oil at oil temperatures of 30^oC and 90^oC. The lines show the anticipated effect of oil viscosity on engine friction.

Figure 2. Friction torque for pressurised motored 1.6L EcoBoost engine at oil temperature of (a) 30oC and (b) 90oC. Peak cylinder pressure (PCP) was 20 bar in all cases. The squares represent the experimental points. The lines show the estimated impact of oil viscosity on the friction torque.

Note that the friction torque curves have a minimum of around 1,500-2,000 rpm engine speed. In the speed range of 1,000-3,000 rpm that covers more than 95% of driving conditions, friction “steals” 3 to 6 Nm from the engine output at normal oil temperature. The figure may be slightly higher for a firing engine due to much higher stresses after top dead centre (TDC) firing and local hot spots due to combustion. However, it does not likely exceed 10 Nm.

The viscometric characteristics of the oils used in this study are presented in Table 2.

Table 2. Viscometric characteristics of crankcase lubricants used in this study

The viscometric characteristics of the oils used in this study are presented in Table 2.

All viscosity grades used the same additive package, as well as identical base oils and viscosity index improvers, though at different percentages.

At high rpm, oil viscosity starts to play a bigger role. Moving from the legacy grade SAE 10W-40 to the lighter weight grade SAE 0W-16 is predicted to bring about a 10% reduction in engine friction at high rpm and 90^oC oil temperature. However, the effect gets progressively smaller when going to lower rpm. It is interesting that for the older engine, the lowest viscosity oil gives the highest friction in the low rpm end. Once again, this shows that the hydrodynamic lubricant film collapse may be a real problem. Of course, one should keep in mind that motored tests cannot simulate the temperature map in a firing engine. However, even though there may be some uncertainty present regarding the absolute values, the general trend is obvious: high viscosity grades are associated with higher losses at high engine speeds, above 2,000 rpm or so, but may actually lead to lower losses at low engine speed.

Figure 3 shows the approximate distribution of friction losses between different engine subsystems.

Figure 3. Mechanical friction losses for different engine subsystems in a 1.6L i4 GTDI engine (SAE 5W-30 motor oil, 90oC oil temperature, PCP = 20 bar).

Different engine designs show significant differences in their tribology. In general, engines that use a direct acting mechanical bucket (DAMB) valvetrain and conventional cast iron cylinder bores see an increase in friction at low rpm because of lubricant film collapse. In contrast, engines that use an RFF valvetrain and thermally sprayed bores have nearly linear friction torque vs rpm graphs and are much better fit for use with ultra-low viscosity oils.

Using ultra-low viscosity oil puts higher requirements on the quality of machined parts and may require significant changes in engine design. For instance, piston-to-bore clearance, bearing clearance, cylinder bore surface characteristics, ring pack characteristics, journals, oil pump, oil filter, oil squirters, chain tensioners, etc. may need to be modified. Newer engines using spray-coated cylinder bores and RFF valvetrain show less asperity friction than older flat-tappet-cammed engines with conventional honed bores. This allows safe migration to lower viscosity oils, as long as the oil pump can maintain oil pressure. The use of superfinished or mechanochemically finished bearing journals addresses the bearing wear issue but comes with a price penalty.

In conclusion, internal combustion engines will remain in use for decades to come, especially in commercial fleets. The continuing powertrain technology development and tribological optimisation of internal combustion engines have allowed significant improvements in fuel efficiency and emissions.

References

B. Zhmud, U. Morawitz, D. Basiri, D. Schulz, Twenty Years of Engine Tribology Research: Some Important Lessons to Learn, SAE Tech. Paper 2023-24-0102.

P. Lee, B. Zhmud, Low Friction Powertrains: Current Advances in Lubricants and Coatings. Lubricants 9 (2021) 74.

B. Zhmud, A. Coen, K. Zitouni, Fuel Economy Engine Oils: Scientific Rationale and Controversies, SAE Tech. Paper 2021-24-0067.

B. Zhmud, Y. Chizhevskiy, E. Tomanik, Digital AI Based Formulation Development Platform for Crankcase Lubricants, SAE Tech. Paper 2022-01-1096.

D. Chobany, B. Zhmud, Mastering the Art of Cylinder Bore Honing. SAE Tech Paper 2020-01-2238.