In the drive to improve fuel economy, the primary focus has been on reducing friction. But testing at Lubrizol suggests that whats happening at the surface of engine parts is critical to improving fuel economy. Their findings show that protecting engine parts by using surface-active additives not only helps reduce friction, but also improves durability. This is leading to what Tom Curtis, vice president, engine additives, called a revolution in the way we formulate fuel economy lubricants for todays cars and trucks.
Fuel economy has been a significant driver of engine lubricant development for decades, said Lubrizols Mike Sutton at the 19th International Tribology Colloquium at the Technische Academie Esslingen, Germany, in January 2014. Sutton, technical fellow, engine oils, added that the cost of vehicle ownership, energy security and the need to limit greenhouse gas emissions are all factors behind legislation promoting fuel economy improvements.
In a presentation at the ICIS World Base Oils and Lubricants Conference in London in February 2014, Curtis explained, According to a user survey we ran in 2011, heavy-duty trends are similar to those for passenger car engine oils. In particular, viscosity grades are shifting in the European Union with operators moving to lighter grades.
End users cite a number of reasons for the shift. The main concern, of course, is fuel economy. A 1 percent saving on fuel economy for a 250 truck fleet, for example, equals a savings of more than 160,000 (U.S. $175,000) annually," Curtis noted.
In the past, legislators focused primarily on passenger cars, but that is changing, Sutton said. New laws related to heavy-duty vehicles are beginning to be put in place.
In fact, Curtis said, specification changes are coming thicker and faster with numerous major industry lubricant upgrades forthcoming. He added that similar trends have also been seen in driveline lubricants, and the demand for lighter viscosity grade oils is expected to continue.
Besides fuel economy, low viscosity lubricants must also provide improved protection; maintain cleanliness, oxidation control and emissions system protection; and create demonstrable customer differentiation. What this means is that todays lubricants must perform in a different operating regime from those in the past, Curtis said.
Traditional lubricants operate in the hydrodynamic regime. But lighter viscosity fluids improve fuel economy by operating in the lowest friction regimes, away from the hydrodynamic. In short, Curtis said, Boundary performance is the new focus.
These demands have placed increased pressure on formulators and additive suppliers to develop additives packages that can deliver the required balance of performance. Curtis said that testing by Lubrizol shows additives can deliver up to 1 percent fuel economy improvement at a fixed viscosity without sacrificing durability.
While lowering viscosity has been a primary route to improving fuel economy, Sutton said, Additive chemistry also plays a critical role. He then summarized a study Lubrizol conducted to determine the magnitude of fuel economy savings possible from a range of engine oil chemistries.
Worldwide Test Cycle
Lubrizol tested different additive chemistries in an engine running the Worldwide Harmonized Transient Cycle. The goal was to understand the range of fuel economy that can be delivered by the additive system alone, Sutton said. The study showed that fuel economy can be improved by more than one percent by the additives, which highlights the need to take the additive chemistry of the oil into account along with viscosity to deliver the maximum level of fuel economy.
Sutton explained that transportation accounts for 29 percent of the total energy used in the European Union. This is comparable to the United States at 41 percent and Japan at 29 percent. Additionally, over 80 percent of the energy used in transportation within the EU is attributed to road transport. Therefore, it is not surprising that governments are introducing legislation concerning fuel economy in the transport sector.
Heavy-duty trucks are a significant and growing sector in transportation, Sutton noted. For example, in the U.S., heavy-duty trucks accounted for 22 percent of greenhouse gas emissions in 2008, and this sector is growing at the fastest rate: by 72 percent from 1990 to 2008.
As a result, the U.S. has issued greenhouse gas and fuel economy standards for heavy-duty vehicles. Europe currently has no heavy-duty fuel economy legislation, Sutton said, but the European Commission has reported its intention to develop a methodology for controlling fuel economy from these vehicles [sometime in] 2014.
But legislation alone is not enough. Also needed are industry standard test cycles that ensure repeatable results. These tests are typically based on chassis dynamometer tests, such as the New European Drive Cycle for passenger cars, or are engine dynamometer based, such as the WHTC for trucks.
The WHTC was developed in 2004 by the United Nations Economic Commission. Over the past several years, Sutton said, the test has been modified in the hope that it would become the global standard for heavy-duty engine emission measurement.
The WHTC is a highly transient cycle that operates for 30 minutes over a range of speed and torque conditions. In general, it operates at lower speeds and loads than the European Transient Cycle that it replaced in Euro VI emission legislation, Sutton noted. Therefore, this cycle is of importance to vehicle OEMs and is now used in lubricant specifications to measure the fuel economy potential of different lubricants.
The Additive Effect
Sutton explained that while thinner viscosity oils tend to reduce fuel consumption, it is worth considering which viscosity measurement most likely correlates with fuel use. Engine friction is often highest in the piston/bore assembly and the bearings. The proportion of friction from these combined areas is in the range of 66 to 81 percent, he said. Therefore, [we looked at] the temperature and shear environment of these component systems.
Typically, temperature is greater than 100 degrees C, and peak shear between the piston ring and cylinder bore has been estimated at more than 107 inverse seconds. When considering the standard viscosity measurements most likely to correlate with fuel consumption, high-temperature high-shear viscosity measured at 106 s-1, corresponds most closely to fuel consumption, said Sutton. And, in fact, this was found to be the case.
A typical lubricant additive package is composed of a range of components, including dispersants, detergents, antioxidants and antiwear chemistry. Each performs a specific function as part of the overall package. However, significant interactions can take place within the oil and on the engine surfaces that can change performance.
With the ranges of individual components available, Sutton said, there can be significant variation in oils formulated to meet different performance requirements. Therefore, balancing the components is critical, and to achieve that balance, there is a need to understand the interactions of the additives with specific hardware in different operating cycles.
He continued, Surface-active chemical components can affect friction; some raise it, others reduce it. For example, friction modifiers have been used in lubricants for well over 80 years, and these additives significantly reduce the coefficient of friction between two mating surfaces. The key, said Sutton, is to understand which additives have a beneficial effect on improving fuel economy, then to combine those components into a balanced package that delivers the required performance.
In investigating the effect of additive chemistry on fuel economy at equal HTHS, Sutton said, It is likely that only a few components have a large effect. Therefore, Lubrizol used what is known as a supersaturated design matrix in the testing. The advantages of using a supersaturated design are that many factors that might otherwise be omitted can be investigated in relatively few experimental runs. The disadvantages of such a design are that it inevitably introduces confounding between factors, which can lead to difficulty in interpreting the data and increases the possibility of some incorrect conclusions.
Finally, Sutton explained, One issue in engine fuel economy tests is the potential for some surface-active components in one test lubricant to remain in the engine after oil is drained, affecting the results for subsequent lubricants. This is why most standard tests measuring fuel economy use high-detergency flushing oil between tests. However, the WHTC does not include such an oil as part of the procedure, and this should be taken into account when analyzing the data, where subsequent oils can be affected by the preceding oil.
Lubrizols testing focused mainly on surface-active chemistries and those known to interact with surface-active chemistries. Therefore, the main groups considered were friction modifiers, antiwear agents, detergents and dispersants. Within these groupings, different types of molecules and percentages of different molecules were tested.
To minimize the effect of viscosity, base oil ratio and viscosity modifier were adjusted to produce an HTHS viscosity of 3.2 milliPascal-seconds for each blend. Although this introduces extra variables into the study, it effectively eliminates the one variable already known to impact fuel economy, HTHS viscosity.
Twelve oils were blended for the test, and the results showed that the percentage fuel economy improvement for nominally constant HTHS viscosity is more than 1 percent. The data were analyzed using Bayesian Variable Assessment in which the probability of each being the correct model was evaluated. Typically, a probability over 35 percent indicates that a factor is too important to ignore, Sutton said, and a probability over 50 percent indicates a likely effect.
The most likely active factors were found to be a detergent with 27 percent probability, a dispersant with 51 percent probability, a friction modifier with greater than 99 percent probability and an antiwear additive with 50 percent probability. Sutton cautioned, The development of a model is useful to aid in the future development of lubricant additives, but it is worth highlighting again that this supersaturated design has many confounding problems. Therefore, to determine more subtle differences and refine the model, further work is necessary.
Sutton concluded, [The testing] highlights the potential improvement available from additive chemistry and that to achieve maximum fuel economy potential of a lubricant, careful balancing of the additive system is necessary.
Curtis said, Insights gained in this testing are leading to a revolutionary paradigm shift in our view of how additives work at the surface. Surface-active additive systems … impart superior performance by synergistically managing friction and wear. The results show that certain surface-active additives form macro layers on the engine surfaces that not only reduce friction but also protect the surface to improve engine durability.
Curtis displayed micrographs showing that proprietary combinations of additives create controllable multilayer composites in the boundary contact zones. This results in the formation of durable, persistent and tenacious wear resistant additive layers.
These novel systems can be used in a variety of lubricants, he said. For example, testing in over-the-road trucks shows fuel economy gains under multiple driving conditions. Also, passenger car engines are protected even in ultra-low SAE 0W-16 formulations in severe start/stop operation.
The same holds true for additive systems developed for axle lubricants. The optimized fluid system demonstrated statistically significant improvement in efficiency, Curtis noted. It also reduced operating temperature and extended hardware life.
Lube Report Asia occasionally republishes articles from its sister publications. This article originally appeared in the July 2014 issue of Lubes'n'Greases Europe-Middle East-Africa under the title "Fuel Economy - Its Not Just about Friction."