But reducing the SAE viscosity grade reduces viscosity at all temperatures, he pointed out. What formulators should actually aim for is reduced viscosity in the intermediate temperature range of 20 degrees Celsius to 100 C-the temperatures that occur during fuel economy testing, including the New European Drive Cycle and the Worldwide Harmonized Light Vehicles Test Procedure.

Viscosity Index Improvers

Use of viscosity index improvers can reduce viscosity at intermediate temperatures while maintaining engine protection by keeping viscosity high at high temperatures, where the oil film is most likely to break down, Hutchinson said. An oils viscosity index indicates how much its viscosity varies with temperature, and viscosity index improvers are lubricant additives that reduce such variation.

Typically, formulators have chosen VIIs to provide low-cost engine protection, particularly in terms of kinematic viscosity at 100 C and cold cranking simulator targets, Hutchinson continued. For fuel economy, HTHS viscosity at 150 C is the focus, with an aim for the minimum value permitted within the SAE viscosity grade.

When developing light oils that target fuel economy, the key criteria for selecting the base oil is often no longer CCS viscosity. Now other requirements, in particular Noack volatility, fill this role. This represents a major shift in the criteria used for selecting the VII, he emphasized.

Viscosity index in high-temperature high-shear conditions is increasingly important as formulators search for ways to minimize HTHS viscosity at 100 C and 80 C to improve fuel economy test results. Therefore, the choice of VII is increasingly influenced by how well the additive controls the viscosity-temperature relationship under both low and high-shear conditions, rather than simply achieving a minimum viscosity value for protection.

The best viscosity-temperature performance is the least change in viscosity as temperature fluctuates. The ideal would be zero change, Hutchinson explained. Performance in this parameter can be improved by increasing the amount of VII in the oil, but it must be countered with a lower base oil viscosity, which can introduce volatility and other concerns. With this in mind, Hutchinson defines a high-performance VII as one that achieves improved viscosity-temperature performance at the same base oil viscosity.

Viscosity index improvers that have been used in engine oils include hydrocarbon types such as crystalline or amorphous olefin copolymers; hydrogenated styrene diene, known as a star type because of its shape; and oxygen-containing ester types, such as polyalkylmethacrylates (PAMAs).

The PAMA types perform best in terms of viscosity index and viscosity-temperature performance because of their polymer architecture, solubility and other factors, according to Hutchinson. The polymers exist as hydrodynamic spheres when dissolved in oil. Benefits depend on the expansion and contraction of these spheres with temperature. A more pronounced effect gives greater control over oil viscosity variation.

Building on this idea, Hutchinson reported that Evonik modified PAMA type VIIs with hydrocarbon components, calling them comb polymers. The polymers have a similar molecular weight to conventional VIIs but better shear stability. The core or backbone of these combs changes from soluble to insoluble form, causing a dramatic change in the hydrodynamic sphere volume with temperature. Normally an insoluble polymer is a bad thing, but the whole polymer itself is kept soluble by the inclusion of these hydrocarbon side chains, he explained, adding that the new VIIs allow for significantly reduced low temperature viscosity to improve fuel economy while still providing sufficient HTHS viscosity.

One challenge is that oils with the comb VIIs can fall below the target range of kinematic viscosity at 100 C for a particular SAE grade. This tends to happen for SAE 30 multigrades but not for SAE 0W-16 oils. Hutchinson believes this is not a concern for engine protection, but can become a problem when assigning the correct SAE J300 grade. It may also cause low oil pressure and raises concerns for variable valve timing actuators. However, the combs can be reconfigured to meet the KV 100 limit with some small compromise in fuel economy potential.

Hutchinson also noted that the benefits of VII are greater in heavier viscosity grades. But within a given viscosity grade, researchers observed improvement in fuel economy with oils containing the comb VII as opposed to an OCP VII. With the SAE 0W-16 grade in an OEM-recognized engine test, were finding about 0.5 percent fuel economy advantage for the best comb VII compared to a conventional OCP type VII, showing there is still measurable fuel economy improvement due to the VII in this light grade, he said.

Friction Modifiers

Formulators are beginning to understand that different strategies can affect fuel economy in different tribological regimes. According to Guillermo Miro of Atten2 Advanced Monitoring Technologies in Eibar, Spain, engine oil viscosity affects hydrodynamic friction while additives such as friction modifiers have an influence in boundary and mixed lubrication regimes. Low-viscosity oils need a combined approach to achieve optimum fuel economy, Miro told the group gathered at the Technische Akademie Esslingen in Ostfildern, Germany.

For example, the benefits of low-viscosity oils during a fuel economy test driving cycle can be reduced when engine technologies designed to increase the warm-up rate of coolant and engine oil minimize the amount of time that an oils low-temperature viscosity contributes to fuel efficiency, Chris Warrens, expert technologist with BP, stated in an abstract submitted to the colloquium. At higher temperatures, the effects of viscosity are less important, and controlling friction under mixed and boundary lubrication conditions play a more significant role.

In a series of tests exploring fuel economy in various engines, Warrens also discovered that the benefit that might be expected from very-low-viscosity oils doesnt always materialize without friction modifiers. Instead, he told the gathering, use of these oils can result in more contact between asperities in the engine, creating more friction.

Frank Lauterwasser of Evonik concurred in his abstract. The lowest viscosity lubricants are not always the best in terms of fuel economy. When the engine runs in the mixed and boundary lubrication regime, the addition of friction modifier could be beneficial for fuel economy.

Hutchinson also pointed to the importance of friction modifiers. When oils containing one of the comb VIIs were tested without a molybdenum friction modifier that was in other formulations, researchers found that fuel economy worsened by about 0.65 percent. So whilst we demonstrated that VII choice can make a difference to fuel economy, the friction modifier also looks to be extremely important in these light oils, he said.

In the extra-urban driving cycle, the last cycle of the NEDC, the oil is hotter and therefore at a lower viscosity. As a result, friction modifiers have a significant effect, most likely due to a higher level of boundary lubrication at the higher temperatures, Hutchinson speculated.

However, Lauterwasser cautioned in his paper, for some engines, increasing oil viscosity leads to higher efficiency than increasing the amount of friction modifier in a lower viscous fluid.

In the end, stated Warrens, The description of oil viscosity, in terms of the parameters defined by SAE J300, does not give an accurate indication of the behavior of the lubricant. Being able to predict the viscosity of an oil in the various critical components of an engine allows better correlation with measured results [in fuel economy test drive cycles].

HTHS in Heavy-duty

Research presented by Atten2s Miro, which was conducted with Repsol SA and CMT-Motores Termicos at the Polytechnic Institute of Valencia, showed that the trends found for gasoline passenger car engines are also present for commercial diesel engines.

Miro and his team tested five commercial diesel engine oil formulations in a stationary engine test using a medium-duty four-cylinder engine meeting Euro VI emissions standards with 3-liter displacement, a variable geometry turbocharger, maximum effective power of 111.3 kilowatts at 3,600 revolutions per minute and maximum effective torque of 350 Newton meters at 2,000 rpm. The reference oil was an SAE 5W-30 with 3.5 cP HTHS viscosity made with an API Group III base oil.

Two of the oils, an API CJ-4 and an API FA-4, both had 5W-30 viscosity and HTHS of 3.05 cP but different additive packages. Interestingly, the CJ-4 oil produced a maximum reduction of 1.6 percent in brake-specific fuel consumption (the rate of fuel consumption divided by power produced), while the FA-4 oil-specifically formulated to provide better fuel economy-produced a 1.2 percent reduction over the reference oil.

These oils represented both the lowest HTHS value and the most significant fuel savings of the tested oils, particularly in operating conditions with low load and high speed where hydrodynamic lubrication dominates. While the CJ-4 oil produced the highest total fuel savings, fuel consumption reduction began only after engine speeds exceeded 2,000 rpm. For the FA-4 formulation, fuel consumption was lowered throughout the engine speed range, Miro and his co-authors stated in their abstract. The additive package of the FA-4 category would have expanded the benefit range of this low viscosity oil, enabling fuel consumption reduction even when working conditions favor mixed and boundary lubrication regimes, the paper explains.

A different API CJ-4 oil and an API CK-4 oil, both with 3.5 cP HTHS-the same as the reference oil-showed no significant change in fuel consumption. Even with the same additive package as the FA-4 category, benefits obtained with CK-4 were masked by its HTHS viscosity, Miro wrote.

An SAE 10W-40 API CI-4 oil with HTHS of 3.85 cP saw an increase of 3.5 percent fuel consumption over the reference oil.

Then the researchers ran the oils in a WLTP test cycle on a chassis dynamometer in a compressed natural gas engine. The results corresponded to those obtained under the stationary test conditions. The first API CJ-4 oil produced a 0.67 percent reduction in fuel consumption over the reference oil and the API FA-4 oil showed a 0.64 percent reduction, while the 10W-40 API CI-4 oil yielded a 0.68 percent increase.

The low-viscosity oils also reduced carbon dioxide emissions compared to the reference oil. The first CJ-4 oil reduced emissions by 0.69 percent, the FA-4 oil reduced them by 0.66 percent and the CI-4 produced a 0.7 percent increase.