Sales of hybrid vehicles have risen modestly in the United States while, according to AVERE France (a European network promoting the use of battery, hybrid and fuel cell electric vehicles), sales in Europe increased by 77 percent year-on-year during the first half of 2014. But percentages can be deceiving because the base is very low.

With such a diverse marketplace in Europe, trends vary significantly by country. Yet, it is in China, the largest single country car market in the world, where hybrids have the greatest potential. China has high green transportation aspirations, and to encourage sales, the government has exempted hybrid and electric cars from its 10 percent purchase tax from September 2014 to the end of 2017. Despite these measures, electric and hybrid cars combined account for just 0.2 percent of the 11.7 million cars purchased in China during the first half of 2014.

The market for these vehicles is still young, and consumers need to be educated to allay misconceptions about the efficiency and longevity of the technology. Yet, it is the lubricant industry, in partnership with original equipment manufacturers, that is driving the greatest advances in the application of hybrid technology and holds the potential to unlock the true potential of the market to improve average fuel economy and reduce greenhouse gas emissions.

Unique Challenges

A distinguishing feature of hybrid electric vehicles is the ability to turn off the conventional engine when the power available from the electrical system exceeds that required to propel the vehicle. This engine-off approach not only saves fuel and engine hours, it also results in cooler operating temperatures for the internal combustion engine. However, it also increases stress on the engine through more frequent starts.

On the other hand, engines operating under traditional cycles regularly inject more fuel than is necessary to cool the combustion chamber and stop the catalytic converter from overheating. This process preserves components to the detriment of fuel efficiency.

As a result, hybrid vehicles are considerably more efficient but require lubricants that decrease the increased stress on engines by reducing friction and managing the range of temperature changes within the engine. These requirements have driven the demand for lower viscosity oils in recent years.

Delicate Balancing Act

The primary challenge with adapting oil viscosity levels is striking an equilibrium between fuel economy and protecting the longevity of engine components. The thinner oil films associated with low-viscosity fluids make it harder for the oil to keep the loaded contact surfaces in the engine sufficiently separated from each other. This can lead to accelerated wear rates and even locally increased friction.

Infineum studies indicate that, in some engines, reducing high-temperature, high-shear viscosities to 2.3 centiPoise presents little risk of engine component wear. Below this level, however, specific engine components, for example the top ring and bearings, suffer higher wear rates.

To ensure that ultra-low viscosity lubricants deliver fuel economy and wear protection, it is becoming increasingly important to co-engineer the vehicle hardware and lubricant system. In many cases, these lubricants also act as hydraulic oils to enable the hardware and engine innovations to deliver increased fuel economy.

Viscosity grades are fragmenting, adding significant complexity to the market. Close collaboration with OEMs is increasingly important to ensure that changes in the design of hardware and lubricant systems deliver enhanced fuel economy performance without compromising engine durability.

Developments in this area are built on existing knowledge and new research conducted into the unique properties required to maximize fuel efficiency and reduced wear in hybrid powertrains. For example, lighter oil grades generally require lower treat rates of viscosity modifier polymers, but the characteristics of these polymers remain critical in maximizing fuel economy and providing both low and high temperature protection.

Lubricating Innovation

Potential developments in the application of lubricants in hybrid vehicles span a wide range of possibilities. This includes the potential for oils specifically designed for hybrid applications and the expected lighter viscosity grades that OEMs are likely to recommend.

In addition, lubricants hold the potential to greatly reduce the harmful emissions produced by the internal combustion engine in hybrid vehicles. Continued investment and research could also provide the solution to implementing fluids that are capable of both lubricating the engine and cooling battery systems simultaneously. Ultimately, a combination of the right lubricant designed for specific hardware will provide the most benefit to improve fuel economy and reduce greenhouse gas emissions.

Another key challenge facing the lubricant industry, OEMs and the large-scale adoption of hybrid vehicles is the issue of backward compatibility. At present, engine and transmission lubricants are formulated to meet industry-wide or OEM specifications. They become increasingly general over time to meet the needs of many additional and more challenging parameters. This subsequently restricts lubricant formulation flexibility and, thus, has an impact on opportunities to optimize performance.

Hybrids, with their focus on reducing CO₂ emissions, create an incentive to break the specification mold and develop lubricants that are specifically tailored to promote fuel economy and increased range – a prime consideration for consumers and OEMs alike. Infineum believes that more OEM-specific formulations will be developed as we drive toward lighter viscosity grades such as SAE 0W-16 and lower.

Understanding Hybrids

To better understand the distinctive stresses placed on lubricants in hybrid vehicles, Infineum conducted a fact-finding project that compared the stresses on the engines of a 2006 Toyota Prius taxi from Winnipeg, Manitoba, Canada (400,000 miles), a 2009 Toyota Camry Hybrid taxi from New York City (264,000 miles) and a nonhybrid limousine from the New York metro area (200,000 miles).

The vehicles all used SAE 5W-30 oils for the trial to ensure parity. The 2006 Prius oil was of ILSAC GF-4 quality, and the newer vehicles used oils meeting the current ILSAC GF-5 specification.

The initial results indicated that the Prius hybrid had poor engine cleanliness, but low wear was observed overall considering the vehicle ran for 400,000 miles. For the 2009 Toyota Camry Hybrid, average sludge and varnish, as well as wear on the value train and piston rings, were all marginally worse in the hybrid engine relative to the nonhybrid.

Subsequently, Infineum started a follow-up project in collaboration with a New York City taxi firm to assess lubricants with varying rheological and performance properties in their hybrid vehicles. We are assessing six test oils that have different HTHS viscosities and performance additive technologies.

Thus far over, 100,000 miles have been completed, and regular oil sampling and engine inspections show the impact of each lubricant. Key findings to date include:

Lack of carbon/varnish on the engine parts

No engine distress noted for any of the oils

Higher fuel economy as engine oil HTHS is reduced from 2.6 to 2.0 cP

Cylinder head decks look clean after 100,000 miles

Minimal fuel dilution

In the next phase of the project, Infineum will conduct a final engine tear down to inspect whether the engine oils with HTHS viscosity below 2.6 cP provided adequate protection to the engine parts throughout the 200,000 mile test. In addition, we will learn if any of the test lubricants can deliver improved cleanliness in the hybrid electric vehicle engines compared to conventional ILSAC GF-5 oils. Full results are expected in early 2016.

Conclusion

It is clear that the demand for hybrid vehicles is growing, and increasingly strict legislation around vehicle emissions is likely to drive more OEMs toward hybrid and plug in hybrid production as opposed to fully electric vehicles in this time frame. While considerable advances have already been made and the increasing adoption of hybrid electric vehicles demonstrates the pay-off, there is still much to be done to improve the reliability of such vehicles and the lubricants in them.

The key to overcoming the current issues faced by the industry is close collaboration between those in the lubricant sector and OEMs. Only by working together can solutions be found to issues of range, fuel efficiency and component wear. The future for the hybrid vehicle industry is a bright one, and many exciting opportunities lie ahead.

Steve Haffner is crankcase market manager for Infineum, based in Abingdon, Oxfordshire, United Kingdom.