Surface treatment may be the key to heavy-duty fuel economy
For the past two decades, light-duty vehicle makers have pushed the lubricant industry to help them improve fuel economy, and the lubes industry did so mostly by steadily reducing engine oil viscosity.
Now heavy-duty truck manufacturers are looking for similar assistance, and the lubes industry is pulling out the same recipe—attempting to improve engine efficiency through lighter crankcase oils. With trucks, however, that approach poses a risk that is significantly greater than it was for cars: the potential for unacceptable levels of engine wear. As a result, some lubrication engineers say solutions will require a systems approach that uses strategies such as component surface treatments in combination with lighter lubricants.
“Low-viscosity oil comes with one major risk,” Boris Zhmud, chief technology officer of Applied Nano Surfaces, said in January during a presentation at the International Colloquium on Tribology, organized by Technische Akademie Esslingen in Ostfildern, Germany. “You may not be able to provide adequate lubrication. Triboconditioning [a type of surface treatment] allows one to switch to lower-viscosity lubricants for improved energy efficiency without accruing risk of wear-related failures.”
Applied Nano Surfaces is based in Uppsala, Sweden, and develops surface treatment technologies that reduce friction and wear.
Heavy-duty Catching Up
The current push to improve fuel economy in trucks began a decade ago, although it took a few years for lubricants to begin contributing to the cause. In 2011, United States regulators announced plans to impose fuel economy mandates on heavy-duty vehicles, as they had for light-duty vehicles since the mid-1970s. Truck manufacturers immediately began looking for fuel economy improvements in a variety of places, especially powertrain optimization, and efficiency did improve—by 24 percent from 2011 to 2016, according to a report that year by the U.S. Department of Transportation.
“Many different approaches to reduce friction losses in the engine came into focus,” Zhmud said during a May email exchange, “new cylinder liner honing techniques, low friction coatings, piston ring pack optimization, etc.”
Original equipment manufacturers also looked for contributions from engine lubricants, stipulating in 2011 that fuel economy would for the first time become a parameter for the next API heavy-duty diesel engine oil category, PC-11, which the industry was then starting to develop. The specification came to market in 2016, commercialized as API CK-4 and API FA-4.
It was the first time that the American Petroleum Institute divided a specification into two categories, and the decision to do so reflected concerns about wear in lower-viscosity heavy-duty oils. FA-4 includes requirements for fuel economy performance and is intended for use only in model year 2016 and newer trucks because of concerns that it may not provide adequate wear protection in older engines. CK-4 is also intended for use in 2016 and newer trucks but does not include fuel economy performance and is safe for use in older engines.
Industry organizations in other regions have likewise looked to develop heavy-duty oils that help improve fuel economy. In 2017, the Japanese Automotive Standards Organization updated its JASO M355 standard for heavy-duty engine oils to include a new category, DH-2F, with requirements for fuel economy performance for fresh and aged engine oil. Fuel economy is also being factored into the heavy-duty side of the European Automobile Manufacturers Association’s upcoming upgrade to its ACEA engine oil sequences.
Heavy-duty oils have started trending toward lighter viscosity. SAE 15W-40 oils are the dominant grade, but SAE 10W-30 products are getting some use, and lighter products are being developed. Zhmud noted that Swedish commercial vehicle manufacturer Scania developed a specification for an SAE 5W-20 oil—LDF-3 FS—and that oils meeting it are commercially available. He added that SAE 0W-16 oils are being tested on new heavy-duty diesel engines.
In this way, heavy-duty engine oils are following the same track as their light-duty counterparts. At the turn of the century SAE 10W-30 was the dominant grade for passenger cars, but SAE 5W multi-grades—5W-30 and 5W-20—are now more popular, and passenger car OEMs are increasingly recommending 0W multi-grades, which have steadily pushed the viscosity envelope from 0W-20 to 0W-16 and 0W-12. Japanese automakers are now developing 0W-8 products. Fuel economy was the main motivation behind this migration.
Boosting Fuel Economy
There is consensus that thinner oils can impart similar benefits to the efficiency of heavy-duty engines. In 2019, Peter Kleijwegt and colleagues at Chevron Oronite reported the results of research into the relative fuel economy contributions of heavy-duty engine oils of varying viscosities. Based on tests conducted under the European Stationary Cycle, Kleijwegt found that an SAE 10W-30 oil could increase fuel economy by 0.6 percent compared to a standard SAE 15W-40 product. Switching to an SAE 5W-20 oil yielded a 1.3 percent improvement, and SAE 0W-16 an improvement of up to 2 percent.
Zhmud said low-vis heavy-duty oils bring other benefits, too: They have greater capacity to cool engines than heavier grades, lead to less cavitation, release air more easily and reduce the need for viscosity index improver additives.
Officials from Lubrizol agreed that there are multiple reasons to move toward lighter engine lubricants for trucks.
“Engines are being designed differently than they have been in the past,” the company said in response to questions for this article. “There are tighter tolerances within engines that are creating an environment where the engines are running hotter than they ever have before, and there is less clearance for the oil to flow through the engine. Lower-viscosity lubricants are designed to operate in these harsher engine conditions and to maximize the efficiency and performance of the engine.”
The main drawback is that lighter oils provide less protection from wear than SAE 15W products. Formulators say this is a bigger concern with heavy-duty trucks than with light vehicles because engines in the former are subject to greater loads. A typical heavy-duty truck has a 750 horsepower engine that can generate torques of up to 3,550 Newton-meters, compared to 350 hp and 1,250 Nm for trucks that were on the road in 1965. Stresses of that level increase potential for scuffing of piston rings and the liners on piston cylinders, as well as wear of rocker pads, roller followers, crossheads, bearings and cam lobes. That wear is most likely to occur when engines are operating at low speeds under high loads, Zhmud said.
Solutions on the Surface
Zhmud’s research has focused on piston rings and liners, and he contends that the key to preventing them from wearing is to reduce friction in the boundary lubrication regime, where sliding surfaces come into direct contact. A variety of surface treatment techniques have been developed to aid in this, including plateau honing, helical slide honing and laser structuring, which generally optimize surface textures and apply chemical films, all in order to reduce friction between sliding contacts.
Applied Nano Surfaces offers several types of treatments, including Triboconditioning, a patented method that combines burnishing with depositing of friction- and wear-reducing chemicals. The burnishing reduces surface asperities while the deposition fills in valleys to make a low-friction film.
ANS has conducted tests in which the use of a liner treated with Triboconditioning mostly eliminated wear on piston rings and moderately reduced wear of the liner, when compared to results using a standard liner, Zhmud said.
The impact of Triboconditioning illustrates the complex nature of friction. As seen on Page 34, the treatment did reduce asperity friction, which occurs in the boundary regime, but it also increased friction in the hydrodynamic regime, where a film of engine oil is maintained between the piston rings and liner, keeping them from coming into contact. In this respect, surface treatments have the opposite impact of reducing oil viscosity, which reduces hydrodynamic friction while increasing asperity friction.
The net result of Triboconditioning was a slight reduction in overall friction—not enough to be a significant contribution to fuel economy.
“Triboconditioning may not have a huge effect on fuel economy, but it enables safe deployment of low-viscosity lubricants,” Zhmud said. “Changing to lower viscosity does indeed help reduce the hydrodynamic friction, but unfortunately, asperity friction then increases. Higher asperity friction means more wear, which is not good. Triboconditioning helps control the asperity friction.”
The administration of U.S. President Donald Trump has said it wants to roll back scheduled increases in fuel economy requirements for heavy-duty trucks, but most of the world remains on track to continue raising targets. That means OEMs are likely to continue looking for help from engine oils, but engine oils themselves may need help to provide it.