Annex IV: Integrated Surface Technology for Friction Reduction in Engines

Overview of Scope:
Within the moving parts of an engine, parasitic frictional losses consume about 17% of the mechanical energy, depending on the engine design, and duty cycles. While some frictional resistance is needed for the operation, the parasitic energy losses can be reduced to increase fuel efficiency. The Annex focuses on research and development activities to demonstrate that surface material technologies can be used to improve fuel economy. This includes surface textures to reduce friction, coatings to protect the textures, and ultralow viscosity lubricants to reduce the drag of engine components in relative sliding motion.

Annex Participants:
US: Led by Dr. Stephen Hsu, George Washington University, USA (Chair)
China: Led by Dr. Junyan Zhang, Lanzhou Institute of Chemical physics, CAS, China
Dr. Lawrence Li, City University of Hong Kong
Australia: Led by Dr. Gwidon Stachowiak, Curtin University, Australia
Israel: Led by Dr. Izhak Etsion, Technion, Israel
UK: Led by Dr. Mark Gee, NPL, UK
Korea: Led by Dr. Joon Won Min, Korea Automotive Technology Institute, Korea

Accomplishments and Impact:
Working with industrial partners, we have conducted industrial standard engine tests on an ultralow viscosity advanced lubricant to achieve 2.4% fuel economy improvement over the current commercial lubricant. The world uses 35,600 million barrels of oil per year and produces 36,061,000 kilotons of CO2. A 2 % reduction translates to an oil consumption reduction of 711 million barrels of oil and a CO2 reduction of 721,000 kilotons per year. The surface textured engine was tested in an engine chassis dynamometer test stand. It achieved a 3% energy efficiency improvement.

Surface texture was first used in engine cylinder liners during the 1950s to prevent seizure in diesel engines. Cross-hatching of the liner is now standard practice throughout the engine industry today. Serious research and development into surface texturing as an academic topic began in the 1990s on mechanical seals where the load is low and speed is high, and the contact is flat on flat. The application was hugely successful, resulting in significant friction reduction and much longer durability. Theory and models followed to firmly establish the surface texturing as an emergent science and technology area. During the ensuring decade, surface texturing appears to have limited application successes. Within this environment, Annex IV was formed to share common experiences and conduct cooperative research and development activities among interested researchers and application engineers. Today, Annex IV has developed comprehensive database and mechanistic understanding of how surface texturing can be used to control friction and wear in sliding components.

Annex Task Structure:
Main task:
1) Surface texture design and fabrication: US, Germany, Israel, Korea
2) Characterization of surface textures: US, Australia, UK
3) Measurement of surface textures: US, UK, China
4) Bench test measurement and engine test validation: US, China

Subtask 1: shear resistant diamond-like-carbon coatings (US, China)
Subtask 2: ultralow viscosity lubricants (US, China)

Technical barriers:
There are several barriers facing the wide spread use of surface texturing: 1) the cost of fabrication on engine components; 2) durability of the surface textures; 3) design guidelines for complex motions sliding pair; 4) how to quantify benefits in engine operating environment.

In 2014, some original equipment manufacturers (OEMs) have reported significant fuel economy increase
(4-7%) by using ultra-low viscosity lubricant in vehicle driving tests. While some OEMs have employed surface material technology to fortify their engines to be more wear resistant, this is not the case for all OEMs. A new ultra-low viscosity lubricant classification of the Society of Automotive Engineers (SAE) has been established in 2013 (SAE 0W-16) and a new lubricant classification GF-6 has been created to allow the use of ultra-low viscosity oils, pending the development and issuance of new engine sequence dynamometer tests to certify this class of oils. Because of the low viscosity, wear emerge as the key challenge.

A new subtask on developing robust low viscosity lubricants was established in 2014 to explore novel base oils (ionic liquid, polyglycols, and liquid crystals, etc.) and more robust friction modifiers and lubricant film enhancers as a means to achieve the fuel economy gains without wear. With the introduction of ultralow viscosity lubricants in the market, concern of potential durability loss from the use of this class of new lubricants looms over the landscape. As a result of this concern, materials technology such as surface texture, diamond like coatings, and bonded chemical films are being examined closely to see whether they can alleviate wear and durability loss.

At the same time, the Annex began working with engine manufacturers to identify a modern engine with current fuel efficient features to test the effect of ultralow viscosity lubricants on long term durability.

• V8 engine
• Aluminum block to reduce weight
• Direct fuel injection
• Cylinder deactivation (AFM), seamlessly switches to 4 cylinder operation SS loading
• Continuous variable valve timing
• Advanced combustion
• Oil jet piston cooling
• transmission

Engine being used for measurement of surface materials technology effect on fuel economy and durability.

Activities and Accomplishments:
The Annex activities include surface texture design, fabrication process development, characterization of textures, bench performance assessment, and engine test validation. A topical report has been published on the topic (accessible from the link above)

Texture fabrication technique development: The requirements for surface texture fabrication on engine components are: 1) low cost; 2) reproducible; 3) control of size, shape, and depth of dimples; 4) no unintended side effects on performance or durability. We have chosen the use microlithography mask making coupled with electrochemical etching to fabricate complex, mixed geometrical shaped discrete dimples, or lines, or grooves on steel (from cold rolled steel to bearing steel with hardened surface layers). The original method is to fabricate a hard mask (containing the texture pattern) on a silicon wafer in a clean room using photoresist and UV irradiation. The pattern is then transferred to a glass substrate and a second UV irradiation is needed to transfer the pattern onto a solid surface. This technique is typically used in making MEMs devices. However, engine component surfaces are irregular in shape, with highly curved (both convex and concave) surfaces, projection of the micropatterns onto such surfaces are not feasible. We have developed a soft mask using polymeric thin films, avoiding the need of the second UV irradiation and textures can be directly fabricated on bearing steel surface with precision.

A) Optical images of the photoresist pattern on polymeric thin film surface; B) soft mask fabricated by dry etching based on the mask patter; C) circular dimples fabricated on the bearing steel surface.

Mathematical surface descriptions: With surface texture design methodology developed, there is a need to address the issue of how textured surfaces can be specified for qualify control. Australia leads the effort on this aspect in the Annex. The aim is to develop a mathematical description of the textured surface which has directionality and multiple length scales. This can be described by using fractal mathematics.

Performance measurement: the effect of surface textures on friction reduction is demonstrated in the piston ring and liner interface. The test was conducted using a Plint ring and liner simulator rubbing a production piston ring against a production cross-hatched liner of a diesel engine. Several surface material technologies were evaluated: the effect of surface textures; the effect of diamond-like-carbon (DLC) coating; and the effect of bonded chemical films. The cumulative effect was evaluated. As shown the figure below, the area under the curve between the baseline friction vs time was calculated and compared when DLC was added on top of the textures, and the bonded chemical film was added on the DLC coating.

55% energy reduction

67% energy reduction

71% energy reduction

Surface topography, including surface textures, grooves, dimples, and lines, can be fabricated on engine surfaces to reduce friction. The friction reduction mechanisms involved include debris trapping, hydrodynamic pressure generation, and hydrostatic pressure reactive force generation. These surface features also increase apparent roughness, reduce contact area, and promote edge stresses; which may increase friction and wear. To maximize friction reduction, proper dimple size, shape, depth, and pattern are needed to balance the lift forces and roughness to achieve the benefits. The use of surface textures is limited by the cost benefit ratio (fabrication cost vs realized benefits). Annex IV has focused on fabrication cost reduction to make the technology viable. A novel one-step soft mask technology, when coupled with electrochemical etching, complex texture shapes and depth and textural patterns can be fabricated onto modern engine component surfaces. This new fabrication technology reduces the texturing cost by an order of magnitude; paving the way for commercialization.

Ultralow viscosity lubricant has been proposed by the Japanese automakers. They reported that the use of ultralow viscosity lubricants (specification calls for low viscosity at 150ºC) achieve 4-7% fuel economy increase in their vehicles in urban driving cycles. A new ultra-low viscosity lubricant classification was established by the Society of Automotive Engineers (SAE) in 2013 (SAE 0W-16) and the new lubricant classification system has been created. A new international lubricant classification was established in 2013 to develop new engine tests for qualification for fuel efficient lubricants. This new category of lubricant is available effective May 2020.

Annex IV has been working with an engine manufacturing company to validate the new lubricant and the surface texture technology in modern vehicle engines. For the testing, a new 3.3 Liter V8 engine with direct fuel injection, active fuel management (cylinder deactivation), and dual-equal camshaft phasing (variable valve timing), and advanced combustion system was used. The tests include: 1) to measure the fuel efficiency of 0W-16 ultralow viscosity lubricants; 2) to measure the effect of surface textures on fuel economy; 3) to measure the durability of the engine with textures.

The test engine was mounted in a light truck and a series of engine chassis dynamometer tests was conducted. The fuel efficiency test procedure used consists of 5 test cycles per day for 5 days. The test cycle started with a cold start FTP-75 (EPA city driving cycle) followed by a double highway test cycles (FFE) and US06 cycles. Six home developed 0W-16 formulations were tested and are summarized in Table 1. These formulated low viscosity oils showed fuel economy improvements when compared to the current 5W-30 commercial oil. The new oils were formulated using components from additive suppliers. A top-tier commercial 0W-16 was also tested as an additional reference point. They show improvement in cold start city driving cycle but much less improvement in highway test cycle (still showing improvements). GW G3 formulation has the best overall fuel economy results by balancing the city cycle driving with the highway cycle chemistry. The candidate oil was always tested with reference oil test before and after the candidate oil.

Table 1. Percent improvement of fuel efficiency of 0W-16 formulations

These engine test results show that ultralow viscosity lubricant, properly formulated, can provide 1-2% fuel economy improvement in a modern US vehicle under typical US driving cycles. The effect appears to be concentrated in the low speed low load city driving cycles. This agrees with the Japanese results in urban driving cycles but the improvement is much less than the 4-7% improvement. This probably is due to tighter tolerance, smoother surfaces (including surface textures) in Japanese vehicles.

For surface texture testing, the engine was installed on an engine dynamometer test stand which can run motored test and fired test runs at set dyno speeds. The lubricant used for this test sequence was the 5W-30 baseline oil. The baseline test in this case was using untextured parts and the candidate test was using textured parts. The engine was manually moved to various speed set points from 1200rpm to 5600rpm, with a step increase of 200rpm. At each speed set point, the torque (N/m) and power (KW) were measured and compared. Three baseline runs were conducted and the average results were taken. For motored engine runs, an inline torque meter was used to measure the torque necessary to maintain the speed. Fired engine tests were conducted with wide open throttle (WOT).

The figure below shows torque outputs at various speed set points. The figure is shown at a compressed torque scale data. The texture has much higher torque at low engine speed range. In mid-speed range, no effect is observed, then at high speed range it shows some improvements. The engine test is similar to actual driving when you increase the speed to accelerate, the load correspondingly is also increased, hence higher severity at higher speeds. For normal driving, the speed range is from 1200rpm to 3000rpm most of the time. Therefore, the baseline data shown here was the average of three tests. Using area under the curve, the overall torque increase due to textures is about 2.9%, or 3% approximately. All tests were conducted in triplicates to improve test precision.

Torque as a function of speed set points in fired wide open throttle engine dynamometer tests

We decided to look into the effect of torque change at each speed set point using the experimental data points. The next figure shows the influence of speed (corresponding loads) on surface texture influence on friction reduction (resulting in higher torque). In the low load region, the percent of torque increase dramatically reach for specific speed range and adjust the torque scale to closely examine the data. This figure shows at lower speeds range of 1200 to 2000, the torque increases to about 13%. At high speed range of 4000 rpm to 5600rpm, the texturing shows about 2% increase.

Percent increase in torque over the baseline case at each speed set point, showing the surface texture effect

One way to resolve this issue is to look into the motored engine test data, the following figure shows the increase in the brake mean effective pressure (BMEP) by 5.5% across the entire speed range of the test. This BMEP increase data are independent of engine displacement. This suggests that the texturing reduces friction to boost the power by 5.5%. If one accounts firing engine would increase the resistance, the 3% torque increase is reasonable. In surface texturing, hydrodynamic and elastohydrodynamic lubrication regimes enhance fluid pressures to reduce friction. The transition basically shows no benefit, and as the speed increases, a moderate effect appears due to the boundary lubrication textures.

Motored wide open throttle speed vs brake mean effective pressure (BMEP) showing the texture increase the pressure, indicating more efficiency in the cylinders

Significance and Impacts:
In the US, the total vehicle population is about 260M (EIA, 2015) and transportation accounts for 31% of the carbon emission. Annually about 140.43B gallons of gasoline were consumed in 2015. Improving fuel economy by 2.4% translates into a potential saving of 3.4B gallons of gasoline reduction per year and about 30M metric tons of CO2 emission per year. Extending to the world gasoline consumption of 1,387B gallons of gasoline/year (2012), the impact will be much greater. Of course, not all cars can use ultralow low viscosity lubricant due to climate and engine design. Nevertheless, friction surface technology, once proven by engine tests, could play significant role in energy efficiency and global warming.