Annex IV: Integrated Surface Technology for Friction Reduction in Engines
Overview of Scope:
Parasitic frictional losses in engines account for about 15-18% mechanical energy. While some frictional resistance is required for the efficient operation, excess parasitic energy losses can reduce fuel efficiency. This Annex focuses on the study of surface material technologies, such as surface texturing, coating, and ultralow viscosity lubricants to reduce friction and enhance durability.
Annex Participants:
Australia: Led by Dr. Gwidon Stachowiak, Curtin University, Australia
China: Led by Dr. Junyan Zhang, Lanzhou Institute of Chemical physics, CAS, China
Israel: Led by Dr. Izhak Etsion, Technion, Israel
Korea: Led by Dr. Joon Won Min, Korea Automotive Technology Institute
UK: Led by Dr. Mark Gee, NPL, UK
US:
Accomplishments and Impact:
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. Figure 1 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.
Figure 1. 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. Figure 2 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.
Figure 2. 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, Figure 3 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.
Figure 3. 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.