P a g e | 1 Continuously Variable Transmission Fluids James L. Sumiejski In the previous article, we discussed stepped automatic transmissions and their fluid needs. This article explains how a continuously variable transmission functions and the fluid performance requirements. Continuously Variable Transmissions (CVTs) have been and continue to be successfully used in small and mid‐sized vehicles, mainly by Asia‐Pacific Original Equipment Manufacturers (OEMs). CVTs have demonstrated good fuel economy benefit in highly populated areas, especially in stop‐start traffic conditions. Due to population increase in and around cities and the global desire for enhanced fuel economy, growth of CVTs is expected to steadily rise over the next several years, comprising about 9% of global transmissions by 2017. The largest market share increase will be in China and Japan. CVTs were originally promoted as providing equal or better fuel efficiency over the 3‐, 4‐, and 5‐speed automatic transmissions. However, the recent introduction of 6‐, 7‐, and 8‐speed automatics and the conversion from manual transmissions to Dual Clutch Transmissions (DCTs) in Europe has provided convincingly equal or better alternatives to CVT units. Consequently, it is imperative that CVT technology demonstrate further improvements in fuel efficiency and overall performance in order to remain popular among OEMs and consumers. Some of these CVT improvements involve hardware changes, such as: 
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Expansion of the variator (belt and pulleys) gear ratio by incorporating an auxiliary transmission (planetary gear set) to achieve a wider ratio coverage Redesign of the oil pump to improve efficiency and reduce churning losses Transmission weight reduction by using a smaller, more efficient variator system Introduction of new belt designs and metallurgy Reduction of parasitic losses throughout the transmission As a result of these CVT hardware changes, a CVT fluid must be developed that is optimized for performance with these improved, next‐generation transmissions. How a CVT Functions CVTs incorporate either a steel belt or chain to transfer power between two split pulleys. Hydraulic pressure generated by a vane or gear pump is used to create the force necessary to move the pulleys. One of the pulleys, known as the drive pulley, is connected to the engine crankshaft. The drive pulley is also called the input pulley, because it's where the energy from the engine enters the transmission. The second pulley is called the driven pulley, because the first pulley is turning it. The driven pulley is also called the output pulley, because it transfers energy to the driveshaft. When one pulley increases its radius, the other decreases its radius to keep the belt tight. As the two pulleys change their radii relative to one another, they create an infinite number of gear ratios, from low to high and everything in between. Thus, in theory, a CVT has an infinite number of "gears" that it can run through at any time, at any engine or vehicle speed. The two main types of CVT systems currently used in the market are: 
The push belt CVT developed by Van Doorne Transmissie, now Robert Bosch GmbH (Figure 1), which is used in approximately 85 to 90% of all vehicles equipped with a CVT © 2014 The Lubrizol Corporation P a g e | 2 
The chain CVT developed by LuK (Schaeffler Group), designed for higher torque applications (Figure 2) Figure 1 Figure 2 CVT Fluid Performance Requirements Historical CVT fluids were based on Automatic Transmission Fluid (ATF) technology, which is why CVT fluids require many of the same performance characteristics as an ATF (as highlighted in the left box of Figure 3). However, as CVT designs became more sophisticated, specialized fluids were needed to provide the unique metal coefficient of friction for the belt or chain and pulley system. High stable metal friction was required to reduce clamping forces for improved efficiency. And because the CVT environment is more severe, CVT fluids must have improved shear stability, wear protection, foam control and oxidation stability, above what is typically needed for a conventional ATF. Recently, the focus for next‐generation CVT fluids has shifted to improvement in anti‐shudder durability performance. While CVTs don’t have shifting clutches like an automatic transmission, most CVTs are equipped with either a Torque Convertor Clutch (TCC) or a Launch or Wet Start Clutch (WSC), which couples the transmission to the engine. OEMs now want good wet clutch friction characteristics for the TCC or WSC, similar to an ATF, but still require the high metal friction performance. The challenge for CVT fluid formulators is to find the proper friction balance in the fluid in order to provide acceptable metal friction performance for the belt (chain) and pulley system, while improving the anti‐shudder durability performance required by the converter or launch clutch. © 2014 The Lubrizol Corporation P a g e | 3 Figure 3. Comparison of ATF and CVT Fluid Requirements CVT Fluid Metal Friction Performance Let’s now take a closer look at some of the unique CVT fluid performance requirements, starting with metal friction. As mentioned earlier, CVTs need a high metal‐metal friction coefficient for the belt or chain and pulley system. This high metal friction allows for a reduction in the clamping force exerted on the belt (chain) and pulleys, which in turn improves the overall efficiency of the CVT. Over the years, there have been many different test methods developed to measure the metal coefficient of friction required for CVTs. However, the Japanese Automotive Standards Organization (JASO) test procedure M358 LFW‐1 block on ring test (Figure 4) is a widely accepted test method for measuring metal friction and is often a requirement in many OEM CVT fluid specifications. The test utilizes a Falex™ Block on Ring test machine that measures the average metal friction coefficient at six different slip speeds, at a test temperature of 110˚C under high load (1,112N) and low load (445N) conditions, between a block of metal and a rotating steel ring. The graph shown in Figure 5 represents typical LFW‐1 results on three fluids, two commercial Continuously Variable Transmission Fluids (CVTFs) and a current commercial ATF. The desired result from this test is a high stable metal coefficient of friction curve over the range of sliding speeds evaluated. Commercial CVT fluids A and B provide acceptable performance in this test. However, the commercial ATF provides low, unstable metal friction, which could lead to possible belt or chain slippage under certain load conditions in the actual transmission. For next generation CVT fluids, the goal is to maintain or even slightly increase this metal friction performance. © 2014 The Lubrizol Corporation P a g e | 4 Figure 4. LFW‐1 Test rig Figure 5. LFW‐1 data on CVTFs and ATF The LFW‐1 block on ring test is a good bench screen test for evaluating numerous potential CVTF candidates in a quick, inexpensive way. However, once a promising candidate fluid has been evaluated in several CVT related tests, the next step is to evaluate the fluid’s performance in an actual variator test. The CVT belt box test (Figure 6) is one such test that utilizes the Short Durability Cycle test method, as developed by Van Doorne Transmissie (Bosch Group) and described in their 2003‐01‐3253 SAE paper. Using a belt/pulley set‐up, the test is run at TOP, Overdrive (OD) and LOW ratios under steady‐state conditions, for a total test time of around 115 hours. These three ratios correspond to the variator system running at various positions, from its minimum to maximum ratio. TOP ratio for this test corresponds to a ratio of 0.617. This is the ratio in the vehicle that would provide the highest vehicle speed. Overdrive ratio corresponds to a ratio of 0.437. This ratio is where the belt is riding on its largest running radius on the primary (drive) pulley and its smallest running radius on the secondary (driven) pulley. In a vehicle, this ratio would correspond to high gear (5th or 6th) and would provide the least amount of torque for highway cruising. LOW ratio for this test corresponds to a ratio of 2.61. Here, the belt is riding on its smallest running radius on the primary pulley and its largest running radius on the secondary pulley. In a vehicle, this ratio corresponds to low gear (1st gear) and would provide the most torque for vehicle launch. Torque capacity tests are conducted at these three ratios and the metal coefficient of friction is then calculated. Figure 6. CVT Belt Box Test rig and set‐up The bar graph in Figure 7 shows the friction coefficients for the same three commercial fluids that were tested in the LFW‐1 test from Figure 5. The two CVT fluids give high metal friction coefficients to maintain an acceptable torque between the belt and pulley. However, the ATF provided lower metal © 2014 The Lubrizol Corporation P a g e | 5 friction at the TOP and OD ratios and during the severe LOW ratio phase of the test, could not even provide enough torque to be measured, which led to belt slippage. This type of ATF performance could possibly lead to damage in an actual vehicle equipped with a CVT. Could not measure LOW Ratio COF; belt slipping
Figure 7. Belt Box friction data on CVTFs and ATF CVT Fluid Wet Clutch Friction As mentioned earlier, CVTs usually contain a Torque Converter Clutch or Wet Start Clutch where a lower friction coefficient is required. For the next‐generation CVT fluids, the OEMs have set goals for improvement in fluid anti‐shudder durability in order to reduce noise, vibration and harshness issues and improve driver comfort. The widely accepted test for evaluating anti‐shudder durability performance is a low velocity friction apparatus (LVFA) tester utilizing the JASO M349 test method. The JASO M349 test provides the slope of the μ‐V friction curve at various points in time. The test is run by performing a speed sweep (ramp up and hold, then ramp down) from 0‐250‐0 RPM. During the entire speed sweep, the clutch pressure is held constant and the torque is measured. The key relationship is friction coefficient versus sliding speed during the down portion of the sweep. If the friction decreases as speed drops to zero, a positive μ‐V slope is achieved. This is desired for good torque converter or launch clutch performance. However, if the friction increases as sliding speed drops to zero, we achieve a negative μ‐
V slope. This type of friction performance is not desired and can lead to shudder in the torque converter lock‐up clutch or launch clutch in the vehicle. Shudder is a stick‐slip phenomenon that occurs in wet clutch devices when the static friction levels are higher than the dynamic friction levels. This stick‐slip can propagate vibrations through the vehicle’s driveline and in some cases can even be felt by the driver. The μ‐V speed slopes are measured in between 24‐hour long, steady‐state intervals. During these intervals the clutch is rotated at a constant speed, temperature and pressure to stress the fluid and friction material. After the 24‐hour period, another μ‐V speed sweep is conducted. This alternating pattern of 24‐hour, steady‐state intervals and speed sweeps continues until the slope of the μ‐V curve © 2014 The Lubrizol Corporation P a g e | 6 goes negative. The amount of time it takes for the fluid to achieve a negative slope is reported as the anti‐shudder durability service life. This test typically measures the fluid’s μ‐V characteristics at three temperatures (40°C, 80°C and 120°C). The test is stopped when the slope is less than zero at 80°C, indicating friction deterioration that could lead to shudder issues. The standard JASO friction material is D‐0512 for JASO test procedure M349‐
2001 and D0600‐2 for JASO test procedure M349‐2010, but other friction materials can also be used in this test based on specific OEM requirements. The bar graph in Figure 8 shows the hours until negative slope that the two commercial CVT fluids provide in this test with the standard D‐0512 friction material. CVTF A averaged about 96 hours and CVTF B averaged about 144 hours. While this level of performance was considered acceptable for first‐
generation CVTFs, it would be deemed unacceptable for modern ATFs and CVTFs. However, for the next‐generation CVT fluid, the OEM target is to provide over 2½ times the anti‐shudder durability performance as compared to today’s CVT fluids, but with no loss of metal friction performance. Attaining this type of balanced performance will require new friction modifier additive technology to provide the enhanced wet clutch performance without compromising metal friction. Figure 8. JASO LVFA friction performance of CVT fluids Lower Viscosity CVT Fluid Requirements Next‐generation CVT fluids are following the trend of ATFs in moving from conventional high viscosity fluids that have a kinematic viscosity at 100°C greater than 7.0 cSt to lower viscosity fluids with a kinematic viscosity at 100°C in the range of 5.5 cSt to 6.2 cSt. The main reason for this change is to gain additional fuel economy benefit. This movement to lower viscosity will increase the demand for better wear and pitting protection, improved thermal stability, stronger anti‐foam performance and better shear stability, which will be covered in the following sections. © 2014 The Lubrizol Corporation P a g e | 7 CVT Fluid Anti‐wear and Pitting Protection CVT fluids have always required enhanced wear protection, due to the differences in types of metal‐
metal contact in the CVT compared to a stepped automatic transmission. However, the movement to lower viscosity CVT fluids will require even stronger anti‐wear performance because of the thinner fluid film thickness protecting the metal parts in the transmission. This enhanced performance can be achieved by the proper balance of the anti‐wear additive technology – typically boron, phosphorus and sulfur chemistry. Some of the typical industry tests that are used to measure the wear performance of ATFs are also used for CVT fluids, but the requirements for CVT fluids could be different based on OEM specifications. OEMs often require the FZG Visual Scuffing test, based on ASTM D5182, which measures scuffing resistance, and the FZG Pitting test, which measures the fatigue wear (pitting) performance of the fluid. Examples of scuffing and pitting on the gears are shown in Figure 9. The CVT push belt or chain box test can also provide additional information on wear protection. Measuring the wear profiles on the belt elements or chain pins and the pulleys after the durability test can provide valuable information on how the CVT fluid provides wear protection in an actual variator system. The two element photos shown in Figure 9 provide examples of desired and undesired belt element wear after the belt durability test. FZG Visual Scuffing Gear
FZG Pitting Gear
Pitting
Micro ‐pitting Scuffing
Belt Elements After CVT Durability Test
Very Light Wear
Acceptable
Medium Wear
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Figure 9. Examples of scuffing, pitting and belt element wear CVT Fluid Oxidation Stability As a result of the new CVT design changes, coupled with lower fluid viscosity, greater emphasis on fluid stability will be required. Next‐generation CVT fluids must demonstrate improved oxidation stability for higher operating temperatures, lower sump volumes and longer warranties. Two common industry tests used for assessing oxidation stability are the DKA and ISOT oxidation tests. These glass tube tests accelerate the oxidation of the test oil in the presence of catalysts and air. The tests typically are run from 96 to 192 hours at temperatures ranging from 150°C to 170°C, depending on the requirements and conditions specified by the OEM. The goal is to minimize fluid change with regard to viscosity increase, acid formation and sludge/varnish buildup. Figure 10 shows the DKA oxidation glass tubes at the end of test between a newly developed, more thermally stable CVT fluid (clean tube) © 2014 The Lubrizol Corporation P a g e | 8 and an older, commercially available CVT fluid, which gave heavy sludge deposits. More thermally stable additive technology will be required to achieve better thermal stability of the fluid. Clean DKA tube at EOT
Heavy sludge
deposit at EOT
Figure 10. DKA oxidation glass tubes at end of test CVT Fluid Anti‐foam Performance Another key requirement for CVT fluids is enhanced foam control. CVTs are very effective foam generators. The hydraulic pump pushes the fluid throughout the transmission and sprays the oil onto the belt and pulleys. This sets up a more severe environment for the fluid due to all the churning of the oil, which allows for greater air entrainment and foam generation. Improved foam control durability is needed to prevent inefficient lubricant film, cavitation in the hydraulic system and loss of oil due to fluid expulsion. To insure better foam control for next‐
generation, low viscosity CVTFs, many OEMs may require foam evaluation on both new fluids and stressed fluids after oxidation tests and/or variator tests. CVT Fluid Shear Stability Performance Finally, minimizing fluid viscosity loss over the life of the transmission is always desired and will be even more critical for lower viscosity CVT fluids in order to maintain adequate film thickness and pump efficiency. The KRL tapered roller bearing test is typically used to measure the shear stability of the fluid. A goal for the next‐generation CVTF is to minimize viscosity loss after a 100‐hour KRL shear test, which has been correlated to about 100,000 miles in some vehicle fleet test studies. Summary Fuel economy is the key driving force behind current and future improvements in both CVT hardware and fluid technology. For the next‐generation CVT fluid, one of the critical challenges facing the CVT lubricant formulator is to maintain higher stable metal friction to reduce clamping forces between the belt or chain and pulleys. In addition, the fluid must provide improvement in anti‐shudder durability for the TCC or WSC to reduced noise, vibration and harshness issues, as well as provide driver comfort. These contrasting requirements will probably require new additive technology to achieve the right friction balance. © 2014 The Lubrizol Corporation P a g e | 9 As the interest in low viscosity fluids also increases, the next‐generation CVTF will require improved wear and pitting protection of the belt or chain and pulley contact areas, and improved oxidation stability to compensate for higher operating temperatures. It will also need to provide excellent anti‐
foam durability and improved shear stability of the fluid viscosity for the life of the transmission. Because of these unique performance requirements, current and future continuously variable transmissions will continue to need specially designed fluids. The next article in the series explains how dual clutch transmissions (DCTs) function, and why the lubricant performance requirements differ from an MT or conventional AT. To read additional articles in the series, go to www.drivelinenews.com/resources. © 2014 The Lubrizol Corporation