A shiftless transmission?
Dr. Edward P. Becker | TLT Automotive Tribology August 2015
Tribologists have a great opportunity to further extend the capabilities of CVTs.
The tribological challenge of CVTs is to transmit the energy from the drive to the driven shaft with minimal loss or slip.
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A MAJOR DRAWBACK OF THE INTERNAL COMBUSTION ENGINE is that it generates zero torque at zero speed—that is, the engine must continue to turn just to overcome its own internal friction or it will stall. Also, to generate sufficient power to accelerate the vehicle, the engine needs to turn relatively fast, even when the vehicle is moving slowly. These facts necessitate some mechanism to vary the relationship between the angular velocity of the wheels and the engine over quite a great range. The solution is a device known as a transmission.
As previously noted in my December 2014 column, in the U.S. more than 95% of vehicles sold have some form of automatic transmission—that is, a transmission that does not require input from the driver to select the speed ratio between the wheels and the engine. This is usually accomplished by means of gears, with each combination of gears providing a fixed speed ratio. Modern transmissions have as many as nine combinations of forward speeds, and the act of moving between the various combinations is called shifting.
The need to improve the fuel efficiency of automobiles has driven the increase in the number of gears (so the engine can operate close to its peak efficiency more of the time). However, more gears means a more complex (and hence larger, heavier and with more friction) transmission. Ideally, we would like a simpler arrangement that would permit the device to vary the speed ratio continuously and seamlessly.
Such a device is the continuously variable transmission (CVT). With very little fanfare, CVTs accounted for less than 3% of automotive transmission in 2006 and rose to an estimated 12% in 2014. The most common arrangement uses two split pulleys and a metal belt or chain to vary the speed ratio between two shafts. The tribological challenge is to transmit the energy from the drive to the driven shaft with minimal loss or slip. In other words, the belt must ride on the pulley with high friction (to transmit energy) but also minimal wear (for durability).
So, CVT fluids have been developed, and continue to be developed to meet these very demanding requirements. The task is far from complete, so automotive tribologists have a great opportunity to keep developing the fluids and additives that extend the capabilities of these devices.
The CVT is known by a variety of names, including the single-speed transmission, gearless transmission, one-speed automatic, variable pulley transmission and infinitely variable transmission. In my opinion, the most accurate name would be the shiftless transmission. Ironically, that word
shiftless already has a common meaning, which is precisely the opposite of what this device is designed to do!
Ed Becker is an STLE Fellow and past president. He is president of Friction & Wear Solutions, LLC, in Brighton, Mich., and can be reached through his website at www.frictionandwearsolutions.com.