Continuously Variable Transmission

Posted by murmini Tue, 10 Oct 2006 08:29:39 GMT

With the onset of the new new MINI models in 2007 comes the demise of the CVT option on the MINI Cooper.

The MINI Cooper S has the option of 6 speed automatic using the Agitronic paddels on the steering wheel verses the standard 6 speed Getrag gearbox. The Cooper has a Continuously Variable Transmission or CVT option in lieu of the standard 5 speed stick shift.

In 2007, MINI will remove the CVT as an option and will rely on the more traditional automatic version as seen in the Cooper S today. What is interesting about the CVT gearbox is the principle that it employs and the adaptability that it brings to a city-style car where pure performance driving is competing with the prospect of continuous city stop/go motoring.

In a manual gearbox, you select gears, by decoupling the engine from the drive wheels, using a clutch. Gears of different sizes are selected, 1, 2, 3 and so on, and the gears are meshed together, the clutch re-engaged and the power is transferred from engine to wheel using the selected ratio.

In an automatic gearbox, it selects the gears as the engine speed increases. Gears of increasing ratios are selected to deliver power to the wheels. Thus keeping the engine in its most efficient power band as the car increases in speed. The selection of each specific gear is dictated by the speed that the driver wants the car to go.

In the Continuously Variable Transmission approach, there is effectively an infinitely variable gear ratio providing a continuos and smooth transfer of power from the engine to the wheels. Again with the engine being optimized against increasingly faster road speed. One, long, variable ratio gear.

The CVT approach is achieved by two shafts, one connected to the engine and one connected to the differential and subsequently the wheels. Each shaft has a conical shape and the two are opposing each other. A belt drive running over the cones is moved and subsequently transfers energy from the large end of cone 1, to the small end of cone 2 (a low ratio for taking off from a standing start) all the way to where the belt is transferring power from the small radius of the cone 1, to the large radius of cone 2 (a high ratio for cruising at high speed). Its a complicated device, with challenges at minimizing slippage, lubrication, heat build up and so forth.

The methods of transferring the power across the cones has evolved over the years, but the principle remains the same. Historically speaking, Leonardo da Vinci developed a step-less transmission theory in 1490 and again, the principle allows for high speed rotation to be gradually applied from a 'drive shaft' to a 'driven shaft'. First patented in 1886, CVT type transmissions come in a variety of designs.

There are a number of fascinating designs out there such as the NuVinci technique, the ToroTrak system and a design nicely evolved from the conical CVT system called A+CVT - Anderson Continuously Variable Transmission. Developed by Larry Anderson, it overcomes the inevitable slipping belt found in a traditional CVT system. Larry has developed and patented a remarkably efficient system that employs floating sprocket bars running the length of the cones, improving the normal CVT's efficiency closer to that of a standard manual transmission.

Larry has a video on his site and he emailed me with these comments explaining it.

"...what your looking at is not a conventional CVT. The difference is the patented 'floating sprocket bars' which allow a specially designed chain to move along the length of the bars. This is what makes the A+CVT positive-drive, not friction-dependent as other CVTs that are now on the market. CVTs currently being used have a fairly high internal energy loss in the neighborhood of 20%. This bites into the potential greater fuel efficiency which would be realized with a low energy loss CVT, coupled with the advantage of having the vehicle's engine running at the optimum RPMs for any given load condition. My CVTs should have an energy loss which would be about the same as a geared transmission, which is in the 4 to 5% range."

Again, this is just a demonstration of the principle of the conical ACT system and shows how the two opposing cones with a drive belt between them, transfers power from the 'drive' shaft to the 'driven' shaft and how the result is a continuously variable gear ratio.

The CVT transmission in the MINI is manufactured by ZF in Belgium and will cease to be offered in the MINIs after the end of this year.

Thank you Larry !