CVT Kicks Ass, Banned for Being too Awesome.
Dear car-buyer of the early 21st century, meet the Continuously Variable Transmission - They're not just for golf carts anymore! In fact, the current auto market has been quietly adding CVTs as an option for several years now... under the radar so to speak. (If you're interested, here is a list of cars options.)
The Continuously Variable Transmission or CVT does the job of the traditional automotive transmission, but better. Better in every respect but how it 'feels', which is interesting. Very interesting.
Engines need to be pampered
Internal combustion engines have been the prime mover for land vehicles for over a century, and for good reason. Even a modest Model T engine was capable of providing some 20 horsepower, or around 15KW of mechanical energy. The curious thing about engines, however, is that they like to run in a very limited range of speed. Too slow, and the engine stalls, too fast and the combustion starts blasting out the exhaust pipe! An efficient powerful engine is one that is running within its power band, or range of speed in which the combustion process can happen to its fullest. Driving a car, however, requires moving your vehicle through a wide range of speeds, often times many times in a single minute. An engine connected directly to a wheel would stall before you even pulled out of the driveway, and would probably fly apart in a back-firing mess at freeway speeds. Bottom line, engines need pampering with respect to their RPM if you want to get the most power out of them.
Enter the transmission
A transmission is a layer of abstraction. A mathematical multiplier. It trades RPM for torque (speed for force, in the rotating sense.) The traditional transmission has a set of some dozen or so gears that provide around five different speed settings. Really big trucks need even more speeds, sometimes as many as 18! The traditional transmission is a lever operated box that lets the driver select which gear teeth are engaged with one another, effectively rearranging the spinning wheels so that the output speed of the engine roughly matches the speed of the wheels on the road.
It takes some skill to shift gears! My first experience with a standard transmission was my high school girlfriend's rusted Datsun in the pothole filled parking lot of my high school. Plenty of stalls, ground gears, and over-revving before I was competent enough to drive out onto the road... The automatic transmission makes life easier, but is essentially the same thing: A set of five or so speed settings. I just selects them for you, without the need to operate a clutch and listen to the engine speed.
Now the CVT
The cleverly sidesteps much of the complexity of an automatic gearbox. With relatively fewer parts, it can be theoretically more reliable and inexpensive. (In reality, because they use newer technology and require more advanced material science, they are more finicky and more expensive... currently.)
There are a few different types of CVTs. The most common sort used in cars involves a hybrid belt-chain that rides in a grove between a couple of split pulleys. The diameter of the pulley experienced by the the belt-chain depends on how wide the split in the pulleys is set. If one split pulley is widely separated, the belt-chain rides in near the middle, acting like a small pulley. If the halves of the split pulley are brought nearly together, the belt is pushed out to the maximum diameter. Here is a pretty decent video that shows it in action: Understanding CVT!
A low speed, high torque situation is one where the motor's pulley is small and the wheel's pulley is big. A high speed, low torque situation is just the opposite: big pulley to small.
Nissan has invented another sort (pictured in the top photo of this article) which uses rollers that move against inverse toroidal tracks. It accomplishes the same thing as a belt-chain, but with more style.
Trading five speeds for Infinity
What if there was a way for an engine's power and speed to be automatically matched exactly to the speed of the wheels at-all-times? Rather than having to shift up and down through gears, the car's power was somehow cleverly scaled and coupled directly to the road... think of the power! Think of the efficiency if that internal combustion engine was never outside its most efficient operating conditions! The CVT's ability to match the engine's power to the changing power demands turn out to be not just a good idea, but a little too good. Too good for Formula 1 racing anyway. Their use in Formula 1 race cars was banned by 1993 because it easily shaved seconds off a lap, disrupting the entire sport. Rather than allowing one innovation to drive change across the board, Formula 1's governing body opted to go with tradition and limited the total number of allowable speeds. The CVT, having an infinite number of speeds is quite contrary to this tradition.
The feel of a CVT in an ordinary car is likewise very nontraditional. It's powerful. It is fuel efficient - especially in urban driving! You'll win the street races. You'll sail past the truck stops. So why do people hate them?
It's boring! And it feels like the clutch is slipping... There is no hint of that quintessential hallmark of power, the engine rev. Instead, the engine climbs up to a middle-range roar and stays there, fixed in its power-band and ready for use. Drivers who know driving know how to get the most power out of their engine. They have no way to cope with something like this. Their skills are obsolete, by premise and not just by better gear shifting.
It is such a fun and clear example of a disruptive technology in a world that is slow to change. In many ways, the CVT is the final keystone in the perfection of the internal combustion vehicle. How fitting that it is finally beginning adoption, at the very moment in history when internal combustion is being displaced by the electric motor.
Obsolete, once more, by premise and not by just a better engine. It is worthy of some lament that the very best, most powerful car motors will be those operated exclusively by driverless cars... what a century of speed, power, and precision to come!