The 2013 Honda Accord has both 4 cylinder and 6 cylinder cars, however, only the 4 cylinder cars have a continuously variable transmission (CVT). I was wondering... is there any particular reason for this?
There's no fundamental reason you can't have a CVT with a 6-cylinder engine. The simplest explanations I can think of are:
The market for CVT is people who want modern efficient vehicles, the opposite demographic of people who want 6- and 8-cylinder engines, and
The CVT helps deliver the right amount of torque at a wide range of speeds, reducing the need for an over-powered engine.
I discussed the popularity of CVTs with a local vendor during my wife's shopping for a new car. The vendor in question was trying very hard to convince us that a CVT was just as good as a regular automatic (which, in certain circumstances, might be true). I asked him why the model that we were looking at did not come in a regular auto or manual configuration. His answer was "fuel efficiency."
Their official corporate message was that they were best able to meet their fuel consumption goals by continuously controlling the revs of the engine. This meant that they essentially needed continuous control over the gear ratio (also known as a CVT). Unfortunately, this also means that an accelerating CVT car sounds very much like a car with a slipping clutch.
For what it's worth, we didn't purchase that vehicle and instead went with a conventional six-speed auto with paddles that the wife loves very much.
The CVT was developed as an alternative to normal automatic transmissions. An automatic has relatively high power losses due to friction and torque converter slip. This was especially noticeable in small cars, so the first CVTs were developed for small cars.
A CVT generally uses a belt to transmit power between two variable-diameter wheels. The tensile strength of this belt, and slip between the belt and wheels, limit the amount of torque that can be transmitted. Scaling the design up with enough reliability for road use has proven difficult.
CVTs have advantages: lower power losses, and you can run the engine closer to its most efficient rev range for more of the time. Both help reduce fuel consumption.
In 1993, the Williams Formula 1 team experimented with a CVT. Unfortunately this was banned from competition before they finished development. Some owners of classic DAFs with first-generation CVT using rubber belts run much larger engines than original, reportedly the CVT keeps working in those circumstances but belts wear out really quickly (with twice the power, replacement was required at 1500 km according to one owner).
Subaru is the only company so far that has done this with the Subaru Legacy 3.6R limited,Outback, and the discontinued Tribeca. I am actually interested in knowing if the CVT version of the Legacy 3.6R limited is as fast or faster than the manual. Theoretically CVTs could be faster. I forgot about V6 engines. Nissan likely has a V6 automobile with a CVT.