Benefits of CVT: Fuel Efficiency and Optimal Gear Ratios
A continuously variable transmission (CVT) is an automatic transmission that does not have a fixed number of gears. Instead, it can vary the gear ratio infinitely between its lowest and highest ratios. This allows the engine to operate at its most efficient RPM for the vehicle speed, road grade, or load condition.
One of the main advantages of a CVT is its impact on fuel economy. The earliest automatic transmissions had two speeds, which were quickly replaced by three-speed and then four-speed transmissions. Each increase in the number of gears improved fuel efficiency and drivability. As a result, most current automatic transmissions have eight, nine, or even 11 speeds. However, the logical endpoint of this trend is infinite speeds, which is exactly what a CVT offers.
Having more gears in a transmission reduces the size of the steps between adjacent gears. This has multiple benefits. Firstly, fuel economy improves because the engine is more likely to operate at its most efficient RPM. Additionally, having more gears reduces the need for constant shifting on grades as smaller steps keep the engine closer to its ideal power and torque range. Furthermore, smaller steps also reduce the need for a torque converter, which can be inefficient and rob power from the engine.
A CVT maximizes these benefits by offering an infinite gear count, removing the need for steps altogether. This stepless transmission provides maximum fuel efficiency and drivability on varying road conditions. It also eliminates the need for a torque converter to ensure smooth gear changes.
Disadvantages of CVTs: Perception and Engine Sound
Despite the numerous advantages of CVTs, there are some drawbacks that contribute to their mixed reception among drivers. One major issue is the perception that CVTs fundamentally change how an engine sounds and how a powertrain responds under acceleration.
Traditionally, drivers are accustomed to a rising engine note punctuated by shifts when accelerating. However, CVT powertrains tend to keep the engine RPM steady and alter the gearing instead. This can result in the engine sounding like it’s perpetually stuck in a low gear, which some drivers find disagreeably odd.
To address this concern, many newer CVTs now artificially create stepped gear changes. These simulated gear shifts can be activated in a “Sport” driving mode or even during normal operation. While this may provide a more familiar experience to drivers, it can come at the cost of some fuel economy.
The Inner Workings of a CVT Transmission
Understanding how a CVT works can shed light on its unique design and operation. Imagine a bicycle with multiple gears on its rear wheel. Now, place the same gearset at the pedals, but flip it around. Each front sprocket is designed to work only with a specific rear sprocket, maintaining a fixed chain length through eight gear combinations.
Replace the chain with a V-belt and turn the gearsets into pulleys with V-flanges that can be pinched together or spread apart. Varying the gap between the V-flanges changes the belt’s operation diameter. One of the pulleys is controlled by a computer, while the other is passive with spring tension clamping its flanges together.
As the controlled pulley’s flanges widen or compress, the V-belt’s diameter shrinks or grows accordingly. In response, the spring-loaded pulley’s flanges and belt diameter are forced to do the opposite because the belt’s length is fixed. This creates a stepless transmission with infinite gears or “speeds” – a CVT.
In summary, CVTs offer benefits such as improved fuel efficiency and optimal gear ratios for varying driving conditions. However, their unique characteristics, including perceived engine sound changes and lack of traditional gear shifts, have contributed to some drivers’ negative opinions. Nevertheless, advancements in control strategies and design have significantly improved CVT performance and enhanced the driving experience.
