The application of CVT (Continuously Variable Transmissions) in modern passenger vehicles is not exactly a new development, and early versions of the technology were plagued with serious reliability issues. However, while the technology cannot be described as “mature” by any stretch of the imagination, recent developments in the fields of design/engineering and control systems have rendered modern versions of CVT technology somewhat more reliable and efficient than was the case ten years ago. Therefore, this article will briefly discuss what CVT transmissions are, how they work, what their advantages / disadvantages are, and why they fail, starting with this question-
Also known as, stepless transmissions, single-speed transmissions, or pulley transmissions, CVT transmissions use a system of two pulleys and a pliable belt to transmit torque to the driving wheels. Unlike other types of transmissions that use gears that can be arranged in different patterns to produce fixed gear ratios, CVT transmissions offer an almost infinite range of “ratios” that are not necessarily linked to, or governed by the engine speed.
Different versions of CVT technology go by many different names, including, Lineartronic (Subaru), Xtronic Jatco (Nissan / Renault), K CVT, CVTi, eCVT (Toyota), INVECS-III (Mitsubishi), and Multitronic (VAG-group vehicles), and while some design specifics vary between automotive applications, the underlying principles of operation are the same in all cases.
In practice, automotive CVT transmissions consist of two variable pulleys, and a belt with a fixed length that runs on both pulleys. One pulley is attached to the engine (typically via a torque converter to provide a neutral stance), while the other is linked to the final drive via a set of gears, with the belt being the means whereby power is transmitted from the engine to the final drive via the two pulleys.
In simple terms, “gear ratios” are established by means of varying the effective diameters of both pulleys in opposite directions at the same time, which is accomplished by causing pressurised transmission fluid to act on the pulleys through computer-controlled actuators. Thus, since the belt has a fixed length, the effective diameters of the two pulleys must be altered by exactly the same amount at exactly the same time to prevent either belt slippage, or belt failure.
Both pulleys consist of moveable sheaves that slide on a shaft; by moving the sheaves of the drive pulley (the pulley connected to the engine) apart, the sheaves of the driven pulley (the pulley connected to the final drive), has to be moved closer together by the same amount at the same time to maintain the proper belt tension.
The practical effect of altering the effective diameters of the two pulleys is roughly analogous to how engaging gears with different diameters produce different gear ratios in a manual transmission. However, unlike a manual, or for that matter, an automatic transmission that is limited to a fixed numbers of gear ratios, a CVT transmission can seamlessly produce any ratio between the opposite maximum and minimum effective diameters of the two pulleys.
NOTE: At this point, it is perhaps worth noting that the word “belt” is used rather loosely. While some applications do use a reinforced rubber belt that provides reasonable levels of efficiency and reliability, other manufacturers use a steel belt, or more precisely, a steel chain, instead of rubber belt. In these designs, the edges of the chain links engage with indentations in the pulleys, and while this arrangement does offer improved efficiencies and durability, this comes at the cost of having had to develop an extremely complex, and largely fault-intolerant control system.
For example, to keep the chain engaged with the indentations on the pulleys, stepped adjustments to the effective diameters of the pulleys must typically be made in about 1/60th of a second to prevent damage to both the chain and the pulleys.
Overall, CVT transmissions require fewer mechanical components to work than both manual and automatic transmissions. Viewed logically, this should have increased both reliability and ease of use of CVT transmissions, but for several reasons, neither has happened. Below are the main reasons why CVT transmissions have not completely replaced conventional transmissions-
Although CVT transmissions have found a wide application in passenger vehicles, including many SUV’s, and particularly SUV’s made by Nissan, all earlier designs suffered from what many drivers described as “an unsatisfactory driving experience”. Below are some of the major drawbacks of CVT transmissions-
Poor driveability
The root of the problem is that CVT transmissions are not mated to engines with respect to how any given mechanical gear ratio affects power delivery and fuel economy. With CVT transmissions, the transmission control system “locks” the engine into a speed that ensures maximum power is delivered to the driving wheels for the longest possible time.
In practice, the throttle position sensor(s) provide(s) the primary input data for the purpose of transmission control. For instance, when the throttle is opened suddenly from a standing start, the transmission control system will decrease the effective diameter of the drive pulley, while increasing that of the driven pulley to create a ratio that is roughly analogous to first gear on a conventional transmission.
However, as the vehicle gathers speed, the engine speed will remain constant at the speed it had when the vehicle took off, while the transmission control system gradually adapts the relative effective diameters of the pulleys. Only when the control system detects that the current engine speed is no longer providing maximum torque to the driving wheels will it allow the engine speed to decrease to the level where it will again provide maximum torque, based on the now-changed relative effective diameters of the pulleys.
The practical effect of this is that the engine speed can remain constant through a wide range of ratios, which phenomenon has become known as the “rubber band effect”, an unsettling experience that can be defined as “waiting for the vehicles’ road speed to catch up to its engine speed”. However, to counteract consumer resistance against the rubber band effect, car manufacturers have developed software in recent years that allow changes to the effective diameters of the pulley to be made only at predefined engine speeds.
While this goes some way towards simulating or mimicking conventional shifting patterns, either automatically or in manual mode via paddle shifters, the nature of CVT transmissions is such that the rubber band effect cannot be eliminated altogether.
Poor reliability
Despite aggressive marketing tactics by manufacturers to demonstrate the opposite, CVT transmissions are inherently unreliable, purely on account of their design/nature. The most common failure modes include programming errors that prevent effective control of the transmission, belt failures that cause poor acceleration, mechanical noises, and very high maintenance and/or repair costs compared to conventional transmissions.
Moreover, since most mechanical failures occur at around the 100 000 km mark, and very often well short of that, most manufacturers of CVT transmissions have developed aggressive routine maintenance / servicing schedules for these transmissions. In fact, the average cost of ownership of a CVT transmission-equipped vehicle is significantly higher (up to 25% higher) than that of a similar vehicle with a conventional transmission.
Poor towing ability
Perhaps the greatest disadvantage of CVT transmissions is the fact that usable power to the driving wheels is limited to the power that can be transmitted through the small contact areas between the belt and the pulleys.
This limitation is partially offset by the fact that the engine is always running at the speed best suited to the current operating conditions, based on input data from the throttle position sensor(s). However, while the effective diameter of the drive pulley relative to the effective diameter of the driven pulley can create a ratio that is analogous to first or second gear on a conventional transmission, excessive loads cause the belt to overheat very quickly, with belt failure usually following soon after.
This is the primary reason why CVT transmissions are not found on heavy trucks or powerful, high-end applications where effective power transmission is more important than seamless acceleration. In fact, Ford discontinued the use of CVT transmissions many years ago because of their limited power transfer abilities, poor reliability, and high maintenance costs.
Despite the limitations of CVT transmissions listed above, most manufacturers go to great lengths to advertise the positive aspects of their CVT transmissions. Whether or not these aspects constitute tangible advantages is debateable, but in the interests of fairness, we will list the most commonly claimed advantages anyway-
Seamless acceleration
If the rubber band effect is ignored, CVT transmissions do indeed provide seamless acceleration, but so do most modern automatic transmissions, which can provide gearshifts that are practically undetectable.
Peak power is available all of the time
Since the engine speed is not linked to a fixed gear ratio as on conventional transmissions, the engine can maintain maximum power through a wide range of ratios, or more precisely, pulley settings. Proponents of CVT transmissions claim that this is a distinct advantage over conventional transmissions since it eliminates repeated gear changes in driving conditions such as steep hill climbs or heavy city traffic.
Fewer moving parts
In theory, fewer moving parts should have increased reliability of CVT transmissions as a corollary, but experience has shown this not to be the case. Poor reliability remains a valid concern with CVT transmissions, and it is likely to remain so for at least the immediate future.
Improved fuel economy
While it is true that CVT transmissions provide a measureable improvement in fuel economy on many applications because the engine is almost always running at its most economical speed, any savings in fuel costs are largely offset by the increased service and maintenance costs of CVT transmissions.
CVT transmissions are a common feature of hybrid vehicles, but in these applications, the technology uses two power inputs, unlike conventional CVT technology that uses only the input from the engine.
On hybrids, the technology is commonly known as eCVT (electronically Controlled Continuously Variable Transmission) and it uses inputs from both the engine and the electric traction motor. Depending on current driving conditions, only a part of the engine’s power is linked to the drive train; the other part of the engine’s power is diverted to a generator and an electric motor, with the amount of power that that is channelled through the electrical path determining the effective ratio of the CVT transmission.
In practice, this important difference between conventional CVT technology and the variant used on hybrid vehicles means that the two applications are fundamentally different, and the variants should therefore be approached differently in terms of diagnostics, and maintenance / service procedures.
