Continuously Variable Transmission (CVT) Transmission System

Welcome to my blog Explore Automotive, Here we see the automotive transmission gearbox which is latest and finest gearbox that’s CVT.

Due to continuously increasing global demand and need for reducing fuel consumption and emissions, there have been continuous improvements going on power-train and transmission efficiency. This is the main thing is taken into consideration, different types of transmission have been developed to meet environmental requirements, emission legislation and customer’s demands for driving pleasure in the next generation of vehicles.

Types of Transmission Gearbox:

There are five types of transmission gearboxes which are as follows:

1) Manual Transmission Gearbox (MT)

2) Automatic Transmission Gearbox (AT)

3) Automatic Manual Transmission Gearbox (AMT)

4) Dual Clutch Transmission Gearbox (DCT) or Dual Shift Gear Transmission Gearbox (DSG)

5) Continuously Variable Transmission Gearbox (CVT)

Here I just introducing the five types of transmission gearbox one by one but in this blog we see the latest and finest type of transmission that’s Continuously Variable Transmission (CVT).

1) Manual Transmission Gearbox:

Various types of automobiles are equipped with manual transmissions. Small economy cars predominantly featured with manual transmission gearbox because they are cheap and efficient, although many are optionally equipped with automatic transmission. 

Economy cars or budget cars are also often powered by the small engines, and manual transmissions make more efficient use of the power produced. Manual transmission gearbox is more efficient than other gearbox such as conventional automatic gearbox, belt driven continuously variable transmission (CVT). Generally this gearbox comes in all type of vehicle because it has less weight that makes vehicle light in weight. 

Manual gearbox operated by using pedal of clutch to change the gears manually by the engagement and disengagement of clutch. The engine which is coupled with a manual transmission can often be started by the method of push starting. This is particularly useful if the starter is inoperable or defunct.

2) Automatic Transmission Gearbox:

Control of automatic transmissions represents an integration of electrics, electronics and hydraulics. The basic functions are achieved by use of sensors, actuators and appropriate software in the transmission control unit (TCU). 

Planetary gears are used in automatic transmission which is always in mesh as a basic means for multiplying the torque obtained from the engine. And the main component is that torque converter with fluid coupling to be placed in the position of clutch. The torque converter serves two primary functions. 

First, is that it act as a fluid coupling to connect engine power smoothly through oil to the transmission gear. Second, it multiples the torque from the engine when additional performance is required.

3) Automatic Manual Transmission Gearbox (AMT):

Upgrading the Manual transmission into automatic in that way, electronic equipment with software is placed to control the gears according to vehicle speed variation among the driving conditions. 

In AMT, coded software is containing the speed limits on which the gear shifted to another when particular limit exceeds that’s the actual way of operation of automatic manual transmission. 

Currently, vehicle manufacturers of includes AMTs focusing on the customer’s demand that means better gear shifting quality (jerk free) and more efficient transmission system, in addition to further reduction in fuel consumption of the vehicle.

4) Dual Clutch Transmission Gearbox (DCT) or Dual Shift Gear Transmission Gearbox (DSG):

At very first, dual clutch transmissions are one kind of hybrid system of a manual and automatic transmission. They’re more similar to a manual transmission system, however, in DCT use two clutches (hence dual clutch) to synchronize the gear changing process. 

To know the how DCT works, for that we must know about manual transmission working phenomenon. In manual transmission driver continuously use the clutch to disengage engine drives from the other driveline to change the gear smoothly. 

In DCT, instead of that two clutches are used to change the gears and both are controlled by means of computer system so there is no need of clutch pedal for shifting gears.

5) Continuously Variable Transmission Gearbox (CVT):

A basic of CVT works just like a variator. Its consisting that a primary pulley, a secondary pulley and a rubber V-belt which connect these two pulleys. 

Each of driver and driven pulley consists of a fixed and a movable pulley is given in fig. 1.(a). The fixed pulleys are fixed on the shafts and the movable pulleys are able to move in the axial direction on the shafts. 

Continuously variable transmission can be achieved by controlling the pulley axial distance between the fixed and the movable pulleys. When the movable pulley of the driver shaft is moving towards the fixed pulley, that time the V-belt is forced to be pushed in the radial outward direction, which causes the belt pitch diameter to be increased. 

Since the belt length and the center distance between the shafts are to be fixed, the belt pitch diameter of the driven pulley get decreases. Therefore, the speed ratio varies in a continuous manner. The variator geometry is given in Fig. 1.(b)

In the last few years, V-belt CVT (Continuously Variable Transmission) is a transmission which having a speed ratio that can be varied continuously over its allowable speed range. Its speed ratio may take on any value between its operating limits, i.e. infinite number of speed ratios are possible. 

In a gearbox transmission, on the other hand, has a discrete number of fixed speed ratios obtained. This property of V-belt CVT gives a better fuel economy as compared with that of classical gearbox transmission. 

Besides that, V-belt CVT has many advantages such as compact, light weight, low manufacturing cost because it has a relatively small number of parts. 

V-belted continuously variable transmission (CVT) has been widely used in low-powered vehicles such as snowmobiles and scooters because of its significant advantages over the other transmission system, including its simple in construction, smooth in operation, easy drivability, low cost, easy maintenance, etc. 

CVT allows that an engine to operate near at maximum power point by automatically varying the speed, so theoretically, rubber V-belt CVTs have advantage of an economic efficiency over the other transmissions. 

However, in spite of the several advantages proposed by a CVT system, the goals of higher fuel economy and better performance have not been realized significantly in a real production vehicle. 

To achieve the objective of lower emissions of vehicle and better performance, it is necessary to capture and understand the detailed dynamic interactions in a CVT system so that efficient controllers could be designed to overcome the existing losses and enhance the fuel economy of the vehicle. 

To overcome this type of energy loss, the electromechanical actuated CVT system becomes an effective solution, since this system only operates during changing the transmission ratio. An electro-mechanical actuated CVT system with a single acting pulley system was introduced.

Continuously variable transmission (CVT) allows that driver can simplify the operations according to driver’s intention and vehicle driving conditions. By combining with the electronic control technology, speed ratio can be changed continuously according to the control strategy, which could be reduce the driver's labor intensity and eliminate the shift shock.

The engine and external load can be achieved the best match, which could improve the fuel economy and emission performance. So it’s an ideal way to drive a car.

According to driver’s intention and vehicle driving conditions, CVT speed ratio can be controlled easily to change continuously, which make vehicle movement and driving resistance achieve a sense of the best match. So, the speed ratio control system is the core control system of CVT.

Principal of CVT

CVT is a transmission having a speed ratio that can be varied continuously over its allowable operating range. Its speed ratio may take on any value between its allowable operational limits, i.e., an infinite number of ratios are possible. 

In a gearbox transmission, on the other hand, has a discrete number of fixed speed ratios. Continuously variable transmission can also usually implies that torque may be controlled independently of speed ratio and vice versa.

In other words, the torque converter of the conventional automobile should not be considered as a CVT because of the speed ratio is set by the torque to be transmitted. 

The term infinitely variable transmission (IVT) means basically the same as CVT, with the added restriction that a speed ratio of zero must be available, i.e., it must be possible to have zero output velocity for any input speed producing an infinite ratio range. 

A CVT providing negative as well as positive speed ratios would also be considered an IVT since its range passes through a speed ratio of zero. Even though this definition of IVT is generally accepted, IVT is often used as a synonym for CVT by those not familiar with the difference. 

Ratio range is one of the most important parameters of a CVT in terms of characterizing it for possible applications. Ratio range is defined as the numerical ratio of the maximum to the minimum output speeds possible for a given fixed input speed. 

For example, if a CVT can be controlled to operate in the range between 3000 and 1000 rpm for a given fixed input speed, its ratio range is 3.0. 

Ratio range is usually more significant than the speed ratios themselves, since the latter can normally be adjusted if necessary by other components in the drive line (e.g., the rear axle ratio of an automobile). The ratio range of an IVT is infinite, since it is calculated as a finite ratio divided by zero. 

Types of CVT

Anciently, different types of automobiles are exclusively use either a conventional manual transmission or transmission with having multiple planetary gears sets that use integral clutches and bands to achieve optimum gear ratios at its allowable ranges. 

A typical automatic transmission uses four or five such gears, while a manual normally employs five or six gears. The continuously variable transmission (CVT) replaces the discrete gear ratios with infinitely adjustable gearing through one of several basic CVT designs.

The CVT system can be of following three types:

 1. Pulley based CVT

 2. Toroidal CVT

 3. Hydrostatic CVT

1. Pulley Based CVT:

A pulley based CVT system is very simple in design & working. The cone pulleys (variable diameter pulleys) are the heart of CVT system.

It has only three basic components:
   ·   A high power metal belt
   ·  A variable input driving cone pulley
   · An output driven cone pulley

The driving pulley is connected to the crankshaft of engine, while the driven pulley is connected to the output or driveshaft of the transmission system. Each conical pulley is made of two 20-degrees cones facing each other. The V-belts rides in the groove of cones. 

The V-belt gets their name from the fact that belts bear a V-shaped cross section, which increases the frictional grip of the belt. 

When the two cones of the pulley are far apart, then the belt rides lower in the groove, & thus the radius of the belt loop going around the pulley get smaller. When the cones are closed together the belt rides higher in the groove, & the radius of the belt loop going around the pulley get larger.  

When the one pulley increases its radius, the other decreases its radius to keep the belt tight. As the pulleys change their radii relative to one another, they create infinite number of gear ratios from low to high & everything in between. 

When the pitch radius s small on the driving pulley & large on the driven pulley, then the rotational speed of the driven pulley decreases, resulting in a lower gear.

As of now, when the available pitch radius is large on the driving pulley & small on the driven pulley, then the rotational speed of the driven pulley increases, resulting in a higher gear. Thus, in theory, CVT has an infinite number of gears ratios. 

2. Toroidal CVT:

These types of transmissions are used the high shear strength of viscous fluids to transmit torque between an input torus and an output torus. As the movable torus slides in linear direction, the angle of a roller changes relative to the shaft position, as seen in Fig.(4). This results in a change in gear ratio.

Half toroidal CVTs have a bit higher efficiency than full toroidal transmissions. But their production seems to be relative costly in comparison to automatic transmissions. One of the reasons therefore are their expensive roller suspensions with a lot of prismatic precise and grinded parts. 

On the other hand the design of full toroidal variators are much lighter and less complex. But up to now because of the higher thermal mechanical surface stress the discs and rollers of full toroidal variators do not achieve reliably comparable lifetimes as half toroidal systems and as automatic transmissions.  

The power delivered by the engine is firstly transmitted to the input discs via the torque converter. Secondly, it is transmitted to the output discs by the power rollers sustained by the trunnion. 

Lastly, the power is delivered to the wheels via the output shaft which is mechanically linked to the output discs of the T-CVT unit.

In order to increase the chances for toroidal transmissions in the market, it needs lighter gearbox designs. These designs should fit into the increasing slim driveline tunnels and offer a very stable operating behaviour as well as the highest possible torque capacities.  Furthermore their production costs should be significant lower than today.

3. Hydrostatic CVT:

Hydrostatic transmissions transmit power through the use of high-pressure oil, typically at pressures up to about 5000 psi. A hydrostatic transmission (Fig. 6) consists of a hydraulic pump and hydraulic motor connected together by two hydraulic lines and with the other required hydraulic components (such as reservoir, check valves, and relief valves). 

The pump creates the hydraulic power (pressure and flow rate), and the motor converts the hydraulic power to mechanical power (torque and speed). 

The basic system is a CVT if 3 the pump is designed to have a displacement (in. /revolution) that can be varied. (Sometimes the motor is also given a variable displacement to give additional versatility.).

The stroke of the pump can usually be reversed so that the hydraulic motor rotation can be either positive or negative.

Straight hydrostatic transmissions (the power-split version will be discussed later) will almost always have a ratio range of infinity, i.e., be IVTs. (Since the stroke of the pump can be set to zero, the output speed of the motor will vary from zero to its maximum value.). 

The torque of the hydrostatic transmission can be reversed (the high-pressure line changing to the low-pressure line and vice versa), with the "pump" then acting as a motor and the "motor" as a pump. 

The hydrostatic transmission is thus quite versatile, and has a number of features desirable for an automobile transmission. There are many commercially available units in sizes appropriate for automobiles. 

The major disadvantages are size, weight, and relative inefficiency (especially when compared to gears) of the straight hydrostatic transmission over a wide speed and torque range.

Other CVT Varieties

Other few types of CVTs have been developed over the course of automotive history, but these types have become less prominent than the push belt and toroidal CVT system.  

The mutating traction drive uses a pivoting and conical shaft to change the “gears” in a CVT system.  As the cone changing an angle, the inlet radius decreases and at same time the outlet radius increases, and vice versa, resulting in an infinitely variable gear ratio which is to be desired. 

Variable geometry CVT system uses the adjustable planetary gear sets to change the gear ratios, but this is more similar to a flexible traditional transmission system than the conventional CVT system.

Inherent Advantages and Benefits

Certainly, the clunky dull sound of a shifting transmission is familiar to all the drivers. On contrast to that, a continuously variable transmission is perfectly smooth gives luxurious ride—as it naturally changes “gears” discreetly and minutely such that the driver or passenger feels only a steady acceleration. 

In a theory, CVT would gives a causeless engine fatigue and would be a more reliable transmission system, as the harshness of gear shifting and discrete gears force the engine to run at a less-than-optimal speed. 

Moreover, CVT system offered an improved efficiency and performance of the vehicle. In Table (1), below showing the power transmission efficiency of a typical five-speed automatic, i.e. denoting the percentage of produced engine power translated through the transmission. This yields an average efficiency of 86%, as compared to a typical manual transmission with 97% efficiency. By comparison, Table (2) below gives an efficiency ranges for several CVT designs.

Table (1) Efficiency vs. Gear Ratio for Automatic Transmission

Gear	Efficiency Range
1	60-85%
2	60-90%
3	85-95%
4	90-95%
5	85-94%

                                           Table (2) Efficiency of Various CVT Designs

CVT Mechanism	Efficiency Range
Rubber Belts	90-95%
Steel Belts	90-97%
Toroidal Traction	70-94%
Nutating Traction	75-96%
Variable Geometry	85-93%

These CVT systems offering improved efficiency over conventional automatic transmission systems, and their efficiency depends less on driving habit of drivers than transmissions. 

Moreover, because of the CVT allows that an engine to running at its most efficient point virtually independent on the vehicle speed, a CVT equipped vehicle yields fuel economy benefits when compared to the conventional transmission. 

Testing by ZF Getriebe GmbH several years ago, it is found that “the CVT uses at least 10% less fuel consumption than a 4speed automatic transmission” for U.S. Environmental Protection Agency city and highway cycles. 

Moreover, the CVT system was more than one second faster in 0-60 mph acceleration testing. The potential for fuel efficiency gains can also be seen in the CVT currently used in Honda’s Civic car model.  

Honda Civic with a traditional automatic averages 28/35 miles per gallon (mpg) city/highway and seen in comparison with the same car with having a CVT gets 34/38 mpg city/highway. 

Honda has used continuously variable transmissions (CVT) in the Civic for several years, but these are model of 1.6 liter cars with having limited torque capabilities.  

Ongoing research and development will inevitably expanding the applicability of CVTs to a much broader range of the engines and automobiles.

Challenges and Limitations:

CVT development has progressed slowly for the variety of reasons, but much of the delaying in the development process can be attributed to the lack of demands for conventional manual and automatic transmissions have long offered sufficient performance and the fuel economy.  

Thus, the problems encountered in CVT system development usually stopped said progress. “Designers are still not have tried successfully to develop the [CVT] system that can match the torque capacity, efficiency, size, weight, and manufacturing cost of step-ratio transmissions”. 

One of the major complaints having with the previous CVTs has been slippage in the drive belts and rollers. This is caused by the lack of discrete number of gear teeth, which is forming a rigid mechanical connection between the gears; friction drives are inherently prone to slip, especially at higher torque. 

With early CVTs of the 1950s and 1960s, engines equipped with CVTs would run at excessively high RPM trying to “catch up” to the slipping belt.  This would be occurred at any time the vehicle was accelerated from a stop at peak torque: “For the compressive belts, in the process of transmitting the torque, micro slip occurring in between the elements and the pulleys.  

This micro slip tends to be increased sharply once the transmitted torque exceeds a certain value”. For many years, the simple solution to this problem is that it has been to use CVTs only in cars with having relatively low-torque engines.  

Another solution is that to employ a torque converter (such as those used in conventional automatics), but this results in reduces the CVT’s efficiency. Perhaps more than anything else, CVT development has been makes difficult by cost.  

Low volume and a lack of infrastructure have been affecting the driven up manufacturing costs, which inevitably yielding in higher transmission prices. 

With increased development, most of these problems can be addressed simply by improvements in manufacturing techniques and materials processing.  

For instance, see the Nissan Motor’s Extroid model “which is derived from a century-old concept, perfected it by using the different modern technology, metallurgy, chemistry, electronics, engineering, and precision manufacturing techniques”. 

In addition to that, CVT control must be addressed.  Even if a CVT can operate at the optimal gear ratios at any variable speeds, how does it “know” what ratio to select?  

Manual transmissions have always a manual controls, where the driver shifts in respective gears when he or she so desired; while in an automatic transmissions have relatively a simple gear shifting algorithms which is accommodate between three and five gears. 

However, CVT systems required far more complex algorithms to accommodate an infinite division of speeds and gear ratios.