A continuously variable transmission (CVT) transfers power through a range Contrast with either manual or conventional automatic transmissions that use. The continuously variable transmission. (CVT) is a transmission in which the ratio of the rotational speeds of two shafts, as the input shaft and output shaft of a. Continuously Variable Transmissions. An Overview of CVT Research Past, Present, and Future. Kevin R. Lang. 21W. May 3,
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Abstract. This MQP defines an intuitive protocol for the tuning of the continuously variable transmission. (CVT) for competition applications including the FSAE. PDF | Nowadays, automakers have invested in new technologies in order to improve the efficiency of their products. Giant automakers have. PDF | On Jan 1, , Sanjog Kumar Panda and others published Design of Continuously Variable Transmission Mechanism for Economy.
However, they are modelled as a part of the mechanical subsystem. Model of the Mechanical Subsystem Vp Figure 3 shows the simplified scheme of the mechanical subsystem which is used for the modelling. The torque sensors are replaced in this scheme by the torques M I and M2. For small torques transferred by the CVT or for high fluid flows through the torque sensors, the input and the output shaft of the torque sensor are rigidly connected by squeezing the balls from both sides.
In this case the torque sensor represents a rigid connection. By increasing the torque or reducing the flow, a relative motion between the input and the output shaft of the torque sensor is possible. Now the torque sensor represents an elastic connection, and the torques M I Figure 1: The scheme of the Cv[ considered A torque at the input of a torque sensor causes an axial sliding of its piston shaft which is enabled by a rolling of the balls in the torque sensor Figure I.
This displacement of the piston shaft changes the drain orifice as depicted in Figure 2. Depending on the drain orifice area and the fluid flow into the torque sensor, a pressure results which is a measure for the torque transferred by the CVT. As shown in Figure I. Their dependence is given by the transmission ratio and M2 are determined by the pressure in the torque sensors as well as by the friction torque MR as follows: 13 I where r and r 2 are the relevant radii for the transmission ratio.
The conditions for a torque sensor to be elastic or replaceable by a spring-damper element rigid as discussed before, are given as follows : where c F is a characteristic quantity of the torque sensor which relates the rotation of the shaft to the translatory motion of the piston ring Figure 2. A change from an elastic to a rigid connection or vice versa causes a change in the structure of the mechanical scheme.
Since the simulation software cannot deal with structural changes, a stiff spring-damper element is used instead of a rigid connection. For reasons of packaging it is also an advantage to use a small radial pulley. The ratio of the variator is thus between 0. Pitch radius of driven pulley Rpri: Pitch radius of drive pulley The following figure shows the change in the transmission ratio of the given pulley pairs in a CVT: Stress analysis of CVT Pulley: In order to investigate the stresses in the pulley, the flexional stiffness was calculated by creating a three-dimensional finite element FEM model of the pulley and applying a belt load statically.
The pulley displayed deformation in both radial and circumferential directions according to its rigidity. The greatest flexional deformation, however, was in the radial direction. Consequently, attention was focused solely on pulley deflection. The following assumptions have been made for the dynamic analysis of pulleys: This belt consists of a large number around of V-shaped steel block elements, held together by a number between 9 and 12 of thin steel tension rings.
The belt runs on the primary pulley at the engine side and the secondary pulley at the wheel side. Each pulley consists of one axially fixed and one moveable sheave, operated by means of a hydraulic cylinder. The cylinders can be pressurized, generating axial clamping forces or thrusts on the belt, necessary for transmission of torque without macro-slip of the belt and for ratio change. The control system are generally of two types: The hydraulic part of the CVT essentially consists of a roller vane pump directly connected to the engine shaft , two solenoid valves and a pressure cylinder on each of the moveable pulley sheaves.
The volume between the pump and the two valves including the secondary pulley cylinder is referred to as the secondary circuit, the volume directly connected to and plus the primary pulley cylinder is the primary circuit.
Excessive flow in the secondary circuit bleeds off towards the accessories, whereas the primary circuit can blow off towards the drain. Pressures are defined relative to the atmospheric drain pressure pd.
As the model will only be used to determine the hydraulic system constraints needed for the feed forward control, the following assumptions have been made: The clamping forces FP and FS are realized mainly by the hydraulic cylinders on the moveable sheaves. Since the cylinders are an integral part of the pulleys, they rotate with an often very high speed, so centrifugal effects have to be taken into account and the pressure in the cylinders will not be homogeneous.
Furthermore, a pre-stressed linear elastic spring with stiffness kspr is attached to the moveable secondary sheave. This spring has to guarantee a minimal clamping force when the hydraulic system fails. Stepper Motor-Driver Screw System: This system includes an AC motor, set of power screw, an electromagnetic pick-up sensor, and two DC motors as shown in the figure below.
Each of the pulleys movable sheaves is operated by a DC motor in order to maintain the correct transmission ratio with the requirement of the driving torque of the car accompanied with electromagnetic pick-up sensor. Electromagnetic pick-up sensor produces the voltage with the travelling speed of the vehicle which operates the DC motor with a double acting magnetic pull-in solenoid switch.
The function of power screw mechanisms is to shift movable sheaves axially along the shafts by the DC motor. Both movable pulley sheaves move at the same time exactly the same linear distance but in the opposite direction. The system activates only when the transmission ratio change occurs which consume less power compared to the conventional hydraulic CVT.
The power screw mechanism makes sure that the pulley is held at its place when there is no changing in gear ratio. No work is extracted from the DC motor during this time. Indeed, it is one of the ideal characteristics for a CVT system.
In order to demonstrate the working principle of the proposed CVT system, several designs and components selection were made.
The stroke length of the moveable sheave, the driving motor torque and power, torque required to move the moveable sheave were estimated by accounting the vehicle load, travelling speed, tires size, wheelbase, and the location of the CG. Figure 2 shows the 3-D model of the proposed CVT. Although the mechanism of the two control system are different, however, their basic algorithms are the same.
The main goal of the CVT control is to achieve fast and accurate tracking of the desired ratio trajectory speed ratio. An important sub-goal is to max efficiency and to minimize belt and pulley wear slip control and rate of change ratio. Hydraulic control system is responsible on the control of the clamping forces and the change of the CVT ratio. The clamping forces are generated on the belt to keep the slip level remains in the intermediate slip region such that torque can transmit and by controlling the rate of ratio change the desired output torque can be realized.
Hence, the two primary goals of the CVT control are: This is required to keep the slip level in the intermediate slip region for effective torque transmission. Variator dynamics for Slip Control: In slip control, the clamping forces are actively controlled to maximize the efficiency of the CVT.
This achieve by maintaining an amount of slip, where the traction coefficient near its maximum. This means that slip is controlled in the transition area of the micro and macro-slip regions intermediate slip region. An increase in the torque level will lead to an increase in belt slip, but by adjusting the clamping force to keep the slip in the optimized value, and meanwhile the slip will not reach destructive levels and therefore damage can be avoided.
The most important requirement of the slip controller is that it has the ability to attenuate the load disturbance caused by torque peaks in the driveline. The slip dynamics thus derived is: A simple representation of the CVT variator dynamics is shown in the figure.
Model of a Continuously Variable Transmission
On the input side of the variator Te represents the engine torque and Je describes the equivalent engine and CVT inertia on the primary shaft. At the output side Td represents the road load torque, defined by road load conditions, and JS describes the equivalent vehicle inertia on the secondary shaft.
The dynamics of the primary and secondary shaft of the CVT variator are given by: These torques generated on both shafts of the variator are described as: Solving the above set of equations together we get: In this description torque losses are neglected.
It is assumed that these losses are not significant for the modelling of variator dynamics. The block diagram obtained for the above mathematical analysis for slip control is: Variator Dynamics for Position control: Since the primary axial pulley position xp is measured and to avoid the nonlinear calculation from xp to xg, this linear position is used as control variable.
Considering this, the dynamic equations are rewritten to: PID controller is a control mode that has most mature technology and the most widely used in continuous system. Because of its simple structure, easy adjustable parameters, and has accumulated rich experience during long-term application in practice, so it has been widely used.
Ratio PID controller model is designed to realize the actual ratio's tracking of goal ratio, to make the engine working in optimal performance set-point or optimal economic set-point ,according to the driver's intentions.
Continuously variable transmission
CVT transmission ratio control includes performance model and economic mode. According to the road condition, the driver chooses expectant drive mode, when drive presses down the accelerator pedal, it means that goal working speed and goal speed ratio of engine are set.
Therefore, we must design a goal speed ratio depends on a specific mode that is decided according to the engine test data, to make sure the practical ratio can effectively follow goal ratio in different working conditions and road conditions. Goal ratio is defined as the ratio of engine goal speed and driven wheel actual speed.
Thus, the CVT goal speed ratio is certain after the engine goal speed determined. Whether economic mode performance model is chosen, the goal speed ratio can be easily defined according to throttle Angle and speed of vehicle.
According to the vehicle driving conditions, the engine goal speed is looked as the target parameter of controller, so the CVT transmission ratio and then the working point of engine can be adjusted, finally, to realize the optimal performance and fuel economic of vehicle.
Speed ratio control is a process of CVT speed ratio control. If only control speed ratio of CVT system without considering transient value caused by the ratio variation, then system transient characteristics is difficult to control. If directly control the variation of speed ratio as control parameter, this problem can be solved. Therefore, in this paper variation of speed ratio is controlled as PID control object to realize speed control.
To determine ratio variation reasonably is the key of CVT speed ratio matching, it will directly affect accelerating performance and smooth acceleration of the car. PID controller model are as follows: They boast several advantages that make them appealing both to drivers and to environmentalists.
The table below describes some of the key features and benefits of CVTs. Feature Benefits Constant, step less acceleration Eliminates "shift shock" jerk experience when the from a complete stop to cruising convention manual transmission shift it gear speed Works to keep the vehicle in its Improved fuel efficiency optimum power range regardless of how fast its traveling Responds better to changing Eliminates gear hunting decelerates, as a Car conditions, such as changes in especially going up a hill.
The CVT's biggest problem has been user acceptance. Because the CVT allows the engine to rev at any speed, the noises coming from under the hood sound odd to ears accustomed to conventional manual and automatic transmissions.
The gradual changes in engine note sound like a sliding transmission or a slipping clutch — signs of trouble with a conventional transmission, but perfectly normal for a CVT.
Flooring an automatic car brings a lurch and a sudden burst of power, whereas CVTs provide a smooth, rapid increase to maximum power.
To some drivers this makes the car feel slower, when in fact a CVT will generally out-accelerate an automatic. Automakers have gone to great lengths to make the CVT feel more like a conventional transmission.
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The continuously variable transmission is a promising transmission for all kinds of drive trains, good results can be obtained in the field of emissions, efficiency and drivability. The pulley based CVT can be divided in two categories, the metal push belt and the metal chain. The first step involved in the design analysis was determining the parameters that helped us decide the engine specification. Followed by it was determination of the dimensions of the other components to suffice the power requirements based on the parameter values and then was the stress analysis for individual components.
At the end was the portion that discussed about the control strategy of different segments of the CVT and their synchronization. Download pdf. Remember me on this computer. CVTs are increasingly found on small cars, and especially high-gas-mileage or hybrid vehicles. On these platforms, the torque is limited because the electric motor can provide torque without changing the speed of the engine.
By leaving the engine running at the rate that generates the best gas mileage for the given operating conditions, overall mileage can be improved over a system with a smaller number of fixed gears, where the system may be operating at peak efficiency only for a small range of speeds. CVTs are also found in agricultural equipment; due to the high- torque nature of these vehicles, mechanical gears are integrated to provide tractive force at high speeds.
The system is similar to that of a hydrostatic gearbox, and at 'inching speeds' relies entirely on hydrostatic drive. CVT technology uses only one input from a prime mover, and delivers variable output speeds and torque; whereas PST technology uses two prime mover inputs, and varies the ratio of their contributions to output speed and power.
These transmissions are fundamentally different. The gear ratio is changed by moving the two sheaves of one pulley closer together and the two sheaves of the other pulley farther apart.
Model of a Continuously Variable Transmission
Due to the V-shaped cross section of the belt, this causes the belt to ride higher on one pulley and lower on the other. Doing this changes the effective diameters of the pulleys, which in turn changes the overall gear ratio.
The distance between the pulleys does not change, and neither does the length of the belt, so changing the gear ratio means both pulleys must be adjusted one bigger, the other smaller simultaneously in order to maintain the proper amount of tension on the belt. The discs can be pictured as two almost conical parts, point to point, with the sides dished such that the two parts could fill the central hole of a torus.
One disc is the input, and the other is the output. Between the discs are rollers which vary the ratio and which transfer power from one side to the other.
When the roller's axis is perpendicular to the axis of the near-conical parts, it contacts the near-conical parts at same-diameter locations and thus gives a 1: The roller can be moved along the axis of the near-conical parts, changing angle as needed to maintain contact. This will cause the roller to contact the near-conical parts at varying and distinct diameters, giving a gear ratio of something other than 1: Systems may be partial or full toroidal. Full toroidal systems are the most efficient design while partial toroidals may still require a torque converter, and hence lose efficiency.
The mCVT is of particular interest as a highly efficient power-split device for blended parallel hybrid vehicles, but also has potential applications in renewable energy, marine propulsion and industrial drive sectors. The magnetic CVT cannot generate greater torque than an electric motor of the same size, so it is not a replacement for mechanical automobile transmission. All power is transmitted by hydraulic fluid. These types can generally transmit more torque, but can be sensitive to contamination.
Some designs are also very expensive. However, they have the advantage that the hydraulic motor can be mounted directly to the wheel hub, allowing a more flexible suspension system and eliminating efficiency losses from friction in the drive shaft and differential components. This type of transmission is relatively easy to use because all forward and reverse speeds can be accessed using a single lever. A zero output speed low gear with a finite input speed implies an infinite input-to-output speed ratio, which can be continuously approached from a given finite input value with an IVT.
Low gears are a reference to low ratios of output speed to input speed. Unlike neutral in a normal automotive transmission, IVT output rotation may be prevented because the back-driving reverse IVT operation ratio may be infinite, resulting in impossibly high back-driving torque; in a ratcheting IVT, however, the output may freely rotate in the forward direction.
Design and analysis of components Most of the CVTs that have been designed and are in use are mostly for low torque ended four wheelers or electric hybrid cars. However, in this project we are extending our discussion and analysis to a two wheeler with low torque requirements. For this reason we chose a Honda Activa as the vehicle to understand the engine requirements for the same and extend it to CVT modelling.
Basic Design Parameters: We have tried to extend the approach of usage of CVT to a two wheeler since vehicles of these kinds usually have a low torque and power requirement. We have chosen the vehicle Honda activa with the following specifications: The Van-Doorne metal pushing V-belt CVT comprises an endless composite metal belt with two pair of variable V- shape pulleys secondary and primary.
The endless metal pushing V-belt is wound around both pulleys. The input pulley is usually called the primary pulley that driving the transmission driven pulley , which in then connected to the vehicle wheels through the final drivetrain driving pulley. The construction of the pulleys such way that, one- half of each pulley are fixed to the shaft while the other half are adjustable as it can be slide along their respective shafts.
Thus, the effective belt radius can be sleeplessly adjusted. General control has been designed in such way that the primary pulley determine the speed ratio while at the secondary pulley, ensured that the proper belt tension always be maintained to prevent slipping. One sheave of each pulley is connected with a hydraulic circuit, these controlled sheaves are on the opposite side of the belt.
With the hydraulic circuit the clamping force on each pulley can be varied, by modifying the clamping force the radius of each pulley can be changed, and so the transmission ratio. This kind of transmission can provide a speed ratio from 0. Movement of the movable sheaves in and out cause the change of gear ratio of a given CVT system.
Following is the working principle of the V-belt type variator illustrated by the shifting process from low ratio to overdrive ratio: These are connected by the CVT belt. The CVT system works through the changing of the distance between the plates on the two pulleys. The clutch pulley plate width increases, and vice versa. This creates an infinite number of possible gear ratios, as the transmission is altering itself on the fly to adapt to the current driving condition. The variator is driven directly by the engine.
Inside the variator are 6 rollers that are positioned in individual slots with ramps that they will move along outward when centrifugal force is applied. As the rollers move outward, they press against the ramp plate which causes the pulley plates of the variator to move toward one another, compressing the belt. This "V" shape created by the pulley plates pushes the belt outward, which draws the belt inward on the driven clutch side, increasing the gear ratio. At idle, the rollers are at their innermost position, the variator pulley plates are at their farthest apart, and the CVT belt is low on the variator side and high on the clutch side.
With increasing RPMs, the rollers move outward along their ramps applying pressure to the ramp plate, which compresses the variator pulley plates and squeezes the CVT belt outwards. The clutch in a CVT system engages when the centrifugal forces of the spinning clutch overcomes the tension of the clutch arm springs and allow the clutch pads to engage with the clutch bell, creating movement. The main clutch torque spring compresses the clutch pulley plates together, forcing the belt outward and acting against the variator.
As the rollers compress the variator side pulley plates when RPMs increase, the belt is forced outward on the variator.
Since the belt is a constant length, this causes the belt to be pulled inward on the clutch, overcoming the tension of the torque spring. The rotational torque is transmitted via belts from drive pulley to the driven pulley and thus acts as a link connecting the two. The belt appears in several forms.
The most important belt types are: Belt Type and its Analysis As the name goes this type of belt are V shaped. However, rubber V-belt CVTs are not well suited for automotive applications, because of their limited torque capacity. Push belts are of interest because a much higher friction coefficient is established between belt and pulleys than in lubricated variants. It consist of segmented , thick-stamped steel blocks configured with horizontal cutouts on both sides that contain stacked ribbons of steel termed as bands that shape the segments into an overall belt assembly.
Its function is to transfer rotational motion and torque from one pulley driver to other driven smoothly, quietly. The belt that was designed by Van Doorne basically comprises two sets of thin metallic band strips and a number of thin metallic plates segment as illustrated.
The entire segment a bend together by two sets bands through the segments location slots.
This situation allowed the segment to slide freely along the sets of bands. The flexible belt consists of many high-strength, bow-tie-shaped metal pieces that are held together by two packs of layered steel bands. Generally each pack has 4—12 layers of bands, and each band is about 0. In contrast to flat belts, V —belts are used with similar sheaves. However they have several advantage over chain drive like flexibility, inexpensive, smooth operation, ease of installation, usability in more than one plane etc.
Noted, the number and the size segments and the number of band strips determine the MPVB power capacity class.
The bending stiffness of the bands is very small and may be neglected, so that only a tension force can be present in the bands. The blocks can transmit torque when they are under compression, hence the name push-belt. The compression force can never exceed the tension in the bands, otherwise the contact between the push-belt and the pulleys could be lost and buckling could occur.
The function of a V belt drive is to transmit rotational motion and torque from one pulley to another, smoothly, quietly and inexpensively.
Belt provides overall combination of design flexibility, low cost and maintenance, ease of assembly and space savings. A V belt is made of fabric and cord, usually cotton, rayon, or nylon, and impregnated with rubber.
In contrast with flat belts, v belt are used with similar sheaves and at shorter center distances. V belt are slightly less efficient than flat belts, but a number of them can be used on a single sheave, thus making a multiple drive.
V belt are made only in certain lengths and have no joint. The amount of power which can be transmitted is determined by tensile strength in bands as the belt squeeze. Because the bands are in tension, they hold the blocks in line with each other.
The compression force can never exceed the tension in the bands, otherwise the contact between the push belt and the pulleys could be lost and buckling could occur.
The torque that is transmitted results from the combination of: Apart from the losses in the bearings of the shafts and losses due to slip, there are friction losses in the pushbelt. The bands and the blocks do not run at the same radius, causing a speed difference between the blocks and the bands.
This results in friction losses in the pushbelt, which lowers the efficiency of the pushbelt. Because of the continuous bending and stretching of the bands, fatigue issues are important. Fatigue resistance specifications limit the torque capacity of the variator, because the maximum clamping forces are limited. There are much more blocks in a pushbelt than there are pins in a chain.For two identical torque sensors, the sensor which measures the higher torque determines the pressure in the cylinder.
Stress analysis on the belt: Finite element analysis for the various components of the stresses was done on ANSYS software on an individual link that comes under maximum stress conditions during the operation.
To determine ratio variation reasonably is the key of CVT speed ratio matching, it will directly affect accelerating performance and smooth acceleration of the car. A continuously variable transmission CVT , also known as a shiftless transmission,.
Instead of the nine-speed automatic, an electrically controlled continuously variable transmission of Chrysler's own design will divvy up power to the front wheels. The pulley displayed deformation in both radial and circumferential directions according to its rigidity.
Keywords: Hybrid vehicles, continuously variable transmission, modelling, simulation 1. One drawback here is that the input to output ratio is sinusoidal and not constant.