Origin of the Variomatic
In 1897 the American H.C. Spaulding is an infinitely variable drive consisting of a belt and two conical pulleys, intended for propelling a car.
By means of a lever system operated by the rider, a flat belt was forced from a larger diameter to a smaller diameter on both pulleys and vice versa to function as a speed variation.
As far as is known, this system has never been put into production. An invention by the Frenchman Fouillaron, who used two pairs of pulleys, was applied. Each pair of pulleys consisted of two conically spoked discs, the spokes of which interlocked like spread fingers of clasped hands. By now inserting the discs by means of moving a lever apart or together created a V-shaped groove whose diameter became smaller or larger. A trapezoidal belt was placed in this groove, consisting of thick slices of leather strung like beads on a metal cord. They proved to have a good service life, which was the reason for serial production of the Fouillaron drive in 1900 and installation in dozens of cars.
The materials and production techniques at the time, but also the fact that the mechanism had to be operated by hand, meant that this form of speed control fell into disuse. At least for cars, because this system was long used by Lang and Flender for the drive and speed control of the main spindle of their lathes.
In addition to the commonly used conventional, manual gearboxes, a serious attempt was first made in 1925 to automate gear changes by means of a hydraulic clutch and a hydraulically switched gearbox. Although this type of transmission turned out to work well, with a hydraulically switched gearbox you still have to deal with a "stepped" system.
So, just like with the manual gearbox, only a limited number of fixed gear ratios are available. DAF founder Hub H.J. van Doorne retrieved Mr Spaulding's invention from oblivion in the conviction that a continuously variable transmission was a fundamentally better solution compared to the conventional manual or automatic transmissions.
Based on Spaulding's idea, a fully automatic transmission system was developed, which was introduced in 1958 under the name Variomatic on the Daf 600 passenger car. In the following years it turned out that this Variomatic, as it was later produced and used by Volvo under the name CVT, is the ideal way to propel a car in city traffic as well as on the motorway.
Basic theory of the Variomatic
Each powertrain has the task of adapting the power supplied by the engine to the driving conditions and transferring it to the driving wheels in such a way that the car can develop its maximum power at different speeds, with both extremes of maximum tractive effort or maximum torque. maximum speed. Figure 1 shows how this goal is achieved using fixed reductions. When accelerating, you must ensure that by choosing the correct transmission ratio (the "gear"), the speed of the engine remains between the above-mentioned values (see figure 2). The Variomatic has a completely different shifting characteristic (figure 3). This means that any gear ratio between an upper and a lower limit is automatically set in relation to the selected engine power and the occurring driving resistance. For a better understanding of this property, we distinguish between two different modes of driving:
1: drive off at full throttle from standstill; you try to reach the top speed as quickly as possible. This requires the greatest possible acceleration (or driving force to the wheels), at maximum engine power. The Variomatic meets this requirement to a great extent. The first part of the curve is run through at full throttle in the same way as with a manual transmission. The automatic clutch engages and speed and rpm increase according to the largest gear ratio (imax line). When the maximum power is almost reached, the engine speed remains approximately constant and each gear ratio is automatically engaged until the smallest gear ratio (imin line) is reached (figure 3, line A).
2: driving at a constant speed, where no maximum power is required from the engine, but only what is necessary to continue driving at the same speed under the current driving conditions (part load). The engine must be as economical as possible, make no noise and produce exhaust gases that are as clean as possible. These requirements can best be met if the engine speed remains low. The points obtained by driving at partial load (certain position of the accelerator pedal) therefore lie on the drawn curve, the so-called partial load curve (figure 3, line B). All reduction settings within the extremes outlined above are possible (partial load range). In summary, it can be said that thanks to the Variomatic an ideal adjustment is obtained between the available power on the one hand and the required power on the other, while the adjustment is also fully automatic. An automatic clutch in conjunction with a shock-absorbing drive shaft thus ensures a smooth transmission of the driving forces.
The principle of the Variomatic
The power from the motor (1) is transferred via an automatic clutch (2) and a primary drive shaft (3) to the first or primary box of the Variomatic (see figure 4). This primary part consists of a distribution box, in which there is a switching mechanism. On the output shaft of the primary gearbox you will find 2 pairs of discs (4), which, together with 2 pairs of discs on the second or secondary gearbox (6) and the V-shaped belts (5) installed in between, form the essential part of the Variomatic. and together provide a continuously variable gear ratio. The left and right sheave pairs of the primary cabinet each consist of a fixed and a movable sheave; the outer disc can slide on the shaft on which it is mounted. This can cause a variation in the belt running diameter; the sheaves apart for a small belt orbital diameter, and the sheaves towards each other for a large belt orbital diameter. The pairs of discs of the secondary cabinet are each also provided with a fixed and a movable disc. In the primary case, the outer disk is the moving one, while in the secondary case, the inner disk is the movable one. This is important to ensure that, when changing the belt running diameter, the belt always runs straight between the pulleys. The center distance between primary and secondary sheaves has been chosen in such a way that, depending on the circumstances, there are two extreme positions: primary on the smallest diameter, where the belts run secondarily on the largest diameter, or vice versa, with an infinite number of intermediate positions between these two extremes , which therefore create as many gear ratios between the primary and secondary part of the Variomatic. Depending on the type of DAF or Volvo, the total transmission ratio between engine and rear wheels is a minimum of 3.60:1 and a maximum of 28.83:1. Thanks to the idea behind the Variomatic, any gear ratio is possible between these limits, whereby the most favorable gear ratio is automatically selected for every operating condition.
Factors affecting the reduction setting
Centrifugal force
In figure 5a the position of the pulleys and the belt is indicated in the largest transmission ratio. This is the position in which you must drive away from a standstill. When the sheaves are moved primarily toward each other, the belt is forced to a large diameter (Figure 5b). Because the length of the belt has a fixed size, the belt is pulled in at the secondary sheaves. The primary disks therefore float by means of the belts to the secondary sheaves, whereby no slip should occur. To do this, the belt must be squeezed forcefully between the sheaves. On the secondary side, this pinching force is provided by a coil spring and a set of disc springs; on the primary side by some regulating elements, of which the centrifugal and vacuum forces are the most important. During upshifts, the squeeze force on the primary discs will therefore have to be greater than the squeeze force on the secondary discs, while conversely, when downshifting, the squeeze force on the secondary discs should be the greatest.
The drum-shaped movable discs of the primary box include 2 pairs of centrifugal weights (figure 6). These centrifugal weights are hinged to the driver housing, which in turn is attached to the distribution shaft.
When the (motor) speed increases, the centrifugal weights swing outwards about their pivot points. The centrifugal forces occurring in this process are converted into an axial force on the movable disc. When this primary squeezing force overcomes the secondary squeezing force, the movable sheave is moved toward the fixed sheave, forcing the belt in the primary sheaves to a larger belt running diameter.
Because the belt length does not change, the belt in the secondary sheaves is forced to a smaller diameter, which means a change in the transmission ratio: the Variomatic shifts up.
If the pinching force in the primary discs remains greater than the pinching force in the secondary discs, the Variomatic will continue to upshift until the smallest possible transmission ratio is reached (figure 7).
As soon as an equilibrium situation arises between the forces in the primary and secondary discs, the Variomatic will not shift any further and the transmission ratio then achieved will be maintained (figure 8).
When the engine speed decreases (until the car comes to a near standstill), the equilibrium situation is disturbed. Due to the now greater pinching force on the secondary discs, they move towards each other and the primary discs are pulled apart (figure 9).
The Variomatic downshifts until the maximum gear ratio has been reached near standstill. Because when the car is stationary, the primary discs must be "open" so that the car can drive away smoothly (again).
Belt pull
There is a second very important factor that influences the reduction setting, namely the belt pull which depends on:
- the driving resistance
- the engine torque
- the transmission ratio of the Variomatic
The following can be said about the driving resistance: as long as the car is driving at a constant speed on a flat road, the tension in the belts will be limited to "supplying" the required thrust to the rear wheels, which is necessary to keep the car with maintain the speed selected under those circumstances.
But when the driving resistance increases, for example when driving up a slope, when the road surface changes or when a headwind picks up, in order to maintain the selected speed, a greater thrust at the rear wheels and therefore a greater tractive effort in the belts are needed.
As an example, the tensile force in one belt is shown schematically in figure 10.
Assuming that the primary sheaves drive the belt and that the belt drives the secondary sheaves, this belt pull will pull the belt between the primary sheaves to a smaller diameter, while the belt between the secondary sheaves will then run at a larger diameter ( see figure 11).
The tension in the belts is therefore rarely constant and, in addition to the conditions outlined above, also depends on acceleration or deceleration. Furthermore, the tension in the belts is influenced by the air resistance (roof rack), the towing of a trailer and the load of the car. The situations in which the interplay of forces between the belt pull forces and the centrifugal force takes place can be explained in more detail using the following examples:
- At a constant driving speed, a certain reduction setting has been reached, where the influence of the centrifugal force of the weights and the tension in the belts are balanced.
- With increasing driving resistance, but with the same engine power, the speed of the car decreases and the tension in the belts increases. As a result, the balance between centrifugal and tensile force is broken, with the result that the belts in the primary sheaves move to a smaller belt running diameter; the Variomatic shifts down.
- In order to maintain the speed reached when the driving resistance increases, the engine has to make more revolutions as a result of that "downshifting". So all the driver has to do is press the accelerator a little further, so that the speed of the car is maintained.
- When moving from climbing a slope to a flat section of the road, the opposite occurs as under b. was described; the speed increases, the tension in the belts decreases and the Variomatic "upshifts". Now to maintain the selected speed, the accelerator pedal must be released until the engine speed has adjusted to the changed reduction setting.
- If you want to suddenly increase the speed, it is sufficient to fully depress the accelerator pedal. This increases engine speed and belt tension, resulting in greater thrust at the rear wheels. As we explained earlier, the Variomatic downshifts strongly under these conditions, allowing strong acceleration. We call this phenomenon the "kick-down" effect. When the accelerator pedal is then released again so far that the speed increased by the acceleration remains constant, the reverse occurs: engine power, belt pull and thrust decrease, so that the gear is automatically switched to a smaller gear ratio. We call this the "overdrive" effect.
It is precisely this continuously variable force that plays such an important role in setting a particular gear ratio. In this way, a gear ratio is obtained fully automatically, which continuously adapts to the ever-changing driving conditions.
Under pressure
A third factor that influences the reduction setting of the Variomatic is the negative pressure that prevails in the intake manifold of a running engine. The movable discs of the primary part of the Variomatic are both divided into two halves by means of a diaphragm mounted on the distribution shaft (figure 12). The two halves are referred to as "chambers"; we speak of an outer and an inner chamber.
The starting point is that the pressure in both chambers is atmospheric. By creating negative pressure in one of these rooms at the right time, you obtain negative pressure support to amplify the aforementioned "kick-down" and "overdrive" effects. By creating an underpressure in the outer chamber (figure 13a), the movable disc will want to move towards the fixed disc. The belt is thereby forced to run on a larger belt running diameter; the Variomatic therefore shifts up. Conversely, creating an underpressure in the inner chamber will push the movable sheave outwards (figure 13b), i.e. away from the fixed sheave, causing the belt to run at a smaller diameter and the Variomatic to shift down (accelerated).
In conclusion, there are three factors that make the Variomatic switch:
1. The centrifugal force (depending on the engine speed)
2. The belt pull (which is mainly influenced by the vehicle resistance)
3. The negative pressure, which supports the switching of the Variomatic.
Operation conditions Variomatic
In practice, the aforementioned three factors will never occur alone. Below we will further discuss the influences that these factors exert on each other.
1. Acceleration
When driving away from standstill at full throttle to, for example, 80 km/h, the following happens (figure 14): by increasing the engine speed, the centrifugal weights in the discs of the primary part of the Variomatic swing outwards, causing the movable disc to is printed to the hard disk. However, the vehicle resistance to be overcome is large; the entire mass of the car has to be set in motion from a standstill, so that the tension in the belts will be great. Because the influence of this tractive force is opposite to that of the centrifugal force, the transmission will therefore be held in a somewhat downshifted position. Despite the fact that the outer chambers of the primary variomatic are in communication with the intake manifold in this position, no noticeable underpressure support will take place, because in this position of the throttle valve (acceleration) there is virtually no underpressure in the intake manifold.
2. Overdrive position
Now when the car has reached a speed of 80 km/h, you want to maintain this speed. Then you let the accelerator pedal up slightly and because the speed, and therefore the power, is not increased any further, the belt pull will decrease. The centrifugal weights now have the opportunity to swing out further: the Variomatic shifts up (figure 15).
To reinforce this effect, negative pressure is now simultaneously created in the outer chamber of the movable disc, which significantly supports the upshifting of the Variomatic. We call this the overdrive position.
3. Kick-down position
When you then increase the speed further from 80 to 110 km/h, the following happens (figure 16): to accelerate the mass of the car again, a greater belt pull is required. Therefore, the accelerator pedal is fully depressed. Due to the now increasing tension in the belt, the Variomatic will downshift slightly. To enhance this effect, the negative pressure support of the outer chamber is switched off and the outer chamber is also brought into open communication with the outside air by fully depressing the accelerator pedal. The outer chamber is "aerated". We call this the kick-down effect. If the throttle is taken back slightly at 110 km/h, which reduces the pulling force in the belts and also creates a negative pressure in the outer chamber, the Variomatic will shift up again to the overdrive position.
4. Braking with the braking pedal
When decelerating sharply from 110 km/h to a standstill, the centrifugal weights and belts (figure 17), which have a certain inertia, take some time to return to the standstill position. The belt pull, as discussed earlier in acceleration, now works in reverse: the secondary sheaves (rear wheels) now drive the primary sheaves. As a result of the belt pull, the belt wants to run on a larger diameter in the primary pulleys and a smaller diameter in the secondary pulleys. The two reasons mentioned above will certainly not make the Variomatic accelerate downshifts. To prevent the Variomatic from being in a slightly upshifted position when the car comes to a stop, an underpressure is created in the inner chamber during braking. This negative pressure generates an axial force, which pushes the movable disc off the fixed disc (Figure 17).
The distance between the primary sheaves increases and the pinching force in the secondary sheaves will pull the belt deeper between the two primary sheaves, speeding the Variomatic to the full downshift position. This rapid downshift also causes an increase in engine speed, resulting in strong engine braking.
5. Engine braking
A gear ratio that is as large as possible is desirable both when driving up a mountain and when descending. When driving uphill, the belt pull is so great that the Variomatic will be held in the downshifted position. But when descending, this is not the case; with the accelerator pedal completely removed and driving down the mountain, the rear wheels (secondary discs) will drive the primary discs and the Variomatic will tend to shift up just like with the "brakes" position. By now creating an underpressure in the inner chamber, an axial force is generated in the movable disc, which counteracts the influence of the centrifugal force and pushes the movable disc away from the fixed disc (figure 18). As a result, the belt "primarily" runs on a smaller diameter and is then kept at the smallest possible diameter. The engine speed therefore remains high and strong engine braking is possible.
Conclusion
Three forces, the centrifugal force, the belt pull and the force in favor of the negative pressure, provide the reduction setting. These forces are regulated and controlled by a simple movement of the accelerator pedal, making the Variomatic a fully automatic transmission that ensures that the correct thrust is available to the rear wheels under all driving conditions.