Automatic transmission

5.3.2  Automatic transmission with torque converter and planetary gears

One of the first challenges that the designers had to solve was to relieve the driver of the constant need to depress the clutch pedal while shifting gears. The fluid coupling came to the rescue, which in the subsequent improvement turned into a torque converter.

5.3.3 Purpose of the torque converter

The availability of a torque converter in the transmission, in addition to simplifying control, also ensures smooth starting of the car and its uniform acceleration. Due to the fact that the traction on the wheels increases gradually, the likelihood of their slipping decreases. At the same time, the vehicle's cross-country ability improves. The torque converter, which is integrated into the vehicle's power train also helps to

damp load fluctuations, thereby reducing engine and power train wear.

Figure 5.18 Torque converter components.

A torque converter is a hydraulic mechanism, which is connected between the engine and the gearbox of a car and provides an automatic change in the torque transmitted from the engine, in accordance with changes in the load on the driven shaft.

5.3.4 Torque converter device

Simple torque converter consists of three impellers with blades (Figures 5.19 and 5.20): rotating pump, turbine wheels and a stationary impeller-reactor. From the inside, the impeller blades are closed with round walls. Thus, an annular cavity of circular cross-section with a small diameter is formed inside the blades. In case if all three wheels with blades are located close to each other, an annular closed cavity is formed in which the working fluid (special oil), which is poured into the torque converter, circulates (as shown by the arrow in Figure 5.19).

The pump wheel is connected to the casing (rotor) and through it to the crankshaft of the engine. The turbine wheel is connected to the gearbox via the driven shaft. The reactor is fixed stiffly on a bushing, which is connected to the crankcase (or mounted on a freewheel*, as shown in Figure 5.19). The torque converter rotor with impellers that are located inside of it is mounted on bearings inside a closed crankcase.

Figure 5.19 Torque converter layout.

Figure 5.20 Simplified view of torque converter.

5.3.5 Torque converter operation

Note
In order for the working fluid (oil) to constantly fill the cavity of the impellers and cool, during the operation of the torque converter, it is continuously pumped from the oil box into the working cavity of the wheels by a pump and drained back into the oil box.

During the operation of the torque converter, the oil pumped into the working cavity of the wheels is captured by the blades of the rotating impeller and thrown outward by centrifugal force. Then it falls on the blades of the turbine wheel and, due to the pressure created in this case, sets it in motion together with the driven shaft. Further, the oil enters the blades of a fixed reactor, which changes the direction of the fluid flow, and then enters the pump wheel again, continuously circulating in a closed circle of the inner cavity of the impellers (as indicated by the arrow in Figure 5.19), while participating in the general rotation of the wheels.

Note
The availability of a stationary reactor, the blades of which are located so that they change the direction of the liquid flow passing through it, contributes to the creation of a certain force on the reactor blades. This force causes the creation of a reactive moment, which acts through the liquid on the turbine wheel blades, in addition to the moment transmitted to it from pump wheel. Thus, the availability of this reactor makes it possible to obtain a torque on the shaft of the turbine wheel, which is different from the torque transmitted by the engine.

The slower the turbine wheel rotates in comparison with the impeller (for example, when the load from the car's movement downhill), the more the reactor blades change the direction of the working fluid flow passing through it, and the more additional torque is transferred from the reactor to the turbine wheel, due to which increases the torque on its shaft.

Note
The property of torque converters to automatically change (or transform) the ratio of torques on the shafts, depending on the ratio of the number of revolutions on the driving and driven shafts (and, therefore, on the magnitude of the external load) is their main feature. Thus, the action of the torque converter is similar to the action of a gearbox with an automatic change in gear ratios.

Interesting
The maximum value of the coefficient of performance (COP) for existing designs of torque converters ranges from 0.85-0.90.

5.3.6 Torque converter and fluid coupling

However, the designers made a decision not to stop there and decided that it was necessary to expand the capabilities of the torque converter in order to improve its adaptability to various operating modes. The solution was simple and interesting at the same time. They installed the reactor wheel on the freewheel clutch, making it possible for the torque converter to work in the hydraulic clutch mode. Such torque converters are called complex.

Note
A freewheel clutch (another name - "overrunning clutch") is a unit, which, under certain conditions, excludes the transfer of torque from the driven shaft back to the driving shaft if, for any reason, the driven shaft starts rotating faster. The most vivid example is the drive of the drive wheel in a bicycle: when you need it, you rotate the pedals - and the thrust from them is transmitted to the wheel through the clutch. Then after accelerating, you stop doing hard work - you do not rotate the pedals, and the wheel rolls along the road without problems. Another example: a wrench with a "ratchet". In one direction it is locked and tightens the bolt, but during the return stroke, the head of the wrench remains in place, and the handle returns to its original position with a characteristic crackle.

5.3.7 Complex torque converter operation

The peculiarity of the operation of such a torque converter is as follows: when the number of revolutions of the driven shaft is much less than the number of revolutions of the drive shaft, which corresponds to an increased load on the output shaft (for example, the car starts moving), then the fluid flow leaving the turbine wheel hits the reactor blades from the inner (with respect to the direction of rotation) side. At the same time, while trying to rotate the wheel in the opposite direction from the general rotation, the flow, which is created by the force, jams the reactor motionless on the freewheel clutch. With the reactor stationary, the entire system acts as the torque converter described above, providing the required torque transformation and helping to overcome changing loads. In case if the load on the output shaft decreases (the car travels at a constant speed along the highway), and the number of revolutions of the turbine wheel rises sharply, then the liquid coming from the turbine blades hits the reactor blades from the outside, trying to rotate it in the direction of the general rotation. In this case, the freewheel clutch is unlocked, and the reactor wheel starts rotating freely in the same direction with the impeller. Due to the absence of stationary blades in the path of the fluid flow, the transformation (change) of the moment stops, and the whole system works as a hydraulic clutch. That is, in fact, it connects the crankshaft with the gearbox shaft directly, like a mechanical clutch.

5.3.8 Automatic transmission assembly

However, the torque converter is only one part of the system. It is necessary to introduce stages with different gear ratios in order to achieve all those parameters that a manual transmission has. And the more of these same stages, the more efficient the use of engine power and torque in various road conditions. Planetary gears are used in order to provide such conditions.

Note
Figures 5.21 and 5.22 illustrate the complexity of an automatic transmission.

5.3.9 Main operating modes of automatic transmission

Almost any automatic transmission has the following modes, that have become, so to speak, standard since the late 1950s:

5.3.9.a Park

Р (Park) — parking lock (the drive wheels are locked, the shafts inside the gearbox are locked directly, this is in no way related to the parking brake);

5.3.9.b Reverse

R (Reverse) — reverse (it is forbidden to turn on this mode until the car comes to a complete stop, there is a blocking on modern automatic transmissions);

5.3.9.c Neutral

N (Neutral) — neutral mode (required for complete separation of the engine and drive wheels in case of short-term parking or towing a short distance);

5.3.9.d Drive

D (Drive) — forward movement (movement with automatic gear shifting from the first to the last, depending on the traffic situation and the driving mode).

5.3.9.e «Kick down»

Note
"Kick down" operating mode is acceleration mode. It can’t be turned on in no way using the selector for switching the automatic transmission operating modes. It turns on only when the accelerator pedal is pressed hard almost all the way, while the electronics switches to one or several gears "down" (decrease) in order to get the maximum acceleration of the car, for example, when overtaking.

5.3.9.f «Low»

L (Low) — low gear, "slow speed" (for driving in difficult road conditions or when the car is parked).

Additional operating modes are sometimes used on the automatic transmission. Thus, in case if there is a mode marked with the digit 2, this means that the gearbox will work only within the first and second gears, that is, "not higher than the second". And if there is additionally position "1", then the gearbox will work "not higher than the first gear"

Figure 5.23 Example of the selector for switching the operating modes of automatic transmission.

5.3.10 Tiptronic: automatic shifting and vehicle control

Originally this idea belonged to Porsche Company. Engineers and designers wondered what if the driver of a car with automatic transmission was given the opportunity to choose gears on his own. Having done minor operational and design improvements, we got a very interesting mode of operation of the automatic transmission: it seems that the driver himself switches gears, but there is no clutch pedal. Though, there is a feeling of complete control over the car.

Having picked up such an interesting solution, all car manufacturers started installing Tiptronic automatic transmissions on their cars. Some manufacturers went further - control in “manual mode” was transferred to paddle shifters, thus making it akin to ordinary road cars with sports cars.

Note
A characteristic external feature of the automatic transmission with Tiptronic is the additional position of the shift selector, which can be moved forward/backward or to the right/left and the availability of “+” and “-” icons on the selector panel, which means gear shifting “up” and “down” correspondingly.

Figure 5.24 Example of the selector for switching the operating modes of automatic transmission with Tiptronic system.

5.3.11 Variator - description

In the case of the automatic transmission described above, although the gears are engaged automatically, they still shift within a given range. And what if this range is preserved, and to get rid of the gears (not in the literal sense of the word) – just make a smooth change in the gear ratio from the lowest to the highest gear. The variator is a solution of this challenge. The most widespread is a friction V-belt variator, and at present - a chain variator (an example of which is shown in Figure 5.25).

Figure 5.25 Example of arrangement of variator automatic transmission.

The variator is a very simple mechanism. This is why, in fact, in recent years it has started gaining more and more popularity among automakers.

This mechanism consists of two pulleys and a belt (chain), which is stretched between them. A feature of these pulleys is that each of them consists of two halves (in terms of similar to truncated cones), which can move apart and move (as shown in Figure 5.26). One of the pulleys is a drive one (connected through a clutch to the engine crankshaft), the second is a driven one (transfers traction further to the wheels).

5.3.12 Вариатор - принцип работы

During the start of the movement, when it is necessary to supply the maximum torque to the drive wheels, the halves of the drive pulley are separated maximally, and the driven pulley is installed with a minimum clearance (left side of Figure 5.26). In this case, the belt seems to fall between the two halves of the drive pulley. Thus, the actual working diameter of the belt on the drive pulley is minimal. And on the driven pulley it is maximum, which achieves the highest gear ratio and, accordingly, the highest torque. As the speed increases, the parts of the driving pulley draw together a little, and the parts of the driven pulley move apart. The gear ratio changes smoothly (right side of Figure 5.26).

Примечание
In variators, the range of gear ratios is limited only by the size of the pulleys.

Figure 5.26 Variator operating principle.

One of the main disadvantages of this type of variator was the small resource of the V-belt, which is located between the pulleys. With the development of technology, the V-belt was replaced by a chain (an example of a chain in Figure 5.27) or, in some cases, by a composite metal belt.

Figure 5.27 Chain, which is used in modern variators.

5.3.13 Manual variator control mode

The so-called virtual gears started being used in modern cars, when using a variator. Shift paddles for shifting these gears up and down are installed under the steering wheel. There are separate positions on the selector that are marked, as in the case of the Tiptronic, by the symbols "+" and "-". Gears are called virtual, because there are no gears as such, but there are only a few (depending on the manufacturer's imagination) fixed positions of the driven and driving variator pulleys.

Good to know
It is necessary to get used to driving a car with a variator, because due to the lack of gears, the electronics tries to maintain the engine speed in the most efficient range of operation. Therefore, it can produce the impression that the engine is “howling” and is constantly overloaded. However, many manufacturers manage to get rid of this shortcoming by introducing more advanced control systems.

5.3.14 Toroidal variator

A toroidal variator is rarely used. But nevertheless it is an excellent attempt by automakers to get rid of the main drawback of V-belt variators - the belt itself. A toroidal variator consists of two oppositely installed cones with concave walls. Two rotors are installed between these cones. They are pressed with a very high force and have the ability to move along the surfaces of the cones, as shown in Figure 5.28.

Figure 5.28 Operating principle of the toroid variator.

One of the cones is the drive, the second is the driven one. Rotation from the driving cone is transmitted to the driven cone via pulleys. At the beginning of the movement, when the gear ratio should be maximum, the pulleys are turned and installed so that they are as close as possible to the top of the driving cone, and to the base of the driven cone. It turns out that the actual diameter of the driving pulley is less than the actual diameter of the driven pulley by the value of the gear ratio. One cone rotates. This rotation is transmitted through the pulley to the second cone. In order to change the gear ratio, it is necessary to move the pulleys in the opposite direction - in the direction of increasing the actual diameter of the drive pulley.

Note
The toroidal variator is named after the figure torus, and in no way is it connected with the ancient Scandinavian ace, the god of thunder and lightning - Thor.

5.3.15 Disadvantages toroidal variator

One of the disadvantages of this type of variators is that the main working parts are made out of metal. And as we all know from the school physics course, metal-to-metal friction has a very low efficiency. In order to compensate for this, all working parts must be made out of

high-strength materials and pressed against each other with great force. These disadvantages are supplemented by the high cost, and the high cost of both manufacturing and maintenance.

Figure 5.29 Appearance of toroidal variator.

5.3.16 Preselective robotic transmission

Nowadays, such types of gearboxes as preselective robotic (automated) are becoming more common. What does it mean? Their design has drive shafts with gears rigidly mounted on them and there are two driven shafts with freely rotating gears and speed synchronizers. Gears of even and odd gears are installed on each shaft, respectively. That is, on one shaft only, for example, 1st, 3rd and 5th gears are switched on, and on the second shaft - 2nd, 4th and 6th. Each of these two shafts has its own clutch. Everything together is controlled and switched by electronics and servo mechanisms.

The benefit of this design is that in fact we have a manual transmission with all its advantages, but with the ability to change gears without interrupting the power flow from the engine to the wheels.

Note
One of the main drawbacks of all manual transmissions is a gap in the power flow, traction from the engine to the wheels, while shifting gears, which is associated with the need to disengage the clutch.

5.3.17 Automatic transmission work

Figure 5.30 shows a diagram of the operation of a preselective gearbox. There are two clutches: one for the shaft with "even" rows of gears, the other for the shaft with "odd" rows of gears. The clutch shafts, in turn, transmit rotation to the driven shafts, and those to one output shaft, as shown in the diagram.

Note
We can call a shaft "even" or "odd" depending on which gears are engaged on that shaft.

How can be smooth switching achieved without interrupting the power flow?

The driver turned on the first gear, the clutch of the “odd” shaft closes and transfers traction from the crankshaft to the driven shaft and then to the output shaft and wheels. When shifting to second gear, the "odd" clutch is not disengaged yet, but the "even" clutch starts to engage. That is, the "even" shaft starts receiving part of the thrust from the engine. At a certain moment, the "odd" clutch is completely disengaged, and the "even" clutch is already fully transferring the thrust from the engine to the second driven shaft. Gear were shifted without interruption in power flow.

Figure 5.30 Layout of preselective transmission.

Thus, we got a great way to combine the advantages of a manual transmission and an automatic transmission.

However, not everything is as rosy in practice as in theory and on the test benches of automakers. Many manufacturers still have not been able to get rid of the jolts during the start of the movement of the car and increase the resource of the clutches.

Figure 5.31 Appearance of preselective transmissions.