The torque converter – the principle of operation, the main elements, causes and consequences of wear.
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The torque converter (GT) – one of the elements of the automatic transmission, performs an important function – the transfer of torque from the engine to the automatic transmission mechanism. The main task of the GT at the initial stage, when it first appeared in the design of an automatic transmission, was to have a non-rigid connection between the engine and the transmission mechanism. At that time, the GT consisted of two parts and was called a hydraulic clutch. And this is the first function of GT. Further on, an additional reactor wheel was introduced into the design of the transmission, and the transmission started to change torque (about 2 times) during acceleration. This is where the name hydro transformer comes from. The next step was the introduction of a mechanical interlock, which allows you to physically “hitch” the pump (drive) wheel and the turbine (driven) wheel. This is done to transfer torque without slipping, directly. In this way, GT is installed in most modern types of automatic transmissions. So, the main GT functions are: – to provide not rigid connection between engine and gearbox. It protects the transmission from sharp jolts at changing revs, allows to stop the car while the engine is running. – Converts torque in some modes. – The transmission is capable of blocking and unblocking the connection between the engine and the transmission at the right moment (at the control unit’s command).
Main malfunctions of transmission and external signs
The most common malfunction is lock wear. Friction layer of friction linings or disc is worn out or “getting greasy”. Or decreases pressure in the locking mechanism (also due to various reasons). The lock starts to “slip”. For the owner it is felt in the form of vibration, jolts. Sometimes it may look like driving on a “washboard”. In fact, there is periodic slippage of the interlock and the box gets shock alternating loads, which are perceived as “jolts” while driving. The blocking algorithm is different for different transmissions. For some types (e.g. ZF) the lockup is triggered already at 1st speed. And further the clutch of the lockup works in the mode of planned slippage. The designer has given the driver a sensation of “sportsmanship”, while sacrificing reliability. That’s why, vibrations from wear of lock-up clutch can start already at low speeds. It is necessary to note, that at first the GT lockup was conceived as an element for increase of economy, and it was switched on when driving with constant speed in higher gears. The GT lockup provided no loss of torque transmission and increased fuel efficiency.
Other malfunctions of GT – often go as “consequences” of lock-up clutch wear: – dirt from wear gets into the oil, intensively contaminates it. Dirty oil – quickly puts other elements of the automatic transmission out of operation, in particular friction bearings – bushings on which other elements in the box rotate. – Friction clutch – overheats the oil. This in turn leads to increased wear of other mechanisms and accelerated oil degradation. – Worn clutch leads to vibrations of adjacent elements, their mechanical wear. Intensive wear of rolling bearings – heavy wear of bearings and bushings – can eventually lead to mechanical wear of the rotating turbine itself – scuffing, fatal breakdowns…
ZFcenter performs a full range of automatic transmission overhauls. Each automatic transmission overhaul includes complete disassembly and overhaul of the transmission. If the customer wishes we can perform only the repair of automatic transmission that was taken off the car and brought to us or automatic transmission stripping-installation with the following repair of automatic transmission. We also accept GTs sent to us from other regions by an external transportation company. Important note! As described above, wear and tear of transmission elements leads to intensive wear and tear of other elements in the automatic transmission “chain”. That’s why repairing only GB does not always solve the problem completely. We recommend our Clients to do full overhaul of automatic transmission, even if there are signs of deterioration of transmission.
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How a Torque Converter Automatic Transmission Works
The advantage of the torque converter transmission is, of course, the convenience of controlling the car’s traction. As a reproach to such transmissions can be put slowness, low efficiency and a relatively short resource. Though it is necessary to give them credit – modern boxes differ with enviable “promptness”.
Do not faint, there is nothing complicated here. Now we will explain everything. But first let’s define the terminology. The point is that many people mistakenly define automatic transmission as two units combined into one: the box itself and the torque converter.
The torque converter consists of two vane machines – a centrifugal pump and a centrifugal turbine. Between them there is a guide apparatus – the reactor. The pump wheel is rigidly connected with the engine crankshaft, the turbine – with the gearbox shaft. The reactor, depending on the operating mode, can rotate freely or be blocked by overrunning clutch.
Useful energy in the torque converter transmission is spent on shoveling (and heating) the oil by the torque converter. Also a lot of energy is “consumed” by the pump, which creates working pressure in the control lines. Hence the lower efficiency. This is the reason why mechanical robotized boxes and variators are more preferable.
The torque converter is an ideal torsional vibration damper and is able to dampen the strong jolts that are transmitted from the engine to the transmission and vice versa. This, by the way, has a very favorable effect on the engine, transmission and running gear life. But the torque converter can also cause a lot of troubles. For example, it does not allow to start the car from a “push start”.
Transfer of torque from the engine to the gearbox is carried out by flows of working fluid (oil), which is thrown by the blades of the pump wheel to the turbine wheel blades. Minimum clearances are provided between the pump wheel and the turbine, and their blades are given a special geometry, which forms a continuous circle of working fluid circulation. So it turns out that there is no rigid connection between the engine and the transmission. This keeps the engine running and the car stopping with the gear engaged, as well as contributing to a smooth traction.
The oil in the torque converter moves along this intricate path. To increase the speed and increase the torque on the turbine wheel, the reactor is blocked. True, in this case the efficiency of the transmission is somewhat reduced.
It should be said that according to the scheme described above, the hydraulic clutch works, which simply transmits torque, without transforming its value. In order to change the torque, the torque converter is designed with a reactor. This is the same wheel with blades, but it, having a connection with the crankcase (housing) of the gearbox, does not rotate (note, up to a certain torque). The reactor blades are located on the path along which the oil returns from the turbine to the pump, and they have a special profile. When the reactor is stationary (torque converter mode), it increases the flow rate of the working fluid circulating between the wheels. The higher the speed of the oil flow, the higher its kinetic energy, the greater its effect on the turbine wheel. Due to this effect, the torque developed on the turbine wheel shaft can be significantly increased.
Imagine a standard situation – the gear in the box is already engaged, and we stand still and depress the brake pedal! What happens in this case? The turbine wheel is at a standstill, and the torque on it is twice higher (depending on the design) than what the engine develops at these revs. By the way, the torque on the output shaft of the torque converter will be the greater the higher the engine speed. It is necessary to release the brake pedal, and the car will accelerate. The acceleration will continue until the torque on the wheels is equal to the moment of resistance to the movement of the car.
When the turbine wheel approaches the speed of rotation of the pump wheel, the reactor wheel is released and begins to rotate together with the two “partners”. In this case, the torque converter is said to have gone into fluid coupling mode. This reduces losses and increases converter efficiency.
Since there is no need in some cases to convert torque and speed, the converter can be locked out at certain times with a friction clutch. This mode helps to bring the efficiency of the transmission practically to unity, slippage between the vanes in this case is excluded by definition.
But imagine this situation. You are driving in a straight line at a constant speed and suddenly start going uphill. Your speed will start to drop and the load on the drive wheels will increase. The torque converter reacts immediately to this change. As soon as the turbine starts to decrease in speed, the reactor wheel will automatically start to slow down, resulting in an increase in fluid circulation rate, which will automatically result in an increase in torque, which will be transmitted to the shaft from the turbine wheel (read to the wheels). In some cases, the increased torque will be enough to overcome a hill without shifting to a lower gear.
Since the torque converter is not able to convert speed and torque transmitted within a wide range, a multistage gearbox is attached to it, which, on top of everything else, is also able to provide reverse rotation (in other words – reverse). Those transmissions that are paired with torque converters usually include a number of planetary gears and have much in common with the “manual” transmissions we are familiar with.
When the transmission is running in upshift mode, the engine rotates the driver. The output shaft is connected to the sun gear, at which time the ring gear is locked in place.If the ring gear is released and at this time the friction locks it against the driver, the transmission is straight.The transmission is downshifted when the motor drives the sun gear, and the driver is locked in place. The power is taken from the ring gear.
In a manual transmission, the gears are in constant mesh, with the driven gears rotating freely on the secondary shaft. By engaging a gear, we mechanically lock the corresponding pinion on the slave shaft. The operation of automatic transmissions is based on the same principle. But planetary gears (or gearboxes) have some interesting features. They include several elements: the driver, satellites, sun and ring gears.
By rotating some elements and fixing others, such reducers allow to change gear ratios, i.e. rotation speed and the force transmitted through the planetary gear. The planetary gears are driven from the output shaft of the torque converter, and their respective elements are fixed using friction belts or friction packs (in a manual gearbox this role is played by synchronizers and locking clutches).
The transmission is engaged as follows. The friction is pressurized by a hydraulic pusher, which in turn is actuated by the pressure of the working fluid, the same that is used in the torque converter. This pressure is created by a special pump, and it is distributed between the respective friction clutches of the gears under the indefatigable control of electronics by means of a special system of solenoid valves – solenoids according to the box operating algorithm.
Friction clutch packs consist of several rings – fixed and movable. They rotate freely in relation to each other until it is necessary to engage the gear. A hydraulic pusher will clamp the friction clutches when the operating pressure is built up in the corresponding line. The moving elements of the friction, for example, rigidly connected to the driver of a planetary gear, will be clamped, the driver will stop and the gear will be engaged.
The essential difference between automatic transmissions and conventional manual transmissions is that the gears in them are shifted almost without any break in the flow of power. One is switched off, the other is switched on almost at the same moment. Strong jerks during shifting are practically excluded, as they are dampened by the already mentioned torque converter. Though, it should be noted, modern boxes with sports adjustment can’t brag of smooth work. Jolts during their work are caused by faster change of gears: such arrangement allows playing back some time during acceleration, but results in accelerated wear of friction clutches. It also affects the transmission and the running gear as a whole not in the best way.
The first generation automatic transmissions were entirely hydraulic. Later, the hydraulics remained only as the executive part of the control system, and the electronic system became the algorithm setting. Due to it, one can implement different algorithms of box’s work – the mode of sudden acceleration, sports, economy, winter…
One of the latest developments of ZF is the 8-speed hydro-mechanical gearbox. The creators say that the box allows saving up to 6 % of fuel in comparison with a similar six-graded automatic and 14 % in comparison with a five-graded one. Everything is logical, a large number of gears allows increasing the time at which the engine operates in the most “efficient” mode and the specific fuel consumption is minimal. Time lost for unnecessary shifts? Not much at all.
In sport mode, for example, engine power is used to the full. Each successive gear is engaged at a speed close to that of the maximum torque. Further acceleration brings the crankshaft speed up to the maximum values at which the engine develops maximum power. And so on. The car in this case develops considerably more acceleration as compared with that which is carried out at work “economy” or “normal” programs.
On most modern cars with automatic transmissions, these or those control algorithms are activated depending on the manner of driving. The electronics adapt the engine-transmission tandem independently. The computer analyzes information from numerous sensors and decides whether or not to shift gears at certain moments depending on the desired shift pattern. If the driving style is measured and smooth, the controller makes appropriate corrections, which does not lead the engine to power modes, which positively affects the fuel consumption. As soon as the driver gets “nervous” and starts to press the gas pedal more frequently and sharply, the artificial intelligence immediately understands that acceleration and acceleration should be performed more quickly, and the power unit will immediately start working according to the “sports” program. If the driver begins to pedal smoothly, the “smart” electronics will switch the gearbox and the engine to the normal mode of operation.
More and more vehicles are being equipped with gearboxes with semi-automatic control mode in addition to automatic control mode. Here, the driver gives the commands to change gears and the control system takes care of the shifts. But it does not mean at all that the electronics will allow you to be wildly unrestrained. Often, the speed of shifting from one gear to another in this mode is increased, but many manufacturers, caring about the resource of the power unit, leave the time of shifts the same as in automatic mode. Machine builders call these systems – Autostick, Steptronic, Tiptronic.
The Americans like to mount the selector of automatic transmission on a steering column. Europeans and Japanese put them on the central tunnel.
By the way, recently some automatic transmissions can be tuned. It became possible due to reprogramming of control units of the engine and gearbox. For the sake of acceleration speed, the automatic transmission control program changes moments of gear change and considerably reduces time of shifts.
It is possible to control the box in a manual mode in the new Mitsubishi Lancer both with the help of a selector and convenient magnesium under wheel switches.
Electronics becomes smarter from year to year. Computers have learned to analyze the degree of friction clutch wear and generate the corresponding pressure required to engage each clutch. By recording the pressure, it is possible to predict the degree of wear of the friction discs and, consequently, of the box as a whole. The control unit constantly monitors the health of the system, recording in its memory fault codes of those elements in which malfunctions occurred during operation.
The four-speed gearbox and torque converter 2002 (M76) of GM as a part of the power unit are installed in the vehicle transversely.
In some cases, the control unit begins to work in a bypass program. Usually in the emergency mode in the gearbox all shifts are prohibited, and one gear is engaged, as a rule – second or third. Operate, in this case, the car is not recommended (and will not work), but to drive on their own to the workshop program will help.
All types of gearboxes can please the car owners with their service at run distance of over 200 thousand km. But there is one “but” – trouble-free operation is possible with proper operation and regular qualified maintenance.
Automatic transmission modes
“P” – parking. In this mode all gears are off, the gearbox output shaft and the transmission “branch” connected with driving wheels are braked by the locking mechanism of the box. When the engine is running, the crankshaft speed limiter is triggered much earlier than during acceleration. Such “foolproofing” prevents the engine from “overclocking” and shoveling the transmission fluid to no avail.
“R” – reverse, – reverse.
“N” – neutral. In this mode the engine and the driving wheels are not connected. The car can coast, it is also possible to tow it without driving the driving axle out.
The mode “D” or “Drive” allows driving. In this mode, gear shifting is automatic.
“S” , “Sport” , “PWR” , “Power” or “Shift” – the sport mode. The most dynamic and the most wasteful. During acceleration the engine is “driven” to the maximum power mode. The speed of transition from one gear to another (depending on the design and program) can be increased. The engine in this case is always in a tone, as a rule, working at revolutions which are not lower than those at which the maximum torque is developed. Forget about economy.
“” – a mode in which transition to a lower gear is carried out for realization of intensive acceleration, for example, at overtaking. A sharp pickup occurs due to the fact that the engine is brought into the maximum output mode, and at the expense of a larger gear ratio of a downshift gear. For the transmission to enter this mode, it is necessary to stomp on the accelerator pedal. In older generation transmissions it was necessary to press the accelerator pedal “to the floor” up to a characteristic click in order to activate “kickdown”.
When working in “Overdrive” or “O/D” mode, the upshift gear will be engaged more often, shifting the engine to lower revolutions. “Overdrive” provides economical driving, but its activation can lead to a significant loss in dynamics.
“Norm” realizes the most balanced driving mode. Shifts to upshift gears usually occur when average rpm is reached and at rpm slightly above average.
If the selector is set to “1” (L , Low), “2” or “3”, your box will not go above the selected gear. The modes are required in heavy road conditions, for example, when driving on mountain roads, when towing a trailer or another vehicle. In this case the engine can operate in the area of medium and high loads without shifting to a higher gear.
“W” , “Winter” , “Snow” – the so-called “winter” mode of automatic transmission operation. In order to prevent slipping of the driving wheels, starting up from the second gear. In order not to provoke unnecessary slippage, the transition from one gear to another in this case can also be done more softly and at lower revolutions. Acceleration in this case may not be too dynamic.
Presence of signs “+” and “-” defines not at all poleness, but possibility of manual gear shifting. Various manufacturers “mix” transfers allow: with the selector of control of automatic transmission, with buttons on a wheel or under wheel switches… In this mode, electronics will not allow switching to those transfers which, in its opinion, are inappropriate at present time. When working with “addition” and “subtraction” signs, the speed of gear change will not be higher than that defined by the program in “Sport” mode. The advantage of the manual mode is the possibility to act in advance.