Automatic transmissions

Genesis and application.

The history of automatic transmissions can be traced back to the early 20th century, when engineers began experimenting with methods of automatically changing gears in cars. The first trials took place in the 1920s and 1930s, but it was not until 1939 that General Motors introduced the first truly automatic transmission, known as the Hydra-Matic. This was a ground-breaking development that made it possible to change gears smoothly without the need for manual intervention by the driver.

The Hydra-Matic quickly gained popularity and its design was used by many other car manufacturers. In the 1950s and 1960s, automatic transmissions became increasingly common in the United States, becoming a symbol of modernity and convenience. In Europe and Asia, their popularity grew somewhat more slowly, mainly due to different driver preferences and differences in road design and driving styles.

The 1970s and 1980s saw further developments in automatic transmission technology. More advanced hydraulic systems and the first versions of electronic control were introduced, allowing even better control over gear changes and reducing fuel consumption. During this period, automatic transmissions became more reliable and available in a wide range of vehicles, from cars to trucks and buses.

The 1990s saw a real technological revolution, when computers and electronics began to play a key role in the management of transmission operation. The first electronically controlled automatic transmissions appeared, which could adapt their operation to road conditions and the driver's driving style. This made automatic transmissions even more efficient and user-friendly.

In the 21st century, automatic transmission technology continues to evolve. Today's transmissions offer a range of modes, including sport, economy or off-road modes, as well as systems that allow manual control via steering paddles. There are also new designs such as Continuously Variable Transmission (CVT) and Dual-Clutch Transmission (DCT), which combine the advantages of automatic and manual transmissions.

Construction and principle of operation.

Classic automatic transmissions (with torque converter and planetary gear set).

The classic automatic transmission, often referred to as a planetary transmission, is a complex mechanism that consists of several key components. Each of these components plays a vital role in transmitting power from the engine to the wheels, while also allowing the gear ratios to change smoothly without driver intervention.

At the heart of a classic automatic transmission is a planetary gear set. Each consists of three main parts: a sun wheel, planet wheels (also called satellites) and a ring wheel. The sun wheel is in the centre, the planet wheels (satellites) rotate around it and are connected to the ring wheel. This arrangement allows for different combinations of gear ratios depending on which wheel is driven, which is stopped and which transmits power.

The clutches and brakes, which are hydraulically actuated, are responsible for controlling the power flow and shifting gears. In a classic automatic transmission, there is an oil pump that supplies pressurised oil to various parts of the transmission, including the clutches and brakes. When a gear change is required, the corresponding clutch is engaged or disengaged, allowing the ratio in the planetary transmission to be changed.

A key component of the classic automatic transmission is also the torque converter. It is located between the engine and the transmission and acts as a hydraulic clutch. The torque converter consists of three main parts: a pump, a turbine and a stator. The pump is connected to the crankshaft of the engine via the flywheel, the turbine to the input shaft of the transmission and the stator is located between the two. When the engine is running, the pump sets the oil in motion to transfer power to the turbine, after which the stator directs the flow of the returning oil in such a way as to increase the efficiency of torque transmission.

The operation of a classic automatic transmission is controlled by a hydraulic system and an electronic system. The hydraulic system manages the flow of fluid to the clutches and brakes, while the electronic system monitors various driving parameters such as vehicle speed, engine load or accelerator pedal position. Based on this data, the control computer (TCU - Transmission Control Unit) decides when and what gear ratio to select. A TCU without an electronic controller is called a valve body, while mechatronics is the component that integrates the valve body and the electronic controller of the automatic transmission.

In summary, the classic automatic transmission is a high-tech device that combines mechanical, hydraulic and electronic components to ensure smooth and efficient gear changes. The planetary gear set, clutches and brakes, torque converter and control systems work together to provide a comfortable and convenient driving experience.

Continuously variable transmissions (CVTs).

The CVT (Continuously Variable Transmission), works on the principle of continuously variable transmission, which distinguishes it from traditional manual and automatic transmissions. A key component of the CVT design is a chain and two sets of pulleys (known as variators).

The chain is made of durable metal or composites and transmits power from the motor to the drive wheels. Each pair of pulleys, one connected to the input shaft and the other to the output shaft, consists of two conical discs that can move along the shaft axis. When the discs change their distance from each other, the active diameter along which the chain moves changes. Decreasing the distance between the discs causes the chain to move to a larger diameter, which increases the gear ratio, while increasing the distance causes the chain to move to a smaller diameter, which decreases the gear ratio.

The movement of the cones in relation to each other is controlled by a hydraulic system that adjusts the active diameter at which the chain operates, enabling smooth gear changes. The CVT transmission is controlled by a sophisticated electronic system that monitors engine performance, vehicle speed, accelerator pedal position and other factors. Based on this data, the system decides on the optimum variator setting to provide the best possible gear ratio for a given situation.

A planetary gear set can be found in CVT transmissions, which allows additional gear changes or the realisation of operating modes such as reverse. The planetary gear set increases the flexibility and operating range of the CVT transmission.

Thanks to its design, the CVT transmission offers flexible and stepless gear changes, resulting in a smoother ride, better performance and optimised fuel consumption. The CVT is particularly popular in hybrid and urban vehicles where comfort and efficiency are priorities. The CVT transmission's cooling and lubrication system ensures that the pulleys and chain are properly lubricated and cooled, minimising friction and wear and ensuring the reliability and durability of the transmission.

Dual-clutch transmissions (DCT/DSG).

The dual-clutch automatic transmission, also known as DCT (Dual-Clutch Transmission) or DSG (Direct-Shift Gearbox), is an advanced mechanism that combines the features of both automatic and manual transmissions. 

The basic element of the DCT transmission are two clutches: one responsible for the even gears (2, 4, 6) and the other for the odd gears (1, 3, 5, 7). These clutches operate on the principle of wet clutches (immersed in oil) or dry clutches, depending on the design and purpose of the transmission. The operation of the clutches is controlled by electronics, which enables them to be engaged and disengaged precisely.

Another important component is the two input shafts, one of which is located inside the other. One shaft is responsible for the odd gears and the other for the even gears. This allows the next gear to be prepared for engagement at the same time, minimising interruptions in power delivery during gear changes.

The gearshift mechanism of the DCT consists of a set of gears that are located on both input shafts. These gears are responsible for the different ratios and their engagement and disengagement is controlled by a hydraulic circuit and synchronisation mechanisms. The gears are changed by moving the forks and connection the corresponding gears together, allowing the required ratio to be selected.

The operation of the dual-clutch transmission is controlled by a sophisticated electronic system known as the TCU (Transmission Control Unit). The TCU monitors various driving parameters such as vehicle speed, engine load, accelerator pedal position and driver preference (driving mode). Based on this data, the TCU decides which clutch to engage, when to change gear and which gear ratio to select for optimum performance and driving economy.

In a dual-clutch transmission, the lubrication and oil cooling system is also a very important component. In the case of wet clutches, oil is used to lubricate and cool the clutches for longer life and reliability. The oil circulates in a closed circuit, and a pump and cooler maintain its proper pressure and temperature.

Remanufacturing process.

The automatic transmission is first pre-cleaned and then disassembled. After disassembly, its components are cleaned in special equipment such as chemical and ultrasonic washers. The clean parts are then carefully inspected for wear, damage and other defects. Components that do not meet the requirements are replaced with new or reconditioned parts.

The next stage is to replace worn or damaged parts. In a classic automatic transmission, these are usually clutch and brake discs, brake bands, seals, sealing rings, hydraulic valves and bearings. In some cases, the oil pump may need to be remanufactured or replaced, as well as torque transmission components.

In DCT/DSG type transmissions, the double clutch is always replaced with a brand new one. Mechanical or electronic damage to the mechatronics, bearing damage are also common problems. Less frequently, synchronisers, gears and other mechanical components need to be replaced. During the remanufacturing process, all oil filters used in a given transmission are always replaced. It is worth noting that, in addition to the oil filters accessible from the outside, some models of transmissions also have filters located inside the housing, the replacement of which is not possible without dismantling into a parts.

The hydraulic control unit (valve body/mechatronics) removed from the automatic transmission is also disassembled into its component parts and thoroughly cleaned. After replacing seals and other worn or damaged components, it is reassembled and tested on a special test bench. Dynamically, the controller's parameters are tested in a test reproducing its operation in the transmission - above all, the pressure of the oil reaching the individual actuators that control the gear changes. 

The torque converter, which is part of a classic automatic transmission (and some CVT transmission designs), also undergoes a remanufacturing process. This involves cutting the converter apart, cleaning all the components and then inspecting them. All seals are replaced, as well as worn or damaged parts, such as the lock-up clutch linings or plates/discs, bearings, stator one-way clutch, etc. After the installation of the new components, the torque converter is prepared for welding, which is carried out on a special automated device. The torque converter's tightness is then checked, followed by measuring and, if necessary, correcting its balance.

The final stage in the remanufacturing of an automatic transmission is its completion and evaluation of the tightness and operation of the components, as well as a final quality check.

Causes and types of damage

Symptoms that may indicate a faulty automatic transmission:

1. Lack of smooth gear changes - delays, jerks or gear changes that are too abrupt.
2. No gear change or limited gear change, no gears.
3. No response or delayed response when switching between P-R-N-D modes.
4. Jerks or bumps when switching between P-R-N-D modes.
5. Drive slippage - the engine increases speed, but the vehicle does not increase speed in line with the engine speed.
6. Lack of power transmission - the engine revs up but the vehicle does not move.
7. Noise, vibration audible/perceptible while driving.
8. Oil leakage from the transmission.

The most common defects in automatic transmissions are:

1. Wear of friction discs, damage to clutches and brakes
2. Damage to the torque converter
3. Worn or damaged oil pump
4. Damage to seals
5. Damage to the valve body/mechatronics
6. Worn or damaged bearings
7. Malfunction of the electronic transmission control unit