Automatic transmission has a number of undeniable advantages. It makes driving a car much easier. Shifts are smooth and jerk-free, improving ride comfort and increasing durability. Modern automatic transmissions have the ability to shift gears and modes manually and can be adjusted to the driving style of a particular driver. But even the most advanced hydro-mechanical gearboxes are not without disadvantages. These include: the complexity of construction, high price and the cost of maintenance, lower efficiency, worse dynamics and increased fuel consumption in comparison with manual gearboxes, slowness of shifts.
Design and principle of operation
Automatic transmission consists of the following main components: torque converter, planetary series, control and monitoring system. The box of front-wheel drive cars additionally contains the main gear and differential inside the case. To understand how the automatic transmission works, it is necessary to imagine what a hydraulic clutch and a planetary gear are. The hydraulic clutch is a device consisting of two paddle wheels installed in one housing, which is filled with special oil. One of the wheels, called the pump wheel, is connected to the engine crankshaft, and the second, turbine wheel, is connected to the transmission. As the pump wheel rotates, the oil streams it throws off spin the turbine wheel. This design allows the torque to be transmitted in about a 1:1 ratio. This is not suitable for a car, because we need the torque to vary over a wide range. Therefore, between the pump and turbine wheels, another wheel was installed – the reactor wheel, which can either be stationary or rotate, depending on the mode of the car. When the reactor is stationary, it increases the speed of the working fluid flow circulating between the wheels. The higher the speed of the oil flow, the greater the impact it has on the turbine wheel. Thus the torque on the turbine wheel increases, i.e. we transform it. Therefore, a device with three wheels is no longer a fluid coupling, but a torque converter.
But also the torque converter can not convert the speed of rotation and the transmitted torque in the necessary limits. And it is not able to provide reversing movement. Therefore, a set of separate planetary gears with different gear ratios – as if several one-stage gearboxes in one body – are attached to it. The planetary gear is a mechanical system consisting of several gears – satellites that rotate around a central gear. The satellites are locked together by a driver. The outer ring gear is internally meshed with the planetary gears. The satellites, attached to the driver, rotate around the central pinion like planets around the sun (hence the name planetary gear), the outer pinion rotates around the satellites. Different gear ratios are achieved by locking the various parts in relation to each other. The gear change is made by the control system, which on the early models was fully hydraulic, and on the modern models, the hydraulics are replaced by electronics.
Torque converter operating modes
The pump wheel rotates before moving, the reactor wheel and turbine wheel are stationary. The reactor wheel is attached to the shaft by means of an overrunning clutch, and therefore can only rotate in one direction. We switch on a gear, press the accelerator pedal – the engine rpm increases, the pump wheel gains momentum and the oil flows to spin the turbine wheel. The oil, thrown back by the turbine wheel, hits the fixed blades of the reactor, which additionally “twist” the oil flow, increasing its kinetic energy, and directs it to the blades of the pump wheel. Thus, with the help of the reactor, the torque is increased, which is required when the car accelerates. When the car has accelerated, and is moving at a constant speed, the pump wheel and turbine wheel rotate at approximately the same speed. At the same time, the oil flow from the turbine wheel hits the reactor blades already on the other side, so that the reactor starts rotating. There is no increase in torque, the torque converter switches to the fluid coupling mode. If, however, the resistance to car movement has increased (for example, the car is going uphill), the rotation speed of the drive wheels, and, accordingly, the turbine wheel drops. In this case, the oil flows again stop the reactor – the torque increases. In this way, the torque is automatically adjusted depending on the driving mode.
The lack of rigid coupling in the torque converter has advantages and disadvantages. Pros: torque changes smoothly and continuously, torsional vibrations and jerks transmitted from the engine to the transmission are damped. The disadvantages are low efficiency, since part of the energy is lost while “shoveling oil” and is spent to drive the automatic transmission pump, which ultimately leads to an increase in fuel consumption. The torque converter uses a lockup mode to eliminate this disadvantage. During cruising in higher gears, the torque converter automatically engages in a mechanical lockup mode, allowing it to function as an ordinary “dry” clutch. This provides a rigid direct connection between the engine and the drive wheels, as in a manual transmission. In some automatic transmissions, lockup mode is also available in lower gears. Interlocked driving is the most economical mode of automatic transmission operation. When the load on the drive wheels increases, the lockup is automatically released. The torque converter generates heat in the transmission, which is why it is designed with a cooling system with a radiator either built into the engine radiator or fitted separately.
How the planetary gear works
Why does the vast majority of automatic transmissions use planetary gears rather than pinion shafts like manual transmissions? Planetary gears are more compact, they provide faster and smoother shifting of gears without a gap in engine power transmission. Planetary gears are durable because the load is transmitted by multiple satellites, which reduces stress on the gears. In a single planetary gear, the torque is transmitted by means of any (depending on the gear selected) two of its elements, of which one is the master and the other is the slave. The third element is stationary. To obtain a direct transmission it is necessary to fix between themselves any two elements which will play the role of a slave link, the third element in this connection is a master. The total ratio is 1:1. A single planetary gear can thus provide three gears for forward movement (downshift, upshift and straight ahead) and a reverse gear. The ratios of a single planetary series do not allow optimum use of engine torque. Therefore, it is necessary to connect two or three of these mechanisms. There are several variants of the connection, each named after its inventor. The Simpson planetary mechanism, consisting of two planetary gears, is often referred to as a double row. The two groups of satellites, each rotating inside its own crown gear, are combined into a single mechanism by a common sun gear. A planetary series of this design provides three stages of transmission ratio change. Another planetary series is installed in series with the Simpson series to produce a fourth, upshift transmission. Simpson’s layout is the most widely used in automatic transmissions for rear-wheel drive vehicles. High reliability and durability with a relatively simple design are its undeniable advantages. The Ravigné planetary series is sometimes called a one-and-a-half planetary series, emphasizing the features of its design: the presence of one crown gear, two sun gears and a driver with two groups of satellites. The main advantage of the Ravignot scheme is that it allows four stages of change in the gear ratio. The absence of a separate planetary series of upshift transmission allows to make the gearbox very compact, which is especially important for the transmissions of front-wheel drive cars. The disadvantages should include a reduction in the life of the mechanism by about one and a half times compared to the Simpson planetary series. This is due to the fact that the gears of the Ravinho gearbox are loaded continuously, in all modes of the box, while the elements of the Simpson series are not loaded during driving in high gear. The second disadvantage is the low efficiency in lower gears, which leads to a decrease in the acceleration dynamics of the car and noisy operation of the box. The Wilson gearbox consists of 3 planetary gears. The crown gear of the first planetary gear, the driver of the second planetary gear, and the crown gear of the third planetary gear are permanently connected to form a single unit. In addition, the second and third planetary gears share a sun gear, which drives the forward gears. The Wilson scheme provides five forward gears and one reverse gear. The Lepelletier planetary gear combines an ordinary planetary series and a Ravigné planetary series docked behind it.
In spite of its simplicity, it provides six forward gears and one reverse gear. The advantage of the Lepelletier circuit is its simple, compact and lightweight design. The designers are constantly improving the automatic transmission, increasing the number of gears, which improves the smoothness of operation and efficiency of the car. Modern “automatics” can have up to eight gears.
How the control system works
There are two types of automatic transmission control systems: hydraulic and electronic. Hydraulic systems are used on obsolete or budget models, while modern automatic transmissions are controlled electronically. The “life-support” device for any control system is the oil pump. It is driven directly from the engine crankshaft. The oil pump creates and maintains constant pressure in the hydraulic system, regardless of crankshaft speed and engine load. In case of pressure deviation from the nominal pressure, automatic transmission operation is disturbed due to the fact that actuators of gear engagement are controlled by pressure. The moment of gear shifting is determined by vehicle speed and engine load. There are two sensors in the hydraulic control system for this purpose: the speed regulator and the valve – throttle or modulator. The speed regulator or hydraulic speed sensor is installed on the output shaft of the automatic transmission. The faster the car is going, the more the valve opens, the higher the pressure of the transmission fluid passing through this valve. Designed to determine the engine load, the valve – throttle is connected by a cable either to the throttle valve (in gasoline engines) or to the HPF lever (in diesels). Some cars do not use a cable to pressurize the throttle valve, but a vacuum modulator, which is actuated by vacuum in the intake manifold (as the engine load increases, the vacuum drops). Thus, these valves generate pressures proportional to the vehicle speed and engine load. The ratio of these pressures determines the shift points and torque converter lockup. The range selection valve is also involved in “deciding” which gear to change, which is connected to the automatic transmission selector lever and, depending on its position, prohibits engagement of certain gears. The resulting pressure created by the valve – throttle and speed regulator causes the corresponding shift valve to operate. Moreover, if the car is accelerating quickly, the control system will engage a higher gear later than when accelerating calmly. How does this happen? The shift valve is under oil pressure from the speed regulator on one side and from the throttle valve on the other side. If the car accelerates slowly, the pressure from the hydraulic speed valve builds up, causing the shift valve to open. Since the accelerator pedal is not fully depressed, the throttle valve does not put much pressure on the shift valve. If the car is accelerating fast, however, the throttle creates more pressure on the shift valve, preventing it from opening. To overcome this opposition, the pressure from the speed regulator must exceed the pressure from the valve-throttle, but this will happen when the car reaches a higher speed than when it accelerates slowly. Each shift valve corresponds to a certain pressure level: the faster the car is moving, the higher the gear will be engaged. The valve block is a system of channels with valves and plungers in them. The shift valves supply hydraulic pressure to the actuators: friction clutches and brake belts, by means of which the various elements of the planetary series are locked and, consequently, the various gears are engaged (disengaged).
The brake is the mechanism that locks the elements of the planetary series to the stationary housing of the automatic transmission. The friction locks the moving elements of the planetary series against each other. The electronic control system, as well as the hydraulic one, uses two basic parameters for operation: vehicle speed and engine load. But electronic sensors, rather than mechanical ones, are used to determine these parameters. The main ones are sensors: gearbox input speed, gearbox output speed, working fluid temperature, selector lever position, accelerator pedal position. In addition, the automatic transmission control unit receives additional information from the engine control unit and other electronic systems of the vehicle (e.g. ABS). This allows for a more accurate determination of shift timing and torque converter lock-up than in a conventional automatic transmission. The gearshift program can easily calculate the vehicle’s driving resistance based on the character of the speed change at a given engine load and make appropriate adjustments to the shifting algorithm, for example, to engage higher gears later when the vehicle is fully laden.
Electronically controlled automatic transmissions, as well as simple hydromechanical boxes, use hydraulics to engage clutches and brake belts, but each hydraulic circuit is controlled by an electromagnetic rather than a hydraulic valve. The use of electronics has significantly expanded the capabilities of automatic transmissions. They received different modes of operation: economy, sport, winter. The sharp growth of popularity of automatic transmissions was caused by the appearance of Autostick mode, which allows the driver choosing the necessary gear independently. Each manufacturer gave this type of transmission its own name: Audi – Tiptronic, BMW – Steptronic. Thanks to electronics, the modern automatic transmissions can now also “teach themselves”, i.e. change the algorithm for shifting depending on the driving style. Electronics have provided ample opportunities for automatic transmission self-diagnostics. We are not only talking about memorizing fault codes. The control program monitors the wear of friction plates and oil temperature, making the necessary corrections to the automatic transmission.
Planetary gearbox in detail
Planetary Manual Transmission (MPT) is a type of transmission that uses planetary gears. It was widespread at the beginning of the twentieth century (Ford T car), nowadays it is quite widespread in tracked vehicles – military and civilian, as well as on bicycles and in cars with hybrid transmission.
Design and principle of operation
Planetary MCPs use a system of satellite gears that rotate around a central sun gear. Most often, the satellites are placed inside a large crown gear (epicycle), with which they are in constant mesh. In turn, the satellites are fixed to the driver. Change of transmission ratio of the planetary MCP depends on which of the three main elements – the sun gear, satellites with the driver and the crown gear – is fixed stationary, on which torque is supplied and from which element is removed by the transmission. In either case, one of the three main elements of the planetary box (and the satellites are treated as one piece with the driver) will be stationary, the other two will rotate. A system of band brakes and locking clutches is used to stop and lock one of the gear elements. But there are planetary mechanisms in which there are no brakes and clutches – we are talking about differentials, which also belong to the planetary mechanisms, built with the use of bevel gears. Variants of planetary systems used in the MCP, quite a lot. Description of the principle of operation refers to the simplest system with three satellites, fixed to the driver at an angle of 120 degrees. Reduction gear. The first option. If you stop the epicycle, the torque from the engine is fed to the shaft of the sun gear, and the torque is removed from the driver, as a result, the rotation frequency of the shaft of the driver will be less than the speed of the sun gear. Second option. If you feed motor shaft torque to the epicycle, block the sun gear, and the torque is taken from the driver, you get the same effect (but with a gear ratio close to one). Boost gear. First option. Epicycle is blocked, the torque is fed to the driver with satellites, and removed from the central sun gear. As a result, the gearbox works as a booster. Second option. The sun pinion is blocked, the torque is applied to the driver and removed from the big crown gear. The effect is the same, the gearbox works as an overdrive gear. Reverse gear. First variant. Torque is input to sun gear, and is taken off the epicycle, with the driver fixed at standstill. In this case, the gearbox works as a negative ratio reducer, i.e. torque reversal mode is engaged. Second variant. The torque is input to the epicycle, and is removed from the sun gear shaft; the driver, again, is fixed motionless. The gearbox works in reverse mode with a negative transmission ratio.
Application of Planetary CVTs
Manual transmission (foot control, to be exact) was out of use in motor transport as early as 1928, when production of the legendary Ford T was discontinued. This car used a planetary mechanical two-speed gearbox. The gears were shifted by pedals that engaged the box’s band brakes. First gear was engaged by pressing the right pedal, the second – the middle and reverse – the left pedal (there were a total of three pedals, instead of the “gas” pedal was used the undercarriage lever). In the 30’s and later years, manual transmission was replaced by semi-automatic and automatic planetary gearboxes. Semi-automatic transmissions used hydraulic clutches instead of clutch and automatic transmissions used torque converters instead of clutch.
Today planetary MCPs are widely used in tracked vehicles, including military ones – in tanks, tractors, transporters. In aircraft turbines, in metal-cutting machines – as gearboxes.
Planetary mechanical gearboxes built into the rear hub of a bicycle wheel are very popular. These boxes are lightweight, durable, efficient and easy to use because they do not require any maintenance. At the same time, they increase the cost of bicycles and are not used in sports models – because of the large mass (about 1.5-2 kg) and less maintainability in comparison with the open chain transfer devices of the parallelogram type.