【正文】 s accelerator pedal (or engine throttle). The control system in the HydraMatic transmission operated entirely by hydraulic pressure, and it contained a plex assemblage of valves, oil passageways, and pistons. The valves moved in response to the inputs of speed and throttle in order to cause the transmission to shift to a new speed setting at the appropriate time. The HydraMatic39。 (2) the engine speed。s output)． In fourth gear, the HydraMatic’s highest setting, the transmission has a velocity ratio of 1， and the engine and driveshaft rotate at the same speed. Together with those speed changes, automatic transmissions also modify the torque that is supplied by the engine to the driveshaft according to the ideal geartrain principle (VR) (TR)＝ 1， as discussed in Chapter 7. Of course, automobile transmissions also have a neutral setting, in which the engine is disconnected from the driveshaft, and a setting for reverse. Modern transmissions incorporate electronic and puter control， and although they are more sophisticated than the HydraMatic, they are similar in their basic operating principles． As depicted in Figure 8． 18， a fluid coupling serves as the clutch between the engine and the transmission. The HydraMatic transmission in Figure has four forward speeds, a neutral setting, and one reverse speed. The velocity ratio for each setting is determined by the operation of the three interconnected plaary geartrains, which are called the front, rear, and reverse stages． A hydraulic control unit, which is not shown in Figure 8． 18, performs the shifts between transmission settings. Each shift is acplished by engaging or disengaging the two band brakes, the two clutches, or the reverse lock. The entire transmission prises a fluid coupling, three plaary geartrains, two clutches, two band brakes, a reverse lock, and a hydraulic control system. Those ponents operate as follows. Fluid Coupling This ponent transfers power from the engine to the transmission without a rigid mechanical connection, a feature that is advantageous for two reasons. First, when the brakes are applied and the transmission is placed in gear, the engine can continue to .operate at its idle speed even though the wheels do not rotate. Second, when there is a sudden speed change, the fluid coupling acts as a buffer to isolate other ponents in the drivetrain from shock. You can view the fluid coupling as being analogous to two household fans that face one another. Imagine that one fan (which we call the pump) is powered up to speed and blows air directly at the (otherwise unpowered) second fan (which we call the turbine). In the ideal situation, the second fan would spin up to the same speed as the first fan and be powered entirely by the air blown on it. In a similar manner, the pump and turbine blades in the fluid coupling of the transmission smoothly transfer power from the engine to the plaary geartrains. The pump and turbine in Figure (a) are made up of multiple blades, and they are connected to separate shafts placed along the centerline of the transmission. The main shaft is connected to the turbine, and it rotates within the hollow intermediate shaft, which in turn is connected to the pump. The housing in which the pump and turbine rotate is filled with a light oil called transmission fluid. As the pump rotates, its blades force oil against the blades on the turbine, causing it to rotate. When the automobile is traveling at a steady speed, the rotation of the turbine on the main shaft follows the rotation of the pump on the hollow intermediate shaft, and they rotate in the same direction and with the same angular velocity. Plaary Geartrains The front, rear, and reverse plaary geartrain stages each include a sun gear, pla gears, a ring gear, and a carrier. The tooth numbers for each gear are listed in Figure (b). The front and rear geartrains set the velocity ratios of the transmission in the first through fourth speed settings, and the reverse stage is used only for the reverse setting. A noteworthy aspect of the transmission is the manner in which the cascade of three plaary geartrains is interconnected through the band brakes and clutches, which in turn are switched by hydraulic control to change speeds. The engine crankshaft supplies power to the transmission by rotating the housing of the fluid coupling. In Figure (a), the housing is connected to the ring gear of the first plaary geartrain. Therefore, at the input side of the transmission, the automobile engine directly drives the ring gear in the first plaary stage. On the other hand, at the output of the transmission, the driveshaft connects to the carriers on both the rear and reverse plaary stages. The main shaft rotates with the turbine in the fluid coupling, and at its other end, the shaft is connected to the sun gear in the rear plaary stage. Band Brakes and Reverse Lock Band brakes in the transmission wraparound drums in the front and rear plaary geartrains. Those brakes are applied or released by the two tensioners shown in Figure (a), which are activated by the hydraulic control system to effect a speed change. When either brake is applied, it locks the plaary geartrain ponent to which it is attached and prevents it from rotating. When the band brake in the front stage is applied, for instance, the sun gear is prevented from rotating. Similarly, when the second band brake is applied, the ring gear in the rear plaary stage, and the sun gear in the reverse stage, are both locked stationary. When the reverse lock is applied, the ring gear in the reverse plaary geartrain does not rotate． Clutches Like the band brakes and reverse lock, the two clutches in the HydraMatic transmission engage or disengage under hydraulic control to change speed settings. The clutches are interleaved disks and plates, and they are