51 Automatic transmission considerations

Because it is difficult to achieve silent and smooth gear ratio changes with a conventional constant mesh gear train, automatic transmissions commonly adopt some sort of epicyclic gear arrangement, in which different gear ratios are selected by the application of multiplate clutches and band brakes which either hold or couple various members of the gear train to produce the necessary speed variations. The problem of a gradual torque take-up when moving away from a standstill has also been overcome with the introduction of a torque converter between the engine and transmission gearing so that engine to transmission slip is automatically reduced or increased according to changes in engine speed and road conditions. Torque converter performance characteristics have been discussed in Chapter 3.

The actual speed at which gear ratio changes occur is provided by hydraulic pressure signals supplied by the governor valve and a throttle valve. The former senses vehicle speed whereas the latter senses engine load.

These pressure signals are directed to a hydraulic control block consisting of valves and pistons which compute this information in terms of pressure variations. The fluid pressure supplied by a pressure pump then automatically directs fluid to the various operating pistons causing their respective clutch, clutches or band brakes to be applied. Consequently, gear upshifts and downshifts are performed independently of the driver and are so made that they take into account the condition of the road, the available output of the engine and the requirements of the driver.

5.1.1 The torque converter (Fig. 5.1) The torque converter provides a smooth automatic drive take-up from a standstill and a torque multiplication in addition to that provided by the normal mechanical gear transmission. The performance characteristics of a hydrokinetic torque converter incorporated between the engine and the gear train is shown in Fig. 5.1 for light throttle and full throttle maximum output conditions over a vehicle speed range. As can be seen, the initial torque multiplication when driving away from rest is con siderable and the large gear ratio steps of the conventional transmission are reduced and smoothed out by the converter's response between automatic gear shifts. Studying Fig. 5.1, whilst in first gear, the torque converter provides a maximum torque multiplication at stall pull away conditions which progressively reduces with vehicle speed until the converter coupling point is reached. At this point, the reaction member freewheels. With further speed increase, the converter changes to a simple fluid coupling so that torque multiplication ceases. In second gear the converter starts to operate nearer the coupling point causing it to contribute far less torque multiplication and in third and fourth gear the converter functions entirely beyond the coupling point as a fluid coupling. Consequently, there is no further torque multiplication.

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  • Brigitte
    What are the considerations of automatic transmission?
    2 years ago

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