25.15.
Automatic Transmission
An Overview.
The idea of using automatic and semi-automatic gear-changing systems to reduce driver’s fatigue is almost as old as the car itself. German engineers were most active in the area of transmission design, and during the period up to World War I a variety of gearless drive systems were developed. The 1901 Manurer-Union car incorporated two friction discs, mounted at a steep angle to each other, the create a stepless transmission system, an idea which is known today as continuously variable transmission (CVT). The driver still had to effect ratio changes by manually moving the edge of the driven disc radially across the drive disc, but was relieved of the task of manipulating gears, clutch and throttle.
In addition to the increase in the power output of engines, crude friction drives proved unreliable and subsequently a variety of semi-automatic transmissions were developed with automatic operation of the clutch, but mostly based around conventional stepped-ratio gearboxes. Unfortunately most of these developments were short-lived, due largely to poor reliability and inadequate materials. •
American car manufacturers wear particularly interested to eliminate the need for gear changing on their luxury vehicles, and with this objective General Motors Corporation developed a type of fully-automatic transmission, which they called Hydramatic. Hydramatic utilized variations in hydraulic fluid pressure to control an epicyclic gear train, driven from the engine through a fluid coupling. The transmission used a low gear to start the vehicle moving, prior to shifting to a higher gear for cruising. The project was spectacularly successful and the world’s first automatic transmission, with two self-selecting forward gears, was introduced on the 1939 Oldsmobile saloon.
Competition between manufacturers resulted in rapid improvements in transmission design, and three-speed units were soon introduced. Since these transmissions were ideally suited to American motoring conditions the market for this system grew fast, so that today all most all cars sold in the US are equipped with automatic transmission.
In Europe automatic transmissions have never been specifically popular and even today. This is partly because automatic gearboxes are less efficient than their manual counterparts, giving rise to a noticeable deterioration in vehicle performance when coupled with the small capacity engines that are common in Europe. Another factor is that European roads tend to be less well graded than those of North America, and so the additional driver control afforded by a manual gearbox is often to be valued. Finally, automatic transmission are costly and are generally available as an extra-cost option, which often deters potential purchasers.
Although automatic transmissions are usually less fuel-efficient than their manual counterparts, they do offer many driving advantages, especially in urban road conditions.
(a) Driver fatigue is reduced since there is no clutch or gear lever to manipulate. This is
specifically significant when driving in dense traffic.
(6) Both hands can remain on the steering wheel at all times, so there is increase in driving safety.
(c) Since the transmission always engages the correct gear for the prevailing driving conditions, the possibility of labouring or over-reviving the engine is eliminated.
The automatic transmission systems are efficient, convenient, easy to operate, durable and reliable, but they are relatively expensive to manufacture and service compared to standard transmission systems. Current automatic transmission designs are lighter, smaller, and less expensive to manufacture and have superior operating characteristics when compared with earlier versions. Automatic transmission system used in passenger cars has a three-member torque converter driven through a two or three-speed automatic shifting planetary gear train. This combination provides smooth torque characteristics from starting to the designed peak operating conditions. A typical modern three speed automatic transmission is shown in Fig. 25.32.
Features.
Since a conventional gear train can not provide silent and smooth gear ratio changes, automatic transmissions commonly adopt some sort of epicyclic gear arrangement. 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 provide the necessary speed variations. A torque converter introduced between the engine and transmission gearing, automatically reduces or increases the engine to transmission slip according to changes in engine speed and road conditions.
Hydraulic pressure signals supplied by the governor valve and a throttle valve control the actual speed at which gear ratio changes occur. The governor valve senses vehicle speed whereas throttle valve senses engine load. These pressure signals are directed to a hydraulic control block consisting of valves and pistons, which translates this information in terms of pressure variations. The fluid pressure supplied by a pump then automatically directs fluid to the various operating pistons causing their respective clutches or band brakes to be applied. Consequently, gear upshifts and downshifts are automatically carried out taking into account of the condition of the road, the available output of the engine and the acceleration/speed requirements of the driver.
Other than the continuously variable transmission (CVT) systems, most modern automatic transmissions have two main units such as a fluid clutch and a main gearbox.
Fig. 25.32. A typical automatic transmission system.
Fluid Clutch.
This unit automatically disconnects the drive when the engine speed is low and gradually connects it as the vehicle is moved from a stationary position. It is either a fluid coupling or fluid converter. The latter is often called a torque converter because, in its simple form, it can double the engine output torque. Also the converter provides a gradual reduction of the torque amplification as the output speed of the converter increases.
Main gearbox.
This automatically provides a series of stepped gear ratios to assist the vehicle to overcome road conditions requiring a large driving torque. Also, it enables the vehicle to operate overj^wide road speed range.
In addition to the provision of neutral and reverse, the modern gearbox also has a positive parking lock and a mechanism for the selection, and hold of a particular gear.
