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Automatic



What Is Automatic Revalidation?The Department of Homeland Security (DHS) U.S. Customs and Border Protection (CBP) has the authority and the responsibility over the admission of travelers to the United States. Under the automatic revalidation provision of immigration law, certain temporary visitors holding expired nonimmigrant visas who seek to return to the U.S. may be admitted at a U.S. port-of-entry by CBP, if they meet certain requirements, including, but not limited to the following:




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Many nonimmigrants will need to reapply and be reissued visas to reenter the U.S. when their existing visas have expired, even if they are in possession of valid admission stamp or paper Form I-94, because automatic revalidation applies to limited categories of travelers. Refer to the Automatic Revalidation Fact Sheet on the CBP website. The following temporary visitors whose nonimmigrant visas have expired, but who have a valid admission stamp or paper Form I-94, must reapply for and be issued nonimmigrant visas prior to their reentry to the United States, if one or more of the following situations exists (this is not a complete listing):


An automatic transmission (sometimes abbreviated AT) is a multi-speed transmission used in motor vehicles that does not require any input from the driver to change forward gears under normal driving conditions.


The most common type of automatic transmission is the hydraulic automatic, which uses a planetary gearset, hydraulic controls, and a torque converter. Other types of automatic transmissions include continuously variable transmissions (CVT), automated manual transmissions (AMT), and dual-clutch transmissions (DCT).


The 1904 Sturtevant "horseless carriage gearbox" is often considered to be the first true automatic transmission.[1][2] The first mass-produced automatic transmission is the General Motors Hydramatic four-speed hydraulic automatic, which was introduced in 1939.


Globally, 43% of new cars produced in 2015 were manual transmission, falling to 37% by 2020.[3] Automatic transmissions have long been prevalent in the United States, but have only started to become common in Europe much more recently. In Europe in 1997, only 10-12% of cars had automatic transmission.[4]


In the United States, over 80% of new cars had automatic transmission by 1957.[3] Automatic transmission has been standard in large cars since at least 1974.[5] By 2020 only 2.4% of new cars had manual transmission.[6] Historically, automatic transmissions were less efficient, but lower fuel prices in the US made this less of a problem than in Europe.[7]


In the United Kingdom, a majority of new cars have had automatic transmission since 2020. Several manufacturers including Mercedes and Volvo no longer sell manual transmission cars. The growing prevalence of automatic transmission is attributed to the increasing number of electric and hybrid cars, and the ease of integrating it with safety systems like Autonomous Emergency Braking.[8][9]


The most common design of automatic transmissions is the hydraulic automatic, which typically uses planetary gearsets that are operated using hydraulics.[10][11] The transmission is connected to the engine via a torque converter (or a fluid coupling prior to the 1960s), instead of the friction clutch used by most manual transmissions.[12]


A hydraulic automatic transmission uses planetary (epicyclic) gearsets instead of the manual transmission's design of gears lined up along input, output and intermediate shafts. To change gears, the hydraulic automatic uses a combination of internal clutches, friction bands or brake packs. These devices are used to lock certain gears, thus setting which gear ratio is in use at the time.[13]


A sprag clutch (a ratchet-like device which can freewheel and transmits torque in only one direction) is often used for routine gear shifts. The advantage of a sprag clutch is that it eliminates the sensitivity of timing a simultaneous clutch release/apply on two planetary gearsets, simply "taking up" the drivetrain load when actuated, and releasing automatically when the next gear's sprag clutch assumes the torque transfer.


The aforementioned friction bands and clutches are controlled using automatic transmission fluid (ATF), which is pressured by a pump and then directed to the appropriate bands/clutches to obtain the required gear ratio.[12][13] The ATF provides lubrication, corrosion prevention, and a hydraulic medium to transmit the power required to operate the transmission. Made from petroleum with various refinements and additives, ATF is one of the few parts of the automatic transmission that needs routine service as the vehicle ages.


The main pump which pressurises the ATF is typically a gear pump mounted between the torque converter and the planetary gear set. The input for the main pump is connected to the torque converter housing, which in turn is bolted to the engine's flexplate, so the pump provides pressure whenever the engine is running. A disadvantage of this arrangement is that there is no oil pressure to operate the transmission when the engine is not running, therefore it is not possible to push start a vehicle equipped with an automatic transmission with no rear pump (aside from several automatics built prior to 1970, which also included a rear pump for towing and push-starting purposes). The pressure of the ATF is regulated by a governor connected to the output shaft, which varies the pressure depending on the vehicle speed.


The valve body inside the transmission is responsible for directing hydraulic pressure to the appropriate bands and clutches. It receives pressurized fluid from the main pump and consists of several spring-loaded valves, check balls, and servo pistons. In older automatic transmissions, the valves use the pump pressure and the pressure from a centrifugal governor on the output side (as well as other inputs, such as throttle position or the driver locking out the higher gears) to control which ratio is selected. As the vehicle and engine change speed, the difference between the pressures changes, causing different sets of valves to open and close. In more recent automatic transmissions, the valves are controlled by solenoids.[13] These solenoids are computer-controlled, with the gear selection decided by a dedicated transmission control unit (TCU) or sometimes this function is integrated into the engine control unit (ECU). Modern designs have replaced the centrifugal governor with an electronic speed sensor that is used as an input to the TCU or ECU. Modern transmissions also factor in the amount of load on an engine at any given time, which is determined from either the throttle position or the amount of intake manifold vacuum.[13]


The multitude of parts, along with the complex design of the valve body, originally made hydraulic automatic transmissions much more expensive and time-consuming to build and repair than manual transmissions; however mass-production and developments over time have reduced this cost gap.


The 1904 Sturtevant "horseless carriage gearbox" is often considered to be the first automatic transmission for motor vehicles.[15][16] Developed in Boston in the United States, this transmission had two forward gear ratios and engine-driven flyweights which controlled the gear selection.[citation needed] At higher engine speeds, high gear was engaged. As the vehicle slowed down and engine RPM decreased, the gearbox would shift back to low. However, the transmission was prone to sudden failure, due to the transmission being unable to withstand forces from the abrupt gear changes.


The adoption of planetary gearsets was a significant advance towards the modern automatic transmission. One of the first transmissions to use this design was the manual transmission fitted to the 1901-1904 Wilson-Pilcher automobile.[17] This transmission was built in the United Kingdom and used two epicyclic gears to provide four gear ratios. A foot clutch was used for standing starts, gear selection was using a hand lever, helical gears were used (to reduce noise) and the gears used a constant-mesh design. A planetary gearset was also used in the 1908 Ford Model T, which was fitted with a two-speed manual transmission (without helical gears).


An early patent for the automatic transmission was granted to Canadian inventor Alfred Horner Munro of Regina in 1923.[18] Being a steam engineer, Munro designed his device to use compressed air rather than hydraulic fluid, and so it lacked power and never found commercial application.[19]


In 1923, a patent was approved in the United States describing the operation of a transmission where the manual shifting of gears and manual operation of a clutch was eliminated. This patent was submitted by Henry R. Hoffman from Chicago and was titled: Automatic Gear Shift and Speed Control. The patent described the workings of such a transmission as "...having a series of clutches disposed intermediate the engine shaft and the differential shaft and in which the clutches are arranged to selectively engage and drive the differential shaft dependent upon the speed at which the differential shaft rotates". However, it would be over a decade later until automatic transmissions were produced in significant quantities. In the meantime, several European and British manufacturers would use preselector gearboxes, a form of manual transmission which removed the reliance on the driver's skill to achieve smooth gear shifts.


The evolution towards mass-produced automatic transmissions continued with the 1933-1935 REO Motor Car Company Self-Shifter semi-automatic transmission,[22] which automatically shifted between two forward gears in the "Forward" mode (or between two shorter gear ratios in the "Emergency low" mode). Driver involvement was still required during normal driving, since standing starts required the driver to use the clutch pedal.[23] This was followed in 1937 by the Oldsmobile Automatic Safety Transmission. Similar in operation to the REO Self-Shifter, the Automatic Safety Transmission shifted automatically between the two gear ratios available in the "Low" and "High" ranges and the clutch pedal was required for standing starts. It used a planetary gearset.[24][25][26] The Chrysler Fluid Drive, introduced in 1939, was an optional addition to manual transmissions where a fluid coupling (similar to a torque-convertor, but without the torque multiplication) was added, to avoid the need to operate a manual clutch.[27][28] 041b061a72


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