最好字数在5000左右!谢谢了
加上翻译好的中文更好!
Machine Transmission
要机械变速器的
第1个回答 2008-02-25
An automatic transmission (commonly abbreviated as "AT") is an automobile gearbox that can change gear ratios automatically as the vehicle moves, thus freeing the driver from having to shift gears manually (similar but larger devices are also used for railroad locomotives).
Most automatic transmissions have a set selection of possible gear ranges, often with a parking pawl feature that will lock the output shaft of the transmission.
However, some simple machines with limited speed ranges and/or fixed engine speeds only use a torque converter to provide a variable gearing of the engine to the wheels. Typical examples include forklift trucks and some modern lawn mowers.
Recently manufacturers have begun to make continuously variable transmissions commonly available (earlier models such as the Subaru Justy did not popularize CVT). These designs can change the ratios over a range rather than between set gear ratios. Even though CVTs have been used for decades in two-wheeled scooters and in a few cars (e.g. DAF saloons and the Volvo 340 series that succeeded them, and later the Subaru Justy), the technology has recently gained greater acceptance among manufacturers and customers.
Comparison with manual transmission
Most cars sold in the United States since the 1950s have been equipped with an automatic transmission. This has, however, not been the case in Europe. In most Asian markets, automatic transmission became very popular from the 1990s. Automatic transmission provides lower fuel efficiency and power than manual transmission. Automatic transmission is easier for drivers, especially beginners. In some jurisdictions, drivers passing their driving test in an automatic-transmission vehicle will not be licensed to drive a manual-transmission vehicle. An example of this is in Australia. However, a driver holding a manual license is permitted to drive an automatic.
Automatic transmission modes
Conventionally, in order to select the mode, the driver would have to move a gear shift lever located on the steering column or on the floor next to him/her. In order to select gears/modes the driver must push a button in (called the shift lock button) or pull the handle (only on column mounted shifters) out. Some vehicles (like the Aston Martin DB9) position selector buttons for each mode on the cockpit instead, freeing up space on the central console. Vehicles conforming to US Government standards must have the modes ordered P-R-N-D-L (left to right, top to bottom, or clockwise). Prior to this, quadrant-selected automatic transmissions often utilized a P-N-D-L-R layout, or similar. Such a pattern led to a number of deaths and injuries owing to unintentional gear mis-selection, as well the danger of having a selector (when worn) jump into Reverse from Low gear during engine braking maneuvers.
Automatic Transmissions have various modes depending on the model and make of the transmission. Some of the common modes are:
Park (P) – This selection mechanically locks the transmission, restricting the car from moving in any direction. A parking pawl prevents the transmission, and therefore the vehicle, from moving (although the front or rear wheels, if the vehicle is rear- or front-wheel-drive respectively, can still spin freely). For this reason, it is recommended to use the hand brake (or parking brake) because this actually locks the (in most cases, rear) wheels and prevents them from moving. This also increases the life of the transmission and the park pin mechanism, because when parking on an incline with the transmission in park without the parking brake engaged will cause undue stress on the parking pin. An efficiently-adjusted hand brake should also prevent the car from moving if a worn selector accidentally drops into Reverse gear during early morning fast-idle engine warmups.
A car should be allowed to come to a complete stop before setting the transmission into park to prevent damage. Usually, PARK is one of only two selections in which the car's engine can be started. In some cars (notably those sold in the US), the driver must have the footbrake depressed before the transmission can be taken out of park. The Park position is omitted on buses/coaches with automatic transmission, which must be placed in neutral with the parking brakes set.
Reverse (R) – This puts the car into the reverse gear, giving the ability for the car to drive backwards. In order for the driver to select reverse they must come to a complete stop, push the shift lock button in (or pull the shift lever forward in the case of a column shifter) and select reverse. Not coming to a complete stop can cause severe damage to the transmission. Many modern automatic gearboxes have a safety mechanism in place, which does to some extent prevent (but doesn't completely avoid) inadvertently putting the car in reverse when the vehicle is moving. This mechanism usually consists of a solenoid- controlled physical barrier on either side of the Reverse position, which is electronically engaged by a switch on the brake pedal. Therefore, the brake pedal needs to be depressed in order to allow the selection of reverse. Some electronic transmissions prevent or delay engagement of reverse gear altogether while the car is moving.
Neutral/No gear (N)– This disconnects the transmission from the wheels so the car can move freely under its own weight. This is the only other selection in which the car can be started.
Drive (D)– This allows the car to move forward and accelerate through its range of gears. The number of gears a transmission has depends on the model, but they can commonly range from 3, 4 (the most common), 5, 6 (found in VW/Audi Direct Shift Gearbox), 7 (found in Mercedes 7G gearbox and in BMW M5) and 8 in the newer models of Lexus cars. Some cars when put into D will automatically lock the doors or turn on the Daytime Running Lamps.
OverDrive ([D], OD, or a boxed D) - This mode is used in some transmissions, to allow early Computer Controlled Transmissions to engage the Automatic Overdrive. In these transmissions, Drive (D) locks the Automatic Overdrive off, but is identical otherwise. OD (Overdrive) in these cars is engaged under steady speeds or low acceleration at approximately 35-45 mph (approx. 72 km/h). Under hard acceleration or below 35-45 mph, the transmission will automatically downshift. Vehicles with this option should be driven in this mode unless circumstances require a lower gear.
Second (2 or S) – This mode limits the transmission to the first two gears, or more commonly locks the transmission in second gear. This can be used to drive in adverse conditions such as snow and ice, as well as climbing or going down hills in the winter time. Some vehicles will automatically upshift out of 2nd gear in this mode if a certain rpm range is reached, to prevent engine damage.
First (1 or L) – This mode locks the transmission in first gear only. It will not accelerate through any gear range. This, like second, can be used during the winter season, or towing.
As well as the above modes there are also other modes, dependent on the manufacturer and model. Some examples include;
* D5 – In Hondas and Acuras equipped with 5-speed automatic transmissions, this mode is used commonly for highway use (as stated in the manual), and uses all 5 forward gears.
* D4 – This mode is also found in Honda and Acura 4 or 5-speed automatics and only uses the first 4 gears. According to the manual, it is used for stop & go traffic, such as city driving.
* D3 – This mode is found in Honda and Acura 4-speed automatics and only uses the first 3 gears. According to the manual, it is used for stop & go traffic, such as city driving. This mode is also found in Honda and Acura 5-speed automatics.
* + − and M – This is the manual selection of gears for automatics, such as Porsche's Tiptronic. This feature can also be found in Chrysler and General Motors products such as the Dodge Magnum and Pontiac G6. The driver can shift up and down at will, like a semi-automatic transmission. This mode may be engaged either through a selector/position or by actually changing gear (e.g. tipping the gear-down paddle).
Hydraulic automatic transmissions
The predominant form of automatic transmission is hydraulically operated, using a fluid coupling or torque converter and a set of planetary gearsets to provide a range of torque multiplication.
Parts and operation
Parts and operation
A hydraulic automatic transmission consists of the following parts:
* Fluid coupling or torque converter: A hydraulic device connecting the engine and the transmission. It takes the place of a mechanical clutch, allowing the engine to remain running at rest without stalling. A torque converter is a fluid coupling that also provides a variable amount of torque multiplication at low engine speeds, increasing "breakaway" acceleration.
* Planetary gearset: A compound planetary set whose bands and clutches are actuated by hydraulic servos controlled by the valve body, providing two or more gear ratios.
* Clutches;bands: to effect gear changes, one of two types of clutches, or bands are used to hold a particular member of the planetary gearset motionless, while allowing another member to rotate..transmitting torque and producing gear reductions or overdrive ratios. These clutches are actuated by the valve body (see below), and their sequence controlled by the transmission's internal programming. Principally, a type of device known as a a sprag or roller clutch is used for routine upshifts/downshifts. Operating much as a ratchet, it transmits torque only in one direction, freewheeling or "overrunning" in the other. The advantage of this type of clutch is that it eliminates the sensitivity of timing a simultaneous clutch release/apply on two planetaries, simply "taking up" the drivetrain load when actuated ,and releasing automatically when the next gear's sprag clutch assumes the torque transfer.
The bands which were mentioned come into play for manually selected gears, such as low range or reverse, and operate on the planetary drum's circumference. Bands are not applied when drive/overdrive range is selected, the torque being transmitted by the sprag clutches instead.
* Valve body: hydraulic control center that receives pressurized fluid from a main pump operated by the fluid coupling/torque converter. The pressure coming from this pump is regulated and used to run a network of spring-loaded valves, check balls and servo pistons. The valves use the pump pressure and the pressure from a centrifugal governor on the output side (as well as hydraulic signals from the range selector valves and the throttle valve or modulator) to control which ratio is selected on the gearset; as the car and engine change speed, the difference between the pressures changes, causing different sets of valves to open and close. The hydraulic pressure controlled by these valves drives the various clutch and brake band actuators, thereby controlling the operation of the planetary gearset to select the optimum gear ratio for the current operating conditions. However, in many modern automatic transmissions, the valves are controlled by electro-mechanical servos which are controlled by the Engine Management System or a separate transmission controller. (See History and improvements below.)
* Hydraulic & Lubricating Oil: called automatic transmission fluid (ATF), this component of the transmission provides lubrication, corrosion prevention, and a hydraulic medium to convey mechanical power. Primarily made from refined petroleum and processed to provide properties that promote smooth power transmission and increase service life, the ATF is one of the few parts of the automatic transmission that needs routine service as the vehicle ages.
The multitude of parts, along with the complex design of the valve body, originally made hydraulic automatic transmissions much more complicated (and expensive) to build and repair than manual transmissions. In most cars (except US family, luxury, sport-utility vehicle, and minivan models) they have usually been extra-cost options for this reason. Mass manufacturing and decades of improvement have reduced this cost gap.
Most automatic transmissions have a set selection of possible gear ranges, often with a parking pawl feature that will lock the output shaft of the transmission.
However, some simple machines with limited speed ranges and/or fixed engine speeds only use a torque converter to provide a variable gearing of the engine to the wheels. Typical examples include forklift trucks and some modern lawn mowers.
Recently manufacturers have begun to make continuously variable transmissions commonly available (earlier models such as the Subaru Justy did not popularize CVT). These designs can change the ratios over a range rather than between set gear ratios. Even though CVTs have been used for decades in two-wheeled scooters and in a few cars (e.g. DAF saloons and the Volvo 340 series that succeeded them, and later the Subaru Justy), the technology has recently gained greater acceptance among manufacturers and customers.
Comparison with manual transmission
Most cars sold in the United States since the 1950s have been equipped with an automatic transmission. This has, however, not been the case in Europe. In most Asian markets, automatic transmission became very popular from the 1990s. Automatic transmission provides lower fuel efficiency and power than manual transmission. Automatic transmission is easier for drivers, especially beginners. In some jurisdictions, drivers passing their driving test in an automatic-transmission vehicle will not be licensed to drive a manual-transmission vehicle. An example of this is in Australia. However, a driver holding a manual license is permitted to drive an automatic.
Automatic transmission modes
Conventionally, in order to select the mode, the driver would have to move a gear shift lever located on the steering column or on the floor next to him/her. In order to select gears/modes the driver must push a button in (called the shift lock button) or pull the handle (only on column mounted shifters) out. Some vehicles (like the Aston Martin DB9) position selector buttons for each mode on the cockpit instead, freeing up space on the central console. Vehicles conforming to US Government standards must have the modes ordered P-R-N-D-L (left to right, top to bottom, or clockwise). Prior to this, quadrant-selected automatic transmissions often utilized a P-N-D-L-R layout, or similar. Such a pattern led to a number of deaths and injuries owing to unintentional gear mis-selection, as well the danger of having a selector (when worn) jump into Reverse from Low gear during engine braking maneuvers.
Automatic Transmissions have various modes depending on the model and make of the transmission. Some of the common modes are:
Park (P) – This selection mechanically locks the transmission, restricting the car from moving in any direction. A parking pawl prevents the transmission, and therefore the vehicle, from moving (although the front or rear wheels, if the vehicle is rear- or front-wheel-drive respectively, can still spin freely). For this reason, it is recommended to use the hand brake (or parking brake) because this actually locks the (in most cases, rear) wheels and prevents them from moving. This also increases the life of the transmission and the park pin mechanism, because when parking on an incline with the transmission in park without the parking brake engaged will cause undue stress on the parking pin. An efficiently-adjusted hand brake should also prevent the car from moving if a worn selector accidentally drops into Reverse gear during early morning fast-idle engine warmups.
A car should be allowed to come to a complete stop before setting the transmission into park to prevent damage. Usually, PARK is one of only two selections in which the car's engine can be started. In some cars (notably those sold in the US), the driver must have the footbrake depressed before the transmission can be taken out of park. The Park position is omitted on buses/coaches with automatic transmission, which must be placed in neutral with the parking brakes set.
Reverse (R) – This puts the car into the reverse gear, giving the ability for the car to drive backwards. In order for the driver to select reverse they must come to a complete stop, push the shift lock button in (or pull the shift lever forward in the case of a column shifter) and select reverse. Not coming to a complete stop can cause severe damage to the transmission. Many modern automatic gearboxes have a safety mechanism in place, which does to some extent prevent (but doesn't completely avoid) inadvertently putting the car in reverse when the vehicle is moving. This mechanism usually consists of a solenoid- controlled physical barrier on either side of the Reverse position, which is electronically engaged by a switch on the brake pedal. Therefore, the brake pedal needs to be depressed in order to allow the selection of reverse. Some electronic transmissions prevent or delay engagement of reverse gear altogether while the car is moving.
Neutral/No gear (N)– This disconnects the transmission from the wheels so the car can move freely under its own weight. This is the only other selection in which the car can be started.
Drive (D)– This allows the car to move forward and accelerate through its range of gears. The number of gears a transmission has depends on the model, but they can commonly range from 3, 4 (the most common), 5, 6 (found in VW/Audi Direct Shift Gearbox), 7 (found in Mercedes 7G gearbox and in BMW M5) and 8 in the newer models of Lexus cars. Some cars when put into D will automatically lock the doors or turn on the Daytime Running Lamps.
OverDrive ([D], OD, or a boxed D) - This mode is used in some transmissions, to allow early Computer Controlled Transmissions to engage the Automatic Overdrive. In these transmissions, Drive (D) locks the Automatic Overdrive off, but is identical otherwise. OD (Overdrive) in these cars is engaged under steady speeds or low acceleration at approximately 35-45 mph (approx. 72 km/h). Under hard acceleration or below 35-45 mph, the transmission will automatically downshift. Vehicles with this option should be driven in this mode unless circumstances require a lower gear.
Second (2 or S) – This mode limits the transmission to the first two gears, or more commonly locks the transmission in second gear. This can be used to drive in adverse conditions such as snow and ice, as well as climbing or going down hills in the winter time. Some vehicles will automatically upshift out of 2nd gear in this mode if a certain rpm range is reached, to prevent engine damage.
First (1 or L) – This mode locks the transmission in first gear only. It will not accelerate through any gear range. This, like second, can be used during the winter season, or towing.
As well as the above modes there are also other modes, dependent on the manufacturer and model. Some examples include;
* D5 – In Hondas and Acuras equipped with 5-speed automatic transmissions, this mode is used commonly for highway use (as stated in the manual), and uses all 5 forward gears.
* D4 – This mode is also found in Honda and Acura 4 or 5-speed automatics and only uses the first 4 gears. According to the manual, it is used for stop & go traffic, such as city driving.
* D3 – This mode is found in Honda and Acura 4-speed automatics and only uses the first 3 gears. According to the manual, it is used for stop & go traffic, such as city driving. This mode is also found in Honda and Acura 5-speed automatics.
* + − and M – This is the manual selection of gears for automatics, such as Porsche's Tiptronic. This feature can also be found in Chrysler and General Motors products such as the Dodge Magnum and Pontiac G6. The driver can shift up and down at will, like a semi-automatic transmission. This mode may be engaged either through a selector/position or by actually changing gear (e.g. tipping the gear-down paddle).
Hydraulic automatic transmissions
The predominant form of automatic transmission is hydraulically operated, using a fluid coupling or torque converter and a set of planetary gearsets to provide a range of torque multiplication.
Parts and operation
Parts and operation
A hydraulic automatic transmission consists of the following parts:
* Fluid coupling or torque converter: A hydraulic device connecting the engine and the transmission. It takes the place of a mechanical clutch, allowing the engine to remain running at rest without stalling. A torque converter is a fluid coupling that also provides a variable amount of torque multiplication at low engine speeds, increasing "breakaway" acceleration.
* Planetary gearset: A compound planetary set whose bands and clutches are actuated by hydraulic servos controlled by the valve body, providing two or more gear ratios.
* Clutches;bands: to effect gear changes, one of two types of clutches, or bands are used to hold a particular member of the planetary gearset motionless, while allowing another member to rotate..transmitting torque and producing gear reductions or overdrive ratios. These clutches are actuated by the valve body (see below), and their sequence controlled by the transmission's internal programming. Principally, a type of device known as a a sprag or roller clutch is used for routine upshifts/downshifts. Operating much as a ratchet, it transmits torque only in one direction, freewheeling or "overrunning" in the other. The advantage of this type of clutch is that it eliminates the sensitivity of timing a simultaneous clutch release/apply on two planetaries, simply "taking up" the drivetrain load when actuated ,and releasing automatically when the next gear's sprag clutch assumes the torque transfer.
The bands which were mentioned come into play for manually selected gears, such as low range or reverse, and operate on the planetary drum's circumference. Bands are not applied when drive/overdrive range is selected, the torque being transmitted by the sprag clutches instead.
* Valve body: hydraulic control center that receives pressurized fluid from a main pump operated by the fluid coupling/torque converter. The pressure coming from this pump is regulated and used to run a network of spring-loaded valves, check balls and servo pistons. The valves use the pump pressure and the pressure from a centrifugal governor on the output side (as well as hydraulic signals from the range selector valves and the throttle valve or modulator) to control which ratio is selected on the gearset; as the car and engine change speed, the difference between the pressures changes, causing different sets of valves to open and close. The hydraulic pressure controlled by these valves drives the various clutch and brake band actuators, thereby controlling the operation of the planetary gearset to select the optimum gear ratio for the current operating conditions. However, in many modern automatic transmissions, the valves are controlled by electro-mechanical servos which are controlled by the Engine Management System or a separate transmission controller. (See History and improvements below.)
* Hydraulic & Lubricating Oil: called automatic transmission fluid (ATF), this component of the transmission provides lubrication, corrosion prevention, and a hydraulic medium to convey mechanical power. Primarily made from refined petroleum and processed to provide properties that promote smooth power transmission and increase service life, the ATF is one of the few parts of the automatic transmission that needs routine service as the vehicle ages.
The multitude of parts, along with the complex design of the valve body, originally made hydraulic automatic transmissions much more complicated (and expensive) to build and repair than manual transmissions. In most cars (except US family, luxury, sport-utility vehicle, and minivan models) they have usually been extra-cost options for this reason. Mass manufacturing and decades of improvement have reduced this cost gap.
参考资料：
