Automatic watering hunter
Automatic watering hunter
automatic transmissionautoself-shifting transmissionn-speed automaticAT
The obvious advantage of an automatic transmission to the driver is the lack of a clutch pedal and manual shift pattern in normal driving. This allows the driver to operate the car with as few as two limbs (possibly using assist devices to position controls within reach of usable limbs), allowing amputees and other disabled individuals to drive. The lack of manual shifting also reduces the attention and workload required inside the cabin, such as monitoring the tachometer and taking a hand off the wheel to move the shifter, allowing the driver to ideally keep both hands on the wheel at all times and to focus more on the road. Control of the car at low speeds is often easier with an automatic than a manual, due to a side effect of the clutchless fluid-coupling design called "creep" that causes the car to want to move while in a driving gear, even at idle. The primary disadvantage of the most popular hydraulic designs is reduced mechanical efficiency of the power transfer between engine and drivetrain, due to the fluid coupling connecting the engine to the gearbox. This can result in lower power/torque ratings for automatics compared to manuals with the same engine specs, as well as reduced fuel efficiency in city driving as the engine must maintain idle against the resistance of the fluid coupling. Advances in transmission and coupler design have narrowed this gap considerably, but clutch-based transmissions (manual or semi-automatic) are still preferred in sport-tuned trim levels of various production cars, as well as in many auto racing leagues.
In 1956, GM introduced the "Jetaway" Hydra-Matic, which was different in design than the older model. Addressing the issue of shift quality, which was an ongoing problem with the original Hydra-Matic, the new transmission utilized two fluid couplings, the primary one that linked the transmission to the engine, and a secondary one that replaced the clutch assembly that controlled the forward gearset in the original. The result was much smoother shifting, especially from first to second gear, but with a loss in efficiency and an increase in complexity. Another innovation for this new style Hydra-Matic was the appearance of a Park position on the selector. The original Hydra-Matic, which continued in production until the mid-1960s, still used the reverse position for parking pawl engagement.
Parts and operation
Hydraulic automatic transmissions
Planetary gears train
Not to be confused with the impeller inside the torque converter, the pump is typically a gear pump mounted between the torque converter and the planetary gearset. It draws transmission fluid from a sump and pressurizes it, which is needed for transmission components to operate. The input for the 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 and there is enough transmission fluid, but the disadvantage is that when the engine is not running, no oil pressure is available to operate the main components of the transmission, and is thus impossible to push-start a vehicle equipped with an automatic transmission. Early automatic transmissions also had a rear pump for towing purposes, ensuring the lubrication of the rear-end components.
governorthrottle cablevacuum modulator
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.
Continuously variable transmissions
Automatic transmission modes
Most automatic transmissions offer the driver a certain amount of manual control over the transmission's shifts.
Conventionally, in order to select the transmission operating mode, the driver moves a selection lever located either on the steering column or on the floor (as with a manual on the floor, except that automatic selectors on the floor do not move in the same type of pattern as manual levers do). In order to select modes, or to manually select specific gear ratios, the driver must push a button in (called the shift-lock button) or pull the handle (only on column mounted shifters) out. Some vehicles position selector buttons for each mode on the cockpit instead, freeing up space on the central console.
Depending on the model and make of the transmission, these controls can take several forms. However most include the following:
Mode selection allows the driver to choose between preset shifting programs. For example, Economy mode saves fuel by upshifting at lower engine speeds, while Sport mode (aka "Power" or "Performance") delays upshifting for maximum acceleration. Some transmission units also have Winter mode, where higher gear ratios are chosen to keep revs as low as possible while on slippery surfaces. The modes also change how the computer responds to throttle input.
As well as the above modes there are other modes, dependent on the manufacturer and model. Some examples include:
Some early GMs equipped with HYDRA-MATIC transmissions used (S) to indicate Second gear, being the same as the 2 position on a Chrysler, shifting between only first and second gears. This would have been recommended for use on steep grades, or slippery roads like dirt, or ice, and limited to speeds under 40 mph. (L) was used in some early GMs to indicate (L)ow gear, being the same as the 2 position on a Chrysler, locking the transmission into first gear. This would have been recommended for use on steep grades, or slippery roads like dirt, or ice, and limited to speeds under 15 mph.
Comparison with manual transmission
A conventional manual transmission is frequently the base equipment in a car, with the option being an automated transmission such as a conventional automatic, semi-automatic, or CVT.
Effects on vehicle control
Unexpected gear changes can affect the attitude of a vehicle in marginal conditions.
Maintaining constant speed
Torque converters and CVT transmissions make changes in vehicle speed less apparent by the engine noise, as they decouple the engine speed from vehicle speed.
Climbing steep slippery slopes
In situations where a driver with a manual transmission can't afford a gearshift, in fear of losing too much speed to reach a hilltop, automatic transmissions are at a great advantage — whereas the manual driver depends on finding a gear that is not too low to enter the bottom of the hill at the necessary speed, but not too high to stall the engine at the top of the hill, sometimes an impossible task, this is a non-issue with automatic transmissions, not just because gearshifts are quick, but they typically maintain some power on the driving wheels during the gearshift.
Manual transmissions use a mechanical clutch to transmit torque, rather than a torque converter, thus avoiding the primary source of loss in an automatic transmission. Manual transmissions also avoid the power requirement of the hydraulic control system, by relying on the human muscle power of the vehicle operator to disengage the clutch and actuate the gear levers, and the mental power of the operator to make appropriate gear ratio selections. Thus the manual transmission requires very little engine power to function, with the main power consumption due to drag from the gear train being immersed in the lubricating oil of the gearbox.
The on-road acceleration of an automatic transmission can occasionally exceed that of an otherwise identical vehicle equipped with a manual transmission in turbocharged diesel applications. Turbo-boost is normally lost between gear changes in a manual whereas in an automatic the accelerator pedal can remain fully depressed. This however is still largely dependent upon the number and optimal spacing of gear ratios for each unit, and whether or not the elimination of spooldown/accelerator lift off represent a significant enough gain to counter the slightly higher power consumption of the automatic transmission itself.
Automatic transmission models
Some of the best known automatic transmission families include:
- General Motors — Dynaflow, Powerglide, Turboglide, "Turbo-Hydramatic" TH350, TH400 and 700R4, 4L60-E, 4L80-E, Holden Trimatic
- Ford: Cruise-O-Matic, C4, CD4E, C6, AOD/AODE, E4OD, ATX, AXOD/AX4S/AX4N
- Cummins 68 RFE (fitted to the Ram diesel segment)
- Chrysler: TorqueFlite 727 and 904, A500, A518, 45RFE, 545RFE
- BorgWarner (later Aisin AW)
- ZF Friedrichshafen automatic transmissions
- Mercedes-Benz transmissions
- Allison Transmission
- Voith Voith Turbo
- Aisin AW; Aisin AW is a Japanese automotive parts supplier, known for its automatic transmissions and navigation systems
- Volkswagen Group — 01M
- Drivetrain Systems International (DSI) — M93, M97 and M74 4-speeds, M78 and M79 6-speeds
- Hyundai Hyundai Powertech — 4F12, 4F16, 4F23 4-Speeds, 5F25, 5F16, 5F23 5-Speeds, 6F17, 6F26, 6F40 6-Speeds, 8R40, 8R50 8-Speeds, Mini Cooper — Automatic or manual transmission, all models
- Thomas W Birch (2012). Automatic transmissions and transaxles. Upper Saddle River, N.J.: Pearson Education. ISBN 9780132622271.
- Munro's original Canadian Patent
- How Automatic Transmissions Work on HowStuffWorks
- How To Drive a Car with Automatic Transmission
- The death of an automatic transmission
- US5370589 Lepelletier's concept is shown on this patent[permanent dead link]
- "What Makes Automatic Transmission Automatic." Popular Mechanics, February 1955, pp. 169–173, the basics in simple terms with drawings.
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