24-07-2014, 01:35 PM
AUTOMATIC TRASMISSION SYSTEM
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ABSTRACT
The modern automatic transmission is by far, the most complicated mechanical component in today’s automobile. Automatic transmission content mechanical systems, hydraulic systems, electrical systems & computer controls, all working together in perfect harmony which goes virtually unnoticed until there is a problem.
INTRODUCTION
How Automatic Transmissions Work
If you have ever driven a car with an automatic transmission, then you know that there are two big differences between an automatic transmission and a manual transmission:
• There is no clutch pedal in an automatic transmission car.
• There is no gear shift in an automatic transmission car. Once you put the transmission into drive, everything else is automatic.
Both the automatic transmission (plus its torque converter) and a manual transmission (with its clutch) accomplish exactly the same thing, but they do it in totally different ways. It turns out that the way an automatic transmission does it is absolutely amazing!
Gears
This automatic transmission uses a set of gears, called a compound planetary gearset, that looks like a single planetary gearset but actually behaves like two planetary gearsets combined. It has one ring gear that is always the output of the transmission, but it has two sun gears and two sets of planets.
Let's look at some of the parts
First Gear
In first gear, the smaller sun gear is driven clockwise by the turbine in the torque converter. The planet carrier tries to spin counterclockwise, but is held still by the one-way clutch (which only allows rotation in the clockwise direction) and the ring gear turns the output. The small gear has 30 teeth and the ring gear has 72, so referring to the chart on this page, the gear ratio is:
Ratio = -R/S = - 72/30 = -2.4:1
So the rotation is negative 2.4:1, which means that the output direction would be opposite the input direction. But the output direction is really the same as the input direction -- this is where the trick with the two sets of planets comes in. The first set of planets engages the second set, and the second set turns the ring gear; this combination reverses the direction. You can see that this would also cause the bigger sun gear to spin; but because that clutch is released, the bigger sun gear is free to spin in the opposite direction of the turbine (counterclockwise).
Second Gear
This transmission does something really neat in order to get the ratio needed for second gear. It acts like two planetary gearsets connected to each other with a common planet carrier.
The first stage of the planet carrier actually uses the larger sun gear as the ring gear. So the first stage consists of the sun (the smaller sun gear), the planet carrier, and the ring (the larger sun gear).
The input is the small sun gear; the ring gear (large sun gear) is held stationary by the band, and the output is the planet carrier. For this stage, with the sun as input, planet carrier as output, and the ring gear fixed, the formula is:
1 + R/S = 1 + 36/30 = 2.2:1
The planet carrier turns 2.2 times for each rotation of the small sun gear. At the second stage, the planet carrier acts as the input for the second planetary gear set, the larger sun gear (which is held stationary) acts as the sun, and the ring gear acts as the output, so the gear ratio is:
1 / (1 + S/R) = 1 / (1 + 36/72) = 0.67:1
To get the overall reduction for second gear, we multiply the first stage by the second, 2.2 x 0.67, to get a 1.47:1 reduction. This may sound wacky, but it works.
Overdrive
By definition, an overdrive has a faster output speed than input speed. It's a speed increase -- the opposite of a reduction. In this transmission, engaging the overdrive accomplishes two things at once. If you read How Torque Converters Work, you learned about lockup torque converters. In order to improve efficiency, some cars have a mechanism that locks up the torque converter so that the output of the engine goes straight to the transmission.
In this transmission, when overdrive is engaged, a shaft that is attached to the housing of the torque converter (which is bolted to the flywheel of the engine) is connected by clutch to the planet carrier. The small sun gear freewheels, and the larger sun gear is held by the overdrive band. Nothing is connected to the turbine; the only input comes from the converter housing. Let's go back to our chart again, this time with the planet carrier for input, the sun gear fixed and the ring gear for output.
Ratio = 1 / (1 + S/R) = 1 / ( 1 + 36/72) = 0.67:1
So the output spins once for every two-thirds of a rotation of the engine. If the engine is turning at 2000 rotations per minute (RPM), the output speed is 3000 RPM. This allows cars to drive at freeway speed while the engine speed stays nice and slow
Hydraulic System
The automatic transmission in your car has to do numerous tasks. You may not realize how many different ways it operates. For instance, here are some of the features of an automatic transmission:
• If the car is in overdrive (on a four-speed transmission), the transmission will automatically select the gear based on vehicle speed and throttle pedal position.
• If you accelerate gently, shifts will occur at lower speeds than if you accelerate at full throttle.
• If you floor the gas pedal, the transmission will downshift to the next lower gear.
• If you move the shift selector to a lower gear, the transmission will downshift unless the car is going too fast for that gear. If the car is going too fast, it will wait until the car slows down and then downshift.
• If you put the transmission in second gear, it will never downshift or upshift out of second, even from a complete stop, unless you move the shift lever.
The Governor
The governor is a clever valve that tells the transmission how fast the car is going. It is connected to the output, so the faster the car moves, the faster the governor spins. Inside the governor is a spring-loaded valve that opens in proportion to how fast the governor is spinning -- the faster the governor spins, the more the valve opens. Fluid from the pump is fed to the governor through the output shaft.
The faster the car goes, the more the governor valve opens and the higher the pressure of the fluid it lets through.
Electronic Controls
Electronically controlled transmissions, which appear on some newer cars, still use hydraulics to actuate the clutches and bands, but each hydraulic circuit is controlled by an electric solenoid. This simplifies the plumbing on the transmission and allows for more advanced control schemes.
In the last section we saw some of the control strategies that mechanically controlled transmissions use. Electronically controlled transmissions have even more elaborate control schemes. In addition to monitoring vehicle speed and throttle position, the transmission controller can monitor the engine speed, if the brake pedal is being pressed, and even the anti-lock braking system.
Using this information and an advanced control strategy based on fuzzy logic -- a method of programming control systems using human-type reasoning -- electronically controlled transmissions can do things like:
• Downshift automatically when going downhill to control speed and reduce wear on the brakes
• Up shift when braking on a slippery surface to reduce the braking torque applied by the engine
• Inhibit the up shift when going into a turn on a winding road
Let's talk about that last feature -- inhibiting the up shift when going into a turn on a winding road. Let's say you're driving on an uphill, winding mountain road. When you are driving on the straight sections of the road, the transmission shifts into second gear to give you enough acceleration and hill-climbing power. When you come to a curve you slow down, taking your foot off the gas pedal and possibly applying the brake. Most transmissions will up shift to third gear, or even overdrive, when you take your foot off the
CONCLUSION
We can conclude from this seminar that, the automatic transmission system is more advantageous over the conventional gearing mechanism. We can have a smooth drive and easy gear changing system. This consists of all automatic controls which detect problems occurred in system early on and warn the driver.
COMPANIES WHICH MANUFACTURE AUTOMATIC TRANSMISSION CARS:
TOYOTA, SUZUKI, VOLVO, MITSUBISHI, FIAT, FORD, DAEWOO, ACURA, MERCIDIES BENZ, MERCURY, NISSAN etc.