seminar code
06-08-2014, 01:06 PM
modern automatic transmission
[attachment=66655]
ABSTRACT
The modern automatic transmission is by far, the most complicated mechanical component in today's automobile. Automatic transmissions contain mechanical systems, hydraulic systems, electrical systems and computer controls, all working together in perfect harmony which goes virtually unnoticed until there is a problem. This article will help you understand the basic of the transmission, about automatic transmission, differences between manual and automatic transmission, the concepts behind what goes on inside these technological marvels and what goes into repairing them when they fail.
What is a transmission?
The transmission is a device that is connected to the back of the engine and sends the power from the engine to the drive wheels. An automobile engine runs at its best at a certain RPM (Revolutions Per Minute) range and it is the transmission's job to make sure that the power is delivered to the wheels while keeping the engine within that range. It does this through various gear combinations. In first gear, the engine turns much faster in relation to the drive wheels, while in high gear the engine is loafing even though the car may be going in excess of 70 MPH. In addition to the various forward gears, a transmission also has a neutral position, which disconnects the engine from the drive wheels, and reverse, which causes the drive wheels to turn in the opposite direction allowing you to back up.
INTRODUCTION
One of the biggest things that has always separated drivers is whether they prefer an automatic transmission (sometimes known as a "slushbox", because of the characteristic "slushy" feel it creates when shifting between gears) or a standard transmission (often called, for more obvious reasons, a "stick shift").
There are a number of benefits to the standard transmission, but the prevailing reason that most people get them is because they prefer them. Car enthusiasts enjoy the feel of being able to control what gear they're in; it makes them feel closer to the vehicle. People who just want to drive and don't care much about how the car works usually prefer the automatic trannie.
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 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!
The need to shift from manual to automatic transmission
The key difference between a manual and an automatic transmission is that the manual transmission locks and unlocks different sets of gears to the output shaft to achieve the various gear ratios, while in an automatic transmission the same set of gears produces all of the different gear ratios. The planetary gear set is the device that makes this possible in an automatic transmission. Driving a manual transmission is especially painful in stop-and-go traffic, where you constantly have to change your gear state. You might also make the case that having to concentrate on shifting your transmission takes some of your attention away from the actual driving, which is dangerous, because you might be concentrating on your tachometer or your gearshift when you really should be focusing on the traffic. An automatic transmission bypasses these problems entirely.
Front wheel
On a front wheel drive car, the transmission is usually combined with the final drive to form what is called a transaxle. The engine on a front wheel drive car is usually mounted sideways in the car with the transaxle tucked under it on the side of the engine facing the rear of the car. Front axles are connected directly to the transaxle and provide power to the front wheels. In this example, power flows from the engine, through the torque converter to a large chain that sends the power through a 180 degree turn to the transmission that is along side the engine. From there, the power is routed through the transmission to the final drive where it is split and sent to the two front wheels through the drive axles.
COMPONENTS OF AUTOMATIC TRANSMISSION
The automatic transmission is composed of three major components.
1. Planetary gear unit
2. Hydraulic control unit
3. Torque converter
For a full understanding of the operation of the automatic transmission, it is important to understand the basic role of these components.
The torque converter provides a means of power transfer from the engine to the input shaft of the transmission. It acts like an automatic clutch to engage engine torque to the transmission and also allows the engine to idle while the vehicle is standing still
with the transmission in gear.
The planetary gear unit provides multiple gear ratios in the forward direction and one in reverse. The design includes two simple planetary gear sets and a common sun gear.
These ratios are provided by use of holding devices which hold members of the planetary set. These holding devices can be multiplate clutches or brakes, brake bands or one-way clutches.
The hydraulic control unit regulates hydraulic pressure and shift points based on vehicle speed and throttle position. It is made up of a highly precision housing and spool valves which are balanced between spring tension and hydraulic pressure. The spool valves in turn control hydraulic passages to holding devices and regulate pressure.
Hydraulic System
It uses a special transmission fluid sent under pressure by an Oil Pump through the Valve Body to control the Clutches and the Bands in order to control the planetary gear sets. Both the band and clutch piston are pressurized by the hydraulic system. The part connecting the band or clutches to the hydraulic system is called the shift valve, while the one connecting the hydraulic system to the output shaft is called the governor (see Figure 4). The governor is a centrifugal sensor with a spring loaded valve. The faster the governor spins, the more the valve opens. The more the valve opens, the more the fluid goes through and the higher the pressure
The Torque Converter
Unlike a manual transmission system, automatic transmission doesn’t use a clutch to disconnect power from the engine temporarily when shifting gears. Instead, a device called a torque converter was invented to prevent power from being temporarily disconnected from the engine and also to prevent the vehicle from stalling when the transmission is in gear. Consider two fans facing each other: when one of them is turned on and starts spinning, the other one will also start spinning at a lower speed (see Figure 6). That’s because the first fan moves the air to drive the second fan to spin. This is the same idea as the torque converter of an automatic transmission system, except that it uses fluid instead of air as the transportation media
CONCLUSION
Being able to drive a stick-shift means more than just understanding your gears, however. That's because to shift gears, you usually don't just move the shift lever and be done with it. (You can do this sometimes, but it's not recommended in day-to-day driving, and we'll get to it later.) Instead, you have to press the clutch pedal while you shift. Keeping the transmission and its fluid cool, and in the correct operating-temperature range is crucial for long life.
[attachment=66655]
ABSTRACT
The modern automatic transmission is by far, the most complicated mechanical component in today's automobile. Automatic transmissions contain mechanical systems, hydraulic systems, electrical systems and computer controls, all working together in perfect harmony which goes virtually unnoticed until there is a problem. This article will help you understand the basic of the transmission, about automatic transmission, differences between manual and automatic transmission, the concepts behind what goes on inside these technological marvels and what goes into repairing them when they fail.
What is a transmission?
The transmission is a device that is connected to the back of the engine and sends the power from the engine to the drive wheels. An automobile engine runs at its best at a certain RPM (Revolutions Per Minute) range and it is the transmission's job to make sure that the power is delivered to the wheels while keeping the engine within that range. It does this through various gear combinations. In first gear, the engine turns much faster in relation to the drive wheels, while in high gear the engine is loafing even though the car may be going in excess of 70 MPH. In addition to the various forward gears, a transmission also has a neutral position, which disconnects the engine from the drive wheels, and reverse, which causes the drive wheels to turn in the opposite direction allowing you to back up.
INTRODUCTION
One of the biggest things that has always separated drivers is whether they prefer an automatic transmission (sometimes known as a "slushbox", because of the characteristic "slushy" feel it creates when shifting between gears) or a standard transmission (often called, for more obvious reasons, a "stick shift").
There are a number of benefits to the standard transmission, but the prevailing reason that most people get them is because they prefer them. Car enthusiasts enjoy the feel of being able to control what gear they're in; it makes them feel closer to the vehicle. People who just want to drive and don't care much about how the car works usually prefer the automatic trannie.
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 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!
The need to shift from manual to automatic transmission
The key difference between a manual and an automatic transmission is that the manual transmission locks and unlocks different sets of gears to the output shaft to achieve the various gear ratios, while in an automatic transmission the same set of gears produces all of the different gear ratios. The planetary gear set is the device that makes this possible in an automatic transmission. Driving a manual transmission is especially painful in stop-and-go traffic, where you constantly have to change your gear state. You might also make the case that having to concentrate on shifting your transmission takes some of your attention away from the actual driving, which is dangerous, because you might be concentrating on your tachometer or your gearshift when you really should be focusing on the traffic. An automatic transmission bypasses these problems entirely.
Front wheel
On a front wheel drive car, the transmission is usually combined with the final drive to form what is called a transaxle. The engine on a front wheel drive car is usually mounted sideways in the car with the transaxle tucked under it on the side of the engine facing the rear of the car. Front axles are connected directly to the transaxle and provide power to the front wheels. In this example, power flows from the engine, through the torque converter to a large chain that sends the power through a 180 degree turn to the transmission that is along side the engine. From there, the power is routed through the transmission to the final drive where it is split and sent to the two front wheels through the drive axles.
COMPONENTS OF AUTOMATIC TRANSMISSION
The automatic transmission is composed of three major components.
1. Planetary gear unit
2. Hydraulic control unit
3. Torque converter
For a full understanding of the operation of the automatic transmission, it is important to understand the basic role of these components.
The torque converter provides a means of power transfer from the engine to the input shaft of the transmission. It acts like an automatic clutch to engage engine torque to the transmission and also allows the engine to idle while the vehicle is standing still
with the transmission in gear.
The planetary gear unit provides multiple gear ratios in the forward direction and one in reverse. The design includes two simple planetary gear sets and a common sun gear.
These ratios are provided by use of holding devices which hold members of the planetary set. These holding devices can be multiplate clutches or brakes, brake bands or one-way clutches.
The hydraulic control unit regulates hydraulic pressure and shift points based on vehicle speed and throttle position. It is made up of a highly precision housing and spool valves which are balanced between spring tension and hydraulic pressure. The spool valves in turn control hydraulic passages to holding devices and regulate pressure.
Hydraulic System
It uses a special transmission fluid sent under pressure by an Oil Pump through the Valve Body to control the Clutches and the Bands in order to control the planetary gear sets. Both the band and clutch piston are pressurized by the hydraulic system. The part connecting the band or clutches to the hydraulic system is called the shift valve, while the one connecting the hydraulic system to the output shaft is called the governor (see Figure 4). The governor is a centrifugal sensor with a spring loaded valve. The faster the governor spins, the more the valve opens. The more the valve opens, the more the fluid goes through and the higher the pressure
The Torque Converter
Unlike a manual transmission system, automatic transmission doesn’t use a clutch to disconnect power from the engine temporarily when shifting gears. Instead, a device called a torque converter was invented to prevent power from being temporarily disconnected from the engine and also to prevent the vehicle from stalling when the transmission is in gear. Consider two fans facing each other: when one of them is turned on and starts spinning, the other one will also start spinning at a lower speed (see Figure 6). That’s because the first fan moves the air to drive the second fan to spin. This is the same idea as the torque converter of an automatic transmission system, except that it uses fluid instead of air as the transportation media
CONCLUSION
Being able to drive a stick-shift means more than just understanding your gears, however. That's because to shift gears, you usually don't just move the shift lever and be done with it. (You can do this sometimes, but it's not recommended in day-to-day driving, and we'll get to it later.) Instead, you have to press the clutch pedal while you shift. Keeping the transmission and its fluid cool, and in the correct operating-temperature range is crucial for long life.