The automatic transmission is a type of a motor vehicle transmission that changes the gear ratio automatically as the vehicle moves. this kind of transmission is also referred to as n-speed or self-shifting transmission. This system helps the driver operate free from a frequent change of gear ratio. The number of forwarding gear ratios is also expressed in the same ways as manual transmission e.g 6-speed manual. Contrary to the manual transmission, this transmission system uses hydraulic fluid coupling instead of a friction clutch, and accomplish its gear selection by locking and unlocking its planetary gear system hydraulically.
This system is equipped with a series of gear ranges, and also a parking pawl which isolates the output shafts from rotating gears and keeps the vehicle from moving backward or forward.
System parts and operations.
The automatic transmission system comprises three major components. These components predominantly operate hydraulically using a torque converter and a planetary gear set that give a wide range of gear selection ratio. The three main parts are; hydraulic controls, planetary gear trains, and torque converter.
The transmission hydraulic control system comprises of special fluids usually referred to as Automatic Transmission Fluid (ATF). the fluid is sent under high pressure by an oil pump to modify the vehicle running speed and to control various bands and clutches.
The transmission oil pump is located in between the planetary gear sets and a torque converter so that it can draw ATF oil from its sump and compress it at high pressure, and transmit to the other components. The pump input is connected to the torque converter housing then bolded to the engine flex tape.
The system pump provides pressure to the entire system only when the engine is on, but no pressure is available when the engine is off. This is the reason why vehicles using automatic transmission cannot be push started.
The vehicle speed regulators such as speed governors are connected to the system output shaft to regulate the hydraulic pressure and hence the vehicle speed.
Automatic Transmission Fluid provides system lubrication, prevents corrosion and acts as a medium that conveys system hydraulic power. This fluid is processed to give properties that promote increase is vehicle service life and power transmission. The system service requires regular monitoring of the ATF level to improve vehicle transmission system performance.
The torque Converter.
The torque converter is a type of fluid coupling system that provides a hydraulic connection from the transmission system to the engine. It acts as a clutch in manual transmission since it decouples and transmits the engine power across the planetary gears thus allowing the vehicle to come to stop while the engine is still running.
A torque converter provides multiple torque variation at low engine speed contrary to fluid coupling since it provides an increase in breakaway accelerations. a fluid coupling operates well when the turbine and impeller rotate at the same speed but it gives inefficient initial acceleration especially when rotation speeds are different.
The torque multiplication is provided by a stator which is a third member in the coupling assembly. it provides the regulated and rational speed of the turbine and impeller.
Planetary gear train.
Planetary gear train a set of planetary gears, clutches, and bands. This mechanical system gives a gear ratio that provides a wide range of speed to the output shaft depending on the locked gears.
During the gear change, a particular member of a planetary planet gear set is held motionless by one or two clutches or bands while others are allowed to keep moving and hence provide gear reduction ratios.
These clutches are controlled by a valve body that is operated sequentially by the transmission’s internal programming using a device known as a sprag or roller clutch which enables downshifts and upshifts and transmits torque in one direction while overrunning the others.
The bands are used for manually selected gears such as reverse, low range, and brakes since it operates only within the circumference of the planetary drum but when it comes to driving or overdrive, the sprag clutches come into play.
Other types of automatic transmission systems.
· CVT. (Continous Variable Transmission)
This type of automatic transmission system operates by smoothly altering the gear ratio by varying the diameter of a pair of belts, wheels or cones. CTV systems mostly use a hydrostatic drive which consists of hydraulic motor and variable displacement pumps in transmitting power without gears. CVT systems are fuel-economical as a manual transmission.
This is an electronically controlled CVT transmission. This type of transmission is usually used by vehicles with hybrid engines. Examples of these vehicles are Lexus and ford motor company. In this system, the engine speed is controlled by a third input to a differential by use of electronically controlled motor generators, while the transmission has fixed gears.
· Dual-clutch Transmission. ( DCT).
The dual-clutch transmission is also referred to as twin-clutch transmission or double-clutch transmission. It is one of the modern types of electrohydraulic manual transmission and semi-automatic transmission.
This kind of transmission uses a separate clutch for even and odd gear sets. This system operates in fully automatic mode but it also gives the driver an option to shift to manually shift gears using a semi-automatic mode.
· Automated manual Transmission.
This type of transmission is also known as semi-automated transmission or (SAT). In this mode, the clutch and gear selection are done by the use of actuators, sensors, pneumatics, and processors. The AMTs were made to help minimize the rate of fuel consumption so that automatic cars can be fuel-efficient.
AMTs are also not expensive to repair. This type of transmission helps the driver to change gears in an automatic mode without using a clutch.
Automatic Transmission modes.
The transmission operating mode is selected using the selection lever located alongside the steering column or on the floor. The selection lever of an automatic transmission is not moved in the same way as a manual transmission. It has the letters indicate the mode of operation you need and it is indicated below the lever side.
The selection modes are in order P-R-N-D-L and each letter represents the actions as follows
P- This is the parking mode that locks the output shaft and prevents the vehicle from moving in any direction.
R- This is a reverse mode. It allows the vehicle to be driven backward. During this selection, the beeper is activated and the beeping sound alerts the vehicle at the back or pedestrians that the car is reversing.
N- N is a neutral mode. It disengages all gear trains and disconnects the transmission from all movable wheels. It allows the vehicle to move freely depending on its weight and slope of the road. The neutral mode does not support braking, so the drive must use brakes to stop the vehicle in motion.
D- D is the drive mode. It allows the transmission to engage all the available gear ratios and allows the vehicle to move forward with any range of gear ratios.
L- L mode locks the transmission to operate only within its first gear. This will disable the transmission change of gear range to prevent engine damage. This mode is usually used when a vehicle is moving downhill to support engine braking.