Automatic Transmission | Components of the Automatic Transmission System

Automatic transmission

There are two types of automatic transmission (gear box), namely, semi-automatic transmission and transmission and fully automatic transmission. These are distinguished according to their effect on vehicle handling dynamics.

Semi-automatic transmissions are the gear boxes on which all operations normally performed by the driver when changing gear are carried out by electronically controlled actuator systems. This implies that a gear change always involves disengagement of the clutch and hence of the drive to the driving wheels. Semi-automatic transmissions are found on long distance haulage trucks, passenger coaches and more recently on small cars and sports cars.

01 - Automatic transmission system - GEAR BOX

Fully automatic transmissions (normally called automatic transmission) change gear under load. This means, the power continues to flow to the driving wheels even during a gear shift operation. In this system, the drive engagement and gear ratio selection operations are performed with no additional driver input.

The main components of the automatic transmission are as follows

(1) Hydrodynamic torque converter.

(2) A number of planetary gear sets located downstream the hydrodynamic torque converter.

(3) Hydraulically actuated multiplate clutches, plate or band brakes, assigned to the individual elements within the planetary gear sets.

(4) One way clutches with shift elements.

(5) Transmission control system.

(6) An engine driven hydraulic fluid pump.

01 - Automatic transmission system - torque converter

Fully automatic transmissions are used in situation where disengagement of power transmission may cause a significant reduction in comfort (particularly in cars with powerful accelerations) or where power flow interruption cannot be accepted for reasons of vehicle handling dynamics (ie. on-off road vehicles).

Mercedes Benz has introduced a seven speed automatic transmission unit, which would make future Mercedes models more economical apart from performance and driveability. This unit employs seven gear rations: These allow the automatic transmission to retain the small increases in engine speed which are important in ensuring optimum gear ratios, while at the same time offering a larger ratio spread between the lowest and highest gear.

An outstanding feature of the new seven speed transmission is the lockup clutch in the hydrodynamic torque converter, which largely eliminates slip between the pump and turbine rotor. Unlike conventional automatic transmissions where the torque converter lock-up is only possible is higher gears, the lock-up clutch in the new seven speed automatic transmission is active from the first gear up.

Electronic transmission control

01 - Automatic transmission system - electronic transmission control

Automatic gear boxes are controlled by electronically operated hydraulic sysems. Hydraulic system actuates the clutches. Electronic units do gear selection and adapt the hydraulic pressure in accordance with the torque flow.

Sensors detect the transmission output shaft speed, engine load and speed, gear selector lever position and positions of the program selector and kick down switch. The control signal processes this information according to a predefined program and uses the results to determine the control variables which are to be transmitted to the gear box.

Electrohydraulic converter elements form the link between the electronic and hydraulic circults. Solenoid valves activate and disengage the clutches; Pressure regulators control the pressure levels at the friction surfaces, which influence on shift quality.

Intelligent shift programs supplement the standard transmission control data wih additional parameters such as forward and lateral acceleration, and the speed with which the accelerator and brake pedals are pressed. This improves driveability.

Automatic transmission is found in the vehicles namely, Honda Accord (five speed), Mercedez Benz C200K (5 speed) and Mitsubishi Pajero (5 speed).

Safety Systems in Vehicles | Seat Belts | Air bags

Seat belts Safety Systems in Vehicles

The function of the seat belts safety systems in vehicles is to restrain the occupants of a vehicle in their seats when the vehicle hits an obstacle.

Three point seat belt

In motoring history, this is the single most significant advance. All credit goes to Volvo’s Nils Bolin for devising it and to Volvo for introducing it in 1959. The three pointer afforded unrivalled restraint. Its use was a quick, easy one handed operation. Many improvements have been made to the webbing, mountings, latches, and inertia reels and belts have got smarter in relating to accident severity and occupant weight. In the present day vehicles seat belts have become mandatory for the driver and front passenger. Some vehicles have seat belts for rear passengers also.

01 - Safety Systems in Vehicles - Seat Belts

Seat belt pretensioner

A spring loaded or explosive device that reacts to a severe frontal impact by automatically snugging the seat belt tight for fully effective restraint.

Seat belt tighteners improve the restraining characteristics of a three-point inertia real belt and increase the protection against injury. In the event of a frontal effect they pull the seat belts tighter against the body and thus hold the upper body as closely as possible against the seat backrest. This avoids excessive forward displacement of the occupants caused by mass inertia. The maximum forward displacement with tightened seat belts is approximately 1cm and the duration of mechanical tightening is 5.. 10ms.

01 - Safety Systems in Vehicles - Seat Belt tightners

On activation, a pyrotechnical propellant charge is electrically fired. The explosive pressure acts on a piston, which turns the belt reel via a steel cable in such a way that the belt rests tightly against the body.

Air bags Safety Systems in Vehicles

The function of front air bags is to protect the driver and the front passenger against head and chest injuries in a vehicle impact with a solid obstacle at speeds of up to 60 km/h. In the frontal impact between two vehicles, the front air bags afford protection at relative speeds of upto 100km/h.

01 - Safety Systems in Vehicles - air bags

To protect driver and front passenger, pyrotechnical gas inflators expand the front air bags in pyrotechnical, highly dynamic way after a vehicle impact is detected by sensors. For the affected occupant to enjoy maximum protection, the air bag must be fully inflated before the occupant comes into contact with it. The air bag needs approximately 40ms to inflate completely.

To maximize the effect of both protective devices (seat belt tightner and front air bag), They are activated with optimized time response by a common ECU (triggering unit) installed in the passenger cell. The ECU’s decleration calculations are based on data from one or two electronic acceleration sensors used to monitor the decelerative forces that accompany in impact. Depending on the impact type, the first trigger threshold is reached within 5 … 60ms.

Seat belt load limiter

A means by which the seat belt relaxes its hold during heavy deceleration, continuing to restrain the occupant while reducing the risk of belt inflicted injury.


Rating: 5 out of 10.