Automatic Gearboxes

Development

Characteristics

Parts and Operations

Significant improvement has been noticed in the Automatic Gearboxes in recent few years. This improvement has been occurred in the field of their increasing fuel efficiency. But, if we compare the efficiency level of the automatic and manual gearboxes, we will find that still, the manual gearboxes are more efficient.

Development reasons of Automatic gearboxes
There are two things which are responsible for the development of Automatic gearboxes. These two things are as follows:

• The arrival of electronics in the 1990's.
• The want of high-speed gear. This want/expectation results into the invention of power gears and automatic gearshifts.

Characteristics of the Automatic gearboxes
Handling of automatic Gearboxes is easy and the driving experience of these gearboxes is also pretty. The only two manual things which have to be done manually in such gearbox are: pressing the accelerator to start the gearbox and pressing the brake to stop these gearbox. The up-down of the gears in automatic gearboxes is done through hydraulic fluid pressure. Therefore, a regular check-up of hydraulic fluid should be done. The presence of Automatic gearbox in the automobile, helps in changing the gear ratios automatically when the vehicle moves. This enables the drivers not to change the gears manually. The automatic transmission in such kind of gears decreases the fuel efficiency as well as power efficiency.

Hydraulically Operated Form of Automatic Gearboxes
The most dominated form of automatic gear is hydraulically operated form. In this form, a fluid coupling or a torque converter and a set of planetary gear is used. This provides a scope of torque (mechanical rotation) multiplication.

Parts and Operations of Hydraulic Automatic Gearboxes
A hydraulic automatic gearbox consists of the following parts:

• Fluid Coupling or Torque Converter
  This is a hydraulic device which connects the gearbox and the engine. It is used by replacing the mechanical clutch. It helps the engine to run continuously without stalling. A torque converter also allows different amount of torque multiplication and that is also at low engine speeds. This thing helps in enhancing the break way acceleration.
  
  
• Planetary Gearset
  In a Planetary Gearset, by a hydraulic servos, the clutches and bands of the gearset are turned on. This turning on process is controlled by the valve body. This process results into two or more gear ratios.
  
  
• Valve Body
  It is the hydraulic control center. The fluid which is operated by the fluid coupling/torque converter, comes through a main pump in pressurized form and reaches to the valve body. The pressurized fluid that comes from the main pump is regulated and then used to operate the network of the spring-loaded valves, servo pistons and check balls. The valves use two kinds of pressure, one is from the pump and another from a centrifugal governor on the output side. The other things which the valves use are the hydraulic signals from the range selector valves and from the modulator.
  The pressure and hydraulic signals help the valves in controlling the selected ratio on the gearset. The selection of ratio on the gearset is required to control because with the changing speed of the engines in cars and difference between the pressure changes in different engines. This two things induce to open and close different sets of valves.
  The controlling process of the hydraulic pressure by the valves, induces different clutches and actuators in controlling the operation of the planetary gearset. Thus, the optimum gear ratio can be determined by the controlled operation conditions of the planetary gearset. A new Engine Management System has been introduced in controlling indirectly the valves in many modern automatic gearboxes.
  
  
• Hydraulic and Lubricating oil
  Hydraulic and Lubricating oil is also called Automatic Gearbox Fluid (AFT). Refined petroleum is primarily used for manufacturing AFT. Later on, it is processed for several times to bring all other properties. As a component of Automatic gearbox, AFT requires routine check-up.
  
  AFT is considered as a very important constituent of the gearbox due to the following functions:
  • Provides lubrication for the gearbox.
  • Protects the gearbox from corrosion.
  • Conveys mechanical power as it is a hydraulic medium.