Automatic Gear-shifting Gyaan

What is it?

A type of motor vehicle transmission that can automatically change gear ratios for a moving vehicle, freeing the driver from having to shift gears manually.

Who dunnit?

It’s been around for over a century!!

It was first developed in 1904 by Boston’s Sturtevant brothers for horseless carriages but had major problems.

In 1934, both REO and General Motors developed semi-automatic transmissions that were less difficult to operate than a fully manual unit.

Later developments in automatic gearbox and fluid coupling technology eventually led to General Motors’ “Hydra-Matic”, the world's first mass-produced automatic transmission in 1939.

The automatic transmission that resembles what’s on today’s cars got its start in 1948 by General Motor’s Buick Motor Division as two-speed “Dynaflow”.

By the late 1960s, the fluid-coupling transmissions gave way to three-speed torque converters.

In the 80’s, overdrive transmissions with four forward speeds and a lockup feature were introduced that increased fuel efficiency. Transaxles and CVTs (continuously variable transmissions) are recent developments.

Technology

To understand the concept of automatic transmission it makes sense to understand manual transmission first.

A standard transmission or manual transmission is the traditional type of transmission used in automobiles. It consists of a series of heliacal gears, synchro rings, roller bearings, counter shafts and gear selectors. The main clutch assembly is used to engage and disengage the engine from the transmission. Heliacal cut gears are used to select the ratio desired while the selector fork moves the gears from one to another by using the gearshift knob. Synchro rings are used to slow the gear to a stop before it is engaged to avoid gear grinding. The counter shaft holds the gears in place and against the main input and output shaft. A stick shift transmission has no torque converter so there is no need for a transmission cooler. A stick shift transmission needs a simple fluid change for proper service.

Besides standard transmission, there are automatic transmissions mechanisms which can be classified into three basic types:

1. Hydraulic Automatic Transmission: In a hydraulic automatic transmission, instead of a clutch, a fluid coupling or “torque converter” is used to connect the transmission and engine hydraulically. Also a combination of brake bands and clutch packs control the planetary gearset which selects the gear ratio. The key difference from a standard 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. Such systems are usually less energy efficient than manual transmissions owing to pumping losses during torque conversion but might provide more on-road acceleration in turbocharged diesel applications. This technique has been used by famous manufacturers like Ford, Honda, Nissan, General Motors, Volkswagen etc.

2. Continuously Variable Transmission: A continuously variable transmission system can steplessly alter through infinite gear ratios by varying the diameter of a pair of belt or chain-linked pulleys, wheels or cones. Some CVTs use a hydrostatic drive consisting of a variable displacement pump and a hydraulic motor to transmit power without gears. CVT designs are usually as fuel efficient as manual transmissions in city driving, as it enables the engine to run at its most efficient revolutions per minute (RPM) for a range of vehicle speeds. It is being deployed in latest hybrid cars by manufacturers like Nissan, Toyota, Honda, Ford, Suzuki etc.

3. Semi-automatic Transmission: If there is a provision to select a gear ratio manually in automatic transmissions, the system is called a semi-automatic transmission. It uses electronic sensors, pneumatics, processors and actuators to execute gear shifts on the command of the driver. This removes the need for a clutch pedal, since the clutch itself is actuated by electronic equipment which can synchronize the timing and torque required to make gear shifts quick and smooth. It has been deployed for racing and conventional uses by manufacturers like Mercedes-Benz, Chrysler, Volkswagen, Renault, Honda, Ferrari etc.

Applications

Automatic cars are more expensive, tend to wear down faster, and are more complicated than manual transmissions but are far better convenient in city traffic. Think about avoiding all the manual effort of gear changing every second minute!

Although they are less popular in other parts of the world including India, most cars sold in America since the 1950s have been automatics. The main reasons for less popularity are:

• Standard transmissions are more efficient and less expensive to produce than automatic transmissions.
• Standard transmissions are generally stronger than automatic transmissions and off road vehicles take advantage of a direct gear selection so they can withstand rough conditions.
• Manual transmission requires less active cooling because less power is wasted.

Check out a list of latest automatic transmission cars in India like Hyundai i10 and Maruti A-Star:

http://www.cardekho.com/cars-search/automatic-transmission-cars-in-india-2010

With automatic transmission becoming more energy efficient like cars with manul transmission, the trend is surely set to prevail on Indian roads in the near future.