What is Transmission all about ?

The job of a car's transmission is to make sure that the right amount of power goes to your wheels to drive at a given speed. It works by shifting gears in a very similar fashion to what you see on a multi-speed bicycle. On a bicycle, if the chain is off, the car won't go, and if the chain is in too high a gear, you'll have trouble getting started from a stop. The same principle applies to your transmission, and if you don't keep it well-maintained, you'll lose fuel economy or maybe not even be able to drive at all. 

There are two major types of transmissions in most cars on the road today: automatic and manual. Several other types of transmission have also been gaining ground in recent years. The dual-clutch transmission and The continuously variable transmission (CVT). An electric cars often don't have a transmission at all. 

Before we delve into the way the manual and automatic transmissions work, we should define some key terms:

• Gear: a set of toothed wheels that work together to determine or alter the relation between the speed of a vehicle's engine and the speed of its wheels; it is also the term used to describe each of the gears a driver can select, which is in fact a ratio of the selected gears on the input and output shafts.
• Gear ratio: the ratio between the rates at which the input and output of a set of gears rotate
• Clutch: a mechanism for connecting and disconnecting a vehicle's engine from its transmission system
• Transmission: a mechanism for transmitting power from a vehicle's engine (or motor) to its wheels
• Shift lever: a control lever which the driver uses to manage the current gear (or gear range) of a vehicle's transmission
• H pattern: an arrangement of gears, usually designated on the shift lever knob, where in the gears are arranged in a series of parallel rows, reminiscent of an "H" with extra legs

Manual Transmission :
Manual transmissions often feature a driver-operated clutch and a movable gear selector. Most automobile manual transmissions allow the driver to select any forward gear at any time, but some,only allow the driver to select the next-higher or next-lower gear ratio. This second type of transmission is sometimes called a sequential manual transmission.

Manual transmissions are characterized by gear ratios that are selectable by locking selected gear pairs to the output shaft inside the transmission.

The driver selects gears in a manual transmission by moving a shift lever, which engages a linkage that controls the movement of the gears along the input shaft. Moving the lever forward or rearward chooses between the two gears available on a given linkage; cars with four gears, or speeds, use two linkages; cars with five or six speeds use three linkages. The driver changes between linkages by moving the shift lever left and right.

To engage a gear in a manual transmission, the driver pushes in the clutch pedal, disconnecting the engine from the input shaft of the transmission. This frees the gears on the input shaft to move, as when the engine is sending torque through the input shaft, the gears on it are engaged. Once the clutch has disconnected the power from the engine to the transmission, the user selects the appropriate gear (i.e. first, third, reverse) and releases the clutch, re-engaging engine power to the input shaft and propelling the car with the selected gear ratio.

Contemporary automotive manual transmissions are generally available with four to six forward gears and one reverse gear. Tractor units have at least 10 gears and as many as 24.

Automatic Transmission :
In an automatic transmission, the same essential process is going on within the transmission itself, but it's all done behind the scenes. One of the primary differences between an automatic transmission and a manual transmission is that automatics don't use clutches (typically, anyway). Instead, the automatic transmission relies on a torque converter to de-couple the engine and the gear set.

The torque converter is why automatic transmission cars "creep" forward when idling and in "drive", as a small amount of the engine's torque is being supplied to the transmission's input shaft.