(C) TRANSMISSION SYSTEM

The power generated in the engine of the automobile is transmitted to the wheels with the help of transmission system.

The basic functions of transmission system.

(1) To transfer the engine output power to the final driven gear,
(2) To provide sufficient torque at starting, ascending, accelerating, braking etc. situations,
(3) To provide low speed to high speed driving capability,
(4) To change the direction of wheel rotation (forward or reverse).

The engine rotation is transmitted to the transmission main shaft through the action of a clutch. This main shaft then drives a counter shaft using a set of meshing gears which are selected by the gearshift lever. The counter shaft's output gear transfers the motion to the differential assembly final gear, and the differential transmits it to the left and right drive shafts. These shafts ultimately turn the left and right wheels respectively. The important components of transmission system are briefly discussed below:

1. clutch.
An automobile has to operate under varying load conditions on the road. It may be noted that when starting engines and also during gear changes, it is necessary to cut off the load upon the engines. The clutch is a device to continue or discontinue load on the engine as necessary to solve these problems.

The clutch is installed between the engine and transmission. It transmits the power of the engine generally through the use of friction. The clutch is linked to the clutch pedal in the passenger compartment. When the driver presses down the clutch pedal, the linkage cause the clutch to disengage. This uncouples the engine from the transmission. When the driver releases the clutch pedal, the springs in the clutch causes it to engage again. Now power can flow from the engine, through the clutch, to the transmission and the gear train.

2. Gear box.
This is a component which houses the complete gear mechanism. The purpose of the gear box is to change the engine torque and speed into the torque and speed required by the wheels of an automobile. It also provides the means to allow the automobile to be reversed. The suitable pairs of meshing gears are used for reducing the engine crankshaft speed before it is applied to the driving wheels. The speed ratio or gear ratio between the crankshaft and the wheels is selected in steps. The following are the three types of gear boxes used in automobiles:

(a) Sliding mesh gear box,
(b) Constant mesh gear box,
(c) Epicyclic type gear box.

All modem manual transmissions are of the constant mesh type. The characteristic of this type of transmission is that the drive and driven gear combination for each speed, with the exception of reverse, are constantly engaged with each other. Thus, even when the gearshift lever is in neutral position, all gear combinations are in mesh and tuning. However, no power is transmitted in this condition as one of the gears of each gear combinations are free wheeling on their shafts.

When the gearshift lever is used to select a different gear, the free wheeling gear from the appropriate gear combination is locked to its shaft using the corresponding synchromesh mechanism.
This action allows the transmission of power using these gears to commence.

(3) Bevel pinion and crown wheel.
This is also known as final drive gear. These are used to tum the drive round through 900 and also provide a permanent reduction in speed. This is required because of the fact that the speed of the engine has to be maintained at optimum level at all the times, Yet a minimum torque has to be made available at the road wheels..

(4) Differential.
The purpose of the differential is to generate the difference in inner and outer wheel speeds when the vehicle is taking a turn. If the right and left wheels are connected without differential, turning would not be possible unless one of the wheels started to be cornering in such a condition Would be extremely difficult and would also result in an increased amount of Tyre wear.

(5) Drive shaft.
The purpose of the drive shaft is to transfer the torque from the differential to the wheels. Accordingly, one end of each shaft is splined to the differential's pinion drive gear  (or side gear), while the other end is splined to the wheel stub axle. The drive shafts are connected to the differential and the wheel stub axles through universal joints.

AUTOMATIC TRANSMISSION

The automatic transmission (briefly written as AT) allows the vehicle to perform automatically  the gear shift operations in response to load, speed, and other relevant conditions. The principle components of an automatic transmission are the torque converter, the gear unit, & hydraulic unit.

The torque converter is mounted to the crankshaft and operates as a flywheel. A starter ring gear is fitted on the outside of the converter, which is used by the starter motor. The basic function of the torque converter mechanism is to transfer the engine's output power to the gear unit through the action of a special fluid. However, it also operates in the complete reverse situations also. It may be noted that when the vehicle is stopped, it is important for the torque converter's fluid to absorb any variation in speed and not to transfer engine power in such a situation.

However, it is not possible to deal adequately with all possible situations of driving using torque increases generated by the torque converter alone, and for this reason, further changes to the driving force are made by the gear unit in accordance with the current driving conditions.

The most fundamental difference between the gear units of automatic transmissions and of manual transmissions is that in case of automatic transmissions, hydraulically controlled multiple disc clutches are used instead of synchronisers. These clutches are used to connect gears to the shafts so that gear shifts can be implemented.

But, in case of reverse gear, sometimes hydraulic pressure is used to actuate a shift fork so that the counter shaft and counter reverse gear can be engaged.

The hydraulic control unit converts the engine load, speed, and other similar variables into hydraulic signals and controls the shifting of the gear unit in accordance with the corresponding hydraulic pressures.