TRANSMISSION REQUIREMENTS

The internal combustion engine, as used in road vehicles, has the following requirements and characteristics—
(1) To start it, some form of external energy must be applied.
(2) Its maximum torque is small compared with that of a steam engine or electric traction motor of the same maximum horse power.
(3) Its maximum power is developed at a relatively high speed – ranging from about 1700 rev/min for heavy commercial vehicles to 12000 rev/min and more in racing cars.

In consequence, it must be used in conjunction with a transmission that differs in many respects from those of either steam- or electrically-powered vehicles.
Various methods have been used for starting engines – for instance, from energy stored in a spring or flywheel, or chemical energy in a cartridge – but the general rule of course is to use a battery-electric starter.
During starting, provision should be made to disengage engine from the drive line and consequently engaging the drive should be smooth and without shocks.
In passing down the drive line the torque is modified, stage by stage, until it becomes propulsive force at the interface between tyres and road.For rapid acceleration more force is to be applied or to overcome inertia in rest more torque is required.This is achieved by changing gear ratios mainly.In this context, gear-changing can be likened to altering the leverage between the engine and the road wheels, so that the relatively small torque available can be translated into a large tractive effort. A large leverage may also be required for climbing hills or traversing very soft or rough ground.

While the basic principles of transmission remain the same for virtually all classes of road vehicle, the actual arrangements vary – for instance, some may have four-wheel drive and others either front- or rear-wheel drive. Where the engine is installed at the front and the axis of its crankshaft is parallel to, or coincident with, the longitudinal axis of the vehicle, ultimately, the drive must be turned through 90° in order that it may be transmitted out to the wheels the axes of which are of course perpendicular to that longitudinal axis. Such a turn, however, is not necessary if the engine is installed transversely, though other complications, such as a need for dropping the drive line to a level below that of the crankshaft while turning it through 180°, may arise.
Another issue is, when the vehicle is cornering the outer wheels have to traverse more distance than the inner wheels due to difference in radii.Then again, it has to accommodate relative motion between the driving wheel and the engine.

Therefore, requirements for the transmission are as follows—
(1) To provide for disconnecting the engine from the driving wheels.
(2) When the engine is running, to enable the connection to the driving wheels to be made smoothly and without shock.
(3) To enable the leverage between the engine and driving wheels to be varied.
(4) It must reduce the drive-line speed from that of the engine to that of the driving wheels in a ratio of somewhere between about 3 : 1 and 10 : 1 or more, according to the relative size of engine and weight of vehicle.
(5) Turn the drive, if necessary, through 90° or perhaps otherwise realign it.
(6) Enable the driving wheels to rotate at different speeds.
(7) Provide for relative movement between the engine and driving wheels.

Now transmission fall mainly in three categories-
(1) Mechanical
(2) Hydraulic
(3) Electric/Electromagnetic

While the first one is the most common among the three, a combination of Mechanical and Hydraulic is gaining popularity.All hydraulic transmissions have been developed but not gone on a large scale production yet.But the most interesting part is that this kind of transmission is not at all uncommon in tractors and other construction and agricultural equipment, like diggers, where transitions from idle to full load are to be achieved instantly and repeatedly.
This video gives an overview what a transmission is and its components.It would be beneficial to see this video before the next post so as to get acquainted with different terms and components.

Next up will be "Types of Mechanical Transmission", so stay tuned..
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