Automotive Transmission System

Introduction:

Transmission system (or also Power Train) is known as the mechanism that transmits the power developed by the engine of automobile to the driving (either rear or front) wheels. Power Transmission System is the next and final stage of the engine generated power before it hits the wheels. The whole system is responsible to couple engine and wheels, driving and adapting the output shaft rotation to a desired speed and torque ratio, allowing a wider range of speed and better performance as the engine has its own RPM limit (redline) and maximum performance value. Basically there are four types;

1.       Manual gear box,

2.      Dual Clutch Transmission (DCT),

3.      Continuous Variable Transmission (CVT) &

4.       Automatic Transmission.

 All vehicles, aircrafts and watercraft included, require transmission in order to convert torque and engine speed. Transmissions are distinguished in accordance with their function and purpose- e.g. selector gearboxes, steering boxes and power take –offs. The task of a transmission system is to convert the traction available from the driver unit, satisfying requirements placed on it by the vehicle, road conditions, the driver and the environmental effects. In addition to the driving and automobile performance of passenger and commercial vehicles, transmission system are of central importance with respect to reliability, fuel consumption, ease of operation and road safety to the pedestrians also. The main goal when designing an automotive transmission system is an optimal conversion of the traction available from the engine into the traction force of the vehicle over a wide range of road speeds.

                                                   Figure: Basic Transmission System.

Following table shows the basic comparison between industrial and automotive transmission system:

Transmission	Number of Speeds (forward)	Ratio 1st gear / overall gear ratio	Power (kW)	Input torque (Nm)	Mass (Kg)	Specific power (kW/Kg)
Industrial	1	12.5 -	330	2100	680	0.48 100%
Commercial vehicle (AMT)	16	14.1 17.0	397	2600	266	1.49 300%
Passenger car (MT)	6	4.2 5.1	294	500	46	6.39 1300%

Table 1: Comparison between industrial and automotive transmission systems.

It is composed of following parts:

a.       The gear box

b.      Clutch

c.       Propeller shaft

d.      Universal joints

e.       Rear axle

f.       Wheels

g.      Tyres

Power Transmission Systems are divided in three major blocks:

a.       Clutch

b.      Gearbox (Transmission)

c.       Differential

Each of them has a specific role transmission power from the engine to the wheels ensuring correct rotation speed and torque conversion.

Requirements of a Transmission System:

a.       When required provide means of connection and disconnection of engine speed with the rest of power train without any shock and smoothly,

b.      Bear the effective torque conversion & reaction, driving thrust and braking effort efficiently,

c.       Provide a varied leverage between the engine and the drive wheels,

d.      When necessary, it should provide means to transfer power in opposite (reverse) direction,

e.       Enable power transmission at varied angles and varied lengths,

f.       Enable speed reduction between engine and the drive wheels in the proper ratio of 5:1,

g.      Enable diversion of power flow from engine to the drive wheels at right angles,

h.      Provide means to drive the driving wheels at different speeds when necessary.

Let’s start exploring each types of transmission systems;

1.      Manual Transmission System

In this type of transmission system, the driver has to manually select and engage the gear ratio according to driving condition. Most of the automobile we see, are equipped with manual transmission system. In this system, on the gear lever we can see marking like 1,2,3…R where notation refers to gear ratio.

Manual transmission in cars is usually controlled by an “H” pattern lever. This is because it has H architecture. Modern transmission in modern passenger cars use synchronizers to eliminate the need for double clutching. A synchro’s purpose is to allow the collar and the gear to make frictional contact before the dog teeth make contact. This lets the collar and the gear synchronizer their speeds before the teeth need to engage.

Following are the stages of manual transmission:

a.      Clutch fully depressed

The clutch is fully disengaged when the pedal is fully depressed. There will be no torque being transferred from the engine to the transmission and wheels. Fully depressing the clutch allows the driver to change gears or stop the vehicle.

b.      Clutch slips

The clutch slips is the point that vary between being fully depressed and released. The clutch slip is used to start the vehicle from rest. It then allows the engine rotation to adjust to the newly selected gear ratio gradually but it is recommended not to slip the clutch for a long time because a lot of heat is generated resulting in energy wastage and tear of clutch.

c.       Clutch fully released

The clutch is fully engaged when the pedal is fully released. All the engine torque will be transmitted to the transmission. This results in the power and speed being transmitted to the wheels.

2.      Dual Clutch Transmission (DCT):

In this type of transmission, the synchronizers of this gearboxes are done with a more abrasive material of friction to get very low synchronization times and to allow down shifts like 6^th to 2^nd. This type of transmission is used in races since 1985 used for Audi and Porsche. The main advantage of this transmission is her capability to work sequentially with two clutches. One of them is connected to the odd gears and to the reverse gear, and other is connected to the even gears. This fact enable the vehicle of have the possibility to change gears very quickly, promotes lowers consumptions and a longer live than the other transmission.

3.      Continuous Variable Transmission (CVT):

In this type of transmission system, it can generate infinite gear ratios without any kind of gears, instead of that it uses a pair of pulleys capable of changes its diameter, thus, changing gear ratio and allowing an infinite variability between highest and lowest gears without discrete steps or shifts while it doesn’t have to lock toothed wheels reducing chances of wear and tear of parts.

4.      Automatic Transmission:

Automatic transmission systems are the most advanced system in modern vehicles which driver’s mechanical efforts are reduced very much and different speeds are obtained automatically. This system is generally also called as Hydramatic transmission system. It contain epicyclic gear arrangement, fluid coupling and torque converter. In this planetary gears sets are placed in series to provide transmission. This type of transmission are used by Skoda, Toyota, Lexus, etc.

It was invented in order to transfer the need of shifting gears from the driver to the wheels, allowing an easy and more fluid driving experience with having same objective of all the other types of transmission: convert the narrow range of engine speeds into a wide range to the output.

Stages of Automatic Transmission:

Park (P):

Selecting the park mode will lock the transmission, thus restricting the vehicle from moving.

Reverse (R):

The reverse mode puts the car into reverse gear, allowing the vehicle to move backward.

Neutral (N):

Neutral mode disconnects the transmission from the wheels.

Low (L):

Low mode will allow driver to lower the speed to move on hilly and middy areas where low speed and high torque is required.

Drive (D):

Drive mode allows the vehicle to move and accelerate through a range of gears.

Parts of transmission system will be discussed in next blog.

YV Nitesh

11^th Jan 2019