Subject: Automobile Engineering

Topic: Transmission Systems, Live Axle & Differential

Difficulty: Medium

i) The principle of operation of overdrive Overdrive is a device to set up the gear ratio in the car. It is fitted in between transmission & propeller shaft. This results in less wear of the engine parts & decreases vibration & noise. As the friction lesses at lower speeds are less. There is a saving of fuel also with the overdrive. Overdrive is generally fitted on top gear only.

Working:
To understand the working of an overdrive. It consists of an epicyclic gear train in which the Sun gear is free to rotate on the input shaft,while the cartier can move on splines on the input shaft. A free wheel clutch is also fitted on the input shaft splines. The output shaft is connected to the ring.
When the Sun gear is locked with the casing i.e it becomes stationary, the speed of the output shaft is increased i.e overdrive is engaged. When, however, the sun gear is locked to the carrier or to the ring. Solid drive through the gear train is obtained. Thus, depending on the locking of the sun gear with casing or with carrier, the overdrive is the normal direct drive is obtained.
There is another possible control of mechanism i.e when the sun wheel is kept free to rotate on the input shaft. In this case, there is direct drive through the free wheel clutch when the engine develops power.

ii) The Working & application of torque converter The construction of torque converter is similar to that of fluid flywheel: the only difference being that it has an additional stationary member called the stator or the reaction member & all the members have blade or vane of specific shape.

The operation of the two however is not similar, where as the fluid flywheel transmits the same torque as given to it by the engine shaft. The torque converter increases the torque in a ratio of about 2:1 to 3:1. Thus, it serves the same purpose as that of a gearbox & that too in a better way where as in the gearbox the torque variation is only in finite no. of steps, in the case of torque converter torque output variation is continuous. However, the efficiency of a torque converter is high only with narrow limits of speed.

A Single stage torque converter is shown in the fig:9.2. It consists of 3 main parts:
i) Impeller or the driving member which is connected to engine.
ii) turbine or driven member which is connected to road where through the transmission gears & driveline.
iii) starter fixed to the fame through a free wheels.
In addition, there is a transmission oil pump which keeps the converter full of oil under pressure. Pressure is necessary to keep the converter full of oil when rotating. Due to rotation the centrifugal force pushes the oil in the outward direction & this tends to form air pockets near the center of the converter . This phenomenon of forming air pocket duo to low pressure is called cavitation, to avoid which the converter pressure is kept between 200 to 1200 KPa. Working: When the engine is started, the impeller starts rotating. Initially, the oil from the impeller is pushed into the turbine because of the higher centrifugal force at the impeller, due to it being driven by the engine & turbine being stationary. Thus oil, having taken high kinetic energy from the engine through the impeller, hits the outer edge of the turbine.

This flow of high energy oil provides the force that tends to rotate the turbine. This force increases with the increase of engine speed. When it is great enough, the turbine starts rotating & the vehicle moves. The turbine blade angle is such that it changes the direction of oil flow so that when it comes out of the turbine at the centre , its directions is effectively backward.
If there were no stator & it were to enter the impeller directly, it will push the impeller in the opposite direction & will thus cause a loss of power.