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Vehicle Powertrain Systems by Behrooz Mashadi, David Crolla

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Chapter 4

Transmissions

4.1 Introduction

The function of the vehicle transmission is to transfer engine power to the driving wheels of the vehicle. Changing gears inside the transmission allows matching of the engine speed and torque with the vehicle's load and speed conditions. In manual transmissions the driver must shift from gear to gear, whereas in automatic transmission the shifting is performed by a control system. There has been a gradual refinement in gearbox design over recent decades and a move towards an increasing number of gear ratios to improve overall performance and efficiency.

The chapter starts by analyzing conventional transmissions – manual gearboxes, clutches and gear ratio calculations. Then, recent developments in transmissions are reviewed and analyzed – these include Automated Manual Transmissions (AMT), Dual Clutch Transmissions (DCT) and Continuously Variable Transmissions (CVT). Their technical benefits are summarized along with the commercial issues which influence their use in current vehicle design. Two of the main trends which have influenced the increasing use of more sophisticated gearboxes have been the demands for both improved driveability and improved fuel efficiency.

4.2 The Need for a Gearbox

Vehicles are traditionally equipped with gearboxes and differentials. The number of gears in vehicle transmissions range from three for older cars to five, six and even eight in newer ones. The differential provides a constant torque amplification ...

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