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    CarWale Gyan: Types of transmissions


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    Ajinkya Lad

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    Introduction

    Gear-Lever

    If you've driven vehicles or have read about them, you would've come across a term transmission. But, what does it exactly means? Well, simply put, a transmission or a gearbox is a mechanical machine that controls power generated from a vehicle's engine/powertrain and transmits it to the wheels, as and when required. Now, there are various kinds of automotive transmissions and in this article, we will take a brief look at the different types of gearboxes.

    Manual Transmission

    Gear-Lever

    A manual transmission or a stick-shift unit is a type of gearbox that requires driver-intervention to shift the gears. The driver has to engage the clutch by pressing/de-pressing the foot pedal and select an appropriate gear ratio by using the gear-selector lever to regulate the torque from the engine to the wheels.

    Conventionally, modern cars come with five- or six-speed manual transmissions, depending on the engine capacity and segment. However, earlier cars came with three- or four-speed manuals, while there have been instances of seven-speed manuals as well. The speeds indicate the gear ratio.

    Exterior

    Usually, a manual transmission allows to select any gear ratio at a given time, but a sequential manual transmission only allows to shift to the next higher or lower ratio. The gear lever is usually floor-mounted, but some cars also get a column-mount or a console-mounted (on dashboard) gear selector.

    Torque Converter Automatic

    Gear-Lever

    An automatic transmission, as the name suggests, is a self-shifting transmission, meaning that the driver doesn't need to manually shift the gears. In this case, a torque converter has the ability to multiply torque even when the output rotational speed is low. This is done by deflecting the fluid coming from the turbine off the stator, while it is locked against a one-way clutch.

    Exterior

    Unlike usual fluid coupling, which is incapable of multiplying torque output, a torque converter has a stator, which alters the drive characteristics during high slippage to produce increased torque output. A torque converter gearbox finds its application in large-capacity engines that produce higher torque output. These transmissions are better built and less prone to failure.

    CVT Automatic

    Gear-Lever

    A CVT or a continuously variable transmission (CVT), also known as a pulley transmission or a step-less transmission, is an automatic gearbox that seamlessly changes through a continuous range of gear ratios. A CVT, virtually, has infinite gear ratios that can be continuously altered on-the-fly to offer a seamless driving experience. 

    Unlike other automatic transmissions, a CVT isn't torque-dependent and uses its infinite gear ratios to transfer power to the wheels. A continuously variable transmission uses four pulleys (cones) in its construction that are connected by rubber belts and dangled on two parallel axles. These cones move up or down and the axles move closer or apart to create infinite ratios.

    Exterior

    Modern CVTs come with pre-defined steps (for eg. six-step or seven-step) to control the movement of the axles and pulleys to offer better control to the driver and provide a smooth power delivery.

    DCT Automatic

    Gear-Lever

    A DCT or a dual-clutch transmission is a type of an automatic gearbox that uses two separate clutches, each for a set of odd and even gear ratios. The larger outer clutch drives the even-numbered gears, while the smaller inner clutch drives the odd-numbered gears.

    Since either of these alternate gear ratios (odd and even) can be pre-selected while being driven in any gear, without interrupting the torque distribution to the wheels, DCTs are the fastest-shifting road car transmissions. DCTs are mainly of two types - wet multi-plate clutches or dry single-plate clutches.

    Exterior

    A wet-clutch DCT, which uses oil for cooling, is designed for engines that develop high torque output of 350Nm or more. A dry-clutch DCT is suitable for smaller engines that produce a lower torque output of up to 250Nm. However, dry-clutch DCTs are more fuel efficient as there are no fluid pumping losses.

    AMT

    Gear-Lever

    Simply put, an AMT is an automated manual transmission, which is also known as an auto gear-shift transmission. It is nothing but a manual gearbox with a hydraulically or electronically controlled actuator, which automatically shifts the gears. The gear transitions depends on the throttle input and the speed of the vehicle.

    Exterior

    AMTs are much more affordable than conventional automatic transmissions. These transmissions are developed to offer driving convenience and aren’t really centred towards driving pleasure. And that’s exactly why AMTs are slow-to-react to throttle inputs and induce a pronounced head-nod when the car is driven with a heavy foot. That said, one upside of AMTs is that these are fuel-efficient.

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