Automatic Transmissions – AMT takes on AT, CVT, DCT

Automatic Transmissions – AMT takes on AT, CVT, DCT

The transmission is, of course, a critical part of a vehicle's powertrain, since it transfers the engine's power to the driven wheels. Automotive engineers aim to minimise power losses in the transmission, while maximising efficiency. An efficient transmission system can help improve fuel economy in a big way, while ensuring that most of the power produced by the engine actually gets to the wheels, thereby maximising acceleration and top speed.

The transmission traditionally consists of gears that move and slot into position when they are engaged, to deliver torque to the drive shaft of the vehicle. Conventional transmissions make use of a manually operated clutch, which temporarily disengages gears from the engine, and then re-engages the next chosen cog as smoothly and seamlessly as possible.

In India, the manual transmission has been the norm for decades, since it provides a combination of lower costs and better fuel economy compared to the regular, torque-converter automatic. Globally, General Motors (GM) was one of the first companies to introduce an automatic transmission, which did not require the use of a manual clutch. GM introduced its 'HydraMatic' transmission in the 1940 Oldsmobile, which it then also offered on various Cadillacs. The HydraMatic system combined the hydraulic operation of a planetary gearbox with a fluid coupling instead of a friction clutch, which eliminated the need for de-clutching. The transmission had four forward speeds and a reverse, and also incorporated a parking pawl which was engaged when the gearshift selector was placed in reverse, with the engine off.

With decades of evolution, today we have many different kinds of automatic transmissions, including the traditional automatic, the automated manual transmission (AMT), dual clutch automatic transmission (DCT) and the continuously variable transmission (CVT). These automatics are not limited to passenger vehicles alone, but are also seen in various types of commercial vehicles and off-highway vehicles.

AT REMAINS RELEVANT

The conventional automatic transmission (AT) remains the most popular, most common form of automatic transmission globally. The AT makes use of a torque converter to transfer the power generated by the engine to the gearbox. They also use gearing that is similar to those used in a manual transmission, but the gears are changed in a different manner. The torque converter is the device that changes gears in a traditional AT system. Torque converters are made up of three main components – impeller, turbine and stator, and is filled with fluid. These three components lock and unlock with the help of multiple clutches, in order to control the movement of planetary gears of the transmission and maintain predetermined gear ratios.

A well-engineered AT delivers power in a smooth, linear manner and with the additional of modern-day electronics, can be operated in different driving modes (for example, 'sport' or 'eco') that alter shift patters, thereby providing either a more dynamic driving experience or improved fuel economy. Since the AT does not have a clutch pedal, it also makes driving easier and is hence especially suitable for driving in the city, where dense stop and go traffic is often the norm. However, conventional ATs can sometimes sap a fair bit of power, resulting in power losses in the drivetrain, and also provide poorer fuel economy as compared to regular, manual transmissions.

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AMT MAKES A COMEBACK

Automated Manual Transmission (AMT) is an older technology, which some manufacturers were providing in their cars back in the 1970s. However, with the latest advances in electronics, which allows better control and calibration, the AMT seems to be making a comeback of sorts, especially in developing markets like India, since it provides better fuel economy as compared to a regular AT. The AMT is actually a regular, manual gearbox, but with the addition of an electro-hydraulic system, which comprises an electronic control unit and hydraulic system. This system, which is also known as the Transmission Control Unit (TCU) monitors parameters like accelerator pedal position and vehicle speed, among others, and controls the hydraulic operations, which changes gears automatically, without the driven needing to use a clutch. Some AMTs also offer a manual shift mode, where the driver can change gears by using the gear shifter, without having to use a clutch.

The hydraulic assembly in an AMT operates the clutch and gears in a manner that is similar to that of an actual human driver, with the hydraulic kit interacting mechanically with the gear shaft. This movement is carried out by a combination of hydraulic actuators and hydraulic electro-valves, which are all assembled into a single kit by OEMs. The engine control system pairs with the TCU for every gear shift, with the interface between the AMT unit and the engine control acting as a torque generator. This results in gentle transfer of torque to the wheels, thereby reducing jerks and sudden deceleration during shifts to some extent. AMT systems are based on relatively old technology but are cheaper to manufacture when compared to conventional AT and are more affordable for the entry-level car buyer. Companies are also working on enhancing the role of electronics in AMT modules, where the transmission is able to communicate with the car's on-board software, helping in improved efficiency.

In India, the first car to be equipped with an AMT was the Maruti Suzuki Celerio, a budget hatchback. Maruti was focussing on bringing this technology to the masses as an alternate to pure ATs, with most of their convenience features but a lower price point and without hurting fuel economy. Since then, MSIL has itself launched three other models with AMT as an option. Other companies have followed in Maruti's footsteps, offering their own AMT-equipped vehicles. Renault, Nissan and Tata are some auto OEMs offering AMT in their cars today, with Renault claiming that the AMT in the Duster compact SUV is the most advanced unit in the country, while the Renault Kwid is the first car in its segment to offer AMT with a rotary shift dial on the dashboard, instead of the conventional stick shift system. Most AMTs have a gear lever that allows the driver to shift between reverse, neutral and drive, while some units also allow the driver to shift manually without having to use a clutch. However, the AMT featured in the Kwid only allows the driver to shift between reverse, neutral and drive. Still, this is an innovation which many buyers are likely to appreciate, since it makes the task of gear selection even easier and simpler.

CVT, THE SMOOTHEST ONE

Unlike a traditional AT, a CVT does not have pre-determined gear ratios or separate gear cogs. Instead, CVT uses a belt that runs between two variable-diameter pulleys, which provides an infinite set of 'gear ratios.' One pulley is driven by the engine and the other is connected to the wheels, and the transmission manipulates the ride height of the belt across the pulleys, while keeping the engine operating in the rpm range where it works most efficiently. This means that there really are no 'gear changes' as such, leading to smooth, seamless power delivery. Some CVTs use a fluid-filled torque converter to transfer power from the engine to the transmission, while others use an automatic activation clutch similar to that used for a manual transmission.

CVTs have gained popularity in recent times due to their benefits of being lightweight and returning good fuel economy. The downsides of CVT is that it's difficult to use with engines that provide very large power output, can have higher losses of power in the drivetrain, can have sluggish response and under sudden, hard acceleration, can exhibit what's sometimes called the 'rubber band effect,' where the engine seems to be revving high, without showing a corresponding increase in vehicle speed. CVT is an economy-focussed AT, not offering the driver a great deal in terms of improving the driving dynamics. OEMs like Nissan and Honda offer CVTs on various cars in India, some of which remain fairly popular.

DCT TAKES THE LEAD WITH TECHNOLOGY

Dual clutch transmissions (DCT), which are mostly used by European manufacturers in high-end cars, are technically the most advanced, can be expensive to produce and often provide the best user experience among all forms of automatics. DCT uses two separate clutches that are arranged concentrically, with one clutch each dedicated to odd and even gears. This provides very fast gear shift speeds, since the next gear is already pre-selected and engaged by the time the previous one is released.

While going through the gears, the DCT makes the engine engage with each of the two gearboxes alternatively, via two drive shafts. The gear shifts are executed with the help of a mechatronic module that is made up of an electronic control unit, sensors and hydraulic control elements. Unlike CVTs, DCTs are capable of handling higher amounts of power and torque, provide the best fuel economy as well as the best dynamic performance. However, since the technology involved in the system is fairly complex and DCT consists of a number of electronics components, the costs associated with manufacturing a DCT is relatively higher, which is the main reason for its lower levels of adoption.

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THE ROAD AHEAD

The concept of ATs is not very new to India, since automatic variants of cars have been offered here for almost two decades. But older ATs were more expensive, more maintenance intensive and offered poorer fuel economy compared to MT. Of course, some people still opted for these as ATs provide a much easier, hassle-free driving experience by doing away with the clutch pedal and eliminating the need to shift gears manually.

Now, with the development of the AMT, CVT and DCT systems, automatics have seen a surge in popularity, not just with premium car buyers but even with those looking at entry level cars. With ever increasing traffic congestion in cities, ATs are now preferred over MT due to their sheer ease of operation. Also, each AT system comes with its own set of pros and cons. While AMTs and CVTs are best suited to city traffic, conventional ATs and DCTs work better with sportier, more powerful cars.

However, we have to conclude by saying that the most notable development, in the Indian context, is the new-age AMT. This system offers all the convenience of conventional ATs, but at a lower price point and without hurting the all-important fuel efficiency. Yes, AMTs are not as smooth and seamless as the AT, but with ongoing developments in calibration and ECUs, the systems seem to be improving. With continued development from both OEMs and suppliers, the day might not be far off when AMTs start offering dynamic performance that is on par with conventional ATs.

TEXT: Naveen Arul