Snapshot – An automatic transmission in a car was rare in the 90s and even in the early noughties. Fuel efficiency was be-all and end-all for most budget car buyers in India, and old-generation, torque converter automatic transmissions meant poor mileage. A definite no-no for both product planners and car buyers. Now that we are in 2016, things have changed.
From having three and four speed torque converter hydraulic automatic gearboxes as the only options on offer, the Indian car buyer now has no less than four different types of automatic transmissions to choose from. As with everything that involves choices, the more, the merrier. The merry making also comes with confusion. So, we’ll help you pick the automatic transmission that’ll best suit your needs.
Torque converter hydraulic automatic gearbox
The torque converter hydraulic automatic is one of the oldest form of the automatic transmissions used on cars. These gearboxes are also known to be power sapping due to heavy transmission losses and are much less efficient than manuals. Cars such as the Maruti Suzuki Swift Dzire petrol, the Honda Brio and the Hyundai Grand i10, Xcent, use torque converter automatics. Gear shifts can get jerky with low end torque converters with 3 and 4 speeds, while high end 8 and 9 speed transmissions such as the ones made by ZF and used by luxury car makers such as BMW and Land Rover, are extremely smooth and quick shifting. To sum it up, the more speeds the automatic gearbox has, the better driving pleasure and fuel efficiency it will deliver.
How this works is that there are 2 rotary parts, a turbine and an impeller. There is fluid that is present in the impeller that gets pushed outwards due to the centrifugal forces and lands on the turbine, thus rotating it. It is mainly due to this reason that there are a lot of losses and this system isn’t as efficient as a manual transmission or other automatics. But that is not all, the turbine then rotates a stator, which causes torque multiplication. A proper working video of the same can be found below:
Certain new cars like the soon to be launched Tata Hexa (6 speed) and the current favorite Mahindra XUV500 use this kind of gearbox. These are much improved units.
Dual clutch automatic transmission
The Volkswagen Polo, in GT TSI trim, is one of the two hatchbacks in India to use a twin clutch automatic gearbox. The other is the Ford Figo Automatic, which uses a 6 speed twin clutch unit. A DSG has the same gear set like a manual transmission, but houses 2 multi plate clutches which control odd and even gears separately. Gear shifting is controlled by a mechatronic unit. There are 2 types of DSGs, wet and dry. Wet means the clutch packs have enough lubrication and are placed in oil where as dry means that there is no lubrication. Based on the torque, companies decide whether to go for wet or dry clutches.
The 1st, 3rd, 5th, 7th and reverse gears are controlled by the outer clutch, while the 2nd, 4th and 6th gears are controlled by the inner clutch. There are 2 separate input shafts that house the gears and hence controlling them is easy. The DSG gearbox offers better fuel efficiency than conventional torque converters and in some cases are as efficient as manuals as well. Transmission losses are minimized while shifts are smoother and quicker.
Among affordable cars, such twin clutch automatic cars are also on offer on the Ford Figo Aspire and Ecosport, the Volkswagen Vento TSI and TDI, Ameo TDi, Skoda Octavia TSi and TDi and the Skoda Rapid Diesel. In the VW range, only the Jetta TDi and the Octavia TDi get the wet clutch system, the others have to make do with a dry clutch unit. On the other end of the spectrum, Porsche uses dual clutch transmissions (PDK) and so does Mercedes (DCT on the CLA). Here is the proper functioning of one along with the parts involved:
Automated manual transmission/semi automatic gearbox
The automated manual transmission (AMT)/semi automatic gearbox is soon gaining popularity in India. Known as the poor man’s automatic, the AMT’s big advantages are that of low cost, light weight and high fuel efficiency (Renault claims that the AMT version is more fuel efficient than the manual version). Since the AMT is essentially a servo motor that automates gear shifting and the working of the clutch, it can be mounted as an extension to manual gearboxes. Due to this, the cost of the AMT is minimal among all the automatic transmissions.
In functionality, the AMT has the same hardware as a manual box, thus helping to keep costs down. The only difference being that there is no clutch pedal and that is controlled by a motor, along with an actuator for the gear shifting.
AMTs are now being used as low cost alternatives to other forms of automatic transmission technologies. For instance, the Maruti Alto K10 AMT is the least priced automatic car in India, and the world. This is now followed by the Kwid AMT, the Celerio, the Tata Zest Diesel, the WagonR StingRay, the Swift Dzire diesel, the Mahindra TUV300 and the Renault Duster Diesel, Nissan Terrano diesel and a host of other cars in India use AMTs. Jerky gear shifts are a disadvantage. If you think only low end cars use it, think again. The sporty Abarth 595 also comes with an AMT.
One very important thing that needs to be kept in mind while driving an AMT is that it should be driven like a manual and not an automatic. While in an automatic, there is continuous power delivery, including when gear changes are happening. In a manual, you have to lift off the accelerator to shift gears, the same has to be applied on the AMT as well to make sure smooth driving. Also, most AMTs don’t have the creep function (ability to roll ahead without pressing the accelerator) like in a manual car which means hill starts will require the handbrake.
Continuously variable transmission (CVT) gearbox
Perhaps the most refined of automatic transmissions, the continuously variable transmission (CVT) consists of hydraulically operated steel pulleys and a belt. The belt transmits torque between the pulleys. When the accelerator is pressed, the sides of the pulleys come together, and the torque from the engine is transmitted to the wheels. During deceleration, the pulley’s go apart and the torque transmitted to the wheels is reduced. Since this process does not involve gears, there is no gear shifting as such.
CVTs are very smooth to operate, with seamless torque delivery. What a CVT has is infinite gear ratios, just like that on an Activa. The only downside to it is the feel of a rubber band effect when revving hard. The Nissan Micra and Sunny, and the Honda Jazz, Amaze and the City, BR-V are some of the cars in India that feature a CVT.