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Old 11th August 2016, 11:28   #16
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Default re: Videos: Understanding the different types of Gearboxes

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Originally Posted by smartcat View Post
Good informative thread. Any guesses as to why a CVT is never mated to a diesel engine?


Audi has diesel CVTs on the A4 and the A6
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Old 11th August 2016, 12:02   #17
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Default re: Videos: Understanding the different types of Gearboxes

AFAIK, letting go of the throttle and then reapplying it, gives a bit of a "manual" and "mechanically involving" feel to the whole auto gear shifting process. I usually do this in my Chevy even when I select the manual mode. I have noticed lower transmission temps when I follow this method rather than simply stepping on the the loud pedal. In the local Dubai heat high temps can potentially ruin the internals faster. Even though I have a towing package with a transmission cooler and an altered diff ratio, I prefer to choose mechanical empathy over mechanical ignorance!
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Old 2nd January 2018, 13:20   #18
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Default Re: Videos: Understanding the different types of Gearboxes

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Originally Posted by ani_meher View Post
Thanks for the excellent info and interesting compilation. What are the 4-speed automatics, 5-speed automatics etc used by Maruti, Hyundai etc? Which category is that?
Really great topic and discussion! A long time has passed since you raised your queries, but better late than never

1. Except CVTs, all other transmissions have fixed gears a la manual transmissions. These are the steps (so to speak) via which engine power is transferred to the wheels. After the second gear, your car will shift to the third (or sometimes fourth) gear. You cannot have a second and four tenths gear. The process of shifting though, is handled by the transmission itself requiring no driver inputs. Ergo, automatic transmissions.

2. 4-speed automatics have 4 fixed gears, 5-speed ones have 5 fixed gears and so on. Obviously, greater the number of gears, better is the distribution of power & torque resulting in greater efficiency and smoother drives.

3. In an AMT, the entire setup is just like a manual transmission. Only the deployment of the clutch and shifting of the gears is automated, thereby giving it the typical head-nod behaviour.

4. In a torque converter setup, the output of the TC is not connected to the wheels directly. It is instead routed through planetary gears and ring type clutch pads. As per my limited knowledge on this subject, the transmission control unit, based on the torque flow and power requirements at that particular time, selects the best gear and engages it. Although the entire transmission assembly is rotating all the time, other gears are kept disengaged by means of the clutch pads. Since there are a lot of moving parts and the shifting process is also quite sophisticated, this type of transmission tends to produce low FE numbers.

5. Similarly, in a DC setup, while one gear is actively transmitting power to the wheels, the transmission control unit pre-selects the next gear depending on the power requirements and keeps it ready for deployment at the shortest of intervals (tenths of a second). The process of shifting is nothing more than releasing one clutch and engaging the other. As the shifts are quick and precise and as it is a manual transmission at its heart, the performance and FE is very refined on this transmission.

6. In an AMT, of course the entire setup is just like a manual transmission. Only the deployment of the clutch and shifting of the gears is automated, thereby giving it the typical head-nod behaviour.

7. In a CVT however, there are no fixed ratios as there are no toothed gears between the input and output shafts. Instead you have two pulleys linked by a chain. These pulleys have conical shafts so the diameter of rotation can be altered and you can develop infinite gear ratios. This also gives good FE numbers though you will have to contend with the infamous rubber band effect.

8. Then coming to the type of automatics used by various car makers, the list is as follows (though not exhaustive by any means)
a. Maruti: 4 speed TC on the Ciaz, CVT on the Baleno and AMT on all others.
b. Hyundai: 4 speed TC, though they are graduating to a 6 speed one on their recent launches (2017 Verna for example).
c. Honda: CVT linked to a TC
d. VAG: The enhusiast's delight DCT
e. Ford: DCT, though they have opted for a 6-speed TC on the latest EcoSport.
f. Tata: AMT, as far as I know.
g. Renault / Nissan: Both AMT and CVT depending on the car.
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