Automatic transmission vs manual transmission

Nice explanation there sarthak!

Yes, most of the automatic scooters use CVT transmission. Most automatics we have now use the torque convertor based system. They are quite technologically simple compared to DSGs and cheaper to produce and maintain. Regarding DSGs, it is not yet proven in our conditions and also, as of now customers are getting free warranty replacements. Dont know the conditon after warranty expires! There is no technical skill to repair, and a replacement costs 1.5lakh upwards!  

That means we dont even have a "solid" technology for AT!

We are relying on the viscocity of some liquid for transmission? That is equivalent to half-clutch driving. That is news to me!

That explains the poor mileage of current AT cars.

Tomy

You are right. The ATs use fluid coupling. They are inefficient and some amount of 'clutch slipping' is present at all times. However, the torque convertors in the present day cars are featuring a 'lock-up' clutch, which will come into action if the car is in constant speed for some time. Once the lock up clutch comes into action, there will be direct coupling across the torque convertor. Hence, driving at a constant speed may give you good FE in an automatic also, but I am afraid that our conditions might not permit the same. I would love a DSG type, only if it is proven to be reliable here. So far, it is not. Blame our conditions for the same!

Also since torque converters use fluid coupling, their maintenance aspects also turn out to be cheaper than the clutch plate based manual counterparts. This is because there is no "friction" force which is being responsible for transferring motion and hence no wear and tear takes place. Just a fluid change at about 1 lakh or 2 lakh miles does the job.

It is very easy to understand the gear reduction mechanism (and the increase in torque) in manual transmission using simple physics or mechanics.

However, though I have used half-clutching in MT cars for very steep slopes, I have never understood the theory behind increase in torque due to clutch slippage.

We can apply the following simple principles of school level Physics for explaining increase in torque by doing reduction using gear or belt:

1. Load x Load-arm = Effort x Effort-arm

2. Mechanical Advantage (MA) = Effort / Load = Load-arm/Effort-arm

I wonder how we explain increase in torque by a slippage? Do the droplets of the liquid work like reduction gears?

The irony is that slipping clutches are also used as torque limiters in some other applications. I guess we can look at the example of the fuel tank lid, in which slippage prevents excess torque to be transmitted and prevents over-tightening or breaking of the lid.

Tomy

Throretically I dont know the explanation. IMHO I feel that once you increase the input shaft speed, it increases the pressure of the fluid. And let us imagine that there is a heavy load on the output that has to move. The force created by the liquid starts moving the output shaft slowly and accelarates it to match the speed. So, it can reduce wear and tear. In the same case, if we had a direct clutch, then maybe clutch slipping would have been at extremes and thus would have led to failure. Most special purpose vehicles like the excavator, road roller and even the old railway locomotives used fluid coupling. 

Sorry. MA = Load / Effort = Effort arm / load arm