Auto Transmission - I

With the advent of technology man has always asked for better systems to lower his efforts in doing things. The same story goes for Transmissions. Manual transmission served the purpose very well. However, soon the customer demands became higher and he wanted a system that could change gears on its own. No one wanted the clutch anymore!! Well atleast not the folks who always tended to over the engine by downshifting too early or lead the vehicle to stall by upshifting too early. This lead designers to come up with a system in which the clutch pedal could be totally eliminated. There were lot of solutions and some have been described here. Unfortunately, i dont know the order of their evolution but guess that that is not really all that interesting...
Lets take the case of the simple Hero puch or the TVS 50.....Can we call it an automatic transmission....?
Not really...however to some extent yes.
How does based on throttle the vehicle start moving?
This is because of a clutch that engages as the RPM increases. Such a clutch is called Centrifugal Clutch. Normally the clutch is disengaged. Which means that no power from the engine will be fed to the chain drive. However as the RPM increases a certain threshold the clutch engages. With the clutch engaging the power is transferred to the wheels. This is a very basic of how we can eliminate the manual clutch ( our initial aim ....). As it is clear that the centrifugal clutch will wear out with time and that means RPM points at which the power will get transferred to you chain drive will vary with time. The second disadvantage is that this method cannot transfer huge amount of torque from the engine shaft to the output shaft. I am not really using the term Transmission shaft because this is not a "Transmisison" in classical sense.
What are the other methods?
How about Kinetic Honda that has been ruling the roads since i was a kid?
Well a Kinetic Honda uses a different mechanism. It uses what is known as a belt drive...( atleast that is what i was told. I am not 100% sure!!). This is fed to a Stepped pulley or Coned Pulley. This coned pulley will be able to provided different Pulley Ratios based on which point of the pulley is used to Tap the output. A the most broad end of the pulley the Torque transmission will be less and a the thinnest end of the pulley the Torque transmission maximum. Centrifugal clutch is still used here.
Does that mean CVT ( The stuff we talk ed about...Continuous Variable Transmission) is a cheap and dirty technology?
No absolutely not. Like any other technology based on how much complexity is added one can make the system very sophisticated. The commonly used CVT's in bikes like Honda Activa, Honda Dio, Kinetec etc are fairly simple. However, some really complicated stuff from Audi can be found here and remember the core is still same....a coned pulley...

After taking care of these two aspects...i.e. some kind of autoclutch and a automatically shifting gearbox the other biggest challenge was to ensure that high amounts of torques are transmitted through the shafts. When you talk of cars like Mercedes, BMW's and high end Maybach's Torque can touch much more than 100 Nm's. 12 cylinders V's could not be "Auto-Transmitted" using a simple or complicated CVT...

Something new was needed...and that will be apart of the next blog ofcourse :P

Tansmissions & Gears

Well long break i would say before i wrote something on my this blog....the other one still gets my attention at times. However, the reason for that was lack of time and also lack of interest i could garner from my readers. However, looks like i want to try again and give it one more shot to make this blog useful enough to folks in order to get some interest from them.
The topic for today is "Transmissions"
Why do we need a transmission...or do we need one?
Frankly we would not need one, we could simply control the speed of a motor the same way we control the speed in our ceiling fan. However, unfortunately our automobiles use motors which are not electricity powered. They are IC's or Internal combustion engines which run most efficiently in a fixed narrow range of RPM. They are not able to produce the same amount of torque at all the RPM's and of course are not efficient at all RPM ranges. At very low RPM the engines suffer from lower dynamic torque and hence we need to somehow use the same RPM to get the needed torque demands. At cruising speeds we require lesser torque as compared to when we are climbing a hill. All these means that the IC engine has to meet various requirements of torque, speed and efficient operation. This can be achieved very easily using a Transmission. A transmission ensure that the Engine can be run in a narrow band of RPM where it runs most efficiently and uses gears to meet the driver request of torque or speed.
How does it do it?
A simple transmission "conceptually" is a set of gears with different gear ratio's. If the engine is running at x RPM using a gear ration you can get y RPM at you transmission shaft based on the following formula:
X * GR = Y
GR = Gear Ratio
X =   Engine RPM
Y =   Transmission Shaft RPM
However, this is an ideal case....losses will come into picture like in any system.
if GR>1 then Y is greater than X. This is rarely done since this means that Transmission shaft RPM is higher than engine RPM. This basically means that your car is completely free. This condition is called Overdrive and mostly seen in higher end car with ECU's and TCU's which keep monitoring if everything is still under control.
What are gears and gear ratio's?
A picture speaks more than words...so here is the picture

Ever wondered why the car or bike will not climb a steep incline when we are on a higher gear as compared to a lower gear? At a higher gear the transmission transfers most of the engine speed to the tires as compared to the torque. If we consider power delivered by engine constant then either we can have higher torque or angular velocity. Hence at the higher gear ( more angular velocity) we need to really rev the engine hard to make the vehicle climb ( more revs more power output) in order to meet the higher torque requirement.

In coming blogs it will try to explain what different transmission can do and how they do it. But basics are these...
1. For a constant power by the engine i.e. constant RPM on engine shaft we should be able to get different torques and output speeds.
2. Gears are one of the mechanism's for doing what we need in point (1). There are other methods like use of pulleys, belts and electro mechanical couplings.