Perils of the start stop system

Today after a long time i am posting on this blog. The reason is simple...I just noticed that the number of visits to the technical blog have been far more compared to the other "time pass" blog that i write even though the technical one has had just  30 odd blog posts. I thought, let me rekindle the technical fire and write a few things about software, automotive and sorts. 

I also thought that it would be quite a joke if i re-write what is already written in numerous websites and re-iterate the same story again. However, i would still do it if i think i can explain things in a simpler or shorter way. In the meanwhile, for those of who who do not follow my other "timepass" blog, i just moved to Germany, the land of innovation. This would also give me with some more additional time to do stuff which i would have missed because of lack of time and interest when i was in India. However, there are also quite a big bunch of new responsiblities that one has to take into consideration when he moves to a new country. This is not just doing all kinds of registrations and getting your papers in order but also adjusting to the culture and lifestyle of the place. 

Today, i would be talking a bit about engine start-stop or MSS as Merc's would call it. In India most of us would have experienced that the bus drivers and the auto-rickshaw drivers turn of the engine at the red traffic lights when they have to wait. Normally this is done in order to save fuel and get better milage figures. In Europe most premium cars do offer start-stop as a default fitting. At BMW all cars come with an automatic start stop. How does this work? 

In a manual transmission you would stop the car, switch to neutral and brake. This would be then used by the software as the trigger to cut off the engine. In an auto transmission though you would have to keep the brake depressed while the transmission is still set to "D" position. Although to the user this would seem quite a simple function because even an uneducated indian autodriver can do this, for the software it is quite a challenge. 
The challenge poses itself as two questions:
1. When should the engine be cut off ?
2. When should the engine be started again ? 

The first question translates to a list of sub questions the answers of which are evaluated by the software to decide if the engine can really be switched off. Some of them are here...you could think of some more and leave them in the comments. 

• In an automatic, is the brake depressed above a threshold? 
• Is the gear still in "D" when the driver stopped the car. i.e. He stopped it when it was running. 
• Is the door closed? . If the door is opened, mostly the driver wants to get out and in that case it is better to turn on the engine and let him know that he should safely lock the vehicle before he exits. 
• Is the engine too warm or too cold? In which case it would either need the coolant pump to cool the engine and if too cold would need to run to reach operational temperature. Especially with turbocharged engines, it would still make sense to run the oil pump a couple of more seconds to ensure that the bearings are still lubricated as the turbo charger speed drops down. Note that unlike the engine, the turbocharger may quickly reach rpm's in 30,000 and above range. Some of the high end turbo's may be spinning at 60,000 and above at higher loads.

The second question is not just the opposite of the first one though in some cases it is . Again, this translates into its set of sub questions. 

• Has the passenger compartment temperature dropped too low or is it too high, in which case the air conditioning might need to be turned on which would need the engine to run in some cases. 
• Is the brake booster pressure too low, in which case the engine needs to be turned on to generate the pressure. 
• Has the driver changed the gear lever position from "D" to say " R". In which case the engine needs to be started to change the solenoid positions in the gearbox. 
• Has the brake been released? 
• Is there a demand by the battery with a charging request.

In the end what looks like a very simple functionality to the driver and most likely for which he isn't going to pay a lot...the software really has to work a lot to be able to give the right kind of user experience. You wouldn't want the car to start when you honk for example and at the same time, you would want the car to start as soon as possible when you step on the gas after the red goes orange. 

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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