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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / August 2006Engine brake in Automatic gearbox
Hello. In an Automatic gearbox, instead of a clutch, there's a torque converter, right? Now, I know that in manual gearbox, engine brake happens because the engine is 100% connected to the wheels - so engine revs down = car slows down. However, in automatic gearbox, there's the torque converter - which is generally two parts in which the engine part moves the wheel part, and it's done by oil pressure. The question is - how can the engine slow down the wheels - when there's no direct connection betw/ them, only an oil pressure one? thank you. > In an Automatic gearbox, instead of a clutch, there's a torque > converter, right? [quoted text clipped - 6 lines] > down the wheels - when there's no direct connection betw/ them, only an > oil pressure one? You might as well ask, how can an engine drive the transmission through the torque converter, since there's no direct mechanical connection between them? In other words, the torque converter works both ways. Well, it's not exactly the same, because you can, by increasing the pressure, increase the speed of the wheels, but how can you, by reducing the pressure, slow it down? won't it just "ignore" the change? for example (I know it's not the same) - in a bicycle - you can accelerate by the pedals, but you can't slow down by turning them slower... Mark Olson כתב: > > In an Automatic gearbox, instead of a clutch, there's a torque > > converter, right? [quoted text clipped - 12 lines] > > In other words, the torque converter works both ways. > Mark Olson ???: >> [quoted text clipped - 13 lines] >> >> In other words, the torque converter works both ways. > Well, it's not exactly the same, because you can, by increasing the > pressure, increase the speed of the wheels, but how can you, by > reducing the pressure, slow it down? won't it just "ignore" the change? > for example (I know it's not the same) - in a bicycle - you can > accelerate by the pedals, but you can't slow down by turning them > slower... No offense intended, but the problem is that you have a mistaken idea about how a torque converter actually works. Perhaps this will help. http://auto.howstuffworks.com/torque-converter.htm arthurdent44@gmail.com wrote: > Well, it's not exactly the same, because you can, by increasing the > pressure, increase the speed of the wheels, but how can you, by > reducing the pressure, slow it down? won't it just "ignore" the change? > for example (I know it's not the same) - in a bicycle - you can > accelerate by the pedals, but you can't slow down by turning them > slower... Right. That is what the torque converter does. You can think of the torque converter as a thing that slips... the speed of the input and the output don't have to be the same. Unlike a slipping belt, though, it transmits the full amount of power across. It's not like a direct chain connection like you might have in a bicycle. If you slow the input down, the output doesn't necessarily follow in speed, but if you reduce the input POWER, the output power is reduced. I know this is a weird idea to wrap your head around. --scott  Signature"C'est un Nagra. C'est suisse, et tres, tres precis." > Well, it's not exactly the same, because you can, by increasing the > pressure, increase the speed of the wheels, but how can you, by > reducing the pressure, slow it down? won't it just "ignore" the > change? for example (I know it's not the same) - in a bicycle - you > can accelerate by the pedals, but you can't slow down by turning them > slower... You can on a track bike. Track bikes have no freewheeling, and are thus more analogous to a torque converter. SignatureTeGGeR® > Well, it's not exactly the same, because you can, by increasing the > pressure, increase the speed of the wheels, but how can you, by > reducing the pressure, slow it down? I think you misunderstand how a torque convertor works. The engine drives an impeller that drives fluid through a turbine that is attached to the input shaft of the transmission. The moving fluid (not "pressure" per se) drives the transmission. Under engine braking, the roles reverse and the driven turbine now acts as the impeller. It is less efficient working "backward" this way, but it works and provides engine braking just fine. > > In an Automatic gearbox, instead of a clutch, there's a torque > > converter, right? [quoted text clipped - 12 lines] > > In other words, the torque converter works both ways. Answered the question with a question that was a great answer. Good job. You are correct a bicycle is not the same as a torque converter. Think back to your tricycle for a more accurate illustration. Torque converters do not continue to slip at higher RPM as they are so designed to slip at low RPM. Hydraulic pressure locks the converter input to output when it turns fast enough. This lock-up point is called stall speed. Think about centrifugal force when you put water in a bucket and crank it around in a circle with your arm. The real high hydraulic pressure in a torque converter is generated by centrifugal force of the fluid being driven to the outward edges of the turbine fins inside of the converter by the same principle as the bucket of water rather than oil pump pressure. As torque converter RPM increases, hydraulic pressure increases. Torque converters are still hydraulically locked by the rpm of the trans being driven by the wheels as the car slows down. For the past 30 years most automatic transmissions have an electronically controlled Torque Converter Clutch to take advantage by eliminating slip under desirable conditions as the torque converter RPM drops below the stall speed RPM. SignatureJustSayGo http://www.automotiveforums.com thank you all for the answers |
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