Auto operating basics? (68RFE)

[Wolfy]

Ignorance confession here



My understanding of auto-transmissions might fill a thimble halfway.



Rotating, sliding gears in a manual, and clutch-plate slipping-drive, I can visualize and understand.



But I am nearly clueless as to what is going on in an auto ("Lock-up"?, etc. ).



Using simplistic examples defined this way:



[A "fan" is a power-driven wind-blowing machine,... and a "windmill" is a wind-blown power converting machine],



- and considering air to be a fluid in the same sense as auto-trans fluid,



... my imagined basic principle is this: An auto trans is an engine-driven fan blowing on a windmill connected to the drive shaft.



Can anyone offer a "for dummies" explanation of the basic workings (and correct my simple example, if required)?



Thanks for whatever education is offered,

 

[Jeff_K]

Howstuffworks "How Automatic Transmissions Work"

 

[DGStrate]

From Dave Goerend:

"Here is some information on our transmissions, converters, and valve bodies and how they work. It is probably more info than you need, but better more than not enough. If you have any questions, or if I failed to answer something, just let me know.

To make these transmissions stand up to the torque of the Cummins Diesel engine will depend on a few things. The first question has to be: what is the truck used for? A truck that is used for a daily driver only, and with an engine that has not had the power turned up, will not need the same transmission as one built to tow with gross weights of 20,000 pounds plus and 800 lbs ft. torque with 450 horse power or a full race trans. A brief description of our different transmissions follows:

We like to start with a low stall triple disc converter that will keep the engine in its torque range of about 1700-2200 rpm, or put another way, the converter will have the engine working at about 500-800 rpm lower than a stock converter. The stock converter usually lets the engine rev to 2300-2800 rpm, this is past its peak power rpm. On the 24 valve engines and the Common Rail this torque range will be higher.

To explain how a torque converter works, let's start with 2 wall fans facing each other, (a drive fan and a driven fan). If we turn one fan on, the wind from this fan will make the other fan turn. In the case of a torque converter, the drive fan is bolted to the engine and the driven fan is connected to the input shaft of the transmission. When we are at a stop sign, with the trans in gear and the engine at idle, the drive fan is spinning so slow that is will not “blow” enough oil at the driven fan to make it turn. As we increase engine and drive fan rpm, it blows more oil at the driven fan and the driven fan starts to turn and moves the vehicle.

The drive fan ALWAYS goes a little faster than the driven fan, just like the wall fans. If you would stick a feather or straw into the driven fan blades it would slow the driven fan down but not the drive fan. This would be just like pulling a heavier trailer. (The straw in the driven fan would be just like adding a load. A heavier trailer is more load. )

When we get up to speed, we have a lock up clutch that will “lock” the fans together. (It actually locks the driven fan to the front cover, which is bolted to the engine. The driven fan is now “locked” to the engine. )

When the drive and driven fan are not locked together, heat is generated in the converter. The greater the load and rpm difference, the greater the heat generated.

Lets say, we have a converter that the drive fan (impeller) is going 2500 rpm and the driven fan (turbine) is going 1800 rpm, (efficiency would be 72%, 1800 divided by 2500 this efficiency is constantly varying depending on the rpm of the converter, the power input to the converter and the output load, or towed weight).

When the converter clutch locks the fans together, the engine rpm will drop 700 rpm. If we use a converter that is more efficient, like a “low stall” converter, say 88%, the impeller would be going 2500 rpm and the turbine 2200 rpm, then when the converter clutch locks the turbine to the front cover, the rpm will drop only 300 rpm. This is much easier on the converter clutch lining and will reduce glazing of the clutch lining, and because the fluid coupling of the converter is more efficient it will deliver more power to the wheels before the converter locks up.

To explain the “stall speed”, let's start with true full stall. If we put the Trans in drive and hold the brakes so the vehicle will not move, and then hold the throttle at wide open, the torque converter will “stall” the engine at a certain rpm, the engine will not be able to spin any faster unless the vehicle is allowed to move. This is true full stall.

The next stall speed can be called “break away” stall speed. Let's imagine we are stopped on a hill and rather than hold the vehicle stationary with the brakes, we do it by giving it enough throttle so we don't move forward up the hill OR backwards down the hill. Let's say, the engine rpm required to “hold” the truck there is 1100 rpm. If we increase the rpm to 1125 and the truck starts to move up the hill, the “break away” stall speed was 1125 rpm.

“Flash stall” is when you floor the throttle from a standing start… The engine starts to accelerate quickly, then the engine rpm “pauses” and starts to pull the truck. Let's say the engine gets from idle to 1500 rpm in 1. 5 seconds when you floor it, then it takes another 2 or 3 seconds to get from 1500 to 1700 rpm, this would mean the “flash stall” was 1500 rpm. When we lower the stall we want to lower the break away speed and the flash stall speed. This makes the engine work at a lower rpm for a given road speed and in most cases, will increase fuel mileage because the converter is more efficient.

Once we are up to speed, the computer will command the lock up clutch “on”, and the driven fan will lock to the front cover of the converter. Now the drive and driven fan will be going the same speed as the engine and all the engine power will be delivered to the Trans and back to the wheels.

In a stock torque converter, the clutch has 1 clutch plate with about 37 square inches of clutch lining. We like to use 3 clutch surfaces that total about 105 square inches of lining and call this a triple disc converter. This triple disc will hold much more torque than a single disc can. "

 

[n7gxz]

Ignorance confession here



My understanding of auto-transmissions might fill a thimble halfway.



Rotating, sliding gears in a manual, and clutch-plate slipping-drive, I can visualize and understand.



But I am nearly clueless as to what is going on in an auto ("Lock-up"?, etc. ).



Using simplistic examples defined this way:



[A "fan" is a power-driven wind-blowing machine,... and a "windmill" is a wind-blown power converting machine],



- and considering air to be a fluid in the same sense as auto-trans fluid,



... my imagined basic principle is this: An auto trans is an engine-driven fan blowing on a windmill connected to the drive shaft.



Can anyone offer a "for dummies" explanation of the basic workings (and correct my simple example, if required)?



Thanks for whatever education is offered,

How this for a simple explanation.



Set your way back machine for the 1910 time frame and the little car called the “Tin Lizzie” or if you wish the Ford Model T. The simple transmission that was used in them has two forward gears and one reverse. The gears where of a planetary type just like that used in an automatic transmission. There was no clutch or torque converter; gears where selected by either pushing on a peddle on the floor or pulling a lever on the side of the car. The pulling on the lever or pushing on the peddle is the manual method of engaging the gears which is replaced by the torque converter and valve body pushing oil against servos in the automatic transmission.



The torque converter is kind of like a food processor or blender with and extra set of blades in it. If you take a blender and fill it up with water and turn it on the water starts to spin with the blades in the bottom. Now if you put a second set of blades in the top that sit down into the water but do not touch the bottom blades the top ones will start to rotate also.



This all make since to you?



Kevin

 

[Wolfy]

Ignorance confession here



My understanding of auto-transmissions might fill a thimble halfway.



--------------------------------------------------------------------



Can anyone offer a "for dummies" explanation of the basic workings (and correct my simple example, if required)?



Thanks for whatever education is offered,

Thanks Guys



I appreciate the link and the time and thought that went into these replies.



Good stuff!



I'll keep studyin'.