How an automatic transmission works

[Linctex]

Originally Posted by Kestas
My dad made a career of designing automatic transmissions for the automotive industry. They were once tasked to build transmissions for racing. He said they could not design something to outperform existing manual transmissions and the drivers who use them.


That had to have been a Long, Long, LONG time ago.

I had a full manual C4 in my Mustang drag car. I guarantee no one on earth can shift faster than it can, and the torque converter I was using is better (made the car faster) than any clutch I could have ever purchased.

 

[Kestas]

Originally Posted by Linctex
I remember back in the 70's and 80's... everybody with a fast car desired a 4-speed more than anything.

Maybe so with the fast car crowd. But I also remember a lot of this was due to the high price of gas. The big V8 cars with slushbox automatics were averaging 12-15 mpg. People were looking for something more like the europeans and japanese were driving, a 4-cyl with 4-spd. That's when I got my '82 Omni and started experiencing 30 mpg.

I also remember the EPA and auto manufacturers were going nuts trying to deal with the shot of rich fuel that went through the engine at every shift.

 

[CR94]

Originally Posted by Linctex
I remember back in the 70's and 80's... everybody with a fast car desired a 4-speed more than anything. ...
I remember 4-speed manuals were obsolescent by the early 80s, even on slow cars.

 

[JLTD]

Originally Posted by CR94
Originally Posted by Linctex
I remember back in the 70's and 80's... everybody with a fast car desired a 4-speed more than anything. ...
I remember 4-speed manuals were obsolescent by the early 80s, even on slow cars.


Yup; 5-speed w/overdrive was coming into the picture. Although I did have a 1977 Plymouth Volare station wagon with four on the floor....

4th was overdrive.

 

[Kestas]

I remember when the overdrive craze hit in the late 70s. People wanted it because they could remember the feature in the 50s, where it would save a significant amount of gas. Ford took their production 4-speed, and modified it so the 3rd and 4th gears were reversed, with the 3rd gear in direct drive. They then changed the 4th gear ratio so it would be overdrive. The differential was changed so the net output in top gear was the same as the old transmission with the same acceleration. But now they could market the transmission as having overdrive!

A wonderful example of hollow engineering.

 

[supton]

Originally Posted by Kestas
I remember when the overdrive craze hit in the late 70s. People wanted it because they could remember the feature in the 50s, where it would save a significant amount of gas. Ford took their production 4-speed, and modified it so the 3rd and 4th gears were reversed, with the 3rd gear in direct drive. They then changed the 4th gear ratio so it would be overdrive. The differential was changed so the net output in top gear was the same as the old transmission with the same acceleration. But now they could market the transmission as having overdrive!

A wonderful example of hollow engineering.

On the differential, I wonder if they had been making the pinion huge and thus driving the ring very thin. Bumping back out to what the original design was for might have been a good move.

But yeah, if the overall final drive ratio was the same... Then again, people today gripe about their automatic transmission downshifts on hills--imagine the griping if they had to manually shift!

Probably more lossy than the non-overdrive setup too.

 

[john_pifer]

Originally Posted by RDY4WAR
I made a version of that a while back...

A torque converter does something that a clutch simply can't and that's multiply torque.

The NHRA Pro Stock cars will always have a special place with me though.



I've read that about torque converters. But, if that's the case, why don't vehicles with automatic transmissions show more torque at the wheels at a given RPM on the dyno?

 

[eyeofthetiger]

Originally Posted by john_pifer
Originally Posted by RDY4WAR
I made a version of that a while back...

A torque converter does something that a clutch simply can't and that's multiply torque.

The NHRA Pro Stock cars will always have a special place with me though.



I've read that about torque converters. But, if that's the case, why don't vehicles with automatic transmissions show more torque at the wheels at a given RPM on the dyno?


Maximum torque multiplication should occur right around the torque converter's stall speed, and that is typically under 2000 RPM.

 

[RDY4WAR]

The torque multiplication is why it's calles a torque converter and not a fluid coupler. The pressure of the flowing across turbine and stator creates more torque than the engine is supplying. Different converter sizes and stator designs generate different amounts of torque. Like mentioned above, the highest torque multiplication occurs at stall speed with the vehicle sitting still as that's when the pressure (resistance) in the converter is highest. As the vehicle starts to move, that multiplication starts to diminish. By ~80-90% coupling (depending on the converter), that multiplication is gone completely and you just have fluid coupling.

Most OEM converters are ~1.8:1 ratio multiplication. Racing converters can be 3.0:1 or higher. The 9.5" 4000 stall converter in my Camaro is ~2.5:1.

 

[A_Harman]

Originally Posted by JohnG
How does a Dual Clutch automatic transmission work?

Probably like the regular automatic, but 2 seconds faster in the end!


It's basically 2 manual transmissions in one assembly with concentric fluid clutches. One transmission contains the odd-number gears, and the other contains the even-number gears. When an odd gear is engaged, the clutch for the even gears is disengaged, and vice-versa. Porsche pioneered this type of transmission for racing in the early 80's.

 

[A_Harman]

Originally Posted by skyactiv


I've said it before, and I'll say it again: Drag racing is abusive only if it's done right.

 

[RDY4WAR]

I love drag racing, no matter what is involved. From kids in Jr Dragsters to casual street cars to Pro Stock and Top Fuel. There's no denying though that it is brutal on every part of the car. The transmission is especially abused. This is especially true for automatics with transbrakes. It's not uncommon, when on the transbrake, to see trans fluid temps climb as much as 60°F per second. Some of those guys sit on the transbrake for 5+ seconds. It's why some turbo cars run JD Hygard or compressor oil in their transmission for the higher flashpoint compared to ATF.

 

[Cdn17Sport6MT]

Originally Posted by sloinker
Dual Clutch automatic transmissions are actually manual transmissions electronically shifted through electromagnetic clutches. They are the fastest shifting transmissions available to consumers and used to be used in Formula 1. Not sure where that technology is nowadays but remember reading about sub 100 millisecond shifts.

Originally Posted by A_Harman
Originally Posted by JohnG
How does a Dual Clutch automatic transmission work?

Probably like the regular automatic, but 2 seconds faster in the end!


It's basically 2 manual transmissions in one assembly with concentric fluid clutches. One transmission contains the odd-number gears, and the other contains the even-number gears. When an odd gear is engaged, the clutch for the even gears is disengaged, and vice-versa. Porsche pioneered this type of transmission for racing in the early 80's.

Not fluid clutches...but rather wet clutches. Clutchplates are immersed in oil and cooled by oil. Fluid clutch is actually a hydraulically / fluid-dynamically linked clutch with always some slippage.

 

[Cdn17Sport6MT]

Originally Posted by sloinker
Dual Clutch automatic transmissions are actually manual transmissions electronically shifted through electromagnetic clutches. They are the fastest shifting transmissions available to consumers and used to be used in Formula 1. Not sure where that technology is nowadays but remember reading about sub 100 millisecond shifts.

I believe NEVER run in F1. Rather, automated-manual SINGLE clutch systems were. Ferrari F1 trans, Maserati CambioCorsa, BMW SMG all are single clutch automated manuals... in road cars. Dual clutch automated manuals are the fastest upshifting transmissions these days... albeit by smaller margins of time than beforehand (witness some Camaro and Corvette performance conventionsl automatics) - but they are sometimes actually slower downshifting than conventional automatics (e.g. if they have, by their internal logic, selected the wrong gear or "shift direction" from what the driver actually wants). Also, in the final analysis, dual clutch automated manuals cannot absorb as much engine torque as conventional performance automatics.

 

[A_Harman]

Originally Posted by Cdn17Sport6MT
Originally Posted by sloinker
Dual Clutch automatic transmissions are actually manual transmissions electronically shifted through electromagnetic clutches. They are the fastest shifting transmissions available to consumers and used to be used in Formula 1. Not sure where that technology is nowadays but remember reading about sub 100 millisecond shifts.

Originally Posted by A_Harman
Originally Posted by JohnG
How does a Dual Clutch automatic transmission work?

Probably like the regular automatic, but 2 seconds faster in the end!


It's basically 2 manual transmissions in one assembly with concentric fluid clutches. One transmission contains the odd-number gears, and the other contains the even-number gears. When an odd gear is engaged, the clutch for the even gears is disengaged, and vice-versa. Porsche pioneered this type of transmission for racing in the early 80's.

Not fluid clutches...but rather wet clutches. Clutchplates are immersed in oil and cooled by oil. Fluid clutch is actually a hydraulically / fluid-dynamically linked clutch with always some slippage.


Ooops. You're right. My fingers were flying faster than my brain was working, I was thinking wet clutch and typed fluid clutch.

 

[A_Harman]

Originally Posted by Cdn17Sport6MT
Originally Posted by sloinker
Dual Clutch automatic transmissions are actually manual transmissions electronically shifted through electromagnetic clutches. They are the fastest shifting transmissions available to consumers and used to be used in Formula 1. Not sure where that technology is nowadays but remember reading about sub 100 millisecond shifts.

I believe NEVER run in F1. Rather, automated-manual SINGLE clutch systems were. Ferrari F1 trans, Maserati CambioCorsa, BMW SMG all are single clutch automated manuals... in road cars. Dual clutch automated manuals are the fastest upshifting transmissions these days... albeit by smaller margins of time than beforehand (witness some Camaro and Corvette performance conventionsl automatics) - but they are sometimes actually slower downshifting than conventional automatics (e.g. if they have, by their internal logic, selected the wrong gear or "shift direction" from what the driver actually wants). Also, in the final analysis, dual clutch automated manuals cannot absorb as much engine torque as conventional performance automatics.


I once instructed a driving student that was running a BMW M3 (I think it was) with the SMG transmission. It had a major drawback in that every upshift resulted in a momentary decal/accel that caused my head to bob forward and backward. At first, it wasn't very noticeable, but it became annoying after about 5 laps. How long did the SMG survive in production? I hope not long, if BMW never got the head-bob situation worked out.

As GM puts more powerful engines in the C8 Corvette, we'll find out how much torque the dual-clutch gearboxes can take. I have heard about engines up to 850 HP to go in future ZR1's. 5.5L DOHC twin turbo, maybe 640 lb*ft of torque.

 

[Cdn17Sport6MT]

Yeah, I agree, I will be interested in seeing how much torque / HP they apply to the C8 Corvette's (undoubtedly wet) concentric clutches in its DCT. It was a recent BMW executive (or a Design Lead Executive) who said that in the future, torque capacity limitations would cause BMW to migrate totally away from DCT transmissions... to conventional torque converter based automatics... which per this fellow - are increasingly close to being equal to a DCT for (general) shifting speed. I say general, because a dual clutch transmission can sometimes be fooled into going into the wrong gear or in the wrong shift-direction... and then it fumbles about (a little). Porsche's ZF based PDK 'box has, to date, the most-advanced programming to make this a rare occasion (apparently).

One thing I should like to mention, here (and some of Y'All may roll your eyes... "Here he goes again..." - sorry) - is that dual clutch transmissions do NOT double-clutch (actually, like the Magnetti-Marelli hardware/software driven single-clutch automated manuals sometimes can - Ferrari F1, Maserati CambioCorsa and DuoSelect, and the BMW SMG boxes). What I mean, here, is these single-clutch automated manuals go into neutral (with clutch engaged) and rev the countershaft to the correct speed so they actually are (under those somewhat rare occasions) easy on the synchromesh. I say rare, 'cuz on the DuoSelect... when you command it to downshift (when the engine rpm is greater than 5000 rpm - crazy, that! - it will actually double-clutch down to save the synchro's. Re DCT's.... what this means when upshifting... is that, so what that they don't double-clutch up... Even though the shifts are lightening-fast {when swapping from even to odd gears (or vice-versa)}... actually, internally, when going up - the non-engaged gearset can lazily engage a gear... 'cuz it is, in fact, not needed yet. Easy-Peasy. But going down.... when in manual mode... and you have a "downshift artist" flipping paddles down at high speed 'cause he / she loves to hear that rev-matching / engine note... well, the non-double-clutching internal-shifting of the DCT box is down-right BRUTAL by way of speed and positive action. Now DCT boxes have double and even triple cone synchronizers.... But still,as time goes on, the synchro's do get knackered. That is why I do not like DCT's.

Having said all of the above, so far, empirical results with DCT's have not been that bad in terms of synchro life.

Time will tell, I suppose, ultimately how good and long-lived DCT's prove to be. We are getting to the high mileage realm of knowing pretty darned soon.