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Old 06-26-2008, 01:36 AM   #31
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How an A/T works

OK, I'm learning... Here's my own wording paraphasing "howstuffworks". Another good explanation is on FamilyCar

planetary gearset
Cutting a cross-section of the trany it's composed of:
- (S) sun gear in the center,
- (R) the ring outside
- (C) the planet carrier (see P)
- (P) the planet gears which are 4 smaller gears working together in lockstep, sandwiched between (S) and (R) and held together by (C)


Animation

Any of (S), (R), (P) can be engaged to another (or all engaged to each other), changing the gearing ratio, to provide reverse, reduction and overdrive. Depending on the mode, the planet carrier (C) will be stationary (planets gears can rotate on themselves but won't move around the sun) or will rotate too. I don't think it matters for the resulting gearing. It's a mechanical side effect.

To get a full gear set, two planetary gearsets are combined (compound planetary gearset). The design means that there's another set of planets of a different diameter. The planets are all working in lockstep, but now there's 2 gears on the sun of different diameters, contacting either the 1st or the 2nd set of planets. There I get lost.

Gearing gets engaged/released by 2 bands and 4 clutches.

Bands are metal brake bands that wrap around sections, actuated by hydrolic cylinders to lock that section.

Clutches are actuated by hydrolic pistons, and held off normally with a spring.

Park: there's a tooth actuated by a lever going out of the tranny that engages and locks the output shaft so it doesn't move.

Neat details:
- In "2" the tranny never down or upshifts.
- If dowshifting, the tranny goes down only if the car speed is low enough
- Upshifts will happen at lower speeds if you accelerate genlty vs hard.
- The tranny downshifts if you floor the gas
- In overdrive the tranny picks the best gear

Gear pump
Spins at the speed of the engine/ torque converter housing, and forces oil from a sump to the hydrolic system.

The governor
A valve connected to the output shaft. Faster rotation the more the valve opens, the higher the fluid pressure in it.

Throttle
Either through a throttle valve has increased pressure as the pedal is depressed, or a vacuum modulator detects the engine load from the manifold pressure.

manual valve
Hooked to the shifter, it inhibits selected gears.

shift valves
pressurises the clutches and bands, each valve detects when to shift from one gear to the next depending on governor and throttle valves pressure.

Electronic tranny
Shifting is still done with hydrolics, but electric solenoids are used instead of valves.

Note

"Most transmissions will upshift to third gear, or even overdrive, when you take your foot off the gas".

Torque converter

IT sits between the engine and the tranny, and contains
a Pump, a Turbine, a Stator and Transmission fluid.

The centrifugal pump is actually the whole housing with built-in fins, which is bolted to the flywheel and rotates at the same speed of the engine. As the flywheel turns, the fins scoop the oil and pushes it.

The turbine faces the pump, and is linked to the tranny shaft, the
oil is pushed against its fins which makes the turbine and tranny shaft rotate.

the tranny fluid moves from the inside out and outside in as the pump pushes it in, and it escapes through the turbine outwards.
The stator lies in between to optimize the flow of fluids.



A torque converter can multiply 2x to 3x the engine torque when accelerating from a stop, because the engine turns much faster.
On the downside, the tranny shaft will always be a little slower than the engine, yielding lower MPG than a MT, because the trany fluid is moving. A lockup clutch is used in modern cars to avoid the problem.

Wear and tear from P&G, going N to D

Now that I took that crash course, let's see if I understand what happens...

From: Familycar course

"The clutch pack is used, in this instance, to lock the planet carrier with the sun gear forcing both to turn at the same speed. If both the clutch pack and the band were released, the system would be in neutral."

So my take away here is going to "N" to "D" is a simple matter of engaging some clutches and brake bands.
One one side you have the torque converter axle and on the other the driveshaft axle.
When in "N" coasting, the driveshaft rotates fast, and the torque converter rotates at idle RPMs.
When engaging to "D", the torque converter shaft is jolted suddenly from 800RPM to say 3000RPMs.
The torque converter in that case is counter productive because it will oppose a force, increasing the stress on the torque converter shaft.
Once in gear, the torque shaft turns at 3000RPMs, and the torque converter handles the decoupling with the engine flywheel which before that was rotating the torque converter shaft.

So I see that you could prematurely wear the tranny's brake bands and clutches that way, because the "N" to "D" shift is not really protected by the torque converter so those take the force by slipping... Unless indeed you rev-match the engine before engaging to "D".

So would those be designed to handle that kind of beating?

Maybe yes? I expect they take that beating when shifting between gears, unless the shifts happen in a way that when we go from one lockdown to another, the parts that lock together are close to sync'ed when that happens, even when you kick the pedal.

Note: The gears are not impacted because they never disengage from each other. They either get locked together to stop their relative movement or not. So I would not expect a sudden breakage there.

Note2: Going D to N does not seem to be a problem then, as you release all gearing so they just desynchronize smoothly with the planetary gears just moving freely to adjust for the variations.
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Old 06-26-2008, 04:41 AM   #32
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Here are some things that you (sonyhome) have stated as problems that don't actually have any effect for my vehicle:

1. Hold the decel button and letting the cruise re-engage actually causes a large jump in rpms, sometimes such that it cause jolts. Therefore, I don't let it re-engage; I set my speed with my foot much more gently and then reset the cruise.

2. I never worry about P or R. I have to press the button to go beyond N and 3. So, that means N, D, and 3 are available without pressing the button. It saves me from accidentally shifting fast and being in a gear I don't want: I have to actively think about it.

3. Haha! shifting gears is slow and inpractical? What do you think a manual tranny is? Only dual clutch autos can shift faster than a human, standard slushboxes are slower. No offense meant of course.

4. Shifting from N to D while coasting, to exceed 3000rpms I would need to be traveling at excess of 95mph or stuff it in 3, which puts it in 4th no lockup (knowing these details about your auto is important). Usually when returning to my speed, I match rpms to the speed. I've had much practice to know what rpms = what speed to avoid potential damage.

So a lot of the problems of Neutral coasting in an auto are negated by knowing your vehicle and how the tranny and engine interact. I'm sure I'm causing premature wear to the tranny, and it will likely fail before the motor does. How much sooner than a typical failure? No way to tell, as these trannys have lasted over 200K miles without major service, only fluid/filter changes.

I have also taken 1st, 2nd, and 3rd (4th hits the limiter at 112mph first)to the redline of the motor, to know where the tranny shifts given the physical variables of my car. Bad for it? sure. The knowledge gained about how it works? Worth it.
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Old 06-26-2008, 06:10 AM   #33
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Here's some details about how the "gold standard"* ( ) in hydraulic automatic transmissions engages gears ...

http://www.allpar.com/mopar/transmis...-tom-hand.html

(*Seriously though, it was much copied and many trannys around today are almost straight cribs off the general design, with an extra gear set cobbled on and a solenoid pack instead of the valve body.)
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Old 06-26-2008, 07:07 AM   #34
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Quote:
Originally Posted by sonyhome View Post
- Shifting gears is awkward, and maybe even dangerous (P and R are a sort distance away)
The lockout (button on a console shifter, or pulling back on a column shifter) should prevent this accident. In my GMC, I end up pulling past the lockout on every shift anyway, but it requires a lot of force to go between any selection so once I get out of D there's not a strong chance I'll even knock at R's door.

Quote:
- Shifting gears annoys passengers not used to this (I went into an argument about that - pfff), the shift clunks are noisy and cause the driver to make a lot of movement.
Lots of hypermiling practices are annoying for passengers. I try to be extra-smooth with passengers, but sometimes they just have to deal with it...at $4/gallon, they can damn well deal with it!

Quote:
- Shifting gears is slow and impractical because the levers are not designed for doing that constantly.
With a console shifter, if the lever is in a decent position, D->N and N->D should be a tiny nudge away. Waiting for the transmission to shift can get annoying, though.

Quote:
BTW, friends that are car "savvy" but not hypermiling savvy claim that I'll destroy my tranny shifting back and forth.
I think that's just FUD. They've never tried it and don't know. I've been shifting my auto trannies around for hundreds of thousands of mile. I had a friend with a rusted out '82 Mustang who did the hardest, jerkiest shifts he could, downshifting for engine braking every time he stopped (because the brakes on that car were TERRIBLE); I was sure it would blow the tranny in no time, but that car lasted forever.

They wouldn't put all that functionality there if it was going to make stuff blow up under moderate conditions. Don't forget, hypermilers aren't pushing huge amounts of power or using high RPMs. We are moving slow and operating (usually) gently, using little power. Some kinds of stuff that would break it under drag racing conditions will not even wear it under our conditions.

Quote:
Originally Posted by sonyhome View Post
- In "2" the tranny never down or upshifts.
This behavior varies greatly from one car to another. Some are obedient, some don't care what you've asked for. Most, I think, will use 2 except under extreme conditions, such as bouncing against the rev limiter.

Quote:
- If dowshifting, the tranny goes down only if the car speed is low enough
This also differs from vehicle to vehicle. I wouldn't depend on it.

Quote:
- Upshifts will happen at lower speeds if you accelerate genlty vs hard.
- The tranny downshifts if you floor the gas
Those are very universal, true of all automatic (and new automated manual / sequential manual / dual clutch) trannies.

Quote:
- In overdrive the tranny picks the best gear
That depends on your priorities. It rarely picks the gear I'd prefer, but it almost always picks an acceptable gear.

[qote]Gear pump
Spins at the speed of the engine/ torque converter housing, and forces oil from a sump to the hydrolic system.[/quote]

Very important! This is how the transmission is kept cool, as it circulates the oil to a cooler in or near the radiator. It is why EOC is not acceptable for most automatic transmissions.

Quote:
"Most transmissions will upshift to third gear, or even overdrive, when you take your foot off the gas".
Yes, but unless you're going slow, it will probably already be in 3rd or 4th gear. This, combined with the torque converter, may be why some people think they're automatically shifting to neutral.

Quote:
Torque converter
That is an in-depth explanation. If you want a simplified one, imagine two window fans pointing at eachother. One is turned on, blowing at the other; you'll see the other spin as a result (just like when the fan is in the window, turned off, and a gust of wind comes through). Modern TCs can lock, which means that both sides get physically connected as if it was just a shaft.

The torque converter acts like a big pillow for all those jarring shocks that you might fear during shifts. It easily absorbs them, preventing the transmission from getting jerked around.

Quote:
When engaging to "D", the torque converter shaft is jolted suddenly from 800RPM to say 3000RPMs.
I suppose there could be some cars where it would hit 3000rpm. That's 70mph in my manual tranny VW, and probably about 100mph in my auto tranny GMC which is 2000rpm at 70mph.

The TC is pretty robust, and the concern tends to be about the bands/clutches in the tranny. They don't get a hard jolt, though, because they don't have to accelerate the engine right away; instead, they only have to accelerate the TC, which allows that to happen easily. By the time the engine is pushing back, the bands/clutches are fully engaged and the TC does all the work, which is exactly the sort of work it's designed to do.

Quote:
The torque converter in that case is counter productive because it will oppose a force, increasing the stress on the torque converter shaft.
No, the engine crankshaft is what's opposing the transmission. The TC cushions that issue as described above.

Quote:
Once in gear, the torque shaft turns at 3000RPMs, and the torque converter handles the decoupling with the engine flywheel which before that was rotating the torque converter shaft.
It doesn't decouple really; it just lets one side spin faster than the other. Like I said, imagine the fans; the engine is attached to the fan that's off and the transmission is attached to the fan that's on. The TC doesn't lockup when the transmission is in N, so it's not physically connected, only through the oil.

Quote:
So I see that you could prematurely wear the tranny's brake bands and clutches that way, because the "N" to "D" shift is not really protected by the torque converter so those take the force by slipping... Unless indeed you rev-match the engine before engaging to "D".
Yes, theoretically you could. I can't say that I have enough P&G miles to say for sure but I am confident that you WON'T prematurely wear that stuff, especially if you rev-match (which I do). Rev-matching is actually extremely easy here, since you don't need to be accurate at all, you have lots of time, and you've got the torque converter to cushion it.

My GMC takes ~1 second to execute my N->D request after I shift it. So, I shift, then get on the gas a little, and watch as the tach jumps up to exactly the spot it needs to be in, and that's where it stays.

Quote:
Maybe yes? I expect they take that beating when shifting between gears, unless the shifts happen in a way that when we go from one lockdown to another, the parts that lock together are close to sync'ed when that happens, even when you kick the pedal.
It is, indeed, designed to handle high-rpm high-power shifts, that's what the TC and all those bands and clutches are for. You shove the pedal down to the floor to dive out onto that busy highway, your engine produces massive amounts of power, and you NEVER interrupt the power while the tranny does its job. It must use those bands and clutches to shift while under FULL power.
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Old 06-26-2008, 07:08 AM   #35
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Quote:
Originally Posted by almightybmw View Post
3. Haha! shifting gears is slow and inpractical? What do you think a manual tranny is? Only dual clutch autos can shift faster than a human, standard slushboxes are slower. No offense meant of course.
Shifting per the user's request is very slow on most automatics. Just ask anyone who thought paddle shifters were going to respond immediately. Most automatics take a second or two to decide if you were serious about your choice. N->D should be even slower due to the usually larger difference in RPM.
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Old 06-26-2008, 07:34 AM   #36
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Quote:
Originally Posted by theholycow View Post
Shifting per the user's request is very slow on most automatics. Just ask anyone who thought paddle shifters were going to respond immediately. Most automatics take a second or two to decide if you were serious about your choice. N->D should be even slower due to the usually larger difference in RPM.
Well yes, like the newer grand am (before it was G6) when it got the paddles, it was neat, but no faster than just using the stick on the floor. What I was referring to is in high performance vehicles. Look up the Audi R8. It can shift in a tenth of a second use 2 clutches. It's actually really neat because both gears are engaged for a split second, providing seamless power. Technically its called a clutchless manual, as you have the 6 speed like a regular manual, but no clutch pedal. But yeah, 85% of slushboxes aren't that fast, 1 second to shift into gear from N is how mine is too. I say 85% because Audi/VW Tiptronic is actually pretty fast at shifting manual style. I just haven't seen a domestic with the same speed in an auto.
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Old 06-26-2008, 07:48 AM   #37
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Yeah, an automated manual is a very different beast. The dual clutch ones, like VW's DSG, supposedly can execute a planned upshift in as little as 8ms, though I think they can still lag quite a bit when an unexpected request comes in from the driver. To tell you the truth, with paddles, I'd WANT it to lag so I have time to choose a gear before it executes...else it has to shift twice just to go from 5th to 3rd.
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Old 06-26-2008, 07:48 AM   #38
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Quote:
Originally Posted by theholycow View Post
I think that's just FUD. They've never tried it and don't know. I've been shifting my auto trannies around for hundreds of thousands of mile. I had a friend with a rusted out '82 Mustang who did the hardest, jerkiest shifts he could, downshifting for engine braking every time he stopped (because the brakes on that car were TERRIBLE); I was sure it would blow the tranny in no time, but that car lasted forever.
Gawd yeah, I put a shift kit* in my tranny and I am continually getting advice from various relatives, friends, one time passengers, that maybe I should take it into a tranny shop and get it looked at, or that I should scrap it because the tranny is on it's way out.... AND 90% of these people probably can't even find the oil filler!!!!

(*Yeah it's "firm", if I floor the gas it will chirp tires going into second, and sometimes going into 3rd if my foot twitches)
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Old 06-26-2008, 08:11 AM   #39
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Quote:
Originally Posted by RoadWarrior View Post
(*Yeah it's "firm", if I floor the gas it will chirp tires going into second, and sometimes going into 3rd if my foot twitches)
One thing is pretty universal: Nearly ANYONE who is well studied insists that firmer shifts (due to shift kit, servo upgrade, increased line pressure, or ECU flash) WILL make the transmission last longer. This is from people who drive slow, people who drive fast, racers, economizers, etc. I have no choice but to believe it. That would be fine, as I enjoy the feel of a firm shift, but my wife will complain if I do it...
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Old 06-26-2008, 08:31 AM   #40
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That Lucas Tranny fix stuff seems to do good things. It has a bucketload of friction modifiers, increases the viscosity effectively increasing pressure a little, firms the shifts up and appears to run the tranny cooler. Despite being thicker, the other effects appear to improve FE, especially if your tranny is slipping slightly or TC lockup isn't firm. The being thicker improves the TC efficiency out of lockup also I believe. Anyhoo, it's the one bottle of snake oil that actually seems to have real benefits.
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