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) https://www.familycar.com/classroom/I...Planetary2.gif 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. https://www.familycar.com/classroom/I...eConverter.gif 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. |
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. |
Here's some details about how the "gold standard"* ( ;) ) in hydraulic automatic transmissions engages gears ...
https://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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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:
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[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:
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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:
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:
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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:
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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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(*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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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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