h a l f b a k e r yI CAN HAZ CROISSANTZ?
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With all the new sequential manual transmissions out there that brag about cutting shift times and such, why not combine two of the best components for an even better transmission?
Automatic transmissions are smooth and convenient transmissions, but they are quite inefficient mainly due to their torque
converters that they require to keep from stalling from a dead stop. Manual transmissions are not very smooth, but they are very efficient because they are pretty much directly connected to the engine unless they have to be disconnected, which happens via their clutch.
I propose an automatic transmission with a clutch instead of a torque converter. The planetary gears would allow it to shift without disengaging the engine from the transmission, cutting shift times even from all the paddle shifted manuals that claim to shift in as low as hundreths of a second. The clutch would disengage the engine from the transmission when the driver lifts off the gas pedal in 1st gear, and at a dead stop the transmission would shift into a starting gear with an extremely low ratio. When the driver then lets off the brakes, the clutch would be dumped and the car would creep forward at a low yet steady speed until the driver then gave the car a little bit more gas.
This wouldn't provide much to any gain in performance, since cars can launch well when their torque converters are "loaded", however it would improve fuel economy. Aside from the fact that no power would be wasted through a torque converter, the clutch would also allow the car to cruise when the driver lifts off the accelerater since the engine and transmission would be disconnected, a thing torque converters alone don't allow for.
Lenco transmission
http://www.lencorac...ftransmissions.html
Something similar, but with a conventional clutch pedal for pulling away [Ned_Ludd, May 24 2007]
GearVendors over/underdrive
http://www.gearvendors.com/
[wittyhoosier, May 25 2007]
Mercedes SL63 AMG
http://blogs.cars.c...008/03/2009-sl.html
Scroll down to the second paragraph [acurafan07, May 17 2008]
[link]
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Oldsmobile's 1940 models featured Hydra-Matic drive, the first mass-production fully automatic transmissions. Initially an Olds exclusive, Hydra-Matic had a fluid coupling (not a torque converter) and three planetary gearsets providing four speeds plus reverse. |
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Some problems, but [+] just to counteract that unwarranted fishbone. |
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The core of the idea seems to be to compress the clutch function into the first bit of accelerator pedal travel. Otherwise it's a Lenco (link). But would that allow enough clutch modulation to enable a smooth start? Or were you thinking about perfidious electronics doing that job? |
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Hey, right up my alley. I'm a manual trans engineer these days. |
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There are several technical hurdles to overcome: |
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With a clutch, you have to allow for some spin synchronization time before you engage gears. Are you assuming a wet clutch or a dry one? |
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Because the torque converter also acts as a torque magnifier at low speeds and large slip, an automatic transmission doesn't have to be geared as low as a manual transmission in 1st gear. Thus, the conventional gear ratios used with planetary gearsets are insufficient, and there would be some technical limitations on how low they could get. It may be possible, it may not. |
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A large source of the inefficiency of an automatic lies in the necessary gear pump to apply hydraulic pressure for control, (as well as for the torque converter). If electronic systems were more reliable and had some kind of failsafe, the valve body could be removed, possibly resulting in some kind of efficiency gain. The reliability isn't there, however. |
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We are currently working on dual clutch transmissions, automated manuals, and the like. The American customer has not responded favorably to the torque interruption thus far required for shifting an automated manual, however. They do sell in Europe. |
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One of the big pushes these days is for the torque converter lockup clutch to have earlier lockup points; it's a technical challenge due to the NVH issues that present themselves at low RPM's however. Engine balancing, behavior, and proper calibration is key. |
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Drivers of manual transmissions don't always or often want to 'cruise' when their foot is lifted from the pedal; if that were the case then the torque converter itself could simply be disconnected. There is the factor of engine braking to consider, like when you're descending a steep hill or using the engine inertia to slow the vehicle down. |
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I don't know much about it, but I have seen a car or two that used a Chevy Turbo 400 automatic with a conventional clutch in place of the converter. I think it was a piece made back in the 70's, but I'll try to find a link somewhere. |
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//Are you assuming a wet clutch or a dry one?//To be honest I really didn't think of that when creating the idea, but either one should work fine. |
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//Thus, the conventional gear ratios used with planetary gearsets are insufficient, and there would be some technical limitations on how low they could get. It may be possible, it may not.// A big part of the idea is the fact that there would be a starting gear. It could possibly even be a 2-speed final drive with the 1st having an extremely low ratio. That way, the clutch smoothness wouldn't matter, since if 1,500 RPM = 5mph, it would have no trouble keeping from stalling. After the car has reached, say, 7mph, the 2nd final drive ratio would be engaged and that would be 1st gear, with no abnormal planetary gears required. |
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Sort of a good idea, but I'd say it's partially baked with lockup converters. If you add the two-speed rear end like [acurafan07] suggests, or an un-sprung weight saving GearVendors over/underdrive (see link) then you could push the lockup point much lower as [RayfordSteele] mentioned. |
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It's already been done! Centrifugal clutches are used on small engines such as weedeaters, and go-karts. They can be set to disengage at "10% higher than idle," which means that the clutch won't slip like a typ. automatic transmission, but racers set the stall speed higher so they can maintain engine rpm's in the power band. |
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Just get a motor that operates at 0 rpm, like electric or compressed gas! |
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The basic idea is well and truly baked. There are plenty of automatic transmissions with centrifugal clutches rather than torque converters. There are details of this particular version that probably aren't baked though - in particular, I'm not aware of an automatic transmission anywhere that has a clutch with epicyclic (planetary) gears (although I'd be very surprised if there aren't a few somewhere). |
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1) Cutting shift times too short puts high stresses on both engine and transmission. You have to allow time for the engine revs to change before locking up completely in a new ratio. |
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2) You say, "The planetary gears would allow it to shift without disengaging the engine from the transmission." |
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This isn't strictly true. Planetary gears, just like other gears, have a neutral position (with all gears disengaged), and it's essential to go through this position during gear changes. They even effectively have a clutch built in (or in fact, a whole series of separate clutches, one for each ratio): the braking rings that select ratios allow a small amount of slip as they engage. This slip is essential to avoid the aforementioned excessive stresses on engine and transmission that would otherwise result from the change of ratio. |
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The only way to avoid this is some kind of CVT - there are several options. |
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Just wanted to point out for all you nay-sayers that this idea has just been baked in the 2009 Mercedes SL63 AMG. |
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