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Old 05-27-2013, 08:47 PM   #1 (permalink)
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More Efficient Transmissions

I've read a few times here on EM that the biggest efficiency hog in today's drivetrain is the transmission itself. My first question is if this is true or not. The second one is, if it is true, what can be done to make them more efficient. What in the system is not currently efficient that can be improved upon?

I understand manuals are generally more efficient than automatics, but I am not asking about that specifically, more of the mechanical inefficiencies. I'm thinking about this from a generic perspective, although, any insight into individual transmissions (manual, auto, manumatic, CVT, etc.) would be greatly appreciated!

Keep in mind, my understanding of transmissions is limited, so you might have to explain it to a 5 year old!

Thanks!

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Old 05-27-2013, 09:51 PM   #2 (permalink)
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Just in general, the automatics of old that use a traditional torque converter are the ones that are inefficient because they are constantly slipping. It's like swimming, you can expend a lot of energy, but because your arms are effectively slipping in the water, you are also wasting a lot of energy pushing the water around.

Manuals are simply solid shafts (oversimplified) and directly hooked to the wheels when the clutch is engaged.

CVTs are the same story except that they use steel bands (or rubber belts in small ones) that hook the transmission directly to the wheels. I don't know if CVTs now use a torque converter so I can't comment on the efficiencies of big ones, but I know my Honda Metropolitan uses a centrifugal clutch, which is a "non slipping" engine to trans engagement mechanism.

Double clutch automatics are much more efficient because they are essentially two manual transmissions in one, so they don't slip like traditional automatics.

Honestly though, anything that isn't a traditional automatic is going to be the most "efficient".
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Old 05-27-2013, 10:01 PM   #3 (permalink)
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Yep, the torque converter is like swiming in syrup. However, *IF* it can be "locked" once 1:1 ratio has been (roughly) achieved, the automatic transmission "loss" can be less than before--ie: close to a manual--but never equal to the manual.

The advantage of CVT is it's ability to infinitely 'adjust' engine speed to coincide with engine *load* so as keep the engine within its BSFC "sweet-spot," something that discrete gears can't do.

Last edited by gone-ot; 05-27-2013 at 11:38 PM..
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Old 05-27-2013, 11:01 PM   #4 (permalink)
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Transmissions are not a big hog tho... sure automatics can be, but manuals have slipped down because automakers are marketing manuals to people who want sporty cars.... giving them lousy gear ratios and put the engine at an uncomfortable speed.

Chevy and Ford are teaming up to build a better transmission because if you can build a transmission that mimics a CVC transmission by letting the engine stay in that "sweet spot for how fast it's spinning then you can make the engine get better mileage... but only in that narrow speed range, to do that you need to improve the transmission! so it's not that the transmission is wasting energy it's that it's causing the engine to waste energy, so their goal is an 8 or 9 speed gear box that keeps the engine happy without wasting energy like old automatics did.
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Old 05-27-2013, 11:42 PM   #5 (permalink)
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But, unfortunately, MANY of the current 6-speed trannys STILL use torque corverters-- unlike Dual-Clutch Transmissions (DCT)--so, they're NOT as efficient getting UP to those "sweet-spot" engine speeds in each gear as a manual transmission.
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Old 05-28-2013, 12:25 AM   #6 (permalink)
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Most modern automatics now have a locking torque converter that automatically locks once the car is in gear (after slipping for a bit to smooth out the shift). This provides a huge gain over older automatics that continuously slipped as long as you were below the stall point for the converter (typically 2,500-3,000 rpm).
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Old 05-28-2013, 05:50 AM   #7 (permalink)
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cvts are a compromise though, they are more lossy than a manual since they have a lot more contact area (plus cost and reliability issues). So there are higher peak efficiencies in a manual which can be capitalized on by a good driver who shifts and throttles to stay within bsfc peak range.

Also most descriptions of CVT's indicate that they keep the engine at max power, not max efficiency (accelerating assumed), so trying to out-think the cvt controller for max efficiency might be daunting. This is always a concern with automation (my prius is a nightmare to drive for max efficiency compared to a stick shift).

FYI, my prius acts like a cvt at times, in "heretical" mode it takes power from the engine using mg1 as a generator and applies it to the wheels via mg2.

Last edited by P-hack; 05-28-2013 at 05:59 AM..
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Old 05-28-2013, 06:02 AM   #8 (permalink)
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but I would love to see what a 6 speed prius could do in the right hands, with regen only electrics and manual control of electric use (so you can plan for hills and traffic repositioning and etc). While not a direct comparison, the best prius in the garage gets 84 mpg, the best insight gets 100 (is a manual too).

Last edited by P-hack; 05-28-2013 at 06:10 AM..
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Old 05-28-2013, 08:35 AM   #9 (permalink)
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Just installed 2" larger od tires on my 6spd rwd nissan 350z. No problem starting in 1st and all the gears feel just right. Big improvement on my highway mpg where I do most of my driving.
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Old 05-28-2013, 04:28 PM   #10 (permalink)
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Heat dissipation from the internal frictions is a way to measure inefficiency from a transmission.

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