How effcient are current automatic transmissions
How effcient are current automatic transmissions
(OP)
I have just read that the Chrysler 904 Torque Flite is one of the most effcient automatic transmissions in use. Is this true?
How is this tested?
Are automatics still 4% to 6% less effcient than a manuel transmission?
Michael
dadof275103
How is this tested?
Are automatics still 4% to 6% less effcient than a manuel transmission?
Michael
dadof275103





RE: How effcient are current automatic transmissions
Mike Halloran
Pembroke Pines, FL, USA
RE: How effcient are current automatic transmissions
Volkswagen's new 7-speed DSG with dry clutches is supposed to be equal in efficiency to a manual - no compromise. The 6-speed DSG (with hydraulic "wet" clutches) is really close. No torque converter on those. But even torque-converter-equipped transmissions can have good efficiency IF they have an aggressive lock-up program. The new transmissions with many closely-spaced forward gears (5 or more) don't need to unlock the torque converter to fine-tune the engine RPM, and they can use the drive-by-wire throttle to cover up variations in torque output that a torque converter normally masks when changing gears.
In many cases, if you go by the EPA ratings, the modern designs have negligible difference between automatic and manual.
RE: How effcient are current automatic transmissions
RE: How effcient are current automatic transmissions
I would not classify any DSG gearbox as an "automatic" even though they have taken over the shifting. Unfortunately, it is another case of language not keeping up with technology & being imprecise. When most people use the term "automatic" they are thinking "torque converter".
RE: How effcient are current automatic transmissions
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: How effcient are current automatic transmissions
You may want to check out the following link from EPA on test procedures.
http://www.epa.gov/otaq/cfeis.htm
Kyle
Kyle Chandler
www.chiefengineering.net
"To the Pessimist, the glass is half-empty. To the Optimist, the glass is half-full. To the Engineer, the glass is twice as large as it needs to be!"
RE: How effcient are current automatic transmissions
I doubt that most people even know what a torque converter does. When they say "automatic" they mean "I'm not shifting it."
RE: How effcient are current automatic transmissions
That alone will drop light throttle engine rpm when up to the required road speed. The transmission itself may be more or less mechanically efficient, but what probably matters more, is the overall package and how it is programmed to operate.
RE: How effcient are current automatic transmissions
On the other hand, more and more ratios increase weight and that decreases fuel efficiency in direct proportion to the overall weight increase of the car. There must be a crossover point where the extra weight fully offsets the extra drive line efficiency.
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers for professional engineers
RE: How effcient are current automatic transmissions
RE: How effcient are current automatic transmissions
Either way there is no doubt that the six speed boxes are more efficient that the 4 speed boxes, whether that is from better mechanical efficiency or better shift scheduling.
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: How effcient are current automatic transmissions
RE: How effcient are current automatic transmissions
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: How effcient are current automatic transmissions
Camaros so equipped since 1992 have a shift lockout, with solenoids in the transmission, controlled by a computer, that forces a 1-4 shift instead of a 1-2 or 1-3 shift at certain speeds at moderate throttle settings. It's clearly intended to improve ratings in the EPA drive cycle (and briefly documented as such in the owners manual).
Some other upshifts are also locked out, but I've only hit one once that I know of; it feels like a really balky synchro.
The computer graciously illuminates a lamp (not prominently placed) on the dash to tell you what it's doing. If you don't notice the lamp and/or act soon enough, the 1-4 shift produces a major bog. Since ~1997, disconnecting the solenoid produces a check engine code and lamp.
Tactics to deal with the unmodified system include:
- Short-shift even earlier. The LS1 tolerates it; the LT1 was less polite.
- Wait 5 seconds (typically while coasting through an intersection with the car behind you still accelerating), at which time the lamp goes out and the transmission will go into the gear you want.
- Drive it hard, and you'll never see the lamp.. and never get near the EPA numbers.
Mike Halloran
Pembroke Pines, FL, USA
RE: How effcient are current automatic transmissions
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: How effcient are current automatic transmissions
http:/
RE: How effcient are current automatic transmissions
There's mention somewhere above about variable-displacement hydraulic pumps in modern automatic transmissions. Can anyone point me to a link to a supplier?
Also, does anyone know if any modern shift strategies need to use PWM or similar control of hydraulics solenoids, maybe for shift smoothing? Or is the electronic side of the control simply on / off?
Thanks, Ian
RE: How effcient are current automatic transmissions
PWM is widely used nowadays. A lot of the time, the line pressure is controlled that way. Sometimes the torque converter lockup is like this.
RE: How effcient are current automatic transmissions
RE: How effcient are current automatic transmissions
CVT's are nowhere close to MT's with regards to mechanical efficiency. Although friction CVT's (like Audi's link-plate chain or VDT's push belt) are more efficient than traction CVT's (like Jatco's half toroidal or Torotrak's full toroidal), even the best friction CVT's can only get about 95% efficiency, input to output, when driving through the variator. This is mostly due to the parasitic losses caused by having to constantly drive the hydraulic pump for the variator pulley clamping and positioning.
Even an average MT will easily achieve 97% efficiency, input to output.
RE: How effcient are current automatic transmissions
RE: How effcient are current automatic transmissions
RE: How effcient are current automatic transmissions
RE: How effcient are current automatic transmissions
RE: How effcient are current automatic transmissions
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers for professional engineers
RE: How effcient are current automatic transmissions
RE: How effcient are current automatic transmissions
RE: How effcient are current automatic transmissions
RE: How effcient are current automatic transmissions
I'm not criticizing this development for what it is (even though I'm kind of hardcore about keeping my traditional manual transmissions).
The presentation for implying more than it should while providing less basis for independent comparison, on the other hand . . .
Norm
RE: How effcient are current automatic transmissions
Great points.
Would you believe? This material as outlined on that SAE page is the same information that is presented in the new product class for service technicians.
RE: How effcient are current automatic transmissions
Enough knowledge to diagnose a component or system < enough knowledge to develop same. Particularly when mandated testing is involved and there might be some choice in what to optimize for.
Training for techs is only going to go to a certain depth even with the newer developments. While I do know a couple of techs who would understand this particular topic in much greater detail than that SAE page goes into (from briefly working at a dealership between engineering assignments), the majority probably would not, or have insufficient interest in even attempting to learn it (I met a few at the other end of the spectrum, too).
Norm
RE: How effcient are current automatic transmissions
You are correct, the Audi Multitronic CVT variator does employ a mechanical device called a "torque sensor" to vary the pulley clamp load in response to torque. This allows the clamping force (and system pressure) produced by the hydraulic system to be lower, thus requiring less power. The torque sensor is basically a roller and helical ramp mechanism on the pulley shaft that creates an increasing axial force on the pulley sheave as torque tries to drive the rollers up the helical ramp. However, the torque sensor only reduces the parasitic loss of the hydraulic system. It doesn't eliminate it.
Consider this, if your CVT equipped car requires 30hp to cruise down the highway at 55 mph, and the hydraulic pump in your CVT is sucking up 1.5hp, that's a 5% loss. A MT under the same conditions would only have about 1.5% loss at most.
RE: How effcient are current automatic transmissions
IIRC, the 904 also does something different internally compared with other 3-speeds, resulting in less lost energy during a shift. It's been a long time since I paid attention to drag racing in general or automatics in particular.
RE: How effcient are current automatic transmissions
[url]http://www.torvec.com/products_ivt.html[/url]
RE: How effcient are current automatic transmissions
Torvec's claims of fuel efficiency improvements seem based on big reductions in engine speed, particularly at low vehicle speeds. The improvements are smaller as speed gets higher. It's possible to do that with practically any CVT and it's possible to do this with a normal transmission by upshifting really early, but most don't do it because of driveability concerns. (The 4 speed auto-box that they're comparing to, isn't a design that's going to set the world on fire.) You can have the economy, but it might not be worth the unpleasantness. I can get about 10% better economy than "normal" with my TDI by upshifting to keep the engine barely above idle, but it requires a feather touch on the accelerator to avoid lugging, and actual acceleration or hill climbing is nonexistent without a downshift.
RE: How effcient are current automatic transmissions
RE: How effcient are current automatic transmissions
At a certain operating condition, an AT with its coaxial input/output architecture, can actually be more efficient than a MT. With the torque converter locked-up and the driving planetary gearset locked-out, the AT simply becomes a "shaft". Passing the power thru without any speed change and with virtually no efficiency loss. The only parasitic losses coming from windage, clutch drag, bearing and seal friction and driving the pump. But they are minor losses.
A MT always must always pass power thru at least one gear mesh by design. So the MT would have slightly higher losses than a "locked-out" AT.
As to your comments about driveline polar inertias, the MT does likely have a lower system rotational MOI than the typical AT. But whether MT or AT, the transmission inertias are probably a relatively small component of the total drivetrain inertia. Especially when one accounts for wheels, tires, brake drums/rotors, etc.
RE: How effcient are current automatic transmissions
I thought 4 speed autos effectively had an OD top so third gear is 1:1 just as fourth is often 1:1 on a 5 speed MT
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers for professional engineers
RE: How effcient are current automatic transmissions
This is all immaterial to drag racing, of course. Automatics have the advantage of not having to shift through Neutral to change gears, so you can never miss a shift, and the torque converter allows a much narrower powerband to be used with only two or three gears.
Incidentally, there are racing transmissions for circle track use that incorporate the clutch into the layshaft, and actually disengage the layshaft altogether when in top gear. No flywheel mass and no rotating gear mass or clutch mass when in top. These also usually have only two gears, one for use in the pits and yellow flags, and one for racing.
RE: How effcient are current automatic transmissions
I find it amusing that BrianPetersen posted "... NOWADAYS, it's a variable-displacement pump..." [re: automatic transmissions]. The original GM 4-speed Hydramatic first offered in 1939 (Cadillac, I think), and up through early 1956 (and even later in some trucks and Rolls Royces) all had a variable-displacement front pump. And they didn't need a lock-up clutch for cruising efficiency, as the fluid element (a 2-element coupling, not a torque converter) was out-of-the-circuit (just along for the ride) in 3rd and 4th gears.
I guess it's obvious that I'm a fan of the DualRange Hydramatic. I ran my first HydroStick (a DualRange modified for racing) back in '65. And I just recently built one for the pickup that I'm putting together.
Just for argument's sake, I'd love to work with a transmission-dynamometer shop on fixturing one of my HydroSticks to get some hard numbers on its efficiency.
RE: How effcient are current automatic transmissions
RE: How effcient are current automatic transmissions
Anyway, I found out it had four forward gears once I swapped in a set of straight plugs and solid copper ignition wires. It was amazing how well the AM radio took up the task of being an auditory tachometer.
Norm
RE: How effcient are current automatic transmissions
RE: How effcient are current automatic transmissions
Yes 4 speed.
Yes vairable displacement pump.
Yes a lockup clutch
No Torque converter a fluid coupling instead.
Not sure how long offered in cars but I think they were still offered in trucks into the 60's.
The Dual coupling Hydramatic is a different transmission.