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How effcient are current automatic transmissions

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  

RE: How effcient are current automatic transmissions

Most of them have lockup torque converters now, so you're only faced with gear losses most of the time.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: How effcient are current automatic transmissions

Traditional automatics also have a hydraulic pump that's running all the time, to generate the hydraulic pressure that's used for holding the internal clutches engaged. Nowadays, this is a variable-displacement pump, to cut the parasitic loss to a minimum - but it's a loss that's not there with a manual.

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

efficiency of automatic transmissions may vary quite a bit depending on the driving pattern. as mike and brian point out a lot of steps are taken to enhance the efficiency, but if the driver has a heavy right foot most of those enhancements come not into play, since the torqueconverter is heavily used without lockup. especially in urban driving and stop&go traffic the penalty may be still high and also when the "sports" type of shifting is selected by the driver. nevertheless, modern 5,6 or 7 speed tranmissions are more efficient then previous types with 4 or less gears. they will not however, as long as a torqueconverter is used as a flexible ratio component, be as efficient as mechanical or "robotosized" mechanical tranmissions like the VW DSG transmissions.   

RE: How effcient are current automatic transmissions

I've seen quite a few vehicles with 6 & 7 speed automatics with HIGHER EPA fuel economy ratings than the manual gearbox.  That may be a ratings fluke & your mileage may vary but obviously they have gotten very good.  

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

Yes, that's an interesting point. Sorry I'm feeling lazy, does anybody know how the shift schedule for manual gearboxes is set for EPA testing?

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

Hey Greg,

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

When most people use the term "automatic" they are thinking "torque converter".

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

The later automatics with six or even seven available ratios, also typically have more aggressive overdrive ratios in the top two gears.

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

More and more ratios make the drive line more efficient, but I would expect a law of diminishing returns to apply.

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

the latest automatic transmissions with more then 5 speeds use a very sophisticated setup combining several planetary gear sets in such a way that both the number of physical gears and the required number of clutches and brakebands is less then in previous designs. this way the overal dimensions of a modern 6 speed automatic can be substantially smaller and lighter then previous designs with 4 speeds. therefore there is not necessaryly a weight penalty in using more gears. there is a tendency though to design more heavy vehicles to comply with various safetyrequirements and luxury wishes, and that usually takes the fuel consumption into a higher bracket, notwithstanding more efficient engines and transmissions.   

RE: How effcient are current automatic transmissions

Well, kinda, sorta. Engineering being the art of compromises it seems unlikely that a smaller, more complex, device will outperform in all respects, such as strength, for example.

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

Local newspaper today had an overview of the new Mazda 6 in it, and for the 4 cylinder model, the auto-trans model has better Transport Canada consumption ratings (9.7 L/100 km city 6.7 highway) than the manual-trans model (10.4 city 6.9 highway). Both 6-speeds.

RE: How effcient are current automatic transmissions

So my point about the manual shift schedules is that it seems to be defined according to the number of gears you have if I have not misinterpreted the last file on kchan's link, whereas for an auto you get to use whatever you feel is appropriate.


 

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

About 6-speed shift schedules:

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

Corvette had the same idiotic system. They also had an 'interesting' attitude to the driveby noise tests. I learned a lot about Federal Government and big business on that program, can't say it improved my opinion of either.


 

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

Apologies if this is too off-topic...

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

I know that Stackpole (now a division of Gates) builds transmission oil pumps of many different designs, and I'm sure there are other suppliers.

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

Modern CVT's are achieving efficiency that exceeds even the manual trans-axles. Check out AUDI's newest design for the A6.

RE: How effcient are current automatic transmissions

Thecardoc,

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

Now granted I don't have percentages to throw out, but Audi's claims stand as THEY make them. The CVT out accelerates their manual transmission (and their automatic) , and achieves better fuel economy than both as well. Their claims, not mine.  :)

RE: How effcient are current automatic transmissions

a car can get better fuel economy while using a less-efficient transmission, even if all other design parameters and operating conditions are held constant...  
 

RE: How effcient are current automatic transmissions

How does that work?  If you're reducing the efficiency of your only variable, how can you increase overall efficiency?

RE: How effcient are current automatic transmissions

The efficiency of your engine is not constant, and the transmission you pick will influence the load/speed histogram seen by the engine for a given (whole vehicle) driving cycle.  Use a less efficient transmission that puts the engine more frequently into an efficient operating regime, and you may have a net benefit in fuel economy.  
 

RE: How effcient are current automatic transmissions

To take Isaac's point to the ridiculous if you have a direct drive or 1:1 with no gears you obviously have a very efficient gear box, but when you need to operate in a variety of conditions and speed ranges, more gear ratios obviously help despite the drop in gearbox 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

Thanks for clearing some of that up. Now part of what Audi has told us is through the use of direct torque measurements as well as computed ones, plus monitoring input and output speed sensors they alter the pressure on the "chain" as required to improve efficiency. Under heavy loads more pressure can be applied to the chain through the pulley system to prevent slippage, under cruising conditions, this pressure is reduced, while still preventing slippage.  The CVT even mimics a gearbox with pre-set ratios that can be commanded manually. (6 speeds)

RE: How effcient are current automatic transmissions

CVT more effcient? How?  No positive drive there, all slipage and wear. All that = loss

RE: How effcient are current automatic transmissions

Those Audi comparisons are still a little bit "apples vs oranges" in that no mention is made of any gear ratios, including final drive.  Only a general note about gear spread inside the box.  There is no way to tell how closely to optimum each transmission has been configured with respect to maximizing performance in the EU fuel consumption test (note that "optimum" was clearly in their quotes with respect to the manual transmission).

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

Hi Norm.

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

I'm not at all surprised.  

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

Thecardoc,

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

In response to the original poster, I believe the "efficiency" they were talking about was the amount of energy it takes to accelerate the internals.  So, for racing purposes, more engine power is available to accelerate the car.  (Which is why you see people taking Chrysler's light-duty 3 speed and adapting it to, say, a Chevy big-block)

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

Honda has a similar hydraulic-CVT as the above post that they developed for scooters and small motorcycles, and they have a 2009 model with it in production. I can't speak for the one above since it gives no details, but Honda's system combines a hydraulic swash-plate pump and motor in a single housing that rotates together with output shaft speed, and the whole deal is arranged so that at highway speed, the hydraulic part of it is locked up (1:1 ratio). At lower speeds when accelerating, the hydraulics transmit more of the load, and there is some inefficiency, but this is a smaller part of most drive cycles.

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

can't edit: should note in above post that in my car (VW Jetta TDI) it's a 5-speed manual so no shenanigans required. The DSG version of the same car won't allow even the slightest engine load below 1500 rpm without downshifting.

RE: How effcient are current automatic transmissions

izzmus,

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

MT at least used to have a 1:1 top gear where the main shaft was locked out and the lay shaft just spun with load except bearing and windage.

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

I cannot believe that any automatic can be more efficient than a rear drive transmission in Fourth gear (or whatever ends up being the 1:1).  I believe this is why BMW used to use non overdrive 5 speed transmissions and unusually long-ratioed final drives, instead of a more normal final drive and an overdrive.  (Of course, a transaxle must always transmit power through the geartrain since the output is not co-axial with the input)

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

"What goes around, comes around"...

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

It's interesting that auto tranny design went backwards before it went forwards again. I've heard of the early 4-speed automatics but know next to nothing about them. I think the Powerglide replaced them - 2 speeds with torque converter that was always in the circuit. 4-speed automatics didn't become popular again until the 1980's, nor did the capability of taking the fluid element out of the system during cruising.

RE: How effcient are current automatic transmissions

I think that early 4A may have been fitted into some 1957 cars. Briefly (and many years ago) I owned a 1957 Pontiac, might have been a Star Chief, and it had the 347 CID engine with a 4 barrel carb.  

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

Yes, Norm, your '57 Pontiac had a 4-speed Hydramatic. It was the 2nd-generation design called Dual-Coupling Hydramatic, first offered in mid-1956. It used a second, smaller fluid coupling to replace the front planetary clutch of the Dual-Range unit. The intent was to "soften" the 1-2 and 3-4 shifts- which it did- and thus wasn't useful as a modified race transmission (no way to instantaneously fill a coupling!). This secondary coupling remained in the circuit while cruising in 4th, so I'm sure it gave away some efficiency. I don't remember whether it retained the earlier variable-displacement pump.

RE: How effcient are current automatic transmissions

Dual Range Hydramatic!

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.

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