Mike Kluger manager-automatic transmission technology section at Southwest Research Institute (SWRI), has authored or co-authored numerous SAE papers relating to transmission technology and SWRI tests. He also evaluates transmissions for many of the world's major automakers. Mr. Kluger recently shared his thoughts on near-term automatic-transmission development with WAW.
The industry has experienced numerous interesting developments in automatic transmission configurations in recent years, some of which appear almost contradictory. In the next five years we'll see an accelerated degree of engineering research focused on transmissions.
In particular, there will be increased demand to, reduce manufacturing costs while simultaneously improving transmission efficiency. These goals will partially be realized through material advaces and with a heavy emphasis on reducing parasitic, or "spin" losses.
A sampling of some near-term automatic transmission trends and advances: Future transmission configurations At the top end of the market, a recent trend has been the move toward 5-speed automatics, now beginning to appear in a number of luxury and near-luxury passenger cars. The impetus behind this development is a typical 5% improvement in fuel economy over vehicles with 4-speed transmissions. To transition to 5-speed configuration, some existing 4-speed automatics comprised of three planetary gear sets will obtain the additional gear ratio by "swap" shifting, which allows two clutch packs to be shifted simultaneously. During swap shifting, one clutch pack is engaging while the other is disengaging. The additional precision hydraulic control required for this procedure is provided with fast-acting solenoids and by installing increased memory in the powertrain control module.
Coincidentally, consumers concerned about the cost of new vehicles show an emerging industry need for inexpensive vehicles with simple, serviceable automatic transmissions.
For this reason, we will see a return to traditional 3-speed automatic transmissions. Although they reduce fuel efficiency by approximately 12%, they are significantly less costly because of the reduced number of parts.
To obtain improved shift quality in the future, automatic transmission clutch bands will no longer be used - they do not provide the precise engagement control clutch packs offer.
Automatic transmission fluid (ATF)
In future transmission designs, the automatic transmission will incorporate a fill-for-life ATF system. In addition to eliminating the need for a dipstick and tube, sealing prevents incorrect ATF levels, a condition that today accounts for a large degree of serious transmission damage.
Sealing transmission fluids, however, places a considerable burden on the quality and composition of the ATF. More stable friction modifiers are necessary to withstand continuously slipping torque converter operation. Sealing the ATF also requires increased oxidation protection, as well as the use of thinner, less viscous transmisision fluids that may need to include synthetic base stocks for improved efficiency - and will certainly make ATF more expensive.
In an effort to improve fuel economy, shift quality and NVH characteristics, there have been attempts in recent years to adopt continuously slipping torque converter they allow transmisisons to transition easily from the "unlocked" to the "locked" mode. Current transmission designs are torque-limited because the friction load-carrying capabilities of today's standard lock-up clutch permits low-torque operation only in third and fourth gear.
Future advances will incorporate locked-up converter operation in all but the lower two-thirds of first gear. Test results using the recommended Environmental Protection Agency (EPA) city-cycle guidelines show a 7% improvement in fuel economy in the locked converter mode.
Fast-acting solenoids - providing better control and more "precise" transmission performance - were first introduced eight years ago for torque converter lockup clutch engagement. Since then, the use of solenoids has increased exponentially to the point where certain quality transmissions contain more than eight, overseeing such tasks as gearshift selection and pressure control. Most current transmissions use only four solenoids, but the number undoubtedly will increase in the future.
Pumping loss accounts for a disproportionately high percentage of the overall power consumed by an automatic transmission and can be as high as 20% in some cases. To reduce these losses, manufacturers have developed new, duocentric and hypocycloid gear designs for the fixed-displacement, internal gear tooth forms.
The advantage of a hypocycloid design, for example, is that it requires only half as many teeth as a conventional internal-external gear form, is at least 15% more efficient and is less expensive.
In some new automatics, there is increasing use of a fluid-recirculation "boost" loop that returns unused, pressurized fluid to the pump inlet. The boost loop dramatically improves fluid flow at crankshaft speeds greater than 3,500 rpm and helps forestall the damaging effects of cavitation at high speeds. Cavitation can be a critical problem for new vehicles with advanced engines delivering shift speeds as high as 7,200 rpm.
New transmission applications employ multiple pump systems to balance the variety of flow requirements that exist in a transmission, for example, between idle and wide-open throttle-shifting. Such systems may include several low-volume, continuously operated lubrication flow pumps and one high-volume, on-demand shift supply pump.
New material applications
More new plastic transmission parts are being developed to provide cost and weight improvements. Glass-filled polypropylene already has been successfully used in torque-converter turbines. Other possible applications are for pump housings, valve bodies and spool valves, center supports and tail housings.
Advances in materials will increase the following typical transmission efficiencies:
1st gear: 60%-85%
2nd gear: 60%-90%
3rd gear: 85%-95%
4th gear: 90%-95%
As these figures show, transmission efficiencies vary dramatically depending on input, torque and speed conditions. Gearbox torque losses commonly run 2 to 10 ft.-lbs., and pump losses are usually 1 to 6 ft-lbs.
Most transmission gearboxes, however, are more torque- than speed-limited. Reduction of torque-dependent losses, however, requires increased structural rigidity for housings, shafts and gears. Such stiffening itself adds additional weight, making improved efficiencies more difficult to realize.
Speed-dependent losses - typically associated with rotation and churning, the "draft" caused by rotation and churning and the drag associated with the clutch pack, seals and bushings - are aggressively targeted by manufacturers as areas where efficiency gains can be more easily realized.
Transmission officiency at SwRI
By its very nature, a transmission should be viewed as three major component systems: a pump, a torque converter and a gearbox. Separate tests of components under controlled conditions that replicate in-use operating conditions are performed at SwRI to assist engineers from a design perspective.
With the ever-increasing demands for improved vehicle fuel economy, automatic transmissions are obvious targets for increased efficiency. To obtain meaningful comparative data, transmission-efficiency testing is performed to extremely precise measurements. This requires accuracies on the order of:
Torque: +/-1 ft-lb. from 0 to 4,750 ft.-lbs.
Speed: +/-1 rpm from 0 to 7,000 rpm.
ATF temp.: +/-2[degrees]F from 140[degrees] to 220[degrees]F.
To eliminate the torsional vibration characteristic of internal combustion engines that would adversely affect the ability to collect reliable, steady-state efficiency measurements, smooth electric dynamometers are used to elimate external sources of transmission vibration.
It is interesting to note that powertrain evolution has followed separate paths in the development of engines and transmissions. Engine development traditionally has focused on overcoming the problems of continuous high-speed, high-load operation. The more difficult operating regime for a transmission involves improvements in continuous shifting to assure lifetime dependability.