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.
Torque converters
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.
Solenoids
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.
Pump systems
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.
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