Smooth in the Clutch

New transmissions combine a manual's simplicity with an automatic's smoothness

By Michael Kluger and Bapiraju Surampudi, Ph.D.

For generations of drivers, the automobile transmission has been defined by two choices: automatic or manual. Each offers its own set of advantages: smoother and easier operation for the automatic versus better economy and more driver control for the manual.

Preferences for one transmission type or the other vary, not only among drivers but also among regions. U.S. consumer preference has swung heavily over the years toward the automatic transmission, while as many as 90 percent of European drivers prefer the manual. Meanwhile, consumers in emerging markets, such as China and other Asian nations, remain uncommitted to one design or the other. It is in these markets that a third transmission type - a hybrid of the other two - offers the potential for future dominance.

Automated manual transmission

Southwest Research Institute (SwRI), in collaboration with an automobile manufacturer, has developed an automated manual transmission (AMT) that merges the convenience of an automatic with the fuel economy and lighter weight of a standard shift.

For this project, SwRI engineers developed software and actuators that can control a variety of powertrains, including electric, hydraulic, and pneumatic systems as well as conventional internal combustion engines and transmissions. The team designed and developed a full-authority vehicle controller that includes an AMT and clutch system with software that automatically shifts gears when engine and road conditions warrant. The shifting is transparent to the driver, much as one would experience with an automatic transmission.

Because the system's hardware is based on a manual transmission, however, AMT-equipped vehicles achieve improved acceleration, fuel economy and emissions in comparison to those equipped with conventional automatic transmissions. The design eliminates the automatic's torque converter, transmission fluid pump and multiple clutches, reducing transmission weight by approximately one-third. The AMT is also less expensive to build than the automatic transmission that dominates the American market.

The heart of the system is the SwRI-developed software AutoShifter™, which monitors engine and road conditions and determines when, and to which gear, to shift. This fundamental technology is particularly useful in hybrid electric vehicles, which emphasize weight savings. SwRI plans to license the AutoShifter technology to vehicle and transmission manufacturers.

The evolution of the AMT into a unit that can rival an automatic transmission's driving experience has been made technically possible because of the commercial availability and acceptance of the following three technologies: sophisticated electronic controllers, fast-responding and compact actuators, and control of engine torque.

From a marketplace perspective, the demand for AMTs should increase because the nature of today's driving conditions is changing due to congested and saturated streets, gridlock conditions and the increasing desire among drivers to use cell phones and perform telematic and other non-driving functions while in their vehicle. In addition, today's driver demands that the transmission, and for that matter the entire powertrain, be more efficient and less costly.

Dual Clutch Transmission (DCT) The DCT substitutes dual clutches for the current production single-sided clutch to allow power from the engine to be transferred in parallel through two power paths. The housing containing both clutches would be connected through a concentric shaft system to two sets of gears, one set being the odd gears (1st, 3rd, 5th) and the other the even gears (2nd, 4th, 6th). Synchronizers mounted on the input shaft would handle similar loads, allowing the output shaft to remain similar in construction to today's conventional two-shaft manual transmission.

Multiple configurations

The configurations for an AMT are quite varied. Some have clutch pedals and some have shift levers, while some have neither, and others have combinations of these. However, even with all these possible derivatives there are only three basic AMT types: single-sided clutch, double-sided clutch and dual clutch.

The single-sided clutch transmission (SSCT) uses a current production manual transmission but incorporates actuators to automatically operate the clutch and gear shifting function. This type of AMT is by far the most cost-effective because it is able to use a conventional transmission without modifications to the existing gears and shafts. It suffers from the manual transmission's characteristic power interrupt during the shift, but the interrupt is shorter than in a conventional manual transmission because the AMT engages/disengages the clutch and moves the shift lever faster than a human can, and also pre-stages the synchronizer in advance of the upcoming shift. Moreover, it precisely adjusts the engine speed - again, faster than a human can - to achieve near-synchronous shifting.

In many applications, specifically those that are cost-sensitive, an SSCT excels. However, in situations where the power interrupt interval must be reduced even further, a double-sided clutch transmission (DSCT) may be used. The shift quality that is derived from a double-sided clutch is superior to that of a single-sided clutch because the total clutch travel time is dramatically reduced. In demanding applications where even better shift quality is required, the continuous power transfer characteristic of a dual clutch transmission (DCT) configuration is required.

Double-Sided Clutch Transmission (DSCT) The DSCT substitutes a double-sided, or dual-acting clutch, for the current production single- sided clutch. This allows power from the engine to be transferred in a series fashion between either of two power paths within the transmission.

Optimum configuration

Faced with the three major choices of clutch configuration within an automated manual transmission, the SwRI team selected the DSCT as the optimal design for a number of reasons, based on both engineering and market considerations.

Performance: In this regard, the DCT rates highest among the three systems because it maintains continuous power during the shift. However, although the DSCT does experience power interruption during the shift, it is typically of much shorter duration than with the SSCT or an unmodified manual transmission. Likewise, although the DCT has excellent shift quality as perceived by the driver due to its smooth operation, the DSCT's shift quality characteristics are perceived as good.

Vehicle fuel consumption: The DCT's lack of power interrupt also contributes to improved fuel consumption relative to the DSCT, but there may be some additional energy loss within the DCT transmission during the clutching event as one clutch disengages and the other engages. Because these clutches are modulated to provide a smooth transition, there would have to be some high-torque slippage of the clutches, resulting in some energy loss.

Cost/weight/packaging: The DCT is more expensive than the DSCT because it is more complex, has a greater number of parts and requires a longer transmission housing. The same factors also make the DCT dramatically heavier than the DSCT. Multiple friction/separator plates, longer clutch plate housings, longer input shafting, an additional piston and additional fluid required for clutch actuation and cooling all contribute to the weight penalty. The DCT contains two clutch assemblies, each comprising a large number of friction plates and separator plates. Consequently, the clutch housings and shafts and their integral splines are longer and more difficult to fabricate. Both systems utilize a pump and control valve to actuate the clutches. However, the amount of fluid required to operate and cool the DCT is dramatically greater than for the DSCT. The DSCT has no special fluid requirements and uses conventional manual transmission fluids. In contrast, the DCT has the requirement of additional clutch capacity associated with the two nested clutches. This is obtained by increasing the kinetic energy loading capability of the clutch and increasing the friction characteristics of the fluid. This, in turn, mandates that current production DCTs use special transmission fluid.

Conclusion

The SwRI team concluded that the DSCT automated manual transmission can provide an economically viable alternative to the manual transmission for new and emerging markets. Compared to competing configurations, it is less expensive and less complex than the DCT and smoother in operation than the SSCT. While its shift quality is not as smooth as that of the DCT, the SwRI team considers the power interrupt of the DSCT as not enough to be perceived as objectionable, especially when one considers its cost savings in comparison to only slightly greater inconvenience compared to the DCT.

Comments about this article? Contact Kluger at (210) 522-3095, or michael.kluger@swri.org.

Published in the Summer 2004 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.

Summer 2004 Technology Today
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