12 February 2007
by Rafael Seidl
Zeroshift, a UK company, claims to have come up with a novel two-step synchronization mechanism for automated single-clutch step-by-step transmissions. When combined with slight feathering of the dry clutch, this is supposed to permit gear changes that are faster than a DSG and smoother than a traditional AT with torque converter. We’ll have to wait for independent confirmation of these aggressive claims.
Step-by-step transmissions are among the simplest—read: cheapest—yet most efficient designs available. Conventional automatic transmissions provide greater torque from a stop because of the torque converter, but they waste significant amounts of fuel until the lockup clutch is engaged, at which point the impeller and pump wheels of the converter are bypassed using a wet clutch.
In many designs, that doesn’t happen until the vehicle reaches approximately 40 mph. Some, e.g. Mercedes-Benz’ 7-gear transmission, do engage the lockup clutch even in first gear, however.
The other advantages of replacing a conventional AT with a sufficiently comfortable single-clutch AMT would be:
Substantially lower cost of manufacture;
Smaller package size and weight;
Same 0-60mph acceleration with a slightly less powerful engine; and
Very efficient freeway cruising via longer gear ratios and rapid kickdown.
On that last point: it’s not immediately clear if Zeroshift’s system can handle aggressive kickdown procedures, e.g. from 6th directly into 2nd gear. The required severe step change in engine speed would require unusually tight integration of engine, clutch and synchronization controls.
Similarly, satisfactory single-clutch AMTs would eliminate the possibility of increased fuel consumption of cars equipped with manual transmissions, at a much smaller premium than dual-clutch designs.
Many Europeans, especially older drivers, still don’t shift into higher gears as early as they could. In part, this is because many of them are still unfamiliar with the substantial low-end torque available in modern turbodiesels and boosted gasoline engines. Another reason may be that maintaining the same vehicle speed in a higher gear currently requires a larger, less comfortable angle of the gas pedal. Also, frequently having to kicking down manually is a chore many choose to avoid.
More details to come at the SAE World Congress in April.
Zeroshift Automated Manual Transmission (AMT) SAE Paper No. 2007-26-061 (Symposium on International Automotive Technology 2007)
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