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Automated
manual transmission (AMT) systems are fine in theory but often less
satisfactory in day-to-day use; the down side is the torque-interruption
effect that accompanies ratio shifts. Driving a car fitted with an
AMT invariably calls for a degree of accelerator control together
with an appreciation of engine speed almost akin to the precision of
playing a musical instrument. Timing is everything; without it,
progress is an uncomfortable experience, with the heads of driver
and passengers nodding in a syncopated rhythm to torque interruption
as gears shift, whether in auto or manual mode.
Improvements to software have
lessened the effect on later AMTs, but the system is invariably
something of a compromise at best. However, the potential benefits
of an AMT are real and the UK company Zeroshift has developed a
system to overcome the torque-interruption effect. That system is
now being integrated into an engineering and technology
demonstration vehicle. At the SAE World Congress earlier this year,
AEI editors singled out the Zeroshift system as being one of the
most innovative technologies to be presented.
Zeroshift was established in
2002 specifically to research and develop a solution to the
drawbacks of regular AMT applications. Said Managing Director, Bill
Martin: “We realized that in many respects
conventional AMTs were close to providing an ideal transmission
solution in terms of cost, packaging, and operating convenience, but
were limited by poor shift quality and emissions issues introduced
by sudden on/off torque required to change gear. We are now
confident that Zeroshift technology can transform almost any manual
gearbox into a compact, low-cost automatic that changes ratio with
zero torque interruption.”
The company’s five-year
development program has now resulted in a secondgeneration solution.
Martin regards the concept as “proven” and said the company is
starting engineering programs with OEMs and transmission suppliers.
Shift quality is equivalent to a traditional planetary automatic
with torque converter,
he claimed, but via a smaller, lighter, and substantially lower-cost
system: “In a typical mixed-drive cycle, the Zeroshift system should
generate a fuel-economy saving of at least 2% compared to a manual
transmission and 7% when compared to a planetary automatic with an
equivalent number of ratios. CO2 emissions are similarly reduced.
Zero torque interruption improves performance, potentially allowing
engine downsizing and a further reduction in CO2. We expect engine
downsizing to become one of the most important solutions to
emissions reduction, particularly in Europe.” |
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Zeroshift’s solution uses control rings to
smooth the operation of an automated manual transmission. |
From manufacturing and vehicle
systems integration aspects, Martin stated that his company’s system
is cheaper to produce than a dual-clutch transmission (DCT) or an
equivalent planetary auto, and that the shift mechanism could be
built into an existing manual gearbox. “It can be assembled on
currently used manual- transmission production lines. The
Zeroshift’s capability centers on its design innovation and control
systems, making it mechanically so simple that only a few
workstations would have to be modified to accommodate a choice of
different components—Zeroshift or conventional synchromesh. We
believe that for an established manufacturer of manual gearboxes, it
is an economically affordable option.”
Martin explained that the second-generation Zeroshift gearbox is
essentially an AMT in which the synchromesh has been replaced by
interlocking drive rings that engage and disengage the drive gears
in a similar manner to dog |
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clutches. Each drive ring incorporates three drive
elements in a single forged component that is moved on a splined hub
shaft by lightweight shift forks. A compact, low-cost electrical
actuator system is being developed to replace the pneumatic system
initially used to prove the concept.
The instant before a shift takes place, the clutch is opened to a
point where it is capable of transmitting only the exact torque
being delivered by the engine. As the shift takes place, inertial
forces cause the clutch to slip, allowing the engine speed to
synchronize with the gearbox input shaft speed. One side of the
drive
element has a retention angle to take up the drive, the opposite
side using a ramp face to smoothly disengage the drive. The
Zeroshift-designed control system coordinates gearshift actuation,
engine management, and clutch operation.
“Shift forces are about 40 N,
compared to some 1000 N for a typical current AMT,” said Martin.
“When shifting from neutral, ring one is engaged to take up drive
while ring two is engaged within a few degrees of revolution to take
up backlash. The next shift is made with ring two taking up the
drive and ring one taking up the backlash. As ring two is unloaded
during the change, it requires less than onetwentieth the axial
force required by a conventional synchromesh. The ratio change is
completed with clutch pressure reapplied. There is zero torque
interruption throughout the shift sequence.”
Stuart Birch |
