BENEFITS OF THE TECHNOLOGY
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ZF has developed a new automatic transmission
for cars with eight speeds that can achieve more than 6% fuel savings. The
priority aim in development, however, was not the maximum number of gears, but
minimum consumption.
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ZF engineers had set the bar high to produce a
new benchmark for automatic car transmissions. The second generation of the ZF
6HP 6-speed transmission, that entered production only in 2006, defines
standards that are hard to top: reaction times faster than human perception,
direct engine linkage by early-stage torque converter lock-up and intelligent,
adaptive control software, that almost reads the driver's intentions from his
foot. But the new transmission is capable of even more - it saves (even more)
fuel. It also guarantees ultimate driving enjoyment, as well as the variability
needed to be able to use future technologies.
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The best currently available technology for
fuel saving by automatic transmissions defines the second generation of ZF
6-speed automatic transmissions (6HP). The new ZF 8-speed-automatic
transmission (8HP) achieves an additional 6% fuel saving and thereby also
reduces CO2 emissions.
Many
improvements - one goal: less consumption!
Several innovations have been introduced in the new 8HP to reduce fuel
consumption: the completely new transmission concept with four gear sets and
only two shift elements open, a higher overall gear ratio, a variable oil pump,
new torque converter, and optimized hydraulic and transmission control. A
stop-start function is offered as an optional function. Idling at a standstill,
the engine automatically switches off. When engaged again, the engine
automatically starts up. Even at standstill times of just 10 seconds, switching
off the engine cuts average consumption noticeably – and also CO2 emissions.
The start/stop function is enabled by the development of the hydraulic
impulse oil storage (HIS). It supplies the hydraulic oil that the
transmission's shift elements need for starting. When the engine is switched
off, it allows for a quick start – as it is required with the start/stop
function. Already 350 milliseconds after starting the engine, the vehicle is
ready for setting off. With the start/stop function of the hydraulic impulse
oil storage, it is possible to reduce fuel consumption by another 5%. Compared
with the worldwide most efficient 6-speed automatic transmission by ZF, the
newly developed 8-speed automatic transmission saves another 11%.
Benefit of HIS
With the HIS it is pssible to
reduce fuel consumption by another 5%
The hydraulic impulse oil storage makes more complex solutions obsolete,
such as a more powerful oil pump in the transmission or an electric hydraulic
pump. As a larger dimensioned transmission oil pump would considerably
neutralize the fuel savings in continuous operation, an electric pump is an
additional burden on the main power supply, has disadvantages in terms of
noise, and leads to a considerably higher integration effort. In contrast, with
the hydraulic impulse oil storage, ZF engineers have already considered the
lowest possible system costs and easy installation of the unit. Other expensive
adaptations of the transmission are not required; after all, with the new
development of the transmission, ZF engineers have already considered the
integration of the hydraulic impulse oil storage by ensuring a favorable design
of the components and the respective routing.
Farewell to conventional gear set concepts
The ZF engineers went on the lookout for a completely new transmission
system and discarded all previous established gear set concepts. Armed with a
comprehensive list of criteria, the development engineers systematically
appraised thousands of possible epicyclic gearing systems and their arrangement
variants. The outcome was an 8-speed automatic transmission system leaving just
four simple gear sets and five shift elements.
Lower drag losses and
higher gear meshing efficiency
The clever
thing, though is the shift plan. There are only 5 shift elements - multi-disc
clutches and brakes in the heart of the transmission - and only 2 are open in
each gear. The fewer open shift elements there are, the fewer transmission
parts there are rotating relative to one another. On the bottom line, this
results in a significant reduction of drag losses in the transmission. The
development engineers were also able to increase the gear meshing efficiency
with the new transmission concept. Energy is lost in some gears when power is
transmitted through gear wheels. In the new automatic transmission, these
losses are below 2% throughout - a further factor helping to reduce
consumption.
Significantly
lower CO2 emissions
The logical consequence of lower fuel consumption is a significant reduction in
CO2 emissions - in real terms, this produces a further reduction of 200 g/km to
178 g/km CO2 emissions in a 3.0-liter 6-cylinder engine - with improved
performance.
Faster acceleration
In addition to its positive effect on fuel consumption, the higher overall gear
ratio improves the acceleration values of the ZF product. Shorter gear steps,
particularly from first to second gear, give vehicles fitted with the 8HP
faster acceleration and enable improved shift quality in the lower gears. Two
additional gears also helped the ZF engineers achieve an increase in the
overall gear ratio, without loading individual gears with excessive torque or
speed. This harmonious torque and speed distribution is less harsh on parts and
allows reliable operation with fewer "hefty" gear sets
Precise gear shifting
200 milliseconds is how long the
8HP 8-speed automatic transmission takes to carry out a gear shift - fast and
smooth. Even a professional driver can't shift as fast as this. This gives gear
shifting that impressively combines driving pleasure, efficiency, and maximum
comfort. Direct gear jumps over two or more gears - including the extreme of
shifting from eighth to second - are also possible. In conjunction with the intelligent
adaptive shift strategies, this guarantees maximum agility and driving fun.
Extendable technologies
In addition to fuel savings and performance improvements, a further objective
of the ZF development engineers was versatility. In a market environment likely
to continue to be characterized by model pluralism, the new transmission series
needs to be able to meet a wide range of different requirements. Right from the
conception of the 8HP, attention was paid to ensuring its suitability as a
modular kit for other starting and all-wheel concepts, without any changes to
the basic transmission concept. In the clutch bell housing, the torque
converter can be replaced by new starting elements (such as a hydrodynamic
cooled clutch) or omitted completely with the assistance of an integrated
starting clutch. On the power take-off end, the new product is compatible with
all ZF all-wheel drive concepts and future developments of these.
Compatible with hybrid
drive system
The new 8-speed automatic transmission is also ideally designed for hybridizing
the driveline: ZF will be able to offer both a mild hybrid with crankshaft
starter generator and a full hybrid as a parallel hybrid on the basis of the
new 8-speed automatic transmission. This will allow all familiar hybrid
functions to be implemented in conjunction with the new 8HP. Up to 25% fuel
savings can be expected from the full hybrid version in urban-cycle traffic.
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New
gear set concept
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Only
5 shift elements with 4 gear sets, of which only 2 are open in each gear. 3
multi-disc clutches and 2 brakes, enabling a compact, highly efficient
design.
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Higher
overall gear ratio
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Overall
gear ratio of 7.0. The transmission consistently adapts to the optimum engine
operating range for improved acceleration and lower fuel consumption.
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Higher
power-to-weight ratio
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Lightweight
components combined with a revolutionary gear set concept: 2 additional gears
transfer more torque with fewer components and unchanged weight
characteristics.
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Lower
drag loss
and greater efficiency
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Only
5 shift elements with 4 gear sets, of which only 2 are open in each gear, and
a new axially parallel vane cell pump increase the efficiency of the
transmission. Energy losses are reduced to a minimum.
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