The questions to ask are no longer ‘Why hybrids?’, but rather ‘What technologies?’ and ‘How to make it a business’...
General Motors,
DaimlerChrysler and BMW Group (the partners of the Global Hybrid Cooperation
group) believe that hybrid development has progressed beyond business necessity
and is now seen more and more as business "common sense”. The questions to ask
are no longer ‘Why Hybrids?’ but rather ‘What technologies?’ and ‘How to make it
a business?’
The Two-Mode Hybrid
developed by the Global Hybrid Cooperation group is a full hybrid system that
enables significant improvement in composite fuel economy while providing
uncompromised performance and towing capability.
In city driving and
stop-and-go traffic, the vehicle can be powered either by the two electric
motors or by the combustion engine, or both simultaneously. The Two-Mode Hybrid
can also drive the vehicle using an input power-split range, a compound
power-split range or four fixed-ratio transmission gears.
The system is flexible and
efficient, with smaller motors, inverter module and battery that enable numerous
cost advantages.
What Type of
Hybrid?
Many types of hybrid
systems were explored by the partners of the Global Hybrid Cooperation in
establishing the Electrically Variable Transmission (EVT) as the basis for the
cooperation. EVTs are currently the dominant hybrid systems in the market place
[ie in Toyota vehicles and those using Toyota licensed technology]
and possess a
number of attractive attributes that contribute to this leading position.
One of these attributes is
smooth and continuous regenerative braking. This maximizes the ability to recoup
vehicle kinetic energy during deceleration meeting the high standards for
refinement of luxury brands.
Another benefit of the EVT
is that it has the ability to continuously vary the speed of the engine, like
the mechanical Continuously Variable Transmission (CVT). The EVT can also store
energy electrically in a battery. The CVT action combined with the ability to
store energy electrically offers the unique opportunity to maximize overall
system efficiency by enabling the operation of the engine at optimal speed and
load points. The ability to manipulate the operation of the engine in this
fashion is also synergistic with the cylinder deactivation systems, thus
allowing the engine to increase operation with deactivated cylinders, thus
reducing fuel consumption.
While the EVT concept has
these and other desirable attributes, EVTs currently in the passenger vehicle
market have shortcomings. The largest shortcoming of the typical EVT is its need
to electrically transmit a large part of the power from the engine under some
operating conditions. While transmission of power electrically enables CVT
action, its magnitude creates inherent efficiency, packaging, and cost issues.
These issues become more pronounced for:
-
high-speed highway and
Autobahn driving,
-
larger engines or
vehicles,
-
higher loads due to towing
or hill climbing.
The objective is to offer
customers a system with the benefits of the EVT while minimizing these
shortcomings, leading to the development of the Two-Mode Hybrid as the focus of
the Global Hybrid Cooperation.
The Two-Mode Hybrid has an
overall level of mechanical content similar to that of a conventional automatic
transmission, yet the Two- Mode Hybrid is capable of both CVT action and full
hybrid functions.
Hybrid Transmission
Designs
The modern automatic
transmission (such as the one pictured) is one of two transmissions concepts
which serve as the basis of the Two-Mode Hybrid. Several fixed gear ratios are
available in the automatic transmission, which allow several options for engine
speed. Its control system selects the best ratio for vehicle performance,
comfort, and fuel economy. Its arrangement of planetary gearing and multiple
wet-plate clutches allows power to be transmitted from the engine to the wheels
at all times, even during shifting. Its torque converter provides a smooth
vehicle launch and can ease the transitions between the fixed gear ratios.
The electrically variable
transmission or EVT is the second of the two transmission concepts serving as
the basis for the Two-Mode Hybrid. The EVT is a power-split transmission
combining planetary gearing and electric motors to provide a continuously
variable ratio between its input and output. Historically, each typical EVT has
been built with only one mechanical option for its powersplit gearing, known as
a one-mode EVT. One-mode EVT development and testing was stopped in the U.S. in
1941, but EVT hybrid concepts appeared in the 1960s and 1970s and eventually
entered automotive production in the 1990s
[in the NHW10 Toyota Prius]
,
following the development of electronic motor controls.
This diagram shows the core
elements of the one-mode input-split EVT concept. The one-mode EVT is currently
the most common hybrid system in the market place. It is mechanically simple and
contains only one planetary gear set and two electric motors, without clutches.
The input power from the engine comes immediately to a planetary gear set. This
gearing splits the power through the transmission between a mechanical path and
an electrical path. A significant portion of the power flows mechanically
through the transmission and is delivered directly to the final drive. The
remaining power from the engine flows to the first electric motor. This first
motor acts as a generator, changing part of the engine power into electric
current. The electric current from this motor can either go into the battery for
storage or on to the second electric motor. The second motor changes electric
current from the first motor or from the battery back into mechanical power for
the output.
The input-split EVT
arrangement is shown schematically in this diagram. At right shows the power
from the engine flowing through both the electrical and mechanical paths of the
EVT to the transmission output. The relative sizes of these electrical and
mechanical power flows are critical, because electrical path efficiency is
typically close to 70%, and mechanical path efficiency is typically greater than
90%.
The amount of power that
must be transmitted electrically varies with vehicle speed. The ratios of the
planetary gearing and speeds of the engine and the motors determine the fraction
of engine power that is transmitted electrically. In the case of a one-mode EVT,
the magnitude of the electrical power can be quite large, driving a requirement
for large motors for any given size of engine and vehicle. Typically, the
combined power rating of both motors must be more than the power of the engine.
The amount of power that must be transmitted electrically with the one-mode EVT
is one of its most critical drawbacks, resulting in reduced efficiency,
packaging issues, higher mass and increased manufacturing costs for a given
level of manufacturing economy.
The high requirement for
electrical power in a typical one-mode hybrid comes from the fact that there is
only a single transmission ratio where the power transmitted through the
electrical power path becomes zero. At this ratio, sometimes called the
"mechanical point" of operation, the speed of the first motor, which controls
the speed ratio through the transmission, reaches zero. With one motor at zero
speed, engine power is not transmitted through the electrical path to the
output. This one mechanical ratio is a characteristic of the one-mode
input-split EVT. This ratio is typically chosen in the design of the one-mode
hybrid for high drive-cycle fuel economy. This forces a compromise, and results
in significant power loss through the electrical path, and ultimately lower
transmission efficiency, at all other transmission ratios. The impact of this
inefficiency is typically more pronounced at higher speeds and with higher load
conditions found in "real world” driving.
The one-mode hybrid has
only proven successful in small to mid-size vehicles carrying light to moderate
loads. As the vehicle size and load rating increase, the electric power flow and
the torque necessary for appropriate output become heavy burdens. The motor
components, especially the motor connected to the output is very large, heavy,
and costly for larger, more powerful vehicles.
One-Mode Hybrid with
Two-Speed Motor Gearing
For larger vehicles, the
one-mode EVT is capable of using reasonably-sized motors only with additional
mechanical gears. One very simple way to apply additional gearing is to provide
two-speed gearing for the larger motor which is connected to the output. This
arrangement requires a pair of clutches and another gear set. This diagram
displays the core mechanical elements for a one-mode hybrid with two-speed motor
gearing.
With the exception of
reducing the maximum torque and speed for one motor, this two-speed gearing does
not change the operation of the one-mode hybrid. The power being transmitted by
the motors is not reduced because there remains only one mechanical relationship
between the engine and output. The two-speed gearing changes only the
relationship of one electric motor to the output. The two-speed motor gearing
can be designed for shifting near the mechanical ratio, but still requires a
sharp change in the speed of the motor during the shift.
The one-mode hybrid with
two-speed motor gearing is shown schematically on the left of this diagram. The
critical power flow is shown on the right. This power flow is identical to that
of the earlier one-mode hybrid concept.
Two-Mode Hybrid with
Input-Split and Compound-Split EVT Modes
The Two-Mode Hybrid
addresses all of the issues of the one-mode hybrid by adding a second EVT mode.
This second mode is a compound-split EVT mode. This is accomplished by adding a
planetary gear set to the one-mode hybrid with two speed output gearing. The
Two-Mode Hybrid is shown in this diagram. In the Two-Mode Hybrid, the two
clutches provide a torque advantage for the motor at low speeds while
fundamentally changing the power flow through the transmission. When the first
clutch is applied and the second clutch is open, the system operates as an
input-split EVT as described earlier. When the second clutch is applied and the
first clutch is released, the system operates as a compound-split EVT. The
Two-Mode Hybrid can shift between these two EVT ranges in a synchronous shift,
involving only torque transfer between elements without sharp changes in the
speeds of any element.
The Two-Mode Hybrid is
displayed schematically here on the right. The combination of a compound-split
EVT range and an input-split EVT range enables a Two-Mode Hybrid system with
electric motors sized for hybrid functionality and provides for more efficient
high speed cruising the size of the electrical power path, as compared with the
one-mode hybrid.
In the two-mode hybrid, the
amount of power transmitted electrically varies with vehicle speed or
transmission ratio. The two-mode hybrid has an input-split EVT range with a
ratio when power through the electrical path from the engine to the output is
zero. It also has a compound-split EVT range with two more of such ratios. The
result is that the two-mode hybrid spans a wide range of transmission ratios
maintaining relatively low power through the electrical path. The first of these
three mechanical ratios or mechanical points can be utilized to maintain
moderate electrical power under acceleration, while another point can be used to
optimize efficiency during high speed cruising such highway or autobahn driving.
The highest overall
efficiency results from channelling the maximum amount of mechanical power while
retaining full hybrid functionality. This approach reduces both the electrical
losses and the size of electric motors necessary. The one-mode hybrid requires
relatively large motors just to transmit engine power to the output. Adding a
compound-split EVT range reduces the need to transmit power electrically and
enables properly sizing the motors for the needs for regenerative braking and
power assist.
The input-split EVT range
and compound-split EVT range allow continuously variable engine speed and full
hybrid functionality throughout the vehicle speed range. In addition to these
two EVT hybrid modes, four fixed gear ratios enable parallel hybrid operation
with electric motors used only for boosting and braking. This parallel hybrid
operation is possible by simply using two additional clutches. The mechanical
core for this Two-Mode Hybrid transmission is shown in this diagram.
As a result, this Two-Mode
Hybrid can operate as a robust CVT or it can select a fixed gear ratio, offering
advantages in fuel economy, comfort and performance. Operating in the fourth
fixed gear ratio as a parallel hybrid provides the opportunity to power down one
of the motors for highest steady-state cruising efficiency and to use the other
for electric boost, supplying accessories and for regenerative braking.
Operating in the other fixed gear ratios enhances acceleration, hill climbing
and towing by enabling mechanical power flow from engine to output, while the
electric motors are in parallel and devoted to electric boost or regenerative
braking. For first and third gears, both motors are available up to the limits
of the battery capacity.
The Two-Mode Hybrid is
displayed schematically on the left of this diagram. The important additional
option of parallel hybrid operation, when engine power flow is supported
entirely by the mechanical gearing, is shown on the right.
Summary of
Advantages
In summary, the six
mechanical choices offered by the Two-Mode Hybrid are:
-
Input-split EVT range
(continuously variable, "mode 1")
-
Compound-split EVT range
(continuously variable, "mode 2")
-
First fixed gear ratio with
electric boost/braking (two motors)
-
Second fixed gear ratio
with electric boost/ braking (one motor)
-
Third fixed gear ratio with
electric boost/ braking (two motors)
-
Fourth fixed gear ratio
with electric boost/ braking (one motor)
Continuously variable
ranges cover all vehicle speeds allowing smooth operation with moderate
acceleration for a comfortable driving experience. The combination of variable
ranges and four actual fixed gears provide strong multiple-ratio acceleration
for a sporty driving experience.
Two-Mode Hybrids for
SUV's, Other Trucks and Luxury Cars
Overall, the Two-Mode
Hybrid offers General Motors, DaimlerChrysler, and BMW Group customers an
optimal combination of performance, fuel economy and comfort in compact,
efficient packages. Motor peak and continuous power are reduced by the options
of operation in input-split, compound-split or fixed gear parallel-hybrid
operation. The fact that motor size is not only smaller but less dependent on
engine size enables a more conventional and smaller hybrid transmission package
and allows the Two-Mode Hybrid to be offered on a wider range of vehicles while
reducing overall system cost.
The first Two-Mode Hybrid
to be offered for sale by the Global Hybrid Cooperation will be for rearwheel-
drive SUV's and other full-size trucks. This transmission will be featured in
General Motors and DaimlerChrysler vehicles, starting in 2007 with the 2008
Chevrolet Tahoe followed shortly after by a Dodge Durango. This photo displays
the exterior of the Two-Mode Hybrid transmission for rearwheel- drive trucks,
which will be installed with large North American V8 truck engines.
Each of the members of the
Global Hybrid Cooperation will also be offering a more compact Two-Mode Hybrid
transmission for rear-wheel-drive luxury cars and other vehicles which require a
more compact transmission. This transmission will be featured in a variety of
General Motors, DaimlerChrysler, and BMW Group vehicles.
Although the Two-Mode
Hybrid design will be jointly developed, the hybrid drive-train system will be
integrated into the vehicles by the manufacturers taking into account the brand
specifications.
Hybrid vehicles will become
one of the cornerstones of future mobility.
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