Featured SUVs - Lexus RX400h
'H' for Performance: Lexus 400h Combines
the Latest Generation Hybrid Synergy Drive With Its Top-Selling
SUV and Finds Improvements in Both Performance and Fuel Economy
Automotive Industries, Feb, 2005 by John Peter
Are you happy with your Lexus RX330, but you'd like a little
more power--maybe enough to dust off the guy down the street
in the Mercedes ML 500.
How about a luxury Lexus SUV that goes from 0 to 60 in 7.3 seconds?
That's about three-tenths of a second faster than your 330 and
puts you on par with your neighbor's ML 500, or 3.4 seconds
30 to 50 mph, a second quicker than competitive vehicles. What
if it had 650 lb.ft. of torque at launch, and on top of all
that performance, it got about 28 to 30 mpg. No, it's not some
kind of super diesel, it's a hybrid-electric SUV. Hybrid isn't
just for fuel economy anymore.
In fact, Denny Clements, group vice president and general manager,
Lexus division, says that the RX400h won't be sold on fuel economy
at all.
"We're going to market it on the basis of performance,
package and quality," says Clements, "and exceed the
buyer's expectations on fuel economy."
Clements says that research shows the fuel economy is not even
on the list when buying luxury SUVs, though it does show up
as a post purchase complaint.
With an expected average of 28 miles per gallon, fuel mileage
should no longer be a factor.
Hybrid Changes
 At
first glance the 400h doesn't look much different from its sister
330. There are three principle differences in the front of the
vehicle. The grille was redesigned and an extra cooling inlet
was added for the hybrid components, which required a redesign
of the front fascia. Round fog lamps replace the rectangular
fog lamps of the 330.
The changes to the rear of the vehicle are even more subtle.
The tail lamps now have horizontal trim pieces and the bulbs
have been replaced with LEDs. LEDs use less energy while giving
off a brighter light and are also a lot more durable than incandescent
bulbs, requiring less maintenance.
The extra cooling inlet in front caused an addition in drag
that needed to be offset. Fairings were installed behind the
front and rear wheels to split up the airflow coming off the
tires. Some new underbody shields were also added to the vehicle.
These tweaks brought the coefficient of drag back in line with
the RX330.
Interior differences consist of brushed-aluminum inserts replacing
the wood inserts on the 330 and a modified IP cluster that replaces
the tachometer with a power meter.
This RX400h rides on specific 18-in. by 7-in. aluminum wheels
fitted with P235/55R 18-in. tires. Lexus chose not to go with
rolling resistance tires. Because of the vehicle's power and
performance characteristics, it was felt that the vehicle needed
sufficient rubber underneath it and a rolling resistant tire
would have been too much of a compromise.
The 400h has front struts with internal rebound springs and
linear control valves, tuned for sportier handling.
Safety First
In the active safety area, the 400h's Electronically Controlled
Braking (ECB) system combines the regenerative braking of a
hybrid vehicle with the cooperative hydraulic braking system
into what is essentially a brake-by-wire system. ECB becomes
an integral component of both the powertrain system and the
handling system.
The RX400h is equipped with Vehicle Dynamics Integrated Management
(VDIM).
VDIM is a proactive stability control system that anticipates
vehicle instability in virtually any direction. The system integrates
a yaw rate sensor, deceleration sensor, speed sensor and steering
sensor that send data to the Electronically Controlled Braking
system (ECB). The ECB system translates brake stroke speed and
pressure and generates the precise amount of combined electric
regeneration and hydraulic pressure needed for virtually any
driving condition. VDIM interfaces with the ABS, brake assist,
vehicle stability control and traction control, allowing more
precise control of these systems. It also uses a new electronic
power steering system to optimize steering assist. The system
uses a 42 VDC electric motor, wrapped around the rack Power
is supplied by a DC/DC converter.
An additional sensor has been added to the vehicle that senses
both side collision and rollover due to side collision, and
triggers the front and rear side curtain airbags. Modifications
have also been made to the body reinforcements and structural
geometry to account for the additional 300 pounds of mass in
the event of a collision.
Quiet Please
Several NVH improvements were made to the 400h. The extra weight,
extra torque and improved handling characteristics required
additional torsional rigidity. Several gussets and reinforcements
were added to stiffen up the body.
The windshield is made of an acoustic damping laminated glass
to keep out unwanted noise when the engine is shut off at rest.
The cooling fan blade tips and shroud were redesigned to reduce
the noise signature and keep it from being transmitted into
the cabin. The 400h also uses direct gear drive on the front
transaxle, which further reduces noise.
Powertrain
The 400h uses the same 3MZ-FE 3.3L 24-valve, V-6 engine that's
found in the RX330. It puts out 208 hp, about 10 percent less
than the 330 due primarily to changes in the intake and exhaust
systems for hybridization.
The engine uses the Otto cycle instead of the Atkinson cycle--another
conscious decision on Lexus' part to lean more toward performance.
It has electronic throttle control, calibrated specifically
for the hybrid application, and the variable valve timing has
been tuned to facilitate hybrid operation. Starting the engine
when the vehicle is moving can create an undesirable noise,
so the intake valve timing has been adjusted to create a later
dosing which allows the engine to start while making less power,
reducing the NVH spike.
The 400h doesn't use the acoustic control intake system from
the 330 as the vehicle gets the majority of its initial torque
from the electric motor. Losing the acoustic intake also made
space for the power electronics.
The hybrid V-6 has no external starter or alternator and no
serpentine belt, which not only deans up the package but reduces
the parasitic losses of a beat system.
Because the engine is frequently off, the 400h uses an electric
power steering system.
This vehicle uses an electric A/C compressor because with a
conventional compressor the engine would need to run just to
keep the cabin cool, reducing fuel economy.
The electric A/C compressor is slightly different than Prius'.
The electric inverter is mounted inside the compressor itself.
DC voltage is sent to the compressor and the onboard converter
supplies AC to the system. It also has an electric water pump
to pump block water to the heater core during engine-off operation
in winter to provide heat to the cabin.
On the intake side of the system there is a charcoal dement
that reduces the trapped vapors during engine shutdown to achieve
a zero evaporation compliance. There is a higher cell density
catalytic converter system on the exhaust side which enables
the vehicle to reach SULEV tailpipe emissions. The vehicle is
certified SULEV in California and meets the Federal standards
for Tier 2 Bin 3.
Unlike Prius, which uses a hot water bottle to heat the intake
manifold, the 400h has a different cold-start emission device
that delays the start while it preheats the air/fuel ratio sensors.
This allows the air/fuel ratio sensors to be active when the
engine does start, reducing cold-start emissions. There is no
need to wait on the engine as there is plenty of electric juice
to launch the vehicle.
Drivetrain
The hybrid transaxle is a three-shaft configuration (as opposed
to the Prius' four-shaft transaxle), which results in a more
compact size with reduced noise. The system uses two water/oil-cooled
permanent magnet electric motors designated MG1 and MG2 and
two planetary gear sets. The direct gear-drive transaxle uses
no clutches, bands, valves, hydraulics, linear actuators or
any of the parts generally associated with either an automatic
transmission or band- or belt-type CVT. It has a final drive
ration of 3.543:1 and weighs 236 pounds, about 26 pounds more
than the five-speed automatic in the RX330. While 26 pounds
may seem like a lot, if the Prius transaxle was scaled up to
handle the level of power made by the 400h, it would be significantly
bigger.
MG1 is principally the generator but it also serves as the starter
for the engine and the control function of the electronic CVT.
It provides 109 kW at 13,000 rpm (146 hp).
MG2 serves as the motor, except under braking when it becomes
a generator. It provides 123 kW at 4,500 rpm (165 hp) and 355
Nm (262 lb.ft.) of peak torque at 0-1,500 rpm. That's even more
impressive when you realize that it's multiplied by nearly two-and-a-half
before it's delivered to the final drive.
The transaxle uses two planetary gear sets. The Power-Split
unit divides engine output to drive the wheels and MG1, the
generator. The Motor Speed Reduction planetary reduces MG2's
12,500 max rpm to a more useable final gear ratio of 2.478:1.
"Mechanically, this transmission is very simple, much simpler
than any other transmission out there. It is a really elegant
way to get variable gear ratios without having to have a lot
of extra pieces in the box," says Dave Hermance, executive
engineer, environmental engineer, Toyota Technical Center.
The 400h's innovative, optional all-wheel-drive system uses
an additional electric motor to drive the rear wheels and can
electronically vary torque front to rear (up to all available
torque to the rear), when extra traction is needed. The rear
motor can also send up to 650 lb.ft, of torque to the rear wheels,
providing V8-type acceleration.
The rear transaxle is also a three shaft configuration with
one motor/generator and no planetary gears. It is also direct
drive and uses no clutches, bands or hydraulics with a 6.86:1
gear ratio and a total weight of 90 pounds, still less than
the weight of the rear axle on a four-wheel-drive RX330. The
system isn't as complex as a conventional 4WD system because
it doesn't include the propeller shaft and support bearings,
just three wires going back to the control box.
The rear drive motor is air/oil cooled as it would be expensive
to route water lines to the rear of the vehicle and makes 50
kW (67 hp) and 96 lb.ft. of torque from 0 to 610 rpm.
The power electronics sit over the top of the front transaxle
and include a system to convert high-voltage DC into three-phase
AC for the three electric motors. It is controlled by the hybrid
ECU and has a boost converter which is a characteristic of all
the Hybrid Synergy Drive systems. This particular system increases
pack voltage from 288 VDC to a system-operating voltage of 650
VDC. (Prius is 202 to 500). Use of this higher voltage holds
the currents down while still making lots of power, reducing
the cost of the electronics.
The DC/DC converter converts 288 VDC to 12 VDC to run the auxiliary
systems. It shares the cooling system with MG1 and MG2 and is
integrated into two stacked radiator systems in the front of
the vehicle.
The motor/generator ECU is now packaged inside the inverter,
reducing one set of inter-connectors, simplifying the system,
The electronics package weighs 70 lb.
There is a separate 12V battery that's used to wake up the computer.
When the system is in the key-off position, the system main
relay opens and separates the high-voltage system from the rest
of the vehicle. The 12V battery is needed to bring the computer
up to energize the system main relay.
The battery pack is the latest-generation NiMH battery. It contains
30 modules with a pack voltage of 288 VDC nominal. Each module
is eight individual cells creating 9.6 VDC with a peak power
of 85 kW, an increase of 80 percent over the Prius.
The metal case improves heat rejection and reduces the size
of the height of the battery by 22 percent, allowing it to be
packaged under the rear seat, while only increasing the rear
seat H-point by 1 in.
The modules are split into three sections to accommodate the
support rails of the 40/20/40 split folding rear seat. Each
module has its own independently variable cooling system The
150 pound battery pack includes the battery ECU, system main
relay and service plug.
The 400h uses a fourth generation hybrid ECU that includes all
of the engine ECU functions, simplifying the interconnection
installations and reducing cost. A CAN-bus is used for all of
the system's high-speed communications.
The electric all-wheal-drive system is principally an on-road
traction enhancer. Its primary job is to manage the large amount
of torque produced by the system and also to ensure superior
performance in slick and wet conditions. It was not designed
to be an off-mad system It has a variable front/rear torque
drive ratio of essentially 30 percent to the rear, on dry surfaces.
Under slick conditions, all of the available torque can be delivered
to the rear axle. Traction control is improved because it's
much faster to control an electrical motor than either modulate
the brakes or throttle in a conventional traction control system
It also allows for four-wheal regenerative recapture and is
managed by VDIM system.
A dosed-loop fuel system replaces the bladder tank used in the
Prius. The system allows no flow path for the vapors to get
out of the tank when the engine is turned off, using a tank
closing valve that sits on top of the tank outlet and doses
the system from the vent standpoint anytime the engine is off,
both at rest and idle stop. The evap canister is only used for
refueling. In order to accommodate the extra pressures, the
tank material has been increased to 2.0 mm from 0.8 mm. Refueling
the 400h requires several steps as the vapors need to be bled
from the system before the cap can be opened When the cap is
released from inside the vehicle, it actuates an electric solenoid
on the electronic fuel door. A series of messages come up on
the IP, letting the driver know when it is safe to refuel. The
tank holds 17.2 gallons and has a direct-read fuel gauge.
The hybrid Lexus is built similar to the Prius. It will use
the same basis body-in-white as the RX330 with modifications
to the floor pan to accommodate placement of the battery beneath
the rear seat. Both the IP and ECU are built on a sub line off
the main assembly line. Like most Toyota vehicles, the powertrain
is brought in on a rolling cart and bolted into place, so sequencing
in the hybrid drivetrain doesn't require any extra steps. The
only extra assembly step needed is the installation of the battery.
The decision to do the hybrid Lexus was made before the second
generation Prius was launched. The RX was picked because Lexus
felt that it would be easier to integrate the hybrid drivetrain
into an SUV. And it would also offer engineers a chance to learn
how to adapt Hybrid Synergy Drive to existing platforms. The
eCVT transmission is reasonably adaptable to any Toyota or Lexus
engine as it's not much bigger than the company's five-speed
automatic. Toyota says the system would be a natural for an
all-wheal-drive Sienna minivan.
"We're getting to the point," says Dave Hermance,
"Where integration will be primarily software development."
The 400h will goes on sale April 15, 2005, and Lexus has already
received more than 9,000 pre-sold orders through its website.
Clements says that each 400h will be built to order at Toyota's
Kyushu, Japan, plant where it will be built alongside the RX330
(Harrier in Japan) and Toyota Highlander (Kluger). A hybrid
version of the Toyota Highlander, which shares the same platform
as the RX330, will begin production in March.
While demand for hybrids is high in North America and the introduction
of the 400h pushed back several months to allow the plant to
ramp up production before hunch, Clements still doesn't see
the 400h taking off like the Prius.
"With the fairly robust premium (about $4,500) we don't
see them selling 10,000 a month," Clements says. "We'll
try to meet demand and if the demand isn't there, we won't build
them."
Build-to-order won't be that difficult, as Clements says the
400h doesn't have a lot of options to choose from.
"Customers can just come in and pick color."
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