Lexus RX400h - In Detail

Press Release

Suspension

 

The new RX 400h employs a substantially enhanced version of Lexus’ proven, front MacPherson strut and rear dual-link strut suspension system to provide impressive handling characteristics, excellent steering response and high speed straight-line stability. The revised system has been tuned to optimise driving dynamics, combining sporty handling with a comfortable ride and minimal Noise Vibration and Harshness (NVH) to guarantee Lexus refinement under all driving conditions.

 

Front suspension

 

The improved front MacPherson strut suspension system features a weight reducing, pressed steel-plated L-shaped lower arm, incorporating high damping bushings, revised shock absorbers with a redesigned valve and oil and seal refinements to promote more linear damping. The

introduction of yoke control coil springs to reduce body roll, realise a flatter cornering attitude and create added vehicle stability during cornering.

 

The optimisation of the caster angle and the adoption of ‘Nachlauf’ geometry, which offsets the kingpin axis ahead of the axle centre, result in increased caster trail which enhances both straight-line stability and steering feel. The location of the kingpin axle outside the wheel, allied to the addition of a small, well-shaped sub-frame, results in excellent tight-turn performance and a compact turning circle of just 5.7 metres.

 

Lower arm and tie rod geometry has been further refined to suppress changes in toe angle under suspension bounce/ rebound, thus ensuring excellent straight line performance.

And the angled location of the lower arm member with the rear side higher than the front creates an anti-dive suspension geometry effectively to suppress nose-diving under braking.

 

In addition, the front suspension also adopts a small level of negative camber, effectively leaning the top of each front wheel inwards when the vehicle is running in a straight line. Under cornering load, the change in vehicle posture naturally reduces the ground contact camber angle of the outside front wheel, optimising contact between tyre tread pattern and road surface for enhanced, high performance cornering.

 

Rear suspension

 

The dual-link strut rear suspension system features a more rigid axle design incorporating third generation axle bearings for high torsional rigidity. The carefully considered tuning of coil spring and damper rates, stabilisers and bushings within the optimised suspension geometry promotes improved handling stability allied to a more comfortable ride.

 

Electric Power Steering

 

The utilisation of Electric Power Steering (EPS) within the RX 400h offers several advantages over comparable hydraulic systems. Powered by a brushless, 42 volt DC motor and a gear reduction mechanism mounted on the coaxial of the rack shaft, the EPS system consumes electricity only when assistance is required, using no energy when, for instance, driving in a straight line, and therefore helping reduce fuel consumption by approximately 1.5%.

 

The system structure, comprising motor, torque sensor and reduction mechanism, is markedly more compact and noise-free in operation. By introducing a ball screw reduction gear, smooth rotation, high efficiency, and low friction and noise have been achieved.

 

The EPS Electronic Control Unit features a 32-bit CPU to increase signal processing speed, and actuates the DC motor based on signals received from various sensors, including steering torque and vehicle speed sensors. Power assistance characteristics are changed particularly smoothly, providing an enjoyable, seamless steering feel that is light at low speeds and increasingly responsive as vehicle speed builds.

 

The absence of power steering fluid in the system makes it environmentally friendly at the time of disposal; servicing and component removal is easier; and bleeding air from the system is no longer necessary.

 

3.3-Litre V6 Petrol Engine

 

The RX 400h employs the 3MZ-FE engine already familiar to drivers of the RX 330 in the US, but features significant revisions to intake, exhaust, cooling and engine control systems to optimise the unit’s compatibility with Hybrid Synergy Drive®.

 

The 3MZ-FE engine is a 3,311cc, four-cam, 24 valve, V6 petrol unit, employing the Otto cycle for performance orientation, and producing a maximum power of 155kW/211DIN hp @ 5,600rpm and 288Nm of torque @ 4,400rpm.

 

Variable Valve Timing - intelligent (VVT-i) guarantees optimum power under all conditions, increasing low-speed torque, boosting overall power at higher speeds and, in coordination with the hybrid control systems, improving fuel efficiency. The use of ETCS-i (Electronic Throttle Control System-intelligent – replaces traditional cable control between the accelerator pedal and the fuel injection) further enhances fuel efficiency and, allied to VVT-i, contributes to smooth engine starting and stopping with a significant reduction in Noise Vibration and Harshness (NVH) during engine start whilst the vehicle is in motion.

 

The unit has no starter, alternator or serpentine belt, and the Hybrid Synergy Drive® system generator is used to start the engine, rather than a conventional starter motor. Moreover, in order to optimise engine speed control for maximum hybrid system fuel efficiency, engine speed is controlled –taking vehicle speed and throttle depression into consideration- by the generator. Depending on vehicle conditions there are, however, instances when the engine speed is controlled independently by the throttle. When its operation is not required by the hybrid drive system, the generator stops the engine immediately after the fuel supply is cut, at which point the fuel pump is also stopped to reduce auxiliary component electric consumption. The installation of both Electric Power Steering and Electric Air Conditioning also reduces power demand from the engine, further optimising fuel efficiency.

 

In order to accommodate the Hybrid Synergy Drive® system inverter within the engine compartment, the intake system air cleaner assembly is installed directly over the engine. The adoption of a slit resonator helps both to reduce noise and enhance performance.

 

Modifications to the cooling system include a modularised radiator for air condenser and inverter use, realising space savings and component number reductions. An all-aluminium engine radiator has been introduced for easier recycling, and the use of a ring fan with Pulse Width Modulation offers high efficiency combined with noise reduction and a saving in fan control system power.

 

Within the exhaust system, the adoption of a tri-metal (platinum, palladium, and rhodium) catalyst offers excellent anti-deterioration performance both at high temperature and times of low temperature activation. A spring-type variable valve within the main exhaust muffler opens to reduce airflow resistance and enhance output power at high engine speeds, remaining closed in the low revolution range greatly to improve soundproofing performance.

 

Front Power Unit

 

The front power unit consists of the front electric motor, generator, power split mechanism and the motor speed reduction planetary gear. All attendant gearings are laid out in a three shaft, transaxle configuration within the

remarkably compact transmission casing, on an extension of the engine crankshaft. One of the notable characteristics of the new, quieter hybrid system is that it has achieved both a higher power output and a reduction in transmission unit size by combining planetary reduction gears with a newly developed, compact, high speed motor.

 

The hybrid transaxle’s 3.137 final drive, direct gear drive configuration is both more compact and quieter running than comparable chain-driven systems, whilst the absence of clutches, bands, valves or hydraulics ensures low­ maintenance and mechanical durability. The unit has a total mass of 118kg which is 25kg heavier than a 5-speed automatic transmission.

 

Front Electric Motor

 

In order to satisfy the high RX 400h performance targets, a compact, high output motor of unprecedented power was required. The Lexus hybrid system employs a water/oil cooled, AC synchronous type motor of a high-efficiency DC, brushless design operating on AC current. A rotor constructed from stacked, electromagnetic steel plates and Neodymium permanent magnets arranged in an optimum, V configuration combine to maximise drive torque and power output.

 

This, allied to a boost in power supply voltage to 650V, a significant increase in motor rotation speed to a maximum of 12,400rpm and the adoption of a planetary gear to amplify torque through motor speed reduction, has increased power output by approximately 2.48 times that of the Prius motor, despite the respective units being of near identical size.

 

With a power output of 123kW/167DIN hp @ 4,500rpm and 333Nm of torque @ 0-1,500rpm, the new unit boasts the highest output per unit of weight and volume in the world.

 

Generator

 

As with the electric moto r, the water/oil cooled generator is also of the AC synchronous type. The generator’s exceptionally high rotational speed generates increased power to the new, high output motor, significantly enhancing the power supply up to the medium-speed range and improving RX 400h acceleration in the low to medium speed arena.

 

Power split planetary gear

 

The power split device is at the very heart of the RX 400h’s Hybrid Synergy Drive® system. It features a planetary gear set, which is aligned with the generator, front electric motor and planetary reduction gear for motor deceleration, on an extension of the engine crankshaft within the transmission casing.

 

Via two output shafts, the power split device is connected on the one hand to the generator, on the other, to the electric motor and the wheels. Thus the power output of the engine may be transmitted along two separate routes; a mechanical route -incorporating torque from the electric motor- to the drive wheels, and an electrical route to the generator.

 

The device employs a planetary gear set (so called because planetary gears orbit a sun gea r, as in the solar system) to divide the power output of the engine between the two routes. This comprises a central sun gear, an outer ring gear, and intermediary, planetary, pinion gears which engage both the former. The pinion gears are themselves attached to a carrier, enabling them to both rotate around their own axis and the central, sun gear.

 

The generator is connected to the sun gear. The engine is connected to the planetary pinion gear carrier. The electric motor is connected to the outer, ring gea r, itself directly connected to the differential, which drives the wheels. Hence, with power transmitted to it either from the engine, the electric motor, or a combination of the two, the rotational speed of the ring gear determines the speed of the vehicle.

 

In this configuration, with a 72:28% engine torque split between ring and sun gears, the planetary gear set allows for the full range of power transmission options inherent in the Hybrid Synergy Drive® system.

 

During initial start up and at slow speeds, the engine is not running and the planetary pinion gear carrier is stationary. Powered by the electric motor, the ring gear revolves, driving the wheels and, via the pinion gears revolving on the stationary carrier, rotates the sun gear attached to the generator.

 

To start the engine as vehicle speed increases, momentarily stopping the rotation of the sun gear generates sufficient force (via the ring gear’s rotation of the planetary gears) to set the planetary pinion gear carrier in motion, turning the engine crankshaft. Once the engine has started, it transmits power back through the pinion gears on the revolving carrier to both the outer ring gear, driving the wheels, and the inner sun gear. Rotated by the sun gear, the generator supplies electric power, via the power control unit, either to recharge the battery or drive the motor.

 

During full acceleration, when both engine and motor operate in tandem to drive the wheels, more power is supplied by the battery to the motor, and engine output increases. Under these circumstances, slowing or even stopping the rotation of the generator’s sun gear diverts more of the engine ’s rotational force through the pinion gears to the outer ring gea r, transmitting additional power to the wheels.

 

Motor speed reduction planetary gear

 

The secret of the new, 123kW/167DIN hp electric motor’s compact dimensions lies in its connection to a second, dedicated, rear planetary reduction gear, also aligned with the power split device and generator, on the engine crankshaft.

 

Once again, this planetary gear system comprises a central sun gear, an outer ring gear, and intermediary, planetary, pinion gears which engage both the former. Within this mechanism, however, sun gear is connected to the motor, and the ring gear is connected to the power output shaft. Moreover, the intermediary, pinion gears are connected to a carrier which, in this case, is fixed to the transmission casing and does not rotate.

 

When the motor drives the sun gear, the planetary pinion gears rotate on their own axis, causing the ring gear to revolve. Due to the gear ratio differential, the rotational speed of the ring gear is slower than that of the sun gear. Therefore, as the motor’s rotary speed is reduced, motor torque is increased.

 

Via the gearing of this motor speed reduction device, the motor torque becomes 1: 2.478. Since motor size is proportional to motor torque, the use of the planetary reduction gear allows for the generation of torque appropriate to the performance requirements of the RX 400h within a remarkably compact motor design.

 

Electronically controlled Continuously Variable Transmission (E-CVT)

 

In conjunction with this unique, hybrid transaxle drive system, the new Lexus RX 400h further benefits from an electronically controlled, Continuously Variable Transmission (E-CVT) which simulates a continuous variation of the transmission ’s current ratio by selectively controlling the rpm of the engine, generator and electric motor. As a result, the engine and the motors operate at the most efficient points regardless of the vehicle’s speed. The E-CVT in the RX 400h provides seamless acceleration without the shift point felt with conventional transmission. Transmission friction losses have also been reduced by some 30% through the use of ball-bearings and low viscosity oil.