Overview: Lexus GS 450h
Lexus’ GS 450h has been on sale in Britain since mid-2006. It is a full series-parallel hybrid, using a 3.5-litre V6 petrol engine together with a high-output electric motor. ‘Series-parallel’ means that the two power units can operate independently or together, depending on the driving conditions.
Because of Japanese engineers’ apparent inability to understand the point of diesel engines, the 450 uses its very complex drivetrain to fight talented European turbodiesels. Lexus in the U.K. emphasises its hybrid’s superior economy to ‘comparable V8 models’. As you can see from the table below, the Lexus puts in a respectable showing — except, of course, that things would look rather different if those European turbodiesels had hybrid drivetrains as well.
The GS 450h’s V6 is both directly and indirectly fuelled. Each cylinder has two injectors: one in the combustion chamber, and one in the intake port. The port injectors have 12 holes and operate at a maximum of 4.0 bar (400kPa). The in-cylinder injectors have twin rectangular slits measuring 0.52mm by 0.13mm, producing a double fan injection pattern at a maximum of 13.0 bar (1.3MPa).
Under cold start conditions, the engine uses both the port injectors and direct injection for an air/fuel mixture of 15-16:1. (A stoichiometric mixture has an air-fuel ratio of 14.7:1.) This richer mixture raises the combustion temperature and contributes to a quicker warm-up of the two thin-wall catalysts.
At idle, the engine runs on direct injection alone. At low to middling loads at lower crankshaft speeds, both direct and port injection systems are used during the intake stroke. This creates a homogenous, 12-15:1 air:fuel ratio. When the engine is running under heavy loads, the direct injection system alone is employed during the intake stroke, again at 12-15:1.
The block, cylinder heads, head covers, oil pan, pistons and bearings for the connecting rods and crankshaft are made of aluminium, while the connecting rods themselves are forged in high-tensile steel. The aluminium block and cast iron liners are produced as a single unit. There is no starter motor, alternator or serpentine belt, as the V6 is started by the hybrid drivetrain’s electric generator.
Timing of both intake and exhaust valves is continuously variable, between 60 and 35 degrees respectively. The settings used are specific to the hybrid car, with valves opening earlier and closing later than usual.
The V6 engine’s exhaust system uses a twin-wall stainless steel manifold to reduce noise and improve thermal insulation. The latter improves the rate at which the catalyst warms up. Pressure control valves in both main silencers close to reduce noise at lower speeds, opening as speed rises to reduce back pressure.
The power output of the V6 — working without any help from the electric motor — looks good next to the turbodiesels, but its torque figure is rather limp. That’s not the whole story, though: the electric motor provides a very substantial boost, at least for short periods, so brief bursts of acceleration can be accomplished with great energy. And even the 0-100km/h sprint amounts to a brief burst for the GS, at 5.9 seconds.
The two power-units drive through an electronically-controlled CVT. This features a ‘sequential mode’ to make it feel more like a conventional transmission.
The hybrid drivetrain
Along with the V6 and the electric motor, the hybrid system also deploys a generator, a 288V
nickel-hydride battery, a power split device, and a compact power control unit.
The hybrid transmission incorporates a two-speed epicyclic motor speed reduction gear. This is aimed at getting the best acceleration in town without sacrificing the usefulness of the electric motor for motorway overtaking: electric motors are at their most efficient at low speed, and at their weakest at high speed.
The electric motor, generator, power split device and two-stage motor reduction gear are housed in a single lightweight, compact longitudinal transmission casing that is one centimetre longer than the six-speed manual transmission in the Lexus IS.
The 450’s electric traction motor is a three-phase, permanent magnet AC synchronous device. It operates at 650V, with its operating voltage overseen by the power control unit. To fit the motor inside the compact transmission casing, the stator wiring is of small diameter and follows a double-star pattern: this allows the unit’s diameter to be reduced as far as possible. The motor delivers 275Nm from the first revolution and a maximum of 199PS.
The Lexus GS 450h has a 50:50 front-to-rear weight distribution, with the NiMH battery pack mounted directly above the rear axle line. A limited-slip differential is fitted.
The electric power steering incorporates a variable gear ratio system, which alters the steering gear ratio according to car’s speed. The variable ratio steering is controlled, like other dynamic systems, by the car’s Vehicle Dynamics Integrated Management (VDIM) system. This also looks after the 450’s adaptive variable suspension, which offers a choice of Sport and Comfort settings.
Wheel location is by double wishbones at the front with a multi-link arrangement at the rear.
Lexus’s pre-crash safety system is available as an extra on the 450. Like other systems we have seen elsewhere, it works in conjunction with the cruise control. In the case of the GS, the seat-belts are primed and the braking system delivers emergency braking if a frontal collision is deemed imminent. Under more normal circumstances, the adaptive cruise control automatically matches the speed of the car to that of the vehicle in front, up to a pre-set maximum, maintaining a safe distance as it does so.
Overall electronic responsibility for coordinating the Lexus’s various dynamic aids falls to the Vehicle Integrated Dynamics Management (VDIM) system. This supervises the ABS, electronic brakeforce distribution (EBD), traction control, and vehicle stability control (VSC), integrating them with the active elements of the system: adaptive variable suspension (AVS), and the electronic, variable gear ratio power steering.
Feeding the GS 450h’s traction motor is a 288V nickel hydride battery pack co-developed with Panasonic. This is managed by a unit which monitors the state of charge and the rate of charging, detects problems and performs fail-safe system management. The battery pack lives in the boot and was designed to last as along as the other principal powertrain components. Lexus claims that the battery is almost 100 per cent. recyclable. It is automatically recharged by the hybrid drive system, and requires no external charging.
An important factor in achieving high durability for any battery pack is the management of its charge and discharge cycles. The hybrid system’s ECU, linked to the battery’s monitoring unit, ensures that the battery pack is never overcharged or ‘deep-discharged’, conditions which cause irreparable damage to battery cells.
Power control unit
This consists of a voltage boost converter and a DC/AC inverter. The converter increases the battery’s voltage from 288V to 650V to drive the motor and — occasionally, when it’s used as a starter motor — the generator. At the same time, the DC/AC inverter converts the DC into AC.
The size and weight of the PCU were minimised by using smaller capacitors and combining components to create a more compact solid state device. The complete unit is comparable in size to a conventional 12V battery.
Power split device
This uses an epicyclic gear train to divide the petrol engine’s output into two. There is a central sun gear, an outer ring gear, and intermediary planetary pinion gears which engage the central and outer gears. The generator is connected to the sun gear, the engine is connected to the planetary pinion gear carrier, and the electric motor is connected to the outer ring gear; this, in turn, is directly connected to the differential. So with power transmitted either from the engine, the electric motor, or a combination of the two, the rotational speed of the ring gear is directly proportional to the speed of the vehicle.
The planetary gear set facilitates the hybrid system’s full range of series and parallel drive configurations.
During initial start-up and at low speeds, the petrol engine is not running. The planetary pinion gear carrier is stationary. Powered by the electric motor, the ring gear drives the wheels and, via the pinion gears rotating on the stationary carrier, drives the sun gear attached to the generator.
To start the engine as vehicle speed increases, momentarily stopping 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, power is transmitted back through the pinion gears on the rotating carrier to both the outer ring gear — driving the wheels — and the inner sun gear. Turned by the sun gear, the generator supplies electric power via the power control unit either to recharge the battery or drive the motor.
The 450’s longitudinal hybrid transmission incorporates a two-speed (3.90 and 1.90) epicyclic motor speed reduction gearbox, coupled directly to the electric motor. In normal conditions, during acceleration, the reduction system will make a gear-change at around 56mph. An electric motor becomes increasingly inefficient as its speed rises, so two-stage gearing allows the motor to remain reasonably efficient — and capable of generating good torque — throughout the car’s speed range. (A CVT that keeps the motor speed low would be the ideal solution in theory, but it would add further mass and bulk to the drivetrain.)
E-CVT with sequential brake shift mode
The GS 450h uses an electronically-controlled CVT with Sequential Shift Mode — marked ‘S’ on the selector — in which the CVT mimics a conventional gearbox with six fixed speeds.
By selecting a lower ratio under braking or deceleration, the brake control ECU increases the amount of engine braking as a percentage of the total brake force applied to the rear wheels, increasing the load on the generator to charge the battery and also applying negative torque to the engine.
Hybrid drive in operation
There are three operating modes. The first is for hybrid operation, balancing power and traction; ‘Power’ gives a livelier response to the accelerator; and ‘Snow’ maximises traction. These are engaged by means of a mode select switch on the centre console.
Left to its own devices over the course of a journey, the powertrain will drive the car in a variety of different ways to maximise fuel efficiency.
When the engine is cold and the ignition is switched on, the system will start the engine to warm the unit. Once it’s warmed up, the stop-start system will shut the engine down whenever the car comes to a stop. But if the car is at idle for prolonged periods, the engine will be restarted to charge the high-voltage battery pack.
In conditions where engine efficiency is low, such as when pulling away from rest and at low speeds, the GS runs on its electric motor alone. The system also gives the car a measure of in-gear ‘creep’, like a conventional automatic transmission.
In normal driving, engine output is divided by the power split device to drive the wheels directly and to power the generator, which in turn drives the electric motor and simultaneously charges the high-voltage battery. In these conditions, the distribution of power is constantly adjusted between the engine and the electric motor to maximise efficiency.
When sudden acceleration is required, the engine and electric motor operate in tandem, with extra power from the battery to boost motor power.
During deceleration and under braking, the engine switches off and the electric motor acts as a high-output generator for the nickel-hydride battery pack.
Under all driving conditions, the battery power level is managed by the engine-drive generator, so there is no need to recharge the system from an external source.
Lexus’s figures show a combined power output of 345PS, standing start acceleration to 100km/h in 5.9s and a maximum speed of 155mph. Because an electric motor is involved, low-end torque is substantial, with 275Nm available from the first revolution of the motor.
Depending on the battery’s state of charge and the loads demanded by the driver, the GS can be driven for up to 1.2 miles under electric power, at speeds up to 20mph and with the climate control system still operating.
The 450’s electric power steering system has a variable ratio. An actuator is attached to the intermediate steering column shaft, which alters the steering gear ratio according to vehicle speed. The adjustment is seamless and continuous, between 11.5:1 and 17.2:1 — a range of about 39 per cent. The number of turns lock-to-lock varies between 2.7 and 3.7.
At very low speeds, the gear ratio is at its highest for manoeuvring. At moderate speeds, the gear ratio is a little lower, with an emphasis on steering response without too much twitchiness. At high speeds the ratio is at its lowest, ensuring gentle response to inputs and maximum vehicle stability. The variable ratio steering control communicates with the Vehicle Dynamics Integrated Management (VDIM) system.
Wheel location is by means of double wishbones at the front and a multi-link arrangement at the back. The wishbones are mounted high, using forged aluminium knuckles with high tensile steel arms. The anti-roll bar is hollow to reduce unsprung weight. The forward-mounted power steering gear and a low toe variation along the suspension stroke are designed to promote a more linear yaw response, steering precision and straight-line stability.
At the back, upper arms help reduce upward spring motion, the mid-arm reduces body roll, and the lower arm provides an anti-lift and anti-squat function.
The GS 450h uses Lexus’s adaptive variable suspension (AVS) system. There are two modes: ‘Normal’ and ‘Sport’. The system’s functionality is fairly self-explanatory, except that Sport mode automatically increases the difference between the inner and outer damping rates in an effort to reduce (or rather, to slow) body roll. At the same time, the variable gear ratio steering automatically increases the steering gear ratio while electrohydraulic assistance is increased.
AVS adjusts the damping force on each wheel independently, in nine stages. It monitors a range of vehicle data, including engine speed, front wheel speed, steering movement and brake-light activation. It also takes readings from three accelerometers.
Inevitably, the suspension system communicates with the VDIM and pre-crash safety systems. One result is that stiffer damping is invoked when these systems are activated.
Vehicle Dynamics Integrated Management (VDIM)
Lexus’s Vehicle Dynamics Integrated Management system uses status data fed from sensors throughout the vehicle, analysing the actions of the driver — such as steering movement, acceleration, braking and gear-changing — and data from accelerometers and sensors for yaw-rate and wheel-speed. The system sets out to to match the car’s dynamics with the apparent intentions of the driver.
VDIM integrates the by-wire electronically controlled braking, brake assist system, electronic brakeforce distribution, traction control and vehicle stability control with the variable-ratio electronic power steering. It also communicates with the adaptive variable
suspension to control damping rates.
One example of the system’s integration is when the driver brakes on a ‘split-mju’ surface. This means that one side of the car has better grip than the other. The VDIM system, like four-channel ABS, monitors the speed of each wheel independently. But in this case, instead of simply modulating brake-line pressure from each wheel that is locking, the system will actively apply torque to the steering to keep the car running straight. The same methodology is applied if the driver accelerates on a split-mju surface.
Rather more radical is the system’s response to putative oversteer. If the car’s yaw-angle sensor detects the onset of oversteer, the VDIM prompts the steering to deliver counter-lock. If the oversteer persists, VDIM uses the by-wire braking system and the hybrid drivetrain’s torque output to bring the vehicle back to its original trajectory.
The system takes a similarly ‘hands-on’ approach to understeer. The VDIM begins by increasing the steering angle. If this isn’t enough to reduce the understeer, the system will use the brakes and powertrain to control the car’s yaw moment. At the same time, if the driver tries to steer too far towards the inside of the corner — potentially increasing understeer — VDIM will limit this steering input through the steering with the intention of helping the front wheels to retain their grip.
Electronically Controlled Braking (ECB)
The 450’s electronically-controlled braking system interacts with the car’s other systems in a way that is’t possible with a conventional, fully hydraulic braking system.
First, the system anticipates braking. As the driver takes his foot off the accelerator, the brakes are put into ‘standby’ mode, bringing the pads as close as possible to the discs. This slightly improves the immediacy of the response when the brake pedal is pressed.
ECB also controls brake energy regeneration for recharging the hybrid system’s battery pack. The system uses a combination of direct hydraulic braking power and regenerative power from the electric motor under deceleration — that is, the electromagnetic resistance to rotation. A hydraulic pressure control function balances the total braking force between that supplied by hydraulics and by the motor.
The ECB system is linked in with the 450’s hill-start assist system. This detects any backward movement during a hill start and automatically increases brake pressure to all four wheels to give the driver time to step from the brake to the accelerator without the vehicle moving backwards significantly, thus proving an option for drivers unable to use the parking brake or both feet.
Adaptive Front Lighting System (AFS)
The adaptive front lighting system improves night-time vision when cornering by swivelling the headlights through up to 15 degrees, in line with the vehicle’s speed and steering angle.
Pre-Crash Safety (PCS) system
As an optional extra, the GS 450h is available with a pre-crash safety (PCS) system that can anticipate certain types of frontal collision.
PCS uses a millimetre-wave radar sensor to detect obstacles up to 150m ahead of the car.
Using vehicle sensors, the PCS monitors vehicle speed, steering angle and yaw rate to help determine in advance whether a collision is unavoidable. If it is, the system pre-emptively activates the pre-crash seatbelt tensioners and adjusts the emergency brake assist system to give maximum braking force once the brake pedal is depressed. Note that the onus is still on the driver to take action here.
At vehicle speeds of more than 9mph, PCS will also activate the front seatbelt pretensioners in cases of severe over- or understeer and during emergency braking. Operating in conjunction with the adaptive suspension, the PCS system also automatically regulates the suspension’s damper rates to minimise nose-dive under emergency braking.
Adaptive Cruise Control (ACC)
The optional pre-crash safety system comes with adaptive cruise control. This vehicle-to-vehicle distance control system can differentiate between vehicles directly ahead and those in an adjacent lane. It uses the PCS millimetre-wave radar sensor, linked to cruise control functions to slow the vehicle automatically to match the speed of the vehicle ahead; once the road is clear, the car will accelerate back to the selected cruising speed.
Steering-guided parking assist sensors
The GS 450h is available with parking assist sensors which respond to steering angle. Four dual sonars are installed in the front bumper, with two back and two corner sonars in the rear bumper. The front sonars change their scanning direction according to steering wheel angle, enabling the system to process obstacle position data and predict the likelihood of contact. The location and proximity of any object is displayed on the car’s touchscreen, together with a suggested steering correction to avoid contact.