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Engine power has been increased by 10 percent and fuel consumption has gone down by 28 percent.
The improvements to achieve improved performance, driveability, fuel economy, noise and emissions include:
| • | Adoption of the Atkinson combustion cycle |
| • | Cooled Exhaust Gas Recirculation (EGR) |
| • | Exhaust Heat Recovery (EHR) |
| • | New Power Control Unit |
| • | Increased front and rear motor torque |
| • | Reduced friction losses in hybrid transaxle |
| • | Eco and EV driving modes |
| • | AI-SHIFT control (Artificial Intelligence - SHIFT) |
Total power output is now 295 bhp - 10 percent more - enabling a top speed of 200 km/h and 0 to 100 km/h acceleration in 7.8 seconds. Conversely, the RX 450h has the lowest fuel consumption of any premium crossover model with a combined cycle figure of 19 km/l. CO2 emissions are also remarkably low at just 148 g/km.
Hybrid Drive System Configuration
The front drive unit is a series/parallel hybrid system featuring a 3.5-litre V6 petrol engine, a powerful electric motor, a generator, a high performance battery, a power control unit and a power split device with a planetary gear set to combine and re-allocate power from the engine, electric motors and generator as driving conditions require.
The rear unit provides four-wheel drive by means of a second electric motor and is mechanically independent of the front unit. The motor is activated and continuously adjusted by the Vehicle Dynamics Integrated Management (VDIM) system, working in conjunction with the power control unit.
V6 Petrol Engine
The 3.5-litre DOHC petrol engine is based on that used in the RX 350, but has been adapted specifically for use in the hybrid powertrain. It produces a maximum 246 bhp at 6,000 rpm and 317 Nm of torque at 4,800 rpm, and features three Lexus hybrid 'firsts' to optimise the fuel efficiency of the Lexus Hybrid Drive: Atkinson cycle combustion, cooled exhaust gas recirculation (EGR) and exhaust heat recovery (EHR).
In an Atkinson cycle engine the valves close late, delaying compression. This creates a high expansion ratio for less compression, reducing intake and exhaust energy losses and converting combustion energy into engine power more effectively. As a result the exhaust temperature is lower than in conventional engines.
EGR reintroduces cooled exhaust gas into the combustion chamber, further reducing the engine's operating temperatures. Together these technologies minimise the need for any fuel enrichment to provide a cooling effect and protect the catalytic converter from overheating damage, thereby improving fuel economy and reducing emissions.
The EHR system uses exhaust gas heat to warm the engine coolant at start-up. This reduces the time the engine needs to warm up, allowing it to be shut off earlier for longer electric motor-only operation, particularly in winter. As a result, the RX 450h achieves an increase of more than 30 per cent in cold weather fuel efficiency compared to the RX 400h.
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Although power from the front and rear electric motors remains the same as it was on the RX 400h, at 165 bhp and 67 bhp respectively, the improved cooling of the PCU has increased their continuous maximum torque band width.
This gives better performance, in particular from a standing start. The front and rear motors now deliver maximum torque of 335 Nm and 139 Nm from zero rpm respectively.
Hybrid Transaxle
Several improvements have been made to the compact transaxle. An oil slinger has been adopted to reduce drive losses in the oil pump; an oil cooler has been fitted; low friction bearings have been installed at key points; and numerous components have been tuned and re-profiled. These modifications combine to increase torque capacity and overall transaxle efficiency.
An improved flywheel damper, with a two-stage, torque absorbing damper, absorbs torque fluctuations and also the shock of engine stop and start. This significantly reduces vibration and noise, notably in urban driving when the engine stops and starts most often.
Artificial Intelligence AI-SHIFT control has been adopted for the first time on a hybrid vehicle. The system determines whether the vehicle is travelling up or downhill by comparing vehicle speed with accelerator pedal angle. The transmission then selects the appropriate shift range - from third to sixth - to maintain engine rpm at an optimum speed for either uphill acceleration or downhill engine braking.
Sequential shift-matic control is also available, by selecting 'S' in the transmission shift gate. In this mode the driver can select any of six speeds to gain more responsive performance, or to add engine braking assistance.
Electric Four-wheel Drive
A unique feature of Lexus Hybrid Drive, the RX 450h's rear wheels are driven by a second motor to provide electric four-wheel drive under certain driving conditions, improving traction and vehicle stability. Operation is controlled by the Vehicle Dynamics Integrated Management (VDIM) system, with automatic engagement via constantly adjusted front and rear electric motor torque under hard acceleration - both from standstill - and at any speed, when driving through corners and whenever the front wheels lose traction.
Aerodynamic Multi-function Design
The reduction of panel gaps and the management of airflow under the vehicle combine with several multi-function aerodynamic elements to give the RX 450h a class leading drag coefficient (Cd) of 0.32. These elements include highly sculpted door handles; a rear spoiler which seamlessly integrates the rear wiper, high-mounted brake light and radio antenna; and door mirrors that incorporate turn indicators, puddle lights and, where specified, a parking camera, while still reducing wind noise by 20 per cent compared to the previous RX.
