Lexus RX 450hApril 7, 2009
Full technical details are now available about the new Lexus RX 450h. The headline is that the hybrid crossover has CO2 emissions of 148g/km, and fuel consumption of 44.8mpg, and this combination is class-leading for this size of SUV.
The second generation Lexus Hybrid Drive actually achieves fuel consumption that is 10mpg better than the outgoing model. All the hybrid system’s components have been revised to achieve dramatically improved efficiency: fuel consumption is reduced by 28 per cent, while at the same time engine power is 10 per cent greater than in the RX 400h. The improvements Lexus has made also contribute to substantially lower emissions, quieter operation and greater driver satisfaction.
Total power output is 295bhp (299 DIN hp), enabling a top speed of 124mph and 0 to 62mph acceleration in 7.9 seconds, whilst the 450h has just a 14 per cent benefit-in-kind company car tax rating.
Lexus Hybrid Drive is a combined series/parallel full hybrid system that comprises a petrol engine and two powerful electric motors. As a full hybrid the vehicle can operate on its electric motors or petrol engine alone, or with a combination of both. Like its predecessor, the RX 400h, the new model’s second, rear-mounted electric motor provides an intelligent four-wheel drive capability. The system combines distinct front and rear driving units, with a power control unit managing their seamless, high-speed interaction.
The front drive unit features a 3.5-litre V6 petrol engine, a powerful electric motor, generator, high performance battery, power control unit (PCU) 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 is mechanically independent from the front. It provides four-wheel drive by means of a second electric motor, which is activated and continuously adjusted by the Vehicle Dynamics Integrated Management (VDIM) system, working in conjunction with the PCU.
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 246bhp (249 DIN hp) at 6,000rpm and 317Nm of torque at 4,800rpm, and features three Lexus hybrid ‘firsts’ to optimise fuel efficiency: 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.
The new power control unit (PCU) is smaller and weighs less than before and is equipped with a new inverter. The inverter’s construction allows for cooling on both sides and this better cooling efficiency has helped increase the power density by 40 per cent. PCU power losses have also been reduced by more than 10 per cent to contribute to better fuel economy.
Although power from the front and rear electric motors remains the same as it was on the RX 400h, at 165bhp (167 DIN hp) and 67bhp (68 DIN hp) 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 335Nm and 139Nm from zero rpm respectively, while the range of the front motor’s maximum torque is extended by 9mph and that of the rear by 22mph.
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.
The fuel efficiency and performance are both further improved by the provision of three new driving modes: ECO, EV and SNOW.
Linked to an indicator in the speedometer, the new ECO mode provides a modified, non-linear throttle action and air conditioning control to support fuel-efficient driving. The relationship between accelerator pedal movement and system output is automatically controlled to reduce throttle response to aggressive driver inputs, improving fuel economy and promoting smoother driving. The air conditioning load on the engine is also reduced sharply under acceleration, further adding to fuel efficiency.
The vehicle automatically runs in EV, electric motor-only mode on start-up, but it can also be selected by the driver using the multi-information switch on the steering wheel. In EV mode the vehicle can be driven around town with minimal noise and zero emissions, with driving range determined by the level of battery charge. As the petrol engine is switched off, the EV drive mode contributes to a significant reduction in overall fuel consumption.
SNOW mode, also selected using the multi-information switch, helps modulate use of the accelerator pedal and improve vehicle stability when pulling away on snow or other slippery road surfaces. Pedal inputs are automatically adjusted to reduce driving torque and minimise wheelspin; should there be any wheelspin, the rear electric motor engages to improve traction.
It’s a shame that the styling of the 450h hasn’t progressed as successfully as the technology.