9 February 2011
A couple of months ago we drove and reviewed Lexus's very capable, if unremarkably new in styling terms, RX350.
At the time we commented that the quoted fuel consumption figure from the 2 tonne SUV was a little hard to achieve in real world driving conditions, and that a diesel engine would be welcome.
We hankered after a diesel for a couple of reasons. Firstly, the immense torque of modern diesels gives that nice punch in the back when accelerating away from the lights, and secondly, more frugal fuel consumption means it's lighter on the pocket at the pump.
Fast forward to August, and the good people at Lexus have thrown us the keys to their frugal RX. However, in Lexus's case, it's not an oil burner, but a hybrid. Oh well, different strokes and all that!
So we were looking forward to the better fuel economy from the luxury RX450h.
The RX450h features a touch of blue around the front as is the tradition with Lexus hybrid models and blue Hybrid logos on either side, setting it apart from its standard RX350 sibling. Otherwise, it's pretty much the same from the outside.
Likewise, interior styling is very similar, with the main difference being the conventional tachometer having been replaced with a hybrid system indicator, which encourages the driver to apply the accelerator in a more environmentally friendly manner.
Both the RX350 and RX450h are powered by 24 valve 3456cc VVT-i V6 petrol engines, but the RX450h differs in that the conventional Otto cycle engine in the traditional petrol version has been replaced by an Atkinson cycle engine.
The key difference here is that the inlet valves are left open for longer, delaying the beginning of the compression stroke creating a high expansion ratio. This reduces energy losses and improves fuel consumption.
Lexus RX450hWhile the Atkinson cycle has its benefits, its traditional weakness is its lack of low down power. However, when combined with electric power, this deficiency is negated, as the advantage of an electric motor is its immediate torque, so it lends itself perfectly to hybrid use.
The RX450h features the Lexus Hybrid Drive system, using a rear electric motor in addition to front electric and petrol motors, combined with a new inverter with high cooling performance to raise fuel efficiency.
The V6 petrol engine produces 183kW of power at 6,000rpm, down from 204kW for the RX350 and 317Nm of torque at 4,800rpm, down from 346Nm.
However, the RX450h is capable of producing a maximum system output of 220kW, 16kW more than the RX350, due to the combination of petrol and electric power, although due to the variable nature of petrol and electric power delivery, seat of the pants acceleration feels on a par with the non-hybrid RX.
The front wheels can be driven by the petrol engine, the electric motor or a combination of both.
The 123kW electric front motor, drives the front wheels via reduction gears to a continuously variable transmission.
A second, 50kW AC permanent magnet motor powers the rear wheels during take-off and acceleration, or in slippery conditions.
When decelerating and braking, a generator captures energy which would normally be wasted. This power is captured as electricity and stored in the high voltage Nickel Metal-Hydride (Ni-MH) battery.
The smooth power and seamless transition from electric to petrol power ensures the RX450h delivers quiet, refined performance, with the vehicle operating predominantly on electric power in slow stop/start traffic. Out on the open road, the petrol engine does most of the work, so depending on driving style, fuel consumption can be heavier on the open road than in traffic.
With a quoted fuel consumption figure of 6.4L/100km, the RX450h offers over 20 percent improvement over the model it replaces. CO2 emissions are improved by 42g/km to 150g/km, making for much more miserly consumption than the RX350.
Braked towing capacity is limited to 1,500kg.
Three models are offered; RX450h priced at $114,990, RX450h SE priced at $124,990 and our test model, the RX450h Limited with a retail price tag of $133,990. Our test vehicle also had options of a Panorama glass roof adding an extra $3,150.
As is the case with all Lexus vehicles, the RX450h comes with a four year / 140,000 kilometre warranty and full maintenance service package, including a new set of tyres if required and Lexus' personal assistance 24 hours a day.
We thoroughly enjoyed the RX450h, and fuel consumption, particularly around town was impressive. But we still can't help wondering how the RX would feel with that punchy torquey diesel we wished for back in June!