BY TIM BRITTEN | 28th Sep 2006
We like:
Invisible operation of complex hybrid system, fuel economy, performance
We like:
Not the most dynamic of SUVs, nor is it an off-roader
IF it wasn’t for the fact the RX400h comes Lexus, the comfort factor involved in being an early adopter of hybrid technology could have been a little lacking.
As it is, the new hybrid SUV comes with the comforting knowledge it’s been developed with the same attention to bulletproof reliability as regular Lexus products.
An example is the 288V sealed nickel metal hydride battery located out of sight, and hopefully out of mind, under the rear seat. Lexus says its operating life is, in normal operating conditions, in excess of ten years. That’s comforting for a number of reasons, not the least of which is the replacement cost of around $3000. More comforting again is the claim by Toyota (the Lexus parent) that it has no knowledge of a battery failure claim in the Prius small hybrid that has been running around now for five years.
The RX400h arrives with the aim of underscoring the Japanese luxury maker’s commitment to hybrid technology, and to supplement an already-successful luxury SUV that is currently leading the market.
It also brings 4WD hybrid technology for the first time, using a system completely different to that used in the RX350.
Rather than using thee differentials to apportion drive to the front and rear, the RX400h does away with a rear driveshaft by using a separate electric motor to power the back wheels. The connotations for future front-drive hybrids are obvious: This is a good way of achieving four-wheel drive without added mechanical weight and complexity.
The RX400h thus gets one more electric motor than the just-launched rear-drive GS450h hybrid, even if the basic principles are similar.
These are getting familiar to some of us today: A conventional petrol engine works in conjunction with an electric motor to minimise exhaust emissions and increase economy by using the electric motor in slow-moving urban situations and combining the two on the highway. In traffic, the petrol engine lies dormant when stationary and, up to certain speeds, even when the car is moving.
One of the really clever things about the Lexus is the use of a planetary gearset to perform the functions of sorting out where the power should be coming from at any given moment, while also operating as a constantly-variable transmission.
In the RX400h, this applies to the front wheels only, as the back wheels have their own discrete operating mode, via their own (MGR) 50kW electric motor.
The system is dubbed E-Four and, in regular on-road situations, operates as a front-drive via the 155kW petrol V6 and the 123kW front electric motor (MG2).
The rear electric motor kicks in to provide extra power and traction under hard acceleration, and when wheel slip is detected up front - similarly to a regular on-demand 4WD system. E-Four sends a maximum of 28 per cent of torque to the back wheels when required.
The upshot of all this is a large SUV (the RX400h weighs just over two tonnes) that will accelerate to 100km/h in a quite creditable 7.6 seconds while returning a diesel-like fuel consumption average of 8.1L/100km. The regular RX350 isn’t as quick, and consumes 11.1L/100km.
The driving experience isn’t that much different to a regular RX.
Through cars like the Prius, Honda’s Civic Hybrid and the new Lexus GS450h, we’ve now had a bit of a chance to experience hybrid motoring, but the eerie silence that descends when the vehicle is sitting stationary still takes some getting used to.
Under way, the V6 is brought so quietly to life by a third electric motor (MG1) that you must concentrate if you want to know when it’s operating.
The RX400h is similar to the Prius and GS450h in its driver information systems, where a large central screen provides five-minute fuel economy averages in one mode, and a complex looking graphic in another that visually informs where the power is coming from, where it’s going to, and if the battery is being recharged. The regular average economy updates also provide a congratulatory readout that specifies how much energy has been sent back to the battery under deceleration and regenerative braking.
The transverse 3.3-litre V6 belongs to the same family as the longitudinally mounted 3.5-litre engine in the RX350 and GS450h and is Lexus-quiet – although seemingly not as much as the GS.
In 3.3-litre form (the RX400h has been on sale in the USA since April 2005 and preceded the 3.5-litre RX350) it delivers a distinct surge helped not a little by the instant torque of the electric motors – 330Nm from zero rpm from the MG2 and 130Nm for the MGR.
The acceleration is satisfyingly brisk considering the SUV’s weight, but it’s a little way short of the distinctly-V8 impressions conveyed by the far more performance-oriented GS450h.
Part of this probably comes from the seamless power delivery provided by the planetary CVT transmission. There’s just a steady push in the back as the power meter creeps towards the vehicle’s overall combined maximum of 200kW. There’s no rational reason why a tacho should be provided and Lexus hasn’t done so.
The drive route for the RX400h launch took us south of Sydney on non-challenging bitumen roads with no forays into the bush. Through traffic, and out on the road, the weighty SUV had no trouble staying below 10L/100km and, on the car we drove, the overall average checked out at around 8.5L/100km, indicating the factory figures are pretty achievable.
Dynamically the RX400h is pretty much the same story as regular RX350s. That is, a soft ride and a slightly reluctant attitude towards tightly winding, relatively fast roads.
The electric power steering is new to RX and has some artificiality to it. But it enables not just a continuation of power assist when the V6 isn’t running, but also allows the vehicle to adopt the all-embracing Vehicle Dynamics Integrated Management (VDiM) stability control system that adds steering correction into the mix if things get out of shape. The 400h also adds 4WD into the equation to send power front or rear to further aid stability when appropriate.
On the road, the on-demand 4WD system operates without the knowledge of the driver, which is something of a relief considering the efforts of early – and, in some cases, still-current – on-demand systems which demonstrated some delay in proving back-up rear-wheel drive.
And the RX400h’s ability to stop, aided by the front and rear electric motors which both provide regenerative braking, is formidably positive, especially under hard applications.
Otherwise there’s not an awful lot to pick a 400h from a regular RX. Only small things like the instrument panel which dispenses with wood trim in favour of a more appropriately technical satin-finish look, a new grille – and the badges – give it away.
The seating, and the general packaging aren’t compromised by the hybrid drivetrain and the battery, as mentioned earlier, is tucked under the back seat, meaning there’s no real evidence of the imposing technology residing within.
Lexus sees the RX400h as a vehicle for early-adopters – or the recently identified New Economic Order “NEO” class. It sees it as an alterative to those who might be seeking the fuel economy advantages of a diesel but don’t want to suffer the perceived downsides of noisiness, smelliness and sooty exhausts.
All those may have been relegated to history by the new-generation diesels, but the Lexus does appear to be an alternative that will appeal to many.
Given the fact that Lexus is planning more hybrids into the future (the V8 Lexus LS hybrid is coming our way next year), there’s no questioning the RX400h makes a convincing argument.