Automotive Engineer

Big SUVs make good hybrid programmes for OEMs. Their high margins make it easier to absorb the additional costs, and offer more packaging space for motors and batteries. They’re good for brand image too.

Lexus has set the benchmark with the RX450h. With a 3.5-litre V6 gasoline engine and a 123kW electric motor under the hood, fuel consumption is only 6.3 litres/100km, and CO2 emissions are 148g/km.

The Lexus has part-time all-wheel drive, courtesy of a 50kW electric motor in the rear axle – there’s no mechanical connection between it and the rest of the powertrain. This cuts parasitic losses but the trade-off is reduced off-road capability.

Volkswagen didn’t want to compromise when developing the Touareg hybrid. It had to have much lower emissions than the gasoline V8 model it replaces but with the same on and off-road performance. Project manager Dr Jochen Böhle says: “This is true Touareg, and that means full towing capacity, full off-road capability, high top speed and a steep climbing gradient – I don’t think the Lexus has this.”

Toyota fits the Lexus with a power-split series/parallel hybrid architecture but VW chose a simpler parallel architecture because, Böhle says, it’s lighter and more efficient. And providing all-wheel drive through an electric rear axle was of no interest: “We couldn’t have any restrictions,” he says.

The hybrid module, comprising a 34kW/140Nm motor and disengagement clutch, weighs 55kg and measures 400mm by 145mm long. It’s housed in the eight-speed ZF automatic transmission, which shares 80% of its parts with the versions used in other Touaregs.

The Touareg’s 288V air-cooled NiMH battery, supplied by Sanyo, stores 1.7kWh of energy, delivering 38kW of peak power.

The battery module, including the cooling ducts and protective steel casing, weighs 79kg and sits in the spare wheel well.