EVEN the rarefied air of luxury sedans must be getting thin thanks to global warming if the latest arrival from Mercedes-Benz is anything to go by.

While the steep rises in the cost of fuel will not be an issue for someone who can afford a $200,000 car, making a statement as an environmentally responsible citizen in these times of global warming can be.

It is the perfect time to introduce a fuel-efficient turbo-diesel to the S-class range, then.

This is Mercedes-Benz’s first S-class diesel in the Australian market. The W221-series S320 CDI shares the same specification and price as the existing $190,900 petrol S350 model.

The W221 S-class arrived here in February 2006. Although Mercedes-Benz Australia has since been touting a diesel S-class, it held off until it believed customer demand was sufficient.

It also knew that a replacement for the long-serving inline six-cylinder turbo-diesel was imminent, so when the 3.0-litre V6 turbo-diesel replacement recently became available, it gave the model the green light.

Currently the S-class still leads its segment, but sales are already down in year-to-date figures. Another reason, no doubt, to spruce up the S-class range with a new variant.

Mercedes-Benz expects 55 sales of the S320 CDI over the rest of 2008. In the first four months of 2008, it has sold 63 S350 models.

The new diesel S-class engine is a 3.0-litre turbo-diesel V6 that develops 173kW at 3600rpm and 540Nm at 1600-2800rpm. This is an optimised version of the engine used in other Mercedes-Benz models that develops peak 165kW/510Nm outputs.

Mercedes-Benz says that changes to the S 320 CDI’s inlet and exhaust systems and engine management including increased boost and fuel delivery are behind the improved results.

Fuel consumption is quoted as 8.3L/100km combined, with C02 emissions of 220gm/km. Claimed performance figures are 0-100km/h in 7.8 seconds and a top speed of 210km/h.

The engine has an alloy block with cast-in grey iron cylinder liners – unusual for a diesel engine, which usually requires the heavier, but stronger cast iron block to cope with the extremely high compression ratios a diesel requires in its combustion chambers. The V6 weighs 208kg.

The third-generation common-rail direct injection diesel engine has piezo-ceramic injectors, providing five fuel injections per power stroke at a peak pressure of up to 1600 bar.

The S320 CDI’s turbocharger is a variable nozzle turbine, with electrically adjustable turbine blades to reduce turbo lag.

Designed to meet Euro IV emissions requirements, the engine has two catalytic converters fitted to the exhaust system and is the first Mercedes-Benz engine fitted with a particulate filter.

The fuel-injection system squirts fuel post-injection when required to increase exhaust temperature to ensure that particles trapped in the filter are burned off to clear the filter.

The rest of the S320 CDI is the familiar S-class package. A seven-speed automatic transmission drives the rear wheels, and the S320 CDI has front, side and side curtain airbags and traction and stability control fitted.

Also on the standard list is air-spring suspension, adaptive damping, Presafe crash preparation system, Adaptive Brake Regulation (include hill start assist, hill-hold function and electric handbrake), 17-inch alloy wheels, bi-Xenon adaptive headlights, trip computer with radio, six-stack CD/DVD player and a port for computer memory cards, analogue TV, 12-way electrically adjustable front seats with lumbar supports and leather interior.

Options available for the S320 CDI include Distronic Plus, BAS Plus, Dynamic Multi Contour seats, Harmon Kardon Surround Logic 7 sound system Night View Assist.

The Active Body Control system (which reduces pitching and bodyroll when driving at high speeds) and the long-wheelbase body are not available for this model variant.