According to most purists, a BMW sports coupe or convertible are supposed to have a smooth gasoline engine –preferably an inline-six- mounted up front and sending power to the wheels at the back.

The new 640d xDrive Coupe and Convertible that will debut at the Geneva Motor Show next month ticks only one of those boxes: the straight-six found under the hood. However, it’s not fed by gasoline but by diesel fuel and it channels power to all four wheels.

It’s not the first diesel-powered 6-Series as that taboo was broken by the previous generation model, but it’s the first to combine an oil-burner with BMW‘s electronically controlled all-wheel drive system.

The 3.0-liter twin-turbocharged diesel engine churns out 308HP (230kW / 313PS) at 4,400 rpm and a maximum torque of 630Nm (465 lb-ft) between 1,500 and 2,500 rpm, and is linked up to an eight-speed Sports automatic transmission.

In Coupe trim, the 640d xDrive accelerates from 0 to 100km/h (62mph) in 5.2 seconds and in Convertible guise in 5.4 seconds, with both models reaching a limited top speed of 250km/h (155mph).

The average fuel consumption of the 640d xDrive Coupe stands at 5.7 to 5.8 liters per 100 kilometers (depending on the tires) in the EU cycle, which is equivalent to 41.2/40.6mpg US, with CO2 emissions of 149 – 153g/km.

Sales of the new 640d xDrive models will start in Europe this spring. BMW has not said if or when it will offer the all-wheel drive, diesel-powered 6-Series variants in other markets.

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