Alongside the hybrid hype, unrelenting progress in diesel engine technology, especially in the reduction of nitrogen oxide emissions, means we're seeing more diesels. Still far from mainstream in the United States, diesels are ubiquitous in Europe; they can claim market share of more than 50 percent in many countries. Now they are starting to make inroads into the performance-car ranks. The latest and perhaps best example of the latter trend is BMW's rapid new 635d. Introduced as part of a lightly refreshed 6 Series lineup, it is one of several BMWs using Munich's twin-turbo, 3.0-liter inline six-cylinder.

When you slot the electronic key into the dashboard, the distant diesel chatter tells you this is no ordinary 6 Series. One stab of the throttle wipes away any lingering doubts about the suitability of a diesel in a sporting BMW. The 635d outguns the 630i easily and has enough torque to make even the 650i feel slow on occasion. Average fuel economy of 34 mpg lets you enjoy the performance without emptying your wallet, while going more than 600 miles on a tankful.

The secret is a complex induction system. A small turbo-charger with minimal boost operates alone from idle to about 1500 rpm, when it is joined by a larger turbocharger running greater boost. The two operate together until 2500 rpm, when the smaller turbo disengages, leaving the bigger unit to carry on up to the 5000-rpm cutout.

The 286 hp at 4400 rpm is generous, but a solid 427 lb-ft of torque at just 1750 rpm really steals the show. That's 66 lb-ft more than the 4.8-liter-V8-powered 650i produces at 3400 rpm. BMW claims 369 lb-ft is available at just 1250 rpm, providing thrust from the first tap on the throttle. BMW's six-speed manual gearbox can't cope with all this twist action, so the latest six-speed automatic is standard. It has a fiddly electronic joystick-style shift lever, remote shift paddles that move with the steering wheel and a different set of ratios from those used in gasoline-fueled 6 Series.

BMW claims 0 to 62 mph (100 km/h) comes in 6.3 seconds, 0.4 second earlier than in the 630i. However, the standing-start performance pales next to the 635d's stomping in-gear ability. The midrange acceleration will convince even the most ardent enthusiast that diesel performance has come of age. No test figures are available, but subjectively, the 635d feels faster than the six-speed manual 650i in the 50-to-75-mph range in fourth gear. There's more to the diesel's character than just urge. The power delivery is linear from idle to redline.

We wouldn't go so far as to say it's as spirited and enjoyable as its gasoline-engined siblings, but there's not much separating them. While it lacks the silky refinement of the 630i and the 650i, the 635d's characteristic diesel growl suits the 6 Series' bullish character.