It's tragic to think the U.S. market, having just gotten a taste of European-style, torque-rich clean diesel engines, could see them disappearing in the future, the victim of extremist emissions regulations proposed in California.

The debate is ongoing, and reasonable voices for auto makers such as BMW AG still have a good shot at convincing the California Air Resources Board that fuel-efficient diesels can play a vital role in reducing carbon-dioxide emissions.

The best way to win the argument is to send these environmental autocrats for a daylong drive along the Pacific Coast Highway in the BMW 335d. They will return with smiles on their faces and a firm grasp of the enthusiast culture that embraces diesel as the perfect balance between performance and fuel efficiency.

For achieving this balance so well, BMW wins its second consecutive 10 Best Engines award for its outstanding 3.0L DOHC inline-6 turbodiesel.

It dazzled the Ward's judges with its quiet deportment at steady-state cruising, thunderous power delivery when required and smoke-free emissions all the time.

Mitigating turbo lag is the unique 2-stage induction strategy that allows a small turbocharger to spool up quickly for low-end response, which triggers a larger compressor to force even more air into the combustion chamber, picking up the slack at higher engine speeds.

This state-of-the-art all-aluminum I-6 relies on high-pressure common-rail fuel delivery, piezoelectric injectors and a raft of exhaust aftertreatment systems to remove diesel particulates, oxides of nitrogen and other emissions, allowing the 335d and the X5 xDrive35d SUV to be sold in all 50 states with the same engine.

Stir all this technology together, and the result is a thrilling sport sedan rated at 23/36 mpg (10.2-6.5 L/100 km) in city/highway driving, while dashing to 60 mph (97 km/h) in 6.0 seconds.

Several Ward's editors managed to get about 30 mpg (7.8 L/100 km). There's nothing shabby about those numbers, or the 26 mpg (9 L/100 km) highway rating for the larger diesel X5.

Specific output of 88 hp/L previously was unheard of for a diesel, but BMW already trumps that number with the 102 hp/L delivered by the 2.0L twin-turbo diesel I-4 in the 123d available in Europe. BMW officially hasn't announced that engine is headed for U.S. shores, but the hints have been unmistakable.

For the meantime, we can revel in the beauty of the 335d, the gift that keeps on giving, even in California.

BMW AG: 3.0L DOHC I-6 TURBODIESEL

Engine type: 3.0L twin-turbo DOHC DI I-6 diesel

Displacement (cc): 2,993

Block/head material: aluminum/aluminum

Bore × stroke (mm): 84 × 90

Horsepower (SAE net): 265 @ 4,200 rpm

Torque: 425 lb.-ft. (576 Nm) @ 1,750 - 2,250 rpm

Specific output: 88 hp/L

Compression ratio: 16.5:1

Assembly site: Steyr, Austria

Application tested: '10 BMW 335d sedan

EPA city/highway (mpg): 23/36