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Driven: 2012 Mazda 6 Diesel Prototype

Volkswagen's monopoly on affordable diesel passenger cars in the U.S. is about to end. Starting in late 2011, American buyers will have at least one fuel-efficient diesel to choose from at the Mazda dealer. While the Mazda team refuses to officially acknowledge that the 6 sedan is the diesel engine's destination, the company has previously announced the engine will first appear in a "next-generation midsize sedan." Furthermore, we're driving Technical Prove-out Vehicles, or TPVs, that are next-generation Mazda 6 sedans, clothed in the current car's sheet metal. The 6, though, might not be the only diesel Mazda we receive. The CX-7 crossover seems a natural candidate and U.S. executives seriously want a Mazda 3 with a diesel, but that compact car presents a pricing challenge.

Two turbos, tons of torque
The new diesel, called Sky-D, is a 2.2-liter four-cylinder with two, sequential turbochargers. In the prototype vehicles, output is rated around 173 hp and a massive 310 lb-ft of torque, but those numbers will likely change slightly with the final calibration. At just 14:1, the compression ratio of the Sky-D is unusually low for a diesel engine. Typically, a low compression ratio causes issues with cold starts and low-load combustion, but Mazda has addressed those problems with increased exhaust-gas recirculation and a bowl in the piston that concentrates fuel below the injector to ensure ignition. Fine control of exhaust-gas recirculation is enabled by variable valve lift on the exhaust side. The low compression ratio also helps keep temperatures down, reducing the formation of nitrogen oxide emissions, and allowing the engine to meet emissions requirements without pricey exhaust after-treatment equipment. Mazda says the engine is good for 43 mpg on the highway in a 6 sedan, 1 mpg more than that in the smaller, less powerful Volkswagen Jetta TDI.

In our first car, the Sky-D was paired to Mazda's revised six-speed manual that's lighter and more compact than the previous gearbox. Mazda also sought to mimic the feel of an MX-5 Miata shifter by shortening throws by five millimeters and reducing shift effort by a little more than one pound. More significant, though, is the shift pattern that's tighter with less side-to-side slop in each gear. Even though we were shifting with our left hand (the prototypes were right-hand-drive cars), it's clear that the new stick offers more precise, better-feeling throws.

Along with 310 lb-ft of torque, the Mazda diesel has one other surprising spec: 5200 rpm. That's the redline of the new engine, which is about 700 rpm higher than that of the typical four-cylinder diesel. On the road, the torque and redline serve harmoniously, creating an engine that has much more flexibility than Volkswagen's 2.0-liter diesel with 140 hp and 236 lb-ft of torque. With Mazda's 2.2-liter, you can comfortably cruise with significantly less shifting, accelerating easily in higher gears and staying in each gear longer. We were already in the left lane when we hit our first stretch of derestricted autobahn. Rather than dropping to fourth gear, we simply left the transmission in sixth and planted the accelerator. The engine pulled strongly and linearly until the 6 found its top speed just north of 130 mph with 600 rpm still available on the tachometer.

Choose to row through the gears, though, and the diesel will charm you with smooth acceleration that lives up to the Zoom-Zoom promise. With the Sky-D, there isn't a performance or refinement penalty for choosing the more efficient engine. The two turbochargers provide thrust throughout the entire rev range that channeled through a sensual six-speed. It's quiet too. The diesel is just marginally louder than today's direct-injected gas engines and these prototypes don't have the meticulous application of insulation like the production vehicles will.

We made our return trip in an automatic-equipped car. Called Sky-Drive, the new six-speed features more aggressive torque-converter lockup to provide better fuel economy and emulate the direct-shifting feel of a dual-clutch or manual transmission. To mask the added shift shock, Mazda has switched from a single-plate to dual-plate clutch and the damper features larger, less rigid springs. Aside from the occasional abrupt one-to-two shift, the gearbox provides quick, yet unobtrusive gear changes. A few times, we would have liked the transmission to downshift earlier on deceleration, but instead we were given a late shift right as the right foot moved back to the gas. There's a manual shift mode, but the calibration isn't quite right as there's a long pause between tapping the shifter and feeling the downshift. Mazda says that the logic hasn't been finalized, though. The company will also have the ability to offer paddle shifters in the next 6, though it's not clear if the diesel car will offer them.

Celebrate diesel diversity
Paddle shifters are a minor detail in the bigger picture here. Mazda's gamble -- and diesel is still a large gamble in the U.S. -- rewards buyers with what should be an affordable, fuel-efficient, and entertaining car. The Sky-D also offers enthusiasts another option in the extremely limited diesel market. With such a compelling character, we hope Mazda's 2012 diesel can convince buyers -- and other automakers -- that diesel engines deserve a bigger role in America.