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.