For decades, Volkswagen and Audi have held the only options for North American shoppers wanting a highly efficient model with a four-cylinder turbo-diesel—their 2.0-liter TDI engine. Now, with Mazda, ...

Note: This is the second in a series that looks at the diesel particulate filter and maintenance best practices that an impact its lifecycle. The first installment, "Diesel coolant heaters boost DPF ...