2009 Volkswagen Jetta TDI

Unlike the diesels of twenty years ago, the new TDI suffers from no severe drawbacks. It will start instantaneously in cold weather, it won't make your teeth vibrate, and it doesn't smoke. Thanks to multiple exhaust-emissions controls (oxidation catalyst, particulate filter, nitrous-oxide storage catalyst, and hydrogen-sulfide catalyst), the 2009 Jetta TDI runs cleanly enough to pass the latest California emissions regulations. And unlike the bigger diesel engines coming from other German manufacturers, it passes those emissions standards without the use of urea injection. The Jetta is also quiet enough that the driver of a 1980s Jetta diesel - or even someone who bought the last Jetta TDI in 2006 - won't believe that it's actually running. At idle, the TDI is nearly as quiet as a gasoline-powered Jetta, and during acceleration, it seems quieter inside than the current Jetta with a gasoline engine. If you listen carefully, you can hear a distant staccato thrum in place of a smooth hum, but it's so hushed that your passengers will probably never notice the difference.

Credit for the quietness goes to a switch to common-rail fuel delivery, a system that uses piezo fuel injectors and 26,000-psi delivery pressures. Precisely metered fuel and fast-reacting injectors allow up to five squirts per power stroke, reducing noise, vibration, and wasted fuel. Displacing 2.0 liters and breathing through sixteen valves, the four-cylinder engine produces 140 hp and 236 lb-ft of torque. For reference, the 2006 1.9-liter, eight-valve diesel produced only 100 hp and 177 lb-ft, and it didn't pass the same emissions standards.

As with most turbocharged engines, there is some lag off idle, and consequently, it's not terribly difficult to stall a Jetta TDI equipped with the standard manual transmission when you're trying to take off from a stop. The optional, $1100 dual-clutch automatic takes care of this problem, and since it changes gears with no interruption in power, there's no waiting for boost to return after a shift.

Nevertheless, the TDI does have some drawbacks. First of all, you'll miss out on the base Jetta's throbbing five-cylinder sound track, and the TDI isn't as responsive or as engaging as the gasoline engine. On a practicality front, only about 40 percent of this nation's service stations carry diesel fuel, and those that do are currently charging some 30 cents per gallon more than regular gasoline. And speaking of price, the Jetta TDI - available as a sedan and a wagon - carries a price premium of about $2000 over a comparably equipped Jetta with the 2.5-liter gasoline five-cylinder. The federal government recently announced that the purchase of a TDI, however, qualifies owners for a $1300 federal tax credit, reducing that effective price premium to a much more palatable $700.

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