There were many reasons why we chose the 2014 Volkswagen Passat TDI for our long-term fleet, but one of the biggest factors was that it is a clean diesel. What does clean diesel mean, and what's so attractive about the technology? Let's get into that subject, starting off with a brief explanation on why clean diesels should tempt you at a time when hybrids and electric vehicles are becoming viable ways to avoid spending money at the pump.

Clean Diesel Technology 101

If you still think they're smelly and environmentally unfriendly, now is a good time to forget everything you thought you knew about diesels. Sure, diesels from the 1970s and 1980s were stinky, smoky, clattering messes that didn't do automakers any favors when it came to environmental regulations or outright performance. While they were durable and offered better fuel economy than their gasoline counterparts, those advantages came at a significant price that made them the black sheep of the automotive world.

As priorities shifted and automakers became more concerned with finding clean-burning alternatives to gasoline powerplants, a new breed of so-called clean diesel engines emerged that run on ultra-low-sulfur fuel. Clean diesels are distinguished by a number of technological innovations that mask or eliminate the unsavory effects of the old engines, including high-pressure injectors which enable quicker reaction times and numerous injection moments within each combustion cycle.

By more carefully managing the amount of diesel introduced for combustion, these types of engines are able to achieve ultra-efficient operation -- up to 43 miles per gallon on the highway in the case of the Passat TDI -- while running nearly as quietly as their gasoline-powered counterparts. If you try hard, you might even be able to approach the Guinness-World-Record-setting 77.99 mpg figure achieved by a manual-transmission-equipped Passat TDI in September 2013.

Diesel Trade-offs

Picking one engine technology over another rarely yields a definitive win-win situation, and clean diesels are certainly no exception to the rule. While some manufacturers (such as Mercedes-Benz) are incentivizing clean diesels by pricing them at or below gas-powered versions, others charge more for the technology. In the case of the Passat, VW's 2.0-liter TDI option starts at $26,675 (only $200 more than the 1.8-liter Sport Edition gasoline model) while running $2,990 less than the V6 version.

Diesels may not wow you with their horsepower figures (our Passat TDI produces a mere 140 hp), but that doesn't mean they're lacking in performance; thanks to their high torque output at low rpms -- in this case, 236 lb-ft at only 1,750 rpm -- this clean diesel feels a whole lot spunkier than its spec sheet might suggest.

While diesel is only available at roughly 50 percent of gas stations, that issue has questionable impact on buyers in dense urban settings and shouldn't cause much concern on road trips because the Passat TDI can go nearly 800 miles on a single tank.

Of greater concern for car shoppers is the fact that fluctuations in the price of diesel don't guarantee that their 15-to-30-percent efficiency improvements will pan out to big savings over spending on gasoline. At the time of this writing, though, diesel happens to be cheaper than gas -- in some cases, by as much as 35 cents per gallon.

The Maintenance Question

While some manufacturers offering clean diesel engines charge a premium for topping off the diesel-exhaust-fluid additive (also known as AdBlue) that reduces NOx emissions, Volkswagen offers free scheduled maintenance for the first three years or 36,000 miles of ownership, eliminating that overhead. Once out of warranty, you'll have to pay for AdBlue top-offs, which are required every 10,000 miles or so and can cost between $10 and $50 at the dealership. The top-offs can also be performed by the owner via a fill-up spout in the car's trunk, and naturally, you'll save money if you add it yourself.

What's the Future of Clean Diesels in the U.S.?

If current trends continue, future-fearing consumers needn't worry about the long-term viability of clean diesels in the U.S. market. Clean diesel sales have been enjoying steady gains and are projected to constitute as much as 10 percent of new car sales by 2018. In 2013, Volkswagen led the charge by claiming more than 75 percent of clean diesel sales in the U.S.

As for our sedan in question, the 2014 Volkswagen Passat TDI is garnering its best year-to-date sales figures yet, with 3,172 clean diesels constituting 35.4 percent of all Passat sales.

As for the future of VW clean diesel engines, Volkswagen announced an upcoming version (with the code name of EA288) that will produce more power and greater efficiency, all but securing this expanding market for the German manufacturer.

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Basem Wasef is an automotive journalist, author, and photographer with two coffee table books under his belt, and is a regular contributor to Popular Mechanics, Robb Report, and Maxim among others. When Basem isn't traveling the globe testing vehicles, he enjoys calling Los Angeles home.

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