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By Nick Chambers

Not long ago the word “diesel” was synonymous with loud, smelly, and unreliable vehicles in the United States. In fact, over the last several decades diesels had all but disappeared as an option for American passenger cars — becoming relegated to the lives of work trucks and heavy-duty pickups.

Throw in a recent fuel price crisis, mix it with a dramatic shift in regulatory policy, and today Americans are being reintroduced to diesels at an increasing pace — but these aren’t your grandpa’s diesels. Here’s a look at some common myths and facts about diesel cars:

Myth: Diesels have dirty tailpipes
No, these “clean diesels” rely on advanced and intricate emissions-scrubbing technologies to make them some of the least-polluting and most efficient cars on the road. When combined with the fact that all diesel fuel now sold in the U.S. is the ultra-low sulfur type, modern diesel toxic and smog-producing emissions are lower than even some efficient gasoline-powered cars. You might think low emissions and high efficiency would go hand-in-hand with horrible performance, but all of today’s clean diesels are turbocharged and have boatloads of power to help them get up and go.

Myth: Diesels are worse for the climate
While diesels have become cleaner when it comes to traditional pollutants that cause smog, acid rain and toxic air pollution, many believe they are still dirty when it comes to greenhouse gases. It’s not as clear cut as that. Diesel fuel does contain more carbon dioxide per gallon than gasoline (diesel=22.2 pounds CO₂/gallon, gasoline=19.4 pounds CO₂/gallon, according to the Environmental Protection Agency), but that doesn’t necessarily mean a given diesel vehicle will pump out more carbon dioxide than a comparable gasoline vehicle. It all comes down to how efficient the engine itself is. Since diesel contains about 14% more carbon dioxide per gallon, if the comparable gasoline engine is more than 14% less efficient than the diesel would be the worse polluter. Many diesels are more than 14% more efficient than a comparable gasoline engine, so they emit less carbon dioxide per mile traveled — although some don’t.

Fact: Diesel engines last a long time
Myth: Diesel cars are therefore cheap to maintain
Diesel engines are built with very robust components, and diesel fuel itself is a good lubricant, reducing wear on the engine’s components. As a result, diesels are known for their longevity, with engines that typically last longer than the rest of the vehicle. But that doesn’t always translate into maintenance savings; they still need regular oil changes and service. Because many of them are made by high-end German car manufacturers, service costs can be astronomical — with pricetags several thousands of dollars higher than their gas-powered counterparts.

Fact: Diesel engines are noisier
Although modern clean diesels are much quieter than their forebears, they still generate clunky knocking sounds that gasoline engines don’t make — which have been known to send a new diesel owner scurrying to the dealer to make sure their car isn’t broken. These special (some might say, endearing) sounds result from how the engine burns diesel fuel: in a gas engine a spark plug ignites the fuel, in a diesel engine fuel is mixed with highly compressed hot air to cause it to ignite. It is this high compression that results in the knocking noises — and also makes a diesel about 20-30% more efficient than a comparable gas engine.

Fact: Fuel economy often exceed expectations
When looking at the window stickers of diesel cars, the fuel economy listed may be lower than the mileage you see in real-world driving. This is due to the way the U.S. Environmental Protection Agency tests cars; in general, most drivers of modern diesels will find their fuel economy is several miles per gallon higher than the window sticker.

Myth: Good fuel economy means low fuel cost
Even though a diesel car is much more efficient than a gas-powered car of the same type, it isn’t always that easy to figure out if it will save you money at the pump. In the U.S., diesel fuel tends to be more expensive than gasoline — thanks in part to lopsided federal tax rates — and diesel rises and falls based on different factors than gasoline. So when gasoline is priced low, diesel can still be high, and when gasoline is high, diesel fuel can be even higher. It is rare that diesel is a priced lower than gasoline. To determine if a given diesel vehicle will save you money at the pump you need to do a little bit of math. Of course, no one can tell you what the price of diesel or gasoline will be in the future, but if trends hold, diesel will always be more expensive.

Myth: If you buy a clean diesel, you get a tax break
Since 2006, there has been a federal tax credit of up to $3,400 that applied to clean diesels. Unfortunately, it applied only to the first 60,000 clean diesel or hybrid cars sold by each manufacturer, and for all diesels on the market, the tax credit has expired.

Myth: It’s easy to burn biodiesel
Some people buy diesels expecting to be able to fill them with biodiesel, but unfortunately, due to the modern advanced emissions equipment on-board, high blends of biodiesel are out. Most manufacturers only allow up to 5% biodiesel mixtures because the biodiesel tends to “gum up” the emissions equipment, causing it to fail over time.

As you can see, it’s not as easy as you might like to determine if clean diesels will save you money in the long run, but you can be sure they will be solidly engineered, fun to drive, reliable, produce low emissions, and last you some time to come.

Related: Reviews of the Best New 2011 Diesel Cars