Dori said:
Much better fuel economy, esp in city driving. That's why even in the UK, which has been traditionally anti-diesel, the diesel car is on the rapid rise, despite the fact that the price of the fuel is about the same as petrol.
Diesel engine costs more as an option:
"Volkswagen's midsize 2005 Passat diesel sedan has a starting MSRP of $23,360. This compares with $22,070 for a comparable gasoline Passat model." (1)
Prohibited by law:
"Diesel models are limited in their availability in the U.S. because five states—California, New York, Massachusetts, Maine and Vermont prohibit their sale due to emission restrictions." (1)
But produce more torque:
"Yet the 2.0-liter four-cylinder engine in the Passat TDI, as the diesel versions are called, puts out an amazing 247 lb-ft of torque at a low 1900 rpm vs. the 166 lb-ft of torque at 1950 rpm in the
1.8-liter four-cylinder gasoline engine in the base Passat sedan." (1)
Get better gas milage:
"Thanks to its higher energy content and its efficient combustion process, diesel performance enables cars to travel at least 30% farther on a gallon of fuel than comparable gasoline models." (2)
But the savings will take time to pay-back the initial increased cost:
"Thus, if drivers maximized the diesel's fuel economy and got the 7 extra miles per gallon, it would take about four years to recoup the approximately $1,200 extra cost for the Passat's diesel engine." (1)
Which will take more time to pay-back if diesel continues to be more expensive to buy than gasoline. And, not every urban filling station has a diesel pump. In some cases, drivers might need to venture farther from suburban neighborhoods to locate a place to fill up.
And they are noisy (more than a comparible gas engine).
Diesel is more popular in Europe because it is taxed less than gasoline:
"The demand for diesel in Europe is fueled by the high cost of gasoline. (Unequal taxation of the two fuels results in diesel costing about one dollar less per gallon in most European countries.)" (2)
It takes more oil to make a gallon of diesel than for gasoline, so the relative efficiencies of diesel and the costs to consumer due to taxation really make the attraction of diesel questionable:
"It should be noted, however, that it takes about 25% more oil to make a gallon of diesel fuel than a gallon of gasoline, so we should really look at how a vehicle does on fuel efficiency in terms of "oil equivalents." Thus, we need to adjust the mileage claims for diesel vehicles downward by about 20% when comparing them to gasoline-powered vehicles." (2)
Although diesel engines generate less carbon dioxide (only because of their slightly better fuel economy), they put out more smog-forming pollutants than gasoline engines:
"when it comes to smog-forming pollutants and toxic particulate matter, also known as soot, today's diesels are still a lot dirtier than the average gasoline car." (2)
It will take more effort and cost than it's worth to make diesel cleaner than gasoline:
"To meet the tougher pollution standards, high-tech diesel engines need low-sulfur diesel fuel. Unfortunately, US Department of Energy modeling has shown this fuel to be more oil and carbon-intensive than reformulated gasoline. Making a gallon of diesel fuel requires 25% more oil and emits 17% more heat-trapping greenhouse gases than gasoline reformulated with MTBE. Similarly, diesel requires 17% more oil and emits 18% more heat-trapping gases than gasoline reformulated with ethanol. This means that diesel fuel's advantages from its higher per-gallon energy content and better performance on greenhouse gases are partially offset by the impact of diesel's fuel-production process."
(1)
http://autos.msn.com/advice/article.aspx?contentid=4022630
(2)
http://www.grinningplanet.com/2005/04-12/diesel-vs-gasoline-article.htm
For more info, see here:
The Diesel Dilemma
Diesel's Role in the Race for Clean Cars
http://www.ucsusa.org/clean_vehicles/cars_and_suvs/page.cfm?pageID=1307