please, do explain/summarize the difference in Euro vs. US diesel regulations... seems the US is generally behind in emissions standards, so I am curious to understand the ball game.
You got some great replies already, but I'll add a little more. You've got three things driving diesel in Europe as well as the common perception that Europe has stronger emissions standards (they don't actually).
Efficiency/CO2 vs. Pollution - Higher MPG results in lower CO2 emissions, recently a big deal in the regulatory environment with climate change concerns. Diesel tends to get better MPG so it reduces CO2 emissions. But CO2 has never really been considered a pollutant because it doesn't affect anyone's health even if it may warm the planet in the long run which would indirectly kill people. Pollution regulations target things like NOx, SOx and particulates which do in fact kill people directly through breathing disorders and cancers. Diesel engines while efficient are
horrible polluters compared to IC unless you wrap the diesel engine in a whole bunch of additional cleaning equipment and also provide controlled very "clean" diesel fuel. Diesel tends to produce way more NOx and it is also harder to remove from the exhaust of a diesel than an IC engine (different exhaust temperatures). Diesel also produces far more particulate pollution and the particles produced by diesels are extremely carcinogenic. So diesel tends to be an efficient if extremely polluting engine. As to the regulations, in Europe the allowed NOx pollution figure is four times higher than in the US. So it is easier to pass emissions for a diesel in Europe than in the US. Sure, the car is 30% more efficient but it belches four times as much crap for every gallon of fuel, so that's about three times as much pollution from a European diesel than a US internal combustion.
Direct regulation vs economic incentive - Pollution is in economics what is referred to as a "tragedy of the commons". Air quality is a common resource shared by all with no direct economic incentive for any individual to protect it, but if individuals don't do something to protect it everyone as a group will regret it in the end. Other examples of the "tragedy of the commons" are common pastures (the origin of the concept actually, common pastures tend to be destroyed by over use resulting in no pasture at all) and oil fields (this was a huge problem in early oil field development in the US, competing wells racing to draw oil from a common field would result in greatly reduced total recovered reserves than if the wells coordinated a slower output). In general the only solution to the "tragedy of the commons" is government regulation, as suboptimal as that often is. So pollution (NOx, SOx, particulates) for vehicles is the domain of direct government regulation. Efficiency on the other hand is quite different between the US and Europe. In the US we have low tax gasoline and thus there is little economic incentive for efficiency. Instead the CAFE standards attempt to directly regulate what vehicles will be available to the consumer as far as efficiency goes. In Europe instead the gas is very highly taxed and so $8/gal gas is typical. In that kind of environment no direct regulation of efficiency is required at all, market forces from the very high cost of has drive consumers to select the most efficient vehicles. We saw a little bit of how effective economic incentives can be when gas prices hit $4/gal in the US in a short period of time, the SUV market collapsed overnight. Lastly, in the US diesel fuel is taxed higher than gasoline and the opposite is true in Europe.
Paper efficiency vs Real efficiency - The dirty little secret of super efficient European cars is that they are in fact not that efficient. The standards and testing for efficiency in Europe are total BS. In Europe you can basically completely reprogram the ECU of the car and make a variety of other changes to the vehicle prior to doing the efficiency test. The result is the vehicle tested bears little resemblance to the vehicles consumers actually get to drive. So when looking to Europe you see not just a diesel boost in their efficiency ratings but also outright fraudulent testing. The US is actually pretty aggressive in policing this and in fact just recently there was a huge penalty levied against a company for listing MPG figures that were just slightly higher than actual.
So put that all together and what do you get? Europeans pay through the nose for their gas and so if something is 30% more efficient they have a large economic incentive to go that route. Furthermore the regulatory environment for pollution is far more lax in Europe than the US (again, four times more NOx allowed in Europe) and so it is easier to get a pollution belching diesel on the road. So manufacturers have large incentives to put diesels on the road in Europe with few barriers. Now throw in the bogus efficiency ratings in Europe and you are left with the typical US consumer wondering "why can't I get that 80 mpg diesel everyone has in Europe". The answer is "because it doesn't actually exist (it is a lot less than 80 mpg), the thing is smearing soot and NOx all over the place to the point that various cities are thinking of banning them and finally you'll pay an extra $3K for it which the European guy will recover quickly in fuel bills but you will never see an ROI on in the US."
