2010 Volkswagen Golf TDI – Click above for high-res image gallery

The question is often asked, "Why is it that drivers in Europe can get all of these amazing high-mileage cars that get 50, 60 or even 70 mpg but we have to celebrate when we get half of that?" There are plenty of reasons, but one of the main ones is the remarkable compression ignition engine, more commonly known by the name of its inventor, the diesel.

In recent years, diesel engined vehicles have accounted for over half of all new vehicle sales in Europe – in some places like France and Italy, up to 80 percent. Yet here in the United States, these high-efficiency wonders represent a tiny minority of car sales. Only in the heavy duty truck segment do diesels grab any significant attention. Read on after the jump to learn why so few diesel cars are available in the U.S.

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There are a number of reasons for limited diesel availability in the U.S. but the two primary hurdles are cost and emissions regulations. Diesel engines are inherently more expensive to build than spark ignition engines that run on gasoline or ethanol. Because of the way a diesel functions, the engine components need to be more robust to withstand the high internal pressures. However, the implementation of much tighter emissions standards has exacerbated the cost problem significantly.

The current U.S. emissions standards fall under what is known as Tier 2 and were phased in between 2004 and 2009. There are currently 8 bins, or categories, that new light duty vehicles can certified for, with bin 1 being zero emissions. California standards are even tighter and for a vehicle to be allowed to go on sale there it must meet the equivalent of at least the national bin 5 standard. Thus Tier 2 bin 5 (T2B5) is the de facto minimum for any automaker that wants to sell a car nationwide.

Here is where the problem lies. T2B5 has significantly tighter requirements than older EU4 European standards when it comes to particulate and nitrogen oxide emissions. The particulate or "soot" emissions are the black clouds that we're all used to seeing pour out of the exhaust of big trucks as they accelerate down the road. This problem has been largely addressed by the use of diesel particulate filters (DPF) in exhaust.

The particulate filter uses a ceramic core to trap the particles that make up the smoke. Pressure sensors on the inlet and exhaust sides of the filter are used to detect when it is full at which point regeneration takes place. To regenerate the filter, the air/fuel mix in the engine is adjusted to temporarily raise the exhaust gas temperature above 600 C, which causes the soot to burn off.

DPFs have been increasingly common installations in Europe over the last several years as the EU5 standard has gone into effect with a limit slightly lower than the T2B5 standard.
The big difference is the NOx emissions. The EU5 standards dropped the limit from .25 g/km to .18 g/km. T2B5 slashes that to only .04 g/km. Even the 2013 EU6 standard brings the standard to only .08 g/km. Eliminating NOx is not easy. Nitrogen oxides are produced when the temperature of combustion gets above about 1600°C. Modern common rail injection systems that allow for multiple fuel injection pulses along with exhaust gas recirculation have helped to reduce combustion temperatures, thus cutting NOx production.

However, that is not enough to meet T2B5 requirements. For that we still need some additional exhaust after-treatment. Depending on the size of the vehicle, this can come in the form of either a lean NOx trap or urea injection (selective catalytic reduction). For the larger vehicles that are more common in the U.S., the urea injection systems are often the only way to go. This requires adding a storage tank for the urea solution that typically holds 4-8 gallons.

Depending on who you ask, the incremental cost of a Tier 2 Bin 5 compliant diesel over a similarly powerful gasoline engine can be $2,000 to $5,000. That's a hefty premium to overcome, especially when fuel prices are hovering at $2.50-2.75 a gallon. While a number of automakers including Honda, Kia and Nissan had indicated or announced outright they would introduce diesels to the U.S. market in 2009-10, all but VW, Mercedes and BMW have canceled or indefinitely suspended those plans.

Since mid-2008, when diesel prices spiked as much $1 a gallon more than gas followed by the economic collapse, all those automakers claimed that diesel just didn't economic sense here. Poor overall sales and the cost of certifying a new, low-volume powertrain mean that those looking for an affordable diesel are pretty much stuck with Volkswagen. So, the U.S. can get diesel cars, just not very many of them.

Thanks to David M. for the question.