In a word, California. Because of the geography and climate there, smog forms very easily in large parts of the state. So, California emissions regulations have been focused - since the mid-20th century - on smog precursors. NOx is a major smog precursor, so they have a very good reason to minimize it.

This is where we get into politics. The US as a whole wants to adopt somewhat tougher emissions regulations, and car manufacturers want to minimize the number of different standards they have to meet in a single market. It turns out that everyone's on board with "just make the California standards the national standards" and so that happens. Then California tightens their standards again and the cycle repeats.

Europe doesn't have a comparable political subdivision with such a high natural propensity for smog formation and the ability to make their own standards. The regulatory differences emerge from a combination of geography and political systems.

The US focuses more on NOx emissions, the EU more on CO2. The first is poisonous, the second is more about greenhouse gas emissions.

http://www3.epa.gov/region1/airquality/nox.html

http://ec.europa.eu/clima/policies/transport/vehicles/cars/d...

> Does the engines comply with EU standards?

No. The engines were (re-)tested at 0.62g/km, EURO5 allows 0.18g/km, EPA Tier 2 / Bucket 5 allows 0.043g/km.

> Why US Standards are different?

US standards are much more stringent on NOx, I expect because historically they regulate gasoline engines which produce relatively little NOx, and the standard bodies could thus easily clamp down on that.

The EURO standards actually have two profiles, one for gasoline and one for diesel, with the diesel profile allowing significantly higher NOx emission than the gasoline profile. Though even with that, the US EPA standard remains more restrictive than the EURO6 gasoline standard.

Here are the limits of the EURO standards: https://en.wikipedia.org/wiki/European_emission_standards#Em...

Here are the limits of EPA Tier II: http://www.dieselnet.com/standards/us/ld_t2.php#bins

(warning, EURO NOx is in g/km, EPA is in g/mi)

US standards are more stringent with respect to NOx emissions. NOx are harmful to health, and their products are also. In addition they form smog which is a blight on many urban areas. Perhaps the problem is worse in USA than in Europe, or perhaps Americans just care more about it.

EU standards reflect the reality that diesel engines are far more common than in the US, so they are less strict with regards to NOx emissions to begin with. About half the km (or miles, or furlong) driven in Europe are on diesel.

Diesel gets better mileage and tends to be cheaper than gas (depending on local taxes), which can make a significant difference with the high fuel prices in Europe.

The oversight is also much more fragmented because it is less centralized than in the US. As I understand it, the oversight is left to the member states, so manufacturers tend to shop around for the most lenient one.

Why is US money different? Why are US plugs and voltage standards different?

Sovereign countries have the right to set their own regulations and bureaucrats don't have a strong track record of international collaboration.