West Virginia University has the Center for Alternative Fuels Engines and Emissions as part of the engineering school, and they test lots of different vehicles and engine technologies. http://www.cafee.wvu.edu

They were commissioned by a European NGO, the international council on clean transportation (ICTT)[1]. The ICTT wanted to include diesel cars from the US market where regulation are stricter with intent to show that it was actually possible to have lower emission from diesel cars.

As stated at the beginning of their report[2]:

  The Center for Alternative Fuels, Engines and Emissions
  (CAFEE) at West Virginia University (WVU) was contracted 
  by the International Council on Clean Transportation 
  (ICCT) to conduct in-use testing of three light-duty 
  diesel vehicles, using a portable emissions measurement 
  system (PEMS), over a variety of pre-defined test routes 
  exhibiting diverse driving conditions pertinent to major 
  United States population centers located in the state of 
  California.

You can also have a look at the ICTT report[3].

[1]: https://en.wikipedia.org/wiki/International_Council_on_Clean...

[2]: http://www.eenews.net/assets/2015/09/21/document_cw_02.pdf

[3]: http://www.theicct.org/sites/default/files/publications/ICCT...