Ok, dare I say it but Wikipedia is as good a place to start as any for the actual emission standards (http://en.wikipedia.org/wiki/European_e ... _standards
), and then you should get some links to other web pages from that.
Also I reckon you want to have a look for some papers (SAE, MTZ are good places to get papers) or info on:
Variable valve trains - BMW valvetronic system, IAV variovalve (http://www.iav.com/en/1_engineering/pow ... /index.php
SCR systems - VW Adblue and Mercedes Bluetec
Stuff on EGR should be pretty widespread on the internet, but also look at internal EGR as well
Also look for some info on HCCI combustion as this is a potential future technology with loads of potential for reducing NOx emissions
These are a few technologies that are currently in development/use that are applicable to an internal combustion engine. Obviously though if you want to go for zero emissions, there's a hydrogen BMW 7 series that came out recently and also I guess you could start looking at some alternative fuels as well.
Also here are a couple of websites that are pretty good:
MTZ - http://www.atzonline.com
Automotive Engineer - http://www.ae-plus.com/
Hope that helps!