What were the car attachments/retrofits in Looper?

Those are probably some kind of modified gasifiers which intake the exhaust fumes and convert them into consumable gas that would in turn run the engine. Gasification, was primarily done by burning fossil fuel like coal or wood. However, in my opinion the movie protrays some kind of an advanced gasifier that helps in fuel preservation, fuel re-cycling in a car and in turn run the engine with the outcome, more like a "Loop" (more appropriate if considered with the theme of the movie).

By the looks of it, I presume some kind of hydrogen-burning technology was implied? Because of the way blue flames were shown? As is, electric was ruled out - we could clearly see the flaming exhaust. The only alternative to be carried (if implied from our times) is the fuel-cell technology...

I think it might be a carbon capture system. However, that could be contradicted by the clear lack of any sort of regulatory infrastructure.

It is supposed to be compressed air, like in a compressor. The first cars using compressed air are here already, shown in the last car shows in Europe. The cars run on air, which is sucked up in front of the car, then compressed, which gives the energy. http://en.wikipedia.org/wiki/Compressed_air_car