Volkswagen’s stunning admission that it sold diesel-powered cars that intentionally faked emissions testing results has left many of their owners — who thought they were buying fuel-efficient and environmentally friendly vehicles — angry and feeling betrayed. The deception could prove a major threat to Volkswagen’s reputation in the United States and could break the trust that the German automaker had cultivated with car owners over the years. It could also have lasting consequences for the future of diesel-powered technology championed by Volkswagen, which has been seeking to make a comeback in the United States.