Cosworth is making its return to F1 as an engine supplier at this weekends Bahrain Grand Prix, after a three year sabbatical, when it will supply powerplants to one-third of the grid. AT&T Williams will be leading the Cosworth challenge, while newcomers HRT F1, Lotus Racing and Virgin Racing will be out to make a positive first impression. Cosworth had also intended to supply the troubled USF1 …