It's called a warp bubble, thank you very much.Uh . . . not to go all geek on this, but at so-called "warp speed", they're exceeding the speed of light by bending space (I can't believe I'm actually writing this). If they were traveling at sub-light speeds, this would be correct, but it's reasonable to assume that the engines are necessary to continue to warp space to maintain their warp speed.
I'm off to my Geeks Anonymous meeting now.