A Ulysses Engine is a type of ship engine. The use of the engine involves Time Travel, but not always in a useful manner . So far all we know about it is a single mention, in passing, by Sanaa, as she proves to Othar that staying at home is more hazardous than adventuring with her big brother. It's a reasonable speculation, although there is so far no other evidence, that it may turn out to be relevant to Agatha's use of time travel as witnessed by Bang.