Infestation - S1-E3
Character mistake: When Maureen Robinson says that the ship is "trillions of light years" away from Earth, she should have said miles instead of years. A later episode correctly changes the distances involved to "trillions of miles."
Answer: The Fortuna didn't have an Alien Engine. The cover story was the Fortuna was to explore Alpha Centauri but it was exploring the rift. The rift is what the Alien Engine creates and explains how the Fortuna was able to travel to a distant planet (what you call "galactic travel").
Bishop73