Engineers improve the power of your ship's thrusters, increasing ship speed as a result. The initial engineer increases travel speed by 10%, but further engineers have diminishing returns. The next engineer increases speed by 5% (15% total cost reduction), then 2.5% (17.5% total), etc. Further engineers will continue to increase ship speed as close to a total of 20%, but this value cannot be reached.
Fighting isn't their strong suit, but engineers will engage in combat when chosen as a travel companion. Their weapons of choice are Pistols and Shortswords, though these are not wielded until the engineer spots a threat.
Engineers originally improved the ship's fuel efficiency by 10% periodically when left on the ship.
Because of this, engineers would endlessly buff fuel efficiency by 10% (that is, fuel cost for a given trip is reduced by ~9.1% of its previous value). Eventually, the ship could travel anywhere for free (and would continue to improve efficiency after this point), though fuel efficiency reset whenever the ship was upgraded.
This effect was later given to Mechanics, but was nerfed to only buff fuel efficiency once per mechanic. Firing the mechanic would remove their buff.