Engineers
Engineers for your ship are recruited by spending Rank Points. Each Rank Point spent recruits 2 Engineers. Each Engineer increases your maximum Energy capacity by 1 point. So spending a levels worth of 5 rank points hires 10 Engineers (5 Rank points x 2 = 10), increasing your max energy by 10.
Experimental Crate adds 4 engineers to your crew.
Prototype Engineering Bay adds 10 engineers to your crew.
Engineers increase total Energy capacity. They have no effect on energy recharge rates. To increase your energy recharge rate, build Relays.
Engineers are included in your ship's crew that will help you in raiding enemy ships, and also help defend your ship from enemy raids.
Engineers take up no ship space whatsoever and have no upkeep cost. How and why they work so frugally is a matter of much debate. But it gives a good reason for considering using rank points for Engineers instead of for increasing Deck Spaces for expensive energy modules.
See Also:
Engineers
Helmsmen
Scientists
Tactical Officers