Remember Engines have a Push/Pull thing going on, so half the time they pull instead of pushing, and only the push energy is actually blowing Air in, which makes it 2000KU total per Steel Ingot (while technically only using 1000KU).
Please be aware that Air instantly escapes the Crucible if it does not get used up in the Steel Process, so if you dont have the total end Temperature needed to actually smelt Steel, then do not run the Engine at all, it would be quite wasteful to do while you cant even make Steel at the too low Temperatures.