smoke
烟 centers on smoke, the visible vapor produced by fire.
烟 combines 火 (fire) with 因, which likely contributes the sound and may suggest something rising or spreading, as smoke does.
Fire 火 on the left and 因 on the right: picture a fire trapped inside an enclosure, producing thick smoke that rises.
For エン, imagine the smoke forming the letter 'N' as it rises: 'N' sounds like エン, and the smoke curls into that shape.