burn, blaze, glow
燃 centers on fire and its visible effects: burning, blazing, and glowing. All three meanings are direct expressions of something being on fire or radiating heat and light.
燃 combines 火 (fire) with 然, which likely contributes the sound and the idea of burning or blazing. The modern form is a straightforward phono-semantic compound.
The left side is fire 火, and the right side 然 looks like a pile of kindling with flames rising. Together they show a fire burning and glowing brightly.
For ネン, imagine a fire burning so intensely that it makes a loud 'nen' sound as it blazes. The crackling 'nen' cues ネン while the flames glow.
fuel
to burn; to get fired up
to burn
burning; combustion
fuel consumption; gas mileage
recurrence; revival; resuscitation
inflammable; flammable; combustible; burnable
to burn out
to flare up; to burst into flames
incombustibility
to blaze; to burn brightly
internal combustion
fuel oil; fuel
Japan Nuclear Fuel Ltd.
mixed oxide fuel; MOX fuel
alcohol fuel
jet fuel
biofuel
rocket fuel
incomplete combustion; imperfect combustion