Looks like an old chevy truck...probably a broken fuel line spilled gas onto hot headers...and poof!
In my old car (85 civic wagon) the carb'ed engine had a fuel pump under the hood (talk about stupid design, huh?). Well I smelled strong gas in the cabin, pulled over and looked under the hood. Turns out the pump seals were oozing raw gas into a little plastic tray, right next to my hot engine. I dried it up, let it cool off, and (stupidly) drove it home. Then to the mechanic the next day.