Heat charcoal, preferably natural chunk, until grey ash appears. In a blender, combine oil, soy sauce, scallions, garlic, lime juice, red pepper, cumin, and sugar, and puree. Place skirt steak in a large heavy duty, zip-top bag and pour in marinade. Seal bag, removing as much air as possible. Allow steak to marinate for at least 1 hour in refrigerator.