Fuel hydrocarbon (6)
(Other definitions for ethane that I've seen before include "Compound" , "Flammable hydrocarbon" , "Ate hen (anag) - hydrocarbon gas" , "Flammable gas; eat hen (anagram)" , "Fuel" .)
(Other definitions for octane that I've seen before include "Anti-knock element in petrol" , "Fuel component in McLaren vehicle" , "Ingredient of petroleum" , "chemical" , "Flammable hydrocarbon - at once (anag)" .)