Hydrocarbons: compare question

A catalytic converter is fitted to a car exhaust. The exhaust gases contain unburnt hydrocarbons, carbon monoxide and oxides of nitrogen. A student claims that the alkanes in petrol could instead be produced by hydrogenation of alkenes.

Compare hydrogenation of an alkene with cracking of a longer-chain alkane as methods of producing alkanes. In your answer include the reagents and conditions for each method, and explain why cracking is used industrially rather than hydrogenation to obtain alkanes of lower relative molecular mass from crude oil.