Abstract: A method is provided of operating an internal combustion engine comprising subjecting a hydrocarbon fuel of relatively low octane number to conversion at elevated temperatures by passing it through a reaction zone containing a fuel system of unidirectional, e.g., substantially parallel, channels with rigid walls, e.g., a monolith or series of monoliths, at least the internal surfaces of which contain an aluminosilicate zeolite catalyst having a Constraint Index in the approximate range of about 1 to 12 and a silica/alumina ratio in a range of about 20 to 5000, e.g., ZSM-5 preferably containing gallium or zinc, such that the fuel leaving said channels has an increased octane number, and passing the fuel to the combustion chamber of said engine.