NOVELTY - A spark ignition engine system comprises: spark ignition engine (18); first unit for introducing fuel from first source; second unit for direct injection of liquid from second source; and fuel management system that varies liquid from second source so as to prevent knock, and it employs information from knock sensor and oxygen sensor (12).    USE - As spark ignition engine system (claimed).    ADVANTAGE - Using a signal from a knock sensor to determine when and how much ethanol or other anti-knock agent must be used at various times in a drive cycle to prevent knock, the fuel management system can be employed to minimize the amount of ethanol or other anti-knock agent that is consumed over the drive cycle. If sufficient ethanol or other ant-knock agent is available, the fuel management system can also be used to employ more ethanol than would be needed to prevent knock. This allows greater gasoline savings (the gasoline savings component from substitution of ethanol for gasoline would increase) and carbon dioxide reduction. In this case it may be desirable to operate at an anti-knock agent fraction which is either varied or constant during the drive cycle.    DETAILED DESCRIPTION - A spark ignition engine system for which fuel is introduced into the engine (18) from a first source and a liquid is separately introduced into the engine cylinders from a second source by direct injection comprises: a spark ignition engine; a first unit for introducing the fuel from the first source into the engine; a second unit for direct injection of the liquid from the second source into the engine cylinders, where during part of the engine operating time, the engine cylinders receive both the fuel from the first source and the liquid that is directly injected from the second source; and a fuel management system which varies the relative amount of the liquid from the second source that is introduced into the engine so as to prevent knock, where a fuel management system employs information from a knock detector and where the engine is operated with stoichiometric fuel/air ratio and: where the stoichiometric operation with the changing of the second source liquid/first source fuel ratio that occurs as the manifold pressure is increased is achieved by use of the fuel management system which employs information from the knock sensor and an oxygen sensor (12) and where the fuel management system enables use of a three way catalyst (10). An INDEPENDENT CLAIM is included for a turbocharged spark ignition engine which is powered by a mixture of gasoline and ethanol and where the ethanol is directly injected in such a way as to provide an octane enhancement that increases at the rate of at least 4 octane numbers for every 20% of the fuel energy that is provided by ethanol and where the vaporization cooling from ethanol provides a greater knock suppression effect than the intrinsic octane of the ethanol.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic diagram of the fuel management system.    Three way exhaust treatment catalyst (10)    Oxygen sensor (12)    Electronic control unit (14)    Turbocharger (16)    Engine (18)