A system and method for fast control of the timing of combustion in an internal combustion engine, comprising actuating a fast-acting fuel-additive supply valve to meter a variable amount of fuel additive into a fuel stream, thereby forming an additive-enhanced fuel with an additive concentration that can be dynamically adjusted as fast as each engine cycle; injecting the additive-enhanced fuel directly or indirectly into a combustion chamber or into an intake port; and combusting the additive-enhanced fuel.