Method and apparatus for controlling combustion timing in an homogenous charge compression-ignition engine

A method for controlling combustion timing in an homogenous-charge compression-ignition engine, comprising: providing a fuel conduit extending between a source of fuel and at least one fuel introduction port in fluid communication with a combustion chamber of said engine; providing at least one source of a fuel additive adapted to alter the reactivity of a fuel, said at least one source of fuel additive being in controllable fluid communication with said fuel conduit; sensing at least one engine operating parameter; and controlling the amount of said fuel additive introduced into said fuel conduit in response to a sensed value of said at least one engine operating parameter.