A premixed charge of air and a low reactivity fuel is created in a combustion chamber space by the time at which a piston comes substantially to top dead center at which time a high reactivity fuel is directly injected into a central zone of the combustion chamber space which is bounded partially by a central bowl cavity in the piston head which is itself surrounded by an upright intermediate wall. The high reactivity fuel combusts by conventional diesel combustion and creates a flame front which propagates into an outer zone of the combustion chamber space which is bounded partially by the upright intermediate wall and an outer bowl cavity in the piston head to initiate combustion of the premixed air-fuel charge in the outer zone of the combustion chamber space.