in an internal combustion engine a fuel - air mixture having compression temperatures and pressures sufficiently low as not to support auto ignition , ignition is achieved by injecting igniting active radicals into the air - fuel mixture . in one embodiment the active radicals are provided by withdrawing a portion of the mixture , treating it to produce active radicals in the portion and returning the portion to the mixture . treatment of the portion typically includes simultaneously injecting , mixing , and compression of a predetermined amount of pilot fuel within the portion .