Apparatus and method for operating a 2-stroke or 4-stroke reciprocating compression-ignition internal combustion engine, of the direct injection type, to reduce fuel consumption and to reduce NO.sub.x emissions. A staged fuel injection process involving two fuel injectors, but only one type of fuel, is used. A low pressure fuel injection device injects a pilot fuel charge into the combustion chamber when the combustion chamber is near its lowest pressure and its maximum volume. The pilot fuel charge stratifies within the combustion chamber, and undergoes a preflame reaction with a resultant release of heat energy. Near the end of the compression stroke of the piston, when the combustion chamber is near its minimum volume, a high pressure fuel injector injects a main fuel charge into the combustion chamber. The main fuel charge is ignited by the burning pilot fuel charge, and combustion of the main fuel charge is accelerated by the heat energy released by the pilot fuel charge preflame reaction. This accelerated combustion process reduces the ignition delay and premix combustion rate, and results in reduced fuel consumption and NO.sub.x emissions, without adversely affecting either combustion noise levels or particulate emission.