A four-cycle, four-cylinder, premixed charge compression ignition internal combustion reciprocating free piston engine with a variable piston stroke and a compression ratio that varies as needed to provide charge ignition, to offer the potential of higher efficiency, lower emissions, and multi-fuel operation. The engine does not have a crankshaft, and therefore does not provide direct rotary output. Instead its free pistons oscillate, in a manner similar to a two cycle free piston engine. For many applications, such as piston pumps and compressors, the engine provides an output directly driven by the oscillating pistons. In other applications, such as but not limited to use as a gas generator for a power turbine, the engine provides an indirect means of producing rotary power. When the engine is used with high-speed power turbines, the power turbine may be directly coupled to a high-speed alternator for electrical power output.