The present invention is directed to a dual engine-compressor system having a crankcase enclosing a crankshaft and having engine cylinder housings and compressor cylinder housings linearly disposed on opposite sides of the crankcase. Combustion pistons are reciprocatingly disposed in the engine cylinder housings and defines alternating combustion chambers on opposite sides of the pistons. Compressor pistons are reciprocatingly disposed in the compressor housings and define alternating low and high pressure compressor chambers on opposite sides of the compressor pistons. The compressor pistons underdo a 4-cycle process to drawn in, re-distribute, and then compress fluid. The compressor cylinder and piston has a series of one-way intakes and reed valves to selectively draw or push fluid in response to movement of the compressor piston.