A computer program package is described for use in simulating the unsteady gas dynamic processes occurring in a reciprocating engine system. The program package is written in Fortran IV and utilizes the Method of Characteristics to solve the general equations of one-dimensional unsteady flow in the presence of heat transfer, wall friction, and entropy gradients. It is arranged in a generalized form to allow ready application to a variety of engine and duct configurations. A subroutine library of standard engine/duct boundary conditions has been developed which, together with the unsteady flow program, can model a desired system configuration with no further programming required from the designer.The paper describes the development of the program package and of some of the duct boundary subroutines. A single cylinder two stroke engine model with intake and exhaust ducts is used to demonstrate the application of the package. Computer run times for typical test runs are of the order of one to three minutes on an IBM 360 Model 50 Computer, confirming that the program is an economical means of predicting system performance.