This paper describes the theoretical and experimental work on the unsteady flow from the cylinder through the exhaust pipe of a reciprocating spark ignition engine. The theoretical work covers the subject of unsteady gas dynamics applied to an internal combustion engine exhaust system. The new development presented here concerning the internal combustion engine simulation is that the computation uses path lines and introduces path line streams. The objectives were to develop a capability of tracing and retaining the identity of the path lines in the exhaust system of an internal combustion engine. Time-resolved hydrocarbon measurements were made in the cylinder and in the exhaust system of a spark ignition engine using fast acting sampling valves. In order to study the mechanisms by which the hydrocarbon mole fraction is reduced in the transport from the cylinder to pipe, the path line calculation in conjunction with a post flame oxidation model was applied to this situation.