In principle the residual material could have an arbitrary composition. The Exhaust Gas model in Ansys CFX restricts the residual material to be products originating from the same fuel and oxidizer as for the combustion. Restricting the residual material to exhaust gas offers two advantages:
The interaction of the residual material with the products can be modeled using a single-mixture fraction chemistry library (that is, the same flamelet libraries can be used as without residual material).
Transformation of newly generated products into residual for simulation of subsequent engine cycles.
For many practical applications it is appropriate to assume that the residuals are recirculated exhaust gas. Exhaust gas recirculation (EGR) may be either externally by guiding the exhaust gas through a duct or pipe into the fresh mixture, or internally by recirculation or incomplete load exchange.
It is still necessary to distinct between products and residuals, because a fresh mixture containing residual material behaves differently than a partially-reacted mixture in the flamefront. The residual material will be less reactive, due to lower temperature and lower radical concentrations, such that no flame develops when merely mixing residuals with fuel and oxidizer.