The effect of nonair mixtures on cycle-to-cycle variations of cylinder pressure characteristics was investigated experimentally with an indirect-injected (IDI) diesel engine, The engine intake temperature and pressure were maintained at normal air-breathing conditions when operated with nonair mixtures, Preliminary results indicate that increases in carbon dioxide concentration can cause significant cyclic variations. Moreover, the extent of such cyclic variations is notably influenced by the oxygen concentration and inert gas constituents of the working fluids.