. Nomenclature. Glossaryâ?¢ A clean engine is one that is not suffering any performance deterioration.â?¢ The deterioration indexDI is an hypothetical term defined for the purpose of thisinvestigation. It combines the effects of any reduction in flowcapacity and efficiency of any gas-path component into a single term.For this investigation, a % reduction in efficiency accompanied by.% reduction in flow capacity equals a % deterioration index. Linearrelationships are assumed and so a % DI represents the combined effectof and % reductions in efficiency and flow capacity, respectively.â?¢ The gas pathis the track through the engine via which air travels from the engineâ??sentrance from the atmosphere into the engine until it emerges onceagain to the atmosphere. All the engine components through which airflows, such as the intake, compressors, combustor and turbines arecalled gas-path components.â?¢ Specific fuel consumption SFC is the fuel flow rate per unit net thrust developed.