The efficiency of separated flow turbofan engine was analyzed based on the definition of gas generator. It proved the existence criterion of the optimum fan pressure ratio and the equivalence of extreme value of the specific thrust

the total efficiency and the specific fuel consumption by theoretical deduction on the basis of the efficiency definitions. This criterion could be used to determine the optimum fan pressure ratio for separated flow turbofan engine in practical design progress. The parametric study results of two different highbypassratio separated flow turbofan engines demonstrated that when the ratio of bypass and primary exhaust velocity equaled to the product of fan and low pressure turbine efficiencies

the overall efficiency and specific thrust reached their maximums and specific fuel consumption reached its minimum

proving the validity of the criterion.