The universal mathematical model of engine was established by the testing model of engine

and the economical operating zone of engine was presented and described.Based on the testing data of a refitted engine

the universal performance curve of the given engine had been obtained

and then economical upper bound and lower bound of the special economical operating zone of engine were demarcated

as well as the optimal torque curve.Analysis showed that in the special economical operating zone of engine

the speed range was about 2300—3850 r·min-1

the torque range was about 17.5—27.9 N·m

and output load region 5—9.5 kW

also the average thermal efficiency could be over 0.25

but the highest thermal efficiency of the engine could not exceed 0.285.