Abstract: Thermal efficiency of pulse detonation engines (PDEs) wasformulated with thermodynamic cycle analysis when the exhaust flow is notperfectly expanded, the quantitative relation of PDEs thermal efficiency andnozzle exit pressure ratio was examined under different initial combustortemperature, the closer the exhaust pressure gets to ambient pressure, thehigher efficiency PDE could achieve. Effects of the CD-nozzle and theD-nozzle on PDE performance are investigated by numerical simulations; theobtained thrust and impulse of PDE are compared with each other fordifferent cases, and nozzle effects on each period of the PDE cycle are alsoinvestigated. In addition, the influence of reflected shock waves from theconvergent section of a CD-nozzle is also studied to show that the reflectedshock waves actually affect the thermodynamic cycle efficiency more or less,but when the shock Mach number is less than 1.5, PDE thermal efficiencyreduction is quite slight and the positive effects of nozzle are dominated.