Using poor-detonable liquid C 8 H 16 /air mixture with low-energy system (total spark energy of 50 mJ) and a new one-step detonation initiation method developed in this study, partial-tube fill experiments have been conducted in a fixed length PDE tube with a fuel/air mixture section of different length, covering a wide range of the detonation tube fill fraction (ratio of fuel/air mixture length to tube length). Impulse was calculated by integrating the pressure differential at the closed end of the tube. Based on the results obtained experimentally, it is found that the fuel-based specific impulse declines when fill fraction increases. On the other hand, the total-mixture-based specific impulse rises as fill fraction increases. A multi-cycle partial-fill model is developed to predict the impulse obtained from a partially-filled detonation tube, which is valuable for the optimization of PDE performance.pulse detonation engine, partial filling, impulse, fill fraction Pulse detonation engines (PDE) are new-concept propulsion systems [1][2][3] , which utilize repetitive detonations to produce thrust or power. PDE offers the potential to provide higher performance while simultaneously reducing engine weight, cost and complexity, relative to conventional propulsion systems currently in service. Due to its obvious advantages, worldwide attention has been paid to the scientific and technical issues concerning PDE.Effective optimization is especially important because the PDE is a new engine competing against the well established turbojet and ramjet [4] . Only high installed performance and low cost will justify the investment needed to bring the PDE to production. While the PDE presents a mechanically simple device, its unsteady thermo-fluid dynamics are quite complex. As may be expected with such a relatively new technology, the study on the approaches for achieving optimum design and performance is only at the initial stage and needs further exploration.In the past few years, numerous studies on the effects of partial mixture filling on PDE performance have been reported analytically, numerically and experimentally [5][6][7][8][9][10][11][12] . Zitoun et al. [5] measured the single-cycle impulse of ethylene-oxygen mixtures under standard conditions in a detonation tube and an extension with the same cylindrical cross section. They directly initiated a detonation with approximately 35 J of energy and the impulse was calculated by integrating the thrust surface pressure differential. Schauer et al. [6] studied the effects of the pressure relaxation rate upon thrust experimentally by adjusting the amount of detonable mixture in the tube while maintaining the same detonation tube length. Significant performance gains were observed. Falempin et al. [8] and Cooper et al. [9] used a ballistic pendulum to measure single-cycle impulse values of ethylene-oxygen mixtures in detonation tubes with attached extensions having a constant cylindrical cross section and also in extensions of varying dimensions. Li and Kailasanath [10] s...