Characterization of an ethylene–oxygen predetonator for high-speed combustors

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Pre-detonators serve as the primary ignition source for pressure-gain combustors, yet systematic investigations into their design parameters and operating sensitivity remain scarce. This work presents the design, fabrication, and experimental characterization of two ethylene–oxygen pre-detonators tested across four feed pressures (0.86–3.45 barg) under both open-valve and closed-valve ignition modes. The two configurations differ in their injection geometry and ignition placement. A transparent polycarbonate detonation tube coupled with high-speed imaging at 310 kfps enabled high-temporal-resolution tracking of the detonation-front velocity along the full tube length. All sixteen test conditions produced self-sustained detonations propagating at or near the Chapman–Jouguet velocity. The top-mounted ignition layout sustained higher overdrive levels over a greater fraction of the tube and exhibited stronger flame luminosity near the exit, indicating more energetic wave delivery to a downstream device. Open-valve operation consistently yielded superior detonation stability and exit-wave strength compared to closed-valve ignition. Numerical calculations attribute this difference to valve-closure-induced rarefaction–shock dynamics that render the downstream portion of the tube fuel-lean prior to ignition. Together, these results establish clear relationships between pre-detonator geometry, valve timing, and detonation-wave strength, providing a basis for ignition system design in pressure-gain combustion applications. Novelty and significance statement: Growing interest in pressure-gain combustors has intensified the need for reliable pre-detonators. Although a few studies have examined pre-detonators, comprehensive assessments of how design choices and operating conditions shape their performance remain limited. This work addresses that gap by designing, fabricating, and experimentally testing two ethylene–oxygen pre-detonators across multiple feed pressures in both open- and closed-valve modes. To the authors’ knowledge, this is the first study to directly quantify how pre-detonator geometry and operating configuration influence the detailed velocity-profile evolution of the detonation front. The results establish clear relationships between operating conditions and detonation-wave quality, identifying the factors that stabilize or weaken the overdriven front and providing a physically grounded basis for selecting valve modes, ignition layouts, and feed pressures in future detonation engine ignition systems.