Ignition criteria for diesel fuel tanks under high-velocity impact of spherical tungsten fragment

Fuel tanks are critical yet vulnerable components in military platforms such as aircraft and ground vehicles, and high-velocity fragment impacts may trigger ignition and pose severe hazards. In realistic combat scenarios, fragments can enter the tank from random directions and follow distinct in-tank trajectories. Four representative impact paths are considered: ullage-only motion, ullage-to-fuel transition, fuel-to-ullage transition, and fuel-only motion. To establish an ignition prediction model that explicitly accounts for path effects, a series of ballistic impact experiments were conducted using spherical tungsten-alloy fragments of three different masses. The experiments reveal a clear path-dependent ordering of ignition thresholds. For a given fragment mass, the minimum impact velocity for ignition increases from the ullage-only path to the ullage-to-fuel, fuel-to-ullage, and fuel-only paths. High-speed imaging further identifies two dominant ignition modes: internal flash ignition and external ignition driven by fuel spray and subsequent atomization. Based on the data, path-conditional ignition probability functions are obtained, and combined with geometry-based path-hit probabilities to estimate ignition risk under random incidence. At a fixed fill ratio, the ignition probability increases with specific kinetic energy and exhibits a double-sigmoid trend with a quasi-plateau in the intermediate energy range between two fitted thresholds. These findings provide experimental evidence and a practical modeling framework for ignition assessment of fuel tanks under diverse fragment impact paths.