Nonlinear characteristics of pilot fuel injection quantity in low-carbon fuel combustion systems

• Micro-pilot injection shows S-shaped lag at small pulses, linearity at large ones. • Higher rail pressure lowers critical pulse width and speeds response. • Critical pulse width hinges on chamber pressure release and needle valve locking. • Nozzle diameter is most sensitive, boosting curve curvature by 10.8%. • Multi-parameter optimization tailors flow–pulse width for micro-ignition. Micro-pilot ignition is essential for low-carbon dual-fuel combustion, relying on precise control of minimal fuel amounts. The curvature of the fuel quantity curve in the pre-critical region dictates the controllability and stability of small injections. This study examines micro-pilot injection characteristics, focusing on fuel calibration, inflection point identification, curve fitting, and injector structural sensitivity. Results show segmented global fuel behavior: S-shaped hysteresis at small quantities and linearity at large quantities. Sensitivity analysis reveals nozzle diameter as the most influential parameter, increasing curvature by 10.8% and crucial for shaping small-quantity performance. Needle lift positively affects curvature via pressure build-up, while reduced return orifice and control valve lift degrade curvature by 12–15%. A “curvature optimization” strategy is proposed: prioritize enlarging nozzle and inlet orifice diameters to boost curvature and flow, while fine-tuning outlet orifice, control valve lift, and needle preload to limit drawbacks. This achieves gradual variation in the small-quantity injection curve without sacrificing global linearity, providing a theoretical basis for high-precision fuel control and tailored injector design in dual-fuel micro-pilot systems.