Combustion mode transitions in methanol-diesel partially premixed combustion under varying injection intervals: CNN-enhanced schlieren quantification

Abstract

Diesel-methanol partially premixed combustion (DM-PPC), characterized by prior mixing and controlled auto-ignition, is a promising strategy for improving combustion controllability and extending methanol utilization in compression-ignition engines. However, the spatiotemporal evolution of ignition and flame development under different injection intervals remains insufficiently quantified. In this work, diesel–methanol dual-fuel direct-injection jet flame tests were conducted in a constant-volume chamber at 750 K and 40 bar. A CNN-based denoising and recognition method centered on the Fast and Flexible Denoising Network (FFDNet) was developed for jet-flame Schlieren images, integrating noise-level map generation-estimation and robust segmentation. Three DM-PPC modes were identified. In the methanol-leading low-temperature DM-PPC mode, the blue flame lasted about 0.7–0.8 ms, the yellow flame was weak and short-lived for about 0.4 ms, the flame-contour area growth-rate peak was about 2.0–3.0 m 2 /s, and the total flame lift-off length (FLOL) was short, about 40–80 mm, toward the methanol side. In the co-injection premixed–diffusion-coupled DM-PPC mode, multi-point ignition kernels formed within the diesel–methanol mixing zone, with flame-contour growth-rate peaks of about 7.0–9.0 m 2 /s and average flame-speed spikes of 170–220 m/s. In the diesel-leading diffusion-flame-dominated DM-PPC mode, the short blue-flame stage was followed by strong yellow-flame clusters; the flame-contour growth-rate peak reached 1.5–1.8 m 2 /s, the initial average flame-speed spike increased to 200–230 m/s, and the early total FLOL was the largest, about 100 mm. The novelty of this study lies in integrating a spatially adaptive FFDNet-based denoising framework with high-speed Schlieren diagnostics to quantitatively identify injection-interval-governed DM-PPC flame modes under high-temperature and high-pressure conditions.