Work

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

Year: 2026

Type: article

Abstract: Diesel-methanol partially premixed combustion (DM-PPC), characterized by prior mixing and controlled auto-ignition, has attracted increasing attention as an effective strategy to improve combustion controllability and extend 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 fuels direct-injection jet flame tests are conducted in a constant-volume chamber (750K and 40bar) with different injection intervals. We proposed a CNN-based denoising and recognition method for jet flame Schlieren images, centered on FFDNet with noise-level map generation-estimation and robust segmentation. Three DM-PPC modes were classified based on two jet flames characteristics with different spray intervals: 1) In methanol early injection premixing leading, low-temperature DM-PPC mode, blue flame lasts about 0.7–0.8 ms, yellow flame is extremely weak and short-lived about 0.4 ms, the flame-contour area growth rate peak is low about 2.0–3.0m²/s, and the total flame lift-off length (FLOL) is short about 40–80 mm toward the methanol side. 2) In diesel-methanol co-injection, premixed-diffusion-coupled DM-PPC, multi-point ignition kernels within the diesel-methanol mixing zone leads to middle flame contour-growth peaks about 7.0–9.0 m²/s and average flame-speed spikes of 170–220 m/s, with the global flame layer located between the diesel and methanol jets, reflecting a balanced coupling of premixed and diffusion flames. 3) In diesel early injection leading, diffusion-flame-dominated DM-PPC, blue-flame stage is shorter but followed by strong, long-lived yellow-flame clusters, the flame contour growth rate peak reaches 1.5–1.8m²/s, the initial average flame-speed spike rises to 200–230 m/s, and the early total FLOL is the largest about 100 mm with a pronounced shift toward methanol side. The novelty of this study is the integration of a spatially adaptive FFDNet-based denoising framework with high-speed Schlieren diagnostics to enable quantitative identification of injection-interval-governed DM-PPC flame modes under high-temperature, high-pressure conditions.

Source: Fuel

DOI: 10.1016/j.fuel.2026.141388