Spray and combustion characteristics of −10 diesel and −50 biodiesel under various injection pressures in a constant-volume combustion chamber

Decarbonizing heavy-duty transport requires biodiesel deployment in extreme climates, yet combustion knowledge for cold-adapted grades under modern high-pressure injection remains absent. This study addresses this gap through the first optical diagnostics of −50 biodiesel (pour point −70 °C) at 180 MPa injection pressure and engine-representative conditions (5 MPa, 500 °C), critical for advanced diesel engine applications. Constant-Volume Combustion Chamber (CVCC) experiments with high-speed shadowgraphy/schlieren imaging systematically decoupled fuel properties effects via parallel nitrogen (evaporation) and air (combustion) tests. Despite biodiesel's 2.6 × higher latent heat causing 6–12 % longer liquid penetration, vapor-phase characteristics converge with diesel after vaporization, indicating thermophysical penalties diminish post-evaporation. Combustion superiority emerges only at ultra-high pressure: at 180 MPa, biodiesel demonstrates 11 % shorter ignition delay (800 vs. 900 μs), 29 % faster peak flame propagation, and 21 % larger flame area, whereas 120 MPa shows negligible differences. Mechanistic analysis reveals cetane-oxygen-atomization synergy. Cetane number reduces induction time, fuel-borne oxygen extends lean-limit combustion, and lower viscosity improves atomization. Pressure escalation from 120 to 180 MPa amplifies biodiesel's ignition advantage by 22 % and flame front propagation velocity gain by 40 %. Results demonstrate that ultra-high-pressure injection (≥180 MPa) is essential for realizing biodiesel's emission-reduction potential in cold regions, providing design criteria for integrating renewable fuels into next-generation advanced diesel engines without compromising cold-start performance.