Optical investigation of natural gas/biodiesel dual direct-injection combustion in a constant volume combustion chamber

Natural gas (NG) and biodiesel are promising low-carbon fuels, and biodiesel-assisted ignition in a dual direct injection configuration provides an effective route for NG combustion. This study optically investigated NG/biodiesel dual direct-injection in a constant volume combustion chamber (CVCC) by varying injection sequence (NG–Biodiesel and Biodiesel–NG), injection interval (Δt), NG energy substitution ratio (ENG), and ambient pressure (P0). Under the NG–Biodiesel strategy, increasing Δt shifted NG combustion from a premixed-diffusion regime toward a more premixed-dominated regime, and the heat release rate (HRR) evolved toward a single-peak profile. At Δt = 2–8 ms, NG-jet-induced turbulence and premixed NG promoted biodiesel evaporation and combustion, which shortened ignition delay and enhanced premixed combustion. At Δt = 10 ms, excessive diffusion time over-leaned the premixed NG mixture, prolonging ignition delay and weakening combustion intensity. Under the Biodiesel–NG strategy, NG remained diffusion-dominated and the HRR exhibited a multi-peak, staged profile. With increasing Δt, both ignition delay and combustion duration increased monotonically, while OH* chemiluminescence weakened during the biodiesel-dominated stage and strengthened during the NG-dominated stage. For both strategies, Pmax and HRRmax increased and then decreased with Δt, whereas the Biodiesel–NG strategy produced markedly less soot. Increasing ENG from 50% to 80% delayed biodiesel ignition and weakened biodiesel combustion while increasing Pmax and reducing soot. When ENG reached 80%, the weakened biodiesel ignition flame reduced its ability to ignite NG, which led to lower NG combustion intensity. Increasing P0 promoted biodiesel ignition and increased Pmax and HRRmax, but it also increased soot formation. At excessively high P0, reduced convergence between the NG jet and the biodiesel flame delayed NG ignition and prolonged combustion duration.