Effects of pilot injection strategy of diesel fuel on combustion characteristics in a premixed methanol-air mixture atmosphere in a CVCC

•Compared with the diffusion combustion of diesel fuel, the combustion of methanol is smokeless.•Ignition delay in DM is longer than that in D, but when t2 advances or mdp increases, it in DM becomes shorter instead.•FLoL in DM is more sensitive to pilot injection compared with in D.•When Δt2 is longer than 2 ms or mdp is more than 3.28 mg, TINL in DM is higher than that in D.•With the increase of mdp, the HRR of methanol combustion increases, but it is lower than that of diesel fuel. The experimental research on effects of diesel pilot injection strategy, including pilot injection timing (t2) and mass (mdp), on the combustion of diesel in a premixed methanol-air mixture atmosphere were conducted in a newly developed constant volume combustion chamber (CVCC), where both methanol and diesel fuel were injected directly into the chamber. Results showed that pilot injection makes the ignition of main injection advance; ignition position move towards the nozzle tip; and the natural flame luminosity become stronger. In diesel/methanol dual fuel mode (DM), the wrinkles on the flame surface are more complicated and the area of natural flame luminosity is larger, especially in earlier t2 or more mdp conditions. Pilot injection shortens ignition delay and ignition delay in DM is longer than that in diesel fuel mode (D). But when the interval between pilot and main injection (Δt2) is longer than 2 ms or mdp is more than 3.28 mg, ignition delay in DM becomes shorter instead. In D the changes of Δt2 and mdp have no significant effects on flame lift-off length (FLoL). But FLoL shortens gradually with the advance of t2 and the increase of mdp in DM. After adding pilot injection, the start of spatially integrated natural luminosity (SINL) rising advances and SINL peak increases. TINL in DM is lower than that in D. But when Δt2 is longer than 2 ms or mdp is more than 3.28 mg, TINL in DM becomes higher instead. Pilot injection can change heat release rate (HRR) from bimodal to unimodal and reduce the HRR peak of main combustion simultaneously.