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

The experimental research on effects of diesel pilot injection strategy, including pilot injection timing (t(2)) and mass (m(dp)), 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 t(2) or more m(dp) 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 (Delta t(2)) is longer than 2 ms or m(dp) is more than 3.28 mg, ignition delay in DM becomes shorter instead. In D the changes of Delta t(2) and m(dp) have no significant effects on flame lift-off length (FLoL). But FLoL shortens gradually with the advance of t(2) and the increase of m(dp) 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 Delta t(2) is longer than 2 ms or m(dp) 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.