Investigation of the injection pressure impact on non-monotonic two-stage ignition delay of diesel engines under cold-start

Poor startability under cold-start conditions is a long-term obstacle of the compression ignition engine and adjustment of the injection pressure is an effective way to improve the startability. Therefore, a detailed investigation of the effect of injection pressure (P_i) on the ignition characteristics of the fuel spray under cold-start conditions is needed. Though numerous studies have been conducted on the effect of P_i on the ignition characteristics of the fuel spray, the two-stage ignition process of the fuel spray under cold-start conditions is still an unknown field. In this paper, the effect of P_i on the two-stage ignition process of the fuel spray under cold-start conditions is investigated in a constant volume combustion chamber through simultaneous Shadowgraphy and high-speed natural luminosity method an analytical model for two-stage ignition delay is established accordingly for further investigation. Based on the experiment results, a monotonic decreasing trend on low-temperature ignition delay time LTI (i.e., form 4.5 ms to 2.5 ms under the ambient temperature of 760 K) and an increasing trend on the cool flame propagation time τ (i.e., form 0.4 ms to 1.6 ms under the ambient temperature of 760 K) are found and finally results in a non-monotonic trend of high-temperature ignition delay (HTI) as the injection pressure raises from 40 MPa to 160 MPa. According to the established model, increased P_i leads to promoted atomization and intensified dilution effect of the fuel spray, which is concluded as the reason for decreased LTI and increased τ, respectively. In this way, the non-monotonic trend of HTI under engine cold-start as P_i increases is quantitatively explained through the promoted model.