Effects of partitioned fuel distribution on auto-ignition and knocking under spark assisted compression ignition conditions

Abstract

Spark-assisted compression ignition (SACI) is a potential way to enhance ignition-control robustness and extend the load range of homogeneous charge compression ignition (HCCI) engines. However, the mechanism underlying combustion mode transition without knocking is not completely elucidated. Using direct high-speed photography and simultaneous pressure acquisition, the proposed study examines different auto-ignition and engine knock scenarios under SACI conditions. A small amount of heptane was directly injected into premixed methane-air mixture to modify local mixture reactivity. Additionally, late side injection was adopted to achieve stable SACI combustion whilst suppressing knocking combustion. Results obtained demonstrate that SACI combustion is essentially determined by the stochastic auto-ignition of the unburned end-gas mixture. End-gas mixture auto-ignition becomes stable thus normal SACI under conditions of high mixture reactivity, but a further rise in mixture reactivity causes engine knock prevail. To suppress SACI knocking, partitioned distribution of heptane via side injection has been found to be significantly effective. Results demonstrate that partitioned fuel distribution via late injection can greatly reduce the knock intensity, thereby implying a transition from SACI knocking to normal SACI combustion. Further analysis demonstrates that knock intensity is determined by the peak heat release rate of auto-ignition and there exists distinct boundaries between different combustion modes. The corresponding threshold value characterizing knocking combustion is approximately 202.30 J/CAD under current conditions. Besides, the peak heat release rate is related to the auto-ignition propagation speed and can be adjusted by the partitioned fuel distribution. The proposed study provides great insight into realizing and controlling SACI combustion to improve engine efficiency.