Strong knocking characteristics under compression ignition conditions with high pressures

ABSTRACT

Knocking combustion has been extensively studied in internal combustion engines in order to achieve higher thermal efficiency. However, the inherent mechanism for knocking combustion under homogenous charge compression ignition (CI) conditions has not been fully understood. In this study, the strong knocking characteristics under CI conditions were parametrically investigated through theoretical analysis and rapid compression machine (RCM) experiments, with addressing knocking intensity (KI) limits and autoignition (AI) modes. The characteristic time and energy release concerned with AI were firstly numerically obtained through constant-volume adiabatic conditions. It is found that despite longer ignition delay time within milliseconds and excitation time within microseconds, iso-octane shows almost the same level of maximum explosive pressure and obvious higher energy density compared with n-heptane with low octane rating, indicating the potential of strong knocking occurrence. Then, knocking characteristics were investigated through RCM experiments operated under different thermodynamic conditions for the different octane rating fuels. It shows that strong knocking combustion characterizing super-knock events is observed for both n-heptane and iso-octane. Meanwhile, initial pressure showed greater impact on knocking combustion, manifesting significant increases in KI at high pressures. The influence from initial temperature becomes more obvious for iso-octane fuel at low pressures with long ignition delay time. Finally, the AI modes for different knocking scenarios were qualitatively assessed through Bradley’s diagram, allowing for the uncertainties of hot-spot sizes and temperature gradients. It shows that most AI modes of strong knocking combustion were located in the developing detonation regime, illustrating the possibility of super-knock under CI conditions with high thermodynamic conditions.

KEYWORDS:

• Super-knock
• primary reference fuel
• compression ignition
• excitation time
• energy density

Acknowledgments

This work was supported by the National Natural Science Foundation of China (grant no. 91641203, 51476114, and 51706152) and Tianjin Natural Science Foundation (grant no. 17JCZDJC31500).

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [51476114, 51706152, 91641203] and Natural Science Foundation of Tianjin City [17JCZDJC31500].