Abstract
Knocking is a destructive and abnormal combustion phenomenon that hinders modern spark ignition (SI) engine technologies. However, the in-depth mechanism of a single-factor influence on knocking has not been well studied. Thus, the major aim of the present study is to study the effects of flame propagation velocity and turbulence intensity on end-gas auto-ignition through a large eddy simulation (LES) and a decoupling methodology in a downsized gasoline engine. The mechanisms of end-gas auto-ignition as well as strong pressure oscillation are qualitatively analyzed. It is observed that both flame propagation velocity and turbulence have a non-monotonic effect on knocking intensity. The competitive relationship between flame propagation velocity and ignition delay of the end gas is the primary reason responding to this phenomenon. A higher flame speed leads to an increase in the heat release rate in the cylinder, and consequently, quicker increases in the temperature and pressure of the unburned end-gas mixture are obtained, leading to end-gas auto-ignition. Further, the coupling of a pressure wave and an auto-ignition flame front results in super-knocking with a maximum peak of pressure of 31 MPa. Although the turbulence indirectly influences the end-gas auto-ignition by affecting the flame propagation velocity, it can accelerate the dissipation of radicals and heat in the end gas, which significantly influences knocking intensity. Moreover, it is found that the effect of turbulence is more pronounced than that of flame propagation velocity in inhibiting knocking. It can be concluded that the intensity of the pressure oscillation depends on the unburned mixture mass as well as the local thermodynamic state induced by flame propagation and turbulence, with mutual interactions. The present work is expected to provide valuable perspective for inhibiting super-knocking of an SI gasoline engine.
Highlights
The increasing severity of the current energy crisis and environmental pollution are leading to technical downsizing and turbocharging in spark ignition (SI) engines due to their superior economy and power performance [1]
Velocity and turbulence intensity on the end-gas auto-ignition as well as pressure oscillation through and a decoupling methodology applied in a downsized SI gasoline engine
The mechanisms of end-gas an large eddy simulation (LES) and a decoupling methodology applied in a downsized SI gasoline engine
Summary
The increasing severity of the current energy crisis and environmental pollution are leading to technical downsizing and turbocharging in spark ignition (SI) engines due to their superior economy and power performance [1]. Kee et al [5] developed a rapid compression/expansion device to explore the essential phenomenon of knocking Their results indicate that increasing the laminar flame speed inhibits the knocking because a fast propagation decreases the time required for low-temperature reactions in the end gas. Yu et al [20] conducted a 1D simulation regarding the effects of the flame propagation velocity on the end-gas auto-ignition in a constant volume combustion chamber by introducing artificial mass diffusivity. They found that increasing the flame propagation can suppress the knocking occurrence if the chamber size is sufficiently small.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
