On autoignition mode under variable thermodynamic state of internal combustion engines

Abstract

Autoignition modes under premixed combustion conditions are usually studied in constant-volume configurations. However, the autoignition events related to knocking combustion in spark-ignition engines do experience variable volumes in combustion chamber and ever-changing thermodynamic states caused by reciprocating piston motion and main flame front compression. Such combustion situations may lead to different autoignition modes from constant-volume scenarios. Using one-dimensional direct numerical simulations with detailed chemistry and transport of H2/air mixture, the autoignition modes during knocking combustion were studied under different engine combustion boundary conditions. It was the first to identify important influence of variable thermodynamic states on the development of autoignition modes through changing critical temperature gradients. Four autoignition modes—thermal explosion, supersonic deflagration, detonation, and subsonic deflagration—were observed, which, however, were quantitatively different from the constant-volume configurations in regime boundaries. Meanwhile, on comparison with intake temperature and equivalence ratio, intake pressure shows greater impact on detonation formation, characterized by a regime extension under high intake pressures. To classify the autoignition modes responsible for various knocking events with different intensities in a straightforward manner, a regime diagram was proposed based on the temperature gradients and the effective energy density used for universally quantifying various intake conditions. This diagram was found useful to determine the distributions of different autoignition modes (especially for detonation) and the potential approaches for achieving maximum thermal efficiency while suppressing engine knock. In addition, detonation mode was prevailing under high effective energy density conditions, and the underlying reasons were ascribed to the significant reduction of excitation time and pre-flame temperature increases by pressure wave.