Dengquan Feng

Influence of injection strategies on knock resistance and combustion characteristics in a DISI engine

The combustion of a direct injection spark ignition engine is significantly affected by the fuel injection strategy due to the impact this strategy has on the gas-mixture formation and the turbulence flow. However, comprehensive assessments on both knock and engine performances for different injection strategies are generally lacking. Therefore, the main objective of the present study is to provide an experimental evidence of how a single injection strategy and a split injection strategy compare in terms of both knock tendency and engine performances like thermal efficiency, torque and combustion stability. Starting from the optimization of a single injection strategy, a split injection strategy is then evaluated. Under the present operating conditions, an optimum secondary injection timing of 100 CAD BTDC is found to have significant improvements on both the knock resistance and the overall engine performances. It should be noted that the present results indicate that the relationship between double injection and anti-knock performance is not monotonous. In addition, the double injection shows superior potential in improving fuel economy and power performance in contrast with the single injection thanks to a more stable combustion when a late injection timing is applied.

On autoignition mode under variable thermodynamic state of internal combustion engines

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.

Combustion Simulations under IC Engine-Relevant Conditions Using Dynamic Adaptive Multi-Zone Method

ABSTRACT The multi-zone (MZ) method is a proven chemical source term solving acceleration algorithm and has been widely used in the numerical research on internal combustion engines. It bins cells with similar thermodynamic states to reduce the number of calls to the reaction step integrator, which is computationally expensive. In this study, a dynamic adaptive multi-zone (DAMZ) method has been proposed. The principal improvement to the existing MZ algorithm is a new binning method in which the binning criteria of the DAMZ method can be chosen automatically as the simulation is performed. A resolution adjustment strategy is also adopted. To validate the DAMZ method under internal combustion engine relevant conditions, two 1D auto-ignition cases with different initial setups and one 3D spark-ignition engine simulation have been conducted. Furthermore, a primary reference fuel chemical mechanism is used to consider the multi-regime oxidation chemistry characteristic of large hydrocarbon molecules. According to the simulation results, the proposed DAMZ method demonstrates a good performance. It is able to mitigate the calculation error induced by the existing MZ method while maintaining the acceleration efficiency.