Experimental study on the effects of mixture homogeneity on stratified-charge compression ignition combustion

Abstract

In this paper, an investigation of the n-heptane stratified-charge compression ignition (SCCI) combustion on a single-cylinder engine using port fuel injection combined with in-cylinder direct injection is reported. The effects of mixture homogeneity, overall equivalence ratio, and fuel delivery advance angle of directly injected fuel on the SCCI combustion characteristics and emissions were evaluated. The experimental results revealed that the heat release curve of SCCI combustion exhibited a three-stage combustion mode, which includes low-temperature reaction (LTR), high-temperature reaction (HTR), and diffusion burn. The operating ranges can be significantly broadened by mixture stratification, owing to the smooth heat release. As the mixture homogeneity increases, the peak value of the heat release in the HTR increased but the maximum heat release in the third stage decreased. For a fixed overall equivalence ratio, with an increase in the mixture homogeneity, the nitrogen oxide (NO x) emissions from SCCI combustion started to decrease initially and attained the lowest level, but carbon monoxide (CO) emissions increased initially and achieved the highest level. Once the mixture homogeneity further increased, NO x emissions began to increase but CO emission decreased. Compared with the traditional direct-injection compression ignition combustion mode, NO x emissions from SCCI combustion can be reduced markedly. Also, hydrocarbon emissions from SCCI combustion were much lower than from neat homogeneous charge compression ignition combustion. The fuel delivery advance angle also had an important influence on SCCI combustion and the emissions. In particular, NO x emissions can be further significantly reduced using the late fuel injection strategy.