Experimental and computational study on recompression reaction of pilot-injected fuel during negative valve overlap in a gasoline-fueled homogeneous charge compression ignition engine

Abstract

Homogenous charge compression ignition (HCCI) engine has been considered as an alternative to conventional spark ignition or compression ignition engines with its high efficiency and low pollutant emissions. However, to lessen detrimental pressure rise by its homogenous nature of combustion process, typical HCCI engine operates in diluted conditions. Especially under low-load operation, significant dilution of the reactant mixture can result in unstable HCCI operation from delayed combustion, or even misfire. In this study, to enhance the mixture ignitability in such conditions, reaction of the direct-injected fuel with trapped residual gas during negative valve overlap (NVO) period, or called recompression reaction, was investigated. In the single-cylinder-engine experiments using research-grade gasoline (RD-387), it is shown that the recompression reaction of the fuel can induce overall earlier HCCI combustion timings (by 3–8 crank angle degrees) than those with native fuel in comparable operating conditions. From in-cylinder pressure measurement, modest exothermicity during NVO is observed, which implies that oxidation of small portion of the fuel with residual oxygen increases overall mixture temperature during NVO and possibly advances the subsequent main combustion timing. To fully understand the effect of recompression reaction on mixture ignitability, zero-dimensional modeling of the recompression stage and the constant-volume-combustion-chamber, using comprehensive primary-reference-fuel chemical kinetics mechanism, was conducted for various equivalence ratio conditions. In low equivalence ratios (0.6–0.75), sufficient oxygen in trapped residual gas leads to the oxidation of the fuel during NVO, thus increasing the mixture temperature. On the other hand, in high equivalence ratios (0.75–1.0), endothermic fuel-pyrolysis-reactions dominate, decreasing the mixture temperature during NVO. The combustion-chamber modeling demonstrates overall shorter ignition delay of the recompression product than that of native fuel at given temperatures. Combining both thermal and chemical effects above, there exist optimum equivalence ratio conditions to achieve the best mixture ignitability from the recompression reaction.