EXPERIMENTAL AND COMPUTATIONAL STUDY ON RECOMPRESSION REACTION OF PILOT-INJECTED FUEL DURING NEGATIVE VALVE OVERLAP IN A GASOLINE-FUELED HOMOGENEOUS CHARGE COMPRESSION IGNITION ENGINE

• Published in: International journal of automotive technology 2014, 15(7), 81, pp.1071-1082
• Main Authors: 이종혁, 송한호
• Format: Article
• Language: English
• Subjects: 자동차공학
• ISSN: 1229-9138; 1976-3832
• DOI: 10.1007/s12239−014−0111−x

Homogenous charge compression ignition (HCCI) engine has been considered as an alternative to conventionalspark ignition or compression ignition engines with its high efficiency and low pollutant emissions. However, to lessendetrimental 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 fromdelayed combustion, or even misfire. In this study, to enhance the mixture ignitability in such conditions, reaction of the directinjectedfuel with trapped residual gas during negative valve overlap (NVO) period, or called recompression reaction, wasinvestigated. In the single-cylinder-engine experiments using research-grade gasoline (RD-387), it is shown that therecompression reaction of the fuel can induce overall earlier HCCI combustion timings (by 3–8 crank angle degrees) thanthose with native fuel in comparable operating conditions. From in-cylinder pressure measurement, modest exothermicityduring NVO is observed, which implies that oxidation of small portion of the fuel with residual oxygen increases overallmixture temperature during NVO and possibly advances the subsequent main combustion timing. To fully understand theeffect of recompression reaction on mixture ignitability, zero-dimensional modeling of the recompression stage and theconstant-volume-combustion-chamber, using comprehensive primary-reference-fuel chemical kinetics mechanism, wasconducted for various equivalence ratio conditions. In low equivalence ratios (0.6–0.75), sufficient oxygen in trapped residualgas leads to the oxidation of the fuel during NVO, thus increasing the mixture temperature. On the other hand, in highequivalence ratios (0.75–1.0), endothermic fuel-pyrolysis-reactions dominate, decreasing the mixture temperature duringNVO. The combustion-chamber modeling demonstrates overall shorter ignition delay of the recompression product than thatof native fuel at given temperatures. Combining both thermal and chemical effects above, there exist optimum equivalenceratio conditions to achieve the best mixture ignitability from the recompression reaction KCI Citation Count: 2