Experimental and computational study on recompress

Authors: J Lee H H Song

Publish Date: 2014/11/26

Volume: 15, Issue: 7, Pages: 1071-1082

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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 lowload 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 directinjected fuel with trapped residual gas during negative valve overlap NVO period or called recompression reaction was investigated In the singlecylinderengine experiments using researchgrade gasoline RD387 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 incylinder 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 zerodimensional modeling of the recompression stage and the constantvolumecombustionchamber using comprehensive primaryreferencefuel chemical kinetics mechanism was conducted for various equivalence ratio conditions In low equivalence ratios 06–075 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 075–10 endothermic fuelpyrolysisreactions dominate decreasing the mixture temperature during NVO The combustionchamber 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

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