Load expansion of stoichiometric HCCI using spark assist and hydraulic valve actuation

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Abstract

A spark-assist homogeneous charge compression ignition (SA-HCCI) operating strategy is presented here that allows for stoichiometric combustion from 1000-3000 rpm, and atloads as high as 750 kPa net IMEP. A single cylinder gasoline engine equipped with direct fuel injection and fully variable hydraulic valve actuation (HVA) is used for thisexperimental study. The HVA system enables negative valve overlap (NVO) valve timing for hot internal EGR. Spark assist stabilizes combustion over a wide range of engine speeds and loads, and allows for stoichiometric operation at all conditions. Characteristics of both spark-ignited combustion and HCCI are present during the SA-HCCI operating mode, with combustion analysis showing a distinctive spark ignited phase of combustion, followed by amuch more rapid HCCI combustion phase. At high load, the maximum cylinder pressure rise rate is controlled by a combination of spark timing and retarding the intake valve closing angle. The latter reduces the effective compression ratio, and therefore the compressive temperatures, allowing the high load limit of the operating range to be expanded. The SA-HCCI operating strategy exhibits improved thermal efficiency at most operating conditions, with fuel consumption improvements up to 9% realized at light engineloads. The SA-HCCI operating strategy presented here doesnot provide an efficiency advantage at all operating pointscompared to SI combustion; a decrease was observed at thehighest speed and at loads above 500 kPa net IMEP. At light engine loads the majority of the heat release takes place during the HCCI phase of combustion, and as such the NOx emissions are very low and are similar to levels observed inpure HCCI. At higher loads, a larger portion of the heat release takes place during the spark ignited phase of combustion, which produces NOx emissions that are much higher than is typically associated with HCCI, but still represent a decrease from conventional SI combustion. By limiting the fuel/air mixture to stoichiometric conditions, thehigher NOx emissions do not represent an implementation barrier to this strategy because compatibility is maintained with very effective conventional 3-way catalysts.

Original languageEnglish
Pages (from-to)244-258
Number of pages15
JournalSAE International Journal of Engines
Volume3
Issue number2
DOIs
StatePublished - Dec 2010

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