Effects of Negative Valve Overlap on the Auto-ignition Process of Lean Ethanol/Air Mixture in HCCI-Engines

Joelsson, Tobias; Yu, Rixin; Sjöholm, Johannes; Tunestål, Per; Bai, Xue‐Song

doi:10.4271/2010-01-2235

Cited by 7 publications

(3 citation statements)

References 24 publications

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“…However, unlike the current work, their study did not examine burn rates, and did not attempt to correlate the effect of stratification to sequential autoignition and burn duration. Joelsson et al [32] performed large eddy simulations (LES) to determine distributions resulting from NVO operation followed by multi-zone kinetics calculations. They found that under higher NVO conditions a zone with a preferred mixture composition and temperature ignited first.…”

Section: Introductionmentioning

confidence: 99%

The effects of thermal and compositional stratification on the ignition and duration of homogeneous charge compression ignition combustion

Kodavasal

Lavoie

Assanis

et al. 2015

Combustion and Flame

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The effects of thermal and compositional stratification on the ignition and duration of homogeneous charge compression ignition combustion In this work, the effects of thermal and compositional stratification on the ignition and burn duration of homogeneous charge compression ignition (HCCI) combustion are studied with full-cycle 3D computational fluid dynamics (CFD) simulations with gasoline chemical kinetics performed during the closed portion of the cycle, from intake valve closing (IVC). The stratification was varied through the use of negative valve overlap (NVO) and positive valve overlap (PVO) breathing strategies. To remove charge energy and phasing effects from the simulation results, the fuel mass and ignition timing were held constant, while mean composition effects were isolated from those of local stratification by maintaining the same mean oxygen concentration, fuel-oxygen equivalence ratio and charge heat capacity. Fuel was premixed with the intake to avoid potential stratification effects arising from direct injection. With NVO, the incomplete mixing of the fresh charge with the large mass fraction of product gases retained within the cylinder from the previous combustion cycle leads to a 23% increase in the preignition thermal stratification, and an order of magnitude increase in the levels of stratifications in fuel to oxygen equivalence ratio and oxygen mole fraction relative to the PVO strategy, which employs a premixed mixture of fresh and product gases. Under the conditions studied, the use of NVO resulted in a 30% increase in the 10-90% burn duration (CA10-90) compared to the PVO condition. Two additional analyses were performed to decouple the effects of thermal and compositional stratification. The first examined the reaction space (based on the ignition delay distribution within the charge prior to ignition) for both breathing strategies to quantify the inherent reactivity stratification. The second examined the more stratified NVO case with a quasidimensional multi-zone model. These analyses revealed that under the conditions studied, HCCI reactivity and combustion duration are governed primarily by thermal stratification and are largely insensitive to compositional stratification at the time of ignition.

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Section: Introductionmentioning

confidence: 99%

The effects of thermal and compositional stratification on the ignition and duration of homogeneous charge compression ignition combustion

Kodavasal

Lavoie

Assanis

et al. 2015

Combustion and Flame

View full text Add to dashboard Cite

show abstract

“…The temperature distribution during compression was dictated by the remaining vortices from the intake stroke and the roll up vortices due to the piston motion. Yu and colleagues 30,31 and Joelsson and colleagues 32,33 used large eddy simulation (LES) modeling of a medium-duty engine and showed that a piston with a combustion bowl resulted in higher thermal stratification than a flat piston. Single-cycle simulations were performed using pre-tabulated data, and a conserved progress variable for combustion modeling, which showed that larger thermal stratification at the same bulk temperature elongated combustion duration.…”

Section: Introductionmentioning

confidence: 99%

Effect of engine size, speed, and dilution method on thermal stratification of premixed homogeneous charge compression–ignition engines: A large eddy simulation study

Sofianopoulos

Boldaji

Lawler

et al. 2019

International Journal of Engine Research

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High heat release rates limit the operating range of homogeneous charge compression–ignition engines to low and medium loads. Thermal stratification has been shown to stagger autoignition, lower heat release rates, and extend the operating range of homogeneous charge compression–ignition engines. However, the dependence of naturally occurring thermal stratification on the engine size, speed, and internal residual dilution is not fully understood. A three-dimensional computational fluid dynamics model with large eddy simulations and detailed chemical kinetics was developed using CONVERGE. This model was used to simulate two different engines: (1) a light-duty 2.0 GM Ecotec Engine modified for homogeneous charge compression–ignition combustion in one of the cylinders and (2) a medium-duty Cummins B-series engine modified for homogeneous charge compression–ignition combustion in one of the cylinders. For the light-duty engine, five consecutive modeled cycles were compared with experimental data from 300 consecutive cycles using residual gas dilution at 2000 r/min. For the medium-duty engine, five consecutive modeled cycles were compared with experimental data from 100 consecutive cycles using air dilution with intake heating at 1200 r/min. In the light-duty engine, it was found that incomplete mixing between fresh charge and residual gas increased thermal stratification early in the compression stroke for residual dilution compared to air dilution. Residual stratification at the onset of ignition was small and not directly coupled with thermal stratification. Heat losses to the walls were the dominant source of thermal stratification at the onset of ignition. The reduced oxygen concentration due to residual dilution, increased the temperature requirement for autoignition, which increased heat transfer losses and increased the thermal stratification around top dead center. The thermal stratification before ignition reduced when the engine speed increased because of the lower heat transfer losses. The light-duty engine was found to have larger portion of the fuel energy lost to heat transfer than the medium-duty engine, which resulted in larger thermal stratification before ignition.

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“…Their single-cycle simulations showed that increasing turbulence intensity reduced temperature stratification, which resulted in shorter combustion duration. Joelsson et al 20,21 simulated the effects of varying NVO duration on thermal stratification by combining LES of the compression process and a multi-zone combustion model. They showed that at low RGFs the hottest regions were the first to ignite, while at high RGFs there was a preferred ignition region, which did not necessarily include the hottest region.…”

Section: Introductionmentioning

confidence: 99%

Investigation of thermal stratification in premixed homogeneous charge compression ignition engines: A Large Eddy Simulation study

Sofianopoulos

Boldaji

Lawler

et al. 2018

International Journal of Engine Research

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The operating range of Homogeneous Charge Compression Ignition (HCCI) engines is limited to low and medium loads by high heat release rates. Negative valve overlap can be used to control ignition timing and heat release by diluting the mixture with residual gas and introducing thermal stratification. Cyclic variability in HCCI engines with NVO can result in reduced efficiency, unstable operation, and excessive pressure rise rates. Contrary to spark-ignition engines, where the sources of cyclic variability are well understood, there is a lack of understanding of the effects of turbulence on cyclic variability in HCCI engines and the dependence of cyclic variability on thermal stratification. A three-dimensional computational fluid dynamics (CFD) model of a 2.0L GM Ecotec engine cylinder, modified for HCCI combustion, was developed using Converge. Large Eddy Simulations (LES) were combined with detailed chemical kinetics for simulating the combustion process. Twenty consecutive cycles were simulated and the results were compared with individual cycle data of 300 consecutive experimental cycles. A verification approach based on the LES quality index indicated that this modeling framework can resolve more than 80% of the kinetic energy of the working fluid in the combustion chamber at the pre-ignition region. Lower cyclic variability was predicted by the LES model compared to the experiments. This difference is attributed to the resolution of the sub-grid velocity field, time averaging of the intake manifold pressure boundary conditions, and different variability in the equivalence ratio compared to the experimental data. Combustion phasing of each cycle was found to depend primarily on the bulk cylinder temperature, which agrees with established findings in the literature. Large cyclic variability of turbulent mixing and spatial distribution of temperature was predicted. However, both of these parameters were found to have a small effect on the cyclic variability of combustion phasing.

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Effects of Negative Valve Overlap on the Auto-ignition Process of Lean Ethanol/Air Mixture in HCCI-Engines

Cited by 7 publications

References 24 publications

The effects of thermal and compositional stratification on the ignition and duration of homogeneous charge compression ignition combustion

The effects of thermal and compositional stratification on the ignition and duration of homogeneous charge compression ignition combustion

Effect of engine size, speed, and dilution method on thermal stratification of premixed homogeneous charge compression–ignition engines: A large eddy simulation study

Investigation of thermal stratification in premixed homogeneous charge compression ignition engines: A Large Eddy Simulation study

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