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Research Papers: Fuel Combustion

Computational Study to Identify Feasible Operating Space for a Mixed Mode Combustion Strategy—A Pathway for Premixed Compression Ignition High Load Operation

[+] Author and Article Information
Chaitanya Kavuri, Sage L. Kokjohn

Department of Mechanical Engineering,
University of Wisconsin-Madison,
Madison, WI 53706

Contributed by the Internal Combustion Engine Division of ASME for publication in the JOURNAL OF ENERGY RESOURCES TECHNOLOGY. Manuscript received February 15, 2018; final manuscript received February 16, 2018; published online March 30, 2018. Editor: Hameed Metghalchi.

J. Energy Resour. Technol 140(8), 082201 (Mar 30, 2018) (18 pages) Paper No: JERT-18-1133; doi: 10.1115/1.4039548 History: Received February 15, 2018; Revised February 16, 2018

A mixed mode combustion strategy with a premixed compression ignition (PCI) combustion event and a mixing controlled load extension injection was investigated in the current study. Computational fluid dynamics (CFD) modeling was used to perform a full factorial design of experiments (DOE) to study the effects of premixed fuel fraction, load extension injection timing, and exhaust gas recirculation (EGR). The goal of the study was to identify a feasible operating space and demonstrate a pathway to enable high-load operation with the mixed mode combustion strategy. The gross-indicated efficiency (GIE) increased with premix fraction, but the maximum premix fraction was constrained by pressure rise rate which confined the feasible operating space to a premix fuel mass range of 70–80%. Injecting part of the premixed fuel as a stratified injection relieved the pressure rise rate constraint considerably through in-cylinder equivalence ratio stratification. This allowed operation with premix fuel mass of 70% and higher and EGR rates less than 40% which resulted in improved GIE of the late cycle injection cases. It was also identified that by targeting the fuel from the stratified injection into the squish region, there is improved oxygen availability in the bowl for the load extension injection, which resulted in reduced soot emissions. This allowed the load extension injection to be brought closer to top dead center while meeting the soot constraint, which further improved the GIE. Finally, the results from the study were used to demonstrate high-load operation at 20 bar and 1300 rev/min.

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Figures

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Fig. 4

Computational mesh shown at TDC

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Fig. 3

NOx and soot emissions comparison between model and experimental data

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Fig. 2

Apparent heat release rate comparison between model and experiments. The AHRR is separated into primary (top) and secondary heat release rate (bottom) to highlight the agreement of the model predictions with the experiments for the secondary heat release rate.

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Fig. 1

Injection strategy for the experimental study used for model validation

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Fig. 5

Injection strategy used for the DOE study

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Fig. 13

Cylinder pressure and AHRR comparison for premix fractions of 0.7–1.0 at an SOI of −10 deg ATDC and 46% EGR

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Fig. 14

NOx versus SOI gasoline at different premix fractions for EGR's 30%, 38%, and 46%

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Fig. 6

Comparison of GIE versus SOI at different premix fractions for EGR's 30%, 38%, and 46%

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Fig. 7

Ignition delay versus local equivalence ratio at different initial temperatures for EGR's of 30%, 38%, and 46%

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Fig. 8

Equivalence ratio of premix fuel versus premix fraction

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Fig. 9

Φ–T plots of the in-cylinder mixture at various crank angles for the 38% EGR case at an SOI of −10 deg ATDC for premix fractions of 0.1, 0.4, and 0.7. Each circle shows the local temperature and equivalance ratio of a CFD cell.

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Fig. 10

Φ–T plots of the in-cylinder mixture at various crank °ASOI for the 38% EGR case at an SOI of −10 deg ATDC and 20 deg ATDC for a premix fraction of 0.4

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Fig. 11

Φ–T plots of the in-cylinder mixture at various crank °ASOI for the 38% EGR case at an SOI of 20 deg ATDC for premix fractions of 0.4 and 0.7

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Fig. 12

Energy flow versus premix fraction at SOI of −10 deg ATDC for EGR's 30%, 38%, and 46%

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Fig. 15

Φ–T plots of the in-cylinder mixture at various crank °ASOI for the 30% and 46% EGR cases at a premix fraction of 0.7 for an SOI of −10 deg ATDC

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Fig. 16

Φ–T plots of the in-cylinder mixture at various crank °ASOI for the 30% and 46% EGR cases at a premix fraction of 0.7 for an SOI of 20 deg ATDC

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Fig. 17

Soot versus SOI gasoline at different premix fractions for EGR's 30%, 38%, and 46%

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Fig. 23

Regions in EGR–SOI operating space that meet constraints of NOx, soot < 2 g/kg-f, PPRR < 20 bar/deg, and combustion efficiency > 80% for the high premix fractions, at different stratified injection fractions

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Fig. 24

Comparison of soot trends for stratified injection fractions of 0 and 0.3 at a premix fraction of 0.7

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Fig. 25

Comparison of the in-cylinder images of equivalence ratio at 0 deg ASOI and 5 deg ASOI for the two cases compared in Fig. 24 at an SOI timing of 15 deg ATDC

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Fig. 26

GIE and UHC as a function of stratified injection fraction for premix fraction of 0.8 at SOI of 0 deg ATDC

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Fig. 18

PPRR versus SOI gasoline at different premix fractions for EGR's 30%, 38%, and 46%

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Fig. 19

Contours of NOx, soot and PPRR as a function of EGR and gasoline SOI timing for premix fractions of 0.6, 0.7, and 0.8

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Fig. 20

Regions in EGR-SOI operating space that meet constraints of NOx < 2 g/kg-f, Soot < 2 g/kg-f, PPRR < 20 bar/deg, and combustion efficiency > 80%. The regions are colored by contours of GIE.

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Fig. 21

Comparison of the PPRR as a function of gasoline SOI timing at different premix fractions, for 30% EGR at stratified injection fractions of 0 and 0.5

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Fig. 22

Comparison of the AHRR for stratified injection fractions of 0 and 0.5 at a premix fraction of 0.8 and an SOI timing of 20 deg ATDC

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Fig. 28

Injection strategy for high load low speed DOE

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Fig. 27

Mass fraction of UHC at different crank °ATDC

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Fig. 29

Cylinder pressure (solid line) and apparent heat release rate (dashed line) of the best case from the DOE study

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Fig. 30

Gross-indicated efficiency, PPRR, soot, and NOx emissions as a function of the fuel split ratio

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Fig. 31

Cylinder pressure, AHRR, and injection velocity for a fuel split ratio of 0.4

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