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Research Papers: Internal Combustion Engines

Controlling n-Heptane HCCI Combustion With Partial Reforming: Experimental Results and Modeling Analysis

[+] Author and Article Information
Vahid Hosseini, W. Stuart Neill

Institute for Chemical Process and Environmental Technology, National Research Council, Ottawa, ON, K1A 0R6, Canada

M. David Checkel

 University of Alberta, Edmonton, AB, T6G 2G8, Canada

J. Eng. Gas Turbines Power 131(5), 052801 (May 22, 2009) (11 pages) doi:10.1115/1.3078189 History: Received June 26, 2008; Revised July 21, 2008; Published May 22, 2009

One potential method for controlling the combustion phasing of a homogeneous charge compression ignition (HCCI) engine is to vary the fuel chemistry using two fuels with different auto-ignition characteristics. Although a dual-fuel engine concept is technically feasible with current engine management and fuel delivery system technologies, this is not generally seen as a practical solution due to the necessity of supplying and storing two fuels. Onboard partial reforming of a hydrocarbon fuel is seen to be a more attractive way of realizing a dual-fuel concept, while relying on only one fuel supply infrastructure. Reformer gas (RG) is a mixture of light gases dominated by hydrogen and carbon monoxide that can be produced from any hydrocarbon fuel using an onboard fuel processor. RG has a high resistance to auto-ignition and wide flammability limits. The ratio of H2 to CO produced depends on the reforming method and conditions, as well as the hydrocarbon fuel. In this study, a cooperative fuel research engine was operated in HCCI mode at elevated intake air temperatures and pressures. n-heptane was used as the hydrocarbon blending component because of its high cetane number and well-known fuel chemistry. RG was used as the low cetane blending component to retard the combustion phasing. Other influential parameters, such as air/fuel ratio, EGR, and intake temperature, were maintained constant. The experimental results show that increasing the RG fraction retards the combustion phasing to a more optimized value causing indicated power and fuel conversion efficiency to increase. RG reduced the first stage of heat release, extended the negative temperature coefficient delay period, and retarded the main stage of combustion. Two extreme cases of RG composition with H2/CO ratios of 3/1 and 1/1 were investigated. The results show that both RG compositions retard the combustion phasing, but that the higher hydrogen fraction RG is more effective. A single-zone model with detailed chemical kinetics was used to interpret the experimental results. The effect of RG on combustion phasing retardation was confirmed. It was found that the low temperature heat release was inhibited by a reduction in intermediate radical mole fractions during low temperature reactions and during the early stages of the negative temperature coefficient delay period.

Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic of the CFR engine experimental setup, described in Table 1

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Figure 2

Combustion characteristic definitions using (a) net rate of heat release and (b) gross cumulative heat release curves

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Figure 3

Selected λ-EGR constant cases, n-heptane HCCI combustion, N=800 rpm, CR=11.8, and Tintake,mix=110°C

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Figure 4

Effect of RG on (a) net rate of heat release and (b) gross cumulative heat release for data set B in Fig. 3

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Figure 5

Effect of RG on start of combustion for operating points in Fig. 3, error bars indicate ±2σ

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Figure 6

Effect of RG on combustion duration for operating points in Fig. 3, error bars indicate ±2σ

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Figure 7

Effect of RG on maximum low temperature heat release for operating points indicated in Fig. 3, error bars indicate ±2σ

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Figure 8

Effect of RG on maximum high temperature heat release timing for operating points indicated in Fig. 3, error bars indicate ±2σ

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Figure 9

Effect of RG on negative temperature coefficient duration for operating points indicated in Fig. 3, error bars indicate ±2σ

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Figure 10

Effect of RG on indicated mean effective pressure for operating points indicated in Fig. 3, error bars indicate ±2σ

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Figure 11

Effect of RG on indicated thermal efficiency (ηth) for operating points in Fig. 3, error bars indicate ±2σ

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Figure 12

Selected pairs of constant λ, similar EGRs, and identical RG blending fractions

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Figure 13

Effect of RG composition on n-heptane HCCI combustion timing retardation, error bars indicate ±2σ

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Figure 14

Effect of RG composition on net rate of heat release

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Figure 15

Comparison of experimental pressure traces of point 1, data set F with ChemComb-SZM, experiment: supercharged n-heptane HCCI combustion, N=800 rpm, CR=11.8, Tintake,mix=110°C, intake pressure=143 kPa, λ=2.94, EGR=19.9%, and RG blend fraction=0.0%

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Figure 16

Comparison of experimental pressure traces of point 1, data set F with ChemComb-SZM, experiment: supercharged n-heptane HCCI combustion, N=800 rpm, CR=11.8, Tintake,mix=110°C, intake pressure=143 kPa, λ=3.00, EGR=21.7%, and RG blend fraction=30.4%

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Figure 17

A comparison of combustion timing (SOC) prediction by ChemComb-SZM with actual experimental combustion timing for data set F in Fig. 3, error bars on experimental SOC show the cyclic variation indicating ±2σ

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Figure 18

ChemComb-SZM simulation results for in-cylinder temperature during and after compression for data set F in Fig. 3

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Figure 19

Typical mole fraction traces of key species of OH, H2O2, and CH2O compared with net rate of heat release in n-heptane HCCI combustion

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Figure 20

Effect of RG on mole fraction of hydrogen peroxide (H2O2), n-heptane HCCI combustion simulated by ChemComb-SZM for data set F in Fig. 3

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Figure 21

Effect of RG on the reaction rates of H2O2, (a) production rate for Reaction 122, (b) production rate for Reaction 123, and (c) total production rate (from all 19 reactions)

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Figure 22

Effect of RG on the ratio of maximum mole fraction during LTHR for OH/H2O2 ratio

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Figure 23

A comparison of SOC prediction by ChemComb-SZM and experimental results for two cases of RG 75/25 (left Y-axis) and RG 50/50 (right Y-axis), error bars indicate ±2σ

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Figure 24

Effect of RG composition on H2O2 mole fraction for (a) RG 75/25 and (b) RG 50/50

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