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

[+] Author Affiliations
Vahid Hosseini, W. Stuart Neill

National Research Council Canada, Ottawa, ON, Canada

M. David Checkel

University of Alberta, Edmonton, AB, Canada

Paper No. ICES2008-1618, pp. 1-14; 14 pages
doi:10.1115/ICES2008-1618
From:
  • ASME 2008 Internal Combustion Engine Division Spring Technical Conference
  • ASME 2008 Internal Combustion Engine Division Spring Technical Conference
  • Chicago, Illinois, USA, April 27–30, 2008
  • Conference Sponsors: Internal Combustion Engine Division
  • ISBN: 0-7918-4813-2 | eISBN: 0-7918-3815-3
  • Copyright © 2008 by ASME and National Research Council of Canada

abstract

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 to realize 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 CFR 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 of intermediate radical mole fractions during low temperature reactions and the early stages of the negative temperature coefficient delay period.

Copyright © 2008 by ASME and National Research Council of Canada

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