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Fuel Combustion

An Experimental Investigation of HCCI Combustion Stability Using n-Heptane

[+] Author and Article Information
Hailin Li

 Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, WV 26506

W. Stuart Neill, Wallace L. Chippior

 National Research Council Canada, Ottawa, ON, K1A 0R6, Canada

J. Energy Resour. Technol 134(2), 022204 (Apr 04, 2012) (8 pages) doi:10.1115/1.4005700 History: Received December 21, 2011; Revised December 22, 2011; Published April 02, 2012; Online April 04, 2012

The combustion stability of a single-cylinder homogeneous charge compression ignition (HCCI) engine operated with n-heptane was experimentally investigated over a range of engine speeds (N), intake temperatures and pressures, compression ratios (CR), air/fuel ratios (AFR), and exhaust gas recirculation (EGR) rates. These parameters were varied to alter the combustion phasing from an overly advanced condition where engine knock occurred to an overly retarded condition where incomplete combustion was observed with excessive emissions of carbon monoxide (CO) and unburned hydrocarbons (UHC). The combustion stability was quantified by the coefficients of variation in indicated mean effective pressure (COVimep ) and peak cylinder pressure (COVPmax ). Cycle-to-cycle variations in the HCCI combustion behavior of this engine were shown to depend strongly on the combustion phasing, defined in this study as the crank angle position where 50% of the energy was released (CA50). In general, combustion instability increased significantly when the combustion phasing was overly retarded. The combustion phasing was limited to conditions where the COVimep was 5% or less as engine operation became difficult to control beyond this point. Based on the experimental data, the combustion phasing limit was approximately a linear function of the amount of fuel inducted in each cycle. Stable HCCI combustion could be obtained with progressively retarded combustion phasing as the fuel flow rate increased. In comparison, stable HCCI combustion was only obtained under very advanced combustion phasing for low load operating conditions. Investigation of the experimental data reveals that the cyclic variations in HCCI combustion were due to cycle-to-cycle variations in total heat release (THR). The combustion completeness of the previous cycle affected the in-cylinder bulk mixture conditions and resultant heat release process of the following engine cycle.

Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Schematic diagram of the test engine

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Figure 2

Cylinder pressure and cumulative heat release versus crank angle (°CA), CR = 10, Tin  = 30 °C, AFR = 50, Pin  = 95 kPa

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Figure 3

COVimep and COVPmax versus engine speed, CR = 10, AFR = 50, Tin  = 40 °C, Pin  = 95 kPa

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Figure 4

COVimep and COVPmax versus air/fuel ratio, CR = 10, Tin  = 30 °C, Pin  = 95 kPa, N = 900 rpm

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Figure 5

COVimep and COVPmax versus intake air temperature, CR = 10, AFR = 50, Pin  = 95 kPa, N = 900 rpm

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Figure 6

COVimep and COVPmax versus intake air pressure, N = 900 rpm, CR = 10, AFR = 60, Tin  = 40 °C, Pin  = 95 kPa

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Figure 7

COVimep and COVPmax versus compression ratio, AFR = 50, Tin  = 30 °C, Pin  = 95 kPa, N = 900 rpm

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Figure 8

COVimep versus compression ratio, see Table 1 for operating conditions

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Figure 9

COVimep and COVPmax versus EGR rate, CR = 10, Tin  = 40–62 °C, Pin  = 95 kPa, m·fuel=0.40kg/h.

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Figure 10

COVimep versus EGR rate, CR = 10, Pin  = 95 kPa, see Table 1 for operating conditions

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Figure 11

COVimep versus CA50. AFR=50. CA50 was changed by varying intake temperature, compression ratio or engine speed. See Table 1 for operating conditions.

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Figure 12

COVimep versus CA50. m·fuel=∼0.40kg/h. CA50 retarded by application of EGR or reduced compression ratio. See Table 1 for operating conditions.

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Figure 13

COVimep versus CA50 with three air/fuel ratios. See Table 1 for operating conditions.

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Figure 14

COVimep versus CA50, with combustion retarded by diluting the intake mixture with fresh air and exhaust gases, respectively. See Table 1 for operating conditions.

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Figure 15

Variation of combustion phasing limit (COVimep  = 5%) with fuel flow rate. Figure includes additional data not included in Table 1.

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Figure 16

COVimep versus COVTHR . See Table 1 for operating conditions.

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Figure 17

Cyclic imep variations for four different AFR ratios, CR = 10, Tin  = 30 °C, Pin  = 95 kPa

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Figure 18

Cyclic imep variations for three different EGR rates, CR = 10, Tin  = 44–62 °C, Pin  = 95 kPa, m·fuel=0.40kg/h.

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