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Research Papers: Internal Combustion Engines

Understanding Ignition Delay Effects With Pure Component Fuels in a Single-Cylinder Diesel Engine

[+] Author and Article Information
Patrick A. Caton

 U.S. Naval Academy, Annapolis, MD 21402patcaton@usna.edu

Leonard J. Hamilton

 U.S. Naval Academy, Annapolis, MD 21402ljhamilt@usna.edu

Jim S. Cowart

 U.S. Naval Academy, Annapolis, MD 21402cowart@usna.edu

J. Eng. Gas Turbines Power 133(3), 032803 (Nov 09, 2010) (11 pages) doi:10.1115/1.4001943 History: Received October 20, 2009; Revised April 01, 2010; Published November 09, 2010; Online November 09, 2010

In order to better understand how future candidate diesel fuels may affect combustion characteristics in diesel engines, 21 pure component hydrocarbon fuels were tested in a single-cylinder diesel engine. These pure component fuels included normal alkanes (C6–C16), normal primary alkenes (C6–C18), isoalkanes, cycloalkanes/-enes, and aromatic species. In addition, seven fuel blends were tested, including commercial diesel fuel, U.S. Navy JP-5 aviation fuel, and five Fischer–Tropsch synthetic fuels. Ignition delay was used as a primary combustion metric for each fuel, and the ignition delay period was analyzed from the perspective of the physical delay period followed by the chemical delay period. While fuel properties could not strictly be varied independently of each other, several ignition delay correlations with respect to physical properties were suggested. In general, longer ignition delays were observed for component fuels with lower liquid fuel density, kinematic viscosity, and liquid-air surface tension. Longer ignition delay was also observed for component fuels with higher fuel volatility, as measured by boiling point and vapor pressure. Experimental data show two regimes of operation: For a carbon chain length of 12 or greater, there is little variation in ignition delay for the tested fuels. For shorter chain lengths, a fuel molecular structure is very important. Carbon chain length was used as a scaling variable with an empirical factor to collapse the ignition delay onto a single trend line. Companion detailed kinetic modeling was pursued on the lightest fuel species set (C6) since this fuel set possessed the greatest ignition delay differences. The kinetic model gives a chemical ignition delay time, which, together with the measured experimental ignition delay, suggests that the physical and chemical delay period have comparable importance. However, the calculated chemical delay periods capture the general variation in the overall ignition delay and could be used to predict the ignition delay of possible future synthetic diesel fuels.

Copyright © 2011This material is declared a work of the US government and is not subject to copyright protection in the United States. Approved for public release; distribution is unlimited.
Topics: Fuels , Delays , Ignition , Diesel
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Figures

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Figure 1

Schematic of experimental engine

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Figure 2

Variation in measured ignition delay with cetane number for all tested fuels. Some disagreement is expected, as the experimental test parameters do not exactly match the ASTM D613 CFR cetane number test, but a strong correlation is observed.

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Figure 3

Conceptual model of ignition delay showing both the physical and chemical delay periods, the processes that occur in each, and some fuel properties that affect each of these processes

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Figure 4

Variation in ignition delay with liquid fuel density. Most of the FT synthetic fuels line up closely with n-alkenes.

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Figure 5

Variation in ignition delay with liquid fuel kinematic viscosity. Among nonaromatic cyclic species, viscosity data were only available for cyclohexane; therefore, only the n-alkane and n-alkene trend lines are shown.

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Figure 6

Variation in ignition delay with liquid fuel-air surface tension

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Figure 7

Variation in ignition delay with liquid fuel normal boiling point (at 101.3 kPa). Most of the FT synthetic fuels and JP-5 line up closely with n-alkenes.

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Figure 8

Variation in ignition delay with liquid fuel vapor pressure (at 300 K). Among nonaromatic cyclic species, vapor pressure data were only available for cyclohexane; therefore, only the n-alkane and n-alkene trend lines are shown.

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Figure 9

Conceptual model of diesel ignition after Refs. 21,29-30. Hot air is entrained with the injected fuel, vaporizing the fuel and ultimately raising the temperature to the point where chemical reactions proceed rapidly to ignition. By choosing a characteristic ignition temperature, consistent with reaction rate models, an equivalence ratio at ignition can be determined.

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Figure 10

Variation in Δh, normalized to Δh of hexadecane, with carbon chain length. Here, Δh represents an approximate enthalpy required for vaporization and heating to a critical ignition temperature and includes the effects of fuel specific heat and enthalpy of vaporization. There is very little variation among the pure fuels tested in this study.

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Figure 11

Variation in ignition delay with carbon chain length. Most of the FT synthetic fuels and JP-5 line up closely with n-alkenes.

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Figure 12

Variation in ignition delay with fuel-air equivalence ratio (computer simulation) for hexane, hexane, and cyclohexane. Perfectly stirred, constant pressure reactor with initial T=770 K and P=55 bars, the latter based on experimental data. The shaded regions indicate the estimated equivalence ratio of the first fuel-air element to reach 770 K based on two different limiting estimates of cylinder air temperature.

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Figure 13

Experimentally measured Ignition delay (CAD) of hexane, 1-hexene, and cyclohexane. The chemical delay period, as estimated by computer simulation in Fig. 1, is indicated by the uppermost bar of each column. The middle shaded region is the uncertainty band based on the low and high estimates of the initial air temperature in the engine. The remaining lower bar is the portion of the ignition delay that remains for the physical delay period. While both the physical and chemical delays change among these three different fuel types, the variation in chemical delay is roughly equal to (1-hexene) and far exceeds (cyclohexane) the physical delay period.

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Figure 14

Variation in ignition delay with effective carbon chain length, Ceffective=C/R, where R is an empirical scaling factor. In the above figure, R values for n-alkenes, FT synthetic fuels, JP-5, cycloalkane/enes, diesel, and the aromatic species were 1.3, 1.3, 1.5, 1.9, 2.5, and 4.0, respectively.

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Figure 15

Correlation between combustion duration (10–90% mass fraction burned) and ignition delay for all tested fuel types

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Figure 16

Correlation between maximum rate of pressure rise and ignition delay for all tested fuel types

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Figure 17

Correlation between emissions of NO and ignition delay for all tested fuel types. While NO emissions can vary significantly even for short ignition delays, as ignition delay rises, best-case NO emissions more than double.

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Figure 18

Correlation between emissions of CO2 and ignition delay for all tested fuel types. Similar to NO emissions, CO2 emissions may be high or low even for short ignition delays; however, as ignition delay rises, best-case CO2 emissions increase by 50%.

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