Internal Combustion Engines

An Experimental and Modeling-Based Study Into the Ignition Delay Characteristics of Diesel Surrogate Binary Blend Fuels

[+] Author and Article Information
Matthew A. Carr, Leonard J. Hamilton

U.S. Naval Academy, Annapolis, MD 21402

Patrick A. Caton

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

Jim S. Cowart

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

Marco Mehl, William J. Pitz

LLNL, Livermore, CA 94550

J. Eng. Gas Turbines Power 134(7), 072803 (May 23, 2012) (10 pages) doi:10.1115/1.4006005 History: Received November 01, 2011; Revised November 01, 2011; Published May 23, 2012; Online May 23, 2012

This study examines the combustion characteristics of a binary mixture surrogate for possible future diesel fuels using both a single-cylinder research engine and a homogeneous reactor model using detailed chemical reaction kinetics. Binary mixtures of a normal straight-chain alkane (pure n-hexadecane, also known as n-cetane, C16 H34 ) and an alkyl aromatic (toluene, C7 H8 ) were tested in a single-cylinder research engine. Pure n-hexadecane was tested as a baseline reference, followed by 50%, 70%, and 80% toluene in hexadecane blends. Testing was conducted at fixed engine speed and constant indicated load. As references, two conventional petroleum-based fuels (commercial diesel and U.S. Navy JP-5 jet fuel) and five synthetic Fischer-Tropsch-based fuels were also tested. The ignition delay of the binary mixture surrogate increased with increasing toluene fraction and ranged from approximately 1.3 ms (pure hexadecane) to 3.0 ms (80% toluene in hexadecane). While ignition delay changed substantially, the location of 50% mass fraction burned did not change as significantly due to a simultaneous change in the premixed combustion fraction. Detailed chemical reaction rate modeling using a constant pressure, adiabatic, homogeneous reactor model predicts a chemical ignition delay with a similar trend to the experimental results but shorter overall magnitude. The difference between this predicted homogeneous chemical ignition delay and the experimentally observed ignition delay is defined as the physical ignition delay due to processes such as spray formation, entrainment, mixing, and vaporization. On a relative basis, the addition of 70% toluene to hexadecane causes a nearly identical relative increase in both physical and chemical ignition delay of approximately 50%. The chemical kinetic model predicts that, even though the addition of toluene delays the global onset of ignition, the initial production of reactive precursors such as HO2 and H2 O2 may be faster with toluene due to the weakly bound methyl group. However, this initial production is insufficient to lead to wide-scale chain branching and ignition. The model predicts that the straight-chain alkane component (hexadecane) ignites first, causing the aromatic component to be consumed shortly thereafter. Greater ignition delay observed with the high toluene fraction blends is due to consumption of OH radicals by toluene. Overall, the detailed kinetic model captures the experimentally observed trends well and may be able to provide insight as to the relationship between bulk properties and physical ignition delay.

Copyright © 2012 by American Society of Mechanical Engineers
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Figure 1

Schematic of experimental diesel CFR engine

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Figure 2

Cetane number versus concentration of toluene (C7 H8 ) with n-hexadecane (C16 H34 ). Toluene-hexadecane blends at 50% and 80% were measured using the ASTM D613 test [11]. The solid data markers represent published cetane values for pure toluene. On the right, five different synthetic fuels and conventional diesel and jet fuel are also shown.

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Figure 3

Typical rate of energy release profile (single cycle). The start of injection (SOI), 10%, 50%, and 90% mass fraction burned locations (CAD10, CAD50, CAD90), and the estimated location of the premixed combustion phase are shown on the figure.

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Figure 4

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

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Figure 5

Ignition delay as a function of toluene (C7 H8 ) volume fraction with hexadecane (C16 H34 ). Three different injection timings are shown for each blend corresponding to advanced (∼20 deg BTC), nominal (∼15 deg BTC), and retarded (∼13 deg BTC) SOI. On the left of the figure, five different synthetic fuels and conventional diesel and jet fuel are also shown at nominal (∼15 deg BTC) SOI.

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Figure 6

Rate of energy release (single cycles) for blends of toluene (C7 H8 ) with hexadecane (C16 H34 ) at nominal SOI (∼15 deg BTC). Each cycle was chosen to be representative of the average. The 10%, 50%, and 90% mass fraction burned points are indicated, as well as the estimated end of the premixed combustion phase (PF).

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Figure 7

Ignition delay versus volumetric percentage of toluene (C7 H8 ) in a binary mixture with Hexadecane (C16 H34 ). The data points in the shaded band correspond to experimentally measured ignition delays. The solid and dashed trend lines are chemical kinetic modeling under these corresponding to fuel-air equivalence ratios from 2 to 8 (55 bars and 770 K).

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Figure 8

Physical ignition delay versus volumetric percentage of toluene (C7 H8 ) in a binary mixture with hexadecane (C16 H34 ). Chemical ignition delays from chemical kinetic modeling were subtracted from the experimentally measured ignition delay in the research engine to show the physical ignition delay associated with that blend.

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Figure 9

Chemical kinetic modeling of ignition delays as a function of temperature with volumetric percentage of toluene (C7 H8 ) in a binary mixture with hexadecane (C16 H34 ) increasing from bottom to top, fuel-air equivalence ratio of 4

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Figure 10

HO2 (hydroperoxy) radical production for 0, 50, and 80% toluene in hexadecane

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Figure 11

Hydrogen peroxide (H2 O2 ) formation for three surrogate blends

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Figure 12

Mole fraction of CH* as a function of time with volumetric percentage of toluene (C7 H8 ) in a binary mixture with hexadecane (C16 H34 ) increasing from left to right

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Figure 13

Model predicted concentrations of toluene (C7 H8 ), hexadecane (C16 H34 ), CO2 , and CO initially at 770 K, 55 bars, fuel-air equivalence ratio 4, and increasing initial volume fractions of toluene in hexadecane

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Figure 14

Temperature history of 0, 50, and 80% toluene in hexadecane mixtures as predicted by chemical kinetic modeling




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