Ignition Characteristics of Diesel Fuel in a Constant Volume Bomb

Jun 30, 2014 - In diesel engines, a shorter ignition delay period has advantages, whereas in gas turbine engines, early autoignition can be a disadvan...
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Ignition Characteristics of Diesel Fuel in a Constant Volume Bomb under Diesel-Like Conditions. Effect of the Operation Parameters Magín Lapuerta,*,† Josep Sanz-Argent,† and Robert R. Raine‡ †

E.T.S. Ingenieros Industriales, Universidad de Castilla-La Mancha, Ciudad Real 13001, Spain Department of Mechanical Engineering, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand

‡

ABSTRACT: A constant volume chamber system was used to characterize the ignition and combustion of a diesel fuel. Fuel is injected in the chamber through a standard diesel common rail injection system. Injection parameters and initial chamber conditions were varied: fuel injection pressure (600 to 1000 bar), injection duration (500 to 3000 μs), initial temperature (808 to 923 K), initial pressures (6 to 21 bar), and oxygen mole fractions (15 to 30% diluted in nitrogen). Multiple measurements confirmed the repeatability of the measurements. Most of the experiments showed a two-stage combustion process, especially at low chamber pressure and highly premixed conditions. This is characteristic of large paraffinic hydrocarbons such as those composing around two-thirds of commercial diesel fuels. Combustion pressure data from diesel fuel have shown a moderate effect of injection pressure and injection duration on the ignition delays, especially when compared to the effect of other operation parameters. A strong effect of initial pressure and temperature was also observed. Increasing the initial pressure leads to nonlinear decreases in the ignition delay, this effect being especially strong at low pressures. Further, an increase in the oxygen mole fraction advances the onset of ignition, this effect being more intense at the lowest oxygen mole fraction in the oxidant. Two separate regions with different temperature-dependence functions were detected, indicating two different fuel oxidation chemistries. Arrhenius-type correlations were obtained for both regions, including the most influential parameters, to predict the first- and second-stage ignition delays.

1. INTRODUCTION The ignition delay period is an important characteristic of diesel fuels and of fuels for gas turbines. In diesel engines, a shorter ignition delay period has advantages, whereas in gas turbine engines, early autoignition can be a disadvantage.1 Many experimental methods are used to measure the ignition characteristics of fuels, for example shock tubes, rapid compression machines, flow reactors, and constant volume combustion chambers. Other methods and variations continue to be developed.2−5 Haylett et al.,2 for example, recently developed an aerosol shock tube to allow for the measurement of the ignition delay of premixed diesel-type fuels. Pickett3 has used a constant volume combustion chamber that is preheated and pressurized to high pressures and temperatures by a premixed combustion process prior to diesel fuel injection. Edenhofer et al.4 and Al-Hamamre and Trimis5 use similar methods of prevaporising and premixing diesel fuel to measure detailed ignition delay data. Recently, Meijer et al. reviewed and classified these methods into constant-pressure flow test rigs and constant-volume preburn vessels.6 In spite of all these diverse measurement methods, there are still many unknowns about the effects of some of the primary variables, such as initial temperature and pressure, on the ignition delay period of diesel-like fuels. Haylet et al.,2 for example, showed ignition delay times for mixtures of air and diesel fuels from various sources covering a range of approximately 10 times (e.g., from about 20 to 200 ms at a temperature of 850 K). This range included both spray ignition and fully evaporated fuel, with the fully evaporated giving generally shorter ignition delay than the spray ignition. These authors and many others have also shown that significant negative temperature © XXXX American Chemical Society

coefficient (NTC) behavior occurs between about 700 and 1000 K for diesel fuels (e.g., see refs 1 and 2). Recently, commercial high-pressure constant-volume combustion chamber systems have become available, some of which are specifically designed to measure the cetane number or the derived cetane number (DCN) of diesel fuels. For this study, one of these systems7 was used over a range of initial conditions to determine the effects of these conditions on the ignition and combustion characteristics of a commercial diesel fuel. This system has several advantages over other methods that are used to measure ignition delay of diesel-like fuels. These include pressure and temperature constant and uniform at the time of fuel injection, thermodynamic analysis simplified due to the constant volume configuration, and a fuel injection process using a high-pressure injection system similar to that of modern diesel engines. However, the system does not lead to purely premixed combustion, which makes the analysis of the ignition delay more difficult. Much of the literature about autoignition reports experimental studies made under conditions very far from those of current diesel engines, especially in relation to the injection system. Ikura et al.,8 as reported by Aggarwal,1 measured ignition delay times of fuel sprays in a heated and pressurized constant-volume chamber. Initial pressure was varied between ambient (1 bar) and 31 bar, initial temperature was varied between 650 and 900 K, and the fuels used included cetane (n-hexadecane) and gas-oil (diesel fuel). They found that the Received: March 7, 2014 Revised: June 27, 2014

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chamber to determine the effects of injection pressure on droplet size and “injection delay” using diesel and biodiesel fuels. For diesel fuelling they showed a decrease in droplet size with increased injection pressure (1000 to 3000 bar) and decreased “injection delay” with increased injection pressure. Injection delay was defined by these authors as the time between the electronic trigger signal being sent to the fuel injector and the first appearance of fuel from the injector. The objective of the present work is therefore to investigate the effects of a range of initial conditions on the ignition delay of a diesel fuel under constant volume conditions using a modern high-pressure fuel injection system, aiming to check whether trends and Arrhenius adjustments from previous works are extensible to this type of injection system or not. This forms part of a larger program aimed toward investigating the ignition delay of different fuels, including biofuels.

higher the pressure or the temperature, the shorter the ignition delay. Injection pressure used was between 100 and 150 bar. Over the temperature and pressure range that they measured, they found a good fit of the ignition delay period to the inverse absolute temperature in an Arrhenius form. Bogin et al.9 gave an overview of the ignition quality tester (IQT) and its application for experiments outside the range for measuring DCN. The IQT is similar in principle to the instrument used in the present work, but it has a lower temperature range and much lower injection pressure; hence, it is not so representative of current diesel fuel injection technology. With that system they varied temperature (723 to 823 K), initial chamber pressure (10 to 30 bar), and O2 mole fraction (15 to 21%). They fitted the results for ignition delay (ID) from eight different diesel-like fuels to an Arrhenius-type relation (eq 1). ⎛ E ⎞ ID = A ·p0 m ·XO2 n·exp⎜ a ⎟ ⎝ R ·T0 ⎠

2. EXPERIMENTAL SETUP (1)

A schematic of the constant-volume combustion chamber system used in this work is presented in ref 14. The instrument is designed to measure the pressure signal and to calculate a DCN from different characteristic times of this signal. DCN correlates with the CN of diesel fuels and diesel-like fuels. For this measurement, the instrument (Cetane ID 510TM7) must be set at very closely specified conditions of coolant temperature, injection pressure, injection duration, initial chamber temperature, initial chamber pressure, and uses synthetic dry air. The instrument incorporates a common rail injector (Bosch part no. 0445110181) with six nozzles with orifice diameter 0.17 mm.14 The instrument admits a fresh charge of high-pressure air into the combustion chamber at the start of each test. The mass of fuel is determined from the injection duration, the mass of charge is obtained from the pressure, temperature, and chamber volume after intake closure, and the equivalence ratio is then calculated. Approximately 1 min is allowed for the air to reach equilibrium temperature with the walls of the combustion chamber. During this time the pressure of the air is also recorded with a static pressure transducer (Honeywell MLH). After this time, a check is made to ensure that the coolant temperature, chamber wall temperature, and the chamber pressure are within a close tolerance with respect to the set values (±2 °C, ± 0.2 °C ± 0.2 bar, respectively). A chamber leakage test is also carried out to ensure that the pressure loss is less than 0.0075 bar/s. Subjected to satisfying these tolerances, a fuel charge is injected, and data is recorded.

where p0 is the initial chamber pressure (bar), XO2 is oxygen mole fraction, and T0 is the initial chamber temperature. Sample values given for the coefficients for one of the diesellike fuels (identified as FD1A) are A = 0.0682, m = −1.002, n = −0.729, and Ea = 44.28 kJ/mol. The Arrhenius relation obtained by Ikura et al.8 can be recast into the same form as that of Bogin (eq 1), and the coefficients to fit Ikura’s data become: diesel: A = 0.0023, m = −1.23, n = −1.6, and Ea = 60.53 kJ/mol. Haylett et al.2 reviewed data for ignition delay times for diesel−air mixtures for spray ignition and for flow reactor measurements with fully vaporised fuels. As expected, the measurements from sprays generally show a longer ignition delay period than they do for premixed mixtures since the ignition delay for sprays may include some physical delay of spray breakup, droplet formation, and evaporation. Although the experimental data of these authors is above the temperature of the present work, they clearly show from their modeling that there is a negative temperature coefficient region from about 870 K to about 700 K. This range is slightly above that found by Dooley et al.10 (between 625 and 723 K) in their kinetic modeling with a surrogate representing a jet fuel, which was confirmed in experiments in a shock tube and a rapid compression machine (RCM). Haylett et al.2 fitted an Arrhenius-type correlation to their experimental data for premixed ignition delays of the form: ⎛ E ⎞ ID = A ·p0 m ·φn ·T0 p·exp⎜ a ⎟ ⎝ R·T ⎠

3. PROCESSING OF RESULTS The instrument records pressure versus time for 15 injection events at each operating condition, and the pressure signals are then averaged. All the characteristic times were defined directly from pressure as in many other experimental studies (e.g., ref 15). The first characteristic time, combustion delay (CD), is determined as the time between the start of fuel injection and the midpoint of the combustion pressure curve, that is, midway between the initial chamber static pressure and the maximum pressure reached during combustion (Figure 1). In cases where the pressure signal shows some oscillations, the maximum pressure is taken by averaging around the maximum value. Two ignition delays were also defined, IDCF and IDm, similarly as in previous works.16,17 This allows distinguishing the onset of first (cool flame) and second (main) stages of combustion, for fuels that show two-stage autoignition. At an intermediate temperature range (aproximatelly from 700 to 900 K), commercial diesel fuels exhibit both cool flame stage, as a consequence of their lowtemperature chemistry (which is characteristic of linear18 or cyclic19 paraffins), and main combustion, which starts when temperature is high enough for branching reactions to take place.

(2)

and found the exponents m = −1.03 and n = −1.02. This fitted a temperature range from 900 to 1300 K for homogeneous mixtures, such that uniform φ values can be identified. The same authors made a review and concluded that, while there is considerable disagreement over the effect of pressure (m ranging from −0.6 to −2), more agreement can be found over the effect of equivalence ratio (n remaining around −1). Ghojel and Tran11 studied diesel sprays in a constant-volume vessel and showed only a small effect of injection pressure on ignition delay for injection pressures from 420 to 1000 bar. Jayakumar et al.,12 from measurements in a single-cylinder diesel engine, showed a decrease in ignition delay period when injection pressure was increased from 600 to 1000 bar. Wang et al.13 carried out fuel injection experiments into a constant-volume B

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reviews the 15 sets of data and eliminates any outliers from the averaging based on Peirce’s criterion.20 Most tests eliminate no outliers, and only one or two outliers are eliminated in a few cases. Multiple repeats of the datum operating condition and several other conditions were made to check the repeatability of the measurements. These repeats were taken on different days and interspersed between other measurements in a random way. Repeats of the datum condition show very good repeatability. For example, measurements taken on the same day show standard deviations from repeat measurements of the CD of less than 0.020 ms. Repeat measurements on different days showed the highest standard deviations for CD of up to 0.081 ms (Table 1). IDCF and IDm showed very similar standard deviations. Table 1. Results of Repeat Measurements at the Datum Condition for Diesel Fuel day

Figure 1. Sample combustion pressure and pressure rate versus time showing the definition of ignition delay and combustion delay.

IDCF is defined as the time lapse from the start of injection to the instant at which pressure rises 0.2 bar above the initial pressure, whereas IDm is defined until the instant at which the line connecting the pressure rise rates at CD and at the instant when the pressure reaches a quarter of the maximum pressure increase equals zero (Figure 1). Results shown in Figure 1 correspond to one test cycle and illustrate the pressure oscillations, which occur after the rapid pressure rise (p0 = 21 bar, T0 = 823 K, diesel fuel). When the increase of pressure from the initial conditions is observed closely (Δp, represented in Figure 2 for the cases

repetitions

1

4

2

3

1+2

7

IDCF (ms)

IDm (ms)

CD (ms)

2.415 0.013 2.285 0.007 2.359 0.070

3.790 0.020 3.640 0.000 3.726 0.081

4.150 0.020 4.000 0.000 4.086 0.081

ave stdev ave stdev ave stdev

4. DIAGNOSTIC AND KINETIC MODELING The pressure trace from each experiment was analyzed using a diagnostic methodology to obtain the heat release rate. The methodology considers a change of composition as a consequence of combustion and estimates the heat transferred through the wall as a consequence of radiation and forced and natural convection. The rest of the details of the diagnostic methodology were explained elsewhere.21 From the heat release trace it is possible to clearly distinguish, in the cases when it occurs, the existence of a two-stage combustion process and to obtain the total duration of the combustion. Constant-volume and constant-pressure kinetic simulations were also carried out to distinguish the influence of the oxidation chemistry of the fuel from the effect of the physical phenomena (such as formation of droplets, vaporization, or formation of the fuel/oxidant mixture). Diesel fuel chemistry was simulated using kinetic mechanism of a fuel surrogate that contains n-heptane and toluene.22 The fuel surrogate is a mixture of 64% n-heptane and 36% toluene (by mass), while its mechanism was constructed from the most recent ones of the individual compounds proposed by Lawrence Livermore National Laboratory (LLNL)23 including 772 species and 3316 reactions. The cooling effect caused by the formation of the gaseous fuel/air mixture was estimated in all the computations using the thermodynamic properties of tetradecane as a physical surrogate of diesel fuel due to its similar molecular formula (Table 2). The simulations were carried out using CANTERA,24 an open-source suite of object-oriented software tools for problems involving chemical kinetics, thermodynamics, and/or transport processes.

Figure 2. Initial pressure variation for all the experiments performed using air (36 experiments).

using air as oxidant) it can be noticed that pressure remains constant for approximately the first 0.6 ms under all experimental conditions. This is interpreted as the time between the electronic trigger of the injector and the actual start of fuel injection into the combustion chamber. Following this initial constant-pressure period, the pressure then decreases almost linearly for a period of time as a consequence of the energy absorption from the bulk gas by droplet evaporation and endothermic reactions. The pressure then increases, as exothermic reactions take place. All the ignition or combustion delays (IDCF, IDm, and CD) were defined using the start of the actual fuel injection. From the 15 injection events, the average IDCF, IDm, and CD are calculated for each operating condition. The software

5. EXPERIMENTAL DESIGN Measurements were made with commercially available diesel fuel. Measured characteristics of the diesel fuel are in Table 2. These properties were determined by the fuel supplier (CEPSA (Spanish Petroleum Company)) and in the laboratories of the University of Castilla-La Mancha. Note that approximately one-third of its C

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Table 2. Diesel Fuel Characteristics property

measurement method

molecular weight C (% w/w) H (% w/w) O (% w/w) aromatic content formula stoichiometric air−fuel ratio FAME content (%v/v) cetane number lower heating value (MJ/kg) density at 15 °C (kg/m3) viscosity at 40 °C (mm2/s) lubricity (μm) distillation (°C) 65% 85% 95%

value

from Aspen Advisor CEN/DS 15104

from elemental composition and molecular weight from molecular formula UNE-EN 14078-10 ASTM D-613/08 ASTM D-240 (b) EN ISO 12185 (b) UNE-EN-ISO 3104-96 (b) UNE-EN-ISO 12156-1-07 ASTM D-86

203.7 86.14 13.20 0.66 29.8 C14.62H26.87O0.08 14.45 5.8 49.6 42.43 844.9 2.51 348 303.5 336 357

composition corresponds to aromatic compounds, while the rest corresponds to linear and cyclic paraffins. For the present work, the instrument was operated over a wide range of conditions, while having due regard to safety constraints. Table 3 presents the range of experimental conditions that were tested

Table 3. Range of Experimental Conditions variable injection pressure (pinj, bar) injection duration (tinj, μs) initial chamber temperature (T0, K) initial chamber presssure (p0, bar) oxygen mole fraction in the oxidant (XO2, % molar)

datum

min.

max.

1000 2500 873 21 0.21

600 500 808 6 0.15

1500 3000 923 21 0.3

Figure 3. Effect of injection pressure on IDCF, IDm, and CD (T0 = 873 K, p0 = 21 bar, tinj = 2500 μs). the instant when injection ends. This instant also considers the trigger delay mentioned above and shown in Figure 2, leading to a final value of 3.1 ms (2.5 + 0.6 ms). It can be observed that, in the studied range of pressure and temperature conditions, injection ends before the start of the main combustion. The rate of heat release increases for higher values of pinj, since the total amount of fuel introduced in the combustion chamber is higher. It can also be observed that all the cases present both cool flame stage (between 2 and 4 ms) and main combustion stage, the rate of heat release during the first stage being independent of the injection pressure. Injection duration was also varied with all other variables at their datum conditions, and the results are presented in Figure 5. As in the previous case, equivalence ratio was also modified for different conditions. The increase of injection duration while the injection pressure is kept constant leads to equivalence ratios from 0.09 to 0.42 as duration goes from 500 to 3000 μs. The results show that there is a small effect of injection duration on all the delays as the injection duration is reduced from 3000 to 1500 μs. In contrast, IDm decreases, CD increases dramatically for an injection duration of 500 μs, while IDCF remains almost constant at all conditions. Examination of the pressure (Figure 6) and heat release rate (HRR, Figure 7a) traces at 500 μs injection duration shows that the combustion of the fuel, rather than being a fast process, proceeds slowly, indicating that heat release and heat transfer occur at almost similar rates, although it is possibly incomplete. Longer injection durations lead to a more visible main combustion. In this figure the end of injection is also indicated with vertical lines. As in the previous discussion, in all cases the main combustion begins after the end of injection and two-stage combustion is observed. It can also be seen, similarly as for different injection pressures, that the rate of heat release and the amount of heat released during the first stage combustion is quite independent (except in the case of the lowest injection duration) from the amount of fuel injected into the combustion chamber (Figure 7b).

in this work and the datum condition that was used. This datum condition corresponds closely (to within 2 °C, and other conditions as specified in the ASTM Standard14) to the condition for which the system was calibrated to provide agreement between DCN and CN (ASTM D613). As noted above, the temperature range covers the NTC region identified by other authors (e.g., refs 2 and 13) and is also a relevant range for typical end-of-compression temperatures in the diesel engine. The pressure range covers from lean combustion conditions (equivalence ratio around 0.35 for 21 bar and around 0.75 for 11 bar) to rich conditions (equivalence ratio around 1.33 for 6 bar), since the pressure variations were made without any modification in the injected fuel mass. When the oxygen mole fraction in the oxidant was varied, the mixture conditions remained lean for all cases (the equivalence ratio ranging from 0.25 to 0.49), except in the cases of 6 bar with oxidant having 15% and 21% of oxygen mole fraction, and 11 bar with 15%.

6. EXPERIMENTAL RESULTS AND DISCUSSION 6.1. Effect of Injection Pressure and Injection Duration. A series of experiments was carried out using diesel fuel at injection pressures of 600 to 1000 bar, with all other variables at their datum conditions. The increase of injection pressure while the injection duration is held constant leads to higher amounts of injected fuel and, as a consequence, higher equivalence ratios (from 0.28 to 0.36 as pressure goes from 600 to 1000). The results are presented in Figure 3 and show that there is a small effect of injection pressure on IDCF, CD, and especially on IDm. In all the cases there is a moderate decrease in delay values as injection pressure is increased from 600 to 1000 bar. Figure 4a shows the heat release rate (HRR) traces for the different injection pressures. A black solid vertical line is also included indicating D

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Figure 4. Heat release rate traces from the experiments at different injection pressures. (a) The whole combustion process. (b) Detail of the first stage heat release. initiates at regions with intermediate stoichiometry (from 0.5 to 1.2). The fuel distribution in the jet depends on physical phenomena such as atomization, evaporation, or mixing, while the oxidation kinetics of the fuel plays a determinant role in the onset of combustion. To identify the causes of the observed different tendencies for different values of pinj and tinj, constant-volume and constant-pressure simulations were carried out at different equivalence ratios. In constant-volume (CV) simulations, a homogeneous mixture is assumed. This is equivalent to the case that the fuel is completely mixed with the oxidant at the end of injection, and therefore, the overall equivalence ratio should be used in the simulations. In constant-pressure (CP) simulations, different local stoichiometries reached in the fuel spray at different times/locations during the mixing process can be simulated. In this case it is assumed that the amount of surrounding air is enough to absorb the expansion of the gas before the combustion chamber restriction becomes sensible. Figure 8 shows the results for IDCF and IDm. Modeled results were obtained from heat release curves because pressure traces would not be useful to define delays in CP simulations. As shown in Figure 8a, differences in IDCF were not significant between CV and CP simulations, since no change in the thermodynamic conditions occurs before cool flame ignition. On the contrary, significant differences can be observed for IDm (Figure 8b). As can be observed, measured IDm values are much lower than in purely premixed autoignition (from constant volume simulations), indicating that autoignition occurred at locally richer mixtures, as proposed in Musculus et al.25 phenomenological model for LTC combustion. However, measured delay values are longer than those obtained in constant pressure simulations for relatively rich stoichiometries (a minimal IDm is obtained for a local equivalence ratio around 1.2, see horizontal gray line), which suggests that a physical delay is also relevant for the ignition. As mentioned before, higher injection pressures result in slightly shorter delay times (Figures 3 and 8). This may be attributed to a reduced physical delay as a consequence of better atomization. Higher injection duration, on the other hand, generally increases ignition delay times (Figures 5 and 8). Since the physical delay should remain constant for all cases (same pinj and thermodynamic conditions in the combustion chamber), this may be attributed to the increasing cooling

Figure 5. Effect of injection duration (μs) on IDCF, IDm, and CD (T0 = 873 K, p0 = 21 bar, pinj = 1000 bar).

Figure 6. Pressure traces from the experiments at different injection durations. The combustion processes observed in Figures 4 and 7 are very similar to those found in partially premixed low-temperature diesel combustion (LTC) processes for heavy-duty engines using late injection. Musculus et al.25 have recently developed a conceptual model to describe this combustion. They show that diesel fuel fully evaporates prior to the second stage combustion forming a jet with local equivalence ratios from lean to rich. Second-stage combustion

Figure 7. Heat release rate traces from the experiments at different injection durations. (a) The whole combustion process. (b) Detail of the first stage heat release. E

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Figure 8. Effect of overall equivalence ratio on (a) IDCF and (b) IDm (T0 = 873 K, p0 = 21 bar). to that observed for CD (Figure 5), which decreases for higher tinj since higher quantities of injected fuel lead to slightly faster combustion processes. In summary, over a wide range of injection pressures and injection durations, variations in IDCF, IDm, and CD are caused by their effect on the atomization of the mixture and on the cooling caused by fuel evaporation. However, this effect is very moderate compared to that of other parameters studied below. These measurements confirm the finding of Ghojel and Tran11 that there is only a small influence of injection pressure on ignition delay for injection pressures between 420 and 1000 bar. Consequently, the following study of the effects of wall temperature, initial combustion chamber pressure, and oxygen mole fraction on IDCF, IDm, and CD was made while keeping injection pressure and injection duration at the datum values (Table 2). 6.2. Effect of the Mixture Initial Temperature and Pressure. Figure 9 shows the effect of varying the initial temperature on IDCF, IDm, and CD when all other variables are held constant at the datum conditions, for three different initial chamber pressures, namely, p0 = 6, 11, and 21 bar. The actual initial pressures varied by ±0.5 bar around these nominal values. Results show a strong effect of both initial pressure and temperature on the delays. Two separate regions showing different temperature dependence can be distinguished when the influence of the initial temperature is analyzed (one region from 808 to 850 K (1000/T from 1.24 to 1.18) and the other from 850 to 923 K (1000/T from 1.18 to 1.08)). As stated above, diesel fuel oxidation exhibits low- and high-temperature chemistries. As a consequence, the relation between ignition delay and temperature for diesel fuel includes three different stages for premixed

Figure 9. IDCF, IDm, and CD versus initial temperature for different p0. effect of fuel evaporation (longer injection process will lead to richer regions, and due to heat exchange between zones, evaporative cooling will be more intense in regions with shortest ignition delay). As can be seen in Figure 8a, this is the same tendency observed for IDCF (a parameter that is very insensitive to the equivalence ratio) at intermediate-to-rich stoichiometries. However, this trend is opposite

Figure 10. Effect of initial wall temperature on combustion progress at different initial pressures. (a) p0 = 6 bar. (b) p0 = 11 bar. (c) p0 = 21 bar. F

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coefficient (NTC, where higher temperatures lead to higher delays). In the previous section it was observed that, in experiments carried out at p0 = 21 bar, ignition takes place at relatively rich local equivalence ratios, since the experimental IDm are much shorter than those obtained from CV simulations considering the overall equivalence ratio. However, delay values at the lowest pressure are practically twice those at p0 = 11 bar, and consequently, a rather homogeneous mixture is formed prior to ignition. Pressure traces at 6 bar (Figure 10) show a lapse of time after cool-flame ignition (and before main ignition) with slow pressure increase, where kinetics is dominated by intermediate temperature reactions (much less reactive than low- or high-temperature ones). This pattern is clearly different at higher initial pressures. Kinetic simulations were carried out to validate this finding. For p0 = 21 and 11 bar, constant-pressure simulations were used assuming that combustion takes place at a local equivalence ratio of 1.2. This value was selected because it provides minimum ignition delay in the range of local equivalence ratios from 0.5 to 1.2 (where, as mentioned in Section 6.1, main combustion is expected to start). For the lowest pressure, it is supposed that the fuel is close to being completely mixed with the oxidant prior to the onset of combustion. For that reason, constant-volume simulations were performed assuming an initial composition that corresponds to the overall equivalence ratio. As can be seen in Figure 11 and considering that the physical delay also contributes to the total delay, the values of the modeled delays are consistent with the experimental results. Simulated IDm show a clear NTC region for the lowest initial pressure, which is also observed in the experimental results, indicating that this process is controlled by the oxidation chemistry. Such NTC region is hardly noticeable at p0 = 11 bar, in both simulated and experimental tests, and not noticeable at all at p0 = 21 bar 2. Modeling is also capable of reasonably capturing the transition temperature from low- to intermediate-temperature regions and the effect of pressure on the delay times. In summary, for the initial temperatures studied in this work, two different temperature regimes are observed corresponding to the lowand intermediate-temperature range of the fuel oxidation chemistry. At the lowest pressure these two regimes are more clearly distinguished, since long ignition delays favor the formation of a rather homogeneous mixture. At higher pressures these different regimes are not so clearly observed because autoignition occurs at locally richer regions. 6.3. Effect of Oxygen Mole Fraction. Figure 12 shows the results of pressure traces from experiments carried out at T0 = 863 K using three different oxygen mole fractions in the oxidant (XO2 being 0.15,

Figure 11. Experimental and modeled IDCF and IDm versus initial temperature for different p0. conditions: low, intermediate, and high temperature. The temperature range considered in this work includes both low temperature and intermediate stages, which explains the presence of two different regions on the experimental results. As can be seen in Figure 9, the higher the pressure, the shorter the delay values. This influence is nonlinear, since an increase in the initial pressure from 6 to 11 bar shortens the delays at a higher rate than an increase from 11 to 21 bar. Besides, the effect of pressure is higher at the highest temperatures analyzed. Almost the same trends are observed for the IDm and CD results. As can be derived from the definition of both times (Figure 1), the longer the main combustion process, the higher the difference between IDm and CD is. Since in most of the cases combustion proceeds pretty rapidly, such a difference is usually small and constant, and therefore, CD is not further analyzed in the rest of this paper. The results shown in Figure 9 suggest that the ignition processes that take place at p0 = 6 bar are different than those occurring at higher pressures, since they show a clear region with negative temperature

Figure 12. Effect of XO2 at T0 = 863 K and different initial pressures. (a) p0 = 6 bar. (b) p0 = 11 bar. (c) p0 = 21 bar. G

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Figure 13. Effect of the oxygen partial pressure on IDm. (a) T0 = 808 K. (b) T0 = 923 K.

Table 4. Coefficients of the Arrhenius-Type Correlations for IDCF and IDm region IDCF IDm

low int low int

A 1.9730 6.3559 3.4605 3.0721

× × × ×

−6

10 104 10−5 105

m

n

Ea (kJ/mol)

−0.1256 −5.4982 −0.3090 −5.0787

−0.4138 −0.5093 −0.7282 −1.0645

98.49

B (kJ/bar·mol)

C (kJ/mol)

2.4487

−9.5594

81.24 2.3613

−17.70

adjusted R2

global R2

0.9748 0.8875 0.9693 0.9599

0.9791 0.9773

ment from the constant Ea assumption (eq 4). Table 4 includes the values of the coefficients for the four correlations (two parameters and two separate temperature ranges) and the R2 values. As can be seen, the global R2 for the two parameters are very good.

0.21, and 0.3) at three different initial pressures. As can be seen, the higher the oxygen mole fraction, the more advanced the combustion. This effect is nonlinear since the decrease of IDm from a fraction of 15% to a fraction of 21% is greater than that from 21% to 30%. Similar results are obtained at different initial temperatures, as can be observed below. It can also be noticed in Figure 12 that the effect on the main ignition timing (IDm) is much greater than the effect on the onset of first stage heat release (IDCF). The delay in the ignition caused by the use of an oxidant with a low oxygen mole fraction also causes a change in the combustion process. As can be observed in Figure 12b, the pressure trace obtained at p0 = 11 for a XO2 = 0.15 are very similar to those obtained at the lowest pressure (Figure 10a). This leads to the appearance of NTC at higher pressures than in the case of an oxidant with higher mole fraction of oxygen, as observed below. Both the change in the oxygen mole fraction and the initial pressure change the availability of oxygen in the oxidant used during combustion. The two parameters can be combined into a single one, the partial pressure of oxygen (pO2). The use of this parameter allows for the separation of the effect of the presence of oxygen from that caused by the presence of inert molecules (in this study N2). Figure 13 shows the effect of pO2 at the two different initial temperatures, corresponding to the lowest and the highest temperatures analyzed in this work. As can be seen in the figure, the presence of inert molecules has a significant influence on IDm at low initial temperatures with a decreasing effect for higher values of T0. It has been suggested that, although the inert gas is not involved in the chemical reaction, it contributes to delay the diffusion of the molecules involved in the reactions,26 and this effect is less noticeable when the molecular mobility is high (high temperature). 6.4. Correlations for the Ignition Delay. The results described above show a strong dependency of the ignition delays on initial pressure, initial temperature, and oxygen mole fraction. It was also observed that the temperature dependency shows two separate ranges (see, e.g., Figure 9), corresponding to different oxidation chemistries. Finally, results from experiments carried out at the lowest pressure show a combustion process mainly controlled by the oxidation chemistry of the fuel, contrarily to higher pressures. Consequently, results from the lowest pressure tested were not considered in the correlations. Two Arrhenius-type correlations are proposed for the prediction of IDCF and IDm for the results obtained at pressures p0 = 11 bar and p0 = 21 bar. Since two separate temperature ranges were observed, the results were separated into two different groups corresponding to lowand intermediate-temperature stages, to obtain different correlations for each region, as suggested by Hernandez et al.27 For the lowtemperature region, a constant energy of activation is assumed (eq 3). However, for the intermediate-temperature region a correlation of the activation energy with the initial pressure showed a significant improve-

⎛ E ⎞ IDlow = A ·p0 m ·XO2 nexp⎜ a ⎟ ⎝ R ·T0 ⎠

(3)

⎛ B·p + C ⎞ IDint = A ·p0 m ·XO2 nexp⎜ 0 ⎟ ⎝ R ·T0 ⎠

(4)

Figure 14 shows both the experimental and the results from correlations of IDCF and IDm. The figure shows a very good fitting for both stages at most of the conditions. The largest error is found at

Figure 14. Correlations obtained for the low temperature (dashed line) and the NTC (solid line) for IDm ((a) p0 = 21 bar, (b) p0 = 11 bar) and IDCF ((c) p0 = 21 bar, (d) p0 = 11 bar). H

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p0 = 11 bar and XO2 = 0.15, conditions for which, as said above, the combustion process is different than it is for the rest of the conditions considered for the correlations.

7. CONCLUSIONS A series of experiments was carried out in a constant-volume combustion chamber equipped with a common rail injection system to analyze the influence of the operational parameters. A two-stage combustion process was observed especially at low chamber pressure and low oxygen mole fraction in the oxidant. The first-stage heat release is caused by the low-temperature reactions characteristic of large hydrocarbons, such as linear or cyclic paraffins, while the main combustion is initiated when branching reactions are the dominating ones during the oxidation. Three different delay times were used in this work: first-stage ignition delay, main-ignition delay, and combustion delay. The delay times observed from the experiments are shorter than those simulated assuming the overall equivalence ratio and a constant-volume process. However, they are longer than those modeled considering relatively rich mixtures (corresponding to local conditions) and a constant-pressure process, indicating that ignition occurs prior to complete mixing and that a physical delay also contributes to the total delay, as a consequence of atomization, evaporation, and mixing. Experiments showed little effect of the injection pressure or duration on this physical delay. It also was observed that chamber pressure has a nonlinear effect on the ignition delays, this effect being stronger at low pressures. Two regions were detected showing different temperature dependencies, corresponding to low- and intermediate-temperature oxidation chemistry of large hydrocarbons. At low initial chamber pressure, the intermediate temperature region even shows negative temperature coefficient, a phenomenon also observed at intermediate pressure when low oxygen mole fraction is used. Similarly to the effect of initial chamber pressure, oxygen mole fraction also showed a nonlinear influence, this being stronger at low fractions. The analysis of the effect of pO2 also showed that the influence of oxygen mole fraction is not only caused by the change of the content of oxygen in the chamber but also by the presence of inert molecules, especially at the lowest temperatures. Finally, Arrhenius-type correlations were proposed to predict IDCF and IDm at initial pressures higher than 11 bar for the ranges of initial temperatures and oxygen mole fraction analyzed in this work.

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Article

NOMENCLATURE A, B, C=constants CD=combustion delay time CN=cetane number CP=constant pressure CV=constant volume DCN=derived cetane number Ea=activation energy ID=ignition delay time LTC=low-temperature combustion m, n, p=exponents NTC=negative temperature coefficient p=pressure R=gas constant RCM=rapid compression machine T=temperature t=time, duration X=mole fraction φ=equivalence ratio

Subscripts

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0=initial conditions CF=cool flame stage inj=injection int=intermediate temperature region low=low temperature region m=main combustion stage O2=oxygen

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +(34) 926295431. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to acknowledge the Spanish Ministry of Science and Innovation for the financial support through the project CINBIOLT (reference code: Ref. TRA2010-18876). Dr. J. SanzArgent also would like to acknowledge the Universidad de CastillaLa Mancha for providing his funding through CYTEMA professorship. Dr. R. Raine gratefully acknowledges the University of Auckland and colleagues in the Department of Mechanical Engineering for support allowing him to take sabbatical leave. Also sincere thanks for the friendly welcome into the Fuels and Engines Group at the University of Castilla-La Mancha. I

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