Homogeneous Charge Compression Ignition Combustion of Primary

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Homogeneous Charge Compression Ignition Combustion of Primary Reference Fuels Influenced by Ozone Addition J.-B. Masurier,*,†,‡ F. Foucher,† G. Dayma,‡ and P. Dagaut‡ †

Laboratoire Pluridisciplinaire de Recherche en Ingénierie des Systèmes, Mécanique, Energétique (PRISME), Université d’Orléans, 45072 Orléans Cedex, France ‡ Institut de Combustion Aérothermique Réactivité et Environnement (ICARE), Centre National de la Recherche Scientifique (CNRS)−Institut des Sciences de l’Ingénierie et des Systèmes (INSIS), 1C, Avenue de la Recherche Scientifique, 45071 Orléans Cedex 2, France ABSTRACT: Homogeneous charge compression ignition (HCCI) combustion applied to the internal combustion engine presents several advantages to meet current automotive objectives. However, controlling the progress of this combustion mode is inherently challenging. This study proposes to use ozone, a strong oxidizer, to control combustion. Experiments were carried out on a HCCI engine fueled with primary reference fuels (PRFs) and seeded in the intake by ozone. Results are presented for six different PRFs, and the effect of ozone on their combustion progress was investigated. Simulations in a constant volume reactor were also performed to explain the effect of ozone based on the chemical kinetics.

1. INTRODUCTION Homogeneous charge compression ignition (HCCI) combustion has been one of the most extensively studied engine combustion modes in recent years. Currently, because of diminishing reserves of fossil fuels and stricter pollutant emission requirements, the main objectives for internal combustion engines are to reduce fuel consumption and pollutant emissions. HCCI combustion can be an interesting solution to achieve these objectives because of its potential to maintain strong efficiency similar to compression ignition engines and to produce very low emissions of NOx and particulate matter. Consequently, it can be one of the most promising combustion modes for internal combustion engines and could replace conventional engines. However, many challenges must be addressed to control the autoignition of the mixture and the phasing of the combustion.1,2 Unlike compression ignition (CI) and spark ignition (SI) engines, HCCI cannot be easily controlled by external means, such as injectors or spark plugs, because the combustion progress is mainly governed by chemical kinetics. This is why many strategies have been studied to control the start of combustion and its phasing. One of the main characteristics of the HCCI engine is that it enables various fuels to be used. Dependent upon their properties, fuels can be a means of controlling combustion because they can autoignite more or less readily depending upon their cetane or octane number. Several recent studies have explored the use of fuel blends. Lü et al.3 conducted investigations with n-heptane isooctane mixtures. According to the octane number (ON), it was shown that the fuel properties act on the ignition timing but also on the maximum pressure and heat release rate. When the octane number increases, both the ignition timing and the combustion phasing are delayed. Yamada et al.4 explored the effect of the HCCI combustion of dimethyl ether with the addition of low amounts of methanol. Results showed that increasing the amount of methanol © XXXX American Chemical Society

decreases the maximum pressure and delays the heat release rate. Similar effects were observed by Saisirirat et al.5 for a nheptane combustion mix with two other alcohols: ethanol and 1-butanol. It is therefore possible to control HCCI combustion by the use of various fuels and fuel mixtures. Other parameters, such as the intake temperature, can be a means to control combustion. Several studies have been performed by varying this thermodynamic parameter. The results observed for HCCI engines fueled with n-heptane or mixtures based on n-heptane6−8 show that increasing the intake temperature can shorten the ignition timing and increase the energy released by combustion. This can be attributed to the chemical kinetics, which are strongly influenced by the temperature in the cylinder and, therefore, the intake temperature. The same results were observed by Mohammadi et al.9 in a natural gas premixed charge compression ignition (PCCI) engine. The implementation of exhaust gas recirculation (EGR) in conventional engines is increasingly used to meet pollutant requirements. Several studies have also been conducted to investigate the effect of EGR as a means of control in HCCI engines. Dubreuil et al.6 studied the effect of EGR with a HCCI engine fueled with n-heptane. Results showed that an increase in EGR can delay the ignition timing and produce an incomplete reaction. To investigate this effect in greater detail, the study was performed with each chemical component of the EGR to identify how they act on the ignition timing. The effects observed can be explained by the dilution of the air/fuel blend, which reduces the fuel oxidation rate, and by the modification in the heat capacity that limits the temperature increase. Lü et al.8 and Fathi et al.10 also conducted experiments with EGR additions on HCCI engines with n-heptane isooctane Received: May 29, 2013 Revised: July 29, 2013

A

dx.doi.org/10.1021/ef401009x | Energy Fuels XXXX, XXX, XXX−XXX

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mixtures and n-heptane natural gas mixtures, respectively. Both authors observed the same effect on combustion. Watanabe et al.11 performed experiments with PRF40 and investigated the effect of cold and hot EGR. Results showed that the effect of cold EGR is greater than that of hot EGR. While these strategies can potentially control HCCI combustion, new approaches have also emerged. Because HCCI combustion is governed by chemical kinetic mechanisms, the use of oxidizing chemical species has been proposed. Among the many oxidizing chemical species, ozone is probably one of the most promising promoters of combustion. Several studies used this molecule in various applications, such as burner setup12−15 and internal combustion engines.4,9,16−19 Ombrello et al.14 conducted experiments with ozone on flame propagation using a propane/oxygen/nitrogen mixture and showed that ozone improved the flame propagation speed because of the decomposition of ozone into O atoms. Other investigations on the laminar burning velocity but in the case of methane were performed by Wang et al.15 and Halter et al.12 and confirmed the enhancement in combustion because of the production of O atoms. Liang et al.13 found the same result but for a hydrogen/oxygen/nitrogen/air mixture. The first work conducted on internal combustion engines was performed by Tachibana et al.19 on a cooperative fuel research (CFR) compression ignition engine and clearly showed that ozone addition can reduce the ignition timing and increase the cetane number of the fuel. Schönborn et al.18 proposed to use ozone to modify the molecular structure of the fuel and control the ignition timing inside the combustion chamber. Mohammadi et al.9 concluded that ozone can improve combustion efficiency in the case of natural gas as fuel but that, with further ozone additions, this enhancement is limited. Nishida and Tachibana17 also studied the effect of ozone addition on natural gas/air mixtures and concluded that ignition timing can be controlled by this chemical species. Yamada et al.4 investigated the effect of ozone on dimethyl ether and confirmed its impact on ignition timing. Finally, recent work performed by Foucher et al.16 on the combustion of n-heptane also indicated that ozone can enhance combustion and can be used to easily control its phasing, even for cycle-to-cycle control. The goal of this study is to use the oxidizing potential of ozone to control the combustion phasing of primary reference fuel (PRF). The effect of ozone seeding in the intake of the HCCI engine is presented, and supplementary experiments were performed to highlight the promoting effect of this oxidizing chemical species. Simulations were conducted to justify the effect of ozone in the internal combustion engine. Lastly, simulations in constant volume combustion cases were carried out to give a kinetic interpretation of the influence of ozone seeding on the PRF combustion progress.

Table 1. Monocylinder Engine Characteristics characteristic

value

bore (mm) stroke (mm) displacement (cm3) rod length (mm) compression ratio

85 88 499 145 16:1

Figure 1. Scheme of the experimental setup. after initial dilution with air, into a plenum with the main air that can be warmed by a heater.

Table 2. Fuel Properties fuel

isooctane (%)

n-heptane (%)

PRF0 PRF20 PRF40 PRF60 PRF80 PRF100

0 20 40 60 80 100

100 80 60 40 20 0

Ozone was generated by an ANSEROS COM-AD-01 ozone generator. This device works on the principle of a dielectric barrier discharge and needs pure oxygen flow to seed the air/fuel mixture in the plenum. Because there is an oxygen addition in the intake, a nitrogen flow was added to maintain the same ratio between oxygen and nitrogen as in air. The intake air was provided by a compressor, and the intake nitrogen was provided by a compressor associated to an air filter. The oxygen was supplied from a gas bottle. Each gas supplied was controlled by a Brooks gas mass flow controller (MFC), while fuel was controlled by a Coriolis liquid MFC. The references of each MFC are given in Table 3. All of the MFCs have an accuracy of ±0.7% (on the measure) and ±0.2% of the full scale, except for the fuel MFC, which has an accuracy of only ±0.2% on the measure. To ascertain the inlet conditions of the engine, the pressure was measured on one of the intake pipes by a Kistler 4075A piezo-resistive absolute pressure sensor with an accuracy of ±0.3% of the full scale. The temperature was

2. EXPERIMENTAL SECTION 2.1. Engine Setup. The engine used for the experiments is a PSA DW10 engine. Its main characteristics are listed in Table 1. Because it is an engine used for research studies, it was modified to work in monocylinder mode and coupled to an electric engine to maintain a constant rotation speed. Moreover, to work in homogeneous inlet conditions, the engine intake was also modified. It is based on the experimental setup developed and studied by Dubreuil et al.6 The present experimental setup is shown in Figure 1. For experiments, six PRFs were used. Each one is defined as a function of volume percentages of isooctane and n-heptane fuels (Table 2). The fuel is stocked in a pressurized tank and is injected,

Table 3. Characteristics of Gas and Liquid MFCs

B

gas/liquid

model

scale

main air air fuel N2 O2 fuel

Brooks 5853S Brooks 5851S Brooks 5850S Brooks 5850S Bronkhorst M13-CORI-FLOW

700 NL/min 30 NL/min 5 NL/min 5 NL/min 2 kg/h


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Table 4. Ozone Reaction Submechanisma reaction

a

A

n

E

reference

O3 + N2 → O2 + O + N2

4.00 × 1014

0.0

22667

14

O2 + O + N2 → O3 + N2

1.60 × 1014

−0.4

−1391

14

O3 + O2 → O2 + O + O2

1.54 × 1014

0.0

23064

14

O2 + O + O2 → O3 + O2

3.26 × 1019

−2.1

0

14

O3 + O3 → O2 + O + O3

4.40 × 1014

0.0

23064

14

O2 + O + O3 → O3 + O3

1.67 × 1015

−0.5

−1391

14

O3 + H ↔ O2 + OH

8.43 × 1013

0.0

934

23

O3 + O ↔ O2 + O2

12

4.82 × 10

0.0

4094

24

O3 + OH ↔ O2 + HO2

1.85 × 1011

0.0

831

25

O3 + HO2 ↔ O2 + OH + O2

9

6.02 × 10

0.0

938

14

O3 + H 2O ↔ O2 + H 2O2

6.62 × 101

0.0

0

26

O3 + CH3 ↔ O2 + CH3O

12

3.07 × 10

0.0

417

27

O3 + NO ↔ O2 + NO2

8.43 × 1011

0.0

2603

24

O3 + N ↔ O2 + NO

6.03 × 10

0.0

0

27

O3 + H ↔ O + HO2

4.52 × 1011

0.0

0

28

O3 + H 2 ↔ OH + HO2

10

6.00 × 10

0.0

19840

29

O3 + CH4 ↔ CH3O + HO2

8.13 × 1010

0.0

15280

29

7

Arrhenius equation: k = ATn exp[−E/(1.9872T)]. Units are in mol, cm3, s, K, and cal.

measured with two K thermocouples, one on each intake pipe, with an accuracy of ±2K. Finally, the ozone concentration in the intake was measured by an ANSEROS MP-6060 ozone analyzer with an accuracy of 0.1 ppm and from a sample taken on one of the intake pipes. For the experimental results, the in-cylinder pressure was measured by a Kistler 6043A piezo-electric pressure sensor with an accuracy of ±2% and positioned by an optical encoder to record 7200 points per cycle, i.e., every 0.1 crank angle degree (CAD). Other results were obtained by analysis of all of the different measures through the sensors. Finally, this setup makes it possible to control several experimental parameters: the engine speed, the equivalence ratio, the intake thermodynamic conditions (pressure and temperature), and the ozone concentration seeded. 2.2. Computations. To explain the experimental results observed, simulations were conducted with the executable Senkin of the Chemkin II package.20 The kinetic scheme used for this paper was proposed by Foucher et al.16 It is composed of a PRF oxidation mechanism21,22 and an ozone submechanism,12 given in Table 4. The entire mechanism involves 4255 reactions, most of them reversible, and 1037 species.

Figure 2. Fuel regions where each PRF can autoignite as a function of the intake pressure and the intake temperature. The 1 and 5 correspond to the iso-CA50 limits.

3. RESULTS AND DISCUSSION 3.1. Experimental Results. For all experiments conducted in these investigations, the engine speed and the equivalence ratio were fixed at 1500 rpm and 0.3, respectively. 3.1.1. Preliminary Experiments. Before studying the effect of ozone seeding in the engine intake, a study was carried out on the HCCI engine previously presented to establish “optimum” regions, where each fuel can autoignite and where 50% of the fuel has burned between 1 and 5 CADs (CA50). Experiments were performed, for each fuel, at different intake temperatures and by varying the intake pressure. For each experiment, the CA50 was determined and only the data points with a CA50 ranging from 1 to 5 CAD were conserved to draw the map. Results are presented in Figure 2 for each fuel previously defined in the experimental setup (Table 2). The upper limit of each fuel corresponds to an iso-CA50 of 1 CAD and the lower limit to an iso-CA50 of 5 CAD. It can be observed that the fuels

Table 5. Intake Thermodynamic Conditions fuel intake temperature (°C) intake pressure (bar)

PRF0

PRF20

PRF40

PRF60

PRF80

PRF100

33

34

34

37

38

180

0.56

0.62

0.74

0.92

1.20

1.20

autoignite more or less easily depending upon the ON. PRF0 with the lowest ON burns easily at low temperature and pressure, whereas PRF100 with the highest ON requires high pressure and temperature to autoignite. Intermediate PRFs can burn at ambient temperature, because of the presence of nheptane in the mixture and its facility of autoignition. However, the intake pressure must be more or less high depending upon the ON. C


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Figure 3. In-cylinder pressure (on the left) and heat release rates (on the right) as a function of the CAD and ozone concentration injected in the intake of the engine. Results are presented downward: PRF0, PRF40, and PRF100.

3.1.2. Effect of Ozone Seeding on PRF Combustion. From the previous results, experiments with ozone seeding were defined. For each PRF, it consisted of starting tests at the lowest pressure and temperature, under the most difficult intake conditions. Starting points are shown as “●” in Figure 2 and are listed in Table 5. These points were chosen to visualize the effect of ozone addition on the HCCI combustion of each PRF in the engine and also because previous studies have shown that ozone addition can move the combustion phasing forward. Experi-

ments were performed by varying the ozone concentration in the intake from 0 to 50 ppm for all PRF, except for PRF100, where variation did not exceed 10 ppm. Figure 3 presents the results of the in-cylinder pressures and the heat release rates calculated as a function of the CAD for three fuels (PRF0, PRF40, and PRF100) and for different ozone concentrations seeded in the HCCI engine intake. The shaded areas correspond to the lower and upper limits of variations over the 100 cycles recorded, and the black curve corresponds to the mean of the 100 cycles. These results show that ozone seeding D


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Figure 4. Shift of the cool and main flame phasing in reference to the phasing of each one without ozone seeding in the intake and as a function of the ozone additions.

Figure 6. Cool flame energy released versus total energy introduced for different fuels as a function of the cool flame shift and the ozone concentration injected in the intake.

Figure 5. Shift of the CA05 and the CA50 in reference to the CA05 and the CA50 without ozone seeding in the intake as a function of the ozone additions.

Figure 7. In-cylinder pressure and heat release rate as a function of the CAD and the ozone concentration in the intake for PRF100 as fuel, for an intake temperature of 100 °C and an intake pressure of 1 bar.

makes it possible to improve combustion and advance its phasing. The same trends can be observed for the other fuels studied. As seen on the in-cylinder pressure curves and heat release rates, the absence of ozone in the intake yields only a slight maximum pressure and slight heat release rate because of

intake thermodynamic conditions (pressure and temperature) that make combustion difficult. With small ozone additions in the intake, maximum in-cylinder pressure abruptly increases and its CAD position moves toward the top dead center. This E


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Figure 8. Phasing of the CA50 versus the ozone concentration for two intake temperatures and intake pressure conditions.

Figure 11. Simulated ozone mole fraction in the engine as a function of the CAD for various intake temperatures at a constant intake pressure of 1 bar.

Figure 9. Computed ignition delays as a function of the temperature inverse for two intake pressures (filled symbols, 20 bar; open symbols, 40 bar) and two fuels (circles, isooctane; squares,n-heptane). Symbols represent simulations with the ozone submechanism, and lines represent simulations without the ozone submechanism.

Figure 12. Main flame ignition timing as a function of the temperature inverse for various ozone concentrations. “□” symbols and curves under this one represent PRF0, and “○” symbols and curves under this one represent PRF100.

means that combustion is enhanced. It can also be seen through the peak increase in the heat release rate, showing that the energy released by combustion also increases. With further ozone additions, it is possible to continue increasing the maximum in-cylinder pressure and heat release rate, but the effects are less pronounced than with the first additions. Moreover, with regard to in-cylinder pressure variations for 100 cycles (shaded areas), ozone seeded in the intake provides a better cycle-to-cycle stabilization of combustion. This can be observed with the covariance of the indicated mean effective pressure (IMEP), which presents values below 2% for all fuels. The effect of ozone seeding in the intake was investigated to control the combustion phasing. In this objective, the cool flame, when it exists, and the main flame phasing are linked to the intake ozone concentration. In this paper, the phasing corresponds to the CAD at which 50% of the total energy of each flame is released. To compare the impact of ozone between each of the fuels tested, the phasing is plotted against the phasing for combustion without ozone in the intake (Figure 4).

Figure 10. Simulated ozone mole fraction in the engine as a function of the CAD for various intake pressures at a constant intake temperature of 30 °C.

F


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Figure 13. Mole fractions of the main radicals and the fuel (at the top) and reaction pathway analysis from rates of consumption (at the bottom) for isooctane (PRF100) at an initial temperature of 800 K, initial pressure of 50 bar, and equivalence ratio of 0.3. Results are presented without ozone on the left and with 10 ppm of ozone on the right.

ozone added in the intake acts directly on the main flame phasing, thus enabling combustion to start more easily and strongly move forward its phasing. Consequently, ozone was seeded with concentrations lower than 10 ppm because CA50 is located before the top dead center and this could damage the engine. Moreover, because of this earlier combustion, experiments were also limited by strong growth of the pressure and, therefore, strong pressure gradients and high levels of noise. However, the effect on the advance of the PRF100 combustion is probably due to the high temperature in the intake, which quickly decomposes ozone. Ozone is a strongly oxidizing chemical species, and it acts mainly on the beginning of combustion. Consequently, the CA05 (crank angle where 5% of the fuel has burned) was observed in this study in comparison to the CA05 for combustion without ozone. Figure 5 shows that, for fuels with cool flame chemistry, the effect is less pronounced than for a fuel without cool flame. Less than 10 ppm of ozone in the intake is necessary to advance the beginning of combustion by approximately 6 CAD with PRF100 as fuel. For other fuels, it is necessary to seed the intake with approximately 30 ppm of ozone to obtain the same shift on the CA05. However, PRF100 does not readily autoignite, and it is necessary to increase the intake temperature to enable its combustion. Knowing that ozone can break down easily under strong temperatures, this effect is probably the main reason for the strong advance

Figure 14. Early reaction paths involved in neat (top) and ozoneseeded (bottom) fuel oxidation.

The present results show that ozone addition lead to earlier fuel combustion. For all of the fuels, except PRF100, where any cool flame occurs on the heat release rate (Figure 3), the effect is more pronounced for the main flame than for the cool flame. This can be explained by the fact that the phasing of the reference combustion used is delayed because of the limited conditions (i.e., without ozone and for limited conditions of intake pressure and temperature). Generally, the advance on the combustion phasing is very visible for ozone concentrations lower than 20 ppm, and the effect is slighter for higher concentrations. Moreover, it can be seen that, for all of the fuels studied, cool flame phasing and main flame phasing follow the same trend, taking into account the standard deviation for each flame phasing, 0.1 and 0.25 CAD, respectively. For ozone concentrations higher than 20 ppm, phasing between each flame follows linear and parallel trends. Finally, for PRF100, G


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Figure 16. Rates of consumption for n-heptane (at the top) and isooctane (at the bottom) with 10 ppm of ozone, initial temperature at 800 K, initial pressure at 25 bar, and equivalence at 0.3.

Figure 15. Mole fraction of the main radicals and rate of consumption of n-heptane (solid line) and isooctane (dash line) for PRF40 with 10 ppm of ozone. The initial temperature, initial pressure, and equivalence ratio were fixed at 800 K, 25 bar, and 0.3, respectively.

Moreover, linear curves between each fuel evolve in parallel. Finally, the maximum cool flame energy released was obtained for PRF0 and then for PRF20 and decreased as a function of the octane number. n-Heptane allows for combustion to release part of its energy in the form of cool flame, while this is not possible for isooctane, as previously observed. Consequently, the PRF mixture contains less and less n-heptane and, hence, releases less and less energy as cool flame. 3.1.3. PRF100 Combustion under Unfavorable Conditions. In view of the effect of ozone on the combustion of PRFs, further experiments were conducted to control HCCI combustion by ozone seeding. Previous results showed that ozone can drastically advance the combustion phasing and the CA50. Moreover, it is well-known that a decrease in the intake temperature or the intake pressure delays the ignition timing. These experiments were carried out with PRF100, because among the PRFs used in this study, only PRF100 easily allows for a decrease in the intake thermodynamic conditions. The intake temperature was set at 100 °C, and the intake pressure was set at 1 bar. According to Figure 2, PRF100 cannot autoignite and it was proposed to use ozone to make its combustion possible. In-cylinder pressure and heat release rate results are presented in Figure 7. As seen, the addition of ozone in the intake of the HCCI engine made it possible to start the autoignition of PRF100; however, a minimum ozone concentration of approximately 56 ppm is required. Moreover, results of the in-cylinder pressure and the heat release rate

observed on the CA05 and the PRF100 main flame phasing compared to the other fuels (Figure 4). With regard to CA50 (Figure 5), the same effect of ozone seeding and similar trends to the main flame phasing (Figure 4) were observed. However, here, CA50 is defined as the CAD over the entire heat release rate presented. Therefore, both parameters are not identical. Because ozone acts on the start of combustion by initiating the first reactions, the effect of ozone addition in the intake on the ratio between the energy released by the cool flame and the energy introduced into the combustion chamber was studied. Results are presented in Figure 6 as a function of the shift of the cool flame phasing and the ozone concentration seeded in the intake of the engine. It was previously shown (see Figure 4) that ozone makes it possible to advance the combustion phasing. With this advance, the ratio between energy released by the cool flame and energy introduced into the combustion chamber linearly increases. As already shown with in-cylinder pressures and heat release rates, the first ozone additions rapidly improve combustion, while further additions will have less effect. For example, in the case of PRF0, an ozone addition of 5 ppm increases the energy released by the cool flame from 8.5 to 10.5%, while with a further addition of 40 ppm, i.e., 45 ppm in the intake, 12% of the energy introduced in the combustion chamber will be released by the cool flame. H


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30 °C for intake pressure variations. Results are presented in Figures 10 and 11. As seen from Figure 10, the intake pressure has a very slight effect on ozone decomposition. Consequently, it is possible to neglect pressure impact during the beginning of combustion, and it explains why the cool and main flame phasing of all of the PRFs except PRF100 follow the same trend. The intake temperature (Figure 11) has a strong effect on the decomposition of ozone. When it increases, ozone decomposition starts earlier and quickly produces O atoms. Therefore, the fuel oxidation by O atoms can start earlier in the cycle. Finally, the intake temperature is the main parameter acting on the sensitivity of the combustion phasing, CA05 or CA50, and it explains why PRF100 is more influenced by ozone addition in comparison to the other PRFs and why the sensitivity decreases with the decrease in the intake temperature. 3.2.3. PRF/Ozone Computations. All of the combustion simulations were conducted for a homogeneous constant volume reactor. Tests were performed for PRF0 and PRF100 at constant intake pressures of 25 and 50 bar, respectively, corresponding to the in-cylinder pressure of the beginning of the main flames and at a constant equivalence ratio of 0.3. Results are presented for these fuels in Figure 12 as a function of the inverse temperature for four ozone concentrations: 0, 1, 10, and 45 ppm. As observed from this figure, the ozone concentration in the mixture makes it possible to act on the main flame ignition timing up to 1000 K. For PRF0, this means that ozone only acts on the cool flame and the negative temperature coefficient, while for PRF100, ozone also acts on the main flame. Moreover, as in the experimental results, a very low ozone concentration (1 ppm) substantially shortens the ignition delay, while when the concentration increases (from 10 to 45 ppm), the ignition delay continues to decrease but the effect is less marked. The description of the oxidation of n-heptane/ozone has already been carried out by Foucher et al.16 In the present paper, experiments were performed with PRF100. Initial temperature, initial pressure, and equivalence ratio were fixed constant at 800 K, 50 bar, and 0.3, respectively. To describe the effect of ozone on isooctane combustion, simulations were carried out with and without ozone. In Figure 13, results show similar trends to those for n-heptane oxidation. Without ozone, isooctane oxidation is achieved by O2 via the reaction C8H18 + O2 → C8H17 + HO2 to form a HO2 radical and an alkyl radical. Then, alkyl radicals (R) rapidly oxidize and OH radicals are produced via the generally accepted hydrocarbon low-temperature oxidation scheme (R + O2 ↔ RO2, RO2 ↔ QOOH, QOOH + O2 ↔ O2QOOH, O2QOOH ↔ HO2Q′OOH, and HO2Q′O2H → 2OH + products). Finally, the fuel is mainly consumed by reaction with OH and HO2. With 10 ppm of ozone, isooctane oxidation is initiated by the reaction C8H18 + O → C8H17 + OH (a), where O atoms come from the decomposition of ozone. Soon after, the fuel consumption mainly proceeds through reactions with O and OH, which is formed from reaction a. Then, the fuel combustion will essentially proceed via reaction with OH. Finally, the fuel is largely and quickly consumed by O and OH compared to the non-seeded case, where the fuel oxidation starts by reaction with O2. Figure 14 gives a schematic view of the early reaction paths involved in neat and ozone-seeded fuel oxidation. Consequently, after ozone addition, OH radicals are formed earlier, allowing for a shortening of the oxidation time and earlier combustion beginning.

present the same trends as the results observed in the beginning of this paper: an enhancement of combustion and an advance of its phasing. However, a higher cycle-to-cycle dispersion was observed (shaded areas). Because these experiments were performed under unfavorable intake pressure and temperature for PRF100 combustion, high ozone seeding (∼50 ppm) was needed. Under these conditions, the ozone concentration in the intake can vary by ±2 ppm, mostly responsible for the observed cycleto-cycle dispersion. A comparison of the shift of the CA50 between these results at low intake pressure and temperature and the results first presented at intake conditions where PRF100 can autoignite without ozone was performed (Figure 8). It can be observed that, at high temperature, low ozone concentrations added in the intake strongly move forward the CA50. However, the sensitivity is better at low temperature, to control the combustion phasing by ozone. Indeed, an addition of 3 ppm of ozone at 180 °C enables an advance of the CA50 by approximately 4 CAD, while at 100 °C, an ozone variation of approximately 35 ppm is necessary to obtain the same improvement. Consequently, the intake temperature is the main cause that influences the sensitivity of the CA50 and the combustion phasing. Moreover, these supplementary experiments show the occurrence of a small cool flame on the heat release rate. In fact, PRF100 cool flame was already observed in the literature30 but not with an internal combustion engine. Without ozone, the ignition timing of the cool flame is high and the rotation speed of the engine is too excessive to allow for the self-ignition of the cool flame. With ozone additions, the ignition timing is strongly reduced and becomes lower than the residence time in the engine, allowing the cool flame to occur. Therefore, this means that ozone additions mainly act on the start of combustion because of the high oxidizing potential of this species. 3.2. Simulation Results. To complete the experimental results and to further investigate the impact of ozone on the PRF, simulations were performed with the help of the Chemkin package20 and the kinetic mechanism described above. 3.2.1. Validation of the Mechanism. First, some computations were conducted to validate the mechanism used in this paper, whereas the PRF oxidation scheme had already been validated for various combustion devices21,22 and the ozone submechanism had been validated on flame speed experiments.12 Ignition delay simulations were performed here for a homogeneous constant volume reactor with and without the ozone submechanism. The results are presented in Figure 9 for the two PRFs at an equivalence ratio of 0.3 and for two intake pressures. It was observed that the presence of the ozone submechanism has no effect on the computed ignition delays. 3.2.2. Ozone Decomposition. To investigate the impact of intake pressure and temperature, simulations were performed using a one-zone internal combustion engine model. The goal of these tests was to show how the intake thermodynamic conditions act on ozone decomposition. Engine characteristics of the model were chosen identical to those of the experimental engine. Simulations were performed in pure compression and for an air mixture seeded by a constant ozone concentration of 10 ppm. The input conditions were similar to the experiments, and simulations were conducted at a constant intake pressure of 1 bar for intake temperature variations and at a constant intake temperature of I


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with a blend fuel (PRF40) was performed and showed that nheptane and isooctane fuels oxidize in the same time but independently. A comparison to single-fuel oxidation showed that the initiation of n-heptane combustion is delayed, whereas that of isooctane combustion is shortened.

Because the experiments performed in this study use nheptane/isooctane mixtures, similar simulations were conducted for PRF40 and 10 ppm of ozone. The conditions were fixed as for isooctane, except for the initial pressure that was fixed at 25 bar, corresponding to the starting pressure for the PRF40 main flame, and the initial concentrations of each fuel were fixed in reference to the definition of the PRF40 and the equivalence ratio of 0.3. Results are presented for the mole fraction of the main radicals and for the rate of consumption of the two fuels in Figure 15. It can be observed that the main radicals produced follow the same trends as isooctane. The only differences concern the characteristic time of the cool flame, which is shortened, and the rate of consumption (ROC), which are increased. With regard to the rates of consumption of each fuel, n-heptane shows the same oxidation routes as isooctane, while the ROC of n-heptane is higher than the ROC of isooctane, because of the initial concentration of each fuel in the mixture and the easier n-heptane oxidation. Knowing that n-heptane can oxidize more easily than isooctane, further simulations were performed to explain the interaction between the two fuels. Simulations were conducted with the same starting conditions as for PRF40 but for two cases: with only n-heptane in the mixture and with only isooctane. Results are presented in Figure 16 and show that the same mechanisms occur but that isooctane combustion occurs later. Consequently, by a comparison to the results of each fuel for PRF40, it can be observed from the PRF40 simulations that the n-heptane mechanism is slightly delayed, while the isooctane mechanism is considerably shortened. For n-heptane, this delay can be explained by its initial concentration in the mixture and the presence of isooctane that consumes a large part of energy to oxidize. For isooctane, its combustion advance is due to n-heptane combustion, because with the start of nheptane combustion, the temperature and the energy increase, making it possible to begin isooctane combustion earlier.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results has received funding from the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007− 2013)/ERC Grant Agreement 291049−2G-Csafe and the region Centre with the European Regional Development Fund.

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REFERENCES

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4. CONCLUSION The effect of ozone on PRF combustion in a HCCI engine has been studied. Experimental results showed that this oxidizing chemical species can improve the combustion of each fuel tested and move forward their phasing. Results on cool and main flame phasing and CA05 and CA50 bring out that injection of low ozone concentrations (below 20 ppm) in the intake of the engine yield a strong combustion advance. For higher ozone additions, the combustion phasing continues to move forward but only moderately. Parallel and linear trends of the increase in the energy released by the cool flame were observed. Moreover, further experiments were performed with PRF100 and showed that ozone can be a promoter of combustion and mainly acts on the initiation of combustion. Finally, ozone seeding in the intake of a HCCI engine represents a new means to control this kind of combustion. Simulations were carried out, first in an internal combustion engine model. Results showed that the main parameter, which acts on the ozone concentration, is the intake temperature and that the effect of the intake pressure can be neglected. These results help explain the experimental similarities and differences observed between PRFs. Other simulations were conducted with a closed constant volume reactor to propose a kinetic interpretation of the effect of ozone on PRF combustion. Results on isooctane combustion with ozone showed that the oxidation paths of this fuel are the same as those for n-heptane oxidation. Consequently, a test J


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