A numerical investigation of the combustion kinetics of

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improved thermal efficiency is primarily due to the low combustion temperature ... Iso-octane (41 mg) was injected through the Bosch gasoline direct injector ...
Fuel 241 (2019) 753–766

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Full Length Article

A numerical investigation of the combustion kinetics of reactivity controlled compression ignition (RCCI) combustion in an optical engine

T

Xinlei Liua,b, Sage Kokjohnb, Yu Lic, Hu Wanga, Hailin Lic, Mingfa Yaoa,

⁎

a

State Key Laboratory of Engines, Tianjin University, No.92 Weijin Road, Nankai District, Tianjin 300072, PR China Engine Research Center, University of Wisconsin-Madison, 1500 Engineering Drive, Madison, Wisconsin 53705, USA c West Virginia University, Morgantown, WV 26506, USA b

ARTICLE INFO

ABSTRACT

Keywords: RCCI Reactivity controlled Combustion kinetics Optical engine Flame structure

This work numerically investigates the detailed combustion kinetics occurring in an optical, reactivity controlled compression ignition (RCCI) engine. Experimental data from the engine combustion network (ECN) relating to spray H and optical RCCI engine spray/combustion were used for model validation. It was found that the RCCI combustion ignition occurred in the squish, bowl rim edge, and downstream of the spray periphery. To provide insight into key reaction pathways, an in-cylinder reaction pathway analysis method was used, and four characteristic RCCI combustion features were selected: (1) initial low temperature heat release (LTHR) from the highreactivity fuel (n-heptane) on the spray periphery; (2) intense LTHR, where both iso-octane and n-heptane were converted to intermediates (e.g., CH2O) through oxygen-related reactions; (3) early stage high temperature heat release (HTHR) with CH2O as the core source species; (4) and intense HTHR, characterized by a substantial energy release. Further analysis of the reactive combustion surfaces demonstrated that the interior flame structure was controlled by OH-CO-O2 combustion kinetics and the exterior was controlled by CH2O-HCO (formyl radical) combustion kinetics. In addition, surrounding the reactive surfaces, iso-octane was consumed primarily through decomposition reactions, forming CH2O, which further fueled the following high temperature combustion.

1. Introduction

ignition (PCI) combustion [4–7]. These strategies typically involve high levels of pre-combustion mixing in order to avoid NOx and soot formation. In the laboratory, numerous strategies have been proposed and have successfully demonstrated high efficiency and ultra-low NOx and soot emissions [1]. For example, homogeneous charge compression ignition (HCCI) is considered an ideal combustion strategy, characterized by a lean mixture, low temperature, and constant-volume combustion. HCCI avoids soot and NOx formation, while maintaining high fuel economy [4]. However, combustion phasing and duration control make its practical application difficult so that most HCCI efforts are confined to low load conditions [8]. To address the control challenges of HCCI combustion, Kokjohn

Future internal combustion engines (ICE) must offer high efficiency, allow for the use of renewable fuels, and achieve ultra-low pollutant emissions [1,2]. Compression ignition (CI) engines typically show a higher efficiency than their spark-ignited (SI) counterparts due to their ability to use a higher compression ratio and their lean operation; however, under conventional, mixing-controlled operation, these engines struggle to meet NOx and soot regulations, while simultaneously maintaining their efficiency advantage over SI engines [3]. Accordingly, numerous researchers have investigated advanced combustion strategies that can be loosely characterized as premixed compression

Abbreviations: AC8H17, Octyl radical; AMR, Adaptive mesh refinement; CA ATDC, Crank angle after top dead center; C7H15-2, Heptyl radical; CFD, Computational fluid dynamics; CH2O, Formaldehyde; CI, Compression ignition; CO2, Carbon dioxide; CR, Common rail; ECN, Engine combustion network; GDI, Gasoline direct injector; H2O, Water; H2O2, Hydroperoxide; HCCI, Homogeneous charge compression ignition; HCO, Formyl radical; HCs, Hydrocarbons; HO2, Hydroperoxyl radical; HTHR, High temperature heat release; ICE, Internal combustion engine; ISDP, Instantaneous species destruction pathway; KH-RT, Kelvin-Helmholtz Rayleigh-Taylor; LTC, Low temperature combustion; LTHR, Low temperature heat release; N2, Nitrogen; NTC, No time counter; OH, Hydroxyl radical; PCI, Premixed compression ignition; ϕ, Equivalence ratio; PLIF, Planar laser-induced fluorescence; PRF, Primary reference fuel; RCCI, Reactivity controlled compression ignition; RCR, Representative creation reaction; RDR, Representative destruction reaction; RNG, Renormalization group; ROP, Rate of production; RXR, Representative exothermic reaction; SI, Spark-ignited; SOI, Start of injection; Spray H, N-heptane spray; 0-D, Zero-dimensional; 3-D, Three-dimensional ⁎ Corresponding author. E-mail address: [email protected] (M. Yao). https://doi.org/10.1016/j.fuel.2018.12.068 Received 4 November 2018; Received in revised form 11 December 2018; Accepted 13 December 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.

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et al. [9,10] proposed the blending of two fuels with different in-cylinder auto-ignition characteristics. They termed this combustion mode reactivity controlled compression ignition (RCCI) combustion. In the RCCI mode, the combustion phasing is controlled by the mass fractions of the two fuels and the combustion duration is controlled by introducing spatial stratification into the fuel reactivity. Numerous researchers have studied RCCI combustion and demonstrated its potential to achieve controlled PCI combustion and ultra-low NOx and soot emissions [5,6,11]. For example, metal engine experiments have shown that US 2010 emission targets (soot and NOx emissions) could be fulfilled by an RCCI engine with indicated efficiencies of greater than 50% [6,11]. Recently, the source of RCCI combustion’s high thermal efficiency has been clarified by exergy analysis [12]. It was found that the improved thermal efficiency is primarily due to the low combustion temperature, which reduces exergy destruction from heat transfer and exhaust losses. To provide insight into the basic fundamentals of the combustion, optical diagnostic techniques have provided an efficient means by which to directly investigate the engine spray, combustion, flame/turbulence interaction and pollutant formation processes, shedding light on the underlying physical and chemical mechanisms and contributing to the development of numerical models [3,13–16]. Kokjohn et al. [8] used a combination of optical diagnostics and kinetics modeling to identify the factors controlling the ignition and growth of the reaction zone for an optically accessible RCCI engine. They revealed the dominant role of the fuel’s PRF (primary reference fuel) number in RCCI combustion ignition. Later, by using multiple laser diagnostics, Tang et al. [14,17] evaluated the impact of mixture stratification on the RCCI combustion process using formaldehyde (CH2O) and hydroxyl (OH) radical planar laser-induced fluorescence (PLIF) imaging. It was demonstrated that the increased stratification resulted in lower rates of heat release due to a staged combustion process. However, the optical engine observations only focused on a number of specific species, making it difficult to fully clarify the engine combustion details. Zero-dimensional (0-D) sensitivity analyses and rate of production analyses can be performed to provide insight into reaction pathways; however, these analyses are typically limited to homogeneous reactor conditions, which can be significantly different from the complicated engine spray-combustion environment [18–21]. Threedimensional (3-D) computational fluid dynamics (CFD) simulations, however, can be used to determine all of the species concentrations [22–24]. To provide additional insight into the combustion details, the present effort will use CFD modeling coupled with in-cylinder reaction pathway analysis [24] to investigate the dominant combustion characteristics of an engine operating in the RCCI combustion mode. The results of the studies will advance our understanding of the dominant reaction pathways controlling RCCI combustion and provide insight into the overall combustion characteristics.

Table 1 Spray H injection parameters. Nozzle diameter (mm) Injection pressure (bar) Injection mass (mg) Injection duration (ms) Discharge coefficient Fuel temperature (K)

0.10 1500 17.8 6.8 0.80 373

2.2. Sandia RCCI combustion 2.2.1. Engine specifications The RCCI combustion experiment that will be analyzed in this work was performed by Kokjohn et al. [8,26] in an optically accessible, single-cylinder engine (Cummins N14 diesel CI engine). Fig. 1 shows a schematic of the engine and Table 2 gives the engine specifications. The original metal piston was replaced with an extended piston with a fused silica bowl, providing a view of the combustion chamber, and one of the two exhaust valves was modified with an optical window and a periscope mirror in the rocker box, providing a view of the squish region. Details of the engine setup can be found in Kokjohn et al. [8]. 2.2.2. Operating conditions During the experiment, the engine was controlled at 1200 rev/min and maintained at 4.2 bar (gross indicated mean effective pressure), corresponding to a low load condition. To protect the optical windows, it was fired 1 out of 10 cycles via control of the electric dynamometer. To achieve RCCI combustion, n-heptane and iso-octane were used as the high and low reactivity fuels, respectively. Table 3 lists the injection parameters. Iso-octane (41 mg) was injected through the Bosch gasoline direct injector (GDI, 100 bar) at −240° crank angle after top dead center (CA ATDC) during the intake stroke, creating a relatively wellmixed charge, and n-heptane was injected using the common rail (CR, 600 bar) at two timings, −57° and −37° CA ATDC, respectively. This injection strategy was developed based on a previous CFD optimization investigation [9]. Intake air pressure and temperature were kept at 1.1 bar and 363 K, respectively, without exhaust gas recirculation, and due to the 9 motored cycles between each fired cycle, it can be assumed that there was no residual burnt gas trapped in the chamber at intake valve closure. 3. Computational setup 3.1. Numerical models The 3-D CFD engine combustion modelling was performed using CONVERGE [27] with the renormalization group (RNG) k- model adopted for the calculation of turbulence [28]. The spray model uses the Lagrangian-drop and Eulerian-fluid methods [29] and a KelvinHelmholtz Rayleigh-Taylor (KH-RT) model without breakup length was used to model the spray breakup process [30]. The no time counter (NTC) algorithm of Schmidt and Rutland was used to model droplet collisions [31], the Frossling correlation was used to model droplet evaporation [32], and a dynamic drag model with a drag coefficient dependent on droplet shape was adopted [33]. Furthermore, the effects of the dispersed phase on the gas-phase turbulence were considered as source terms in the turbulence model [34]. Details of these various submodels can be found in Som’s thesis [35]. For combustion modelling, the SAGE detailed chemical kinetics solver was used to model the reaction chemistry [36], with the physical properties of the fuels represented by those of n-heptane and iso-octane predefined in CONVERGE. The reduced PRF combustion kinetic mechanism developed by Wang et al. [37] and containing 109 species and 543 reactions was utilized to model the fuel combustion kinetics.

2. Experimental data 2.1. ECN non-combustion spray H Prior to the engine combustion modelling studies, experimental data relating to spray H (n-heptane) from the engine combustion network (ECN) [25] were utilized to calibrate the spray models. The noncombustion spray H experiment was performed in a 108 × 108 mm constant volume chamber in which the bulk temperature and pressure were maintained at 967 K and 43.3 bar, respectively. The initial gases were nitrogen (N2, 89.71%, vol.), carbon dioxide (CO2, 6.52%, vol.), and water (H2O, 3.77%, vol.) to inhibit combustion. The injection parameters are listed in Table 1. Both the experimental liquid and vapor penetration lengths were collected for spray calibrations.

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Fig. 1. Schematic of the optically accessible research engine [26]. Left: high-speed chemiluminescence imaging; right: PLIF imaging.

reaction. Furthermore, for a specific species, the representative reactions accounting for its destruction and creation in each computational cell were also calculated and termed the representative destruction reaction (RDR) and creation reaction (RCR), respectively. By visualizing and analyzing the RDRs and RCRs of the important species (such as n-heptane and iso-octane), some significant combustion characteristics, including the combustion ignition and development, can be explained. In the current work, the instantaneous species destruction pathway (ISDP) was calculated using the contribution of each reaction to the whole creation or destruction process for a specific species by

Table 2 Sandia optical engine specifications. Engine type Engine speed (rpm) Number of cylinder Number of valves Bore/stroke (mm) Connecting rod length (mm) Displacement (L) Swirl ratio Compression ratio Common rail injector Bosch GDI injector

Cummins N-14 1200 1 3a 139.7/152.4 304.8 2.34 About 0.5 10.75:1 8 holes, included angle 152°, 600 bar, 0.14 mm nozzle 7 holes, 100 bar, 0.15 mm nozzle

i, j

a

denotes 2 intake valves and 1 exhaust valve. The other (single) exhaust valve has been modified.

i, j

,

i, j

(1)

where i, j is the normalized creation/destruction rate of the ith species by the jth reaction, i, j is the actual creation/destruction rate of the ith species by the jth reaction, and i, j is the total creation/destruction rate of the ith species by all the reactions. The representative exothermic reaction (RXR) in each cell was identified by examining the reaction that made the highest contribution to the total heat release, which reaction was found to be quite significant for elucidation of the flame structure. Table 4 summarizes the major reactions identified by the RDR and RXR analyses in this work.

Table 3 Injection parameters. GDI SOI (CA ATDC) CR SOI1/SOI2 (CA ATDC) Total fuel mass (mg/cyc) Iso-octane mass (%) N-heptane mass (%) SOI1 mass (mg/cyc) SOI2 mass (mg/cyc)

=

−240 −57/−37 62 66% 34% 12.6 8.4

3.3. Mesh details For spray H modelling, a cylinder with both a height and bottom diameter of 108 mm was used and for the optical engine combustion simulation an eighth-sector mesh was adopted [26], considering the high computational memory requirement for the ISDP calculation. To prove the feasibility for the sector mesh, a whole mesh was set as the base case for comparisons. The two approaches were validated by comparing to the measured mixture distribution data. CONVERGE adopts an innovative grid generation method, called adaptive mesh refinement (AMR), which can embed finer cells automatically in regions where the flow field needs to be resolved in more detail [29]. As a result, it is possible to use an appropriate base mesh with a coarser scale, which is beneficial for saving computational time. In this work, a base mesh of 2 mm was used for the modelling cases

3.2. Combustion visualization method Firstly, the calculated results for the temperature, pressure, mass, volume, and all of the species concentrations in each computational cell were exported to MATLAB and were processed into column format. Then, the CHEMKIN code was called by MATLAB with the 3-D CFD results as inputs. The reduced combustion mechanism was coupled with CHEMKIN to calculate the instantaneous combustion details [23,24]. For rate of production (ROP) calculations, each computational cell was considered as a perfectly stirred reactor with three main features: the destruction and creation rates for each species, the forward/reverse reaction rates of all reactions, and the chemical heat release for each 755

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Table 4 List of the key RDR and RXR reactions. R3 R4 R5 R8 R10 R11 R16 R17 R18

NC7H16 + O = C7H15-2 + OH NC7H16 + OH = C7H15-2 + H2O NC7H16 + HO2 = C7H15-2 + H2O2 C7H15O2-2 = C7H15-2 + O2 C7H14OOH2-4O2 = C7H14OOH2-4 + O2 C7H14OOH2-4O2 = NC7KET24 + OH IC8H18 + OH = AC8H17 + H2O IC8H18 + HO2 = AC8H17 + H2O2 IC8H18 + O2 = AC8H17 + HO2

R22 R244 R256 R261 R265 R276 R285 R295

AC8H17O2 = AC8H17 + O2 CH3 + O2(+M) = CH3O2(+M) O + H2O = 2OH H + O2(+M) = HO2(+M) HO2 + OH = H2O + O2 CO + OH = CO2 + H HCO + O2 = CO + HO2 CH2O + OH = HCO + H2O

[29]. Grid sensitivity analyses for the spray calibrations were performed with the AMR scales ranging from 1 to 4, corresponding to minimum mesh sizes of 1.0–0.125 mm. Both the velocity and n-heptane species concentration fields were employed during AMR and for engine combustion cases, the temperature field was also considered for AMR. 4. Results and discussions 4.1. Spray H Data from Sandia spray H [25] was used to evaluate the model predictions and to assess the grid sensitivity. Different minimum AMR sizes from 1.0 to 0.125 mm were tested. For the simulations, the liquid penetration length was defined as the axial distance from the nozzle of 95% of the liquid fuel mass and the vapor penetration length was defined as the maximum distance at 0.1% of the fuel mass fraction from the nozzle. Fig. 2 compares the experimental and predicted penetration lengths. We can see that almost all the simulations were able to capture the liquid penetration length well. However, for the vapor penetration

Fig. 3. Comparison of predicted mixture distributions at 1.0 ms, with the measured vapor boundary (black lines) also depicted.

length, the coarsest mesh (AMR1) exhibited a significantly different prediction performance compared with the other finer meshes. With the AMR scale lower than 2, corresponding to a minimum mesh size of 0.5 mm, the predicted vapor penetration length was found to be much higher than the experimental value, although the predictions did not exhibit much variation with a minimum mesh size lower than 0.25 mm, in agreement with the work of Senecal et al. [29]. Fig. 3 shows the predicted mixture distributions and the measured vapor boundary. In line with the previous discussion, the spray structures penetrate further downstream for the coarser mesh, exhibiting a narrower cone angle, which is believed to be due to under-resolution of the droplet drag in the near nozzle region. At an AMR scale of 3

Fig. 2. Comparison of experimental and predicted liquid and vapor penetrations. 756

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Fig. 4. Comparison of predicted and experimental PRF number distributions at different crank angles at a location 10 mm below the fire deck. Gray lines represent: left, view window border; middle, piston bowl rim; right, cylinder liner.

(minimum cell size equals to 0.25 mm), the simulation adequately reproduces the experimental spray structure. Refining the mesh further, to a minimum cell size of 0.125 mm, yields little change in the spray structure. Accordingly, all results presented here use a minimum cell size of 0.25 mm.

4.2. Optical engine study 4.2.1. Fuel mixture distribution The validation of the spray and mixing models was performed by comparing the modelling results to the experimental data of Kokjohn et al. [8]. Fig. 4 shows the measured and predicted PRF number distributions during the n-heptane injection process. Simulations were 757

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Fig. 5. Comparison of predicted and experimental equivalence ratio (ϕ) distributions at a location 13 mm below the fire deck. Gray lines represent: left, view window border; middle, piston bowl rim; right, cylinder liner.

performed with both a sector and full three-dimensional mesh to assess the impact of the sector assumption on the spray and mixing results. Overall, the spray processes are suitably predicted by both the sector and full mesh. Although the sector mesh has a slightly higher squish region (about 1 mm) due to the absence of an optical slot compared with the full mesh, this has only a minor influence on the mixture distribution. However, despite of the good prediction for the mixture distribution shape, the minimum PRF number is over-predicted by the current models. This may be due to the experimental errors, or alternatively due to the spray sub-models related to the spray break-up and wall interaction processes, which require more improvements in the future. Fig. 5 compares the experimental and predicted equivalence ratio ( ) distributions after −21 CA ATDC, excluding the sector mesh results, as the latter does not have a slot for the introduction of the PLIF laser introduction. Overall, the equivalence distributions are well captured. Owing to the in-cylinder swirl flow, the mixture in the downstream region is pushed downward in a clockwise direction. As seen from −21 to −5 CA ATDC, the charge continues mixing with the ambient

Fig. 6. Comparison of predicted and experimental in-cylinder pressure and heat release rate profiles.

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Fig. 7. Comparison of the predicted temperature distributions colored by iso-volume of temperature higher than 1000 K (red), and experimental chemiluminescence images (left: piston crown window; right: cylinder head window). Note: the sector mesh has been duplicated by 7 times. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 8. Distributions of destruction rate (mole/m3/s), mole fraction and RDR of n-heptane and iso-octane, temperature, heat release rate (W/m3, log), and mole fractions of OH, CH2O, CO, and HO2 at −18.5 CA ATDC.

Fig. 9. Reaction rate constants of R3, R4, and R5 for NC7H16 and R16, R17, and R18 for IC8H18.

mixtures, resulting in the lower equivalence ratios, while the high equivalence ratio region remains in the squish region, indicating the high reactivity there. The above validations confirm that the current spray model can adequately predict the in-cylinder equivalence ratio distributions and

can reasonably describe the mixture reactivity variations. The comparisons between the sector and full mesh show that the sector mesh yields similar mixture distribution predictions as are those obtained with the whole mesh. As a consequence, the sector mesh was adopted for further combustion modelling investigations. 759

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region and inside the piston bowl near the rim. After the ignition, further ignition pockets are observed, and then the high temperature combustion grows rapidly, finally propagating to fill the entire combustion chamber by 5 CA ATDC, which processes were well predicted by the modelling results. However, discrepancies with the experimental results can be observed regarding the ignition location on the spray downstream, for which the experiment exhibits wider distributions that are further downstream while the models exhibit longer and thinner distributions. This may be attributed to the discrepancies in the high/ low reactivity fuel mixture distributions. 4.2.3. Combustion kinetics visualization Kokjohn et al. [8] showed that PRF number stratification has a larger effect on RCCI combustion ignition than equivalence ratio or temperature. Since a lower PRF number indicates a higher n-heptane mass fraction and a shorter ignition delay, n-heptane should be a significant factor affecting the system reactivity. In this section, detailed combustion kinetics analyses are performed to explain the effects of the high reactivity fuel (n-heptane) on the combustion ignition and the following combustion development. Four important combustion periods were selected for the further combustion kinetics analyses, −18.5, −16.5, −5.0, and −1.0 CA ATDC, corresponding to the initial low temperature heat release (LTHR), intense LTHR, initial high temperature heat release (HTHR), and intense HTHR, respectively.

Fig. 10. RXR distribution at −18.5 CA ATDC.

4.2.3.1. Initial LTHR (−18.5 CA ATDC). At −18.5 CA ATDC, the incylinder averaged temperature is about 750 K, corresponding to the typical low temperature combustion (LTC) condition of n-heptane [38]. Fig. 8 shows distributions of the species mole fractions, fuel destruction rate and RDR, temperature, and exothermic heat release rate. The second n-heptane spray impinges on the bowl rim and squish region, leading to a cooler region in the spray core owing to evaporation while the spray periphery has been well mixed with the iso-octane charge at a higher temperature. As a consequence, the spray wings exhibit stronger LTHR, resulting in the higher destruction rates of n-heptane and isooctane and the consequently higher heat release. Comparatively, however, the destruction of iso-octane is much lower than that of nheptane due to its lower reactivity. Fig. 8 also shows the representative destruction reactions for nheptane and iso-octane in each computational cell. Both n-heptane and iso-octane are mainly consumed through the H-abstraction reactions of OH (R4 and R16, respectively). Fig. 9 compares the reaction rate constants of the main consumption reactions for n-heptane and iso-octane. The H-abstraction reactions of OH exhibit the highest reaction rate constants at 750 K, higher than the other three kinds of H-abstraction reactions. For n-heptane, R3 exhibits a slightly lower reaction rate than R4, since the O concentration (not shown) is significantly lower than that of OH. Although the hydroperoxyl radical (HO2) concentration is higher than OH, their levels are comparable. Therefore, the H-abstraction reactions (R4 and R16) by OH account for the majority of the consumption of both fuels.

Fig. 11. Reaction pathway analysis of NC7H16 and IC8H18 at −18.5 CA ATDC. Reactions in blue color represent RXR. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

4.2.2. Combustion evaluation Fig. 6 shows a comparison of experimental and predicted in-cylinder pressure and heat release profiles. Generally, a reasonable agreement with the experimental results is observed, both exhibiting an obvious low temperature heat release and an abrupt subsequent high temperature heat release. Fig. 7 illustrates the experimental RCCI combustion chemiluminescence images, with the left image representing the piston-crown window and the right the cylinder-head window (squish region). The high temperature combustion ignition first occurred in a region downstream of the n-heptane jet, both in the squish

Fig. 12. ROPs of O2 and OH at −18.5 CA ATDC. Reactions in blue color represent RXR. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 760

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Fig. 13. Distributions of destruction rate (mole/m3/s), mole fraction and RDR of n-heptane and iso-octane, temperature, heat release rate (W/m3, log), and mole fractions of OH, CH2O, CO, and HO2 at −16.5 CA ATDC.

Fig. 14. RXR distribution at −16.5 CA ATDC.

To further explain the LTHR occurring during RCCI combustion, Fig. 10 evaluates the representative exothermic reactions in each computational cell, from which information relating to the fuel reactivity interaction can be observed. In addition, Fig. 11 depicts the reaction pathways of n-heptane and iso-octane. Combined with the heat release rate distribution shown in Fig. 8, it is noticed that most of the heat released is derived from reactions R10, R8, and R11, all belonging to the significant low temperature consumption pathway of n-heptane, as shown in Fig. 11. The iso-octane also experiences subtle low temperature consumption, mainly through reaction R22. Due to the higher OH concentration near the n-heptane spray periphery owing to the heptane LTC, some iso-octane is consumed there and converted to octyl radical (AC8H17), which can be categorized as a fuel reactivity interaction via reactive radical pools (OH) [39]. Fig. 12 shows the consumption and production pathways of O2 and OH (radical pool), revealing that the major O2 consumption reactions related to the LTC chemistries of n-heptane and iso-octane contribute to the heat release directly. Most of the OH is produced from the low temperature consumption pathways of n-heptane by the reactions R11 (C7H14OOH2-4O2 = NC7KET24 + OH) and R12 (NC7KET24 = CH3COCH2 + NC3H7CHO + OH), beneficial for iso-octane destruction through H-abstraction by OH (R16).

Fig. 15. Reaction pathway analysis of NC7H16 and IC8H18 at −16.5 CA ATDC. Reactions in blue color represent RXR. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

4.2.3.2. Intense LTHR (-16.5 CA ATDC). Fig. 13 shows the distributions of species concentrations, fuel destruction rate and RDR, temperature, and exothermic heat release rate at −16.5 CA ATDC. By this time, the averaged temperature grows to about 810 K and intense LTHR can be observed, which is characterized by the large amount of CH2O formation. Clearly, n-heptane in the spray core region plays a more important role in intense LTHR than fuel in the spray periphery. Meanwhile, the iso-octane destruction shape is still in good agreement with that of n-heptane, indicating the high reactivity fuel controlled combustion pattern. However, due to the low in-cylinder temperature, consumption of n-heptane and iso-octane are controlled by H-abstraction reactions through OH (R4 and R16). Compared with 761

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Fig. 16. ROPs of O2 and OH at −16.5 CA ATDC. Reactions in blue color represent RXR. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 17. Distributions of destruction rate (mole/m3/s), mole fraction and RDR of n-heptane and iso-octane, temperature, heat release rate (W/m3, log), and mole fractions of OH, CH2O, CO, and HO2 at −5.0 CA ATDC.

reactions changing from the initial LTHR reactions (R8, R10, and R11) to smaller molecule reactions (R95, R97, R244, and R285). In addition, all of these heat release reactions are related to O2, implying the significance of the fuel mixing process for the low temperature combustion characteristics. Fig. 15 illustrates the reaction pathways integrated over the whole combustion region, and Fig. 16 shows the ROP analyses of O2 and OH. In addition to reactions R11 and R12, the OH formation is dominated by reaction R98 (C3KET21 = CH2O + CH3CO + OH), contributing to the consumption of n-heptane and iso-octane and the formation of CH2O. This indicates that the combustion of iso-octane is still controlled through the OH pool, which is mainly derived from the nheptane low temperature pathway. According to the reaction pathway analyses of n-heptane and isooctane, we can see that most of the fuel is eventually converted into CH2O, which is an important LTHR characteristic and forms the basis for the following HTHR [38]. However, major differences can be observed due to the higher temperature atmosphere at −16.5 CA ATDC compared to −18.5 CA ATDC. The formed heptyl and octyl radicals (C7H15-2 and AC8H17) are primarily consumed by decomposition reactions instead of O2-addition reactions, although the LTHR intermediates still play an important role during combustion.

Fig. 18. RXR distribution at −5.0 CA ATDC with a temperature higher than 1000 K.

−18.5 CA ATDC, an obviously different heat release distribution can be observed, with the highest heat release derived instead from the nheptane spray core, bowl rim, and squish regions. As a consequence of the LTHR reactions, substantial CH2O is formed around the high heat release region. Fig. 14 shows the RXR at −16.5 CA ATDC. Compared to the values at −18.5 CA ATDC, heat release from the higher temperature core region exhibits an entirely different feature, with the major heat release

4.2.3.3. Initial HTHR ignition (−5.0 CA ATDC). After the end of LTHR, the in-cylinder combustion continues to develop mildly. By −5.0 CA ATDC, the averaged temperature rises to about 900 K, at which point 762

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1055 K. The high temperature heat release is predominantly controlled by the reaction R285 (HCO + O2 = CO + HO2), which is part of one of the most significant consumption pathways for CH2O. Fig. 19 shows the reaction pathway analysis. Overall, both fuels are predominantly converted to CH2O through decomposition reactions, without experiencing low temperature consumption pathways due to the high temperature, resulting in CH2O accumulation. Afterwards, CH2O is consumed by Habstraction via OH (R295), forming the formyl radical (HCO), which instantaneously generates CO by R285, leading to substantial heat release. Fig. 20 shows the ROP analyses for OH and HO2. Most of the OH is formed via the reaction R268 (H2O2(+M) = 2OH(+M)), a significantly different feature compared to the LTHR, for which OH is mainly produced by the n-heptane low temperature reactions. Meanwhile, HO2 plays an important role in hydroperoxide (H2O2) formation and fuel consumption, which is principally created by R285. Consequently, cyclic reactions are formed by R295, R285, R267, and R268, with CH2O as the source core species, laying the foundation for the intense HTHR. 4.2.3.4. Intense HTHR (−1.0 CA ATDC). Fig. 21 shows the distributions of species mole fractions, fuel destruction rate and RDR, temperature, and heat release rate at −1.0 CA ATDC. After high temperature ignition, hydrocarbons (HCs) confined in the ignition pockets are promptly consumed, producing H2O and CO2. N-heptane, iso-octane, CH2O, and CO are all depleted rapidly in these ignition pockets, generating high temperatures and thus leaving some postcombustion zones, which phenomena are in agreement with the optical PLIF observations of Tang et al. [14]. Consistent with the high temperature ignition, n-heptane and iso-octane are still mainly destroyed by H-abstraction via HO2 (R5 and R17) for the higher incylinder averaged temperature (990 K). Meanwhile, intense exothermic reactions play an important role at the high temperature reacting surface, characterized by the high destruction rates of n-heptane and iso-octane. Fig. 21 showed that the rapid destruction of iso-octane is synchronized with the high heat release rate; accordingly, the region where the destruction rate of iso-octane is higher than 4e−5 (mole/m3/s) is extracted in order to investigate the combustion structure of the high temperature reacting surfaces. Fig. 22 shows the growth of the reacting surfaces for three crank angles (−4, −3, and −1 CA ATDC). The enlarged diagram at −1.0 CA ATDC shows that the high temperature combustion reactive surface is mainly composed of two parts:

Fig. 19. Reaction pathway analysis of NC7H16 and IC8H18 at −5.0 CA ATDC. Reactions in blue color represent RXR. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

the high temperature ignition occurs, as indicated by the abrupt increase of OH and temperature. Fig. 17 shows the distributions of species concentrations, fuel destruction rate and RDR, temperature, and exothermic heat release rate at −5.0 CA ATDC. Obviously, the high temperature ignition occurs in the squish and bowl rim regions, characterized by high destruction rates of n-heptane and iso-octane. By this time, the n-heptane has been consumed to a much lower level, leading to considerable CH2O accumulation, in accordance with the optical observations of Kokjohn [26]. Due to the higher temperature and HO2 mole fraction, most of the fuel in the higher temperature region are consumed by the H-abstraction reactions through HO2 (R5 and R17) instead of OH. Fig. 18 shows the RXR distribution for the region where the temperature is higher than 1000 K, with an averaged temperature at about

1) the interior characterized by the OH-CO-O2 combustion kinetics (reactions R256, R261, R265, and R276) and 2) the exterior characterized by the CH2O-HCO combustion kinetics (reactions R285 and R295).

Fig. 20. ROPs of OH and HO2 at −5.0 CA ATDC. Reactions in blue color represent RXR. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 21. Distributions of destruction rate (mole/m3/s), mole fraction and RDR of n-heptane and iso-octane, temperature, heat release rate (W/m3, log), and mole fractions of OH, CH2O, CO, and HO2 at −1.0 CA ATDC.

Fig. 22. RXR distributions at −4, −3 and −1.0 CA ATDC.

5. Conclusions A numerical investigation was performed to clarify the detailed combustion kinetics on an optical RCCI engine. Firstly, important spray and in-cylinder fuel distribution characteristics were studied. Then, RCCI combustion modelling research was performed. Based on the combustion kinetics visualization method, four important combustion timings were selected to clarify the RCCI combustion characteristics. More insights into the dominant reaction pathways controlling RCCI combustion were presented, which will further advance our understanding of the RCCI combustion characteristics. Major conclusions are as follows:

Fig. 23. Reaction pathway analysis at −1.0 CA ATDC. Reactions in blue color represent RXR. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

(1) The mesh sensitivity study showed that a minimum mesh of 0.25 mm is sufficient to predict the fuel spray distribution. In addition, the RCCI spray and combustion processes were well captured by the model, with ignition occurring in the squish, bowl rim edge, and downstream of the spray periphery, in reasonable agreement with the experimental measurements. (2) The initial LTC ignition of RCCI was controlled by the high-reactivity fuel spray-combustion process, with the primary heat release coming from the n-heptane LTC process on the spray periphery, which generated OH and enhanced the system reactivity accordingly. (3) O2 was a significant factor during the intense LTHR, dominating the representative exothermic reactions. Through the LTC pathways, both of the fuels were converted to CH2O, in preparation for the high temperature ignition. (4) The high temperature ignition was dominated by the reaction R285

Surrounding the reactive surfaces, iso-octane is consumed by decomposition reactions to generate CH2O, which will further fuel the following high temperature combustion propagation. Fig. 23 illustrates the reaction pathway analysis integrated over the reaction surface, and Fig. 24 illustrates the ROP analyses for HO2 and OH at −1.0 CA ATDC. Similar to the high temperature ignition timing, mutual dependence of the reaction pathways of CH2O and O2 can be observed. The H atom in OH is primarily derived from CH2O via reactions R285 and R276, consuming O2 and leading to the formation of CO and HO2, as well as CO2 and H, respectively. While CH2O is mostly consumed via the reaction R295 through OH, of which the O atom is originally from O2. 764

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Fig. 24. ROPs of OH and HO2 at −1.0 CA ATDC. Reactions in blue color represent RXR. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

(HCO + O2 = CO + HO2), an important consumption pathway of CH2O, accompanied by the cyclic consumption and production of OH and HO2 radicals, laying a significant foundation for intense HTHR. (5) The intense HTHR was characterized by the high temperature reacting surfaces, with the interior featuring the OH-CO-O2 combustion kinetics and the exterior the CH2O-HCO combustion kinetics. Surrounding these reactive surfaces, the less reactive fuel (iso-octane) was primarily consumed by decomposition reactions generating CH2O, which fueled the further high temperature combustion propagation process.

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