Modeling knocking combustion in hydrogen assisted compression ignition diesel engines

Energy 76 (2014) 768e779

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Energy journal homepage: www.elsevier.com/locate/energy

Modeling knocking combustion in hydrogen assisted compression ignition diesel engines Amin Maghbouli a, Wenming Yang a, *, Hui An a, Sina Shafee b, Jing Li a, Samira Mohammadi c a b c

Department of Mechanical Engineering, Faculty of Engineering, National University of Singapore, Singapore Department of Mechanical Engineering, Faculty of Natural and Applied Sciences, Middle East Technical University, Ankara, Turkey Department of Chemical Engineering, Amirkabir University of Technology, Tehran, Iran

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 September 2013 Received in revised form 1 March 2014 Accepted 21 August 2014 Available online 16 September 2014

In the present study, effects of hydrogen induction on combustion characteristics of a compression ignition diesel engine were investigated and a comprehensive model for identifying knocking combustion was developed. This was done by defining number of critical local regions within the CFD (computational fluid dynamics) computational domain for a hydrogen assisted compression ignition engine. Regional parameters such as local pressure rise rate, local heat release rate and local concentration change of specific chemical species were used for knock identification. Comprehensive chemical kinetics mechanisms of diesel and hydrogen fuels were used enabling detailed chemistry predictions. After validation of the model for extensive diesel operating conditions; 1%, 3%, 5% and 7% hydrogen induction in volume in intake air was considered for a single case to investigate knocking combustion. Using the developed knock prediction model, results showed knocking combustion for hydrogen-air premixed charges richer than 5% by volume. This was well captured by the regional pressure rise rate and heat release rate diagrams. Moreover, regional data showed that knock occurred in central parts of the piston bowl and above the piston crown, whereas location near to cylinder wall did not show the same trend. In former locations, very high rate of production and consumption for HO2 as a free radical was resulted. This was coincided with higher hydrogen consumption and temperature rise. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Hydrogen Compression ignition Local regions Knock modeling

1. Introduction Gigantic air pollution dilemmas, daily increasing energy demand and fast depletion of conventional fuels requires urgent introduction of alternative fuels with nearly zero environmental impact. In this regard, transportation and internal combustion engines are the main consumers of the fossil fuels. Although various studies on alternative power generation units and technologies such as fuel-cells and hybrid-automobiles are carrying out worldwide, they do not seem to completely take the role of internal combustion engines in the near future. Therefore, a considerable deal of research is concentrated on optimization of conventional engines and introduction of alternative fuels to meet increasing power and low emissions demand. Compression ignition engines operating by gaseous fuels exhibit higher power density and lower specific emissions compared to dedicated diesel engines; however,

* Corresponding author. Tel.: þ65 65166481. E-mail address: [email protected] (W. Yang). http://dx.doi.org/10.1016/j.energy.2014.08.074 0360-5442/© 2014 Elsevier Ltd. All rights reserved.

high intake temperature and high compression ratio combined with high engine load may cause engine experiencing abnormal combustion declared as engine knock. If gaseous fuel is used in the diesel engines, increasing its amount strengthens the possibility of engine knock, thereby reducing output power. This is potentially a limiting factor on engine downsizing and hinders engine manufacturers from achieving higher indicated power. Gaseous fuels such as hydrogen, CNG (compressed natural gas), biogas, producer gas and etc. can be used in compression ignition engines lowering diesel fuel consumption and emissions due to lower carbon to hydrogen ratio in their molecular structures. Among mentioned gaseous fuels, hydrogen holds a noticeable advantage for internal combustion engines due to its renewable nature and zero carbon based emissions. Hydrogen has a wide flammability range which enables hydrogen engines/hydrogen assisted engines to ignite under lean operating mixtures; however, this type of engines are more prone to the engine knock [1]. Hot exhaust gases from previous cycle and glowing combustion products can act as an ignition source and cause early auto-ignition of premixed hydrogen-air mixtures before the flame arrival [2]. Saravanan et al.

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Nomenclature

Abbreviations ATDC after top dead center CAD crank angle degree CFD computational fluid dynamics CO carbon monoxide CO2 carbon dioxide DI direct injection EGR exhaust gas recirculation EVO exhaust valve open HCCI homogeneous charge compression ignition HRR heat release rate IVC inlet valve closure LPG liquefied petroleum gas NOx nitrogen oxides UHC unburnt hydrocarbons PRR pressure rise rate SI spark ignition SOI start of injection

experimentally investigated effects of hydrogen port fuel injection in DI (direct injection) diesel engine [3]. They concluded the brake thermal efficiency can be increased by hydrogen induction; however, further hydrogen addition resulted in knocking combustion. They also achieved simultaneous reduction in NOx (nitrogen oxides) and smoke emission and knock free combustion with proper adjustment in injection timing and duration. Lata et al. studied dual-fuel operation of a multi cylinder turbocharged diesel engine with hydrogen and LPG (liquefied petroleum gas) by conducting extensive experiments [4]. They reported an enhanced brake thermal efficiency at the high engine load condition by hydrogen and LPG induction and reduced UHC (unburnt hydrocarbons), NOx and soot emissions. On the other hand, strong knock was observed by increasing hydrogen/LPG amount at higher engine loads. Szwaja and Grab-Rogalinski also experimentally investigated pure hydrogen combustion under HCCI (homogeneous charge compression ignition) conditions and hydrogen-diesel co-combustion in a compression ignition engine [5]. It was concluded that hydrogen-air stoichiometric mixture under the HCCI mode generated a very high engine knock, whereas for hydrogen-diesel cocombustion, knock was observed when the hydrogen energy fraction was exceeded 16% of the whole dieselehydrogen mixture energy. Akansu et al. [6] extensively studied effect of natural gas and hydrogen mixtures in spark ignition internal combustion engines and also provided costs of using different mixture. In their study it was concluded that UHC, CO2 (carbon dioxide) and CO (carbon monoxide) emissions were decreased with increasing hydrogen induction, whereas NOx tended to increase and there was a need to apply EGR (exhaust gas recirculation) to reduce NOx emission level. Moreover, natural gas had shown knock free combustion and this characteristic was not varied much by addition of small amount of hydrogen. Karim et al. experimentally studied various methane/ hydrogen proportions with different equivalence ratios [7]. They reported large number of data for flame speed, power output, ignition delay, combustion duration, peak pressure and knocking regions and concluded addition of small amount of hydrogen can noticeably enhance engine performance. Liu and Karim used twozone thermodynamic model to investigate auto-ignition and knock in a diesel-hydrogen dual-fuel engine [8]. Diesel pilot-fuel chemistry was neglected and its combustion was considered as an ignition source. Knock was identified through very high cylinder

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pressure and temperature values and it was concluded that knocking operation with hydrogen is wider than those encountered with other gaseous fuels. An et al. numerically investigated effects of hydrogen induction in a compression ignition diesel engine [9]. They used KIVA4 coupled with semi-detailed chemical kinetics to simulate diesel engine and validated their model for extensive engine load and speed. They have also added extensive hydrogen oxidation chemistry into n-heptane/toluene mechanism to enable simulation of hydrogen assisted diesel combustion. Results showed that induction of small amount of hydrogen significantly enhanced diesel engine operation under low load conditions. They also concluded that further addition of hydrogen would increase premixed HRR (heat release rate) especially at lower engine speeds due to relatively longer cycle time which promotes the possibility of engine knock. As discussed, despite many experimental researches on hydrogen assisted diesel engine under knocking conditions [10e13], there is a lack of extensive numerical study and comprehensive knock model to predict onset and strength of knock. Furthermore, most of the earlier numerical engine knock studies were performed by the use of thermodynamic models along with the assumption of uniform temperature, pressure and chemical species distribution inside the considered zones. Therefore, key parameters such as: chemical interaction of the diesel fuel, local rate of pressure rise, local temperature, local HRR and local chemical species concentrations and their effects on knocking behavior were neglected. Use of mean temperature, pressure and HRR values in identification of the knocking combustion is not an appropriate tool and only very strong knock can be captured by using these simplified models. The present work aims to accurately model and demonstrate knocking combustion by developing a 3D CFD (computational fluid dynamics) -Chemistry model. This has been done by taking into account the effects of fluid flow and chemical kinetics simultaneously, not only for in-cylinder mean values of pressure, temperature and HRR, but also for local regions where knock is likely to occur. As at lower speeds engine is more prone to knock due to longer cycle time [14], this study focuses for knock modeling at low engine speeds. 2. Multi-dimensional engine modeling 2.1. 3D-CFD modeling tool Combustion process and knock phenomenon are threedimensional events which rely on numerous physical and chemical aspects [15]. This requires utilization of a multi-dimensional CFD coupled with detailed chemical kinetics to accurately account for the effects of fluid flow, spray combustion and detailed fuel oxidation and auto-ignition chemistry. In this study, fluid flow simulation was carried out using the latest LANL (Los Alamos National Laboratory) CFD code [16], KIVA4, with improved turbulence and spray/wall interaction models. The combustion calculations in the original KIVA4 code are based on a global reaction for fuel oxidation chemistry, and it involves only 12 active chemical species [16]. The assumption of single-step global reaction ignores essential features of the combustion and auto-ignition of hydrocarbon fuels so called “double-stage ignition”. Fuels with tendency of exhibiting double-stage ignition, are more prone to knock as the first stage ignition happens at relatively low temperature, but provides free radicals and enthalpy for the initiation of the main ignition stage. The phenomenon is completely ignored by global single step mechanisms. Obviously, combustion model has a key role in reliable and accurate predictions of the global engine performance data such as: in-cylinder pressure, HRR and emissions. Hence, in this work the gas phase kinetics library of CHEMKIN-II [17] was

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integrated into the KIVA4 code to generate an advanced multicomponent fuel combustion model [18]. Some basic sub-models in the default KIVA4 require further modifications to maximize code predictability. In doing so, KH-RT (KelvineHelmholtz and RayleigheTaylor) spray break up models [19] were implemented in KIVA4 calculations for spray primary and secondary breakup stages. Moreover, O'Rourke collision model [20] was used for fuel particle collision and the RNG (Re-Normalisation Group) k-ε model was used as turbulence model. 2.2. Multi-component fuel combustion model While developing chemical kinetics mechanisms; some researchers include one or two global reactions at the beginning of the chemical kinetics mechanism of the heavy hydrocarbons such as diesel and biodiesel fuels [21e23]. This is because either chemical pathways of molecular break down of the considered fuel surrogate was not available or number of intermediate chemical species and kinetics pathways of their production/destruction would be so large that the final mechanism cannot be used in 3D CFD calculations. This approach still has some deficiencies such as: it is not suitable for the multi-component fuel evaporation model, it considers limited number of fuel components and its difficulty in determination of chemical kinetics rate coefficients of the introduced global reaction which can significantly affect ignition delay time and combustion. In this study, the global reaction in the DOS (diesel oil surrogate) reaction mechanism [21] was removed from the chemical kinetics mechanism and instead multi-component evaporation and combustion were applied in the simulations. Diesel fuel components were considered to consist of n-heptane and toluene with mole basis proportions of two and one as aliphatic and aromatic hydrocarbon compounds of the diesel fuel [18]. 2.3. Engine setup, geometry and computational mesh The experiments were performed on a four cylinder TOYOTA 2KD-FTV common rail fuel injection diesel engine in the engine laboratory of National University of Singapore [24]. The engine was loaded with an AVL DP 160 water-cooled passive eddy current dynamometer which is able to provide a peak brake power of 160 kW and a maximum torque of 400 N-m with an accuracy of ±0.3%. The air flow rate was measured using an AVL Sensyflow P air flow meter with a resolution of 100 ms for sampling frequency. An AVL 733S.18 fuel balance was used to measure the fuel consumption rate with a sampling frequency of 500 ms and an accuracy of ±1%. The cylinder pressure was measured at a resolution of 1 CAD (crank angle degree) by an AVL GH13P water-cooled pressure transducer which was mounted on the cylinder head, and it can sustain a peak pressure of 250 bars. Specifications of the engine were given in Table 1. All the performance parameters are averaged over 50 engine cycles [24]. Heat release rate is one of the most important parameters used to justify the combustion characteristics of a fuel,

and it further influences the overall engine performance and emission characteristics. In the present study, the heat release rate is calculated based on the experimental cylinder pressure curve by applying the first law of thermodynamics as shown in Eq. (1) and it does not take into account the heat loss through the cylinder walls.

dQ g dV 1 dP ¼ P þ V dQ ðg  1Þ dQ ðg  1Þ dQ

(1)

where dQ/dQ is the heat release rate per crank angle, Q is crank angle, P is the pressure, V is the cylinder volume, and g is the specific heat ratio which is taken to be 1.37 during compression and 1.30 during expansion. To perform numerical calculations and reduce the computational time, a 60-degree sector mesh was generated based on the symmetric combustion chamber as shown in Fig. 1. Computational mesh was consisted of 3540 cells at TDC (top dead center) with the piston crevice region included. Periodic faces were considered at sector mesh side faces and moving and fixed walls were used for piston and liner patches. Mesh and time-step independency tests can be found in Ref. [9] and not repeated here. Using the mesh in Fig. 1, run times for a closed cycle simulation took about 40 h at the HPC (High Performance Computing) system in National University of Singapore. 3. Knock modeling 3.1. Theoretical insights into knock modeling Engine knock is unwanted rapid auto-ignition of a pocket of airfuel mixture, before the arrival of the flame front, causing a noticeable increase in local heat release and subsequently pressure and propagation of acoustic waves over the entire combustion chamber. Engine knock is a stochastic phenomenon and its occurrence relies on many parameters. Experimentally, knock can be detected by recording cylinder pressure curves for number of cycles and using different methods to filter the noises and allocating a specific crank angle(s) for the knock onset [25]. On the other hand, numerical modeling of the knocking combustion may provide detailed information about an abnormal knocking cycle but it may not be necessarily coincided with experimental observations. Nonetheless, if a comprehensive numerical investigation can predict knock, it can be concluded that experimentally knock is happening in majority of the engine cycles [26]. Three most commonly used knock models are AnB [27], Shell [28] and Empirical formulations based on an Arrhenius function [29,30]. Paying

Table 1 TOYOTA 2KD-FTV engine specifications. Engine aspiration Engine cycle Number of cylinders Injector orifices Fuel injection system Bore * stroke Displacement volume Compression ratio Max. output power Max. output torque

Turbo-charged Diesel e 4 Stroke 4 e Inline 6 - Centric Common rail, Denso 92 * 93.8 [mm] 2.494 [lit] 18.5: 1 75 [kW] @ 3600 [rpm] 260 [N.m] @ 1600 [rpm]

Fig. 1. Toyota 2KD-FTV 60 sector mesh (at TDC) used in 3D-CFD simulations and its considered walls and periodic faces.

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attention to the governing chemistry is the common feature among the mentioned knock models. Shell model was used by many researchers not only for modeling end-gas knock in spark ignition engines but also for diesel fuel auto-ignition chemistry. Recent advances in computational power enabled researchers to use chemical kinetics in many engineering applications such as combustion chemistry. Similar to the shell auto-ignition model, purpose of applying chemical kinetics mechanisms is to provide detailed and comprehensive basis for chemical reactions taking place during the auto-ignition and combustion of a sample hydrocarbon fuel. In the case of knock modeling, chemistry has a direct effect on knock occurrence and if it is appropriately incorporated with fluid flow and combustion models, detailed numerical predictions can be achieved. This will be further discussed in the next sections. 3.2. Chemical kinetics mechanism In this study, DOS chemical kinetics mechanism developed at Chalmers University of Technology [21] was used for diesel fuel oxidation chemistry and extensive hydrogen reaction mechanism from the Lawrence Livermore National Laboratory [31] was incorporated with DOS mechanism enabling detailed predictions of hydrogen auto-ignition and combustion in compression ignition diesel engine. The original DOS mechanism contains 70 chemical species participating in 305 elementary reactions, including the semi-detailed oxidation pathways of n-heptane (C7H16) and toluene (C7H8) while the hydrogen combustion reaction mechanism contains 8 chemical species participating in 21 elementary reactions. In DOS mechanism C14H28 was considered to be the single component fuel and it was assumed to be decomposed into its constituent components of n-heptane and toluene through a global reaction at the beginning of the mechanism. Nonetheless, this approach still has some deficiencies such as: it is not suitable for the multi-component fuel evaporation model, it considers limited number of fuel components and its difficulty in determination of chemical kinetics rate coefficients of the introduced global reaction which can significantly affect ignition delay time and combustion [32]. In the present study, a multi-component fuel combustion model was used for simulation of a hydrogen assisted diesel engine. Under engine knocking conditions, auto-ignition of gaseous fuel is highly dependent on low temperature combustion and formation of the pool of radicals [33]. Warnatz showed that in knocking conditions auto-ignition could be identified from an overshoot on OH radical concentration and the onset of CO formation [33]. Reaction types were divided into low and high temperature reactions. At low temperatures: (1) Two consecutive O2 additions to alkyl radicals, (2) isomerization of alkylperoxi, alkylhydroperoxi radicals via cyclic structures, (3) OH elimination after internal rearrangement, and (4) b-decomposition of O ¼ RO00 , C ¼ RO00 , O ¼ R00 , and alkenyl radicals. At high temperatures: (1) alkane and alkene decomposition, (2) H atom abstraction from alkanes and alkenes by H, O, OH, HO2, HC-radicals, (3) b-decomposition of alkyl radicals and (4) isomerization of alkyl radicals. About the hydrogen oxidation mechanism developed by Conaire et al. [31], they have used previous works of Yetter et al. [34], Kim et al. [35] and Mueller et al. [36] on CO/H2/O2 reaction mechanism; however, they extended the range of its validity from 298 to 2700 K for temperature, 0.05e87 atm for pressure and the equivalence ratios from 0.2 to 6.0. This mechanism includes shuffle reactions of: H þ O2 4 OH þ O, H2 þ O 4 OH þ H and H2 þ OH 4 H2O þ H and recombination reaction of: H þ O2 þ M 4 HO2 þ M and HO2 consumption reactions of: HO2 þ H 4 2OH, HO2 þ H 4 H2 þ O2 and HO2 þ OH 4 H2O þ O2 which have been shown to accurately predict lean deflagrations [31,37,38]. Reactions: H þ OH þ M 4

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H2O þ M and H þ H þ M 4 H2 þ M as direct recombination reactions were considered as they have an essential role in accurate chemical kinetics predictions for stoichiometric and rich air-fuel as well as non-premixed mixtures. H2O2 chemistry is important as it describes combustion of second explosion limit of H2eO2 mixtures [37]. This is mainly due to controlled rate of radical production through the regular chain branching/termination in reactions: H þ O2 4 OH þ O and H þ O2 þ M 4 HO2 þ M. Moreover, elementary reactions of: HO2 þ HO2 4 H2O2 þ O2 and HO2 þ H2 4 H2O2 þ H generate H2O2 from HO2 as occurs near the lean flammability limit. In conclusion, essential H2 oxidation path ways under wide validity range for temperature, pressure and equivalence ratio were considered in the mechanism [31] enabling accurate hydrogen combustion chemistry predictions under normal and knocking combustion conditions. As it discussed earlier, in this study DOS mechanism [21] was incorporated with the hydrogen mechanism [31] in order to simulated hydrogen assisted compression ignition diesel engine. The equivalence ratio validity range of DOS mechanism which has been used in the present work is 0.5e3.0 and ensures equivalence ratio validity range under diesel engine operating condition. Being valid up to equivalence ratio of 3.0 ensures the simulation that fuel rich regions in stratified charges of air and fuel around the spray cone will be modeled correctly by chemistry. As long as DOS chemical kinetics mechanism was used in this study accurate combustion rate calculations were assured due to mechanisms validity over wide range of equivalence ratio and effect of turbulence parameters were not considered in the model. 3.3. Allocation of regions for knock study In case of hydrogen assisted diesel engine, both premixed hydrogen-air and non-premixed diesel fuel charges are available in the combustion chamber. This condition is very similar to dual-fuel compression ignition combustion; however, in hydrogen assisted diesel engine the amount of gaseous fuel is much lower than normal dual-fuel engine operating condition. Karim and Liu [39] theoretically described that energy release of a dual-fuel dieselnatural gas engine was made up of three distinguished parts namely pilot-fuel combustion, combustion of gaseous fuel in vicinity of pilot-fuel, and combustion of remained gaseous fuel as parts I, II, and III. They explained that combustion was initiated by the energy release of the diesel pilot-fuel, while the maximum heat release occurred in part II and then combustion propagated into the remained natural gas-air mixture near the cylinder wall boundaries. In the present study ten local regions were considered for TOYOTA 2KD-FTV diesel engine to study the knock phenomenon on

Fig. 2. Ten introduced regions for knock study inside the CFD computational domain of TOYOTA 2KD-FTV engine geometry (at TDC).

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Table 2 Initial and boundary conditions as simulation input parameters. Engine speed [rpm] Velocity boundary conditions Engine Load IVC [CAD ATDC] EVO [CAD ATDC] Pressure at IVC [bar] Temperature at IVC [K] Cylinder wall temperature [K] Piston head temperature [K] Heat Transfer model on walls Initial charge EGR species Diesel mass [gr/cycle] Air mass [gr/cycle] Total equivalence ratio SOI [CAD ATDC]

1200 Law-of-the-wall 100% e Turbo-charged 149 150 1.27 374 525 550 Han and Reitz Air þ 5% EGR [Vol] N2, O2. CO2, H2O 0.035 0.7 1.33 5

experimental data was achieved, Fig. 3 is a comparison between experimental results and simulation results for in-cylinder pressure and HRR. As it can be seen, both peak pressure and HRR locations and magnitudes were well predicted by applying comprehensive multi-component fuel evaporation and combustion models. 4. Results and discussion 4.1. Mean in-cylinder results After validating the numerical model, hydrogen induction simulations were conducted. For the experimental case with specifications as indicated in Table 2, 1%, 3%, 5% and 7% hydrogen

local regions in a hydrogen assisted diesel engine. Local values for pressure, temperature, HRR and specific chemical species concentration were used in order to identify knocking combustion. Exact locations and dimensions of these regions were given in Fig. 2. R1 to R5 were considered to be in piston bowl, R6 to R8 near cylinder walls and R9 and R10 on top of the piston crown. For more information regarding to selection of local regions see Ref. [40]. 3.4. Model validation It has been discussed that the chemical kinetics mechanism in this study was a combination of semi-detailed n-heptane, toluene and hydrogen mechanisms for simulation of a hydrogen assisted diesel engine. In order to ensure this combined mechanism was appropriate for combustion calculations in the compression ignition engine environment, pure diesel simulations were carried out. Extensive model validation for pure diesel condition can be found in the authors' published paper [9] for engine loads of 10%, 50% and 100% and speeds of 1600, 2400 and 3200 rpm. This ensures valid predictions for chemical species production/destruction rates calculated by the applied chemical kinetics mechanism. Engine operating conditions, initial and boundary conditions which have been used as input parameters in numerical calculations were given in Table 2. The reason for selecting full engine load and low engine speed was because engine was more turbocharged at higher loads and had longer cycle time at lower speeds, both increase knocking combustion possibility. Closed cycle simulation for this particular case was performed and good agreement with

Fig. 3. Comparison between simulation and experimental results for cylinder pressure and HRR.

Fig. 4. Pressure, Temperature and HRR diagrams for Base, 1%, 3%, 5% and 7% hydrogen induction.

A. Maghbouli et al. / Energy 76 (2014) 768e779 Table 3 Ignition delay, start of combustion and peak HRR for 0%, 1%, 3%, 5% and 7% of hydrogen induction cases. Hydrogen induction in volume in intake air Ignition delay [CAD] Start of Combustion [CAD ATDC] Maximum total HRR [J/CAD]

0%

1%

3%

5%

7%

6 1 71

5 0 79

4 1 65

1 4 93

1 4 197

induction in volume in intake air was considered. It should be noted that the amount of injected diesel fuel was kept constant for all the simulations. Predefined local regions (R1 to R10) were used to record local temperature, pressure, HRR and chemical species concentration changes for 0% (base), 1%, 3%, 5% and 7% hydrogen induction through application of the semi-detailed chemical kinetics mechanism incorporated with 3D-CFD calculations. Fig. 4

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illustrates inecylinder mean pressure, temperature and HRR for hydrogen induction percentages and Table 3 shows ignition delay, start of combustion and maximum total HRR for different hydrogen induction cases. It can be seen by increasing hydrogen induction; maximum pressure and temperature were tangibly elevated. This trend is very noticeable for 5% and 7% hydrogen inductions where unwanted pressure rise is captured by the model before TDC which will result in a reduced indicated output power. Earlier and sharper temperature rise were predicted for higher hydrogen percentages showing more active combustion by increasing hydrogen induction. Total HRR curves show that by increasing hydrogen amount in the intake manifold hydrogen tends to ignite faster and generate very high peaks and possibility of knock before TDC for 5% and 7% hydrogen induction cases. More detailed understanding can be resulted by paying attention to the diesel (lumped value for nheptane and toluene) and hydrogen fuels oxidation histories with

Fig. 5. Concentration history of diesel fuel (lumped values of n-heptane and toluene), hydrogen and oxygen [g] together with temperature curve (only rend) for 0%, 1%, 3%, 5% and 7% of hydrogen induction cases.

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oxygen in Fig. 5. It should be noted only gaseous state of diesel fuel is plotted with only temperature rise trend of each case. For hydrogen concentration within the combustion chamber two stages can be considered. First: premixed combustion of 1%, 3%, 5% and 7% hydrogen induction cases and second: production of hydrogen for all cases due to diesel fuel molecular break down to lighter chemical species such as H2 through applied chemical kinetics mechanism, approximately later than 15 CAD ATDC (crank angle degree after top dead center). For 0% hydrogen induction, premixed combustion of the first stage did not take place as there was no premixed hydrogen charge for 0%. For 1% H2 induction, the amount of premixed hydrogen remained unchanged and no noticeable hydrogen oxidation was observed. On the other hand, hydrogen started to oxidize immediately after the presence of autoignited diesel fuel for 3%, 5% and 7% H2 cases and by increasing hydrogen induction this trend was more pronounced where for 7% H2, all the premixed hydrogen was burnt at the first stage of

premixed combustion, generating very sharp temperature rise. Fast and HCCI like combustion of hydrogen in 5% and 7% H2 induction cases was triggered by auto-ignition of a pocket of air and evaporated diesel mixture and formation of small flame kernels. However, this trend was not observed for 1% and 3% hydrogen induction cases and it might be due to the lack of sustainable combustion and strength of diesel auto-ignited flame kernels near hydrogenoxidizer mixtures. For 3% H2 case, gradual consumption of hydrogen and temperature rise were captured showing enhanced combustion but as discussed, further increase of premixed hydrogen was resulted in rapid HRR, temperature and pressure rise before TDC (Fig. 4) which is not ideal due to generating negative work. As discussed hydrogen concentration had an increasing trend around 15 CAD ATDC. This is due to generated hydrogen molecules through application of semi-detailed chemical kinetics mechanism. Hydrogen and oxygen contours are presented in Fig. 6 to provide

Fig. 6. H2 and O2 contours [mass fraction] for 7% H2 induction at 3, 2, 1, 5 and 10 CAD ATDC.

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their temporal concentration data for 7% hydrogen induction case. It illustrates that at 2 CAD ATDC, hydrogen consumption was initiated and oxygen was also decreased at high reactivity regions simultaneously. Later hydrogen underwent a volumetric HCCI like combustion which was identified as knock and at the same crank angle noticeable reduction in the oxygen amount was observed. After initiation of the diesel fuel injection followed by spray break up and evaporation, the heavy hydrocarbon specie is decomposed to form lighter chemical species such as hydrogen through number of elementary reactions. Because of this, hydrogen contours show high concentration on diesel spray path, and increases as chemical kinetics mechanism proceeds to generate more hydrogen. Oxygen is consumed due to combination with the heavy hydrocarbons and formation of many lighter chemical species and exothermic reactions. This is why in high local temperature regions of the diesel combustion; oxygen amount is noticeably reduced in the contours of 5 and 10 CAD ATDC. Rapid consumption of hydrogen at 2 CAD ATDC can be considered as engine knock but the aim of this study is

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to further refine the observations to allocate the knocking location(s) inside the combustion chamber and its onset CAD by using local knock identification criteria rather than mean in-cylinder values. 4.2. Local in-cylinder results 4.2.1. Regional pressure rise rate It was discussed before that knocking combustion can be identified from very rapid and intense rises in PRR (pressure rise rate) and HRR values. Fig. 7 depicts regional, R1-10, PRR curves for 0%, 1%, 3%, 5% and 7% hydrogen induction cases. It can be seen PRR curves in all cases have two sharp peaks around TDC and 15 CAD ATDC, and by increasing H2 induction first peak is getting stronger. This is mainly due to availability of rich H2-air mixtures at higher hydrogen induction cases and more strengthened HCCI type combustion, generating rapid and high PRR curves. For both H2 induction cases of 1% and 3%, maximum regional PRR is less than 2

Fig. 7. Regional PRR for 0%, 1%, 3%, 5% and 7% of hydrogen induction cases.

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[bar/CAD] and this ensures knock free combustion as it is generally accepted that knock in compression ignition engines will occur for PRR more than 8 [bar/CAD] [41]. However, cases with 5% and 7% hydrogen induction show higher PRR peaks of 8 [bar/CAD] and 16.8 [bar/CAD] at TDC and 2 CAD ATDC, respectively. It can be concluded at this full engine load and low speed further increasing hydrogen from 5% induction in intake manifold will result in knocking combustion. Nonetheless, regional pressure and PRR cannot provide needed data to allocate the knocking location(s) within the combustion chamber. Regional HRR and regional chemical species data in the next sections will be used to see which geometrical locations of the combustion chamber are more prone to knock. 4.2.2. Regional heat release rate HRR analysis is a very useful tool to distinguish knocking combustion as knock is directly related to molecular break down, formation of pool of radicals and exothermic chemical reactions. Despite the fact that onset of knock can be captured by sharp rises in HRR diagrams, mean in-cylinder HRR cannot be used to find the location of knock occurrence inside the combustion chamber. Fig. 8

illustrates the values of total and regional HRR for base and hydrogen induction cases. Introduced regions in the 3D CFD computational domain indwell small volumes (9 mm3); hence their corresponding values of HRR are considerably less than total incylinder HRR value. It is obvious from the diagrams there are notably high peaks and rapid rises in HRR curves in local regions for the knocking cases of 5% and 7% H2 induction compared to other normal combustion cases. For the knocking cases, at TDC for 5% H2 and 2 CAD ATDC for 7% H2, large number of local regions were experienced sharp HRR rises and among them, R1 to R5 show higher peaks at the mentioned knocking crank angles. It can be concluded that due to compression heating, H2 underwent very fast auto-ignition and knock mostly appeared at the middle of combustion chamber (R1 to R5) and slightly above the piston crown (R9 and R10). An evidence for that can be found in temperature contours of Fig. 9. In this figure, temperature contours of no-knock condition (0% H2 induction) were compared to the heavy knocking condition (7% H2 induction) at six engine crank angles. At 4 CAD ATDC, it can be seen that temperature contours for 0% H2 and 7% H2 were almost the same at the early stage of combustion. For 0% H2 induction, by further increasing the crank angle, there was no

Fig. 8. Regional HRR for 0%, 1%, 3%, 5% and 7% of hydrogen induction cases.

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significant change in temperature contours till 4 CAD ATDC, after this crank angle combustion is initiated due to evaporation and auto-ignition of the diesel fuel. On the other hand, temperature contours in 7% H2 case keep increasing in specific locations. More vividly at 2 CAD ATDC high temperature cores can be seen and marked with arrows. These cores were formed mainly in central parts of the piston bowl and above the piston crown, whereas locations near the cylinder walls were not showing the same trend due to higher heat transfer to the cold walls. It can also be seen that HCCI like combustion took place at 2 CAD ATDC for 7% H2 case, resulting in volumetric heat release and temperature rise within the whole combustion chamber. Temperature distribution of both cases at 4 and 10 CAD ATDC were similar, however, 7% H2 case has higher values. This is due to combustion of hydrogen and diesel was then burning in higher ambient temperature comparing to 0% H2

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case, leading to extension of very high temperature (2500 K) regions. 4.2.3. Regional chemical species concentration Chemical species concentration history such H2 and HO2 concentration can provide useful information to identify a knocking cycle. For instance, local values of gaseous fuel and its burning rate together with formation of free radicals can be considered as the evidences of knock occurrence within the combustion chamber. Fig. 10 shows local mole fractions of hydrogen (gaseous fuel) and HO2 (free radical) and local temperature in R4 and R7 for 0%, 1%, 3%, 5% and 7% H2 induction cases. As discussed in local HRR section, regions at the middle of combustion chamber and above the piston crown in the hydrogen assisted diesel engine under study were more prone to knock than regions near the cylinder walls. In order

Fig. 9. Comparison of temperature contours [K] for 0% and 7% hydrogen induction cases at 4, 3, 2, 1, 4 and 10 CAD ATDC.

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to compare local properties of these high and low reactivity regions, R4 and R7 were selected. It can be seen that hydrogen was burnt faster in R4 than R7 in all H2 induction cases and burning rate was elevated with increasing in the amount of hydrogen induction. This shows that richer hydrogeneair mixture promotes the possibility and strength of engine knock. Free radicals have direct effect on oxidation of the fuel species and accelerating exothermic reactions and subsequently heat release. Mole fraction history of HO2 as a free radical was plotted in Fig. 10. It shows high production/ consumption rate of HO2 for high reactivity region of R4 comparing to low reactivity region of R7. Moreover, rapid consumption of HO2 was coincided with local hydrogen burning rate and temperature rise. R4 curves show double stage temperature rise, whereas for all R7 temperature curves such a trend was not observed. It should be noted that for R4 first temperature rise in 5% and 7% H2 induction cases is because of knock occurrence, and the second is due to diesel auto-ignition and combustion.

5. Conclusions Comprehensive 3D CFD-chemical kinetics simulations were performed by developing a predictive model to study the knock phenomenon within a hydrogen assisted compression ignition diesel engine. Semi-detailed chemical kinetics mechanisms were selected and applied for a low speed and full load engine case where the possibility of knock due to longer cycle time and turbocharged condition was high. Thermal and chemical properties of the different combustion chamber locations were taken into specific attention. After discussing the model development, numerical calculations and their results, the main conclusions can be drawn as below: 1. Hydrogen induction up to 3% of the intake charge volume enhanced the diesel engine operation but for 5% and 7% hydrogen induction cases sharp PRR were resulted before top

Fig. 10. H2 and HO2 mole fractions and temperature [K] history of R4 and R7 for 0%, 1%, 3%, 5% and 7% H2 induction cases.

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dead center. This is a limiting factor on engine downsizing as it affects output power by creating negative force while piston is on its way to top dead center. 2. By introducing ten regions, namely R1 to R10, regional data such as PRR, HRR and chemical species local concentrations, were used to identify knocking combustion and knock locations. High regional PRR was considered as the knock identification factor and regional HRR and species concentration were used to spot knocking locations within the combustion chamber. 3. Noticeable knocking combustion were resulted by application of higher than 5% (by volume) hydrogen in the intake charge of a compression ignition diesel engine. Regional PRR, HRR and chemical species concentration data showed that in such an operating condition, knock is occurring at start of combustion rather than end-gas auto-ignition of SI (spark ignition) engines. Moreover, knock took place in central parts of the piston bowl and above the piston crown; whereas locations near the cylinder walls were not showing the same trend.

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