Fuel 90 (2011) Contents lists available at ScienceDirect. Fuel. journal homepage:

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1 Fuel 9 (2) Contents lists available at ScienceDirect Fuel journal homepage: A parametric study on natural gas fueled HCCI combustion engine using a multi-zone combustion model Ali Yousefzadi Nobakht a,, R. Khoshbakhi Saray a,, Arash Rahimi b a Mechanical Engineering Faculty, Sahand University of Technology, Sahand New City, Tabriz, Iran b Chemical Engineering Faculty, Sahand University of Technology, Sahand New City, Tabriz, Iran article info abstract Article history: Received 7 June 2 Received in revised form 2 December 2 Accepted 2 December 2 Available online 4 January 2 Keywords: HCCI combustion Detailed chemical kinetics Multi-zone combustion model Parametric study Homogenous Charge Combustion Ignition (HCCI) is a good method for higher efficiency and to reduce NOx and particulate matter simultaneously in comparison to conventional internal combustion engines. In HCCI engines, there is no direct control method for auto ignition time. A common way to indirectly control the ignition timing in HCCI combustion engines is varying engine s parameters which can affect the combustion. In this work, a parametric study on natural gas HCCI combustion is conducted in order to identify the effect of inlet temperature and pressure, compression ratio, equivalence ratio and engine speed on combustion and engine performance parameters. In this paper, two kinds of parameters will be discussed. First, in-cylinder pressure diagrams and variation of start of combustion which are combustion parameters will be presented and then the second category, indicated mean effective pressure and thermal efficiency which are performance parameters will be studied. A six zone model coupled with detailed chemical kinetics code is used to simulate HCCI combustion. Both heat and mass transfer was considered in the modeling procedure. Results revealed that among the considered parameters, the equivalence ratio and inlet pressure are the most valuable parameters which can improve the combustion and performance characteristics of the HCCI engine. Ó 2 Elsevier Ltd. All rights reserved.. Introduction Ultra low NOx levels and near zero soot emissions while maintaining high thermal efficiency, makes Homogenous Charge Compression Ignition (HCCI) combustion one of the most promising internal combustion engine strategies for the future [ 8]. The advantages of HCCI combustion is due to its nature of very lean and premixed rapid combustion with high heat release rate () and no flame front [9,]. HCCI combustion has high thermal efficiency because of ability to run with high compression ratios, no throttle loss, lean and almost constant volume combustion []. While the potential benefits of HCCI combustion are great, there are many difficulties need to be prevailed over. Producing high levels of CO and HC emissions, problems in cold start and reduced normal operating range of HCCI engines are some of these difficulties [5,6]. One of the most important technical challenges is to control the start of combustion (SOC) across the speed and load range of engine [2,3]. Therefore, finding a reliable way to control HCCI combustion is currently under widespread investigation [4,5]. Some of the controlling methods, among others, are inlet Corresponding author. Tel.: ; fax: addresses: y_ali_n@yahoo.com (A.Y. Nobakht), khoshbakhti@sut.ac.ir (R. Khoshbakhi Saray), arash_rahimi_ma@yahoo.com (A. Rahimi). Tel.: temperature or pressure variation and variation of compression ratio (variable valve timing method) [6,6 8]. In order to design a reliable system for HCCI combustion controlling, it is essential to know the effect of changing each parameter on combustion behavior. For this purpose, in this work a parametric study is performed to determine the effect of inlet temperature, inlet pressure, compression ratio, equivalence ratio and engine speed on in-cylinder pressure trace, SOC, indicated mean effective pressure (IMEP) and thermal efficiency of a Natural Gas fueled HCCI engine. 2. Model description As shown in Fig., the combustion chamber divided into six zones; Core volume is perfectly centered within the charge, forming an inner gas cylinder surrounded by an outer concentric shell of gas with a thickness of t. The outer zones and crevices zone are cylindrical annulus. To model boundary layer, it is assumed that each zone becomes successively thinner as it approaches cylinder surfaces and the value of R indicates how much thinner each successive zone is relative to previous zone s thickness. Values of the core zone s volume as a fraction of total cylinder volume, R and the number of zones are adjustable model parameters. In this study, R =.3 and V Core is 2% of total cylinder volume /$ - see front matter Ó 2 Elsevier Ltd. All rights reserved. doi:.6/j.fuel

2 A.Y. Nobakht et al. / Fuel 9 (2) Nomenclature CR compression ratio c k t specific heat constant of intake mixtures inside the kth zone h c convection heat transfer coefficient height instantaneous cylinder height h i mixture enthalpy of (i )th zone h i mixture enthalpy of ith zone K tot total conductivity m k mass of kth zone m tot in-cylinder total mass m i!i mass transfer rate from zone i to zone i m i!iþ mass transfer rate from zone i to zone i + MW i molecular weight of ith species N s total number of species N z number of zones P in-cylinder pressure Pr Prandtl number P ivc pressure at IVC Q k heat transfer into/out of the kth zone T time T temperature internal energy of the ith species U i T ivc T k m V k y Y i,k temperature at IVC temperature of kth zone gas velocity volume of kth zone distance mass fraction of the ith species in the kth zone Greek symbols x i;k molar rate of production q k density of the kth zone b scaling factor U equivalence ratio Abbreviations CAD crank angle degree EVO exhaust valve open HCCI Homogenous Charge Compression Ignition heat release rate IMEP indicated mean effective pressure IVC inlet valve closure MZCM multi-zone combustion model SOC start of combustion The multi-zone combustion model (MZCM) developed using Fortran9 programming language. Modeling takes place in the closed part of the engine cycle (from IVC to EVO) and the model includes heat transfer, mass transfer and detailed chemical kinetics sub models. The GRI-mech 3. [9] chemical kinetic mechanism is used to describe the natural gas oxidation chemistry. 3. Governing equations of multi-zone combustion model All properties in each zone of MZCM are considered to be uniform and calculated based on ideal gas theory and detailed chemical kinetics. The initial condition of the model was set based on Ref. [2]. It is assumed that the in-cylinder mass is homogenous at IVC. After that point mass is transferred between zones to maintain the pressure uniform inside the combustion chamber: dm k dt X N Z K¼ m k ¼ m tot Energy conservation equation and the equation for net rate of production of each species are as follows: dt m k c k k t dt ¼ m X Ns k i¼ dy i;k dt ¼ _x i;kmw i q k ðþ ð2þ dy i;k U i P dv k dt dt þ dq k dt þ m i!ih i m i!iþh i ð3þ Radiative heat transfer is ignored in this study and Woschni s correlation [2] with modifications which made it be appropriate for HCCI engines [22] is used to model convective heat transfer from in-cylinder mixture to cylinder wall. ð4þ h c ðtþ ¼b HeightðtÞ :2 PðtÞ :8 TðtÞ :73 vðtþ :8 ð5þ Heat transfer between zones is modeled using a mechanism similar to conduction. Therefore, the heat flux is based on the temperature difference of the neighboring zones and on their mean distance as follows: q ¼ K tot dt dy ð6þ In Eq. (6), K tot is total conductivity which is calculated based on Yang and Martin model [23] and is the sum of a laminar and a turbulent component. K tot ¼ K l þ K t The ratio of turbulent to laminar conductivity is calculated using the following formula: ð7þ Fig.. Zone configuration of MZCM. k t k l ¼ Pr l Pr t l t l l ð8þ

3 5 A.Y. Nobakht et al. / Fuel 9 (2) Table CFR engine specifications. Parameter Specification Parameter Specification Engine type Single cylinder Stroke (cm).4 Displacement (cc) 62 Connecting rod (cm) 24 Throttle Fully open IVO (CAD) Main fuel NG IVC (CAD) 24 Compression ratio 7.25: EVO (CAD) 5 Bore (cm) 8.26 EVC (CAD) 735 Table 2 CFR engine operating conditions for four considered cases. Case Case 2 Case 3 Case 4 Equivalence ratio U NG mass flow rate (mg/s) % EGR T ivc (K) P ivc (bar) Eq. (8) presupposes swirl dominated flows and is used in the absence of other data. The viscosity ratio of Eq. (8) is calculated from the following relation: l t ¼ ky l þ n exp 2akyþ n ð9þ l And y þ n ¼ u l w Z u qdy n ðþ where k =.4 is the Von Karman Constant, a =.6 and y þ n is the normal distance from the wall. The characteristic velocity is considered to be proportional to engine speed. 4. set-up and engine specifications Experiments were conducted in University of Alberta engine research facility. Engine was a Waukesha CFR single cylinder research engine coupled to a DC motoring dynamometer. The Case Case Case Case Fig. 2. In-cylinder pressure histories and heat release rate diagrams of four considered cases.

4 A.Y. Nobakht et al. / Fuel 9 (2) Zone Zone Case Case Zone Zone Case Case 4 Fig. 3. In-cylinder temperature histories of four considered cases. CFR engine specifications for the current study are summarized in Table. The engine was maintained at constant speed of 8 RPM and was modified to operate in HCCI mode at wide open throttle by Hosseini and Checkel [24]. A heater was installed inside the intake manifold in order to control the intake mixture temperature when needed. Intake mixture temperature was set at constant value and was monitored just before intake valve. The amount of external EGR was controlled by a manual butterfly valve. A Kistler (643A) water cooled pressure transducer was used to acquire pressure signal on a. CAD resolution. 5. Model validation In this section the model results are compared to experimental data in order to validate the modeling ones. For this purpose four different engine operating conditions have been chosen which are available in Table 2. Since the values of SOC, IMEP and thermal efficiency directly depend on pressure trace then to have a reliable parametric study on these parameters, the model must be able to predict in-cylinder pressure accurately. In Fig. 2, the calculated in-cylinder pressure histories and the experimental ones are compared for four different operating conditions. It can be seen that the developed MZCM is able to predict pressure traces with an acceptable accuracy and the developed MZCM can be used as a powerful tool for parametric Table 3 Variable changing manner in parametric study. Parameter Baseline value Range Step size T ivc (K) P ivc (bar) Compression ratio Equivalence ratio Engine speed (rpm) 8 7

5 52 A.Y. Nobakht et al. / Fuel 9 (2) study. Fig. 2 also shows the heat release rate () graphs which shown underneath of the pressure diagrams. According to diagrams it is evident that HCCI engines have a rapid combustion and the combustion takes place in a short period of time. In-cylinder temperature histories of different zones are shown in Fig. 3 for considered cases. It is noticeable that, temperature of crevices volume assumes to be constant and equal to wall temperature. 6. Parametric study procedure Table 3 presents the variables considered in the parametric study, which are compression ratio, mixture temperature at IVC, mixture pressure at IVC, engine speed and air/fuel equivalence ratio. The case 2 of considered operating conditions in Table 2 is chosen as the base condition and the variables are changed around this point. The parametric study conducted varying one variable at each run, while all others were kept at the base value P(ivc)=2.55(base).5 (bar).55(base) P(ivc)=2.5 (bar) Crankangle (degree) Results and discussion Figs. 4 8 show the effect of T ivc, P ivc, CR, U and engine speed on the in-cylinder pressure and history, respectively. Increasing the inlet temperature relative to its base value advances the ignition because it leads to higher mean mixture temperature (Fig. 4), while decreasing T ivc reduces the reaction rate and retards the ignition. Fig. 5 shows the effect of P ivc variation on pressure and diagrams. Increasing P ivc from its base value increases the mean mixture temperature and raises the reaction rates. Therefore, it advances the ignition, raises heat release rate and increases both compression and expansion pressure (shifts up the pressure diagram). Decreasing the P ivc has the reverse effect and decreases the reaction rates and finally leads to missfiring of the in-cylinder mixture. Fig. 6 shows that increasing compression ratio has the same effect of pressure increment and advances ignition and consequently combustion. Changing compression ratio is one of the most noteworthy ignition controlling methods in HCCI engines [6 8]. In Fig. 7, the effect of varying the equivalence ratio, U, on pressure and traces is shown. The U changes in a way to maintain the input energy of engine constant. Increasing U from its base Fig. 5. Effect of inlet pressure variation on pressure and histories CR= (base) (base) CR= Fig. 6. Effect of compression ratio variation on pressure and histories (base) T(IVC)=42 45 T(IVC)= (base) (K) 4 4(K) phi= (base) (base) phi= Crankangle (degree) Fig. 4. Effect of inlet temperature variation on pressure and histories. Fig. 7. Effect of equivalence ratio variation on pressure and histories.

6 A.Y. Nobakht et al. / Fuel 9 (2) value increases the reaction rate as a result of higher fuel concentration. Decreasing the equivalence ratio lowers the fuel availability inside the cylinder and in the case of U =.3 misfiring phenomenon occurs Engine speed=7 8(base) 9 (rpm) 8(base) 9 Engine speed=7 (rpm) Fig. 8. Effect of engine speed variation on pressure and histories. Fig. 8 presents the effect of engine speed variation on pressure and history. Increasing engine speed reduces the available time for chemical reactions and retards the combustion. The effect of considered parameters on combustion and engine performance parameters (e.g. IMEP, thermal efficiency and SOC) are shown in Fig. 9. In this part of study, the misfired cases are ignored and just the normal combustion ones are considered. In Fig. 9a, the effects of considered variables on SOC are studied. The definition of SOC adopted here was the point of maximum pressure rise rate. As described in pressure diagrams, increasing T ivc increases reaction rates and advances the combustion. Increment of P ivc and CR increase the mean temperature of the mixture, and then these parameters have the same effect of T ivc on SOC. As mentioned before, increasing U increases the available fuel in the mixture and advances the combustion. The increment of engine speed reduces the available time for reactions and retards the ignition and combustion processes. The effects of changing considered variables (e.g. T ivc, P ivc, U, RPM and CR) on IMEP and thermal efficiency are shown in Fig. 9b and c, respectively. According to IMEP and thermal efficiency definition and the figures, it can be seen that the graphs practically has same trends for the variables change. Increasing inlet temperature decreases expansion work, advances the combustion and consequently increases compression work and thus leads to lower IMEP and thermal efficiency. Increasing P ivc increases both compression and expansion work, but the increment of expansion work is dominated and therefore it results higher IMEP and thermal efficiency. IMEP and thermal efficiency decrease when (a) (SOC-SOC(base))/SOC(base) % %Increase CR Tivc Pivc RPM Phi (b) (IMEP-IMEP(base))/IMEP(base) % %Increase -5 CR Tivc Pivc RPM Phi (c) (eff.-eff.(base))/eff.(base) % CR Tivc Pivc RPM Phi %Increase -5 - Fig. 9. Effect of varying considered variables on: (a) SOC, (b) IMEP and (c) thermal efficiency.

7 54 A.Y. Nobakht et al. / Fuel 9 (2) compression ratio is increased. Because, increasing the CR increases compression stroke s work, while keeping expansion work almost constant. Increasing engine speed decreases compression work while keeping expansion work constant or increases it (in the cases main combustion takes place after TDC), therefore both IMEP and thermal efficiency increase. By increasing U, both compression and combustion works increase but the increment of expansion work is more significant and it leads to higher IMEP and thermal efficiency. According to modeling results, it can be seen that the values of IMEP and thermal efficiency are increased by increasing equivalence ratio and inlet pressure. Therefore, among the considered parameters, the equivalence ratio and inlet pressure are the most valuable parameters which can improve the combustion and performance characteristics of the HCCI engine. 8. Conclusions In this paper, a parametric study was conducted in order to investigate the effect of T ivc, P ivc, compression ratio, equivalence ratio and engine speed on a natural gas fueled HCCI engine s combustion and performance parameters using the developed multi-zone combustion model. Results of this work can be summarized as follows:. Increasing T ivc advances the combustion but decreases the engine efficiency and IMEP. 2. IMEP and thermal efficiency are increased by inlet pressure increment. Also, combustion takes place earlier when the P ivc is increased. 3. Combustion is advanced with increasing compression ratio, but it has negative effect on engine performance and it decreases IMEP and thermal efficiency. 4. Increasing U advances combustion because of higher fuel availability in the combustion chamber. It also improves IMEP and thermal efficiency. 5. In higher RPMs, the combustion is retarded and IMEP and thermal efficiency are increased because the combustion takes place in the expansion stroke to produce more expansion work. 6. Among the considered parameters, the equivalence ratio and inlet pressure are the most valuable parameters which can improve the combustion and performance characteristics of the HCCI engine. Acknowledgment The authors wish to gratefully thank Prof. M.D. Checkel, for providing the engine research facility of University of Alberta for the tests to be experimented and also Dr. V. Hosseini for his ongoing scientific support. In addition, the Iranian Fuel Consumption Organization (IFCO) financially supported this study, which is highly acknowledged. References [] Onishi S, Hong SJ, Shoda K, Do Jo P, Kato S. Active thermo atmospheric combustion (ATAC) a new combustion process for internal combustion engines. SAE paper 795; 979. [2] Najt PM, Foster DE. Compression-ignited homogeneous charge combustion. 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