Experimental and stochastic reactor modeling results of an HCCI

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Experimental and stochastic reactor modeling results of an HCCI engine fueled with primary reference fuel Halit Yasar, Enes Usta, and Usame Demir Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03880 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018

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Experimental and stochastic reactor modeling results of an HCCI engine fueled with primary reference fuel H. Yasar1, E. Usta2 U. Demir3 1

Sakarya University, Engineering Faculty, Mechanical Engineering Department, 54050, Sakarya-Turkey 2 Đstanbul Naval Shipyard, 34944, Đstanbul, Turkey 3 Bingöl University, Engineering and Architecture Faculty, Mechanical Engineering Department, 12000, BingölTurkey

Abstract In recent years, many researches have been performed in order to decrease fuel consumption, noise and exhaust emission levels in internal combustion engines. In this study, the effects of excess air coefficient on performance and exhaust emissions (CO, CO2) of an HCCI engine fueled with primary reference fuel (PRF) were investigated for different intake air pressure and temperature values. The simulation studies were performed by using SRM Suite software. The chemical kinetic mechanism, which contains 138 species and 633 reactions that are embedded into the program, was used to simulate the combustion of the PRF fuel during the combustion simulations. The analysis covers the full cycle, and provides data about induction, compression, combustion, expansion and exhaust. The exhaust emissions, cylinder pressure and heat release rate results were compared with the experimental data. The zero-dimensional software (SRM Suite) gives quite reasonable results compared with the experimental data and it has advantages such as the shorter solution time and the unlimited chemical kinetic mechanism compared with the 3-dimensional combustion simulation softwares. Keywords: SRM Suite, Combustion Analysis, HCCI Engine, Engine Performance, Exhaust Emissions

1. INTRODUCTION The detailed chemical kinetic mechanisms, improved for the high carbon fuels, usually have many components and chemical reactions. Moreover, it takes only minutes with today’s computers to perform analysis by using the zero-dimensional solvers and detailed chemical mechanisms for the high carbon fuels, during the steady-state combustion analysis.1 In recent years, it has been trying to improve zero-dimensional combustion analysis softwares to simulate the internal combustion engines effectively and properly. One of these softwares, SRM Suite2, can analyze combustion in internal combustion engines by using the stochastic solution method, as its main development purpose is combustion and emission analysis for the internal combustion engines by using the stochastic solution method. Moreover, it is possible to gain the relative dimensions to the analysis of internal combustion engine created by using stochastic solution equations, and thus the results get closer to the experimental observations. 1

Correspondence to: Halit Yasar, University of Sakarya, Engineering Faculty, Department of Mechanical Engineering, 54187, Esentepe Campus Serdivan-Sakarya-TURKEY. Tel: +902642955879 Fax: +902642955601 (shared fax) E-mail: [email protected], [email protected]

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Also, many publications that examine the reliability and the ability of the software, are available in the literature. It has performed the combustion analysis by working together with some of the studies conducted to determine the usability of the SRM software on different engine concepts and combustion conditions such as; fuel from the manifold injection HCCI engine modeling3-8, modeling the effects of engine combustion of alternative fuel mixture911 ,early and one injection HCCI engine modeling12, double jet HCCI engine modeling [13], multi-cycle non-permanent simulation and control14-16, soot formation17 and CFD program that is KIVA program.18 In the present study, cylinder pressure and net heat release rate predicted by using SRM Suite software are compared with the experimental data obtained from Ricardo Hydra HCCI engine. The analyses were performed for four different excess air coefficients at two different intake air temperature and pressure values. Beside of that, the simulation results of CO and CO2 emissions are compared with the experimental data. Moreover, the simulation results cylinder temperature are also given for different excess air coefficients.

2. EXPERIMENTAL In this study, the experimental data used to validate the simulation results were obtained during the project studies named “Improved engine efficiency-impact of deposits on HCCI”, at Shell Research Centre in the UK, within the support program of the author’s EU Marie Curie Transfer of Knowledge Scheme (FP6). The measurements were carried out on a Ricardo Hydra single-cylinder research engine. The engine specifications and valve timings are given in Table 1. The intake air was pressurized using an electrically driven compressor and the intake air was also heated. The temperature of the fresh charge (Tint) was measured with a K thermocouple located 80 mm from the manifold head face and approximately 200 mm from the back of the inlet valve. A proportional–integral–derivative (PID) regulator held the charge temperature within 276 K of the set value. The mass flow rate of intake air was measured with a Cussons laminar air flow meter. The gravimetric fuel flow-rate was also measured. The mixture strength was measured by a Horiba MEXA 1500 exhaust gas analyzer. Cylinder pressure was measured with a Kistler 6125 piezoelectric pressure transducer located in the side of the pentroof cylinder head. Heat release data was calculated from the pressure signal by using Rassweiler and Withrow heat release model.19 The measurement data was recorded on an AVL data acquisition system. Engine tests were performed for two intake air pressures of 1 bar and 2 bar. During the experiments, engine speed was 1200 rpm, intake air temperature was 353 K for 1 bar and 523 K for 2 bar intake air pressure. The excess air coefficient was set to fixed values of 4.1, 4.5, 4.75 and 5.0 for 353 K and 3.0, 3.75, 4.0 and 4.25 for 523 K by adjusting the amount of fuel injected. Lubricant and coolant temperatures were both held at a constant 363 K. The PRF fuel used in the experimental studies consists of 85% i-octane and 15% n-heptane (PRF-85). The measurement accuracies and uncertainties in the calculated results are given in Table 2.


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3. MODEL In the current study, SRM Suite software (CMCL Innovation, 2014) was used to simulate HCCI combustion. SRM software has a breathing model where the simulations were carried out between intake valve opening (IVO) and exhaust valve closing (EVC). Su et al.20 implemented a new PDF-based computational model to simulate an HCCI engine with DI during gas exchange to the SRM solver. From the experience during the simulation studies, a model calibration process with breathing model simulation has more accurate results than close volume simulations, which are between intake valve closure (IVC) and exhaust valve open (EVO).21 An experimental engine, which has 3% crevice volume of the total cylinder, was implemented to the SRM. To increase the simulation consistency, 20 µm blow-by or ring gap defined from the experimental geometry were used in the SRM. Time steps were chosen as 0.1◦CA with 100 numbers of particles and particle weighting factor was set at 12 for each simulation. Su et al.22 showed that increasing the number of particles for SRM simulations can decrease the statistical errors significantly. Geometrical compression ratio was used as effective compression ratio. The ratio of the connecting rod length to the crank radius was 1.66. Mean mixing time was chosen as 5 ms and the localness mixing model (LMM) was used.23 LMM is particularly recommended for IC engine applications since it takes localness into account. Analysis were performed for the same operating conditions as experimental. SRM software includes the structure of the boundary layer and the crevices models with principles of heat and mass transfer through the cylinder wall, which is translated in the context of the stochastic modeling approach. The heat loss through the cylinder wall leads to a significant temperature gradient in the boundary layer, thus the wall temperature was set to 430 K ± 2. It is needed to use a suitable chemical kinetic mechanism for combustion analysis. In this article, the chemical kinetic mechanism, developed by Andrea et al.1, for the PRF was used. This mechanism includes 138 species and 633 reactions. This model is used to predict both engine combustion behaviors and exhaust emissions. 4. RESULTS AND DISCUSSION In this article, the experimental results were compared with the predicted results in terms of cylinder pressure, cylinder temperature, heat release rate, CO and CO2 emissions. Predicted and measured cylinder pressure traces are shown in Fig.1 for different excess air coefficients for 2 bar and 353 K intake air pressure and temperature values. In Fig. 2, experimental and predicted cylinder pressures are compared for different excess air coefficients for 1 bar and 523 K intake air pressure and temperature values. It can be seen from the figures that maximum cylinder pressure decreases with increasing excess air coefficient. It is also observed that the ignition delay increases and the crank angle at which the maximum cylinder pressure moves away from TDC with increasing excess air coefficient. As the excess air coefficient increases, the air-fuel mixture becomes leaner. As a result, the ignition delay increases and the burning rate decreases. This increases the combustion duration. Thus, the maximum pressure decreases the crank angle at which the maximum cylinder pressure moves away from TDC.


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In Table 3, the maximum cylinder pressure values and the differences between experimental and predicted results were presented. It can be seen from the table that the experimental and predicted maximum cylinder values are adjacent to each other and the differences are between 0.10-0.85 percent. HCCI combustion can only be possible in case of being a lean mixture in a limited range of air-fuel ratio. In an HCCI combustion, maximum cylinder temperature is in the range of 14001600 K and this temperature range is lower than the diesel combustion temperature which is in the range of 1700-2200 K. Low cylinder gas temperature causes low NOx and particulate emissions. On the other hand, it causes higher HC and CO emissions, resulting in misfire and slow oxidation reactions compared with the conventional diesel combustion.24 Fig. 3 and Fig. 4. show the predicted cylinder temperature for different intake conditions and excess air coefficients. In Fig. 3, predicted cylinder temperature traces are shown for 2 bar and 353 K intake air pressure and temperature values. On the other hand, Fig. 4 shows predicted cylinder temperature traces for 1 bar of intake air pressure and 523 K of intake air temperature.

It can be seen from the Fig. 3 that maximum cylinder temperatures are reached around 4-7.4 crank angle degrees after TDC. Maximum cylinder temperature was 1525 K for the excess air coefficients of 4.1 and reached at 4 crank angle degree. On the other hand, maximum cylinder temperatures are reached around 7.5-9.2 crank angle degrees after TDC. Maximum cylinder temperature was 1653 K for the excess air coefficients of 3.0 and reached at 9.2 crank angle degree. For the richer fuel-air mixture, the cylinder temperature starts to rise earlier. It can be seen from the figures that instantaneous and maximum cylinder temperatures decrease with increasing excess air coefficient. As can be seen from Fig. 3, cylinder temperature drops below the complete combustion limit for the excess air coefficients of 5.0 and 4.75. This can cause misfire, higher HC and CO emissions. Furthermore, the cylinder temperature increases with decreasing excess air coefficient and gets close to the NOx limit in the case of 3.0 excess air coefficient. This means that very low excess air coefficients (lower than 3.0) can cause NOx formation as well.

CO and HC emissions are the main issues for HCCI engines. HCCI engines produce higher HC and CO emissions compared to the modern and advanced diesel engines, especially at low loads. Generally, CO emissions are considered to be controlled by the excess air coefficient and CO oxidation reactions are also sensitive to the cylinder gas temperature.25 The averaged experimental and predicted CO and CO2 emissions are given in Fig. 5. In Fig. 5a, CO and CO2 emission traces are shown for 2 bar and 353 K intake air pressure and temperature values. On the other hand, CO and CO2 emission traces are shown for 1 bar, 523 K intake air pressure and temperature values in Fig. 5b. It can be seen from the figures that the difference between model and experiment increases for higher excess air coefficients.


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Predicted and experimental net heat release traces are shown in Figs. 6 and 7 for two different intake air temperatures and four different excess air coefficients. Low temperature combustion (cold flame zone) is formed before 20 crank angles remains to TDC as shown in Fig. 6. Continuously, the premixed combustion phase occurs and the combustion is not completed. So, a typical HCCI heat release curve is formed too. It is known that the main combustion phase begins around 1000 K while the low temperature combustion occurs near 800 K.26 Immediately, after the low temperature combustion negative temperature zone consists, and the temperature increases during the decrease of all reactions in this area. As can be seen from Fig.6, the maximum net heat release values at the excess air coefficients of 4.1, 4.5 and 4.75 occur slightly earlier than TDC. The reason is that in these running conditions, the intake air pressure is high (2 bar). Due to the high intake air pressure, the cylinder temperature significantly increases during the compression stroke, and consequently the ignition delay decreases. As a result, the combustion starts earlier. If the combustion starts too early, the potential for generating useful work from this heat energy reduces and as a result, the thermal efficiency of the engine decreases. These results show that the control of combustion in HCCI engines is a major problem. For this purpose, combustion in HCCI engines can be controlled by using the methods such as variable intake air pressure and temperature, variable compression ratio and variable excess air coefficient depending on the operating conditions. 5. CONCLUSIONS In this article, the effects of excess air coefficient on performance and exhaust emissions of an HCCI engine fueled with primary reference fuel were investigated for different intake air pressure and temperature values. The simulation studies were performed by using SRM Suite software. The chemical kinetic mechanism, which contains 138 components and 633 reactions that are embedded into the program, was used to simulate the combustion of the PRF fuel during the combustion simulations. The analysis covers the full cycle, and provides data about induction, compression, combustion, expansion and exhaust. The following conclusions have made depending on the obtained results: ● The model validation shows that the predictions for in-cylinder pressure agree well with the experimental ones. The difference between the model and the experiment was less than 0.85 percent. ● The cylinder temperature decreases with increasing excess air coefficient and drops below the complete combustion limit for the excess air coefficients of 4.75 and 5.0. This may cause misfire, higher HC and CO emissions. Furthermore, the cylinder temperature gets close to the NOx formation limit in the case of excess air coefficient of 3.0. This means that very low excess air coefficients (lower than 3.0) may cause thermal NOx formation as well. ● SRM Suite software has advantages such as the shorter solution time and the opportunity of unlimited chemical kinetic mechanism usage compared with the 3dimensional combustion simulation softwares.


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FIGURES

Pressure (Bar)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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278 (a)


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Pressure (Bar)

10

(b) Figure 1. Variation of measured and predicted cylinder pressures for different excess air ratios

Pressure (Bar)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

279

(a)


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11 280

281

(b) Figure 2. Variation of measured and predicted cylinder pressures for different excess air ratios

282


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12

Temperature (K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

283 Figure 3. Variation of predicted cylinder temperatures for different excess air ratios


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13

Temperature (K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309

Figure 4. Variation of predicted cylinder temperatures for different excess air ratios


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14 310 311 1.4

Pint = 2 bar,Tint= 353 K

1.3

SRM CO Emission Experimental CO Emission

1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 4

4.1

4.2

4.3

4.4

4.5

4.6

4.7

4.8

4.9

5

Excess Air Coefficient 8

Pint= 1 bar,Tint= 523 K

7.5

SRM CO 2 Emission 7

Experimental CO2 Emission

6.5 6 5.5

CO2 Emission (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 2.75

3

3.25

3.5

3.75

Excess Air Coefficient

312 313 314 315 316 317 318 319 320 321 322 323 324 325

(a) (b) Figure 5. Variation of measured and predicted CO and CO2 exhaust emissions


4

4.25

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Net Heat Release Rate (kJ/m 3degree)

15

326 327 328

(a)


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329 330

(b) Figure 6. Variation of measured and predicted net heat release traces for different excess air coefficients


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16

331 332 333

(a)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

334 (b)

335 336 337

Figure 7. Variation of measured and predicted net heat release traces for different excess air coefficients


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338 339 340

TABLES Table 1. Ricardo Hydra engine specifications

Parameter Bore Stroke Connection rod length Compression ratio Inlet valve diameter Number of valves Inlet valve opening (IVO) Inlet valve closing (IVC) Exhaust valve opening (EVO) Exhaust valve closing (EVC) 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361


Value 86 86 143.5 14.04 32 4 340 612 120 332

Unit mm mm mm mm CAD CAD CAD CAD

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362

Table 2. The measurement accuracies and uncertainties in the calculated results

Measured parameter Temperature CO (% vol.) CO2 (% vol.) Calculated parameter Cylinder pressure Net heat release

Accuracy ±0.1 oC ±0.01 ±0.01 Uncertainity ±0.16% ±1.70%

363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389


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Table 3. Measured and predicted maximum cylinder pressures and the differences

Intake Air Excess Air Conditions Coefficient P = 2 bar, Tint = 353 K int 4.10 Pint = 2 bar, Tint = 353 K 4.50

Pmax (bar) Experiment 102.90

Pmax (bar) Model 102.66

Difference (%) 0.24

94.80

95.17

0.39

4.75

Pint = 2 bar, Tint = 353 K

89.52

89.16

0.40

5.00


86.39

86.48

0.10

3.00


50.19

49.92

0.55

3.75


40.56

40.22

0.84

4.00


39.09

39.06

0.08

4.25


39.59

39.32

0.69

391


Experimental and stochastic reactor modeling results of an HCCI

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