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Numerical analysis of knock during HCCI in a high compression ratio methanol engine based on LES with detailed chemical kinetics
Energy Conversion and Management 96 (2015) 188–196
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Numerical analysis of knock during HCCI in a high compression ratio methanol engine based on LES with detailed chemical kinetics Xudong Zhen a, Yang Wang b,⇑ a b
School of Automotive and Transportation, Tianjin University of Technology and Education, Tianjin 300222, China State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 28 November 2014 Accepted 16 February 2015
Keywords: Methanol engine Knock HCCI LES Detailed chemical kinetics
a b s t r a c t In this study, knock during HCCI (homogeneous charge compression ignition) was studied based on LES (large eddy simulation) with methanol chemical kinetics (84-reaction, 21-species) in a high compression ratio methanol engine. The non-knocking and knocking combustion of SI (spark ignition) and HCCI engines were compared. The results showed that the auto-ignition spots were initially occurred near the combustion chamber wall. The knocking combustion burnt faster during HCCI than SI methanol engine. The HCO reaction rate was different from SI engine, it had two obvious peaks, one was positive peak, and another was negative peak. Compared with the SI methanol engine, in addition to the concentration of HCO, the concentrations of the other intermediate products and species such as CO, OH, CH2O, H2O2, HO2 were increased significantly; the reaction rates of CH2O, H2O2, and HO2 had negative peaks, and whose values were several times higher than SI methanol engine. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction It is generally accepted that knock limits the increase of geometric compression ratio, so it is one of the main obstacles to improve the thermal efficiency of SI engines [1–3]. Nowadays, many new combustion modes are proposed and developed, for instance, HCCI, PCCI (premixed charge compression ignition), RCCI (reactivity controlled compression ignition), DICI (direct injection compression ignition) etc. Compared with the conventional SI or CI (compression ignition) engines, the HCCI combustion mode has many advantages such as low NOx, soot emissions and high thermal efficiency [4]. So, it is most widely studied. However, knock phenomenon also occurred in HCCI engines. Because the HCCI engines do not require external ignition device, it has no spark plugs and it will not produce a propagating flame front, so the normal combustion of SI and HCCI engines are different. Therefore, the knock mechanism occurred in HCCI engines is different with those in SI engines. Meanwhile, there are also a lot of the common characteristics in both SI and HCCI engines, for instance, the in-cylinder pressure will generate high amplitude oscillations, and the oscillation frequency is relatively high during knock in the combustion chamber [5]. Currently, the knock
⇑ Corresponding author. Tel.: +86 22 27406842x8017; fax: +86 22 27383362. E-mail address: [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.enconman.2015.02.053 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.
mechanism in HCCI engines has not been extensively studied, especially for high compression methanol engines. The HCCI combustion mode involves simultaneous heat release in a spatially large region, so it is very fast [6]. Eng [7] investigated the differences between knock in both HCCI and SI engines. He found that the pressure oscillation amplitudes in HCCI combustion were 5–10 times higher than SI knocking combustion. Azimov et al. [8] found that there was a marked difference between the intensity of the flame emissions of knocking and non-knocking events. Shi et al. [9] studied knocking combustion in a diesel HCCI engine. They found that increasing engine load and speed would increase HCCI knocking intensity. Griffiths et al. [10] studied the conditions for the knock origins in HCCI combustion mode by using a rapid compression machine. It was found that knock was originated from localized development of the incandescent hot ignition stages. Lecocq et al. [11] investigated the knock and the pre-ignition in SI engines, a new LES model coupled with flame surface density and tabulated kinetics approach has been used in study. It was found that knock phenomenon could be simulated by using the developed model through the hot spots. Robert et al. [12] studied the knock based on LES and TKI (tabulated kinetics of ignition) model in a spark ignition engine. It was demonstrated that LES model was able to simulate knock in realistic engine, the in-cylinder pressure variability, the knock occurrence timing and frequency could be predicted by LES method. Linse et al. [13] proposed a new knock
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model based on probability density function approach and detailed chemical kinetics to predict knock in turbocharged direct injection SI engines. It was found that the mean knock onset could be predicted by the proposed new knock model with reasonable accuracies, and the impacts of spark timing on the knock cycles also could be simulated. The end-gas auto-ignition occurrence during knock was studied by Kawahara et al. [14] in a compression–expansion engine using high-speed video camera. It was found that the end-gas autoignition was occurred due to the flame front propagation and intensity oscillations caused by shockwaves. Hou et al. [15] studied the knock characterization in HCCI DME (dimethyl ether) engine by using wavelet packet transform. It was found that without knock, mean absolute value of coefficients, wavelet packet energy and entropy for the subsignal 1 were much greater than others. As knock occurrence, three wavelet packet quantifiers for seven subsignals increased greatly. Vancoillie et al. [16] developed a knock prediction model for methanol engines. It was found that compared to order correlations that were developed for gasoline, the developed prediction model could capture the high temperature sensitivity of methanol auto-ignition kinetics, so it had a better prediction of the knock limited spark advance for various compression ratios and loads. Szwaja and Naber [17] studied the dual nature of hydrogen/air mixture knock in a SI engine. It was found that the slight knock was generated due to the combustion instabilities. The serious knock was generated due to the hydrogen/air mixture auto-ignition, and the generated intensity would be several times higher than the slight knock. Sileghem et al. [18] developed a quasi-dimensional knock model for gasoline/alcohol blends. The results showed that compared with the experimental results, the proposed knock integral approach was satisfactory. Chen et al. [19] proposed a new model for predict knock onset in SI engines with cooled EGR (exhaust gas recirculation). The results showed that both with and without EGR, the proposed model was able to accurately predict knock onset. Merola and Vaglieco [20] studied the phenomena of nonknocking and knocking combustion in a single cylinder SI engine, and the study was carried out based on the analysis of flame natural emission imaging and spectroscopy from UV (ultraviolet) to visible. The results showed that OH and HCO radical species were related to the knock onset and duration. Pan et al. [21] studied the knock in a SI engine based on the analysis of in-cylinder pressure oscillation characteristic. It was found that the pressure oscillation was due to the reaction of end-gas auto-ignition, and it could be effectively suppressed through the appropriate cooled EGR technique. Methanol has been recently used as an alternative to conventional fuels for IC (internal combustion) engines in order to satisfy some economical and environmental concerns. The properties of methanol, gasoline and diesel fuels are shown in Table 1 [22]. Despite numerous researches, it is found that the knock mechanism such as methanol fuel during HCCI has not been fully studied. For instance, the knock occurrence timing, the knock occurrence location, the knock combustion intensity and the produced intermediate species and radicals (for instance, the mass fraction and reaction rate etc.) have not been fully studied. The study of the knocking combustion mechanism is the core scientific engineering technology issue for the high compression ratio methanol engine, ant the findings can provide adequate theoretical guidance to the development of the high compression ratio methanol engine. Therefore, it is necessary to study the knocking combustion mechanisms during HCCI mode. HCCI combustion is mainly governed by chemical kinetics. Because the auto-ignition temperature of methanol is relatively high (465 °C), so it is hard to auto-ignite and therefore requires a higher compression ratio, some amount of intake
Table 1 The properties of methanol, gasoline and diesel [20]. Fuel property
Methanol
Gasoline
Diesel
Formula Molecular weight Oxygen content Stoichiometric air/fuel ratio Low calorific value (MJ/kg) High calorific value (MJ/kg) Freezing point (°C) Boiling point (°C) Flash point (°C) Auto-ignition temperature (°C) Research octane number Motor octane number Cetane number Inflammability limit Specific heat (20 °C) (kJ/kg K) Latent heat (kJ/kg) Viscosity (20 °C) (CP)
CH3OH 32 50% 6.45 19.66 22.3 98 64.8 11 465 108.7 88.6 3 6.7–36 2.55 1109 0.6
C5–12 95–120 0 14.6 44.5 46.6 57 30–220 45 228–470 80–98 81–84 0–10 1.47–7.6 2.3 310 0.29
C10–26 180–200 0 14.5 42.5 45.8 1 to 4 175–360 55 220–260
40–55 1.85–8.2 1.9 270 3.9
heating, or some type of pre-ignition. Adjusting inlet temperature and pressure of the mixture can help to control ignition and combustion intensity [23]. In this paper, knock phenomenon during HCCI was studied based on LES coupled with methanol chemical kinetics (84reaction, 21-species) in a high compression methanol engine. The adopted methanol mechanism was validated through many tests [24]. The results from this study should be valuable in explaining the knock mechanism of methanol fuel during HCCI. 2. Numerical methods 2.1. LES models It is important to choose the turbulence model during simulation, and the simulated results will be different when using different turbulence models. Nowadays, LES method is widely used to analyze the turbulent flow in IC engine research fields [25,26], and the knocking combustion research field of IC engines is not exception [11,12,27]. LES is a spatial average of turbulence pulsation (or turbulent vortex), that is, the large-scale vortex and small-scale vortex are separated through some kind of filter functions, and the large-scale vortex is simulated directly, and the small-scale vortex is simulated by using sub-models [28]. Compared with the conventional time-averaged methods such as RANS (Reynolds averaged Navier stokes) models, one of the primary advantages is that there will be more flow eddies, structures and vortices can be simulated. Another advantage is that the predictive capability is better. In addition, it can be used to simulate some new unknown phenomena in engines, so it is becoming a better choice to simulate unsteady turbulent flow [29–32]. The governing equation of LES is illustrated as [33,34]:
2
3
@ðui Þ @ðui uj Þ 1 @p @ 6 @ui 7 þ ¼ þ ðui uj ui uj Þ5 4v @t @xj q @xi @xj @xj |fflfflfflfflfflfflffl ffl{zfflfflfflfflfflfflfflffl}
ð1Þ
sij
where u is the velocity; p is the pressure; t is the time; q is the density. The equation of sub-grid scale stress tensor is illustrated as:
1 3
sij skk ¼ 2tSGS Sij
ð2Þ
where
Sij ¼
1 @uj @ui þ 2 @xi @xj
ð3Þ
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The sub-grid scale viscosity is illustrated as [33,34]:
tSGS ¼ ðC s f DÞ2 jSj
ð4Þ
where C s is a fixed model constant, is defined as 0.1; D is the fil1=3
ter width, and can be defined as D ¼ ðVolÞ , Vol is the volume of calculated cell; jSj is the resolved rate of strain tensor, and can be defined as:
jSj ¼ ð2Sij Sij Þ1=2
ð5Þ
Near wall, it is required a wall-damping function of Van Driest type to vanish the eddy viscosity on the wall, is defined as:
f ¼ 1 exp
yþ 25
ð6Þ
2.2. Engine model The specifications of methanol engine are listed in Table 2. The simulation is performed between intake valve closing and exhaust valve opening. The 1-D simulation model is established through the GT-Power software platform [22,35]. The three-dimensional (3-D) simulations are calculated through the CFD (computational fluid dynamics) code, which is established through the AVL FIRE software platform. The computational mesh of 3-D CFD calculation is shown in Fig. 1, the maximum mesh is exceed 450,000 cells, and the maximum grid size in only 0.2 cm. Knock can be analyzed based on the local pressures of combustion chamber [36,37]. The knock intensity can be defined as the intensity of the amplitude of pressure oscillation. For quantitative comparisons for knock intensities, an index PPmax (peak-to-peak value of vibration signal) is adopted to represent knock intensity [22,35], where:
PP max ¼ maxðjpðhÞjÞ
ð7Þ
While pðhÞ is the knocking pressure. Knock detection threshold can distinguish knocking combustion from non-knocking combustion. Sometimes, an index PP th is adopted to represent knock threshold, it is a predetermined threshold that distinguishes knocking combustion cycles from non knocking combustion cycles. It is difficult and complex to determine the threshold; its value should depend on the engine operating point, but often depend on the empirical values [37]. In the present study, the 3-D CFD calculation and the chemical reaction calculation were performed as separate cases. The exchanged data included the boundary conditions at the surface and source-terms (momentum, mass, energy etc.) in the volume of the respective calculation domains, and it was performed at each iteration when transient calculations. Both non-mesh related data
Table 2 Engine specifications and conditions.
and mesh-related data (e.g. velocity or pressure at cell centers or at cell nodes or at boundary face centers) could be exchanged [25]. The coupling calculations were carried out by the separate module (General Species Transport Module and General Gas Phase Reaction Module) integrated in CFD code, and the chemical reaction kinetics can be inputted in the separate model. In order to save computation time, the simulation was performed on a 24 G memories and 12 Xeon processors workstation. In order to calculate the pressure oscillation caused by autoignition, calculated local pressures at ten different locations in the combustion chamber are used for analysis. The adopted ten local positions are illustrated in Fig. 2. The established 1-D GT-Power calculation model and 3-D CFD calculation model are validated, the 1-D model is validated by comparing the experimental data. Table 3 illustrates the comparison of results of the 1-D model calculated results and the experimental data. It can be seen that the error between the calculated and experimental data is within the permitted (less than 5%), so it is validated. Fig. 3 shows the calculated in-cylinder pressure comparison of the 3-D CFD model and the validated 1-D model. The error between the 1-D and 3-D calculations is within the permitted (less than 10%). Therefore, the established 3-D CFD model can be used to study the engine combustion phenomenon. 3. Simulation results and analysis
Engine type
Methanol 4100 series
Engine configuration
Inline four-cylinder, four-stroke, turbocharged and intercooled 100 127 17.5 3.99 x shape 54.0 18.0 Spark ignition 83 410 2600 Methanol
Bore (mm) Stroke (mm) Compression ratio Displacement (L) Chamber shape Chamber diameter (mm) Chamber depth (mm) Combustion mode Rated power (kW) Maximum torque (Nm) Maximum speed (r/min) Fuel
Fig. 1. Computational mesh of the high compression ratio methanol single-cylinder engine.
The simulation conditions are shown in Table 4. The boundary conditions and initial conditions are also illustrated in Table 4. The simulated ten different local pressure results in the combustion chamber are shown in Fig. 4. It can be seen from the Fig. 4, because the adopted compression ratio of the engine is very high, so the simulated local pressure oscillations are relatively very high. The simulated PPmax values at ten different local positions are illustrated in Fig. 5, the variations of pressure oscillation magnitudes at ten locations are clearly seen. It can be seen from the figure that the most severe pressure oscillations occurred at position 6 (the PPmax value is only about 8.88 MPa), while the most weak pressure oscillation occurred at position 7 (the PPmax value is only about 1.59 MPa). However, in SI methanol engine, the most severe
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X. Zhen, Y. Wang / Energy Conversion and Management 96 (2015) 188–196 Table 4 Operating conditions for knock simulation. Engine speed (rev/min) CH3OH mass fraction O2 mass fraction N2 mass fraction Boundary conditions Head temperature (K) Piston temperature (K) Cylinder temperature (K) Initial conditions Temperature (K) Pressure (Bar) Turbulence kinetic energy (m2/s2)
1200 0.138604 0.199844 0.661552 450 450 450 432 1.49 768.75
Fig. 2. Numerical transducer locations on the high compression ratio SI methanol engine.
Table 3 Comparison of results of 1-D model calculated results and experimental data. Engine performance
Experimental data
1-D calculated results
Errors (%)
Torque (Nm)) Power (kW)
410 83
427.7 85.9
4.32 3.49
Fig. 4. Simulated in-cylinder pressures at local positions during HCCI.
Fig. 3. Comparison of 3-D model calculated and 1-D validated model calculated in-cylinder pressure.
pressure oscillation occurred at position 8 (the PPmax value is only about 37.12 MPa), while the most weak pressure oscillation occurred at position 7 (the PPmax value is only about 2.99 MPa) [35]. So it can be found that the present HCCI engine tends to knock near the combustion chamber wall and the intake valve side. The reason was that with the combustion process, the intake valve temperature was gradually increased. The large differences of PPmax values between HCCI and SI engines are mainly because of its different combustion modes, for instance, the spark timings have significant impacts on the knock intensity [3,22]. Fig. 6 illustrates the temperature evolution results inside the combustion chamber during HCCI. As can be seen from the Fig. 6, auto-ignition spots are initially occurred near the combustion chamber wall, and the spots are gradually expanded and
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Fig. 5. Calculated the PPmax values during HCCI.
propagating from the combustion chamber wall to center. Because the combustion chamber in methanol engine is x shape, and the compression ratio is higher in methanol engine during HCCI mode, so the corresponding in-cylinder temperature near TDC (top dead center) is higher. With the piston nears TDC, the flow velocity of in-cylinder first increased, and then decreased. The maximum of velocity occurred at 6.68 °ATDC as illustrated in Fig. 7. With the overall increases of temperature and pressure in-cylinder make the mixture at the outer region reaches to auto-ignition conditions. So, the surrounding mixture was compressed ignited in multipoint form [38]. For the case with crank angle at 6.84 °ATDC, a rapid increase in combustion chamber can be observed. As the auto-ignition occurrence, the end-gas temperature is above 3000 K. In the following within 0.2 °CA, the residual methanol is consumed quickly. Compared with the SI combustion mode (SI methanol engine) [35], the HCCI auto-ignition is occurred simultaneously at many locations, in the present engine, the knocking combustion duration is shorter, so it burn faster. Cho et al. [39] studied the controlled auto-ignition characteristics of methane/air gas mixture. It was found that the reaction region shapes were irregular, and the locations of reaction initiation were
random. It was also found that spontaneous ignition was a typical feature of CAI (HCCI) combustion mode. Fig. 8 illustrates the OH radicals evolution results inside combustion chamber during HCCI. In Fig. 8, the OH radicals evolution results inside combustion chamber are basically the same with the temperature evolution results inside combustion chamber (see Fig. 8), which is consistent with the results of SI combustion mode (SI methanol engine) [35]. Fig. 9 illustrates the pressure evolution results inside combustion chamber during HCCI. It can be seen from the figure that auto-ignition spots are initially occurred near the positions 6 and 2. As the ongoing of combustion process, the pressure wave is formed in the combustion chamber. When the engine is running at 6.84 °ATDC, the most serious knock is occurred at position 6. It is clearly that the pressure wave is propagating from wall to center of the combustion chamber. The simulated results showed that when the present high compression ratio engine is running with HCCI mode, it tends to knock in the un-burnt area on the chamber wall. The produced intermediate species and radicals (for instance, the mass fraction and reaction rate etc.) will be different during knock between SI combustion modes and HCCI combustion modes. Fig. 10 illustrates the in-cylinder intermediate species and radicals during HCCI. In Fig. 10, it is clearly to observe that the generations of CH2O (formaldehyde), H2O2 and HO2 species anticipate the other species such as CO, HCO and OH during HCCI. The peak generation timings of OH, HCO, CH2O, H2O2, and HO2 are 6.56, 6.56, 6.96, 6.96 and 6.8 °ATDC respectively. Compared with the SI methanol engine [35], the generation of H2O2 anticipates the HO2, which is an opposite result. Species such as H2O2 and CH2O almost generate simultaneously. Compared with the SI methanol engine [35], in addition to the concentration of HCO (the concentration of HCO radicals has a little change, the peak mass fraction of HCO radicals is only up to 5.3E-7), the concentration of the other intermediate products and species such as CO, OH, CH2O, H2O2, HO2 are increased significantly. In terms of the root reasons, because during the combustion, the generation and consumption of species (Fig. 10) is determined by the chemical reaction rate, so it can be explained by the results of chemical reaction rate. Fig. 11 illustrates the reaction intensities of in-cylinder intermediate radicals and species during HCCI. In Fig. 11, it can be seen that compared with the SI methanol engine, the CO and OH have only one peak
Fig. 6. Simulated in-cylinder temperature profiles during HCCI.
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Fig. 7. Simulated in-cylinder flow velocity profiles during HCCI.
(it has two peaks in the SI methanol engine [35]). The reaction rates of OH radicals and CO are positive, which are consistent with the SI methanol engine. However, the reaction rates of CH2O, and H2O2 are negative, which are opposite to the SI methanol engine. The value of reaction rate of OH radicals is increased significantly. The value of reaction rate of CO is increased a little. It also can be seen in Fig. 11, as the reaction proceeds, the reaction rate of HCO is from positive to negative, and it has two obvious peaks, one is positive peak, and another is negative peak, which is different from SI methanol engine [35]. It is clearly that the reaction rate of CH2O has only one positive peak in the SI methanol engine, however,
in the HCCI methanol engine, it has only one negative peak. The negative peak value is up to eight times higher than the positive peak value. Compared with the SI methanol engine, the reaction rate of H2O2 has one obvious negative peak, and its value is several times higher; the reaction rate of HO2 also has one obvious negative peak, and the value is about ten times higher. In addition, it also can be seen from Fig. 11, the reaction time is very short (the reaction time is only 0.2 crank degrees), which was faster than SI methanol engine [35]. From the above analysis, it can be found that there is a clear relation between the in-cylinder pressure oscillation intensity (knock intensity) and the species reaction rate: the stronger the
Fig. 8. Simulated in-cylinder OH profiles during HCCI.
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Fig. 9. Simulated the evolution of in-cylinder pressure profiles during HCCI.
Fig. 10. Calculated the species mass fraction during HCCI.
species reaction rate, the higher the pressure oscillation intensity. The reaction rates of OH, CH2O, H2O2 play a dominant role in the in-cylinder pressure oscillation intensity during HCCI knock.
Fig. 11. Calculated the mean reaction rate during HCCI.
Some mass fractions and reaction rates of species during knocking combustion in SI and HCCI methanol engines are summarized in Table 5.
X. Zhen, Y. Wang / Energy Conversion and Management 96 (2015) 188–196 Table 5 The mass fractions and reaction rates of species during knock in SI and HCCI methanol engines. SI methanol engine The mass fractions HCO Many peaks HO2 Many peaks Many peaks CH2O and H2O2 The reaction rates HCO
Positive, many peaks
CO and OH CH2O and H2O2 HO2
Positive, two peaks Positive Many positive peaks
HCCI methanol engine One peak One peak One peak Two peaks, one positive, one negative Positive, one peak Negative Two positive peak, one negative peak
4. Conclusions In this paper, knock phenomenon during HCCI is studied in a high compression ratio methanol engine based on LES with methanol chemical kinetics. The knock occurrence timing, the knock occurrence location, the knock combustion intensity and the produced intermediate species and radicals (for instance, the mass fraction and reaction rate etc.) were studied. The obtained results from this study can be summarized as follows: (1) It was concluded that the HCCI knock in the present engine tended to occur near the combustion chamber wall and the intake valve side. The reason was that with the combustion process, the intake valve temperature was gradually increased. The HCCI auto-ignition was occurred simultaneously at many locations, in the present engine, the knocking combustion duration was shorter, so it burnt faster than SI methanol engine. (2) The OH radicals could be a good indicator of the temperature during HCCI, which was consistent with the results of SI engine. Compared with the SI methanol engine, the generation of H2O2 anticipated the HO2, which was an opposite result; the reaction rate of HCO was from positive to negative, and it had two obvious peaks, one was positive peak, and another was negative peak, which was different from SI methanol engine. (3) Compared with the SI methanol engine, in addition to the concentration of HCO, the concentrations of the other intermediate products and species such as CO, OH, CH2O, H2O2, HO2 were increased significantly; the reaction rates of CH2O, H2O2, and HO2 had negative peaks, and whose values were several times higher than SI methanol engine. (4) The reaction rates of OH, CH2O, H2O2 play a dominant role to the pressure oscillation intensity of in-cylinder during HCCI knock; the stronger the species reaction rate, the higher the pressure oscillation intensity. The reaction rate of OH was opposite to the CH2O and H2O2. The reaction rates of OH radicals and CO were positive during HCCI knock, which were consistent with the SI methanol engine. However, the reaction rates of CH2O, and H2O2 were negative, which were opposite to the SI methanol engine.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51406135, 51176137) and Tianjin University of Technology and Education (Grant No. KYQD14019).
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Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Numerical analysis of knock during HCCI in a high compression ratio methanol engine based on LES with detailed chemical kinetics Xudong Zhen a, Yang Wang b,⇑ a b
School of Automotive and Transportation, Tianjin University of Technology and Education, Tianjin 300222, China State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 28 November 2014 Accepted 16 February 2015
Keywords: Methanol engine Knock HCCI LES Detailed chemical kinetics
a b s t r a c t In this study, knock during HCCI (homogeneous charge compression ignition) was studied based on LES (large eddy simulation) with methanol chemical kinetics (84-reaction, 21-species) in a high compression ratio methanol engine. The non-knocking and knocking combustion of SI (spark ignition) and HCCI engines were compared. The results showed that the auto-ignition spots were initially occurred near the combustion chamber wall. The knocking combustion burnt faster during HCCI than SI methanol engine. The HCO reaction rate was different from SI engine, it had two obvious peaks, one was positive peak, and another was negative peak. Compared with the SI methanol engine, in addition to the concentration of HCO, the concentrations of the other intermediate products and species such as CO, OH, CH2O, H2O2, HO2 were increased significantly; the reaction rates of CH2O, H2O2, and HO2 had negative peaks, and whose values were several times higher than SI methanol engine. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction It is generally accepted that knock limits the increase of geometric compression ratio, so it is one of the main obstacles to improve the thermal efficiency of SI engines [1–3]. Nowadays, many new combustion modes are proposed and developed, for instance, HCCI, PCCI (premixed charge compression ignition), RCCI (reactivity controlled compression ignition), DICI (direct injection compression ignition) etc. Compared with the conventional SI or CI (compression ignition) engines, the HCCI combustion mode has many advantages such as low NOx, soot emissions and high thermal efficiency [4]. So, it is most widely studied. However, knock phenomenon also occurred in HCCI engines. Because the HCCI engines do not require external ignition device, it has no spark plugs and it will not produce a propagating flame front, so the normal combustion of SI and HCCI engines are different. Therefore, the knock mechanism occurred in HCCI engines is different with those in SI engines. Meanwhile, there are also a lot of the common characteristics in both SI and HCCI engines, for instance, the in-cylinder pressure will generate high amplitude oscillations, and the oscillation frequency is relatively high during knock in the combustion chamber [5]. Currently, the knock
⇑ Corresponding author. Tel.: +86 22 27406842x8017; fax: +86 22 27383362. E-mail address: [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.enconman.2015.02.053 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.
mechanism in HCCI engines has not been extensively studied, especially for high compression methanol engines. The HCCI combustion mode involves simultaneous heat release in a spatially large region, so it is very fast [6]. Eng [7] investigated the differences between knock in both HCCI and SI engines. He found that the pressure oscillation amplitudes in HCCI combustion were 5–10 times higher than SI knocking combustion. Azimov et al. [8] found that there was a marked difference between the intensity of the flame emissions of knocking and non-knocking events. Shi et al. [9] studied knocking combustion in a diesel HCCI engine. They found that increasing engine load and speed would increase HCCI knocking intensity. Griffiths et al. [10] studied the conditions for the knock origins in HCCI combustion mode by using a rapid compression machine. It was found that knock was originated from localized development of the incandescent hot ignition stages. Lecocq et al. [11] investigated the knock and the pre-ignition in SI engines, a new LES model coupled with flame surface density and tabulated kinetics approach has been used in study. It was found that knock phenomenon could be simulated by using the developed model through the hot spots. Robert et al. [12] studied the knock based on LES and TKI (tabulated kinetics of ignition) model in a spark ignition engine. It was demonstrated that LES model was able to simulate knock in realistic engine, the in-cylinder pressure variability, the knock occurrence timing and frequency could be predicted by LES method. Linse et al. [13] proposed a new knock
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model based on probability density function approach and detailed chemical kinetics to predict knock in turbocharged direct injection SI engines. It was found that the mean knock onset could be predicted by the proposed new knock model with reasonable accuracies, and the impacts of spark timing on the knock cycles also could be simulated. The end-gas auto-ignition occurrence during knock was studied by Kawahara et al. [14] in a compression–expansion engine using high-speed video camera. It was found that the end-gas autoignition was occurred due to the flame front propagation and intensity oscillations caused by shockwaves. Hou et al. [15] studied the knock characterization in HCCI DME (dimethyl ether) engine by using wavelet packet transform. It was found that without knock, mean absolute value of coefficients, wavelet packet energy and entropy for the subsignal 1 were much greater than others. As knock occurrence, three wavelet packet quantifiers for seven subsignals increased greatly. Vancoillie et al. [16] developed a knock prediction model for methanol engines. It was found that compared to order correlations that were developed for gasoline, the developed prediction model could capture the high temperature sensitivity of methanol auto-ignition kinetics, so it had a better prediction of the knock limited spark advance for various compression ratios and loads. Szwaja and Naber [17] studied the dual nature of hydrogen/air mixture knock in a SI engine. It was found that the slight knock was generated due to the combustion instabilities. The serious knock was generated due to the hydrogen/air mixture auto-ignition, and the generated intensity would be several times higher than the slight knock. Sileghem et al. [18] developed a quasi-dimensional knock model for gasoline/alcohol blends. The results showed that compared with the experimental results, the proposed knock integral approach was satisfactory. Chen et al. [19] proposed a new model for predict knock onset in SI engines with cooled EGR (exhaust gas recirculation). The results showed that both with and without EGR, the proposed model was able to accurately predict knock onset. Merola and Vaglieco [20] studied the phenomena of nonknocking and knocking combustion in a single cylinder SI engine, and the study was carried out based on the analysis of flame natural emission imaging and spectroscopy from UV (ultraviolet) to visible. The results showed that OH and HCO radical species were related to the knock onset and duration. Pan et al. [21] studied the knock in a SI engine based on the analysis of in-cylinder pressure oscillation characteristic. It was found that the pressure oscillation was due to the reaction of end-gas auto-ignition, and it could be effectively suppressed through the appropriate cooled EGR technique. Methanol has been recently used as an alternative to conventional fuels for IC (internal combustion) engines in order to satisfy some economical and environmental concerns. The properties of methanol, gasoline and diesel fuels are shown in Table 1 [22]. Despite numerous researches, it is found that the knock mechanism such as methanol fuel during HCCI has not been fully studied. For instance, the knock occurrence timing, the knock occurrence location, the knock combustion intensity and the produced intermediate species and radicals (for instance, the mass fraction and reaction rate etc.) have not been fully studied. The study of the knocking combustion mechanism is the core scientific engineering technology issue for the high compression ratio methanol engine, ant the findings can provide adequate theoretical guidance to the development of the high compression ratio methanol engine. Therefore, it is necessary to study the knocking combustion mechanisms during HCCI mode. HCCI combustion is mainly governed by chemical kinetics. Because the auto-ignition temperature of methanol is relatively high (465 °C), so it is hard to auto-ignite and therefore requires a higher compression ratio, some amount of intake
Table 1 The properties of methanol, gasoline and diesel [20]. Fuel property
Methanol
Gasoline
Diesel
Formula Molecular weight Oxygen content Stoichiometric air/fuel ratio Low calorific value (MJ/kg) High calorific value (MJ/kg) Freezing point (°C) Boiling point (°C) Flash point (°C) Auto-ignition temperature (°C) Research octane number Motor octane number Cetane number Inflammability limit Specific heat (20 °C) (kJ/kg K) Latent heat (kJ/kg) Viscosity (20 °C) (CP)
CH3OH 32 50% 6.45 19.66 22.3 98 64.8 11 465 108.7 88.6 3 6.7–36 2.55 1109 0.6
C5–12 95–120 0 14.6 44.5 46.6 57 30–220 45 228–470 80–98 81–84 0–10 1.47–7.6 2.3 310 0.29
C10–26 180–200 0 14.5 42.5 45.8 1 to 4 175–360 55 220–260
40–55 1.85–8.2 1.9 270 3.9
heating, or some type of pre-ignition. Adjusting inlet temperature and pressure of the mixture can help to control ignition and combustion intensity [23]. In this paper, knock phenomenon during HCCI was studied based on LES coupled with methanol chemical kinetics (84reaction, 21-species) in a high compression methanol engine. The adopted methanol mechanism was validated through many tests [24]. The results from this study should be valuable in explaining the knock mechanism of methanol fuel during HCCI. 2. Numerical methods 2.1. LES models It is important to choose the turbulence model during simulation, and the simulated results will be different when using different turbulence models. Nowadays, LES method is widely used to analyze the turbulent flow in IC engine research fields [25,26], and the knocking combustion research field of IC engines is not exception [11,12,27]. LES is a spatial average of turbulence pulsation (or turbulent vortex), that is, the large-scale vortex and small-scale vortex are separated through some kind of filter functions, and the large-scale vortex is simulated directly, and the small-scale vortex is simulated by using sub-models [28]. Compared with the conventional time-averaged methods such as RANS (Reynolds averaged Navier stokes) models, one of the primary advantages is that there will be more flow eddies, structures and vortices can be simulated. Another advantage is that the predictive capability is better. In addition, it can be used to simulate some new unknown phenomena in engines, so it is becoming a better choice to simulate unsteady turbulent flow [29–32]. The governing equation of LES is illustrated as [33,34]:
2
3
@ðui Þ @ðui uj Þ 1 @p @ 6 @ui 7 þ ¼ þ ðui uj ui uj Þ5 4v @t @xj q @xi @xj @xj |fflfflfflfflfflfflffl ffl{zfflfflfflfflfflfflfflffl}
ð1Þ
sij
where u is the velocity; p is the pressure; t is the time; q is the density. The equation of sub-grid scale stress tensor is illustrated as:
1 3
sij skk ¼ 2tSGS Sij
ð2Þ
where
Sij ¼
1 @uj @ui þ 2 @xi @xj
ð3Þ
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The sub-grid scale viscosity is illustrated as [33,34]:
tSGS ¼ ðC s f DÞ2 jSj
ð4Þ
where C s is a fixed model constant, is defined as 0.1; D is the fil1=3
ter width, and can be defined as D ¼ ðVolÞ , Vol is the volume of calculated cell; jSj is the resolved rate of strain tensor, and can be defined as:
jSj ¼ ð2Sij Sij Þ1=2
ð5Þ
Near wall, it is required a wall-damping function of Van Driest type to vanish the eddy viscosity on the wall, is defined as:
f ¼ 1 exp
yþ 25
ð6Þ
2.2. Engine model The specifications of methanol engine are listed in Table 2. The simulation is performed between intake valve closing and exhaust valve opening. The 1-D simulation model is established through the GT-Power software platform [22,35]. The three-dimensional (3-D) simulations are calculated through the CFD (computational fluid dynamics) code, which is established through the AVL FIRE software platform. The computational mesh of 3-D CFD calculation is shown in Fig. 1, the maximum mesh is exceed 450,000 cells, and the maximum grid size in only 0.2 cm. Knock can be analyzed based on the local pressures of combustion chamber [36,37]. The knock intensity can be defined as the intensity of the amplitude of pressure oscillation. For quantitative comparisons for knock intensities, an index PPmax (peak-to-peak value of vibration signal) is adopted to represent knock intensity [22,35], where:
PP max ¼ maxðjpðhÞjÞ
ð7Þ
While pðhÞ is the knocking pressure. Knock detection threshold can distinguish knocking combustion from non-knocking combustion. Sometimes, an index PP th is adopted to represent knock threshold, it is a predetermined threshold that distinguishes knocking combustion cycles from non knocking combustion cycles. It is difficult and complex to determine the threshold; its value should depend on the engine operating point, but often depend on the empirical values [37]. In the present study, the 3-D CFD calculation and the chemical reaction calculation were performed as separate cases. The exchanged data included the boundary conditions at the surface and source-terms (momentum, mass, energy etc.) in the volume of the respective calculation domains, and it was performed at each iteration when transient calculations. Both non-mesh related data
Table 2 Engine specifications and conditions.
and mesh-related data (e.g. velocity or pressure at cell centers or at cell nodes or at boundary face centers) could be exchanged [25]. The coupling calculations were carried out by the separate module (General Species Transport Module and General Gas Phase Reaction Module) integrated in CFD code, and the chemical reaction kinetics can be inputted in the separate model. In order to save computation time, the simulation was performed on a 24 G memories and 12 Xeon processors workstation. In order to calculate the pressure oscillation caused by autoignition, calculated local pressures at ten different locations in the combustion chamber are used for analysis. The adopted ten local positions are illustrated in Fig. 2. The established 1-D GT-Power calculation model and 3-D CFD calculation model are validated, the 1-D model is validated by comparing the experimental data. Table 3 illustrates the comparison of results of the 1-D model calculated results and the experimental data. It can be seen that the error between the calculated and experimental data is within the permitted (less than 5%), so it is validated. Fig. 3 shows the calculated in-cylinder pressure comparison of the 3-D CFD model and the validated 1-D model. The error between the 1-D and 3-D calculations is within the permitted (less than 10%). Therefore, the established 3-D CFD model can be used to study the engine combustion phenomenon. 3. Simulation results and analysis
Engine type
Methanol 4100 series
Engine configuration
Inline four-cylinder, four-stroke, turbocharged and intercooled 100 127 17.5 3.99 x shape 54.0 18.0 Spark ignition 83 410 2600 Methanol
Bore (mm) Stroke (mm) Compression ratio Displacement (L) Chamber shape Chamber diameter (mm) Chamber depth (mm) Combustion mode Rated power (kW) Maximum torque (Nm) Maximum speed (r/min) Fuel
Fig. 1. Computational mesh of the high compression ratio methanol single-cylinder engine.
The simulation conditions are shown in Table 4. The boundary conditions and initial conditions are also illustrated in Table 4. The simulated ten different local pressure results in the combustion chamber are shown in Fig. 4. It can be seen from the Fig. 4, because the adopted compression ratio of the engine is very high, so the simulated local pressure oscillations are relatively very high. The simulated PPmax values at ten different local positions are illustrated in Fig. 5, the variations of pressure oscillation magnitudes at ten locations are clearly seen. It can be seen from the figure that the most severe pressure oscillations occurred at position 6 (the PPmax value is only about 8.88 MPa), while the most weak pressure oscillation occurred at position 7 (the PPmax value is only about 1.59 MPa). However, in SI methanol engine, the most severe
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X. Zhen, Y. Wang / Energy Conversion and Management 96 (2015) 188–196 Table 4 Operating conditions for knock simulation. Engine speed (rev/min) CH3OH mass fraction O2 mass fraction N2 mass fraction Boundary conditions Head temperature (K) Piston temperature (K) Cylinder temperature (K) Initial conditions Temperature (K) Pressure (Bar) Turbulence kinetic energy (m2/s2)
1200 0.138604 0.199844 0.661552 450 450 450 432 1.49 768.75
Fig. 2. Numerical transducer locations on the high compression ratio SI methanol engine.
Table 3 Comparison of results of 1-D model calculated results and experimental data. Engine performance
Experimental data
1-D calculated results
Errors (%)
Torque (Nm)) Power (kW)
410 83
427.7 85.9
4.32 3.49
Fig. 4. Simulated in-cylinder pressures at local positions during HCCI.
Fig. 3. Comparison of 3-D model calculated and 1-D validated model calculated in-cylinder pressure.
pressure oscillation occurred at position 8 (the PPmax value is only about 37.12 MPa), while the most weak pressure oscillation occurred at position 7 (the PPmax value is only about 2.99 MPa) [35]. So it can be found that the present HCCI engine tends to knock near the combustion chamber wall and the intake valve side. The reason was that with the combustion process, the intake valve temperature was gradually increased. The large differences of PPmax values between HCCI and SI engines are mainly because of its different combustion modes, for instance, the spark timings have significant impacts on the knock intensity [3,22]. Fig. 6 illustrates the temperature evolution results inside the combustion chamber during HCCI. As can be seen from the Fig. 6, auto-ignition spots are initially occurred near the combustion chamber wall, and the spots are gradually expanded and
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Fig. 5. Calculated the PPmax values during HCCI.
propagating from the combustion chamber wall to center. Because the combustion chamber in methanol engine is x shape, and the compression ratio is higher in methanol engine during HCCI mode, so the corresponding in-cylinder temperature near TDC (top dead center) is higher. With the piston nears TDC, the flow velocity of in-cylinder first increased, and then decreased. The maximum of velocity occurred at 6.68 °ATDC as illustrated in Fig. 7. With the overall increases of temperature and pressure in-cylinder make the mixture at the outer region reaches to auto-ignition conditions. So, the surrounding mixture was compressed ignited in multipoint form [38]. For the case with crank angle at 6.84 °ATDC, a rapid increase in combustion chamber can be observed. As the auto-ignition occurrence, the end-gas temperature is above 3000 K. In the following within 0.2 °CA, the residual methanol is consumed quickly. Compared with the SI combustion mode (SI methanol engine) [35], the HCCI auto-ignition is occurred simultaneously at many locations, in the present engine, the knocking combustion duration is shorter, so it burn faster. Cho et al. [39] studied the controlled auto-ignition characteristics of methane/air gas mixture. It was found that the reaction region shapes were irregular, and the locations of reaction initiation were
random. It was also found that spontaneous ignition was a typical feature of CAI (HCCI) combustion mode. Fig. 8 illustrates the OH radicals evolution results inside combustion chamber during HCCI. In Fig. 8, the OH radicals evolution results inside combustion chamber are basically the same with the temperature evolution results inside combustion chamber (see Fig. 8), which is consistent with the results of SI combustion mode (SI methanol engine) [35]. Fig. 9 illustrates the pressure evolution results inside combustion chamber during HCCI. It can be seen from the figure that auto-ignition spots are initially occurred near the positions 6 and 2. As the ongoing of combustion process, the pressure wave is formed in the combustion chamber. When the engine is running at 6.84 °ATDC, the most serious knock is occurred at position 6. It is clearly that the pressure wave is propagating from wall to center of the combustion chamber. The simulated results showed that when the present high compression ratio engine is running with HCCI mode, it tends to knock in the un-burnt area on the chamber wall. The produced intermediate species and radicals (for instance, the mass fraction and reaction rate etc.) will be different during knock between SI combustion modes and HCCI combustion modes. Fig. 10 illustrates the in-cylinder intermediate species and radicals during HCCI. In Fig. 10, it is clearly to observe that the generations of CH2O (formaldehyde), H2O2 and HO2 species anticipate the other species such as CO, HCO and OH during HCCI. The peak generation timings of OH, HCO, CH2O, H2O2, and HO2 are 6.56, 6.56, 6.96, 6.96 and 6.8 °ATDC respectively. Compared with the SI methanol engine [35], the generation of H2O2 anticipates the HO2, which is an opposite result. Species such as H2O2 and CH2O almost generate simultaneously. Compared with the SI methanol engine [35], in addition to the concentration of HCO (the concentration of HCO radicals has a little change, the peak mass fraction of HCO radicals is only up to 5.3E-7), the concentration of the other intermediate products and species such as CO, OH, CH2O, H2O2, HO2 are increased significantly. In terms of the root reasons, because during the combustion, the generation and consumption of species (Fig. 10) is determined by the chemical reaction rate, so it can be explained by the results of chemical reaction rate. Fig. 11 illustrates the reaction intensities of in-cylinder intermediate radicals and species during HCCI. In Fig. 11, it can be seen that compared with the SI methanol engine, the CO and OH have only one peak
Fig. 6. Simulated in-cylinder temperature profiles during HCCI.
X. Zhen, Y. Wang / Energy Conversion and Management 96 (2015) 188–196
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Fig. 7. Simulated in-cylinder flow velocity profiles during HCCI.
(it has two peaks in the SI methanol engine [35]). The reaction rates of OH radicals and CO are positive, which are consistent with the SI methanol engine. However, the reaction rates of CH2O, and H2O2 are negative, which are opposite to the SI methanol engine. The value of reaction rate of OH radicals is increased significantly. The value of reaction rate of CO is increased a little. It also can be seen in Fig. 11, as the reaction proceeds, the reaction rate of HCO is from positive to negative, and it has two obvious peaks, one is positive peak, and another is negative peak, which is different from SI methanol engine [35]. It is clearly that the reaction rate of CH2O has only one positive peak in the SI methanol engine, however,
in the HCCI methanol engine, it has only one negative peak. The negative peak value is up to eight times higher than the positive peak value. Compared with the SI methanol engine, the reaction rate of H2O2 has one obvious negative peak, and its value is several times higher; the reaction rate of HO2 also has one obvious negative peak, and the value is about ten times higher. In addition, it also can be seen from Fig. 11, the reaction time is very short (the reaction time is only 0.2 crank degrees), which was faster than SI methanol engine [35]. From the above analysis, it can be found that there is a clear relation between the in-cylinder pressure oscillation intensity (knock intensity) and the species reaction rate: the stronger the
Fig. 8. Simulated in-cylinder OH profiles during HCCI.
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Fig. 9. Simulated the evolution of in-cylinder pressure profiles during HCCI.
Fig. 10. Calculated the species mass fraction during HCCI.
species reaction rate, the higher the pressure oscillation intensity. The reaction rates of OH, CH2O, H2O2 play a dominant role in the in-cylinder pressure oscillation intensity during HCCI knock.
Fig. 11. Calculated the mean reaction rate during HCCI.
Some mass fractions and reaction rates of species during knocking combustion in SI and HCCI methanol engines are summarized in Table 5.
X. Zhen, Y. Wang / Energy Conversion and Management 96 (2015) 188–196 Table 5 The mass fractions and reaction rates of species during knock in SI and HCCI methanol engines. SI methanol engine The mass fractions HCO Many peaks HO2 Many peaks Many peaks CH2O and H2O2 The reaction rates HCO
Positive, many peaks
CO and OH CH2O and H2O2 HO2
Positive, two peaks Positive Many positive peaks
HCCI methanol engine One peak One peak One peak Two peaks, one positive, one negative Positive, one peak Negative Two positive peak, one negative peak
4. Conclusions In this paper, knock phenomenon during HCCI is studied in a high compression ratio methanol engine based on LES with methanol chemical kinetics. The knock occurrence timing, the knock occurrence location, the knock combustion intensity and the produced intermediate species and radicals (for instance, the mass fraction and reaction rate etc.) were studied. The obtained results from this study can be summarized as follows: (1) It was concluded that the HCCI knock in the present engine tended to occur near the combustion chamber wall and the intake valve side. The reason was that with the combustion process, the intake valve temperature was gradually increased. The HCCI auto-ignition was occurred simultaneously at many locations, in the present engine, the knocking combustion duration was shorter, so it burnt faster than SI methanol engine. (2) The OH radicals could be a good indicator of the temperature during HCCI, which was consistent with the results of SI engine. Compared with the SI methanol engine, the generation of H2O2 anticipated the HO2, which was an opposite result; the reaction rate of HCO was from positive to negative, and it had two obvious peaks, one was positive peak, and another was negative peak, which was different from SI methanol engine. (3) Compared with the SI methanol engine, in addition to the concentration of HCO, the concentrations of the other intermediate products and species such as CO, OH, CH2O, H2O2, HO2 were increased significantly; the reaction rates of CH2O, H2O2, and HO2 had negative peaks, and whose values were several times higher than SI methanol engine. (4) The reaction rates of OH, CH2O, H2O2 play a dominant role to the pressure oscillation intensity of in-cylinder during HCCI knock; the stronger the species reaction rate, the higher the pressure oscillation intensity. The reaction rate of OH was opposite to the CH2O and H2O2. The reaction rates of OH radicals and CO were positive during HCCI knock, which were consistent with the SI methanol engine. However, the reaction rates of CH2O, and H2O2 were negative, which were opposite to the SI methanol engine.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51406135, 51176137) and Tianjin University of Technology and Education (Grant No. KYQD14019).
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