The knock study of high compression ratio SI engine fueled with methanol in combination with different EGR rates

Fuel 257 (2019) 116098

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Full Length Article

The knock study of high compression ratio SI engine fueled with methanol in combination with different EGR rates

T

Xiaoyan Lia, Xudong Zhena, , Yang Wangb, Daming Liua, Zhi Tiana ⁎

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

ARTICLE INFO

ABSTRACT

Keywords: Methanol High compression ratio engine Knock EGR Detailed chemical kinetics

Methanol, with a high octane index, is considered to be one of the most ideal fuels for high compression ratio engines. In this paper, the SAGE models combined with methanol chemical reaction kinetics (consisting of 46species and 247-elementary reactions) are used to study knock combustion. Knock combustion is simulated in a high compression ratio SI methanol engine with different EGR rates. The results show that the introduction of EGR into the combustion chamber can reduce the knock intensity and delay the knock onset time. Without EGR, the intake side of the combustion chamber is prone to knocking mostly. As the EGR rate increases, the exhaust side of the combustion chamber is the most likely to knocking. OH radicals can be considered an indicator of the temperature, and H2O2 can be regarded as an indicator of the propagating flame front. OH marks the knock phase duration and HCO marks the onset of knocking. The reaction intensities of CH2O, H2O2, CO and OH species are higher than other species during knocking combustion. The greater the reaction rate is, the greater the knock intensity will be.

1. Introduction The development and application of alternative fuels for internal combustion engines are of great significance in alleviating the shortage of petroleum resources and the air pollution and ecological damage caused by petroleum burning. Methanol is a good alternative fuel for engines, which has great developmental value and applying prospects. In addition, the source of the methanol is aplenty, it is easy to be obtained from a number of raw materials such as waste substances, coal, biomass, natural gas and wood, etc [1]. Because of the high octane number of methanol, it can be used in engines with high compression ratio to improve knock resistance [2]. The methanol molecule has an oxygen content of up to 50%, which makes a faster burning speed and the fuel burns more efficiently [3]. High evaporation latent heat of methanol can improve the thermal efficiency of the engine. The physical and chemical properties of methanol are shown in Table 1. Although methanol has the advantages above, there are still some potential problems when using methanol in engines. Over the years, a lot of research works have been conducted on the engine performance [4,5], combustion characteristics [6,7], cold start characteristics [2,8,9], and emissions [10,11]. Verhelst et al. [12] conducted a detailed study on the feasibility of methanol as a fuel for internal combustion engines. They summed up that by optimizing the engine block, injector, ⁎

valves, cranking rpm, ignition and direct injection could improve the cold start of the methanol engine. The application of methanol fuel has a well-known challenge concerning material compatibility [12]. This requires modifications to the engine fuel system [12]. The California Energy Commission has identified design elements for methanol compatibility [13]. Zincir et al. [14] and Cay et al. [15] found that methanol had better emission characteristics than those of gasoline and diesel. McCoy et al. [16] and James et al. [17] demonstrated that methanol as an engine fuel could improve performance and efficiency of engines. Many research work shows that with the development of engine technology, methanol fuel has a good applying prospect in the engine. The compression ratio is the most important structural parameter affecting the performance and working efficiency of the SI engine. Increasing the compression ratio can increase the output power of the SI engine, reduce the fuel consumption rate, and improve the fuel economy. However, with the increase of compression ratio, the initial temperature and pressure of mixture combustion are all increased, the flame propagation rate becomes higher, the compression effect on the unburned end-gas mixture is enhanced, the ignition delay period is shortened, and ultimately the knock tendency is enhanced [18–20].The knocking combustion phenomenon is an important obstacle to further increasing the compression ratio of the SI engine and improving the

Corresponding author. E-mail address: [email protected] (X. Zhen).

https://doi.org/10.1016/j.fuel.2019.116098 Received 28 July 2019; Received in revised form 19 August 2019; Accepted 27 August 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

Fuel 257 (2019) 116098

X. Li, et al.

Nomenclatures EGR CI SI DFSI DISI RCCI

CFD 1-D 3-D AMR TDC BTDC IVC EVO

Exhaust gas recirculation Compression ignition Spark ignition Dual-fuel spark ignition Direct injection spark ignition Reactivity controlled compression ignition

combustion thermal efficiency of the engine. In order to further optimize the combustion process of methanol and explore effective methods for suppressing knocking, many scholars have done a lot of research works. Most of the studies are focused on the methanol blended fuels. Feng et al. [21] conducted a study on the knock combustion in a downsized spark engine with different methanol-isooctane blends. They found that knocking originated from the strong interaction between the pressure wave and the heat released by spontaneous combustion. Moran et al. [22] studied the effect of engine speed on the anti-knock performance of methanol-isooctane blends by studying the fuel evaporation characteristics of the intake system. The results showed that the act of adding methanol to gasoline causes dramatic cooling at low engine speeds, and this effect would disappear due to the short residence time at high speeds. At low speeds, cooling inhibits the knocking of the methanol-gasoline mixture. Liu et al. [23] studied the effect of methanol/gasoline ratio of DFSI (dual-fuel spark ignition) on knock suppression on a natural aspirated engine with high compression ratio. They found that Methanol-Gasoline DFSI combustion mode not only effectively expand the knock-limit, but also improves fuel economy. Yacoub et al [24] conducted a detailed studies on knocking combustion of C1– C5 alcohol–gasoline blends with matched oxygen content in a SI engine. They found that the addition of low alcohols (methanol, ethanol and n-propanol) to unleaded gasoline could improve the knock resistance of gasoline, and the knock resistance of higher alcohol (n-butanol and n-pentanol) blends decreases, when compared with pure gasoline. Methanol, Ethanol and n-propanol all have good knock resistance, allowing operation at high compression ratios. Zhou et al. [25] developed a reduction mechanism of biodiesel/ methanol and combined it with a three-dimensional CFD model to study the knocking phenomenon of RCCI (reactive controlled compression ignition) engine fueled with biodiesel and methanol. The results showed that cooled EGR, retarded SOI and lower premixed methanol mass fraction can suppress knocking. Breda et al. [26] studied the effects on knock intensity of port water/methanol injection in a high-turbocharged DISI engine. They found that the spark advance increase in order to reach the same knock tolerance would make the cylinder temperature to decrease when the methanol content is poor. Liu et al. [27] studied the detonation suppression strategy for eliminating super-knocking in a rapid compression machine fueled with

propane or methanol mixtures. The results showed that propane and methanol could suppress knocking, thereby suppressing super-knocking directly. There are fewer studies on pure methanol fuel compared to that of methanol blended fuels. Zhen et al. [28] performed a multidimensional study of knock combustion occurrence in a high compression ratio methanol engine using CFD (Computational Fluid Dynamics) method. They found that knocking could be effectively suppressed by retarding spark timing and increasing EGR rate. Blumberg et al. [29] produced simulation results comparing E85 (ethanol blend fuel, consists of 85 percent ethanol and 15 percent unleaded gasoline) and methanol. The results showed that methanol had higher antiknock resistance due to its higher octane number and heat of vaporization. Furthermore, the borderline knock consumption of methanol is lower than E85, for the same knock suppression, only half of the methanol fuel flow is required compared to ethanol fuel. Daniel et al. [30] have done the similar research. They experimentally compared the effectiveness of pure methanol and ethanol as knock mitigation fuels in a dual-injection SI engine. But the results obtained were contrary to the simulation results of Blumberg et al. Vancoillie et al. [31] developed an autoignition delay time correlation for pure methanol fuel and applied it in a knock prediction model based on the knock integral method. They found that accurate characterization of the thermal process in the cylinder is critical for knock prediction. Numbers of valuable research works have been performed to study the knocking combustion of methanol in SI or CI engines. However, little research has been done on the effect of EGR rate on knocking combustion with pure methanol as fuel. In this paper, simulation studies about knocking combustion on a high compression ratio SI engine fueled with methanol is conducted. The effects of different EGR rates on knock intensity, knock onset time and the knocking position are compared. The components of recycled exhaust gas that play a major role in reducing the in-cylinder temperature are discussed. 2. Modeling methodology 2.1. Numerical methodology In this paper, the SAGE model proposed by Senecal et al. [32] is used to simulate the in-cylinder combustion process of a methanol engine. The SAGE Detailed Chemistry Solver is a general combustion model used to solve the detail chemical kinetics in the combustion process. The SAGE model calculates the reaction rates for each elementary reaction while the CFD (Computational Fluid Dynamics) solves the transport equations [33]. A multi-step chemical reaction mechanism by proposed Turns. [34] can be written in the form:

Table 1 The properties of methanol. Property

Methanol

Formula Molecular mass (kg/kmol) Research octane number Boiling point (℃) Density (kg/m3) Cetane number Flammability limit (% volume) Latent heat (kJ/kg) Lower heating value (MJ/kg) Auto-ignition temperature (℃) Stoichiometric air/fule ratio Vapor pressure at 37℃(kPa)

CH3OH 32 111 65 790 3–5 6.7–36 1110 19.6 470 6.5 31.5

Computational fluid dynamics One-dimensional Three-dimensional Adaptive mesh refinement Top dead center Before top dead center Intake Valve Closing Exhaust Valve Opening

M

M

Vm, r xm m=1

Vm, r x m m =1

for r = 1, 2, ….R

(1)

where Vm, r and Vm, r were the stoichiometric coefficients for the reactants and products, respectively, for species and m reaction r , R was the total number of reactions, xm represents the chemical symbol for species m . The net production rate of species m was given as: '

2

''

Fuel 257 (2019) 116098

X. Li, et al.

solved for a given computational cell. The equations were given as:

Table 2 Main technical parameters of the engine.

d [Xm ] = dt

Item

Content

Engine type

Orthostichous, four cylinder, water cooled, port injection 100 127

Bore (mm) Stroke (mm) Type of combustion chamber Displacement (L) Ignition sequence Combustion form Compression ratio Rated power (KW/(r/min)) Maximum torque (Nm/(r/min))

V dT = dt

=

Vm, r qr for m = 1, 2, ….M

Vm,

r

= Vm,

r

Vm,

where M was the total number of species, the rate-of-progress variable qr for the rth reaction was given as: M

qr = kfr

[Xm ]Vm, r

m =1

M

skrr m =1

''

[Xm ]Vm, r

(4)

where [Xm] was the molar concentration of speciesm , kfr and krr were the forward and reverse rate coefficients for reactionr , The forward rate coefficient was expressed by the Arrhenius form:

kfr = Ar T br e ( krr =

Er Ru T )

(5)

kfr K cr

m

(h m

m)

([Xm ] Cp, m )

(8)

In this study, an inline four-cylinder, four-stroke, turbocharged and intercooled engine were used to simulating the combustion process of a high compression ratio SI methanol engine. The engine specifications were listed in Table 2. The engine added and improved the ignition system and fuel injection system based on conventional diesel engines. Initial and boundary conditions for 3-D simulation were obtained from 1-D engine simulation and engine bench experimental. The 1-D engine model used in this work was built upon the GT-Power software platform. The 1-D engine consists of a supercharging system and an EGR control system. Furthermore, the injection strategy of this engine is port fuel injection. The 1-D model of the high compression ratio SI methanol engine was shown in Fig. 1. This model had been validated by the experimental tests [28,35]. The 3-D simulation model was established in CONVERGE software platform and shown in Fig. 2. The shape of the intake valve and exhaust valve were not considered here, which simplified the structure of chamber and saved the cost of simulation. The combustion simulation was carried out between IVC and EVO. Previous studies have shown that methanol-fueled engines can use higher levels of EGR than gasoline and diesel fuels [1]. The methanol is able to work satisfactorily with EGR as much as 33%–40% [36,37]. Under certain conditions, methanol can even use nearly 50% EGR without notable cycle to cycle burn variability [38]. In this study, the EGR as 5%, 15% and 30% were chosen to analyze their effects on knocking combustion of methanol

(3)

r

m

2.2. Engine model

(2)

r=1

dP dt

where V was volume, T was temperature, P was pressure, hm and C p, m were the molar specific enthalpy and molar constant-pressure specific heat of species m, respectively.

3.99 1-3-4-2 SI 17.5 83/2600 410/1600

R m

(7)

m

(6)

where Ar was the pre-exponential factor, br was the temperature exponent, Er was the activation energy, and R u was the universal gas constant, K cr was the equilibrium coefficient was determined from the thermodynamic properties. The governing equations for mass and energy conservation can be

Fig. 1. The 1-D model of the high compression ratio SI methanol engine. 3

Fuel 257 (2019) 116098

X. Li, et al.

Fig. 2. The 3-D model of the high compression ratio SI methanol engine and monitor locations.

Table 3 The operation conditions of methanol engine. Item

Content

Spark Timing (°CA BTDC) Spark pluge energy (J) Port fuel injection timing (°CA ATDC) Engine Speed(r/min) Initial Temperature (K) Initial pressure (Mpa) Lambda Fuel EGR

25 0.2 325 800, 1500 390 0.11 1 Methanol 0%, 5%, 15%, 30%

Fig. 3. (a) Grid-sensitivity analysis, (b) Comparison of model calculated and measured in-cylinder pressure, (c) Comparison of model calculated and measured heat release rate.

engines. The two most commonly used engine speeds on urban roads were discussed in this study (idle condition: 800 rpm, typical city condition: 500 rpm). The operation conditions were listed in Table 3. The grid-sensitivity analysis was shown in Fig. 3(a). In this work, the mesh with 1,200,000 cells as a maximum was used to simulate the 4

Fuel 257 (2019) 116098

X. Li, et al.

Fig. 4. (a) Effect of EGR rate on the laminar flame speed, (b) Effect of EGR rate on the ignition delay.

combustion process. The maximum mesh size in the calculated analysis was 4 mm. In order to improve the accuracy of the simulation calculation, the model was subjected to AMR processing, and the mesh was refined to 5 levels to a minimum grid of 0.125 mm. The maximum calculated time-step increment was only 0.01 crank angle degree during knocking combustion. The methanol chemical reaction kinetics (consisting of 46-species and 247-elementary reactions) developed by Zhang et al. [39] was used in this study.

intake valve side and the exhaust valve side. In order to verify where the knock combustion tended to occur and eliminate the influence of the flame propagation distance, 8 monitoring points are evenly arranged on the combustion chamber wall (the monitoring point 3 was located near the exhaust side, and the monitoring point 7 was located near the intake side.) The location of the 8 monitoring point was shown in Fig. 2. For better analysis of knock characteristics, band-pass filtering with cutoff frequencies 4–47 kHz was used to process the knock oscillated pressure. The maximum amplitude of filtered pressure oscillations (MAPO) was taken as an evaluation index of the knock intensity. This method can intuitively showed the magnitude of the pressure fluctuation caused by knocking combustion and well characterize the destructiveness of knock combustion. MAPO obtained:

2.3. Modeling validation It is necessary to verify the calculation model used before performing the simulation calculation. The obtained simulation results were compared with the experimental results of Gong et al. [40]. The comparison results were shown in Fig. 3(b) (c). It can be seen that the calculated pressure and HRR of the 1-D and 3-D model were in good agreement with the experimental results. As shown, the tendency of these curves is highly consistent. It can be seen that the simulation results and experimental results were still somewhat different. By comparing the experimental results with the simulation results, the reasons for these differences can be explained, including the following aspects: firstly, the methanol/air mixture was not completely consistent in the experiments and simulations; secondly, there were some mechanical loss in the experimental test; thirdly, the simplified combustion chamber and heat transfer model used in the simulation not exactly the same as the actual structure and combustion process of the engine in the experimental test; Finally, the cyclical variation was not considered in the simulation. In conclusion, there was a consistent trend between the experimental results and simulation results. Thus, the established 3-D model can be used to simulate the engine in-cylinder auto-ignition and combustion process.

MAPO = max(|P |

0 + 0

)

(9)

where 0 was the crank angle corresponding to the start of calculation windows, ξ stood for the calculation window, P was the filtered pressure. In order to distinguish between knock and conventional combustion, a knock intensity threshold needs to be selected. The knock intensity threshold was generally set as 0.1 MPa [45]. When MAPO exceeds knock intensity threshold, it was considered that knocking occurs. The larger the value of MAPO is, the greater the knocking intensity will be. 3. Simulation results and analysis 3.1. Laminar flame speed and ignition delay Laminar flame speed is a fundamental property to characterize the combustion process of the mixture [46]. Fig. 4(a) showed the effect of EGR rate on laminar flame speed at two engine speeds (800 rpm and 1500 rpm). As shown, with the increased of EGR rate, the laminar flame speed decreased. As the engine speed increased, the effect of EGR on laminar flame speed increased. Laminar flame speed closely links to the properties of kinetic and temperature [46,47]. Laminar flame speed increased with the increased of the temperature [48]. With the increased of EGR rate, the in-cylinder temperature decreased (Fig. 11). The oxidation of methanol proceeds down the following path: CH3OH → CH2OH/CH3O → CH2O → HCO → CO → CO2. Methanol is

2.4. Evaluation methods for onset and severity of knock As well-known, high frequency pressure oscillations occur during knocking combustion. The local pressure oscillations at different positions in the combustion chamber were different [41–43]. Previous studies have mentioned that the knock combustion tended to occur near combustion chamber wall, the intake valve side and the exhaust valve side [44, 56, 59]. The spark plug is located at the center between the

5

Fuel 257 (2019) 116098

X. Li, et al.

Fig. 5. In-cylinder pressure curves and heat release rate under different EGR rate.

initially consumed through H abstraction reactions with CH2OH and CH3O generated [46]. The H abstraction reactions were as following: CH3OH + H ⇌ CH3O + H2

(10)

CH3OH + H ⇌ CH2OH + H2

(11)

time was affected by OH group, as the OH increased, the ignition delay time is extended [51]. The ignition delay time chemistry is driven by the HO2 chemistry, for which the formation is sensitive to the reaction (13): CH2OH + O2 ⇌ CH2O + HO2, this HO2 will then attack the methanol and form H2O2 via H abstraction reaction (16), closely followed by the decomposition of H2O2 into two OH radicals through reaction (17) [52]. The chemical reactions were as follows:

Less CH3O is generated because of high bond energy between O and H i-n hydroxyl group (-OH) [46]. Both CH3O and CH2OH go to CH2O and generate reactive HCO and finally release high concentration of H radical pool [46,49]. The chemical reactions were as follows: CH3O + M ⇌ CH2O + H + M

(12)

CH2OH + O2 ⇌ CH2O + HO2

(13)

CH2O + H ⇌ HCO + H2

(14)

HCO + M ⇌ H + CO + M

(15)

CH3OH + HO2 ⇌ CH2OH + H2O2

(16)

H2O2 + H2O2 ⇌ OH + OH

(17)

With the increased of EGR rate, the concentration of methanol decreased, HO2 coming from CH3OH decreased, OH decreased, leading to an increased ignition delay time. 3.2. Cylinder pressure and heat release

EGR has a dilution effect [45], with the increased of EGR rate, CH2O coming from CH3OH decreased, HCO generation was suppressed. In a hydrocarbon flame, the concentration of HCO radical is a good indicator of heat release [50]. As HCO generation decreased, heat release decreased, resulting in slow laminar flame speed. Fig. 4(b) showed the effect of EGR rate on ignition delay at two engine speeds (800 rpm and 1500 rpm). As shown, with the increased of EGR rate, the ignition delay time increased. As the engine speed increased, the effect of EGR on ignition delay increased. Ignition delay

Cylinder pressure is one of the most important parameters of combustion process [45]. In this paper, the effect of EGR rate on cylinder pressure during knock combustion at two engine speeds (800 rpm and 1500 rpm) were analyzed. The results were shown in Fig. 5(a) and Fig. 5(b). As shown, with the increased of EGR rate, the peak in-cylinder pressure decreased. This is since the recycled exhaust gas contains some polyatomic molecules (CO2 and H2O) with a specific heat capacity higher than air which lower in-cylinder gas temperature [53]. EGR dilutes the oxygen concentration of engine cylinder, resulting in a

6

Fuel 257 (2019) 116098

X. Li, et al.

Fig. 6. In-cylinder pressure and pressure oscillation (4–47 kHz band pass filtered pressure) during knock combustion (engine speed: 800 rpm).

lower combustion rate [53]. When the engine speed was 800 rpm and 1500 rpm, the maximum difference of peak cylinder pressure corresponds to different EGR rates can reach 2.22 MPa and 4.89 MPa, respectively. The addition of EGR has a significant inhibitory effect on the in-cylinder ignition. When the engine speed was 800 rpm and 1500 rpm, compared with 0% EGR rate, the occurrence time of the peak in-cylinder pressure at 30% EGR rate was delayed by 1.73 °CA and 9.85 °CA, respectively. As the EGR rate increased, the combustion duration was shortened. This is due to the introduction of EGR, which causes the decrease of in-cylinder oxygen concentration and chemical reaction rate [45]. With the increase of engine speed, the influence of EGR on combustion increases. Heat release rate (HRR) is another important parameter to characterize the combustion process. As is shown in Fig. 5(c) and (d), the energy contained in the end-gas that is ahead of the propagation turbulent flame releases its energy rapidly during knocking combustion and resulting in a sharp rise in HRR [45]. Therefore, the sharp increase of HRR can be used to determine the start time and the intensity of auto-ignition, as well as to indicate the onset of knock pressure oscillation [45]. As the EGR rate increased, the peak HRR decreased, and the time of occurrence of peak HRR was delayed. As the EGR rate increased, the in-cylinder temperature decreased, the oxygen concentration decreased and the combustion rate decreased, leading to the decreased of the heat release rate [54]. With the increased of EGR rate,

the flame speed decreased and the ignition delay time increased, leading to the delay of peak HRR. Increasing the EGR rate can reduce the knocking tendency of methanol engine and delay the onset time of knocking combustion. For all EGR rates, as the engine speed increased, the peak HRR increased and the time of occurrence of peak HRR was delayed. 3.3. Onset and knock intensity In order to highlight the pressure oscillations caused by knocking, band-pass filtering with cutoff frequencies 4–47 kHz was used to process the knock oscillated pressure. Using the degree of the peak pressure oscillation, knock intensity was analyzed and quantified [45]. In this section, the effects of EGR rate on the onset time and intensity of knocking combustion at two velocities (800 rpm, 1500 rpm) were studied. The results were shown in Figs. 6 and 7. The black curve in the figure represents the in-cylinder pressure and the red curve represents the pressure oscillation. For the convenience of observation, the blue dashed line in all figures represents the start of the knocking combustion. At any speed, as the EGR rate increased, the knock intensity decreased and the onset time of knocking combustion was delayed. As the engine speed increased, the effect of EGR on knocking combustion was enhanced. When the engine speed was 1500 rpm and the EGR rate was 30%, the MAPO was less than 0.1 MPa, and the knocking combustion

7

Fuel 257 (2019) 116098

X. Li, et al.

Fig. 7. In-cylinder pressure and pressure oscillation (4–47 kHz band pass filtered pressure) during knock combustion (engine speed: 1500 rpm).

disappeared. The knock onset is related to the ignition delay period of the endgas. The auto-ignition time of the unburned fuel–air mixture depends on octane number, local temperature and pressure [45]. When knocking combustion occurred, spontaneous combustion of unburned end-gas will generate pressure waves and even shock waves [55]. EGR

can reduce the amplitude of high-frequency pressure oscillation [56]. This is because the dilution of EGR causes the increase in heat capacity of the mixture in the combustion chamber and the decrease in oxygen concentration. The introduction of EGR slows down the chemical heat release and flame propagation of the fuel–air mixture in the combustion chamber, which reduces the end-gas temperature and combustion

Fig. 8. MAPO in eight monitor points (engine speed: 1500 rpm).

8

Fuel 257 (2019) 116098

X. Li, et al.

Fig. 9. The mass fraction field of O2 during knocking combustion at different EGR rates (engine speed: 800 rpm).

temperature, thereby suppressing the auto-ignition reaction and weakens the pressure wave to some extent [57,58].

temperature. The evolution of O2 and CH3OH was opposite to the evolution of in-cylinder temperature. Fig. 11 showed the variation of the in-cylinder temperature at different EGR rates. As the EGR rate increased, the peak in-cylinder temperature decreased and the time delays in the appearance of the peak in-cylinder temperature. This showed that CO2, H2O and N2 in the recirculated exhaust gas can effectively reduce the temperature in the cylinder of the combustion chamber. Both CO2 and H2O have a high specific heat capacity, and the recirculated exhaust gas contains a large amount of CO2 and H2O. The introduction of EGR can effectively improved the specific heat of the mixture in the cylinder, thereby reducing the in-cylinder temperature. Knocking combustion is an abnormal combustion phenomenon caused by spontaneous combustion of the end-gas before the arrival of the flame front [59,60]. Previous studies have shown that the auto-ignition delay period of the end-gas mainly depends on the octane number of the fuel, the partial pressure and temperature of the unburned area. It’s relationship was as follows:

3.4. Location of knock combustion As shown in Fig. 6, when the engine speed was 800 rpm, 0% to 30% of EGR rate was introduced, the onset time of knocking combustion was 14.47, 12.65, 8.7, and 0.95 crank angle degree before TDC, respectively. Fig. 8 showed the MAPO of 8 monitoring points at different EGR rates when the engine speed was 1500 rpm. As EGR increased, the MAPO of monitoring points decreased overall. When no EGR was introduced, the maximum MAPO appeared at monitoring point 7, and the monitoring point 7 was located near the intake side of the combustion chamber (Fig. 2). When the EGR rate was 5% and 15%, the maximum MAPO still appeared at the monitoring point 7, but the MAPO at the monitoring point 3 was only slightly smaller than the monitoring point 7, and was almost equal to the MAPO of the monitoring point 7, and the monitoring point 3 was located near the exhaust side of the combustion chamber (Fig. 2). When the EGR rate increased to 30%, the maximum MAPO appeared at the monitoring point 3. Fig. 9 showed the mass fraction field of O2 after knocking at different EGR rates. When EGR was not introduced, there was a large amount of unburned mixture near the intake side of the combustion chamber, so this zone was the most dangerous for the rising of the pressure waves swiping the cylinder [43]. In addition, there was also a relatively small amount of unburned mixture near the combustion chamber wall. As the EGR increased, the unburned mixture gradually gathered near the exhaust side of the combustion chamber. When EGR rate was 30%, the unburned mixture was mostly distributed near the exhaust side of the combustion chamber. When EGR was not introduced, knock combustion was most likely to occur near the intake side of the combustion chamber. With the increased of EGR rate, the knock intensity of the intake side detonation was weakened, and the knock intensity of the exhaust side was increased. When the EGR rate was 30%, knock combustion was most likely to occur near the exhaust side of the combustion chamber.

= 17.68

ON 100

3.4042

P

1.7exp

( 3800 T )

(18)

where ON, P and T were the octane number of the fuel, the partial temperature and pressure of the unburned mixture, respectively. With the decreased of temperature, the auto-ignition delay period of the end-gas was obviously prolonged, and the knocking tendency was decreased. Therefore, the introduction of EGR can effectively suppress the tendency of knocking. Fig. 12(a) illustrated the mass fraction of in-cylinder intermediate radicals and species during knocking combustion. As shown, the methanol fuel combustion was one of the results including the intermediate products of CH2O, HCO, OH radicals, and H2O, CO2, CO, H2O2 species [28]. The peak mass fraction of HCO radicals was very small, thus its concentration during knocking combustion almost can be ignored. The OH species was the predominant species. Merola et al. [28] also got the same conclusion. They found that OH marked the knock phase duration and HCO marked the onset of knocking. Kawahara et al has done similar research [56]. They found that the OH oscillations were synchronous with the in-cylinder pressure oscillations [56]. Fig. 12(b) illustrated the mean reaction rate of in-cylinder intermediate radicals and species during knocking combustion. The reaction intensities of CH2O, H2O2, CO and OH species were higher than other species during knocking combustion. This conclusion was consistent with the findings of zhen et al [28,55]. Near the crank angle of 14.7 °CA

3.5. Species change during knock combustion The main components of the exhaust gas produced by methanol combustion were CO2, H2O, O2, CH3OH and N2. Fig. 10 showed the evolution of in-cylinder temperature, CO2, H2O, O2, CH3OH and N2 during knocking combustion. It can be seen that the evolution of CO2, H2O and N2 was almost identical to the evolution of the in-cylinder

9

Fuel 257 (2019) 116098

X. Li, et al.

Fig. 10. The evolution of in-cylinder temperature, CO2, H2O, O2, CH3OH and N2 profiles during knocking combustion.

10

Fuel 257 (2019) 116098

X. Li, et al.

transition from low-temperature chemistry to high-temperature autoignition [56]. The evolution profile of H2O2 was consistent with that of flame front. Therefore, H2O2 can be regarded as an indicator of the propagating flame front. Previous research had also reached the same conclusion [55]. 4. Conclusions In this paper, the effect of EGR on knocking combustion was studied in high compression ratio SI engine with detailed methanol chemical reaction kinetics (contains 46-species and 247-elementary reactions). The knock combustion intensity, the knock onset time, the knock onset location and the species change during knock combustion were studied. The simulation results can be summarized as follows: (1) Recirculated exhaust gas has the function of cooling and dilution. With an increase of EGR rate, both the knock intensity and in-cylinder pressure decrease, and the knock onset time is delayed. (2) Without EGR, the intake side of the combustion chamber is prone to knocking mostly. As the EGR rate increases, the exhaust valve side involves a large part of unburned mixture, so this area is prone to knocking mostly. (3) The main components of the exhaust gas produced by methanol combustion are CO2, H2O, O2, CH3OH and N2. The components that can lower the temperature of the combustion chamber are CO2, H2O and N2. (4) As the EGR rate increases, the temperature in the combustion chamber decreases, the auto-ignition delay period of the end-gas mixture is prolonged, and the knocking tendency is weakened. (5) OH radicals can be considered as an indicator of temperature, H2O2 can be regarded as an indicator of the propagating flame front. OH marked the knock phase duration and HCO marked the onset of knocking. (6) The OH species is the predominant species during knocking combustion. The concentration of HCO radicals almost could be ignored during knocking combustion.

Fig. 11. The variation of the in-cylinder temperature curve at different EGR rates.

BTDC, the reaction intensity was strongly. It can be seen that there was a certain relationship between the reaction rate and the knocking intensity. The greater the reaction rate is, the greater the knock intensity will be [28]. Fig. 13 illustrated the evolution profiles of in-cylinder pressure, OH radical and H2O2 during knock combustion. The propagation of flame in the cylinder can be judged by the evolution of temperature in the cylinder. As shown, the normal spread of the flame front can be clearly seen at the crank angle of 20 °CA BTDC. Subsequently, the flame front begins to show a distinct convex shape. This indicated that the propagating flame front produces self-acceleration phenomenon [55]. The self-acceleration of the flame front caused the shock wave in the combustion chamber. The reflection of the shock wave back and forth from one side of the inner wall surface of the combustion chamber to the other side, thus caused knocking combustion. The evolution profile of OH radical was consistent with that of temperature. This suggested that the OH radical can be considered an indicator of temperature. Kawahara et al. have done similar research [56]. They found that OH radicals were a good indicator of the

The reaction intensities of CH2O, H2O2, CO and OH species were higher than other species during knocking combustion. The greater the reaction rate is, the greater the knock intensity will be.

Fig. 12. (a) Calculated the species mass fraction during knocking combustion, (b) Calculated the mean reaction rate during knocking combustion.

11

Fuel 257 (2019) 116098

X. Li, et al.

Fig. 13. The evolution of in-cylinder pressure, OH radical and H2O2 profiles during knocking combustion.

Acknowledgement

Int J Hydrogen Energy 2016;41:17676–86. [8] Gong CM, Li JB, Peng LG, Chen YL, Liu ZL, Wei FX, et al. Numerical investigation of intake oxygen enrichment effects on radicals, combustion and unregulated emissions during cold start in a DISI methanol engine. Fuel 2019;253:1406–13. [9] Li ZH, Gong CH, Qu X, Liu FH, Sun JZ, Wang K, et al. Critical firing and misfiring boundary in a spark ignition methanol engine during cold start based on single cycle fuel injection. Energy 2015;89:236–43. [10] Gong CM, Huang W, Liu JJ, Wei FX, Yu JW, Si XK, et al. Detection and analysis of formaldehyde and unburned methanol emissions from a direct-injection spark-ignition methanol engine. Fuel 2018;221:188–95. [11] Wang X, Ge YS, Zhang CZ, Tan JW, Hao LJ, Liu J, et al. Effects of engine misfire on regulated, unregulated emissions from a methanol-fueled vehicle and its ozone forming potential. Appl Energy 2016;177:187–95. [12] Verhelst S, Turner JWG, Sileghem L, Vancoillie J. Methanol as a fuel for internal combustion engines. Prog Energy Combust Sci 2019;70:43–88. [13] Bromberg L, Cheng W. Methanol as an alternative transportation fuel in the US: options for sustainable and/or energy-secure transportation. Tech. Rep. Massachusetts Institute of Technology; 2010. [14] Zincira B, Deniz C, Tuner M. Investigation of environmental, operational and economic performance of methanol partially premixed combustion at slow speed operation of a marine engine. J Cleaner Prod 2019;235:1006–19. [15] Çay Y, Korkmaz I, Çiçek A, Kara F. Prediction of engine performance and exhaust emissions for gasoline and methanol using artificial neural network. Energy 2013;50:177–86. [16] McCoy GA, Kerstetter J, Lyons JK. Alcohol-fueled vehicles. Tech. Rep. Washing-ton State Energy Office; 1993. [17] Pearson RJ, Turner JWG. GEM ternary blends: Testing iso-stoichiometric mixtures of gasoline, ethanol and methanol in a production flex-fuel vehicle fitted with a physical alcohol sensor. SAE paper no. 2012-01-1279; 2012. [18] Nakada T, Itoh T, Takagi Y. Application of CARS to development of high

This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51406135, 51776135, 51706155) References [1] Xie FX, Li XP, Su Y, Hong W, Jiang BP, Han LW. Influence of air and EGR dilutions on improving performance of a high compression ratio spark-ignition engine fueled with methanol at light load. Appl Therm Eng 2018. https://doi.org/10.1016/j. applthermaleng.2015.10.046. [2] Gong CM, Wei FX, Si XK, Liu FH. Effects of injection timing of methanol and LPG proportion on cold start characteristics of SI methanol engine with LPG enriched port injection under cycle-by-cycle control. Energy 2018;144:54–60. [3] Xie FX, Li XP, Wang XC, Su Y, Hong W. Research on using EGR and ignition timing to control load of a spark-ignition engine fueled with methanol. Appl Therm Eng 2013;50:1084–91. [4] Nidhi, Subramanian KA. Experimental investigation on effects of oxygen enriched air on performance, combustion and emission characteristics of a methanol fuelled spark ignition engine. Appl Therm Eng 2019;147:501–8. [5] Chen ZM, Wang L, Yuan XN, Duan QM, Yang B, Zeng K. Experimental investigation on performance and combustion characteristics of spark-ignition dual-fuel engine fueled with methanol/natural gas. Appl Therm Eng 2019;150:164–74. [6] Chen ZM, Wang L, Zeng K. Comparative study of combustion process and cycle-bycycle variations of spark-ignition engine fueled with pure methanol, ethanol, and nbutanol at various air–fuel ratios. Fuel 2019;254:115–683. [7] Ji CG, Yang JX, Liu XL, Zhang B, Wang SF, Gao BB. A quasi-dimensional model for combustion performance prediction of an SI hydrogen-enriched methanol engine.

12

Fuel 257 (2019) 116098

X. Li, et al.

[41] Zhen XD, Wang Y, Zhu YS. Study of knock in a high compression ratio SI methanol engine using LES with detailed chemical kinetics. Energy Conver Manage 2013;75:523–31. [42] Galloni E. Dynamic knock detection and quantification in a spark ignition engine by means of a pressure based method. Energy Convers Manage 2012;64:256–62. [43] Galloni E, Fontana G, Staccone S. Numerical and experimental characterization of knock occurrence in a turbo-charged spark-ignition engine. Energy Convers Manage 2014;85:417–24. [44] Zhen XD, Wang Y. Numerical analysis of knock during HCCI in a high compression ratio methanol engine based on LES with detailed chemical kinetics. Energy Convers Manage 2015;96:188–96. [45] Wei HQ, Feng DQ, Pan JY, Shao AF, Pan MZ. Knock characteristics of SI engine fueled with n-butanol in combination with different EGR rate. Energy 2017;118:190–6. [46] Li QQ, Zhang WJ, Jin W, Xie YL, Huang ZH. Laminar flame characteristics and kinetic modeling study of methanol-isooctane blends at elevated temperatures. Fuel 2016;184:836–45. [47] Gu XL, Huang ZH, Wu S, Li QQ. Laminar burning velocities and flame instabilities of butanol isomers–air mixtures. Combust Flame 2010;157:2318–25. [48] Zhang ZY, Huang ZH, Wang XG, Xiang J, Wang XB, Miao HY. Measurements of laminar burning velocities and Markstein lengths for methanol–air–nitrogen mixtures at elevated pressures and temperatures. Combust Flame 2008;155:358–68. [49] Zhang YM, Li QQ, Liu H, Yan ZY, Huang ZH. Comparative study on the laminar flame speeds of methylcyclohexane-methanol and toluene-methanol blends at elevated temperatures. Fuel 2019;245:534–43. [50] Echekki Tarek, Chen Jacqueline H. Structure and Propagation of Methanol-Air Triple Flames. Combust Flame 1998. https://doi.org/10.1016/S0010-2180(97) 00287-3. [51] Zhang JX, Niu SD, Zhang YJ, Tang CL, Jiang X, Hu EJ, Huang ZH. Experimental and modeling study of the auto-ignition of n-heptane/n-butanol mixtures. Combust Flame 2013;160:31–9. [52] Pinzón LT, Mathieu O, Mulvihill CR, Schoegl I, Petersen EL. Ignition delay time and H2O measurements during methanol oxidation behind reflecte d shock waves. Combust Flame 2019;203:143–56. [53] Horng-Wen Wu, Hsu Tzu-Ting, Fan Chen-Ming, He Po-Hsien. Reduction of smoke, PM 2.5, and NO X of a diesel engine integrated with methanol steam reformer recovering waste heat and cooled EGR. Energy Convers Manage 2018;172:567–78. [54] Ibrahim Amr, Bari Saiful. An experimental investigation on the use of EGR in a supercharged natural gas SI engine. Fuel 2010;189:1721–30. [55] Zhen XD, Liu DM, Wang Y. The knock study of methanol fuel based on multi-dimensional simulation analysis. Energy 2017;122:552–9. [56] Merola SS, Vaglieco BM. Knock investigation by flame and radical species detection in spark ignition engine for different fuels. Energy Convers Manage 2007;48:2897–910. [57] Wei HQ, Zhu TY, Shu GQ, Tan LL, Wang YS. Gasoline engine exhaust gas recirculation e a review. Appl Energy 2012;99:534–44. [58] Hunicz J, Geca M, Rysak A, Litak G, Kordos P. Combustion timing variability in a light boosted controlled auto-ignition engine with direct fuel injection. J Vibro Eng 2013;15:1093–101. [59] Cowart J, Haghgooie M, Newman C, et al. The intensity of knock in an internal combustion engine: an experimental and modeling study. SAE Technical Paper, 922327; 1992. [60] Heywood JB. Internal Combustion Engine Fundamentals. New York: Mcgraw-hill; 1988.

compression ratio spark ignition engine. SAE Technical Paper, 932644; 1993. [19] Roberts M. Benefits and challenges of variable compression ratio (VCR). SAE Technical Paper, 2003-01-0398, 2003. [20] Costa RC, Sodré JR. Compression ratio effects on an ethanol/gasoline fuelled engine per-formance. Appl Therm Eng 2011;31:278–83. [21] Feng HQ, Wei JN, Zhang J. Numerical analysis of knock combustion with methanolisooctane blends in downsized SI engine. Fuel 2019;236:394–403. [22] Moran DP. Fuel evaporation and the high speed knock phenomenon of methanolgasoline blended fuels. SAE Technical Paper, 942063; 1994. [23] Liu H, Wang Z, Wang JX. Methanol-gasoline DFSI (dual-fuel spark ignition) combustion with dual-injection for engine knock suppression. Energy 2014;73:686–93. [24] Yacoub Y, Bata R, Gautam M. The performance and emission characteristics of C1–C5 alcohol–gasoline blends with matched oxygen content in a single-cylinder spark ignition engine. Power Energy 1998;212(5):363–79. [25] Zhou DZ, Yang WM, An H, Li J. Application of CFD-chemical kinetics approach in detecting RCCI engine knocking fuelled with biodiesel/methanol. Appl Energy 2015;145:255–64. [26] Liu H, Wang Z, Qi YL, He X, Wang YD, Wang JX. Super-knock suppression for highly turbocharged spark ignition engines using the fuel of propane or methanol. Energy 2019;169:1112–8. [27] Breda S, Berni F, d’Adamo A, Testa F, Severi E, Cantore G. Effects on knock intensity and specific fuel consumption of port water/methanol injection in a turbocharged GDI engine: comparative analysis. Energy Procedia 2015;82:96–102. [28] Zhen XD, Wang Y, Xu SQ, Zhu YS. Numerical analysis on knock for a high compression ratio spark-ignition methanol engine. Fuel 2013;103:892–8. [29] Blumberg PN, Bromberg L, Kang H, Tai C. Simulation of high efficiency heavy duty SI engines using direct injection of alcohol for knock avoidance. SAE paper 200801-2447; 2008. [30] Daniel R, Wang CG, Xu HG, Tian GH, Richardson D. Dual-injection as a knock mitigation strategy using pure ethanol and methanol. SAE paper 2012-01-1152; 2012. [31] Vancoillie J, Sileghem L, Verhelst S. Development and Validation of a Knock Prediction Model for Methanol-Fuelled SI Engines. SAE paper 2013-01-1312; 2013. [32] Senecal PK, Richards KJ, Pomraning E, Yang T, Dai MZ, McDavid RM, et al. A New Parallel Cut-Cell Cartesian CFD Code for Rapid Grid Generation Applied to InCylinder Diesel Engine Simulations. SAE Paper No. 2007-01-0159, 2007. [33] Richards KJ, Senecal PK, Pomraning E. CONVERGE (Version2.1.0) Manual. Middleton, WI: Convergent Science Inc.; 2014. [34] Turns SR. An Introduction to Combustion. McGraw-Hill Inc; 1996. [35] Zhen XD, Wang Y, Xu SQ, Zhu YS. Study of knock in a high compression ratio sparkignition methanol engine by multi-dimensional simulation. Energy 2013;50:150–9. [36] Brusstar M, Stuhldreher M, Swain D, Pidgeon W. High efficiency and low emissions from a port-injected engine with neat alcohol fuels, SAE Tech. Pap. Ser. 2002, 2002–01–2743. [37] De Petris C, Diana S, Giglio V, Police, G. High efficiency stoichiometric spark ignition engines, SAE Tech. Pap. Ser. 1994, 941933. [38] Vancoillie J, Demuynck J, Sileghem L, Van De Ginste M, Verhelst S, Brabant L, et al. Hoorebeke, The potential of methanol as a fuel for flex-fuel and dedicated sparkignition engines. Appl Energy 2013;102:140–9. [39] Zhang F, Shuai SJ, Wang Z, Zhang X, Wan JX. A detailed oxidation mechanism for the prediction of formaldehyde emission from methanol-gasoline SI engines. Proc Combust Inst 2011;33:3151–8. [40] Gong CM, Yu JW, Wang K, Liu JJ, Huang W, Si XK, et al. Numerical study of plasma produced ozone assisted combustion in a direct injection spark ignition methanol engine. Energy 2018;153:1028–37.

13