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Combustion performance improvement of a diesel fueled Wankel stratified-charge combustion engine by optimizing assisted ignition strategy
Energy Conversion and Management 205 (2020) 112324
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Combustion performance improvement of a diesel fueled Wankel stratifiedcharge combustion engine by optimizing assisted ignition strategy
T
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Wei Chena, Qingsong Zuoa, , Jianfeng Panb, Jianping Zhanga, Zhiqi Wanga, Bin Zhanga, Guohui Zhua, Baowei Fanb a b
College of Mechanical Engineering, Xiangtan University, Xiangtan, Hunan 411105, China School of Energy and Power Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Diesel Wankel engine Combustion performance Stratified combustion Assisted ignition strategy
This work aimed to further improve the combustion performance in a diesel Wankel engine by optimizing the assisted ignition strategy. Based on the established and verified dynamic simulation model, the effects of some key assisted ignition parameters, such as the number of assisted spark plug, assisted ignition position (AIP) and assisted ignition timing (AIT) on combustion performance were analyzed systematically. Simulation results indicated that the reaction process occurred in the mid-anterior region of the rotary chamber due to the stratifiedcharge mixture and forward-propagate flame. The combustion performance of the double assisted spark plug schemes was better than that of the single assisted spark plug scheme. The double assisted spark plugs symmetrically distributed relative to the major axis (X-axis) of the rotary chamber, the combustion process was more reasonable. When changing AIP, peak combustion pressure (Pmax) and combustion zone varied greatly, while they changed little when advancing AIT. For the optimized assisted ignition strategies, the optimal AIP scheme was Case5 (0, −88.5, ± 15 mm) and the optimum AIT scheme was Case7 (35°CA BTDC). Comparing with the original scheme (Case1), their Pmax increased by 16.68% and 17.83%, respectively. Meanwhile, the CA50 and CA10-90 were shortened obviously.
1. Introduction Currently, humanity is facing a severe energy crisis and environmental pollution problems, which put forward the requirements of high efficiency, cleaner and high energy density for many power systems [1]. Many scholars believe that the internal combustion engine will be the primary power source in the transportation field at present and in future decades [2]. As a kind of internal combustion engine, the rotary engine has many inherent advantages of small space proportion, strong fuel adaptability and the high power-to-weight ratio [3]. Nevertheless, it also needs to meet the new development requirement and latest market trends. The Wankel engine is the most famous and typical engine among different kinds of rotary engines. Meanwhile, the Wankel engine has a high-speed rotating triangular rotor, and it adopts an eccentric rotation design and converts the rotation motion directly into the driving force. It is mainly applied in the military field of the unmanned aerial vehicle (UAV) presently [4]. Benefiting from its lightweight structure and high power-to-weight ratio, the Wankel engine is very suitable for popularization and application in the civil field of hybrid electric vehicle (HEV) [5]. In recent years, with the vigorous ⁎
development of the new energy automobile market, the high-density power system based on Wankel engine technology has attracted the attention of many scholars and manufacturers [6,7]. However, the core combustion problem of the Wankel engine limits its rapid development and expanding application. Thus, it is urgent to improve the combustion efficiency and enhance the combustion performance of the Wankel engine, and then making it meet the high standards and requirements of the market. Ordinarily, the air intake, air/fuel mixing, mixture formation, ignition and combustion progress are the crucial engine working stages. By reasonably designing and optimizing the intake system, fuel injection system and ignition system, the combustion performance of the Wankel engine can be improved effectively. Recently, for the air intake system, some researchers focused on the intake pressure and angle [8], intake method [9] and the number of intake port [10]. For the fuel injection system, optimizing the injection strategy under port injection (PI) or direct injection (DI) mode to reasonably organize mixture distribution was carried out by some scholars. These included the gasoline [11] PI mode and the natural gas [12], diesel [13] and aviation kerosene [14] DI modes, respectively. Considering the multi-fuel characteristics of the Wankel
Corresponding author. E-mail address: [email protected] (Q. Zuo).
https://doi.org/10.1016/j.enconman.2019.112324 Received 17 September 2019; Received in revised form 10 November 2019; Accepted 19 November 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Three-dimensional model of diesel Wankel engine.
affects the combustion characteristics of the engine directly. Because it can control the start timing of the combustion process, which is essential for the flame kernel formation and growth, and the subsequent flame propagation process during the initial combustion stage [21]. Since this paper pays attention to the effect of ignition strategy on the Wankel engine combustion performance, the next step will introduce literature which focuses on the ignition system of the Wankel engine detailly. The latest research mainly includes: Kawahara et al. [22] adopted the laser absorption method to measure the mixture concentration around the spark plug in a gasoline Wankel engine. They found that after the start of combustion, the fuel concentration near the spark plug had a strong correlation with the combustion characteristics at the initial combustion stage. Boretti et al. [23] proposed the jet ignition technology to accelerate the fuel combustion rate of the gaseous fuel Wankel engine. The results indicated that the combustion efficiency and power output were improved due to the combustion rate became fast. Fan et al. [24] numerical explored the effects of ignition timing and ignition position on a natural gas Wankel engine
Table 1 Specifications of the diesel Wankel engine. Generating radius, (R)
103.5 mm
Eccentricity, (e) Chamber width, (B) Compression ratio Displacement Intake timing Exhaust timing
15.0 mm 79.2 mm 12.94 648 cm3 Advance angle 74 °CA BTDC, Delay angle 45 °CA ABDC Advance angle 90 °CA BBDC, Delay angle 44 °CA ATDC
engine, the dual-fuel injection mode was proposed and studied to further increase the Wankel engine combustion efficiency, which included the hydrogen/gasoline [15], hydrogen/natural gas [16] and natural gas/diesel [17] under PI mode or PI plus DI mode, and the hydrogen/ ethanol [18] and hydrogen/n-butanol [19] under PI mode. For the ignition system, it has a vital effect on the ignition and combustion process in a spark-ignition engine [20]. As a result, it
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Fig. 2. Transient mesh of diesel Wankel engine: (a) Mesh at initial timing; (b) Mesh at fuel injection timing.
combustion performance. The results showed that the engine peak pressure increased by 27.4% when adopting the trailing spark plug scheme coupled with 50 °CA BTDC ignition timing. Zambalov et al. [25] used a laser ignition method to improve the stability of the ignition process in a hydrogen Wankel engine. They pointed out that when using a double laser ignition system, the combustion performance was better than that of the single laser ignition system. Su et al. [26,27] experimentally investigated the influence of ignition timing on the performance of the hydrogen/gasoline and hydrogen/n-butanol Wankel engines. They found that advancing spark timing could increase the engines’ peak combustion pressure and temperature. Ji et al. [28,29] numerically explored the influence of ignition position on combustion characteristics and emission performance of the hydrogen/gasoline Wankel engine by using synchronous and asynchronous ignition method. The results showed that it was better ignition strategy when the dual-spark plugs were symmetrically arranged respect to the minor axis, the L-plug spark timing was 25°CA BTDC and T-plug spark timing was 35°CA BTDC. These interesting and fine papers are all involved with different Wankel engines, which using gasoline, n-butanol, natural gas, and hydrogen or their mixed fuel. Besides, the results show that the engine combustion performance is improved effectively by optimizing ignition strategy. Also, two key ignition parameters, namely ignition position and ignition timing, are generally concerned and studied. However, if the fuel distribution in the narrow rotary is different, the adopted ignition strategy also needs to change. It is because the mixture distribution at ignition timing has a crucial influence on subsequent combustion [12]. For these Wankel engines as above-mentioned, when using fuel PI mode, the mixture at ignition timing is homogeneouscharge. Although sometimes gaseous fuel DI mode is adopted, the mixture distribution and concentration are still quite different from that of using diesel fuel DI mode. In summary, as for the diesel Wankel engine, the optimization of the ignition strategy needs to be carefully reconsidered and deeply explored due to the stratified-charge mixture in the rotary chamber. As known, the diesel Wankel engine has the merits of high thermal efficiency, better power performance and low fuel consumption when comparing to the gasoline Wankel engine. Particularly, its high-security features make it very suitable for military and aviation applications [30]. It should be emphasized that the compression ratio of the diesel Wankel engine is lower due to its structural characteristics. It leads to it always uses an assisted ignition source to ignite the combustible mixture. Generally, the spark plug ignition is adopted widely in the prototype. For the studies on the ignition system of the diesel Wankel engine, Abraham et al. [31] compared the ignition reliability of two
Fig. 3. Different positions during the engine working process: (a) 270°CA BTDC; (b) 0°CA TDC; (c) 270 °C ATDC; 540°CA BTDC. Table 2 Summary of working conditions and key models. Specific objects
Values and results
Engine speed Throttle opening Intake pressure Intake temperature Equivalence ratio Diesel injection pressure Diesel injection duration Diesel injection timing Turbulence model Spray model Combustion model
4000 r/min 100% 1 bar 300 K 0.67 70 MPa 24°CA 80°CA BTDC RNG κ-ε model DPM and KH-RT model EDC model
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Fig. 4. Experimental setup for turbulence model validation: (a) Test rig schematic; (b) Test rig photo.
Fig. 5. Verification results of the turbulence model: (a) Different crank angles; (b) Different turbulence models.
Fig. 6. Experimental setup for spray model validation: (a) Test-bed schematic; (b) Test-bed photo.
performance is not presented and the key conclusions are not obtained. Based on a detailed literature survey and summary analysis, it can be found that there is no published literature focus or involve on comprehensive optimization of assisted ignition strategy for the diesel Wankel engine. Currently, limited information is provided about assisted ignition strategies on the stratified-charge combustion process in the diesel Wankel engine. Thus, the present study tries to fill this gap by conducting an optimization investigation of assisted ignition strategy in a diesel Wankel engine, thereby to better understand the combustion process and further improve combustion performance for its faster development and practical application. Especially, the effects of some key assisted ignition parameters, such as the number of assisted spark plug, assisted ignition position (AIP) and assisted ignition timing (AIT) on engine performance were explored by using computational fluid dynamics (CFD) method. Ultimately, the effective means to enhance combustion performance were proposed and evaluated. Meanwhile, the optimal assisted ignition strategy for the diesel Wankel engine was also
spark plugs with one spark plug. The results showed that engine performance was better when using two spark plugs. Hixon et al. [32] studied the feasibility of using catalytic electrothermal plug ignition mode. They found that the catalytic glow plug could operate steadily at a lower temperature. Muroki et al. [33] researched the influence of spark plug and pilot flame ignition methods on combustion performance in a diesel Wankel engine. The results indicated that the pilot flame ignition method could obtain better combustion stability. Wadumesthrige et al. [34] and Votaw [35] studied the spark ignition and micro-pilot flame ignition strategies of a diesel Wankel engine. They found that the optimized spark location and spark timing could improve engine performance. In summary, the ignition strategy is critical to organize the combustion process, and it can directly affect the performance of the diesel Wankel engine. In this paper, to distinguish the traditional spark ignition Wankel engine, the ignition strategy is defined as “assisted ignition strategy”. However, most of them are too old and the relevant researches are still not enough and comprehensive. Meanwhile, the crucial influence rule on engine combustion 4
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Fig. 7. Verification results of spray model in the constant volume combustion bomb: (a) Penetration length; (b) Spray shape.
Fig. 8. Verification results of spray model in the diesel Wankel engine: (a) Penetration length; (b) Spray shape.
determined and recommended. It provides some theoretical guidance for improving combustion performance and applying other liquid fuel in the Wankel engine. 2. Model establishment and reliability assessment 2.1. Geometrical model and transient mesh The three-dimensional model of the diesel Wankel engine is shown in Fig. 1. According to Fig. 1, the engine operates under diesel highpressure DI and assisted spark plug ignition mode. In Fig. 1, L1 is the distance between the fuel injection position and AIP (X-axis), and the value is 35 mm. Also, ϕ1 means the fuel injection angle, and the value is 60°. Also, L2 is the distance between AIP and the cylinder center point (Y-axis), and the value is 88.5 mm. The prototype parameters are listed in Table 1. Fig. 2 depicts the transient mesh of the diesel Wankel engine. As shown in Fig. 2, only one rotary chamber is chosen and it mainly includes three parts, namely the intake part, exhaust part and rotary chamber part. For all mesh parts, the tetrahedral mesh and 2.2 mm mesh size are adopted. At the initial timing (450°CA BTDC), the specific mesh parameters are 180,212 cells, 73,120 faces and 36,578 nodes. The
Fig. 9. Verification results of the combustion model.
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Table 3 Experimental conditions for model validation. Validated models
Engine speed
Injection pressure
Nozzle diameter
Ambient pressure
Ambient temperature
Injection timing
Equivalence ratio
Turbulence model
400 r/min 600 r/min – – 2000 r/min 3200 r/min
– – 70 MPa 160 MPa 44 MPa –
– – 0.18 mm 0.167 mm 0.217 mm –
– – 2,4,6 bar 50 bar 3.5 bar –
– – – 710 K – –
– – – – 65°CA BTDC –
– – – – – 0.39
Spray model
Combustion model
Fig. 10. Schematic diagram of assisted ignition strategies. (a) AIP schemes; (b) AIT schemes.
intake and exhaust parts are static mesh since they do not need to update during engine working. Moreover, the self-programming function combines with dynamic mesh technology in the professional CFD tool (ANSYS-Fluent) is chosen to realize the transient updating of the rotary chamber part. Fig. 3 shows different positions during the engine working process. According to Fig. 3, the volume of the combustion chamber is the largest at 270 °CA and the smallest at 0°CA or 540°CA. After mesh independence checking [24] and according to Ref. [13], the mesh type and mesh size can meet the computing requirements. Finally, an Intel Xeon E5-2690 CPU @ 2.6 GHz processor is used for case calculation. The calculation time of one single cycle is up to 15 days. It will take more time once a negative grid is generated. Considering the computation time, computation cost and workstation consumption, it is a reasonable choice under the current limited conditions.
Table 4 Summary of different assisted ignition strategies. Schemes
Assisted ignition timings
Spark plug I coordinates
Case1 Case2 Case3 Case4 Case5 Case6 Case7 Case8 Case9 Case10
20°CA 20°CA 20°CA 20°CA 20°CA 40°CA 35°CA 30°CA 25°CA 20°CA
(0, (0, (0, (0, (0, (0, (0, (0, (0, (0,
BTDC BTDC BTDC BTDC BTDC BTDC BTDC BTDC BTDC BTDC
−88.5, −88.5, −88.5, −88.5, −88.5, −88.5, −88.5, −88.5, −88.5, −88.5,
0) mm 0) mm 0) mm 0) mm −15) mm −15) mm −15) mm −15) mm −15) mm −15) mm
Spark plug II coordinates – (50, −88.5, 0) mm (0, −88.5, 15) mm (0, −88.5, −15) mm (0, −88.5, 15) mm (0, −88.5, 15) mm (0, −88.5, 15) mm (0, −88.5, 15) mm (0, −88.5, 15) mm (0, −88.5, 15) mm
Fig. 11. Diesel mass friction and air streamlines at different crankshaft angles: (a) 70°CA BTDC; (b) 60°CA BTDC; (c) 50°CA BTDC; (d) 40°CA BTDC. 6
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Fig. 12. Pmax and MFB under different AIP schemes: (a) Pmax; (b) MFB.
working condition was set up, as shown in Fig. 6. As shown in Fig. 6, the experimental setup consists of the high-pressure fuel pump, BOSCH injector, single-hole diesel nozzle, electronic control unit (ECU), control computers, high-speed camera, grating light source, convex lens, knifeedge, pressure gauge and pressure regulating valves. Diesel fuel was used in the experiment, and the test was repeated five times under each condition to ensure the reliability of experimental results [17]. Moreover, the fuel spray data in the rotary chamber from Ref. [38] was also chosen to validate the diesel fuel spray model further. The data from the spray experiment and the literature were both the average value of the measured data. Fig. 7 and Fig. 8 shows the verification results of the spray model in the constant volume combustion bomb and the diesel Wankel engine, respectively. It should be noted that the color of numerical results means particle traces colored by particle residence time (s). As shown in Figs. 7 and 8, by comparing to the experimental data, the simulated penetration length and spray shape of the diesel fuel show that the error is small and also the curves are well fitted. However, there is an error between the simulation results and experimental data. It is because the vaporization and mixing characteristics of real diesel cannot be fully simulated when using n-heptane. As a whole, the error of the prediction results in the constant volume combustion bomb is controlled at 5%, and that of in the diesel Wankel engine is 7%. When validating the EDC combustion model, the assessed turbulence model and spray model were chosen simultaneously. And the experimental data in Ref. [39] was used to verify the combustion model. In Fig. 9, the error of pressure in the rotary chamber between experimental data and simulation results is satisfactory. And the error of peak pressure is about 5%, and the average error during the combustion process is about 10%. It is because only one combustion chamber is simulated, and the leakage loss in the combustion chamber is ignored and the n-heptane cannot fully simulate the combustion characteristics of real diesel. All the validated work is carried under different engine working conditions (Table 3). What needs to be clarified here is that in the previous studies, the universality and rationality of the combustion model which coupled with different simplified mechanisms, such as diesel [13], natural gas [24], gasoline [40] and aviation kerosene [41] are also verified systematically. They are closely related studies and the verification results are satisfactory, which means that the established dynamic simulation model is accurate and reliable. Finally, this further research is also based on the completed validation work of the key model, and the more detailed information is presented in the documents [13,24,40,41].
2.2. Boundary conditions and key models The diesel Wankel engine operated under the conditions of natural aspirated, 4000 r/min speed, and full throttle opening. When exploring the effects of assisted-ignition strategies on engine combustion characteristics, the fuel injection parameters, namely the injection pressure, injection timing and injection duration kept constant, and their values were 70 MPa, 80°CA BTDC and 24°CA, respectively. Meanwhile, the nozzle is a BOSCH single-hole diesel nozzle and its diameter is 0.18 mm. Firstly, the re-normalization group (RNG) κ-ε turbulence model was chosen. Since it can predict the air evolution process in the rotary chamber of the diesel Wankel engine accurately. Secondly, the prediction accuracy of high-speed droplets of the discrete phase model (DPM) combined with Kelvin-Helmholtz, Rayleigh-Taylor (KH-RT) break-up model (B0 = 0.61 and B1 = 10), dynamic-drag law and walljet type during fuel spray process is reliable [36]. Therefore, it was chosen to predict the crucial air–fuel mixing processes, such as diesel injection, fuel movement, collision, coalescence, droplet-wall interactions. Thirdly, the eddy-dissipation concept (EDC) combustion model coupled with the 29 species and 52n-heptane reactions reduced mechanism [37] was selected to predict the diesel combustion process. The reliability of these models has been verified in detail, and now they are widely used in the Wankel engine numerical study based on different CFD platforms [8,10,13]. All boundary conditions and key models are presented in Table 2. 2.3. Experimental setup and model validation results Different experimental setups were built to validate the reliability of the key simulation models, and a stepwise verification method was adopted under different engine conditions. For turbulence model validation, a visualized Wankel engine was improved to obtain flow fields in-cylinder previously [8]. The engine test rig schematic and photo are shown in Fig. 4. In Fig. 4, the engine test rig mainly includes an optical Wankel engine, particle image velocimetry (PIV) system, frequency converter, shaft encoder, smoke generator and so on. Besides, the verification results of the turbulence model are presented in Fig. 5. As it is shown in Fig. 5(a), the RNG κ-ε model can accurately capture the flow trend and flow pattern of airflow under different crank angles. According to Fig. 5(b), also the prediction results of the RNG κ-ε model is the most reliable by comparing to different turbulence models. As known, the validation of the spray model is crucial for the DI Wankel engine. Therefore, a detailed verification work was carried out by considering the environment in the constant volume combustion bomb and the rotary chamber [13]. The experimental platform of constant volume combustion bomb under the diesel Wankel engine 7
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Table 5 Mixture distribution at AIT, temperature field and combustion intermediate HO2 concentration at TDC under different AIP schemes.
3. Results and discussion
the Z-axis direction. To study the influence of AIT, Case6-Case10 are gradually delaying the ignition timing from 40 to 20°CA BTDC. The specific parameters of different assisted ignition strategies are shown in Table 4.
3.1. Assisted ignition strategy elaboration Since the stratified-charge mixture in the diesel Wankel engine mainly distributes in the front region of the rotary chamber. According to the preliminary optimization experience [13], some key ignition parameters are finally chosen to explore their influence on engine combustion performance. Fig. 10 shows the detailed assisted ignition strategies. In Fig. 10, Case1 is the original scheme of the prototype. In order to research the influence of the number of assisted spark plug, Case2-Case5 adopt double assisted spark plugs. To explore the effect of AIP, Case2 is set to change the ignition position along the X-axis direction, and Case3-Case5 are set to change the ignition position along
3.2. Air-fuel evolution process and mixture formation Generally, before the combustion reaction, the fuel mixing progress will not be influenced when changing the ignition strategies, such as the number of assisted spark plug, AIP and AIT. Therefore, Fig. 11 illustrates the early mixture formation in the rotary chamber at different crankshaft angles. Furthermore, the air streamlines and diesel mass fraction (%) is used to illustrate the air and fuel movement process, respectively. In Fig. 11(a), at 70°CA BTDC the airflow in-cylinder is 8
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Fig. 13. Combustion characteristics under different AIT schemes: (a) Pmax; (b) MFB.
mainly the forward one-way flow, and there are only some small vortexes that exist near the leading pocket of the rotary chamber. During the fuel injection process, high-speed injected diesel collides with the rotary wall near the leading pocket, and some fuel attaches to the rotary wall. According to Fig. 11(b), the space of the rotary chamber further decreases with the rotating motion at 60°CA BTDC, and the airflow incylinder is squeezed. At the same time, there is mainly the forward oneway flow in the rotary chamber, and the small vortices further break up. Besides, diesel continues to diffuse and its distribution area also expands. As it is shown in Fig. 11(c) and (d), when the crankshaft angle is 50°CA BTDC and 40°CA BTDC, the airflow in-cylinder changes completely into the forward one-way flow, and the small vortices disappear. The injection process (injection duration is 24°CA) has ended, and diesel further diffuses to the front region of the rotary chamber under the direct impact of the high speed forward one-way flow. Finally, the distribution area of the mixture is further enlarged, and it mainly concentrates in the mid-anterior region of the rotary chamber.
these double assisted spark plug schemes is significantly faster than that of the single assisted spark plug scheme. Case2 has the fastest burning rate, then that is Case 5. Moreover, the flame propagation periods (CA10-90s) of Case1 to Case5 are 32.98°, 21.50°, 27.12°, 30.45° and 23.56° (ATDC), respectively. For Case2, the mixture has burned completely at this moment, which indicates that the fuel burn too fast and the combustion reaction process in-cylinder is so intense. It may increase the possibility of reducing the engine mechanical performance and service life. According to the principle of the engine reasonable combustion organization, the optimal scheme is Case5 and its combustion process is reasonable. In summary, when adopting double assisted spark plugs, the combustion performance of the diesel Wankel engine is obviously better than that of the single assisted spark plug. Among all double assisted spark plug schemes, Case5 is the preferred AIP scheme. When comparing to the original scheme, the Pmax of Case5 increases by 16.68%, and the CA50 and CA10-90 decrease significantly. It illustrates that the combustion efficiency of the studied diesel Wankel engine is effectively improved. In order to present the combustion development and chemical reactions processes in the rotary chamber clearly, the mixture distribution at AIT, temperature field and combustion intermediate HO2 concentration at TDC under different AIP schemes are given in Table 5. First, by observing the fuel distribution rule of Case1 to Case5 at AIT, the combustible mixture is concentrated at the mid-anterior region of the cylinder and distributed around the assisted spark plug, which provides a favorable burning environment for stable ignition combustion in the early stage. However, if the number of assisted spark plug and the AIP are changed, the presentive combustion processes are completely different. Second, according to the temperature field in Table 5, the area of the combustion zone at TDC of double assisted spark plug schemes is obviously wider than that of the single assisted spark plug scheme. Moreover, the combustion area of Case2 and Case5 is obviously larger than that of Case3 and Case4. It is because more fuel distributes near the assisted spark plug at the beginning of combustion, especially Case2. As for Case2, the flame front has spread to the front end of the rotary chamber. As a result, the flame propagation speed of Case2 is fastest. Nonetheless, its combustion reaction is also too intense. Finally, according to the combustion intermediate HO2 in Table 5, the flame shows an obvious forward propagation rule. Besides that, for double assisted spark plug schemes, the combustion reaction zone has a close relationship with the AIP, that is the flame of Case2 spreads to the region of the front end of the rotary chamber, Case3 is to the front cylinder wall and Case4 is to the rear cylinder wall. It indicates that during the flame development process, the flame presents the rule of propagating to the regions which exist an assisted ignition source.
3.3. Effect of assisted ignition position on engine combustion characteristics The peak combustion pressure (Pmax) and mass fraction burned (MFB) under different AIP schemes as seen in Fig. 12. Firstly, analyze the effect of the number of assisted spark plug on engine combustion characteristics. In Fig. 12(a), the Pmax of Case1 to Case5 is 45.26, 60.99, 47.94, 47.61 and 52.81 (bar), respectively. Moreover, the Pmax of the double assisted spark plug schemes (Case2 to Case5) is higher obviously when compares to the single assisted spark plug scheme (Case1). It indicates that using double assisted spark plugs can improve the initial ignition stage stability for the stratified-charge mixture of the diesel Wankel engine, and then it is beneficial to the following combustion process. Since there is an appropriate mixture concentration around the ignition system, adopting double assisted spark plugs can ensure that more mixture in-cylinder is ignited in time. Especially, under the combustion environment of high speed and forward one-way flow, the ignition stability of using double assisted spark plugs in the narrow rotary chamber is more prominent. Secondly, compare the effect of four different AIPs on engine combustion performance. That is Case2, Case3, Case4 and Case5. Among these double assisted spark plug schemes, the Pmax of Case2 and Case5 are higher, which shows that they are the better power performance schemes. By comparing with the single spark assisted plug scheme (Case1), their Pmax is increased by 34.75% and 16.68%, respectively. According to Fig. 12(b), the central heat release angles (CA50s) of Case1 to Case5 are 15.60°CA ATDC, 5.15°CA BTDC, 11.56°CA ATDC, 13.86° ATDC and 9.26° ATDC, respectively. The fuel burning rate of 9
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Table 6 Mixture distribution at AIT, temperature field and combustion intermediate HO2 concentration at TDC under different AIT schemes.
Case10 are 13.58°CA BTDC, 7.73°CA BTDC, 2.37°CA BTDC, 3.58°CA ATDC and 9.26°CA ATDC, respectively. Meanwhile, the CA10-90s of Case6 to Case10 are 0.62°CA BTDC, 5.61°CA ATDC, 11.17°CA ATDC, 17.59°CA ATDC and 23.56°CA ATDC, respectively. It shows that the curves of MFB integrally move to the left. Besides, CA50 and CA10-90 are close to the TDC, and also fuel burning rate is accelerated. Therefore, MFB increases gradually when the AIT is advanced. Since under an advanced AIT, the combustion process starts earlier and the total combustion time before TDC is longer. However, if the AIT is too early during the compression stage, that may be detrimental to the combustion performance and engine working. Therefore, the Pmax does not present the same increasing trend. In conclusion, advancing AIT, the MFB increases in turn, and Case7 is the optimal AIT scheme. However, due to the overall mixture distribution at AIT changes little, the Pmax of all schemes changes little.
Meanwhile, changing AIP can control the burning zone in the rotary chamber when making fuel use of the characteristics of mixture stratified distribution and flame forward propagation in the diesel Wankel engine. 3.4. Effect of assisted ignition timing on engine combustion characteristics The Pmax and MFB under different AIT schemes are presented in Fig. 13. Accroding to Fig. 13(a), the Pmax of Case6 to Case10 is 52.95, 53.33, 52.73, 51.93 and 52.81 (bar), respectively. With the advance of AIT, the change extent of Pmax is smaller in each scheme, which illustrates that AIT has less influence on the Pmax. In addition, the Pmax of Case7 is the largest, which indicates that when the AIT is 35°CA BTDC, the engine can achieve higher combustion pressure and obtain better power performance. As shown in Fig. 13(b), the CA50s of Case6 to 10
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The mixture distribution at AIT, temperature field and combustion intermediate HO2 concentration at TDC under different AIP schemes are presented in Table 6. As the mixture distribution in Table 6 shown, diesel mainly concentrates in the mid-anterior region of the rotary chamber at different AITs. With the rotor rotating, the distribution range of mixture increases. However, the fuel concentration around the assisted spark plug does not change so much. By observing the temperature field and combustion intermediate HO2 concentration in Table 6, after the assisted spark plug ignites the combustible mixture, the combustion reaction occurs in the region which has the stratified distributed combustible mixture, that is the mid-anterior region part of the rotary chamber. Owing to the forward flame propagation rule of the Wankel engine and the stratified-charge mixture in the front part of the rotary chamber, HO2 concentrated in the same region. Also, by advancing AIT, the total area of the burning zone, contour range of HO2 and airflow velocity change slightly. It further explains the Pmax presents a small fluctuation at different AITs, which is because the flame propagation speed, flow field distribution, combustion temperature and burning intensity in-cylinder change little.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to acknowledge Project (51606162 and 51975503) supported by the National Natural Science Foundation of China, Project (2017JJ4052) supported by the Natural Science Foundation of Hunan Province and Project (19QDZ42) supported by the Ph.D. Scientific Research Startup Foundation of Xiangtan University. Author Contribution. Wei Chen: Conceptualization, Methodology, Software, Data curation, Writing-Original Draft, Writing-Review & Editing. Qingsong Zuo: Supervision, Writing-Review & Editing, Funding acquisition. Jianfeng Pan: Resources, Project administration, Funding acquisition. Jianping Zhang: Supervision, Funding acquisition. Zhiqi Wang: Writing-Reviewing & Editing. Bin Zhang: Writing-Review & Editing. Guohui Zhu: Writing-Review & Editing. Baowei Fan: Formal analysis.
4. Conclusions The core objective of the present study was to improve the combustion performance in a diesel fueled Wankel stratified-charge combustion engine by using a CFD method. Thus, the influence of assisted ignition strategies, such as the number of assisted spark plug, AIP and AIT on the combustion characteristics were systematically explored and optimized. The important conclusions were:
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1. For the diesel Wankel engine, the fuel concentrated at the midanterior region of the rotary chamber, and it presented a state of stratified distribution. Stratified-charge combustible mixture and forward-propagate flame made reaction process mainly occur in this zone during the combustion process. 2. When using double assisted spark plugs, the Pmax and MFB of the diesel Wankel engine were improved obviously. Combustion performance of double assisted spark plug schemes (Case2 to Case5) was better than that of the single assisted spark plug scheme (Case1). 3. Changing AIP, the Pmax and the combustion zone of Case1 to Case5 were quite different, since the distribution and concentration of the mixture around the assisted spark plug is crucial to the following combustion process. The double assisted spark plugs symmetrically distributed relative to the major axis (X-axis) of the rotary chamber was better. For concerned AIPs, the optimal scheme was Case5 (i.e. 0, −88.5, ± 15 mm). 4. Advancing AIT, since little difference in mixture concentration and distribution area, the Pmax, the reaction area and combustion intensity of Case6 to Case10 changed little. Meanwhile, the MFB was increased due to a longer combustion time before TDC. For explored AITs, the optimum scheme was Case7 (i.e. 35°CA BTDC). 5. Comparing with the original scheme (Case1), the Pmax of the optimum AIP and AIT schemes increased by 16.68% and 17.83%, respectively. Also, their CA50 and CA10-90 were shortened obviously.
CRediT authorship contribution statement Wei Chen: Conceptualization, Methodology, Software, Data curation, Writing - original draft, Writing - review & editing. Qingsong Zuo: Supervision, Writing - review & editing, Funding acquisition. Jianfeng Pan: Resources, Project administration, Funding acquisition. Jianping Zhang: Supervision, Funding acquisition. Zhiqi Wang: Writing - review & editing. Bin Zhang: Writing - review & editing. Guohui Zhu: Writing - review & editing. Baowei Fan: Formal analysis. 11
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direct-injection enrichment. Energy Convers Manage 2019;181:372–81. [29] Shi C, Ji C, Wang S, Yang J, Li X, Ge Y. Numerical simulation on combustion process of a hydrogen direct-injection stratified gasoline Wankel engine by synchronous and asynchronous ignition modes. Energy Convers Manage 2019;183:14–25. [30] Abraham J, BracKweon CBM. A review of heavy-fueled rotary engine combustion technologies (No. ARL-TR-5546). Army Research Lab Aberdeen Proving Ground Md; 2011. [31] Abraham J, Bracco FV. 3-D Computations to improve combustion in a stratifiedcharge rotary engine-Part III: Improved ignition strategies (No. 920304). SAE Technical Paper 1992. [32] Hixon D, Burns K, Pfefferle W. Catalytic glow plugs in the JDTI stratified charge rotary engine (No. 930680). SAE Technical Paper 1993. [33] Muroki T, Moriyoshi Y, Takagi M, Suzuki K, Imai M. Research and development of a direct injection stratified charge rotary engine with a pilot flame ignition system (No. 2001-01-1844). SAE Technical Paper 2001. [34] Wadumesthrige B, Asela A. Computational investigation of optimal heavy fuel direct injection spark ignition in rotary engine. Wright State University; 2011. [35] Votaw ZS. Computational study on micro-pilot flame ignition strategy for a direct injection stratified charge rotary engine. Wright State University; 2012. [36] ANSYS Inc (2015). ANSYS-Fluent theory guide. Release 16.0. [37] Patel A, Kong SC, Reitz RD. Development and validation of a reduced reaction mechanism for HCCI engine simulations (No. 2004-01-0558). SAE Technical Paper 2004. [38] Morita T, Hamady F, Stuecken T, Somerton C, Schock H. Fuel-air mixing visualization in a motored rotary engine assembly (No. 910704). SAE Technical Paper 1991. [39] Nguyen HL, Addy HE, Bond TH, Lee CM, Chun KS. Performance and efficiency evaluation and heat release study of a direct-injection stratified-charge rotary engine (No. 870445). SAE Technical Paper 1987. [40] Pan J, Lu Y, Huang M, Otchere P, Chen W, Fan B. Effect of different hydrogen blending ratios on combustion process of gasoline-fueled rotary engine. Environ Progr Sustain Energy 2019;38(5):1–13. [41] Lu Y, Pan J, Fan B, Otchere P, Chen W, Cheng B. Research on the application of aviation kerosene in a direct injection rotary engine-Part 2: Spray combustion characteristics and combustion process under optimized injection strategies. Energy Convers Manage 2020;203:112217.
injection plus natural gas port injection rotary engine. Energy Convers Manage 2018;160:150–64. Chen W, Pan J, Liu Y, Fan B, Liu H, Otchere P. Numerical investigation of direct injection stratified charge combustion in a natural gas-diesel rotary engine. Appl Energy 2019;233:453–67. Amrouche F, Erickson PA, Varnhagen S, Park JW. An experimental study of a hydrogen-enriched ethanol fueled Wankel rotary engine at ultra lean and full load conditions. Energy Convers Manage 2016;123:174–84. Su T, Ji C, Wang S, Cong X, Shi L. Research on performance of a hydrogen/nbutanol rotary engine at idling and varied excess air ratios. Energy Convers Manage 2018;162:132–8. Wang T, Zhang X, Zhang J, Hou X. Numerical analysis of the influence of the fuel injection timing and ignition position in a direct-injection natural gas engine. Energy Convers Manage 2017;149:748–59. Chen Z, Wang L, Zhang Q, Zhang X, Yang B, Zeng K. Effects of spark timing and methanol addition on combustion characteristics and emissions of dual-fuel engine fuelled with natural gas and methanol under lean-burn condition. Energy Convers Manage 2019;181:519–27. Kawahara N, Tomita E, Hayashi K, Tabata M, Iwai K, Kagawa R. Cycle-resolved measurements of the fuel concentration near a spark plug in a rotary engine using an in situ laser absorption method. Proc Combust Inst 2007;31(2):3033–40. Boretti A, Jiang S, Scalzo J. A novel Wankel engine featuring jet ignition and port or direct injection for faster and more complete combustion especially designed for gaseous fuels (No. 2015-01-0007). SAE Technical Paper 2015. Fan B, Pan J, Liu Y, Zhu Y. Effects of ignition parameters on combustion process of a rotary engine fueled with natural gas. Energy Convers Manage 2015;103:218–34. Zambalov SD, Yakovlev IA, Skripnyak VA. Numerical simulation of hydrogen combustion process in rotary engine with laser ignition system. Int J Hydrogen Energy 2017;42(27):17251–9. Su T, Ji C, Wang S, Shi L, Yang J, Cong X. Effect of spark timing on performance of a hydrogen-gasoline rotary engine. Energy Convers Manage 2017;148:120–7. Su T, Ji C, Wang S, Shi L, Cong X. Effect of ignition timing on performance of a hydrogen-enriched n-butanol rotary engine at lean condition. Energy Convers Manage 2018;161:27–34. Ji C, Shi C, Wang S, Yang J, Su T, Wang D. Effect of dual-spark plug arrangements on ignition and combustion processes of a gasoline rotary engine with hydrogen
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Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Combustion performance improvement of a diesel fueled Wankel stratifiedcharge combustion engine by optimizing assisted ignition strategy
T
⁎
Wei Chena, Qingsong Zuoa, , Jianfeng Panb, Jianping Zhanga, Zhiqi Wanga, Bin Zhanga, Guohui Zhua, Baowei Fanb a b
College of Mechanical Engineering, Xiangtan University, Xiangtan, Hunan 411105, China School of Energy and Power Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Diesel Wankel engine Combustion performance Stratified combustion Assisted ignition strategy
This work aimed to further improve the combustion performance in a diesel Wankel engine by optimizing the assisted ignition strategy. Based on the established and verified dynamic simulation model, the effects of some key assisted ignition parameters, such as the number of assisted spark plug, assisted ignition position (AIP) and assisted ignition timing (AIT) on combustion performance were analyzed systematically. Simulation results indicated that the reaction process occurred in the mid-anterior region of the rotary chamber due to the stratifiedcharge mixture and forward-propagate flame. The combustion performance of the double assisted spark plug schemes was better than that of the single assisted spark plug scheme. The double assisted spark plugs symmetrically distributed relative to the major axis (X-axis) of the rotary chamber, the combustion process was more reasonable. When changing AIP, peak combustion pressure (Pmax) and combustion zone varied greatly, while they changed little when advancing AIT. For the optimized assisted ignition strategies, the optimal AIP scheme was Case5 (0, −88.5, ± 15 mm) and the optimum AIT scheme was Case7 (35°CA BTDC). Comparing with the original scheme (Case1), their Pmax increased by 16.68% and 17.83%, respectively. Meanwhile, the CA50 and CA10-90 were shortened obviously.
1. Introduction Currently, humanity is facing a severe energy crisis and environmental pollution problems, which put forward the requirements of high efficiency, cleaner and high energy density for many power systems [1]. Many scholars believe that the internal combustion engine will be the primary power source in the transportation field at present and in future decades [2]. As a kind of internal combustion engine, the rotary engine has many inherent advantages of small space proportion, strong fuel adaptability and the high power-to-weight ratio [3]. Nevertheless, it also needs to meet the new development requirement and latest market trends. The Wankel engine is the most famous and typical engine among different kinds of rotary engines. Meanwhile, the Wankel engine has a high-speed rotating triangular rotor, and it adopts an eccentric rotation design and converts the rotation motion directly into the driving force. It is mainly applied in the military field of the unmanned aerial vehicle (UAV) presently [4]. Benefiting from its lightweight structure and high power-to-weight ratio, the Wankel engine is very suitable for popularization and application in the civil field of hybrid electric vehicle (HEV) [5]. In recent years, with the vigorous ⁎
development of the new energy automobile market, the high-density power system based on Wankel engine technology has attracted the attention of many scholars and manufacturers [6,7]. However, the core combustion problem of the Wankel engine limits its rapid development and expanding application. Thus, it is urgent to improve the combustion efficiency and enhance the combustion performance of the Wankel engine, and then making it meet the high standards and requirements of the market. Ordinarily, the air intake, air/fuel mixing, mixture formation, ignition and combustion progress are the crucial engine working stages. By reasonably designing and optimizing the intake system, fuel injection system and ignition system, the combustion performance of the Wankel engine can be improved effectively. Recently, for the air intake system, some researchers focused on the intake pressure and angle [8], intake method [9] and the number of intake port [10]. For the fuel injection system, optimizing the injection strategy under port injection (PI) or direct injection (DI) mode to reasonably organize mixture distribution was carried out by some scholars. These included the gasoline [11] PI mode and the natural gas [12], diesel [13] and aviation kerosene [14] DI modes, respectively. Considering the multi-fuel characteristics of the Wankel
Corresponding author. E-mail address: [email protected] (Q. Zuo).
https://doi.org/10.1016/j.enconman.2019.112324 Received 17 September 2019; Received in revised form 10 November 2019; Accepted 19 November 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Three-dimensional model of diesel Wankel engine.
affects the combustion characteristics of the engine directly. Because it can control the start timing of the combustion process, which is essential for the flame kernel formation and growth, and the subsequent flame propagation process during the initial combustion stage [21]. Since this paper pays attention to the effect of ignition strategy on the Wankel engine combustion performance, the next step will introduce literature which focuses on the ignition system of the Wankel engine detailly. The latest research mainly includes: Kawahara et al. [22] adopted the laser absorption method to measure the mixture concentration around the spark plug in a gasoline Wankel engine. They found that after the start of combustion, the fuel concentration near the spark plug had a strong correlation with the combustion characteristics at the initial combustion stage. Boretti et al. [23] proposed the jet ignition technology to accelerate the fuel combustion rate of the gaseous fuel Wankel engine. The results indicated that the combustion efficiency and power output were improved due to the combustion rate became fast. Fan et al. [24] numerical explored the effects of ignition timing and ignition position on a natural gas Wankel engine
Table 1 Specifications of the diesel Wankel engine. Generating radius, (R)
103.5 mm
Eccentricity, (e) Chamber width, (B) Compression ratio Displacement Intake timing Exhaust timing
15.0 mm 79.2 mm 12.94 648 cm3 Advance angle 74 °CA BTDC, Delay angle 45 °CA ABDC Advance angle 90 °CA BBDC, Delay angle 44 °CA ATDC
engine, the dual-fuel injection mode was proposed and studied to further increase the Wankel engine combustion efficiency, which included the hydrogen/gasoline [15], hydrogen/natural gas [16] and natural gas/diesel [17] under PI mode or PI plus DI mode, and the hydrogen/ ethanol [18] and hydrogen/n-butanol [19] under PI mode. For the ignition system, it has a vital effect on the ignition and combustion process in a spark-ignition engine [20]. As a result, it
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Fig. 2. Transient mesh of diesel Wankel engine: (a) Mesh at initial timing; (b) Mesh at fuel injection timing.
combustion performance. The results showed that the engine peak pressure increased by 27.4% when adopting the trailing spark plug scheme coupled with 50 °CA BTDC ignition timing. Zambalov et al. [25] used a laser ignition method to improve the stability of the ignition process in a hydrogen Wankel engine. They pointed out that when using a double laser ignition system, the combustion performance was better than that of the single laser ignition system. Su et al. [26,27] experimentally investigated the influence of ignition timing on the performance of the hydrogen/gasoline and hydrogen/n-butanol Wankel engines. They found that advancing spark timing could increase the engines’ peak combustion pressure and temperature. Ji et al. [28,29] numerically explored the influence of ignition position on combustion characteristics and emission performance of the hydrogen/gasoline Wankel engine by using synchronous and asynchronous ignition method. The results showed that it was better ignition strategy when the dual-spark plugs were symmetrically arranged respect to the minor axis, the L-plug spark timing was 25°CA BTDC and T-plug spark timing was 35°CA BTDC. These interesting and fine papers are all involved with different Wankel engines, which using gasoline, n-butanol, natural gas, and hydrogen or their mixed fuel. Besides, the results show that the engine combustion performance is improved effectively by optimizing ignition strategy. Also, two key ignition parameters, namely ignition position and ignition timing, are generally concerned and studied. However, if the fuel distribution in the narrow rotary is different, the adopted ignition strategy also needs to change. It is because the mixture distribution at ignition timing has a crucial influence on subsequent combustion [12]. For these Wankel engines as above-mentioned, when using fuel PI mode, the mixture at ignition timing is homogeneouscharge. Although sometimes gaseous fuel DI mode is adopted, the mixture distribution and concentration are still quite different from that of using diesel fuel DI mode. In summary, as for the diesel Wankel engine, the optimization of the ignition strategy needs to be carefully reconsidered and deeply explored due to the stratified-charge mixture in the rotary chamber. As known, the diesel Wankel engine has the merits of high thermal efficiency, better power performance and low fuel consumption when comparing to the gasoline Wankel engine. Particularly, its high-security features make it very suitable for military and aviation applications [30]. It should be emphasized that the compression ratio of the diesel Wankel engine is lower due to its structural characteristics. It leads to it always uses an assisted ignition source to ignite the combustible mixture. Generally, the spark plug ignition is adopted widely in the prototype. For the studies on the ignition system of the diesel Wankel engine, Abraham et al. [31] compared the ignition reliability of two
Fig. 3. Different positions during the engine working process: (a) 270°CA BTDC; (b) 0°CA TDC; (c) 270 °C ATDC; 540°CA BTDC. Table 2 Summary of working conditions and key models. Specific objects
Values and results
Engine speed Throttle opening Intake pressure Intake temperature Equivalence ratio Diesel injection pressure Diesel injection duration Diesel injection timing Turbulence model Spray model Combustion model
4000 r/min 100% 1 bar 300 K 0.67 70 MPa 24°CA 80°CA BTDC RNG κ-ε model DPM and KH-RT model EDC model
3
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Fig. 4. Experimental setup for turbulence model validation: (a) Test rig schematic; (b) Test rig photo.
Fig. 5. Verification results of the turbulence model: (a) Different crank angles; (b) Different turbulence models.
Fig. 6. Experimental setup for spray model validation: (a) Test-bed schematic; (b) Test-bed photo.
performance is not presented and the key conclusions are not obtained. Based on a detailed literature survey and summary analysis, it can be found that there is no published literature focus or involve on comprehensive optimization of assisted ignition strategy for the diesel Wankel engine. Currently, limited information is provided about assisted ignition strategies on the stratified-charge combustion process in the diesel Wankel engine. Thus, the present study tries to fill this gap by conducting an optimization investigation of assisted ignition strategy in a diesel Wankel engine, thereby to better understand the combustion process and further improve combustion performance for its faster development and practical application. Especially, the effects of some key assisted ignition parameters, such as the number of assisted spark plug, assisted ignition position (AIP) and assisted ignition timing (AIT) on engine performance were explored by using computational fluid dynamics (CFD) method. Ultimately, the effective means to enhance combustion performance were proposed and evaluated. Meanwhile, the optimal assisted ignition strategy for the diesel Wankel engine was also
spark plugs with one spark plug. The results showed that engine performance was better when using two spark plugs. Hixon et al. [32] studied the feasibility of using catalytic electrothermal plug ignition mode. They found that the catalytic glow plug could operate steadily at a lower temperature. Muroki et al. [33] researched the influence of spark plug and pilot flame ignition methods on combustion performance in a diesel Wankel engine. The results indicated that the pilot flame ignition method could obtain better combustion stability. Wadumesthrige et al. [34] and Votaw [35] studied the spark ignition and micro-pilot flame ignition strategies of a diesel Wankel engine. They found that the optimized spark location and spark timing could improve engine performance. In summary, the ignition strategy is critical to organize the combustion process, and it can directly affect the performance of the diesel Wankel engine. In this paper, to distinguish the traditional spark ignition Wankel engine, the ignition strategy is defined as “assisted ignition strategy”. However, most of them are too old and the relevant researches are still not enough and comprehensive. Meanwhile, the crucial influence rule on engine combustion 4
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Fig. 7. Verification results of spray model in the constant volume combustion bomb: (a) Penetration length; (b) Spray shape.
Fig. 8. Verification results of spray model in the diesel Wankel engine: (a) Penetration length; (b) Spray shape.
determined and recommended. It provides some theoretical guidance for improving combustion performance and applying other liquid fuel in the Wankel engine. 2. Model establishment and reliability assessment 2.1. Geometrical model and transient mesh The three-dimensional model of the diesel Wankel engine is shown in Fig. 1. According to Fig. 1, the engine operates under diesel highpressure DI and assisted spark plug ignition mode. In Fig. 1, L1 is the distance between the fuel injection position and AIP (X-axis), and the value is 35 mm. Also, ϕ1 means the fuel injection angle, and the value is 60°. Also, L2 is the distance between AIP and the cylinder center point (Y-axis), and the value is 88.5 mm. The prototype parameters are listed in Table 1. Fig. 2 depicts the transient mesh of the diesel Wankel engine. As shown in Fig. 2, only one rotary chamber is chosen and it mainly includes three parts, namely the intake part, exhaust part and rotary chamber part. For all mesh parts, the tetrahedral mesh and 2.2 mm mesh size are adopted. At the initial timing (450°CA BTDC), the specific mesh parameters are 180,212 cells, 73,120 faces and 36,578 nodes. The
Fig. 9. Verification results of the combustion model.
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Table 3 Experimental conditions for model validation. Validated models
Engine speed
Injection pressure
Nozzle diameter
Ambient pressure
Ambient temperature
Injection timing
Equivalence ratio
Turbulence model
400 r/min 600 r/min – – 2000 r/min 3200 r/min
– – 70 MPa 160 MPa 44 MPa –
– – 0.18 mm 0.167 mm 0.217 mm –
– – 2,4,6 bar 50 bar 3.5 bar –
– – – 710 K – –
– – – – 65°CA BTDC –
– – – – – 0.39
Spray model
Combustion model
Fig. 10. Schematic diagram of assisted ignition strategies. (a) AIP schemes; (b) AIT schemes.
intake and exhaust parts are static mesh since they do not need to update during engine working. Moreover, the self-programming function combines with dynamic mesh technology in the professional CFD tool (ANSYS-Fluent) is chosen to realize the transient updating of the rotary chamber part. Fig. 3 shows different positions during the engine working process. According to Fig. 3, the volume of the combustion chamber is the largest at 270 °CA and the smallest at 0°CA or 540°CA. After mesh independence checking [24] and according to Ref. [13], the mesh type and mesh size can meet the computing requirements. Finally, an Intel Xeon E5-2690 CPU @ 2.6 GHz processor is used for case calculation. The calculation time of one single cycle is up to 15 days. It will take more time once a negative grid is generated. Considering the computation time, computation cost and workstation consumption, it is a reasonable choice under the current limited conditions.
Table 4 Summary of different assisted ignition strategies. Schemes
Assisted ignition timings
Spark plug I coordinates
Case1 Case2 Case3 Case4 Case5 Case6 Case7 Case8 Case9 Case10
20°CA 20°CA 20°CA 20°CA 20°CA 40°CA 35°CA 30°CA 25°CA 20°CA
(0, (0, (0, (0, (0, (0, (0, (0, (0, (0,
BTDC BTDC BTDC BTDC BTDC BTDC BTDC BTDC BTDC BTDC
−88.5, −88.5, −88.5, −88.5, −88.5, −88.5, −88.5, −88.5, −88.5, −88.5,
0) mm 0) mm 0) mm 0) mm −15) mm −15) mm −15) mm −15) mm −15) mm −15) mm
Spark plug II coordinates – (50, −88.5, 0) mm (0, −88.5, 15) mm (0, −88.5, −15) mm (0, −88.5, 15) mm (0, −88.5, 15) mm (0, −88.5, 15) mm (0, −88.5, 15) mm (0, −88.5, 15) mm (0, −88.5, 15) mm
Fig. 11. Diesel mass friction and air streamlines at different crankshaft angles: (a) 70°CA BTDC; (b) 60°CA BTDC; (c) 50°CA BTDC; (d) 40°CA BTDC. 6
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Fig. 12. Pmax and MFB under different AIP schemes: (a) Pmax; (b) MFB.
working condition was set up, as shown in Fig. 6. As shown in Fig. 6, the experimental setup consists of the high-pressure fuel pump, BOSCH injector, single-hole diesel nozzle, electronic control unit (ECU), control computers, high-speed camera, grating light source, convex lens, knifeedge, pressure gauge and pressure regulating valves. Diesel fuel was used in the experiment, and the test was repeated five times under each condition to ensure the reliability of experimental results [17]. Moreover, the fuel spray data in the rotary chamber from Ref. [38] was also chosen to validate the diesel fuel spray model further. The data from the spray experiment and the literature were both the average value of the measured data. Fig. 7 and Fig. 8 shows the verification results of the spray model in the constant volume combustion bomb and the diesel Wankel engine, respectively. It should be noted that the color of numerical results means particle traces colored by particle residence time (s). As shown in Figs. 7 and 8, by comparing to the experimental data, the simulated penetration length and spray shape of the diesel fuel show that the error is small and also the curves are well fitted. However, there is an error between the simulation results and experimental data. It is because the vaporization and mixing characteristics of real diesel cannot be fully simulated when using n-heptane. As a whole, the error of the prediction results in the constant volume combustion bomb is controlled at 5%, and that of in the diesel Wankel engine is 7%. When validating the EDC combustion model, the assessed turbulence model and spray model were chosen simultaneously. And the experimental data in Ref. [39] was used to verify the combustion model. In Fig. 9, the error of pressure in the rotary chamber between experimental data and simulation results is satisfactory. And the error of peak pressure is about 5%, and the average error during the combustion process is about 10%. It is because only one combustion chamber is simulated, and the leakage loss in the combustion chamber is ignored and the n-heptane cannot fully simulate the combustion characteristics of real diesel. All the validated work is carried under different engine working conditions (Table 3). What needs to be clarified here is that in the previous studies, the universality and rationality of the combustion model which coupled with different simplified mechanisms, such as diesel [13], natural gas [24], gasoline [40] and aviation kerosene [41] are also verified systematically. They are closely related studies and the verification results are satisfactory, which means that the established dynamic simulation model is accurate and reliable. Finally, this further research is also based on the completed validation work of the key model, and the more detailed information is presented in the documents [13,24,40,41].
2.2. Boundary conditions and key models The diesel Wankel engine operated under the conditions of natural aspirated, 4000 r/min speed, and full throttle opening. When exploring the effects of assisted-ignition strategies on engine combustion characteristics, the fuel injection parameters, namely the injection pressure, injection timing and injection duration kept constant, and their values were 70 MPa, 80°CA BTDC and 24°CA, respectively. Meanwhile, the nozzle is a BOSCH single-hole diesel nozzle and its diameter is 0.18 mm. Firstly, the re-normalization group (RNG) κ-ε turbulence model was chosen. Since it can predict the air evolution process in the rotary chamber of the diesel Wankel engine accurately. Secondly, the prediction accuracy of high-speed droplets of the discrete phase model (DPM) combined with Kelvin-Helmholtz, Rayleigh-Taylor (KH-RT) break-up model (B0 = 0.61 and B1 = 10), dynamic-drag law and walljet type during fuel spray process is reliable [36]. Therefore, it was chosen to predict the crucial air–fuel mixing processes, such as diesel injection, fuel movement, collision, coalescence, droplet-wall interactions. Thirdly, the eddy-dissipation concept (EDC) combustion model coupled with the 29 species and 52n-heptane reactions reduced mechanism [37] was selected to predict the diesel combustion process. The reliability of these models has been verified in detail, and now they are widely used in the Wankel engine numerical study based on different CFD platforms [8,10,13]. All boundary conditions and key models are presented in Table 2. 2.3. Experimental setup and model validation results Different experimental setups were built to validate the reliability of the key simulation models, and a stepwise verification method was adopted under different engine conditions. For turbulence model validation, a visualized Wankel engine was improved to obtain flow fields in-cylinder previously [8]. The engine test rig schematic and photo are shown in Fig. 4. In Fig. 4, the engine test rig mainly includes an optical Wankel engine, particle image velocimetry (PIV) system, frequency converter, shaft encoder, smoke generator and so on. Besides, the verification results of the turbulence model are presented in Fig. 5. As it is shown in Fig. 5(a), the RNG κ-ε model can accurately capture the flow trend and flow pattern of airflow under different crank angles. According to Fig. 5(b), also the prediction results of the RNG κ-ε model is the most reliable by comparing to different turbulence models. As known, the validation of the spray model is crucial for the DI Wankel engine. Therefore, a detailed verification work was carried out by considering the environment in the constant volume combustion bomb and the rotary chamber [13]. The experimental platform of constant volume combustion bomb under the diesel Wankel engine 7
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Table 5 Mixture distribution at AIT, temperature field and combustion intermediate HO2 concentration at TDC under different AIP schemes.
3. Results and discussion
the Z-axis direction. To study the influence of AIT, Case6-Case10 are gradually delaying the ignition timing from 40 to 20°CA BTDC. The specific parameters of different assisted ignition strategies are shown in Table 4.
3.1. Assisted ignition strategy elaboration Since the stratified-charge mixture in the diesel Wankel engine mainly distributes in the front region of the rotary chamber. According to the preliminary optimization experience [13], some key ignition parameters are finally chosen to explore their influence on engine combustion performance. Fig. 10 shows the detailed assisted ignition strategies. In Fig. 10, Case1 is the original scheme of the prototype. In order to research the influence of the number of assisted spark plug, Case2-Case5 adopt double assisted spark plugs. To explore the effect of AIP, Case2 is set to change the ignition position along the X-axis direction, and Case3-Case5 are set to change the ignition position along
3.2. Air-fuel evolution process and mixture formation Generally, before the combustion reaction, the fuel mixing progress will not be influenced when changing the ignition strategies, such as the number of assisted spark plug, AIP and AIT. Therefore, Fig. 11 illustrates the early mixture formation in the rotary chamber at different crankshaft angles. Furthermore, the air streamlines and diesel mass fraction (%) is used to illustrate the air and fuel movement process, respectively. In Fig. 11(a), at 70°CA BTDC the airflow in-cylinder is 8
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Fig. 13. Combustion characteristics under different AIT schemes: (a) Pmax; (b) MFB.
mainly the forward one-way flow, and there are only some small vortexes that exist near the leading pocket of the rotary chamber. During the fuel injection process, high-speed injected diesel collides with the rotary wall near the leading pocket, and some fuel attaches to the rotary wall. According to Fig. 11(b), the space of the rotary chamber further decreases with the rotating motion at 60°CA BTDC, and the airflow incylinder is squeezed. At the same time, there is mainly the forward oneway flow in the rotary chamber, and the small vortices further break up. Besides, diesel continues to diffuse and its distribution area also expands. As it is shown in Fig. 11(c) and (d), when the crankshaft angle is 50°CA BTDC and 40°CA BTDC, the airflow in-cylinder changes completely into the forward one-way flow, and the small vortices disappear. The injection process (injection duration is 24°CA) has ended, and diesel further diffuses to the front region of the rotary chamber under the direct impact of the high speed forward one-way flow. Finally, the distribution area of the mixture is further enlarged, and it mainly concentrates in the mid-anterior region of the rotary chamber.
these double assisted spark plug schemes is significantly faster than that of the single assisted spark plug scheme. Case2 has the fastest burning rate, then that is Case 5. Moreover, the flame propagation periods (CA10-90s) of Case1 to Case5 are 32.98°, 21.50°, 27.12°, 30.45° and 23.56° (ATDC), respectively. For Case2, the mixture has burned completely at this moment, which indicates that the fuel burn too fast and the combustion reaction process in-cylinder is so intense. It may increase the possibility of reducing the engine mechanical performance and service life. According to the principle of the engine reasonable combustion organization, the optimal scheme is Case5 and its combustion process is reasonable. In summary, when adopting double assisted spark plugs, the combustion performance of the diesel Wankel engine is obviously better than that of the single assisted spark plug. Among all double assisted spark plug schemes, Case5 is the preferred AIP scheme. When comparing to the original scheme, the Pmax of Case5 increases by 16.68%, and the CA50 and CA10-90 decrease significantly. It illustrates that the combustion efficiency of the studied diesel Wankel engine is effectively improved. In order to present the combustion development and chemical reactions processes in the rotary chamber clearly, the mixture distribution at AIT, temperature field and combustion intermediate HO2 concentration at TDC under different AIP schemes are given in Table 5. First, by observing the fuel distribution rule of Case1 to Case5 at AIT, the combustible mixture is concentrated at the mid-anterior region of the cylinder and distributed around the assisted spark plug, which provides a favorable burning environment for stable ignition combustion in the early stage. However, if the number of assisted spark plug and the AIP are changed, the presentive combustion processes are completely different. Second, according to the temperature field in Table 5, the area of the combustion zone at TDC of double assisted spark plug schemes is obviously wider than that of the single assisted spark plug scheme. Moreover, the combustion area of Case2 and Case5 is obviously larger than that of Case3 and Case4. It is because more fuel distributes near the assisted spark plug at the beginning of combustion, especially Case2. As for Case2, the flame front has spread to the front end of the rotary chamber. As a result, the flame propagation speed of Case2 is fastest. Nonetheless, its combustion reaction is also too intense. Finally, according to the combustion intermediate HO2 in Table 5, the flame shows an obvious forward propagation rule. Besides that, for double assisted spark plug schemes, the combustion reaction zone has a close relationship with the AIP, that is the flame of Case2 spreads to the region of the front end of the rotary chamber, Case3 is to the front cylinder wall and Case4 is to the rear cylinder wall. It indicates that during the flame development process, the flame presents the rule of propagating to the regions which exist an assisted ignition source.
3.3. Effect of assisted ignition position on engine combustion characteristics The peak combustion pressure (Pmax) and mass fraction burned (MFB) under different AIP schemes as seen in Fig. 12. Firstly, analyze the effect of the number of assisted spark plug on engine combustion characteristics. In Fig. 12(a), the Pmax of Case1 to Case5 is 45.26, 60.99, 47.94, 47.61 and 52.81 (bar), respectively. Moreover, the Pmax of the double assisted spark plug schemes (Case2 to Case5) is higher obviously when compares to the single assisted spark plug scheme (Case1). It indicates that using double assisted spark plugs can improve the initial ignition stage stability for the stratified-charge mixture of the diesel Wankel engine, and then it is beneficial to the following combustion process. Since there is an appropriate mixture concentration around the ignition system, adopting double assisted spark plugs can ensure that more mixture in-cylinder is ignited in time. Especially, under the combustion environment of high speed and forward one-way flow, the ignition stability of using double assisted spark plugs in the narrow rotary chamber is more prominent. Secondly, compare the effect of four different AIPs on engine combustion performance. That is Case2, Case3, Case4 and Case5. Among these double assisted spark plug schemes, the Pmax of Case2 and Case5 are higher, which shows that they are the better power performance schemes. By comparing with the single spark assisted plug scheme (Case1), their Pmax is increased by 34.75% and 16.68%, respectively. According to Fig. 12(b), the central heat release angles (CA50s) of Case1 to Case5 are 15.60°CA ATDC, 5.15°CA BTDC, 11.56°CA ATDC, 13.86° ATDC and 9.26° ATDC, respectively. The fuel burning rate of 9
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Table 6 Mixture distribution at AIT, temperature field and combustion intermediate HO2 concentration at TDC under different AIT schemes.
Case10 are 13.58°CA BTDC, 7.73°CA BTDC, 2.37°CA BTDC, 3.58°CA ATDC and 9.26°CA ATDC, respectively. Meanwhile, the CA10-90s of Case6 to Case10 are 0.62°CA BTDC, 5.61°CA ATDC, 11.17°CA ATDC, 17.59°CA ATDC and 23.56°CA ATDC, respectively. It shows that the curves of MFB integrally move to the left. Besides, CA50 and CA10-90 are close to the TDC, and also fuel burning rate is accelerated. Therefore, MFB increases gradually when the AIT is advanced. Since under an advanced AIT, the combustion process starts earlier and the total combustion time before TDC is longer. However, if the AIT is too early during the compression stage, that may be detrimental to the combustion performance and engine working. Therefore, the Pmax does not present the same increasing trend. In conclusion, advancing AIT, the MFB increases in turn, and Case7 is the optimal AIT scheme. However, due to the overall mixture distribution at AIT changes little, the Pmax of all schemes changes little.
Meanwhile, changing AIP can control the burning zone in the rotary chamber when making fuel use of the characteristics of mixture stratified distribution and flame forward propagation in the diesel Wankel engine. 3.4. Effect of assisted ignition timing on engine combustion characteristics The Pmax and MFB under different AIT schemes are presented in Fig. 13. Accroding to Fig. 13(a), the Pmax of Case6 to Case10 is 52.95, 53.33, 52.73, 51.93 and 52.81 (bar), respectively. With the advance of AIT, the change extent of Pmax is smaller in each scheme, which illustrates that AIT has less influence on the Pmax. In addition, the Pmax of Case7 is the largest, which indicates that when the AIT is 35°CA BTDC, the engine can achieve higher combustion pressure and obtain better power performance. As shown in Fig. 13(b), the CA50s of Case6 to 10
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The mixture distribution at AIT, temperature field and combustion intermediate HO2 concentration at TDC under different AIP schemes are presented in Table 6. As the mixture distribution in Table 6 shown, diesel mainly concentrates in the mid-anterior region of the rotary chamber at different AITs. With the rotor rotating, the distribution range of mixture increases. However, the fuel concentration around the assisted spark plug does not change so much. By observing the temperature field and combustion intermediate HO2 concentration in Table 6, after the assisted spark plug ignites the combustible mixture, the combustion reaction occurs in the region which has the stratified distributed combustible mixture, that is the mid-anterior region part of the rotary chamber. Owing to the forward flame propagation rule of the Wankel engine and the stratified-charge mixture in the front part of the rotary chamber, HO2 concentrated in the same region. Also, by advancing AIT, the total area of the burning zone, contour range of HO2 and airflow velocity change slightly. It further explains the Pmax presents a small fluctuation at different AITs, which is because the flame propagation speed, flow field distribution, combustion temperature and burning intensity in-cylinder change little.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to acknowledge Project (51606162 and 51975503) supported by the National Natural Science Foundation of China, Project (2017JJ4052) supported by the Natural Science Foundation of Hunan Province and Project (19QDZ42) supported by the Ph.D. Scientific Research Startup Foundation of Xiangtan University. Author Contribution. Wei Chen: Conceptualization, Methodology, Software, Data curation, Writing-Original Draft, Writing-Review & Editing. Qingsong Zuo: Supervision, Writing-Review & Editing, Funding acquisition. Jianfeng Pan: Resources, Project administration, Funding acquisition. Jianping Zhang: Supervision, Funding acquisition. Zhiqi Wang: Writing-Reviewing & Editing. Bin Zhang: Writing-Review & Editing. Guohui Zhu: Writing-Review & Editing. Baowei Fan: Formal analysis.
4. Conclusions The core objective of the present study was to improve the combustion performance in a diesel fueled Wankel stratified-charge combustion engine by using a CFD method. Thus, the influence of assisted ignition strategies, such as the number of assisted spark plug, AIP and AIT on the combustion characteristics were systematically explored and optimized. The important conclusions were:
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1. For the diesel Wankel engine, the fuel concentrated at the midanterior region of the rotary chamber, and it presented a state of stratified distribution. Stratified-charge combustible mixture and forward-propagate flame made reaction process mainly occur in this zone during the combustion process. 2. When using double assisted spark plugs, the Pmax and MFB of the diesel Wankel engine were improved obviously. Combustion performance of double assisted spark plug schemes (Case2 to Case5) was better than that of the single assisted spark plug scheme (Case1). 3. Changing AIP, the Pmax and the combustion zone of Case1 to Case5 were quite different, since the distribution and concentration of the mixture around the assisted spark plug is crucial to the following combustion process. The double assisted spark plugs symmetrically distributed relative to the major axis (X-axis) of the rotary chamber was better. For concerned AIPs, the optimal scheme was Case5 (i.e. 0, −88.5, ± 15 mm). 4. Advancing AIT, since little difference in mixture concentration and distribution area, the Pmax, the reaction area and combustion intensity of Case6 to Case10 changed little. Meanwhile, the MFB was increased due to a longer combustion time before TDC. For explored AITs, the optimum scheme was Case7 (i.e. 35°CA BTDC). 5. Comparing with the original scheme (Case1), the Pmax of the optimum AIP and AIT schemes increased by 16.68% and 17.83%, respectively. Also, their CA50 and CA10-90 were shortened obviously.
CRediT authorship contribution statement Wei Chen: Conceptualization, Methodology, Software, Data curation, Writing - original draft, Writing - review & editing. Qingsong Zuo: Supervision, Writing - review & editing, Funding acquisition. Jianfeng Pan: Resources, Project administration, Funding acquisition. Jianping Zhang: Supervision, Funding acquisition. Zhiqi Wang: Writing - review & editing. Bin Zhang: Writing - review & editing. Guohui Zhu: Writing - review & editing. Baowei Fan: Formal analysis. 11
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