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Parametric investigation of low pressure dual-fuel direct injection on the combustion performance and emissions characteristics in a RCCI engine fueled with diesel and CH4
Fuel 260 (2020) 116408
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Full Length Article
Parametric investigation of low pressure dual-fuel direct injection on the combustion performance and emissions characteristics in a RCCI engine fueled with diesel and CH4
T
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Bo Yang , Qimeng Duan, Bing Liu, Ke Zeng Xi’an Jiaotong Univ, Sch Energy & Power Engn, Xi’an 710049, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Low pressure dual-fuel direct injection Air-assisted direct injection RCCI Dual-fuel Diesel/CH4
Dual-fuel direct injection is one of the most promising ways to control mixing process in a dual-fuel engine. A low pressure dual-fuel direct injection (LPDDI) concept inspired by air-assisted direct injection (AADI) technology is presented in this paper and an experiment has been conducted to evaluate the potential of the LPDDI applied in a diesel/CH4 dual-fuel engine. The effects of diesel injection timings and CH4 injection timings on the combustion performance and emissions characteristics were investigated under a constant engine speed of 2000 rpm in a single cylinder research engine. Under the varying operation condition of diesel and CH4 injection timings, the cylinder pressure, Heat Release Rate (HRR), Peak Pressure Rise Rate (PPRR), CA10, CA50, combustion duration, coefficient of variation of indicated mean effective pressure (CoVimep), as well as regular emissions (THC, CO and NOx) are analyzed. The results indicated that the diesel and CH4 injection timings have significantly influence on the combustion process. An optimized combustion performance obtained at the earliest diesel injection timing of −250 °CA ATDC and the latest CH4 injection timing of −112 °CA ATDC in this work. Furthermore, the results also proved that the LPDDI can be successfully applied in diesel/CH4 dual-fuel engine and a RCCI combustion mode can be achieved.
1. Introduction Recent years, in order to meet the challenge of the more stringent emission regulations, the concept of low temperature combustion (LTC) was investigated by many researchers [1–3]. Because of the LTC can simultaneously reduce the NOx and PM emissions in internal combustion engines, the LTC theory has attracted intensive attention all over the world [4,5]. The Dual-fuel combustion is one of the most promising strategies to achieve the concept of the LTC [6]. In addition, the Reactivity Controlled Compression Ignition (RCCI) is a more recent mode of the dual-fuel combustion and provides a new way to control the combustion process [7,8]. In a RCCI combustion engine, low reactivity fuels are usually induced from intake port and the high reactivity fuels are directly injected into cylinder near the top dead center (TDC). The gasoline and natural gas are the common low reactivity fuels while the diesel and diesel-like fuels, such as bio-diesel are the most popular candidates of the high reactivity fuels [9,10]. Poorghasemi et al. [11] experimentally investigated the effect of diesel injection strategies on combustion characteristics in a natural gas/diesel RCCI combustion engine and they reported that the diesel injection pressure, spray angle
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and diesel fuel fraction as well as the premixed ratio of natural gas play a critical role in the RCCI combustion process. Liu et al. [12] optimized diesel injection parameters and combustion chamber geometries in a diesel/natural gas RCCI engine employed simulation method and indicated that the spatial distribution of diesel spray in-cylinder has significant influence on the combustion and emissions characteristics. Those researches mentioned above confirmed that the control parameters of mixing process have obviously effects in the combustion characteristics by formation different kinds of diesel/natural gas mixture (premixed or stratification) in-cylinder. However, natural gas induced from intake port is usually premixed especially when a pre-mixer is used in the dual-fuel engine, which has little influence on the mixing process. So, the direct injected diesel dominated the dual-fuel mixing process and governed the combustion process. This is why so many scholars pay more attention to the diesel injection strategies to improve RCCI combustion and emissions characteristics [13]. Although a lot of complexity injection strategies have been proposed, there is still lack an efficient method to control natural gas mixing process as we desired. Moreover, the combustion phasing and flame propagation characteristics of RCCI combustion depend on the
Corresponding author. E-mail address: [email protected] (B. Yang).
https://doi.org/10.1016/j.fuel.2019.116408 Received 20 July 2019; Received in revised form 6 October 2019; Accepted 9 October 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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reactivity gradient and its spatial distribution as well as mass ratio of the dual-fuel charge in cylinder [14–16]. In a manner of speaking, the method influenced the reactivity gradient and its spatial distribution in cylinder are the most important for a RCCI combustion. Therefore, recently researchers try to utility dual-fuel direct injection technology to enhance the control of the mixing process and explore the effects of stratification of dual-fuel mixture on the combustion and emissions characteristics in a RCCI combustion engine. Mikulski Maciej et al. [17] conducted a simulation study to understand the role of low reactivity fuel (natural gas) stratification in a dual-fuel RCCI engine. They assessed the effects of natural gas direct injection on the thermal efficiency and methane emissions. They concluded that the stratification provided by natural gas direct injection paly a positive influence on the RCCI combustion and emissions. Martin Wissink and Reitz [18] experimentally investigated the stratification formed by dual-fuel direct injection in a RCCI combustion engine fueled with gasoline/diesel and they indicated that the dual-fuel distribution over the cylinder could be controlled by dual-fuel direct injection strategy. Moreover, the RCCI combustion phasing and combustion noise could be controlled by adjusting the degree of stratification formed by dual-fuel direct injection [19]. The commonly dual-fuel direct injection technology (except Westport HPDI system) is direct injected low reactivity fuels and high reactivity fuels into combustion chamber used two independent injectors with high injection pressure, separately. Most previous researches used the method to achieve dual-fuel direct injection and obtained an expected good result. However, this method required a relative high injection pressure and added too much complexity in the fuel supply system [20,21]. Moreover, two independent injectors required more complex control strategy and more space in cylinder head for installation. In order to overcome those disadvantages of the high pressure dual-fuel direct injection technology, we introduced a new LPDDI method achieved by the AADI technology invented by Orbital Co. Ltd [22]. The AADI firstly applied in a two-stroke engine developed for a unmanned aerial vehicle (UAV) for military purpose by Orbital Co. Ltd [23]. The compressed air is used to blast the liquid fuel droplets so the finer atomization obtained under a relative low injection pressure [24,25]. After that, a lot of studies had been conducted to explore the application of AADI technology in different kinds of spark ignited engines all over the world [6–8] and those studies confirmed the AADI technology is a good low-pressure direct injection system. In this work, natural gas replaced the air in the AADI injector. Natural gas and diesel were simultaneously injected by an AADI injector with a low injection pressure and natural gas was used to enhance the atomization of diesel. So, a good spray characteristic of dual-fuel mixture could be obtained under a relative low injection pressure. Furthermore, by adjusting the injection timings (natural gas and diesel), the dwell time between diesel injection and CH4 injection events can be flexibly controlled. Therefore, the two injection events can be separated or be overlapped in certain time. Consequently, the dual-fuel mixture process will be controlled and different degrees of stratification charge can be obtained in cylinder. In this study, an experimental investigation was conducted to evaluate the potential of the low pressure direct injection system applied in a diesel/CH4 dual-fuel engine. The effects of diesel and CH4 injection timings on the combustion and emissions characteristics were systematically investigated. Moreover, the in-cylinder pressure, CA10, CA50, combustion duration, HRR, CoVimep, PPRR as well as regular emissions have been analyzed. Furthermore, this study proposed a costeffective solution for LPDDI and also achieved more flexible control of the reactivity gradient and its spatial distribution in cylinder by combining CH4 and diesel injection process.
Table 1 Specifications of the test engine. Item
Characteristics
Type
Single cylinder, naturally aspired and air-cooling, dual-fuel engine ω type 92 mm × 75 mm 19:1 0.499 L 0.9 MPa 0.7 MPa AADI injector 8.2 kW/3600 rpm Opening Closing 16°BTDC 44°ABDC 48°BBDC 12°ATDC
Combustion chamber Bore × stroke Compression ratio Displacement volume Diesel injection pressure CH4 injection pressure Dual-fuel direct injection Rated power Valve timing Intake Exhaust
2. Experiment setup and procedure 2.1. Test engine and fuel supply system A single cylinder, naturally aspirated and air-cooling common rail diesel engine was modified to run in diesel/CH4 dual-fuel direct injection mode. The basic specifications of the test engine are listed in the Table 1. The common rail injection system in the original diesel engine was removed and a low pressure direct injection system was equipped in the test engine in order to achieve dual-fuel direct injection. The key part of the system is an AADI injector and the sauter mean diameter (SMD) of fuel spray produced by AADI injector can even reach approximately 4 μm–5μm [22,26,27]. So, improved spray characteristics in the LPDDI system can be expected. A cross-section of the injector (Fig. 1) was provided to illustrate the working principle. As shown in Fig. 1, the AADI actually is a built-up nozzle and it includes a liquid injector (for diesel injection) and a gas injector (for mixture injection). In the experiment, the compressed air was replaced by CH4. During a work cycle, the diesel was injected into the premixed cavity in which full filled by compressed CH4. And then, the diesel fuel breaks down into tiny droplets and distributes into the compressed CH4 in premixed cavity. Finally, the diesel/CH4 mixture was direct injected into cylinder when air-assisted injector opens. The pressure of compressed CH4 should be sufficient to produce a sonic blow down the mixture into the cylinder and an improved spray characteristic in a low injection pressure operation condition can be obtained. Moreover, the diesel injection
Fig. 1. Cross-section of AADI diesel/CH4 dual-fuel injector model. 2
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Fig. 2. The schematic of experimental setup and instruments layout.
and the CO, THC and NOx emissions were measured. A kistler 6058A piezoelectric pressure transducer, as shown in Fig. 2, was installed in the cylinder head and was used to obtain the cylinder pressure. A combustion analyzer (Kistler KiBox) was employed to process the cylinder pressure signal. A high precise electric balance and a gas mass flow meter were used to measure the consumption of diesel and CH4, separately. The more details information about those instruments are listed in Table 3.
and diesel/CH4 injection events can be controlled separately in a certain range by an ECU developed for this work. The schematic of experimental setup and instruments layout was shown in Fig. 2. The pressure of CH4 was reduced from 12 MPa to 0.7 MPa through a two-stage pressure regulator and the low pressure CH4 was connected with the AADI injector at the CH4 supply port as shown in Fig. 1. Additionally, a shut off valve was also applied in the CH4 supply system just for emergency situation protection. The diesel was compressed by a low pressure pump and increased the pressure to 0.9 MPa. And then, it flowed through a surge tank in order to restrain the variation of the pressure during experiment. After that, diesel was connected with the AADI injector at the diesel supply port as shown in Fig. 1. The properties of diesel and CH4 used in this experiment are listed in Table 2.
2.3. Experiment procedure and operating condition sets All experiment study was conducted at a constant speed of 2000 r/ min operating conditions. The intake air temperature was fixed at 20 ± 1 °C and the highest cylinder head temperature was limited within 120 ± 5 °C. To evaluate the effects of LPDDI technology on the combustion and emissions characteristics, the diesel injection timings firstly studied under a fixed CH4 injection timing. And then, under an optimized diesel injection timing operating condition, the CH4 injection timings were varied. The diesel and CH4 injection duration never changed in the experiment. The more information about operating condition sets can be found in Table 4.
2.2. Data acquisition and measurement system The test engine was controlled by an engine control system (Powerlink CAC55). The load and speed of the test engine was controlled by a 55 kW electrical dynamometer. The data of sensors, such as cylinder head temperature, lubricating oil pressure and temperature, exhaust gas oxygen as well as exhaust temperature, was recorded by a data acquisition system based on LabVIEW platform. The regular exhaust emissions were sampled directly from the exhaust pipe. And then, the samples were sucked into an automotive exhaust emissions analyzer
Table 3 Information of the instrumentations used in this study.
Table 2 The properties of test fuels. Fuel properties
Low heating value, MJ/kg Density, kg/m3, (T = 25 °C) Viscosity, mm2/s, (T = 20 °C) Cetane number Octane number Auto-ignition temperature, °C Stoichiometric air–fuel ratio, kg/kg Carbon content, %
Fuel type and value for following Diesel
CH4
42.8 834.8 3.393 52.5 – 316 14.69 87
51.5 0.788 – – 125 595 17.24 75
Variable measured
Device
Manufacturer and Model
Accuracy
Torque Engine speed Temperature Diesel fuel mass flow
55 kW DC dynamometer Thermocouple Electronic gravimetric balance CH4 flow meter KiBox KiBox Exhaust emission analyzer
Powerlink/CAC55
± 0.2% F.S. ± 1 rpm ± 2.5 °C ± 0.1 g
CH4 mass flow Cylinder pressure Crank angle Regular emissions
3
TC direct/K type Beijing Heng Odd Instrument Ltd/HAES30K-1 Seven Star/CS 230A Kistler/6058A Kistler/2619A11 Horiba MEXA-584L
± 0.35% F.S. ± 0.5% 0.1°CA HC/ ± 12 ppm CO/ ± 0.06% NOx/30 ppm
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Table 4 Operating condition sets. NO.
1 2 3 4 5 6 7 8
IMEP/bara
Load/%
−150
6.8
91
−148 −136 −124 −112
5
67
Injection Duration/ms
Flow rate/(kg/h)
Injection Timings/°CA ATDC
Diesel
CH4
Diesel
CH4
Diesel
CH4
1.4
6.5
0.408
0.482
1.65
2.65
0.448
0.152
−250 −160 −120 −100 −250
a IMEP calculated under the diesel injection timing of −250 °CA ATDC and CH4 injection timing of −150 °CA ATDC and −148 °CA ATDC, separately.
Fig. 4. CA10, CA50 and combustion duration under different diesel injection timings.
3. Results and discussions
injection timings and the amplitude of the HRR increased significantly. Moreover, the position of peak value in the HRR curves move forward to the TDC with advanced diesel injection timings. This result also is an evidence of better dual-fuel spray formed by advanced diesel injection timings and in accordance with the above statement. However, there are some unique characteristics of the combustion process in the LPDDI engine compared with conventional dual-fuel engine. First, the HRR curves exhibit a unimodal characteristic in the LPDDI engine and very different from the conventional dual-fuel engine which with a typical three stage HRR curves. Second, the combustion process in the LPDDI engine is obvious faster than that in a conventional dual-fuel engine. Those difference indicated that the combustion process in the LPDDI engine mainly controlled by the chemical kinetics [29–31]. More homogenous mixture and wider space distribution of ignition kernel formed all over the cylinder. The CH4 and diesel were totally mixed before combustion started and very different with pilot-ignited dualfuel combustion process. The CA10, CA50 and combustion duration versus varying diesel injection timings are given in Fig. 4. With advanced diesel injection timings, the CA10 slightly move to the TDC while CA50 drastically shifts to the TDC from about 15 °CA ATDC to about 8 °CA ATDC. The combustion duration just reduced from the 21 °CA to 18°CA when diesel injection advanced from −100 °CA ATDC to −120 °CA ATDC. And then, advanced diesel injection timing, the combustion duration almost keeps constant. It is obviously that the CA10-50 is accelerated by advancing diesel injection timing due to a better dual-fuel spray and a more suitable distribution of ignition kernel [32]. Moreover, it also is the evidence that the combustion phase shifts to the TDC and this is consistent with the analyzed above. Combustion stability and combustion noise are important parameters for evaluation the combustion quality in engines. In this paper, the CoVIMEP and PPRR are used to represent the combustion stability and combustion noise, respectively. Fig. 5 shows the CoVIMEP and PPRR under different diesel injection timings. We observe that CoVIMEP is reduced from around 5.5% to 2.5% with advanced diesel injection timings. The PPRR exhibits a converse tendency and increased from around 2.3 bar/°CA to over 10 bar/°CA, which equal to the knock limitation [33]. The results illustrated that with advanced diesel injection timings, the combustion stability significantly improved while the combustion noise deteriorated. It is reasonable that a better dual-fuel spray is obviously good for combustion stability and a rapid combustion process definitely will lead to increase PPRR. The Fig. 6 provides the brake thermal efficiency (BTE) under different diesel injection timings at part load (IMEP = 5 bar) operation conditions. With advanced diesel injection timings from −100 °CA ATDC to −250 °CA ATDC, the BTE increased from about 31.8% to over
In this section, the combustion and emissions characteristics results of diesel/CH4 engine with the LPDDI were discussed. The effects of diesel and CH4 injection timings on combustion performance and emissions characteristics were analyzed. For better understanding, the injection timing mentioned in this paper is defined as the crank angle when the solenoid of injector was energized. The period of crank angle from 10% to 90% mass fraction burned is referred as the combustion duration. Moreover, the CH4 injection timings actually represented the timings at which dual-fuel mixture is injected into cylinder. 3.1. The effects of diesel injection timings on the combustion and emissions The curves of cylinder pressure and HRR under different diesel injection timings are shown in Fig. 3. With advanced diesel injection timings, an increased tendency in the cylinder pressure curves is observed and the peak pressure increased from 52 bar under −100 °CA ATDC to the 75 bar under −250 °CA ATDC. Moreover, the crank angle, which corresponding to the peak cylinder pressure, move to the top dead center (TDC). The interval time between diesel injection and CH4 injection event increased with advanced diesel injection timing and different distributions of the diesel droplets inside compressed CH4 in premixed cavity are formed. Under the most advanced diesel injection timing (-250 °CA ATDC), diesel spray has more time to mix with compressed CH4 and a more homogenous distribution of the diesel droplets inside CH4 environment is obtained in premixed cavity. As a result, the broken process of diesel droplets is enhanced due to stronger CH4 entrainment and the quality of dual-fuel spray is improved [26–28]. The HRR curves also show an increased tendency with advanced diesel
Fig. 3. Cylinder pressure and HRR under different diesel injection timings. 4
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Fig. 5. CoVIMEP and PPRR under different diesel injection timings.
Fig. 7. HC, CO and NOx emissions under different diesel injection timings.
3.2. The effects of CH4 injection timings on combustion and emissions In an AADI injector, the air injection timing corresponding to the fuel injection timing in a conventional injector. So, the CH4 injection timings in this paper actually represented the timings at which the diesel/CH4 dual-fuel mixture was injected into cylinder not just CH4. Fig. 8 shows the cylinder pressure and HRR curves under variation CH4 injection timings. Observing those curves, we can find that the peak cylinder pressure increased from 57 bar under −148 °CA ATDC to 65 bar under −112 °CA ATDC with retarded CH4 injection timings. Moreover, the position of the peak cylinder pressure in curves slightly moves to the TDC and shape of the pressure curves changes from a typical unimodal to a bimodal one. The HRR have the similar tendency with the cylinder pressure. With retarded CH4 injection timings, the peak HRR increased significantly and the curves shift to the TDC. Furthermore, the shape of the HRR curves also changed from a unimodal curve to a bimodal one as the same with cylinder pressure curves. As we all know, in a typical diesel/natural gas dual-fuel engine (port injection natural gas and direct injection diesel), the combustion phase is dominated by the pilot diesel injection timings. But in the LPDDI engine, as shown in Fig. 8, the combustion phase is controlled by the reactivity of the diesel/CH4 mixture in the cylinder rather than the CH4 injection timings. Therefore, the spatial distributions of the dual-fuel mixture in cylinder play an important role in the combustion process. With retarded CH4 injection timings, the injection backpressure increased and the influence of gas on the diesel droplets broken process is reduced. So, the quality of the dual-fuel spray deteriorated and SMD of the diesel increased [26]. However, the increased SMD is maybe not a negative influence in a LPDDI dual-fuel engine because the large droplets will easily form ignition kernels, particularly under the lean dual-
Fig. 6. BTE under different diesel injection timings under high load.
36.1%. This is because of advancing diesel injection timings, the spray quality is improved as we discussed above and the combustion process is more complete. However, the BTE of the LPDDI engine in the high load (IMEP = 6.8 bar) is still lower compared with the baseline diesel engine in the same operation conditions. The results are also consistence with the HRR curves exhibited above. Moreover, the BTE of LPDDI engine still need improved in the high load. The HC, CO and NOx are the mainly regular emissions in diesel/CH4 dual-fuel engine. In dual-fuel engine, the HC emissions mainly due to incomplete combustion and slower burning rate of CH4. Furthermore, it is believed that 90% of the HC emissions in a diesel/natural gas dualfuel engine is unburned CH4 [34]. CO and NOx emissions are commonly related with temperature-time history. The fuel’s oxidation and decomposition are dominated by in-cylinder temperature, which significantly influences the formation of CO emissions and the maximum temperature in the combustion process directly controls the NOx emissions [35,36]. Fig. 6 shows the HC, CO and NOx emissions results under different diesel injection timings. Observing the Fig. 7, we can find that with advancing diesel injection timings, HC and CO notably decreased while the NOx significantly increased. As we mentioned above that a better-quality dual-fuel spray and a faster combustion process achieved by advanced diesel injection timings. As a result, the better dual-fuel spray will lead a more complete combustion and reduced the unburned CH4. The faster combustion process indicated that most heat of fuel released in a shorter time. So, the heat dissipation is restrained and the local temperature of in-cylinder decreased while the peak temperature of combustion increased. Therefore, the CO reduced due to a lower local in-cylinder temperature and NOx increased under a higher peak combustion temperature.
Fig. 8. Cylinder pressure and HRR under different CH4 injection timings. 5
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Fig. 11. BTE under different diesel injection timings under part load. Fig. 9. CA10, CA50 and combustion duration under different CH4 injection timings.
is obtained. The BTE curves under different CH4 injection timings at part load (IMEP = 5 bar) exhibits in Fig. 11. As we can observed, with advanced the CH4 injection timings, the BTE obviously decreased. The maximum value of the BTE is about 38% which is higher than that in the diesel engines at same operation conditions. This result indicated that the CH4 injection timings paly a very significantly influence on the combustion process of a LPDDI engine. Moreover, with retarded CH4 injection timings, the combustion phase shifts to the TDC and the BTE increased. And the tendency of the CA10, CA50 and combustion duration curves are also consistence with the conclusion. The regular emissions of HC, CO and NOx under different CH4 injection timings are provided in Fig. 12. The HC and CO curves exhibit a decreased tendency with retarded CH4 injection timings while NOx emissions show a converse tendency. The faster initial combustion stage decreased HC emissions and reduced heat dissipation during combustion. So, a lower in-cylinder temperature is obtained and CO emissions are decreased with retarded CH4 injection timings. As the similar analysis, the higher peak combustion temperature is achieved under a faster initial combustion stage and this is the reason that NOx emissions are increased with retarded CH4 injection timings.
fuel mixture operation conditions. The CA10, CA50 and combustion duration curves under variation CH4 injection timings are shown in Fig. 9 and more evidences can be found in CA10 curve, which usually used to evaluate the ignition characteristics. With retarding CH4 injection timings, the CA10 move to the TDC and the earliest CA10 obtained under CH4 injection timing of −112 °CA ATDC. This result is in accordance with the discussions above and increased SMD provides more suitable environment for ignition kernels formation. Consequently, the later injection timings, the shorter time left before ignition. Of course, over retarded the CH4 injection timings will face high backpressure and even failure to direct injection. The CA50 have similar tendency with the CA10 under the variation injection timings and it is proved that the initial stage of the combustion process is enhanced due to a better flame propagation environment. Furthermore, the combustion duration is slightly prolonged with retarded CH4 injection timings and probably more CH4 completely burned with retarded CH4 injection timings. As the combustion stability and noise are concerned, the CoVIMEP and PPRR versus variation CH4 injection timings is given in Fig. 10. Observing the curves, we can notice that the CoVIMEP decreased significantly with retarded CH4 injection timings while the PPRR curve exhibits a totally converse tendency. This result indicated that the combustion stability is improved by retarded CH4 injection timings and combustion noise increased while the maximum value is still within an acceptance range. Moreover, a suitable ignition kernels and flame propagation mixture distributions was produced by retarding CH4 injection timings. Therefore, a more stable and faster combustion process
4. Conclusions In this study, we evaluate the possibility to apply the LPDDI technology in a diesel/CH4 duel-fuel engine. The effects of diesel injection timings and CH4 injection timings on the combustion performance and emissions characteristics have been experimentally studied in this paper. The in-cylinder pressure, HRR, CoVIMEP, PPRR, CA10, CA50 and combustion duration as well as regular emissions are calculated. On the
Fig. 10. CoVIMEP and PPRR under different CH4 injection timings.
Fig. 12. HC, CO and NOx emissions under different CH4 injection timings. 6
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basis of the experiment results and discussions presented above, the main conclusions are summarized as following:
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(1) Both the diesel and CH4 injection timings have significantly influence on the combustion process of LPDDI engine. Moreover, with advanced the diesel injection timings, the positive effect of air (CH4) on the liquid (diesel) droplets breaking process will be enhanced and a more homogenous diesel/CH4 mixture will be expected. The environment more suitable for ignition kernels formation is produced by retarding the CH4 injection timings in cylinder. In this work, an optimized combustion performance can be obtained under an earlier diesel injection timing (−250 °CA ATDC) and a later CH4 injection timing (−112 °CA ATDC). (2) The reactivity gradient and its spatial distribution of dual-fuel mixture dominate the combustion performance and emissions characteristics. By adjusting the relative chronology of the diesel and CH4 injection events, the difference degrees (homogenous or stratification) of dual-fuel mixture will be obtained as we needed. So the LPDDI give us an effectively method to control the reactivity gradient and its spatial distribution in a dual-fuel engine. (3) The LPDDI technology provides a cost-effective solution to achieve direct dual-fuel (liquid/gas) injection and extends the flexibility to control the mixing process of dual-fuel charge in cylinder. Additionally, it is a very promising technology to achieve RCCI combustion mode and enable a potential way to optimize the RCCI combustion process. 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. Acknowledgements This study is financially supported by the National Natural Science Foundation of China (Grant No. 51706171) and the China Postdoctoral Science Foundation (Grant No. 2016M602815). References [1] Imtenan S, Varman M, Masjuki HH, et al. Impact of low temperature combustion attaining strategies on diesel engine emissions for diesel and biodiesels: a review. Energy Convers Manage 2014;80(1):329–56. [2] Kumar Agarwal Avinash, Pratap Singh Akhilendra, Kumar Maurya Rakesh. Evolution, challenges and path forward for low temperature combustion engines. Prog Energy Combust Sci 2017;61(7):1–56. [3] Saxena S, Bedoya ID. Fundamental phenomena affecting low temperature combustion and HCCI engines, high load limits and strategies for extending these limits. Prog Energy Combust Sci 2013;39(5):457–88. [4] Yousefi A, Guo HS, Birouk M. Effect of diesel injection timing on the combustion of natural gas/diesel dual-fuel engine at low-high load and low-high speed conditions. Fuel 2018;235(8):838–46. [5] Hyunwook Park, Euijoon Shim, Choongsik Bae. Improvement of combustion and emissions with exhaust gas recirculation in a natural gas-diesel dual-fuel premixed charge compression ignition engine at low load operations. Fuel 2018;235(8):763–74. [6] Rosha P, Dhir A, Mohapatra SK. Influence of gaseous fuel induction on the various engine characteristics of a dual fuel compression ignition engine: a review. Renewable Sustainable Energy Rev 2018;82(2):3333–49. [7] Pourya Rahnama, Amin Paykani, Vahid Bordbar, et al. A numerical study of the effects of reformer gas composition on the combustion and emission characteristics of a natural gas/diesel RCCI engine enriched with reformer gas. Fuel 2017;209(7):742–53. [8] Benajes J, Garcia A, Monsalve-Serrano, et al. Achieving clean and efficient engine operation up to full load by combining optimized RCCI and dual-fuel diesel-gasoline
7
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Full Length Article
Parametric investigation of low pressure dual-fuel direct injection on the combustion performance and emissions characteristics in a RCCI engine fueled with diesel and CH4
T
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Bo Yang , Qimeng Duan, Bing Liu, Ke Zeng Xi’an Jiaotong Univ, Sch Energy & Power Engn, Xi’an 710049, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Low pressure dual-fuel direct injection Air-assisted direct injection RCCI Dual-fuel Diesel/CH4
Dual-fuel direct injection is one of the most promising ways to control mixing process in a dual-fuel engine. A low pressure dual-fuel direct injection (LPDDI) concept inspired by air-assisted direct injection (AADI) technology is presented in this paper and an experiment has been conducted to evaluate the potential of the LPDDI applied in a diesel/CH4 dual-fuel engine. The effects of diesel injection timings and CH4 injection timings on the combustion performance and emissions characteristics were investigated under a constant engine speed of 2000 rpm in a single cylinder research engine. Under the varying operation condition of diesel and CH4 injection timings, the cylinder pressure, Heat Release Rate (HRR), Peak Pressure Rise Rate (PPRR), CA10, CA50, combustion duration, coefficient of variation of indicated mean effective pressure (CoVimep), as well as regular emissions (THC, CO and NOx) are analyzed. The results indicated that the diesel and CH4 injection timings have significantly influence on the combustion process. An optimized combustion performance obtained at the earliest diesel injection timing of −250 °CA ATDC and the latest CH4 injection timing of −112 °CA ATDC in this work. Furthermore, the results also proved that the LPDDI can be successfully applied in diesel/CH4 dual-fuel engine and a RCCI combustion mode can be achieved.
1. Introduction Recent years, in order to meet the challenge of the more stringent emission regulations, the concept of low temperature combustion (LTC) was investigated by many researchers [1–3]. Because of the LTC can simultaneously reduce the NOx and PM emissions in internal combustion engines, the LTC theory has attracted intensive attention all over the world [4,5]. The Dual-fuel combustion is one of the most promising strategies to achieve the concept of the LTC [6]. In addition, the Reactivity Controlled Compression Ignition (RCCI) is a more recent mode of the dual-fuel combustion and provides a new way to control the combustion process [7,8]. In a RCCI combustion engine, low reactivity fuels are usually induced from intake port and the high reactivity fuels are directly injected into cylinder near the top dead center (TDC). The gasoline and natural gas are the common low reactivity fuels while the diesel and diesel-like fuels, such as bio-diesel are the most popular candidates of the high reactivity fuels [9,10]. Poorghasemi et al. [11] experimentally investigated the effect of diesel injection strategies on combustion characteristics in a natural gas/diesel RCCI combustion engine and they reported that the diesel injection pressure, spray angle
⁎
and diesel fuel fraction as well as the premixed ratio of natural gas play a critical role in the RCCI combustion process. Liu et al. [12] optimized diesel injection parameters and combustion chamber geometries in a diesel/natural gas RCCI engine employed simulation method and indicated that the spatial distribution of diesel spray in-cylinder has significant influence on the combustion and emissions characteristics. Those researches mentioned above confirmed that the control parameters of mixing process have obviously effects in the combustion characteristics by formation different kinds of diesel/natural gas mixture (premixed or stratification) in-cylinder. However, natural gas induced from intake port is usually premixed especially when a pre-mixer is used in the dual-fuel engine, which has little influence on the mixing process. So, the direct injected diesel dominated the dual-fuel mixing process and governed the combustion process. This is why so many scholars pay more attention to the diesel injection strategies to improve RCCI combustion and emissions characteristics [13]. Although a lot of complexity injection strategies have been proposed, there is still lack an efficient method to control natural gas mixing process as we desired. Moreover, the combustion phasing and flame propagation characteristics of RCCI combustion depend on the
Corresponding author. E-mail address: [email protected] (B. Yang).
https://doi.org/10.1016/j.fuel.2019.116408 Received 20 July 2019; Received in revised form 6 October 2019; Accepted 9 October 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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reactivity gradient and its spatial distribution as well as mass ratio of the dual-fuel charge in cylinder [14–16]. In a manner of speaking, the method influenced the reactivity gradient and its spatial distribution in cylinder are the most important for a RCCI combustion. Therefore, recently researchers try to utility dual-fuel direct injection technology to enhance the control of the mixing process and explore the effects of stratification of dual-fuel mixture on the combustion and emissions characteristics in a RCCI combustion engine. Mikulski Maciej et al. [17] conducted a simulation study to understand the role of low reactivity fuel (natural gas) stratification in a dual-fuel RCCI engine. They assessed the effects of natural gas direct injection on the thermal efficiency and methane emissions. They concluded that the stratification provided by natural gas direct injection paly a positive influence on the RCCI combustion and emissions. Martin Wissink and Reitz [18] experimentally investigated the stratification formed by dual-fuel direct injection in a RCCI combustion engine fueled with gasoline/diesel and they indicated that the dual-fuel distribution over the cylinder could be controlled by dual-fuel direct injection strategy. Moreover, the RCCI combustion phasing and combustion noise could be controlled by adjusting the degree of stratification formed by dual-fuel direct injection [19]. The commonly dual-fuel direct injection technology (except Westport HPDI system) is direct injected low reactivity fuels and high reactivity fuels into combustion chamber used two independent injectors with high injection pressure, separately. Most previous researches used the method to achieve dual-fuel direct injection and obtained an expected good result. However, this method required a relative high injection pressure and added too much complexity in the fuel supply system [20,21]. Moreover, two independent injectors required more complex control strategy and more space in cylinder head for installation. In order to overcome those disadvantages of the high pressure dual-fuel direct injection technology, we introduced a new LPDDI method achieved by the AADI technology invented by Orbital Co. Ltd [22]. The AADI firstly applied in a two-stroke engine developed for a unmanned aerial vehicle (UAV) for military purpose by Orbital Co. Ltd [23]. The compressed air is used to blast the liquid fuel droplets so the finer atomization obtained under a relative low injection pressure [24,25]. After that, a lot of studies had been conducted to explore the application of AADI technology in different kinds of spark ignited engines all over the world [6–8] and those studies confirmed the AADI technology is a good low-pressure direct injection system. In this work, natural gas replaced the air in the AADI injector. Natural gas and diesel were simultaneously injected by an AADI injector with a low injection pressure and natural gas was used to enhance the atomization of diesel. So, a good spray characteristic of dual-fuel mixture could be obtained under a relative low injection pressure. Furthermore, by adjusting the injection timings (natural gas and diesel), the dwell time between diesel injection and CH4 injection events can be flexibly controlled. Therefore, the two injection events can be separated or be overlapped in certain time. Consequently, the dual-fuel mixture process will be controlled and different degrees of stratification charge can be obtained in cylinder. In this study, an experimental investigation was conducted to evaluate the potential of the low pressure direct injection system applied in a diesel/CH4 dual-fuel engine. The effects of diesel and CH4 injection timings on the combustion and emissions characteristics were systematically investigated. Moreover, the in-cylinder pressure, CA10, CA50, combustion duration, HRR, CoVimep, PPRR as well as regular emissions have been analyzed. Furthermore, this study proposed a costeffective solution for LPDDI and also achieved more flexible control of the reactivity gradient and its spatial distribution in cylinder by combining CH4 and diesel injection process.
Table 1 Specifications of the test engine. Item
Characteristics
Type
Single cylinder, naturally aspired and air-cooling, dual-fuel engine ω type 92 mm × 75 mm 19:1 0.499 L 0.9 MPa 0.7 MPa AADI injector 8.2 kW/3600 rpm Opening Closing 16°BTDC 44°ABDC 48°BBDC 12°ATDC
Combustion chamber Bore × stroke Compression ratio Displacement volume Diesel injection pressure CH4 injection pressure Dual-fuel direct injection Rated power Valve timing Intake Exhaust
2. Experiment setup and procedure 2.1. Test engine and fuel supply system A single cylinder, naturally aspirated and air-cooling common rail diesel engine was modified to run in diesel/CH4 dual-fuel direct injection mode. The basic specifications of the test engine are listed in the Table 1. The common rail injection system in the original diesel engine was removed and a low pressure direct injection system was equipped in the test engine in order to achieve dual-fuel direct injection. The key part of the system is an AADI injector and the sauter mean diameter (SMD) of fuel spray produced by AADI injector can even reach approximately 4 μm–5μm [22,26,27]. So, improved spray characteristics in the LPDDI system can be expected. A cross-section of the injector (Fig. 1) was provided to illustrate the working principle. As shown in Fig. 1, the AADI actually is a built-up nozzle and it includes a liquid injector (for diesel injection) and a gas injector (for mixture injection). In the experiment, the compressed air was replaced by CH4. During a work cycle, the diesel was injected into the premixed cavity in which full filled by compressed CH4. And then, the diesel fuel breaks down into tiny droplets and distributes into the compressed CH4 in premixed cavity. Finally, the diesel/CH4 mixture was direct injected into cylinder when air-assisted injector opens. The pressure of compressed CH4 should be sufficient to produce a sonic blow down the mixture into the cylinder and an improved spray characteristic in a low injection pressure operation condition can be obtained. Moreover, the diesel injection
Fig. 1. Cross-section of AADI diesel/CH4 dual-fuel injector model. 2
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Fig. 2. The schematic of experimental setup and instruments layout.
and the CO, THC and NOx emissions were measured. A kistler 6058A piezoelectric pressure transducer, as shown in Fig. 2, was installed in the cylinder head and was used to obtain the cylinder pressure. A combustion analyzer (Kistler KiBox) was employed to process the cylinder pressure signal. A high precise electric balance and a gas mass flow meter were used to measure the consumption of diesel and CH4, separately. The more details information about those instruments are listed in Table 3.
and diesel/CH4 injection events can be controlled separately in a certain range by an ECU developed for this work. The schematic of experimental setup and instruments layout was shown in Fig. 2. The pressure of CH4 was reduced from 12 MPa to 0.7 MPa through a two-stage pressure regulator and the low pressure CH4 was connected with the AADI injector at the CH4 supply port as shown in Fig. 1. Additionally, a shut off valve was also applied in the CH4 supply system just for emergency situation protection. The diesel was compressed by a low pressure pump and increased the pressure to 0.9 MPa. And then, it flowed through a surge tank in order to restrain the variation of the pressure during experiment. After that, diesel was connected with the AADI injector at the diesel supply port as shown in Fig. 1. The properties of diesel and CH4 used in this experiment are listed in Table 2.
2.3. Experiment procedure and operating condition sets All experiment study was conducted at a constant speed of 2000 r/ min operating conditions. The intake air temperature was fixed at 20 ± 1 °C and the highest cylinder head temperature was limited within 120 ± 5 °C. To evaluate the effects of LPDDI technology on the combustion and emissions characteristics, the diesel injection timings firstly studied under a fixed CH4 injection timing. And then, under an optimized diesel injection timing operating condition, the CH4 injection timings were varied. The diesel and CH4 injection duration never changed in the experiment. The more information about operating condition sets can be found in Table 4.
2.2. Data acquisition and measurement system The test engine was controlled by an engine control system (Powerlink CAC55). The load and speed of the test engine was controlled by a 55 kW electrical dynamometer. The data of sensors, such as cylinder head temperature, lubricating oil pressure and temperature, exhaust gas oxygen as well as exhaust temperature, was recorded by a data acquisition system based on LabVIEW platform. The regular exhaust emissions were sampled directly from the exhaust pipe. And then, the samples were sucked into an automotive exhaust emissions analyzer
Table 3 Information of the instrumentations used in this study.
Table 2 The properties of test fuels. Fuel properties
Low heating value, MJ/kg Density, kg/m3, (T = 25 °C) Viscosity, mm2/s, (T = 20 °C) Cetane number Octane number Auto-ignition temperature, °C Stoichiometric air–fuel ratio, kg/kg Carbon content, %
Fuel type and value for following Diesel
CH4
42.8 834.8 3.393 52.5 – 316 14.69 87
51.5 0.788 – – 125 595 17.24 75
Variable measured
Device
Manufacturer and Model
Accuracy
Torque Engine speed Temperature Diesel fuel mass flow
55 kW DC dynamometer Thermocouple Electronic gravimetric balance CH4 flow meter KiBox KiBox Exhaust emission analyzer
Powerlink/CAC55
± 0.2% F.S. ± 1 rpm ± 2.5 °C ± 0.1 g
CH4 mass flow Cylinder pressure Crank angle Regular emissions
3
TC direct/K type Beijing Heng Odd Instrument Ltd/HAES30K-1 Seven Star/CS 230A Kistler/6058A Kistler/2619A11 Horiba MEXA-584L
± 0.35% F.S. ± 0.5% 0.1°CA HC/ ± 12 ppm CO/ ± 0.06% NOx/30 ppm
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Table 4 Operating condition sets. NO.
1 2 3 4 5 6 7 8
IMEP/bara
Load/%
−150
6.8
91
−148 −136 −124 −112
5
67
Injection Duration/ms
Flow rate/(kg/h)
Injection Timings/°CA ATDC
Diesel
CH4
Diesel
CH4
Diesel
CH4
1.4
6.5
0.408
0.482
1.65
2.65
0.448
0.152
−250 −160 −120 −100 −250
a IMEP calculated under the diesel injection timing of −250 °CA ATDC and CH4 injection timing of −150 °CA ATDC and −148 °CA ATDC, separately.
Fig. 4. CA10, CA50 and combustion duration under different diesel injection timings.
3. Results and discussions
injection timings and the amplitude of the HRR increased significantly. Moreover, the position of peak value in the HRR curves move forward to the TDC with advanced diesel injection timings. This result also is an evidence of better dual-fuel spray formed by advanced diesel injection timings and in accordance with the above statement. However, there are some unique characteristics of the combustion process in the LPDDI engine compared with conventional dual-fuel engine. First, the HRR curves exhibit a unimodal characteristic in the LPDDI engine and very different from the conventional dual-fuel engine which with a typical three stage HRR curves. Second, the combustion process in the LPDDI engine is obvious faster than that in a conventional dual-fuel engine. Those difference indicated that the combustion process in the LPDDI engine mainly controlled by the chemical kinetics [29–31]. More homogenous mixture and wider space distribution of ignition kernel formed all over the cylinder. The CH4 and diesel were totally mixed before combustion started and very different with pilot-ignited dualfuel combustion process. The CA10, CA50 and combustion duration versus varying diesel injection timings are given in Fig. 4. With advanced diesel injection timings, the CA10 slightly move to the TDC while CA50 drastically shifts to the TDC from about 15 °CA ATDC to about 8 °CA ATDC. The combustion duration just reduced from the 21 °CA to 18°CA when diesel injection advanced from −100 °CA ATDC to −120 °CA ATDC. And then, advanced diesel injection timing, the combustion duration almost keeps constant. It is obviously that the CA10-50 is accelerated by advancing diesel injection timing due to a better dual-fuel spray and a more suitable distribution of ignition kernel [32]. Moreover, it also is the evidence that the combustion phase shifts to the TDC and this is consistent with the analyzed above. Combustion stability and combustion noise are important parameters for evaluation the combustion quality in engines. In this paper, the CoVIMEP and PPRR are used to represent the combustion stability and combustion noise, respectively. Fig. 5 shows the CoVIMEP and PPRR under different diesel injection timings. We observe that CoVIMEP is reduced from around 5.5% to 2.5% with advanced diesel injection timings. The PPRR exhibits a converse tendency and increased from around 2.3 bar/°CA to over 10 bar/°CA, which equal to the knock limitation [33]. The results illustrated that with advanced diesel injection timings, the combustion stability significantly improved while the combustion noise deteriorated. It is reasonable that a better dual-fuel spray is obviously good for combustion stability and a rapid combustion process definitely will lead to increase PPRR. The Fig. 6 provides the brake thermal efficiency (BTE) under different diesel injection timings at part load (IMEP = 5 bar) operation conditions. With advanced diesel injection timings from −100 °CA ATDC to −250 °CA ATDC, the BTE increased from about 31.8% to over
In this section, the combustion and emissions characteristics results of diesel/CH4 engine with the LPDDI were discussed. The effects of diesel and CH4 injection timings on combustion performance and emissions characteristics were analyzed. For better understanding, the injection timing mentioned in this paper is defined as the crank angle when the solenoid of injector was energized. The period of crank angle from 10% to 90% mass fraction burned is referred as the combustion duration. Moreover, the CH4 injection timings actually represented the timings at which dual-fuel mixture is injected into cylinder. 3.1. The effects of diesel injection timings on the combustion and emissions The curves of cylinder pressure and HRR under different diesel injection timings are shown in Fig. 3. With advanced diesel injection timings, an increased tendency in the cylinder pressure curves is observed and the peak pressure increased from 52 bar under −100 °CA ATDC to the 75 bar under −250 °CA ATDC. Moreover, the crank angle, which corresponding to the peak cylinder pressure, move to the top dead center (TDC). The interval time between diesel injection and CH4 injection event increased with advanced diesel injection timing and different distributions of the diesel droplets inside compressed CH4 in premixed cavity are formed. Under the most advanced diesel injection timing (-250 °CA ATDC), diesel spray has more time to mix with compressed CH4 and a more homogenous distribution of the diesel droplets inside CH4 environment is obtained in premixed cavity. As a result, the broken process of diesel droplets is enhanced due to stronger CH4 entrainment and the quality of dual-fuel spray is improved [26–28]. The HRR curves also show an increased tendency with advanced diesel
Fig. 3. Cylinder pressure and HRR under different diesel injection timings. 4
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Fig. 5. CoVIMEP and PPRR under different diesel injection timings.
Fig. 7. HC, CO and NOx emissions under different diesel injection timings.
3.2. The effects of CH4 injection timings on combustion and emissions In an AADI injector, the air injection timing corresponding to the fuel injection timing in a conventional injector. So, the CH4 injection timings in this paper actually represented the timings at which the diesel/CH4 dual-fuel mixture was injected into cylinder not just CH4. Fig. 8 shows the cylinder pressure and HRR curves under variation CH4 injection timings. Observing those curves, we can find that the peak cylinder pressure increased from 57 bar under −148 °CA ATDC to 65 bar under −112 °CA ATDC with retarded CH4 injection timings. Moreover, the position of the peak cylinder pressure in curves slightly moves to the TDC and shape of the pressure curves changes from a typical unimodal to a bimodal one. The HRR have the similar tendency with the cylinder pressure. With retarded CH4 injection timings, the peak HRR increased significantly and the curves shift to the TDC. Furthermore, the shape of the HRR curves also changed from a unimodal curve to a bimodal one as the same with cylinder pressure curves. As we all know, in a typical diesel/natural gas dual-fuel engine (port injection natural gas and direct injection diesel), the combustion phase is dominated by the pilot diesel injection timings. But in the LPDDI engine, as shown in Fig. 8, the combustion phase is controlled by the reactivity of the diesel/CH4 mixture in the cylinder rather than the CH4 injection timings. Therefore, the spatial distributions of the dual-fuel mixture in cylinder play an important role in the combustion process. With retarded CH4 injection timings, the injection backpressure increased and the influence of gas on the diesel droplets broken process is reduced. So, the quality of the dual-fuel spray deteriorated and SMD of the diesel increased [26]. However, the increased SMD is maybe not a negative influence in a LPDDI dual-fuel engine because the large droplets will easily form ignition kernels, particularly under the lean dual-
Fig. 6. BTE under different diesel injection timings under high load.
36.1%. This is because of advancing diesel injection timings, the spray quality is improved as we discussed above and the combustion process is more complete. However, the BTE of the LPDDI engine in the high load (IMEP = 6.8 bar) is still lower compared with the baseline diesel engine in the same operation conditions. The results are also consistence with the HRR curves exhibited above. Moreover, the BTE of LPDDI engine still need improved in the high load. The HC, CO and NOx are the mainly regular emissions in diesel/CH4 dual-fuel engine. In dual-fuel engine, the HC emissions mainly due to incomplete combustion and slower burning rate of CH4. Furthermore, it is believed that 90% of the HC emissions in a diesel/natural gas dualfuel engine is unburned CH4 [34]. CO and NOx emissions are commonly related with temperature-time history. The fuel’s oxidation and decomposition are dominated by in-cylinder temperature, which significantly influences the formation of CO emissions and the maximum temperature in the combustion process directly controls the NOx emissions [35,36]. Fig. 6 shows the HC, CO and NOx emissions results under different diesel injection timings. Observing the Fig. 7, we can find that with advancing diesel injection timings, HC and CO notably decreased while the NOx significantly increased. As we mentioned above that a better-quality dual-fuel spray and a faster combustion process achieved by advanced diesel injection timings. As a result, the better dual-fuel spray will lead a more complete combustion and reduced the unburned CH4. The faster combustion process indicated that most heat of fuel released in a shorter time. So, the heat dissipation is restrained and the local temperature of in-cylinder decreased while the peak temperature of combustion increased. Therefore, the CO reduced due to a lower local in-cylinder temperature and NOx increased under a higher peak combustion temperature.
Fig. 8. Cylinder pressure and HRR under different CH4 injection timings. 5
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Fig. 11. BTE under different diesel injection timings under part load. Fig. 9. CA10, CA50 and combustion duration under different CH4 injection timings.
is obtained. The BTE curves under different CH4 injection timings at part load (IMEP = 5 bar) exhibits in Fig. 11. As we can observed, with advanced the CH4 injection timings, the BTE obviously decreased. The maximum value of the BTE is about 38% which is higher than that in the diesel engines at same operation conditions. This result indicated that the CH4 injection timings paly a very significantly influence on the combustion process of a LPDDI engine. Moreover, with retarded CH4 injection timings, the combustion phase shifts to the TDC and the BTE increased. And the tendency of the CA10, CA50 and combustion duration curves are also consistence with the conclusion. The regular emissions of HC, CO and NOx under different CH4 injection timings are provided in Fig. 12. The HC and CO curves exhibit a decreased tendency with retarded CH4 injection timings while NOx emissions show a converse tendency. The faster initial combustion stage decreased HC emissions and reduced heat dissipation during combustion. So, a lower in-cylinder temperature is obtained and CO emissions are decreased with retarded CH4 injection timings. As the similar analysis, the higher peak combustion temperature is achieved under a faster initial combustion stage and this is the reason that NOx emissions are increased with retarded CH4 injection timings.
fuel mixture operation conditions. The CA10, CA50 and combustion duration curves under variation CH4 injection timings are shown in Fig. 9 and more evidences can be found in CA10 curve, which usually used to evaluate the ignition characteristics. With retarding CH4 injection timings, the CA10 move to the TDC and the earliest CA10 obtained under CH4 injection timing of −112 °CA ATDC. This result is in accordance with the discussions above and increased SMD provides more suitable environment for ignition kernels formation. Consequently, the later injection timings, the shorter time left before ignition. Of course, over retarded the CH4 injection timings will face high backpressure and even failure to direct injection. The CA50 have similar tendency with the CA10 under the variation injection timings and it is proved that the initial stage of the combustion process is enhanced due to a better flame propagation environment. Furthermore, the combustion duration is slightly prolonged with retarded CH4 injection timings and probably more CH4 completely burned with retarded CH4 injection timings. As the combustion stability and noise are concerned, the CoVIMEP and PPRR versus variation CH4 injection timings is given in Fig. 10. Observing the curves, we can notice that the CoVIMEP decreased significantly with retarded CH4 injection timings while the PPRR curve exhibits a totally converse tendency. This result indicated that the combustion stability is improved by retarded CH4 injection timings and combustion noise increased while the maximum value is still within an acceptance range. Moreover, a suitable ignition kernels and flame propagation mixture distributions was produced by retarding CH4 injection timings. Therefore, a more stable and faster combustion process
4. Conclusions In this study, we evaluate the possibility to apply the LPDDI technology in a diesel/CH4 duel-fuel engine. The effects of diesel injection timings and CH4 injection timings on the combustion performance and emissions characteristics have been experimentally studied in this paper. The in-cylinder pressure, HRR, CoVIMEP, PPRR, CA10, CA50 and combustion duration as well as regular emissions are calculated. On the
Fig. 10. CoVIMEP and PPRR under different CH4 injection timings.
Fig. 12. HC, CO and NOx emissions under different CH4 injection timings. 6
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basis of the experiment results and discussions presented above, the main conclusions are summarized as following:
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(1) Both the diesel and CH4 injection timings have significantly influence on the combustion process of LPDDI engine. Moreover, with advanced the diesel injection timings, the positive effect of air (CH4) on the liquid (diesel) droplets breaking process will be enhanced and a more homogenous diesel/CH4 mixture will be expected. The environment more suitable for ignition kernels formation is produced by retarding the CH4 injection timings in cylinder. In this work, an optimized combustion performance can be obtained under an earlier diesel injection timing (−250 °CA ATDC) and a later CH4 injection timing (−112 °CA ATDC). (2) The reactivity gradient and its spatial distribution of dual-fuel mixture dominate the combustion performance and emissions characteristics. By adjusting the relative chronology of the diesel and CH4 injection events, the difference degrees (homogenous or stratification) of dual-fuel mixture will be obtained as we needed. So the LPDDI give us an effectively method to control the reactivity gradient and its spatial distribution in a dual-fuel engine. (3) The LPDDI technology provides a cost-effective solution to achieve direct dual-fuel (liquid/gas) injection and extends the flexibility to control the mixing process of dual-fuel charge in cylinder. Additionally, it is a very promising technology to achieve RCCI combustion mode and enable a potential way to optimize the RCCI combustion process. 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. Acknowledgements This study is financially supported by the National Natural Science Foundation of China (Grant No. 51706171) and the China Postdoctoral Science Foundation (Grant No. 2016M602815). References [1] Imtenan S, Varman M, Masjuki HH, et al. Impact of low temperature combustion attaining strategies on diesel engine emissions for diesel and biodiesels: a review. Energy Convers Manage 2014;80(1):329–56. [2] Kumar Agarwal Avinash, Pratap Singh Akhilendra, Kumar Maurya Rakesh. Evolution, challenges and path forward for low temperature combustion engines. Prog Energy Combust Sci 2017;61(7):1–56. [3] Saxena S, Bedoya ID. Fundamental phenomena affecting low temperature combustion and HCCI engines, high load limits and strategies for extending these limits. Prog Energy Combust Sci 2013;39(5):457–88. [4] Yousefi A, Guo HS, Birouk M. Effect of diesel injection timing on the combustion of natural gas/diesel dual-fuel engine at low-high load and low-high speed conditions. Fuel 2018;235(8):838–46. [5] Hyunwook Park, Euijoon Shim, Choongsik Bae. Improvement of combustion and emissions with exhaust gas recirculation in a natural gas-diesel dual-fuel premixed charge compression ignition engine at low load operations. Fuel 2018;235(8):763–74. [6] Rosha P, Dhir A, Mohapatra SK. Influence of gaseous fuel induction on the various engine characteristics of a dual fuel compression ignition engine: a review. Renewable Sustainable Energy Rev 2018;82(2):3333–49. [7] Pourya Rahnama, Amin Paykani, Vahid Bordbar, et al. A numerical study of the effects of reformer gas composition on the combustion and emission characteristics of a natural gas/diesel RCCI engine enriched with reformer gas. Fuel 2017;209(7):742–53. [8] Benajes J, Garcia A, Monsalve-Serrano, et al. Achieving clean and efficient engine operation up to full load by combining optimized RCCI and dual-fuel diesel-gasoline
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