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Effects of air jet duration and timing on the combustion characteristics of high-pressure air jet controlled compression ignition combustion mode in a hybrid pneumatic engine
Energy Conversion and Management 127 (2016) 392–403
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Effects of air jet duration and timing on the combustion characteristics of high-pressure air jet controlled compression ignition combustion mode in a hybrid pneumatic engine Wuqiang Long, Xiangyu Meng, Jiangping Tian ⇑, Hua Tian, Jingchen Cui, Liyan Feng Institute of Internal Combustion Engine, Dalian University of Technology, Dalian, Liaoning 116024, China National Engineering Research Center of Shipbuilding, Dalian University of Technology, Dalian, Liaoning 116024, China
a r t i c l e
i n f o
Article history: Received 7 June 2016 Received in revised form 7 September 2016 Accepted 8 September 2016
Keywords: Diesel premixture Hybrid pneumatic engine Jet controlled compression ignition High pressure air
a b s t r a c t The high-pressure air jet controlled compression ignition (JCCI) combustion mode was employed to control the premixed diesel compression ignition combustion phasing by using the compound thermodynamic cycle under all operating conditions, which is accomplished in a hybrid pneumatic engine (HPE). A three-dimensional computational fluid dynamics (CFD) numerical simulation coupled with reduced n-heptane chemical kinetics mechanism has been applied to investigate the effects of highpressure air jet duration and the start of jet (SOJ) timing on the combustion characteristics in the power cylinder of HPE. By sweeping the high-pressure air jet durations from 6 to 14 °CA and SOJ timings from 12 °CA ATDC to the top dead center (TDC) under the air jet temperatures of 400 and 500 K, respectively, the low- and high-temperature reactions, combustion efficiency, as well as the combustion phasing and burning duration have been analyzed in detail. The results illustrated that a longer air jet duration results in a higher peak in the first-stage high-temperature reaction, and the short air jet duration of 6 °CA can lead to a higher combustion efficiency. The SOJ timing sweep results showed that there exists an optimum timing for obtaining the highest combustion efficiency and shortest burning duration. Ó 2016 Published by Elsevier Ltd.
1. Introduction Combustion phasing of premixed charge compression ignition (PCCI) is one of the most important factors impacting the overall diesel engine performance, which directly affects thermal efficiency, combustion efficiency, heat transfer, ringing intensity, load limitation, emissions and so on. Nevertheless, it is hard to control the combustion phasing in the PCCI combustion mode [1,2]. The high-quality premixed mixture with early fuel injection is formed before combustion occurs. As a result, the complex interaction of pressure, temperature, fuel concentration and other factors in the cylinder makes the control of combustion phasing more difficult. As proposed in the early 1980s [3], diesel fuel can be directly injected into the combustion chamber with advanced timings to prepare the high-quality mixture before combustion, which was called diesel hot premixed combustion (DHPC). As well-known as diesel PCCI combustion currently, researchers have been contributing lots of efforts to explore the potential for controlling its ⇑ Corresponding author at: Institute of Internal Combustion Engine, Dalian University of Technology, Dalian, Liaoning 116024, China. E-mail address: [email protected] (J. Tian). http://dx.doi.org/10.1016/j.enconman.2016.09.028 0196-8904/Ó 2016 Published by Elsevier Ltd.
combustion phasing. One of the most common methods is external exhaust gas recirculation (EGR) with intercooling [4–6], with which the non-reactive diluents of the residual gases would help to retard the combustion phasing. It was found that the combustion phasing can be retarded at low loads. However, it becomes less sensitive or even insensitive regarding to the external EGR at medium and high loads [4]. In addition, an over-high EGR rate generally leads to a high brake specific fuel consumption (bsfc) [6]. The internal EGR with negative valve overlap (NVO) can help to form the homogeneous mixture and reduce particulate matter (PM) emission. The limitation, however, is that the effect of heating is larger than that of diluting on the mixture for a high internal EGR rate, which leads to an advanced combustion phasing. This phenomenon suppresses the extension of load limitation [7]. The intake temperature is also used as a controlling parameter for the combustion phasing [8,9]. Lower intake temperature decreases the in-cylinder temperature and decelerates the chemical reaction process, which leads to a retarded combustion phasing and slower heat release rate. The control of the compression ratio has also been considered as a potential method to control the combustion phasing. This method can be divided into the control of the geometric and effective compression ratio, both of which are able to
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Nomenclature ATDC BTDC BBDC bsfc CFD CA10
after top dead center before top dead center before bottom dead center brake specific fuel consumption computational fluid dynamics crank angle at 10% burning rate of the accumulated heat release CA50 crank angle at 50% burning rate of the accumulated heat release CA10-CA90 burning duration from 10% to 90% burning rate of the accumulated heat release DHPC diesel hot premixed combustion EGR exhaust gas recirculation EVO exhaust valve opening HCCI homogeneous charge compression ignition
reduce the in-cylinder temperature and pressure [10–14]. The application of variable compression ratio (VCR) technology can extend the high-load range with reduced geometric compression ratio. However, it is constrained by the practical feasibility, expensive costs and machining effort [10,11]. Variable valve timing (VVT) with a late intake valve closing (IVC) timing can also be able to make the extension of load limitation, while it typically reduces the thermal efficiency due to the pumping loss [13]. Besides, fuel-air mixing enhancement [15–17], fuel reactivity [18,19], and fuel injection strategies [20–23] have also been widely investigated to control the combustion phasing in diesel PCCI combustion mode. Diesel blended with high-octane fuel, such as gasoline, n-butanol, can retard the combustion phasing due to the reduction of local reactivity in the chamber. However, the fuel economy turns poorer with a high ratio of high-octane fuel due to the unstable or incomplete combustion [18]. Multiple injections can achieve the mixture stratification to take the advantage of the charge cooling effects of late injection or the chemical reactivity effects of richer regions [20]. Several of these techniques mentioned above have also been applied simultaneously to explore the possibility of the phasing control in diesel PCCI combustion mode [6,8,13,22]. Except traditional internal combustion engine with advanced combustion concept, there is also hybrid pneumatic engine (HPE). HPE was first proposed by Schechter in 1999 [24], which aimed to capture the energy of braking in the form of compressed air and reuse this energy at a later time. This engine requires none of the expensive electric equipment used in the electric hybrid engines [25]. Generally, HPE has a pneumatic storage system combined with the conventional internal combustion engine, and the engine can also have separate compressor and power cylinders. It can recycle the brake energy and charge the compressed air tank during the deceleration and downhill conditions. High-pressure air can be released from the compressed air tank to power the engine under the cold-start and low-load conditions. HPE can also be operated in a range of high-efficiency loads all the time by using firing and charging mode. New thermodynamic cycles in HPE have been widely investigated due to the potential for achieving both the low fuel consumption and emissions [26–30]. The Scuderi group [31,32] proposed a HPE with split-cycle design, which accomplishes the conventional four strokes in two cylinders through a crossover port. This engine can reach an extremely high-expansion ratio by using Miller cycle and turbocharging. Fazeli et al. [33] evaluated the energy recycling capability by using two compressed air storage tank to restore the brake energy. Their
HPE HSDI IEGR IVC JCCI NOx NVO PCCI PM SOC SOI SOJ TDC VCR VVT
hybrid pneumatic engine high-speed direct-injection internal exhaust gas recirculation intake valve closing jet controlled compression ignition nitrogen oxides negative valve overlap premixed charge compression ignition particulate matter start of combustion start of injection start of jet top dead center variable compression ratio variable valve timing
experimental results revealed a 125% improvement in energy recycling in comparison with the single-tank system. As the mentioned technologies used in PCCI combustion have not completely solved the issue of combustion phasing control due to their slow response to load variations and the unavoidable knocking issue at high loads, so high-pressure air jet controlled compression ignition (JCCI) combustion mode based on the compound thermodynamic cycle in HPE has been proposed [34], aimed to control the phasing of diesel premixed combustion directly and precisely through high-pressure air jet under all load conditions. In this combustion mode, the premixed mixture in the local region is ignited by the high-pressure air jet compression, and the combustion phasing can be controlled flexibly through the control of highpressure air jet. Compound thermodynamic cycle in HPE also takes the advantages of multiple operation modes as discussed above. The previous study about the effects of high-pressure-air jet pressure and temperature on the power cylinder of HPE showed that this combustion mode has intensified low-temperature reaction and two-stage high-temperature reaction. Low air jet pressure and high air jet temperature can obtain high thermal efficiency [35]. This paper aims to investigate the effects of high-pressure air jet duration and timing on the combustion phasing control by using the numerical simulation method in the power cylinder of HPE.
2. Hybrid pneumatic engine with compound thermodynamic cycle In order to control the phasing of premixed compression ignition combustion, compound thermodynamic cycle—being capable of low-compression ratio, high pressure-rise ratio and highexpansion ratio—is employed in the HPE to realize the highpressure air JCCI combustion. Compound thermodynamic cycle employs the low effective compression ratio by using early or late IVC to obtain the low compression pressure and temperature, which is able to avoid the autoignition of the original mixture without any additional heat source. Then, high-pressure air is jetted into the combustion chamber and continuously compresses the original mixture near the top dead center (TDC). The temperature and pressure of the original mixture in the local region increased immediately by the high-pressure air jet compression trigger the autoignition. By this way, the combustion phasing of the premixture can be controlled directly and precisely. Because of the low effective and high geometric compression ratio employed in this
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Fig. 1. Schematic diagram of compound HPE used in the study.
combustion mode, it can enhance the thermal efficiency compared to the conventional diesel combustion mode [34]. Compound thermodynamic cycle is implemented in a modified HPE, as shown in Fig. 1. This engine mainly consists of the compressor cylinder, power cylinder, compressed air storage tank and high pressure air jet system. The compressor cylinder without fuel injection or firing is able to compress the air or pressurized gas and charge the compressed air storage tank. The power cylinder with four-stroke cycle works based on compound thermodynamic cycle. Well-mixed mixture is prepared by using early fuel injection, and proper IVC timing is employed to avoid spontaneous combustion. High-pressure air released from the compressed air storage tank is jetted into the combustion chamber through the highpressure air jet system near TDC. The original mixture in the local region is compressed further by the high-pressure air jet, inducing the autoignition and subsequent combustion. As a kind of internal combustion-air hybrid power system, HPE with compound thermodynamic cycle also has several operating modes. It can be operated in the air-power mode by releasing the high-pressure air to both the compressor and power cylinders without fuel injection and firing under the low-load conditions. It can also recycle the braking energy to improve the overall efficiency during the deceleration and downhill conditions. More details were clarified in Ref. [35]. The model of high-pressure air jetted into the power cylinder of HPE through the check valve was established to investigate the effects of air jet duration and timing on the high-pressure air JCCI combustion mode in this study.
check valve. The longitudinal section of the simulated 3D model is shown in Fig. 2, and the computational grid with local refinement is presented in Fig. 3. The diesel injector has a nozzle with spray included angle of 105° and 8 holes, and it was installed beside the check valve with a deviation angle of 15° from vertical. An improved piston geometry with a reentrant-shallow-basin bowl was used in this work, which was designed with the consideration of both the premixture preparation and high-pressure air jet. Relevant engine specifications are summarized in Table 1. A fixed injection pressure of 150 MPa, a fueling rate of 14.5 mg/cycle,
Fig. 2. Combustion chamber section.
3. Engine specifications The engine simulated in this work was a single-cylinder, naturally aspirated, high-speed direct-injection (HSDI) diesel engine with a displacement of 0.418 L. In order to ensure that no autoignition takes place in the combustion chamber without any additional heat sources, the geometry compression ratio of the engine is reduced from 19 to 12. A check valve was added vertically in the central position of the cylinder head, through which highpressure air was jetted into the combustion chamber. More details of the check valve structure can be found in the work of Zhang et al. [36]. The engine model includes the combustion chamber and
Fig. 3. Computational mesh.
W. Long et al. / Energy Conversion and Management 127 (2016) 392–403 Table 1 Engine specifications. Displacement Bore Stroke Compression ratio Piston geometry Nozzle hole diameter Nozzle hole number Spray hole cone angle IVC EVO
0.418 L 86 mm 72 mm 12.0:1 Reentrant-shallow-basin bowl 0.133 mm 8 105° 45 °CA ABDC 55 °CA BBDC
Table 2 Calculation parameters. Engine speed Intake air temperature Intake air pressure Injection pressure Start of injection (SOI) Fuel flow rate Initial temperature Initial pressure Wall temperature Piston temperature
1500 rpm 293 K 0.1 MPa 150 MPa 35 °CA BTDC 14.5 mg/cyc 330 K 1.15 bar 410 K 450 K
and a start of injection (SOI) timing of 35 °CA ATDC were employed in the simulation. The corresponding calculated parameters are listed in Table 2.
4. Numerical model, validation and mesh sensitivity study 4.1. Numerical model Three-dimensional (3D) computational fluid dynamics (CFD) code with a variety of enhanced physical and chemistry submodels was used, and the sub-models are listed in Table 3. K-zeta-f model [37] was employed to simulate the turbulent flows, as it is capable of solving the variable-density engine flows. The Kelvin-Helmholtz Rayleigh-Taylor (KH-RT) model [38] was used to simulate the breakup process of injected droplets. The spray collision model was developed by Nordin [39] with the improvement of grid independence. The spray/wall interaction model was developed by Naber and Reitz [40] with the function of predicting the sprayimpingement phenomena. Heat transfer from the wall was simulated by the Han and Reitz model [41] which accounts for the variations in the gas density and the turbulent Prandtl number in the boundary layer. A reduced n-heptane reaction mechanism, based on the detailed n-heptane reaction mechanism [42], with 29 species and 52 reactions developed by Patel et al. [43] was selected to simulate the diesel fuel chemistry, as it has similar autoignition characteristics to those of diesel. In the highpressure air JCCI combustion mode, high-pressure air jet actually increases the local compression ratio, it is still premixed compression ignition combustion. Therefore, the reduced n-heptane mechanism used in PCCI combustion mode is also suitable for this
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simulation work. Because of the early fuel injection timing ( 35 °CA ATDC) and late high-pressure air jet (near TDC), the mixture is already well-premixed before high-pressure air jet, so these sub-models used in the PCCI combustion mode still suits the highpressure JCCI combustion mode. 4.2. Model validations The relevant computational models were validated by the experimental measurements conducted by Lee [44] in a HSDI diesel engine. A nozzle with spray included angle of 130° and 8 holes, an EGR rate of 55%, a geometric compression ratio of 16, and an open-crater-type combustion chamber were used in the tested engine to realize the diesel PCCI combustion. Fig. 4 shows the comparisons between corresponding experimental and simulated data for the variation of in-cylinder pressure and heat-release rate at the start of injection (SOI) timings of 30 and 40 °CA ATDC. It can be seen that the simulated results reveals a good agreement with the measurements. 4.3. Mesh sensitivity study According to the literature [45], the mesh density might affect the simulation results, consequently, the cell sizes for the mesh sensitivity were studied as listed in Table 4, and the effects of the cell size on the combustion are shown in Fig. 5. The refined grid was used for the whole combustion chamber from 35 to 30 °CA ATDC via fixed embedding, and a finer cell size was used for the check valve. By sweeping the cell size from 0.8 to 1.2 mm for the combustion chamber, it shows that these five cases has very similar results, indicating high grid independence. With the consideration of the grid independence for all the cases, the cell sizes of 0.9 and 0.45 mm were selected for the combustion chamber and check valve, respectively. 5. Results and discussion 5.1. Effects of high pressure air jet duration on the in-cylinder combustion characteristics The high-pressure air jet durations of 6, 8, 10, 12 and 14 °CA are compared in this section under a fixed start of jet (SOJ) timing of 4 °CA ATDC, a fixed jet pressure of 8 MPa, and the jet temperatures of 400 and 500 K, as shown in Table 5. Fig. 6 presents the total air jet mass variations with different air jet durations.
Table 3 Computational models. Turbulent model Breakup model Collision model Spray/wall interaction model Heat transfer from the wall Fuel chemistry
k-zeta-f model [37] KH-RT model [38] Nordin model [39] Naber and Reitz model [40] Han and Reitz model [41] Reduced n-heptane mechanism [43]
Fig. 4. Comparisons of in-cylinder pressure and apparent heat release rate between measurements and predictions for SOI 40 and 30 °CA ATDC.
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Table 4 Mesh details for the sensitivity study. Based grid size [mm] (embedding level 0)
Cell size of the combustion chamber from 35 to 30 °CA ATDC [mm] (embedding level 2)
Cell size of the check valve [mm] (embedding level 3)
Minimum to maximum cell count
4.8 4.4 4 3.6 3.2
1.2 1.1 1 0.9 0.8
0.6 0.55 0.5 0.45 0.4
100,000–125,000 125,000–160,000 140,000–170,000 155,000–190,000 180,000–230,000
Fig. 5. Mesh sensitivity (air jet temperature = 400 K, air jet pressure = 8 MPa, start of jet = 4 °CA ATDC, air jet duration = 10 °CA).
Table 5 High-pressure air jet parameters for jet duration comparisons. Jet temperature Jet pressure SOJ Jet duration
400 and 500 K 8 MPa 4 °CA ATDC 6, 8, 10, 12, and 14 °CA
Fig. 6. Total air jet mass variations with various air jet durations.
Fig. 7 presents the velocity flow field under the air jet duration of 14 °CA and jet temperature of 400 K. At 2 and 3 °CA ATDC, as the high-pressure air is jetted into the combustion chamber through the check valve, it flows along with the piston top and
the bottom of the cylinder head. At 3 °CA ATDC, it can be seen that the velocity is close to zero in Zone A. Because the temperature and pressure increase immediately, so the spontaneous combustion occurs in this region. At 8 °CA ATDC, since the autoignition has occurred in Zone A, the velocity flow field in the combustion chamber is affected by both the high-pressure air jet and combustion. As the high-pressure air jet ends at 10 °CA ATDC, the velocity decreases significantly at 13 °CA ATDC, and a slightly high velocity can be observed near the piston top. Fig. 8 shows the numerical results of cut-planes colored by the in-cylinder temperature and n-heptane mass fraction distributions under the air jet durations of 8 and 14 °CA. As the high-pressure air flows along the piston head and the bottom surface of the cylinder head and compresses the original mixture, the temperature in the central region of the chamber rises up rapidly, subsequently, the spontaneous combustion occurs in this region as shown in Fig. 8a. High-pressure air with a longer jet duration compresses the original mixture for a longer time, leading to a higher compression rate and temperature in the local region in the cylinder. At 6 °CA ATDC, because there is still high-pressure air jet in the 14 °CA duration case, so it shows a higher temperature than that in the 8 °CA duration case in the central area. At 12 and 15 °CA ATDC, the 8 °CA duration case displays high temperature in a larger region due to less mixing of the high-pressure air and original mixture. The mass fraction distributions of n-heptane are presented in Fig. 8b to investigate its reaction process. At 6 °CA ATDC, the observation in the case of 8 °CA duration shows that a larger amount of fuel exists around the upper area of the chamber. Until 9 °CA ATDC, there is still a higher concentration of fuel in the local region compared to that in the case of the 14 °CA duration. At 12 and 15 °CA ATDC, the 8 °CA duration case also shows higher n-heptane concentration in the upper area. The fuel concentration is affected by both the jet mass of the high-pressure air and the reaction rates. More high-pressure air jet makes the mixture leaner, where the air jet flow passes. While more air jet also leads to higher peaks in the high temperature reaction and faster early-stage combustion, which can be observed in Fig. 9. The effects of air jet duration on the in-cylinder pressure and heat-release rate at the air jet temperature of 400 and 500 K are presented in Fig. 9. As discussed in the previous work [35], the high-pressure air JCCI combustion includes the intensified lowtemperature reaction and two-stage high-temperature reaction. The high-temperature reaction contains the spontaneous combustion in the local region and the followed diffused combustion. The spontaneous combustion caused by the high-pressure air jet compression was termed as the first-stage high temperature reaction, and the followed diffused combustion induced by the mixing of high-pressure air and original mixture was termed as the second-stage high temperature reaction. It should be noted that the mixture is well-mixed in both the first and second stages, even though the second stage has long reaction period in some cases. There is no obvious difference in the low-temperature reaction among the various air jet durations. While with the increase of the air jet duration, the high-temperature reaction reveals higher
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Fig. 7. Velocity flow field (air jet duration = 14 °CA, air jet temperature = 400 K).
(a) Temperature
(b) N-heptane mass fraction Fig. 8. Comparisons of in-cylinder temperature and n-heptane mass fraction distributions for the air jet durations of 8 °CA and 14 °CA (air jet temperature = 400 K).
peak values in both of the two stages and also displays a faster early combustion, except the 14 °CA duration case. It can be seen that the 6 °CA duration case shows a retarded first-stage reaction in both of the two air jet temperatures. The high-pressure air jet ends at 2 °CA ATDC in the 6 °CA duration case, while the hightemperature reaction has not occurred at that time, resulting in the slow combustion process. This phenomenon illustrates that the insufficient high-pressure air jet duration would result in over-retarded combustion, or even misfire. It can also be noted that the 14 °CA duration case has lower peak values than the 12 °CA case. This could be explained as much more high-pressure air jet
results in less mixture burning in the first-stage reaction due to more mixing of the high-pressure air and the original mixture. Fig. 10 shows the in-cylinder mean temperature variations with different air jet durations at the air jet temperatures of 400 and 500 K. The in-cylinder mean temperature trend is mainly affected by both the high-pressure air jet and combustion rate. More air jet leads to a lower mean temperature, while a shorter combustion process can make a higher peak pressure and temperature. The increased air jet duration case reveals a more advanced but lower peak value, except for the 12 °CA duration case. This is due to its shorter combustion duration in comparison with the 10 °CA
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(a) Air jet temperature = 400 K
(a) Air jet temperature = 400 K
(b) Air jet temperature = 500 K (b) Air jet temperature = 500 K
Fig. 10. Effects of air jet duration on in-cylinder mean temperature.
Fig. 9. Effects of air jet duration on in-cylinder pressure and heat-release rate.
duration case. In addition, the higher combustion efficiency also contributes to enhance the mean temperature, which can be observed in Fig. 11. As the rapid mixing of the high-pressure air and original mixture results in a small amount of overlean mixture and local lowtemperature region in the combustion chamber, where it would be too lean to combust sufficiently, so the combustion efficiency should be considered in high-pressure air JCCI combustion mode. Fig. 11 shows the heat-release percentage of the total energy under different air jet durations. On the one hand, the spontaneous combustion with more fuel in the first-stage reaction leads to a shorter burning duration in the second-stage reaction, and the faster combustion process is beneficial for reducing the mixing time of highpressure air and original mixture, and therefore improving the combustion efficiency. On the other hand, more air jet could induce more mixing of the high-pressure air and original mixture, thus leading to more overlean mixture and larger low-temperature region, which reduces the combustion efficiency, as shown in Fig. 8. Both of these two reversed factors impact on the combustion efficiency. As the jet duration is increased from 6 to 10 °CA, the combustion efficiency is decreased. This phenomenon indicates that the combustion efficiency is mainly dominated by the jet mass of the high-pressure air during these air jet durations. At the case
Fig. 11. Effects of air jet duration on heat-release percentage of total input energy.
of 12 and 14 °CA duration, the combustion efficiency is improved due to the rapid combustion process both in the first- and second-stage high-temperature reactions. It is worth noting that a higher air jet temperature could help to improve the combustion efficiency significantly.
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the two air jet temperatures, which is primarily caused by the slowdown of the combustion process in comparison with the case of 12 °CA jet duration. For the burning duration comparison, the longer burning duration can be found in all the jet duration cases under the jet temperature of 400 K despite using a shorter burning duration from 10% to 80% burning rate of the accumulated heatrelease. The burning duration is attributed to the combustion rate related to the two-stage high-temperature reaction. More air jet makes the spontaneous combustion occur with more mixture. In the meantime, more mixing of the high-pressure air and original mixture reduces the post combustion rate. These two factors shows the reversed effects on the burning duration. It can be noted that the 10 °CA air jet duration for the 400 K case shows a longer burning duration. This is because the fact that it has very slow post combustion rate due to the mixing of high-pressure air and original mixture. Obviously, the 12 and 14 °CA jet durations can obtain short durations, which indicates that longer air jet duration over a certain point can accelerate the overall combustion process. 5.2. Effects of high pressure air SOJ timing on the in-cylinder combustion characteristics This section investigates the effects of the high-pressure air SOJ timing on the in-cylinder combustion characteristics. The SOJ Fig. 12. Effects of air jet duration on combustion phasing and burning duration.
In Fig. 12, CA50—the crank angle at 50% burning rate of the accumulated heat release—was employed to represent the combustion phasing. CA10-CA90 was used to represent the burning duration for air jet temperature of 500 K, while CA10-CA80 was employed to represent the burning duration for the 400 K cases due to the incomplete combustion. By increasing the air jet duration from 6 to 12 °CA, CA50 is advanced gradually due to the rapid combustion process in both the first- and second-stage hightemperature reactions. Further increasing the air jet duration to 14 °CA, a slight retarded combustion phasing can be found in both
Table 6 High-pressure air jet parameters for SOJ timing comparisons. Jet temperature Jet pressure Start of jet (SOJ) Jet duration
400 and 500 K 7 MPa 12, 9, 6, 3, and 0 °CA ATDC 10 °CA
(a) Air jet temperature = 400 K
(b) Air jet temperature = 500 K Fig. 13. Total air jet mass variations with various SOJ timings.
Fig. 14. Effects of SOJ timing on in-cylinder pressure and heat-release rate.
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timings were swept from 12 to 0 °CA ATDC at a fixed jet duration of 10 °CA, a fixed jet pressure of 7 MPa, and the jet temperatures of 400 and 500 K, as listed in Table 6. The total air jet mass with different SOJ timings and temperatures is shown in Fig. 13. Fig. 14 shows the effects of high-pressure air SOJ timing on the in-cylinder pressure and heat-release rate at the air jet temperatures of 400 and 500 K. In Fig. 14a, the peak of the lowtemperature reaction is reduced gradually with the SOJ timings retarded from 9 to 0 °CA ATDC. This is mainly because that a retarded SOJ timing is more likely to induce a leaner mixture in the region where the low-temperature reaction occurs, and subsequently results in less fuel involved in the cool flame. It can be noted that SOJ 12 has a lower peak value of low-temperature reaction compared to SOJ 9. As SOJ 12 has the lowest incylinder temperature during the high-pressure air jet, the cool flame might occurs in a smaller region in the chamber, resulting in a lower peak. For the high-temperature reactions, SOJ 6 reveals the highest peak value among these SOJ timings in the first-stage reactions. On the one hand, as shown in Fig. 15, the temperature distribution at the end of air jet shows that the spontaneous combustion occurs in a smaller region with a SOJ timing earlier than 6 °CA ATDC. On the other hand, a more homogeneous mixture with a retarded SOJ timing makes the spontaneous combustion occur in the region with a lower equivalence ratio, resulting in a lower peak value for a SOJ timing later than 6 °CA ATDC. For the pressure traces at the air jet temperature of 400 K, it appears that the SOJ 12 and 9 cases present lower peaks than that in the SOJ 6 case due to the longer combustion process and incomplete combustion, which will be discussed below. While at the air jet temperature of 500 K, since the higher air jet temperature enhances the combustion process and combustion efficiency in the SOJ 12 and 9 cases, the peak pressure reveals an expectedly increased trend with an advanced SOJ timing other than the 400 K cases. The effects of high-pressure air SOJ timing on the in-cylinder mean temperature are presented in Fig. 16. Since the amount of the high-pressure air jet is almost constant at a fixed jet temperature, as shown in Fig. 13, the in-cylinder temperature is mainly affected by the SOJ timing, burning duration and combustion efficiency in this section. The fluctuations near TDC are caused by the high-pressure air jet and low-temperature reaction. In Fig. 16a, SOJ 6 presents the highest peak value which is consistent with its pressure trace. It can be noted that SOJ 12 reveals a low peak value even with the most advanced SOJ timing due to the long burning duration and incomplete combustion. With an increased air jet temperature as shown in Fig. 16b, the peak value trend of the in-cylinder temperature is almost consistent with the SOJ timings, while the enhanced peak value in SOJ 12 is still slightly lower compared to that in SOJ 9. There exists an optimum SOJ timing for obtaining the highest combustion efficiency at a fixed jet temperature, as shown in Fig. 17. The combustion efficiency becomes worse with the SOJ timing later or earlier than this optimum timing. This phenomenon
can be explained by using in-cylinder n-heptane mass fraction distributions in combination with the heat-release rate traces. In Fig. 18, it can be seen that most of n-heptane concentrates in the area near the cylinder wall at the SOJ 12 timing. In the highpressure air JCCI combustion mode, the spontaneous combustion occurs in the central region in the chamber, then the combustion propagates to the surrounding regions. While the slow combustion process in SOJ 12 induces more time for the mixing of the highpressure air and original mixture in the area near the cylinder wall. The lean mixture and low temperature in this region limit the chemical reactivity, resulting in the incomplete combustion. In the SOJ 0 case, the retarded SOJ timing makes the mixture more
(a) Air jet temperature = 400 K
(b) Air jet temperature = 500 K Fig. 16. Effects of SOJ timing on in-cylinder mean temperature.
Fig. 15. Comparisons of in-cylinder temperature distribution at the end of air jet for different SOJ timings (air jet temperature = 400 K).
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Fig. 17. Effects of SOJ timing on heat-release percentage of total input energy.
homogeneous, resulting in more fuel involved in the mixing of the high-pressure air and original mixture, which also leads to a more incomplete combustion. Comparatively, SOJ 6 reveals a lower n-heptane concentration in the combustion chamber, as observed at 30 °CA ATDC. While the combustion efficiency can be improved significantly with a higher jet temperature, and it can also be optimized with the adjusted diesel injection timing. It can be seen from the upper portion of Fig. 19 that despite jetting with the advanced SOJ timings of 12 and 9 °CA ATDC, their CA50 s are retarded compared to that in SOJ 6. This is mainly because the fact that they have less fuel burned in the first-stage high-temperature reactions and longer duration in the secondstage high-temperature reactions. While with a higher jet temperature as shown in the 500 K cases, CA50 is consistently retarded with the late SOJ timing. The burning duration comparisons are
Fig. 19. Effects of SOJ timing on combustion phasing and burning duration.
shown in the lower portion of Fig. 19, and because of the combustion efficiency issue, CA10-CA81, CA10-CA89 and CA10-CA77 were employed to represent the burning durations of SOJ 12, SOJ 3 and SOJ 0, respectively. For these two air jet temperatures, it is obvious that higher jet temperature shortens the burning duration significantly. It appears that this combustion mode exists an optimum SOJ timing for getting the shortest burning duration at each jet temperature with other fixed operating conditions. The firststage high-temperature reaction plays an important role in
Fig. 18. Comparisons of in-cylinder n-heptane mass fraction distributions for different SOJ timings (air jet temperature = 400 K).
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accelerating the combustion process, and more mixture involved in the first-stage reaction can leads to a faster second-stage reaction, and therefore an overall shorter burning duration. In the current work, according to the results of combustion efficiency and burning duration, it appears that the optimum SOJ timing is 6 °CA ATDC among these timings under the air jet duration of 10 °CA and jet pressure of 7 MPa.
6. Conclusion This study aims to investigate the effects of air jet duration and SOJ timing on the high-pressure air JCCI combustion characteristics based on the compound thermodynamic cycle in the power cylinder of HPE. The 3D CFD numerical simulation coupled with the reduced n-heptane chemical kinetics mechanism was developed to analyze the high-pressure air JCCI combustion process. Highpressure air JCCI combustion has two-stage high-temperature reaction due to the spontaneous combustion in the local region and subsequent combustion in the cylinder, and the combustion phasing can be controlled directly by adjusting the air jet parameters. According to the results of the air jet duration sweeping from 6 to 14 °CA and the SOJ timing from 12 °CA ATDC to TDC, the following conclusions can be drawn: (1) The high-pressure air jet durations from 8 to 14 °CA have little effect on the start of combustion, however, the 6 °CA jet duration retards the start of high-temperature reaction due to the low air jet compression rate. The combustion phasing (CA50) is advanced gradually with the increased air jet duration from 6 to 12 °CA. While the 14 °CA jet duration presents a slightly retarded combustion phasing compared to the 12 °CA jet duration. (2) The peak values in both the first- and second-stage hightemperature reactions are increased with the air jet durations from 6 to 12 °CA, however, the peaks with air jet duration of 14 °CA start to reduce. It appears that CA50 trend is consistent with the variation of the peaks. Additionally, a longer air jet duration also leads to more mixing of the highpressure air and original mixture, resulting in larger low temperature region in the combustion chamber. While, a higher jet temperature can improve the combustion efficiency and burning duration significantly. (3) The results of SOJ timing sweep reveals that the combustion phasing is controllable. The start of combustion delays progressively with the retarded SOJ timing. For the air jet temperature of 500 K, the combustion phasing is consistently retarded with the late SOJ timing. While for the air jet temperature of 400 K, the combustion phasing is firstly advanced and then retarded, which is mainly caused by the different combustion rates. (4) There exists an optimum timing for obtaining the highest combustion efficiency and shortest burning duration. This optimum timing leads to more mixture involved in the first-stage high-temperature reaction and induces a faster second-stage high-temperature reaction. The rapid combustion process with an optimized SOJ timing shortens the mixing duration of the high-pressure air and original mixture, and accordingly improves the combustion efficiency.
Acknowledgment This work is gratefully acknowledged by the National Natural Science Foundation of China (Grant Nos. 51076024 and 51379034).
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Effects of air jet duration and timing on the combustion characteristics of high-pressure air jet controlled compression ignition combustion mode in a hybrid pneumatic engine Wuqiang Long, Xiangyu Meng, Jiangping Tian ⇑, Hua Tian, Jingchen Cui, Liyan Feng Institute of Internal Combustion Engine, Dalian University of Technology, Dalian, Liaoning 116024, China National Engineering Research Center of Shipbuilding, Dalian University of Technology, Dalian, Liaoning 116024, China
a r t i c l e
i n f o
Article history: Received 7 June 2016 Received in revised form 7 September 2016 Accepted 8 September 2016
Keywords: Diesel premixture Hybrid pneumatic engine Jet controlled compression ignition High pressure air
a b s t r a c t The high-pressure air jet controlled compression ignition (JCCI) combustion mode was employed to control the premixed diesel compression ignition combustion phasing by using the compound thermodynamic cycle under all operating conditions, which is accomplished in a hybrid pneumatic engine (HPE). A three-dimensional computational fluid dynamics (CFD) numerical simulation coupled with reduced n-heptane chemical kinetics mechanism has been applied to investigate the effects of highpressure air jet duration and the start of jet (SOJ) timing on the combustion characteristics in the power cylinder of HPE. By sweeping the high-pressure air jet durations from 6 to 14 °CA and SOJ timings from 12 °CA ATDC to the top dead center (TDC) under the air jet temperatures of 400 and 500 K, respectively, the low- and high-temperature reactions, combustion efficiency, as well as the combustion phasing and burning duration have been analyzed in detail. The results illustrated that a longer air jet duration results in a higher peak in the first-stage high-temperature reaction, and the short air jet duration of 6 °CA can lead to a higher combustion efficiency. The SOJ timing sweep results showed that there exists an optimum timing for obtaining the highest combustion efficiency and shortest burning duration. Ó 2016 Published by Elsevier Ltd.
1. Introduction Combustion phasing of premixed charge compression ignition (PCCI) is one of the most important factors impacting the overall diesel engine performance, which directly affects thermal efficiency, combustion efficiency, heat transfer, ringing intensity, load limitation, emissions and so on. Nevertheless, it is hard to control the combustion phasing in the PCCI combustion mode [1,2]. The high-quality premixed mixture with early fuel injection is formed before combustion occurs. As a result, the complex interaction of pressure, temperature, fuel concentration and other factors in the cylinder makes the control of combustion phasing more difficult. As proposed in the early 1980s [3], diesel fuel can be directly injected into the combustion chamber with advanced timings to prepare the high-quality mixture before combustion, which was called diesel hot premixed combustion (DHPC). As well-known as diesel PCCI combustion currently, researchers have been contributing lots of efforts to explore the potential for controlling its ⇑ Corresponding author at: Institute of Internal Combustion Engine, Dalian University of Technology, Dalian, Liaoning 116024, China. E-mail address: [email protected] (J. Tian). http://dx.doi.org/10.1016/j.enconman.2016.09.028 0196-8904/Ó 2016 Published by Elsevier Ltd.
combustion phasing. One of the most common methods is external exhaust gas recirculation (EGR) with intercooling [4–6], with which the non-reactive diluents of the residual gases would help to retard the combustion phasing. It was found that the combustion phasing can be retarded at low loads. However, it becomes less sensitive or even insensitive regarding to the external EGR at medium and high loads [4]. In addition, an over-high EGR rate generally leads to a high brake specific fuel consumption (bsfc) [6]. The internal EGR with negative valve overlap (NVO) can help to form the homogeneous mixture and reduce particulate matter (PM) emission. The limitation, however, is that the effect of heating is larger than that of diluting on the mixture for a high internal EGR rate, which leads to an advanced combustion phasing. This phenomenon suppresses the extension of load limitation [7]. The intake temperature is also used as a controlling parameter for the combustion phasing [8,9]. Lower intake temperature decreases the in-cylinder temperature and decelerates the chemical reaction process, which leads to a retarded combustion phasing and slower heat release rate. The control of the compression ratio has also been considered as a potential method to control the combustion phasing. This method can be divided into the control of the geometric and effective compression ratio, both of which are able to
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Nomenclature ATDC BTDC BBDC bsfc CFD CA10
after top dead center before top dead center before bottom dead center brake specific fuel consumption computational fluid dynamics crank angle at 10% burning rate of the accumulated heat release CA50 crank angle at 50% burning rate of the accumulated heat release CA10-CA90 burning duration from 10% to 90% burning rate of the accumulated heat release DHPC diesel hot premixed combustion EGR exhaust gas recirculation EVO exhaust valve opening HCCI homogeneous charge compression ignition
reduce the in-cylinder temperature and pressure [10–14]. The application of variable compression ratio (VCR) technology can extend the high-load range with reduced geometric compression ratio. However, it is constrained by the practical feasibility, expensive costs and machining effort [10,11]. Variable valve timing (VVT) with a late intake valve closing (IVC) timing can also be able to make the extension of load limitation, while it typically reduces the thermal efficiency due to the pumping loss [13]. Besides, fuel-air mixing enhancement [15–17], fuel reactivity [18,19], and fuel injection strategies [20–23] have also been widely investigated to control the combustion phasing in diesel PCCI combustion mode. Diesel blended with high-octane fuel, such as gasoline, n-butanol, can retard the combustion phasing due to the reduction of local reactivity in the chamber. However, the fuel economy turns poorer with a high ratio of high-octane fuel due to the unstable or incomplete combustion [18]. Multiple injections can achieve the mixture stratification to take the advantage of the charge cooling effects of late injection or the chemical reactivity effects of richer regions [20]. Several of these techniques mentioned above have also been applied simultaneously to explore the possibility of the phasing control in diesel PCCI combustion mode [6,8,13,22]. Except traditional internal combustion engine with advanced combustion concept, there is also hybrid pneumatic engine (HPE). HPE was first proposed by Schechter in 1999 [24], which aimed to capture the energy of braking in the form of compressed air and reuse this energy at a later time. This engine requires none of the expensive electric equipment used in the electric hybrid engines [25]. Generally, HPE has a pneumatic storage system combined with the conventional internal combustion engine, and the engine can also have separate compressor and power cylinders. It can recycle the brake energy and charge the compressed air tank during the deceleration and downhill conditions. High-pressure air can be released from the compressed air tank to power the engine under the cold-start and low-load conditions. HPE can also be operated in a range of high-efficiency loads all the time by using firing and charging mode. New thermodynamic cycles in HPE have been widely investigated due to the potential for achieving both the low fuel consumption and emissions [26–30]. The Scuderi group [31,32] proposed a HPE with split-cycle design, which accomplishes the conventional four strokes in two cylinders through a crossover port. This engine can reach an extremely high-expansion ratio by using Miller cycle and turbocharging. Fazeli et al. [33] evaluated the energy recycling capability by using two compressed air storage tank to restore the brake energy. Their
HPE HSDI IEGR IVC JCCI NOx NVO PCCI PM SOC SOI SOJ TDC VCR VVT
hybrid pneumatic engine high-speed direct-injection internal exhaust gas recirculation intake valve closing jet controlled compression ignition nitrogen oxides negative valve overlap premixed charge compression ignition particulate matter start of combustion start of injection start of jet top dead center variable compression ratio variable valve timing
experimental results revealed a 125% improvement in energy recycling in comparison with the single-tank system. As the mentioned technologies used in PCCI combustion have not completely solved the issue of combustion phasing control due to their slow response to load variations and the unavoidable knocking issue at high loads, so high-pressure air jet controlled compression ignition (JCCI) combustion mode based on the compound thermodynamic cycle in HPE has been proposed [34], aimed to control the phasing of diesel premixed combustion directly and precisely through high-pressure air jet under all load conditions. In this combustion mode, the premixed mixture in the local region is ignited by the high-pressure air jet compression, and the combustion phasing can be controlled flexibly through the control of highpressure air jet. Compound thermodynamic cycle in HPE also takes the advantages of multiple operation modes as discussed above. The previous study about the effects of high-pressure-air jet pressure and temperature on the power cylinder of HPE showed that this combustion mode has intensified low-temperature reaction and two-stage high-temperature reaction. Low air jet pressure and high air jet temperature can obtain high thermal efficiency [35]. This paper aims to investigate the effects of high-pressure air jet duration and timing on the combustion phasing control by using the numerical simulation method in the power cylinder of HPE.
2. Hybrid pneumatic engine with compound thermodynamic cycle In order to control the phasing of premixed compression ignition combustion, compound thermodynamic cycle—being capable of low-compression ratio, high pressure-rise ratio and highexpansion ratio—is employed in the HPE to realize the highpressure air JCCI combustion. Compound thermodynamic cycle employs the low effective compression ratio by using early or late IVC to obtain the low compression pressure and temperature, which is able to avoid the autoignition of the original mixture without any additional heat source. Then, high-pressure air is jetted into the combustion chamber and continuously compresses the original mixture near the top dead center (TDC). The temperature and pressure of the original mixture in the local region increased immediately by the high-pressure air jet compression trigger the autoignition. By this way, the combustion phasing of the premixture can be controlled directly and precisely. Because of the low effective and high geometric compression ratio employed in this
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Fig. 1. Schematic diagram of compound HPE used in the study.
combustion mode, it can enhance the thermal efficiency compared to the conventional diesel combustion mode [34]. Compound thermodynamic cycle is implemented in a modified HPE, as shown in Fig. 1. This engine mainly consists of the compressor cylinder, power cylinder, compressed air storage tank and high pressure air jet system. The compressor cylinder without fuel injection or firing is able to compress the air or pressurized gas and charge the compressed air storage tank. The power cylinder with four-stroke cycle works based on compound thermodynamic cycle. Well-mixed mixture is prepared by using early fuel injection, and proper IVC timing is employed to avoid spontaneous combustion. High-pressure air released from the compressed air storage tank is jetted into the combustion chamber through the highpressure air jet system near TDC. The original mixture in the local region is compressed further by the high-pressure air jet, inducing the autoignition and subsequent combustion. As a kind of internal combustion-air hybrid power system, HPE with compound thermodynamic cycle also has several operating modes. It can be operated in the air-power mode by releasing the high-pressure air to both the compressor and power cylinders without fuel injection and firing under the low-load conditions. It can also recycle the braking energy to improve the overall efficiency during the deceleration and downhill conditions. More details were clarified in Ref. [35]. The model of high-pressure air jetted into the power cylinder of HPE through the check valve was established to investigate the effects of air jet duration and timing on the high-pressure air JCCI combustion mode in this study.
check valve. The longitudinal section of the simulated 3D model is shown in Fig. 2, and the computational grid with local refinement is presented in Fig. 3. The diesel injector has a nozzle with spray included angle of 105° and 8 holes, and it was installed beside the check valve with a deviation angle of 15° from vertical. An improved piston geometry with a reentrant-shallow-basin bowl was used in this work, which was designed with the consideration of both the premixture preparation and high-pressure air jet. Relevant engine specifications are summarized in Table 1. A fixed injection pressure of 150 MPa, a fueling rate of 14.5 mg/cycle,
Fig. 2. Combustion chamber section.
3. Engine specifications The engine simulated in this work was a single-cylinder, naturally aspirated, high-speed direct-injection (HSDI) diesel engine with a displacement of 0.418 L. In order to ensure that no autoignition takes place in the combustion chamber without any additional heat sources, the geometry compression ratio of the engine is reduced from 19 to 12. A check valve was added vertically in the central position of the cylinder head, through which highpressure air was jetted into the combustion chamber. More details of the check valve structure can be found in the work of Zhang et al. [36]. The engine model includes the combustion chamber and
Fig. 3. Computational mesh.
W. Long et al. / Energy Conversion and Management 127 (2016) 392–403 Table 1 Engine specifications. Displacement Bore Stroke Compression ratio Piston geometry Nozzle hole diameter Nozzle hole number Spray hole cone angle IVC EVO
0.418 L 86 mm 72 mm 12.0:1 Reentrant-shallow-basin bowl 0.133 mm 8 105° 45 °CA ABDC 55 °CA BBDC
Table 2 Calculation parameters. Engine speed Intake air temperature Intake air pressure Injection pressure Start of injection (SOI) Fuel flow rate Initial temperature Initial pressure Wall temperature Piston temperature
1500 rpm 293 K 0.1 MPa 150 MPa 35 °CA BTDC 14.5 mg/cyc 330 K 1.15 bar 410 K 450 K
and a start of injection (SOI) timing of 35 °CA ATDC were employed in the simulation. The corresponding calculated parameters are listed in Table 2.
4. Numerical model, validation and mesh sensitivity study 4.1. Numerical model Three-dimensional (3D) computational fluid dynamics (CFD) code with a variety of enhanced physical and chemistry submodels was used, and the sub-models are listed in Table 3. K-zeta-f model [37] was employed to simulate the turbulent flows, as it is capable of solving the variable-density engine flows. The Kelvin-Helmholtz Rayleigh-Taylor (KH-RT) model [38] was used to simulate the breakup process of injected droplets. The spray collision model was developed by Nordin [39] with the improvement of grid independence. The spray/wall interaction model was developed by Naber and Reitz [40] with the function of predicting the sprayimpingement phenomena. Heat transfer from the wall was simulated by the Han and Reitz model [41] which accounts for the variations in the gas density and the turbulent Prandtl number in the boundary layer. A reduced n-heptane reaction mechanism, based on the detailed n-heptane reaction mechanism [42], with 29 species and 52 reactions developed by Patel et al. [43] was selected to simulate the diesel fuel chemistry, as it has similar autoignition characteristics to those of diesel. In the highpressure air JCCI combustion mode, high-pressure air jet actually increases the local compression ratio, it is still premixed compression ignition combustion. Therefore, the reduced n-heptane mechanism used in PCCI combustion mode is also suitable for this
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simulation work. Because of the early fuel injection timing ( 35 °CA ATDC) and late high-pressure air jet (near TDC), the mixture is already well-premixed before high-pressure air jet, so these sub-models used in the PCCI combustion mode still suits the highpressure JCCI combustion mode. 4.2. Model validations The relevant computational models were validated by the experimental measurements conducted by Lee [44] in a HSDI diesel engine. A nozzle with spray included angle of 130° and 8 holes, an EGR rate of 55%, a geometric compression ratio of 16, and an open-crater-type combustion chamber were used in the tested engine to realize the diesel PCCI combustion. Fig. 4 shows the comparisons between corresponding experimental and simulated data for the variation of in-cylinder pressure and heat-release rate at the start of injection (SOI) timings of 30 and 40 °CA ATDC. It can be seen that the simulated results reveals a good agreement with the measurements. 4.3. Mesh sensitivity study According to the literature [45], the mesh density might affect the simulation results, consequently, the cell sizes for the mesh sensitivity were studied as listed in Table 4, and the effects of the cell size on the combustion are shown in Fig. 5. The refined grid was used for the whole combustion chamber from 35 to 30 °CA ATDC via fixed embedding, and a finer cell size was used for the check valve. By sweeping the cell size from 0.8 to 1.2 mm for the combustion chamber, it shows that these five cases has very similar results, indicating high grid independence. With the consideration of the grid independence for all the cases, the cell sizes of 0.9 and 0.45 mm were selected for the combustion chamber and check valve, respectively. 5. Results and discussion 5.1. Effects of high pressure air jet duration on the in-cylinder combustion characteristics The high-pressure air jet durations of 6, 8, 10, 12 and 14 °CA are compared in this section under a fixed start of jet (SOJ) timing of 4 °CA ATDC, a fixed jet pressure of 8 MPa, and the jet temperatures of 400 and 500 K, as shown in Table 5. Fig. 6 presents the total air jet mass variations with different air jet durations.
Table 3 Computational models. Turbulent model Breakup model Collision model Spray/wall interaction model Heat transfer from the wall Fuel chemistry
k-zeta-f model [37] KH-RT model [38] Nordin model [39] Naber and Reitz model [40] Han and Reitz model [41] Reduced n-heptane mechanism [43]
Fig. 4. Comparisons of in-cylinder pressure and apparent heat release rate between measurements and predictions for SOI 40 and 30 °CA ATDC.
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Table 4 Mesh details for the sensitivity study. Based grid size [mm] (embedding level 0)
Cell size of the combustion chamber from 35 to 30 °CA ATDC [mm] (embedding level 2)
Cell size of the check valve [mm] (embedding level 3)
Minimum to maximum cell count
4.8 4.4 4 3.6 3.2
1.2 1.1 1 0.9 0.8
0.6 0.55 0.5 0.45 0.4
100,000–125,000 125,000–160,000 140,000–170,000 155,000–190,000 180,000–230,000
Fig. 5. Mesh sensitivity (air jet temperature = 400 K, air jet pressure = 8 MPa, start of jet = 4 °CA ATDC, air jet duration = 10 °CA).
Table 5 High-pressure air jet parameters for jet duration comparisons. Jet temperature Jet pressure SOJ Jet duration
400 and 500 K 8 MPa 4 °CA ATDC 6, 8, 10, 12, and 14 °CA
Fig. 6. Total air jet mass variations with various air jet durations.
Fig. 7 presents the velocity flow field under the air jet duration of 14 °CA and jet temperature of 400 K. At 2 and 3 °CA ATDC, as the high-pressure air is jetted into the combustion chamber through the check valve, it flows along with the piston top and
the bottom of the cylinder head. At 3 °CA ATDC, it can be seen that the velocity is close to zero in Zone A. Because the temperature and pressure increase immediately, so the spontaneous combustion occurs in this region. At 8 °CA ATDC, since the autoignition has occurred in Zone A, the velocity flow field in the combustion chamber is affected by both the high-pressure air jet and combustion. As the high-pressure air jet ends at 10 °CA ATDC, the velocity decreases significantly at 13 °CA ATDC, and a slightly high velocity can be observed near the piston top. Fig. 8 shows the numerical results of cut-planes colored by the in-cylinder temperature and n-heptane mass fraction distributions under the air jet durations of 8 and 14 °CA. As the high-pressure air flows along the piston head and the bottom surface of the cylinder head and compresses the original mixture, the temperature in the central region of the chamber rises up rapidly, subsequently, the spontaneous combustion occurs in this region as shown in Fig. 8a. High-pressure air with a longer jet duration compresses the original mixture for a longer time, leading to a higher compression rate and temperature in the local region in the cylinder. At 6 °CA ATDC, because there is still high-pressure air jet in the 14 °CA duration case, so it shows a higher temperature than that in the 8 °CA duration case in the central area. At 12 and 15 °CA ATDC, the 8 °CA duration case displays high temperature in a larger region due to less mixing of the high-pressure air and original mixture. The mass fraction distributions of n-heptane are presented in Fig. 8b to investigate its reaction process. At 6 °CA ATDC, the observation in the case of 8 °CA duration shows that a larger amount of fuel exists around the upper area of the chamber. Until 9 °CA ATDC, there is still a higher concentration of fuel in the local region compared to that in the case of the 14 °CA duration. At 12 and 15 °CA ATDC, the 8 °CA duration case also shows higher n-heptane concentration in the upper area. The fuel concentration is affected by both the jet mass of the high-pressure air and the reaction rates. More high-pressure air jet makes the mixture leaner, where the air jet flow passes. While more air jet also leads to higher peaks in the high temperature reaction and faster early-stage combustion, which can be observed in Fig. 9. The effects of air jet duration on the in-cylinder pressure and heat-release rate at the air jet temperature of 400 and 500 K are presented in Fig. 9. As discussed in the previous work [35], the high-pressure air JCCI combustion includes the intensified lowtemperature reaction and two-stage high-temperature reaction. The high-temperature reaction contains the spontaneous combustion in the local region and the followed diffused combustion. The spontaneous combustion caused by the high-pressure air jet compression was termed as the first-stage high temperature reaction, and the followed diffused combustion induced by the mixing of high-pressure air and original mixture was termed as the second-stage high temperature reaction. It should be noted that the mixture is well-mixed in both the first and second stages, even though the second stage has long reaction period in some cases. There is no obvious difference in the low-temperature reaction among the various air jet durations. While with the increase of the air jet duration, the high-temperature reaction reveals higher
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Fig. 7. Velocity flow field (air jet duration = 14 °CA, air jet temperature = 400 K).
(a) Temperature
(b) N-heptane mass fraction Fig. 8. Comparisons of in-cylinder temperature and n-heptane mass fraction distributions for the air jet durations of 8 °CA and 14 °CA (air jet temperature = 400 K).
peak values in both of the two stages and also displays a faster early combustion, except the 14 °CA duration case. It can be seen that the 6 °CA duration case shows a retarded first-stage reaction in both of the two air jet temperatures. The high-pressure air jet ends at 2 °CA ATDC in the 6 °CA duration case, while the hightemperature reaction has not occurred at that time, resulting in the slow combustion process. This phenomenon illustrates that the insufficient high-pressure air jet duration would result in over-retarded combustion, or even misfire. It can also be noted that the 14 °CA duration case has lower peak values than the 12 °CA case. This could be explained as much more high-pressure air jet
results in less mixture burning in the first-stage reaction due to more mixing of the high-pressure air and the original mixture. Fig. 10 shows the in-cylinder mean temperature variations with different air jet durations at the air jet temperatures of 400 and 500 K. The in-cylinder mean temperature trend is mainly affected by both the high-pressure air jet and combustion rate. More air jet leads to a lower mean temperature, while a shorter combustion process can make a higher peak pressure and temperature. The increased air jet duration case reveals a more advanced but lower peak value, except for the 12 °CA duration case. This is due to its shorter combustion duration in comparison with the 10 °CA
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(a) Air jet temperature = 400 K
(a) Air jet temperature = 400 K
(b) Air jet temperature = 500 K (b) Air jet temperature = 500 K
Fig. 10. Effects of air jet duration on in-cylinder mean temperature.
Fig. 9. Effects of air jet duration on in-cylinder pressure and heat-release rate.
duration case. In addition, the higher combustion efficiency also contributes to enhance the mean temperature, which can be observed in Fig. 11. As the rapid mixing of the high-pressure air and original mixture results in a small amount of overlean mixture and local lowtemperature region in the combustion chamber, where it would be too lean to combust sufficiently, so the combustion efficiency should be considered in high-pressure air JCCI combustion mode. Fig. 11 shows the heat-release percentage of the total energy under different air jet durations. On the one hand, the spontaneous combustion with more fuel in the first-stage reaction leads to a shorter burning duration in the second-stage reaction, and the faster combustion process is beneficial for reducing the mixing time of highpressure air and original mixture, and therefore improving the combustion efficiency. On the other hand, more air jet could induce more mixing of the high-pressure air and original mixture, thus leading to more overlean mixture and larger low-temperature region, which reduces the combustion efficiency, as shown in Fig. 8. Both of these two reversed factors impact on the combustion efficiency. As the jet duration is increased from 6 to 10 °CA, the combustion efficiency is decreased. This phenomenon indicates that the combustion efficiency is mainly dominated by the jet mass of the high-pressure air during these air jet durations. At the case
Fig. 11. Effects of air jet duration on heat-release percentage of total input energy.
of 12 and 14 °CA duration, the combustion efficiency is improved due to the rapid combustion process both in the first- and second-stage high-temperature reactions. It is worth noting that a higher air jet temperature could help to improve the combustion efficiency significantly.
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the two air jet temperatures, which is primarily caused by the slowdown of the combustion process in comparison with the case of 12 °CA jet duration. For the burning duration comparison, the longer burning duration can be found in all the jet duration cases under the jet temperature of 400 K despite using a shorter burning duration from 10% to 80% burning rate of the accumulated heatrelease. The burning duration is attributed to the combustion rate related to the two-stage high-temperature reaction. More air jet makes the spontaneous combustion occur with more mixture. In the meantime, more mixing of the high-pressure air and original mixture reduces the post combustion rate. These two factors shows the reversed effects on the burning duration. It can be noted that the 10 °CA air jet duration for the 400 K case shows a longer burning duration. This is because the fact that it has very slow post combustion rate due to the mixing of high-pressure air and original mixture. Obviously, the 12 and 14 °CA jet durations can obtain short durations, which indicates that longer air jet duration over a certain point can accelerate the overall combustion process. 5.2. Effects of high pressure air SOJ timing on the in-cylinder combustion characteristics This section investigates the effects of the high-pressure air SOJ timing on the in-cylinder combustion characteristics. The SOJ Fig. 12. Effects of air jet duration on combustion phasing and burning duration.
In Fig. 12, CA50—the crank angle at 50% burning rate of the accumulated heat release—was employed to represent the combustion phasing. CA10-CA90 was used to represent the burning duration for air jet temperature of 500 K, while CA10-CA80 was employed to represent the burning duration for the 400 K cases due to the incomplete combustion. By increasing the air jet duration from 6 to 12 °CA, CA50 is advanced gradually due to the rapid combustion process in both the first- and second-stage hightemperature reactions. Further increasing the air jet duration to 14 °CA, a slight retarded combustion phasing can be found in both
Table 6 High-pressure air jet parameters for SOJ timing comparisons. Jet temperature Jet pressure Start of jet (SOJ) Jet duration
400 and 500 K 7 MPa 12, 9, 6, 3, and 0 °CA ATDC 10 °CA
(a) Air jet temperature = 400 K
(b) Air jet temperature = 500 K Fig. 13. Total air jet mass variations with various SOJ timings.
Fig. 14. Effects of SOJ timing on in-cylinder pressure and heat-release rate.
400
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timings were swept from 12 to 0 °CA ATDC at a fixed jet duration of 10 °CA, a fixed jet pressure of 7 MPa, and the jet temperatures of 400 and 500 K, as listed in Table 6. The total air jet mass with different SOJ timings and temperatures is shown in Fig. 13. Fig. 14 shows the effects of high-pressure air SOJ timing on the in-cylinder pressure and heat-release rate at the air jet temperatures of 400 and 500 K. In Fig. 14a, the peak of the lowtemperature reaction is reduced gradually with the SOJ timings retarded from 9 to 0 °CA ATDC. This is mainly because that a retarded SOJ timing is more likely to induce a leaner mixture in the region where the low-temperature reaction occurs, and subsequently results in less fuel involved in the cool flame. It can be noted that SOJ 12 has a lower peak value of low-temperature reaction compared to SOJ 9. As SOJ 12 has the lowest incylinder temperature during the high-pressure air jet, the cool flame might occurs in a smaller region in the chamber, resulting in a lower peak. For the high-temperature reactions, SOJ 6 reveals the highest peak value among these SOJ timings in the first-stage reactions. On the one hand, as shown in Fig. 15, the temperature distribution at the end of air jet shows that the spontaneous combustion occurs in a smaller region with a SOJ timing earlier than 6 °CA ATDC. On the other hand, a more homogeneous mixture with a retarded SOJ timing makes the spontaneous combustion occur in the region with a lower equivalence ratio, resulting in a lower peak value for a SOJ timing later than 6 °CA ATDC. For the pressure traces at the air jet temperature of 400 K, it appears that the SOJ 12 and 9 cases present lower peaks than that in the SOJ 6 case due to the longer combustion process and incomplete combustion, which will be discussed below. While at the air jet temperature of 500 K, since the higher air jet temperature enhances the combustion process and combustion efficiency in the SOJ 12 and 9 cases, the peak pressure reveals an expectedly increased trend with an advanced SOJ timing other than the 400 K cases. The effects of high-pressure air SOJ timing on the in-cylinder mean temperature are presented in Fig. 16. Since the amount of the high-pressure air jet is almost constant at a fixed jet temperature, as shown in Fig. 13, the in-cylinder temperature is mainly affected by the SOJ timing, burning duration and combustion efficiency in this section. The fluctuations near TDC are caused by the high-pressure air jet and low-temperature reaction. In Fig. 16a, SOJ 6 presents the highest peak value which is consistent with its pressure trace. It can be noted that SOJ 12 reveals a low peak value even with the most advanced SOJ timing due to the long burning duration and incomplete combustion. With an increased air jet temperature as shown in Fig. 16b, the peak value trend of the in-cylinder temperature is almost consistent with the SOJ timings, while the enhanced peak value in SOJ 12 is still slightly lower compared to that in SOJ 9. There exists an optimum SOJ timing for obtaining the highest combustion efficiency at a fixed jet temperature, as shown in Fig. 17. The combustion efficiency becomes worse with the SOJ timing later or earlier than this optimum timing. This phenomenon
can be explained by using in-cylinder n-heptane mass fraction distributions in combination with the heat-release rate traces. In Fig. 18, it can be seen that most of n-heptane concentrates in the area near the cylinder wall at the SOJ 12 timing. In the highpressure air JCCI combustion mode, the spontaneous combustion occurs in the central region in the chamber, then the combustion propagates to the surrounding regions. While the slow combustion process in SOJ 12 induces more time for the mixing of the highpressure air and original mixture in the area near the cylinder wall. The lean mixture and low temperature in this region limit the chemical reactivity, resulting in the incomplete combustion. In the SOJ 0 case, the retarded SOJ timing makes the mixture more
(a) Air jet temperature = 400 K
(b) Air jet temperature = 500 K Fig. 16. Effects of SOJ timing on in-cylinder mean temperature.
Fig. 15. Comparisons of in-cylinder temperature distribution at the end of air jet for different SOJ timings (air jet temperature = 400 K).
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Fig. 17. Effects of SOJ timing on heat-release percentage of total input energy.
homogeneous, resulting in more fuel involved in the mixing of the high-pressure air and original mixture, which also leads to a more incomplete combustion. Comparatively, SOJ 6 reveals a lower n-heptane concentration in the combustion chamber, as observed at 30 °CA ATDC. While the combustion efficiency can be improved significantly with a higher jet temperature, and it can also be optimized with the adjusted diesel injection timing. It can be seen from the upper portion of Fig. 19 that despite jetting with the advanced SOJ timings of 12 and 9 °CA ATDC, their CA50 s are retarded compared to that in SOJ 6. This is mainly because the fact that they have less fuel burned in the first-stage high-temperature reactions and longer duration in the secondstage high-temperature reactions. While with a higher jet temperature as shown in the 500 K cases, CA50 is consistently retarded with the late SOJ timing. The burning duration comparisons are
Fig. 19. Effects of SOJ timing on combustion phasing and burning duration.
shown in the lower portion of Fig. 19, and because of the combustion efficiency issue, CA10-CA81, CA10-CA89 and CA10-CA77 were employed to represent the burning durations of SOJ 12, SOJ 3 and SOJ 0, respectively. For these two air jet temperatures, it is obvious that higher jet temperature shortens the burning duration significantly. It appears that this combustion mode exists an optimum SOJ timing for getting the shortest burning duration at each jet temperature with other fixed operating conditions. The firststage high-temperature reaction plays an important role in
Fig. 18. Comparisons of in-cylinder n-heptane mass fraction distributions for different SOJ timings (air jet temperature = 400 K).
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accelerating the combustion process, and more mixture involved in the first-stage reaction can leads to a faster second-stage reaction, and therefore an overall shorter burning duration. In the current work, according to the results of combustion efficiency and burning duration, it appears that the optimum SOJ timing is 6 °CA ATDC among these timings under the air jet duration of 10 °CA and jet pressure of 7 MPa.
6. Conclusion This study aims to investigate the effects of air jet duration and SOJ timing on the high-pressure air JCCI combustion characteristics based on the compound thermodynamic cycle in the power cylinder of HPE. The 3D CFD numerical simulation coupled with the reduced n-heptane chemical kinetics mechanism was developed to analyze the high-pressure air JCCI combustion process. Highpressure air JCCI combustion has two-stage high-temperature reaction due to the spontaneous combustion in the local region and subsequent combustion in the cylinder, and the combustion phasing can be controlled directly by adjusting the air jet parameters. According to the results of the air jet duration sweeping from 6 to 14 °CA and the SOJ timing from 12 °CA ATDC to TDC, the following conclusions can be drawn: (1) The high-pressure air jet durations from 8 to 14 °CA have little effect on the start of combustion, however, the 6 °CA jet duration retards the start of high-temperature reaction due to the low air jet compression rate. The combustion phasing (CA50) is advanced gradually with the increased air jet duration from 6 to 12 °CA. While the 14 °CA jet duration presents a slightly retarded combustion phasing compared to the 12 °CA jet duration. (2) The peak values in both the first- and second-stage hightemperature reactions are increased with the air jet durations from 6 to 12 °CA, however, the peaks with air jet duration of 14 °CA start to reduce. It appears that CA50 trend is consistent with the variation of the peaks. Additionally, a longer air jet duration also leads to more mixing of the highpressure air and original mixture, resulting in larger low temperature region in the combustion chamber. While, a higher jet temperature can improve the combustion efficiency and burning duration significantly. (3) The results of SOJ timing sweep reveals that the combustion phasing is controllable. The start of combustion delays progressively with the retarded SOJ timing. For the air jet temperature of 500 K, the combustion phasing is consistently retarded with the late SOJ timing. While for the air jet temperature of 400 K, the combustion phasing is firstly advanced and then retarded, which is mainly caused by the different combustion rates. (4) There exists an optimum timing for obtaining the highest combustion efficiency and shortest burning duration. This optimum timing leads to more mixture involved in the first-stage high-temperature reaction and induces a faster second-stage high-temperature reaction. The rapid combustion process with an optimized SOJ timing shortens the mixing duration of the high-pressure air and original mixture, and accordingly improves the combustion efficiency.
Acknowledgment This work is gratefully acknowledged by the National Natural Science Foundation of China (Grant Nos. 51076024 and 51379034).
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