Investigation of Effects of Air Jet Pressure and ... - ACS Publications

Article pubs.acs.org/EF

Investigation of Effects of Air Jet Pressure and Temperature on HighPressure Air Jet Controlled Compression Ignition Combustion Based on a Novel Thermodynamic Cycle Xiangyu Meng, Mingqi Zuo, Wuqiang Long,* Jiangping Tian, and Hua Tian Institute of Internal Combustion Engine/National Engineering Research Center of Shipbuilding, Dalian University of Technology, Dalian 116024, China ABSTRACT: To control premixed charge compression ignition (PCCI) phasing in internal combustion engines, a novel method called high-pressure air jet controlled compression ignition (JCCI) based on compound thermodynamic cycle was proposed to achieve diesel premixed combustion in all load operations. The method is implemented in a hybrid pneumatic engine (HPE). The application of HPE is also beneficial to the fuel consumption and emissions because of its several flexible operation modes. The purpose of this paper is to investigate the in-cylinder high-pressure air JCCI combustion processes in the power cylinder of HPE. A three-dimensional (3D) computational fluid dynamics (CFD) model coupled with reduced n-heptane chemical kinetics has been developed to analyze the effects of high-pressure air jet pressure and temperature on the in-cylinder ignition and combustion characteristics. The results demonstrated that the diesel premixture combustion phasing can be controlled near the top dead center (TDC) with the variations of air jet pressure and temperature. For the combustion characteristics, the local region of the in-cylinder mixture is compressed and ignited by the high-pressure air jet. Intensified lowtemperature reaction and two-stage high-temperature reaction can also be observed. Low jet pressure and high jet temperature have the potential of obtaining high efficiency due to the rapid combustion. EGR was often used in diesel PCCI engines.7 The combustion phasing is retarded significantly with the increased EGR rate; consequently, the peak value of heat release rate and in-cylinder pressure are reduced. However, a high EGR rate in PCCI combustion is generally harmful to engine performance. Many researchers have also been focusing on using the multiple injection strategies to implement the PCCI combustion.12,13 Multiple injections including pilot and post injections are wellknown as a beneficial method to control the combustion phasing. Comprehensive application of multiple techniques is usually taken simultaneously to accomplish the combustion phasing control. Park and Bae7 studied the effects of EGR and multiple injections on PCCI combustion in a single-cylinder diesel engine at part loads conditions. Aleiferis et al.5 also experimentally and computationally investigated the possibility of combustion phasing control with both inlet air heating and internal EGR in a high-swirl, low compression ratio engine. They found that, by increasing the inlet air temperature from 25 to 210 °C, the combustion phasing was slightly advanced by 10.6 °CA with 40% internal EGR. The hybrid pneumatic engine (HPE) concept, which combines a conventional internal combustion engine with a pneumatic storage system, instead of using an expensive battery in an electric hybrid engine, has become an interesting method to realize both low fuel consumption and emissions. The HPE concept as a new cycle for automobile engines was proposed by Schechter in 1999.14 Several researchers have estimated the efficiency and emissions based on this new cycle.15−19 Its

1. INTRODUCTION Premixed charge compression ignition (PCCI) combustion for diesel engines has its potential to reduce both nitrogen oxides (NOx) and particulate matter (PM) simultaneously while maintaining high thermal efficiency. For its low volatility, diesel fuel is generally injected into the cylinder directly before top dead center (TDC), which was called diesel hot premixed combustion (DHPC) mode in the early 1980s.1 In PCCI combustion mode, the air−fuel mixture is well mixed before combustion occurs. Therefore, the formation of NOx and PM can be suppressed because of the reduction of rich region in the combustion chamber. Nevertheless, the control of combustion phasing is still one of the crucial issues in determining whether the diesel PCCI engine could be commercialized or not. In recent years, a considerable amount of research effort has been involved in the controlling of diesel PCCI combustion phasing. Several potential control methods have been conducted, like fuel−air mixing enhancement,2−4 intake charge heating system,5 external exhaust gas recirculation (EGR),6,7 internal EGR,5 variable compression ratio (VCR),8,9 variable valve timing (VVT),10,11 and multiple injection strategies.12,13 One potential methodPCCI engines fitted with the VCR technologywas considered to control the combustion phasing and extend the clean combustion operation range with decreased compression ratio at high loads. However, the practical feasibility, expensive costs, and machining effort are the application constraints of the VCR technology.8,9 VVT was supposed to be another feasible approach to be applied in PCCI combustion. With a late intake valve closing (IVC) timing, the combustion phasing can be retarded, and the range of engine load could also be extended, while the combustion efficiency generally decreases due to the pumping loss.10,11 © 2015 American Chemical Society

Received: August 12, 2015 Revised: December 14, 2015 Published: December 15, 2015 674

DOI: 10.1021/acs.energyfuels.5b01842 Energy Fuels 2016, 30, 674−683

Article

Energy & Fuels flexible operation modes, like air compression mode, air power mode, firing and charging mode, and normal engine firing mode, enable the engine to run in a range of high-efficiency loads all the time. The utilization of compressed air power on engine cool starts and low-load conditions could significantly reduce the overall emissions. The Scuderi group20−22 proposed the split-cycle engine with a split-cycle design, which divided the conventional four strokes over two cylinders, connected by a crossover port. This engine could accomplish the Miller cycle with turbocharging and reach an extremely extended expansion ratio with a downsized compressor cylinder. As mentioned by the Scuderi group,20 it could attain a compression ratio of 96 and an expansion ratio of 50. Dimitrova and Maréchal16 also simulated the efficiency of the HPE powertrain with a threecylinder gasoline engine on urban usage. The results showed that HPE is suited for urban usage very well, having an efficiency improvement of 20−50% and low fuel consumption resulting in only 51 g of CO2/km. Fazeli et al.23 evaluated the braking regenerative energy recycling capability by utilizing low-pressure and high-pressure storage tanks to restore the kinetic energy when a vehicle is decelerating. The cylinder compresses the air from both the atmosphere air and the lowpressure tank and then charges the high-pressure tank and the low-pressure tank during the compression stroke. The experimental results showed that it realized 125% improvement in energy recycling by using the double-tank system compared to the single-tank system. The technologies used in PCCI combustion as mentioned above are indirect approaches to control the chemicalthermophysical properties of mixture, resulting in a slow response to load variations and unavoidable advanced combustion at high loads. To directly control the combustion phasing of diesel premixed compression ignition, a novel method called high-pressure air JCCI, which is implemented in HPE, was investigated in this paper. Several engine combustion concepts have been investigated by other researchers to realize the premixed compression ignition combustion with proper fuel properties. Gasoline compression ignition (GCI) concept,24,25 for example, employs low-cetane fuels to offer low emissions and high efficiency with combustion phasing control, implemented in conventional engines. Comparatively, the highpressure air JCCI combustion is implemented in HPE engine with the compound thermodynamic cycle, while it reduces the specific requirements of fuel properties. In the previous work,26 the process of high-pressure air jetted into the constant combustion chamber was studied. The results showed that the mixture temperature and pressure in the local region of the chamber increased rapidly by the high-pressure air jet compression. The maximum temperature rise was more than 150 K, which is sufficient to allow the mixture at the critical conditions to meet the autoignition conditions. By employing this approach, the combustion phasing can be controlled directly by the high-pressure air jet. The main purpose of this paper is to investigate the in-cylinder high-pressure air JCCI combustion process in the power cylinder of HPE.

cycle is called high-pressure air jet controlled compression ignition (JCCI). Figure 1 presents the P−V indicator diagram

Figure 1. Compound thermodynamic cycle.

of the compound thermodynamic cycle. Compared to the conventional thermodynamic cycle of diesel engines, there are three main differences: First, the effective compression ratio is reduced by late IVC; thereby the decreased compression temperature and pressure near top dead center (TDC) are obtained to inhibit the autoignition of the premixture. Point 1b represents the IVC timing. Then, a certain amount of highpressure air is jetted into the cylinder and mixes with the original premixture again near TDC. The in-cylinder temperature and pressure of the local region rise rapidly by the highpressure air jet compression, which could induce the autoignition of the premixture actively to achieve rapid combustion. The curve from points 2a to 2b shows the duration of highpressure air jet near TDC. Besides, as a relatively high geometric compression ratio, the expansion ratio is much larger than the effective compression ratio, leading to a higher thermal efficiency. Compound HPE used in this paper, being a kind of internal combustion−air hybrid power system, is divided into a compressor cylinder and a power cylinder, combined with a compressed air storage tank, as shown in Figure 2. In moderate

Figure 2. Schematic diagram of compound HPE used in this work.

operating conditions, the compressor cylinder has the intake and compression strokes. It compresses the pressurized gas produced by exhaust gas turbocharger, where the gas is composed of fresh air and exhaust gas introduced from the EGR valve. Then it charges the compressed air storage tank. The power cylinder works as a fully four-stroke cycle with naturally aspirated or turbocharged intake. Fuel injection occurs in the early intake stroke or compression stroke to form a wellmixed mixture; meanwhile, suitable valve timing is used to achieve low effective compression ratio and ensure that the

2. COMPOUND THERMODYNAMIC CYCLE AND HPE CONCEPT To directly control the combustion phasing of diesel premixed compression ignition, a compound thermodynamic cycle with low-compression ratio, high pressure-rise ratio, and highexpansion ratio was proposed by Long,27−29 which is implemented in HPE. The method based on the compound 675


Article

Energy & Fuels autoignition of the premixture cannot occur without any additional heat source. High-pressure air supplied from a compressed air storage tank is jetted into the power cylinder through a high-pressure air jet system near TDC. It compresses the in-cylinder premixture, leading to the autoignition of the local region. Thus, the combustion phasing is controllable by the high-pressure air jet, and the premixed combustion could be accomplished in full loads. In the low-load conditions, highpressure air can be released to the compressor cylinder and power cylinder to power the engine without fuel injection or combustion. In the high-load conditions, the compressor cylinder can work as an air-powered cylinder to output power. During vehicle deceleration or downhill operations, the engine can recycle the braking energy, saved as highpressure air in the compressed-air storage tank. The model of high-pressure air jetted into the power cylinder through the check valve was established. The impacts of the air jet parameters, like temperature and pressure, on diesel premixed combustion in the power cylinder were studied.

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 high-pressure air jet temperature high-pressure air jet pressure

3. ENGINE SPECIFICATIONS The engine model simulated in this paper was derived from a single-cylinder, naturally aspirated, high-speed direct-injection (HSDI) diesel engine with a displacement of 0.418 L. One major modification of the original engine was to decrease the geometry compression ratio from 19 to 12, ensuring that no autoignition occurs in the chamber without any additional heat source. High-pressure air was jetted into the combustion chamber through a check valve, which was added in the central position of the cylinder head to prevent backfire. The structure of the check valve can be found in the experimental work of Zhang et al.30 The diesel injector was mounted beside the check valve with a deviation angle of 15° from vertical. A longitudinal section of the simulated 3D combustion chamber model is shown in Figure 3. A new piston geometry with

1500 rpm 293 K 0.1 MPa 150 MPa 35 °CA BTDC 14.5 mg/cyc 300, 400, 500, and 600 K 6, 7, 8, and 9 MPa

Figure 4. Computational mesh.

chemistry submodels, as listed in Table 3, was used in the work. The turbulent flows were modeled by using the k-zeta-f Table 3. Computational Models turbulent model breakup model collision model spray/wall interaction model heat transfer from the wall combustion model

Figure 3. Combustion chamber section.

k-zeta-f model31 KH−RT model32 Nordin model33 Naber and Reitz model34 Han and Reitz model35 reduced n-heptane mechanism36

model31 to deal with variable-density engine flows. The Kelvin−Helmholtz Rayleigh−Taylor (KH−RT) model32 was employed to simulate the breakup process of droplets. The collision model used in this study was proposed by Nordin.33 The spray/wall interaction model adopted here was the Naber and Reitz model,34 capable of predicting the spray-impingement phenomena. Heat transfer from the wall was developed by Han and Reitz35 and accounts for the variations in the gas density and the turbulent Prandtl number in the boundary layer. A reduced n-heptane reaction mechanism with 29 species and 52 reactions developed by Patel et al.36 was adopted to calculate the diesel fuel chemistry, because n-heptane has similar autoignition characteristics to those of diesel. 4.2. Model Validations. The computational models related to this work were validated by the PCCI engine experiments

reentrant-shallow-basin bowl was used in this study, which was beneficial for both mixture preparation and high-pressure air flow. An eight-hole nozzle with a spray included angle of 105° was used, and the injection pressure was kept at 150 MPa. The relevant engine specifications are listed in Table 1. This research focused on the part-load operating conditions with a fueling of 14.5 mg/cycle and an engine speed of 1500 rpm. The corresponding operating conditions tested in this paper are shown in Table 2. A high-quality mesh composed of 159 044 cells at TDC was used to model the tested engine, and the computational grid is shown in Figure 4.

4. NUMERICAL METHOD 4.1. Numerical Model. AVL Fire computational fluid dynamics (CFD) code coupled with various physical and 676


Article

Energy & Fuels from the work of Lee.37 To realize low-temperature premixed combustion in part loads, a nozzle with spray included angle of 130° and 8 holes, ∼55% EGR, a compression ratio of 16.0, and an open-crater-type combustion chamber were designed in the experiments. The comparisons between predicted and measured data are shown in Figure 5. Start of injection (SOI)

Figure 6. Total in-cylinder mass variations with four air jet pressures.

distributions for the air jet pressures of 6 and 9 MPa. Overall, the high-pressure air flows along the piston head and the bottom surface of the cylinder head and mixes with the original mixture. The temperature in the local region of the original mixture starts to rise up rapidly by the compression of highpressure air. At 5 °CA BTDC, as the high-pressure air is jetted into the cylinder, the jet velocity in the 9 MPa case is higher than that in the 6 MPa case, leading to a higher local temperature. At TDC, the temperature distribution displays an obvious stratification with the higher temperature in the upper region. The air with a higher jet pressure compresses the original mixture faster, leading to a higher-temperature rising rate in the local region, which makes the autoignition occur earlier. At 5 °CA ATDC, the 6 MPa case presents a higher maximum temperature and a larger high-temperature region than those of the 9 MPa case, indicating that a lower jet pressure allows spontaneous combustion with more mixture. At 10 °CA ATDC, the 6 MPa case also displays high temperature in a larger region than that in the 9 MPa case. Figure 8 presents the results of cut-planes colored by the incylinder pressure distributions for the air jet pressures of 7 and 8 MPa. The in-cylinder pressure shows an apparent stratification, resulting from the high-pressure air compression. At 5 °CA BTDC, it can be seen that Zone Q in the 8 MPa case displays a higher pressure than that in the 7 MPa case due to its higher compression rate. At 2 °CA BTDC and 1 °CA ATDC, the pressure in the 8 MPa case is still higher than that in the 7 MP case. Because the low-temperature reaction has occurred, the in-cylinder pressure distribution is affected by both the high-pressure air jet and the chemical reactions. As the highpressure air is jetted into the cylinder, the local temperature and pressure in the upper region increase simultaneously; therefore, this method can be considered to increase the local compression ratio. The reduced n-heptane mechanism was used in this work to simulate the diesel combustion; the mass fraction distributions of n-heptane are shown in Figure 9 to investigate the effects of the jet pressure on the reaction process. At 5 °CA BTDC, the higher concentration of n-heptane centralizes in the outer region of the cylinder for each jet pressure case, where the jet air has not passed by. The 6 MPa case presents a higher nheptane concentration in the local region relative to the 9 MPa case. At TDC, as the high-pressure air flows along the piston head, the higher concentration of n-heptane moves to the upper region near the cylinder head. Comparatively, the 9 MPa case displays a lower concentration due to the earlier autoignition and the faster mixing of the high-pressure air and original mixture. While at 5 °CA ATDC, the 9 MPa case reveals a higher concentration because of the slower combustion process. At 10 °CA ATDC, the 9 MPa case still shows a higher

Figure 5. Comparisons of in-cylinder pressure and apparent heatrelease rate between measurements and predictions.

timings of various cases were simulated sweeping from 40 to 5 °CA BTDC. Figure 5 shows the calculated results of pressure and apparent heat-release rate with corresponding experimental data for SOI 40 °CA BTDC and SOI 30 °CA BTDC. It can be seen that the predicted pressure and heat-release rate are wellmatched with the experimental data.

5. RESULTS AND DISCUSSION 5.1. Effects of High-Pressure Air Jet Pressure on the In-Cylinder Combustion Characteristics. This section shows the effects of the high-pressure air jet pressure on the in-cylinder combustion characteristics at a fixed jet mass of 380 mg, a jet timing of 7° BTDC, and a jet temperature of 400 K. The air jet durations of 6, 7, 8, and 9 MPa are 11, 9, 7.75, and 6.75 °CA, respectively. Figure 6 shows the total in-cylinder mass with the variation of air jet pressure. The air flow rates vary with air jet pressure due to the various densities and velocities. 5.1.1. In-Cylinder Temperature, In-Cylinder Pressure, and n-Heptane Mass Fraction Distributions. Figure 7 shows the results of cut-planes colored by the in-cylinder temperature 677


Article

Energy & Fuels

Figure 7. Comparisons of in-cylinder temperature distribution for air jet pressures of 6 and 9 MPa.

Figure 8. Comparisons of in-cylinder pressure distribution for air jet pressures of 7 and 8 MPa. Figure 10. Effects of air jet pressure and nonair jet on in-cylinder pressure.

concentration in the outer region. This observation indicates that the autoignition occurs earlier, but the combustion process develops slowly with higher jet pressure. 5.1.2. Effects of Air Jet Pressure on In-Cylinder Pressure and Heat-Release Rate. The in-cylinder pressure variations with the different air jet pressures and nonair jet are shown in Figure 10. At 7 °CA BTDC and TDC, the in-cylinder pressures increase dramatically caused by the high-pressure air jet. With the increase of jet pressure, the pressure curve presents a higher pressure-rising rate due to the shorter jet duration at the same mass of the high-pressure air, as shown in Figure 6. The decreased and retarded peak pressure trend can also be observed for an increased jet pressure. The 6 MPa case shows the lowest value during the air jet period but presents the highest and most advanced peak pressure in the following period, which demonstrates that its pressure-rising rate is the highest due to the rapid combustion process. In contrast, the peak pressure in the 9 MPa case decreases by 1.7 MPa compared to that in the 6 MPa case. This decline results from the overmixing effects, which can be observed from the temperature and n-heptane mass fraction distributions in the 9

MPa case in Figures 7 and 9. At 5 °CA ATDC, it can be seen that the initial combustion occurs in a small region with a low reaction temperature. At 10 °CA ATDC, the temperature in the rich n-heptane region is below 1000 K due to the fast mixing of the original mixture and the high-pressure air, leading to a slow combustion process. In addition, the overmixing mixture also results in insufficient combustion, which can be observed in Figure 13. The effects of air jet pressure and nonair jet on the incylinder heat-release rates are shown in Figure 11. It can be seen that only a small amount of low-temperature reactions occur with the nonair jet, which can be considered as a misfire. Overall, there are an intensive low-temperature reaction and a two-stage high-temperature reaction for each case. The lowtemperature reaction is intensified due to the rapid temperature rising in the local region and the intensive turbulent motion in the cylinder. For a higher jet pressure, the low-temperature reaction displays a higher reaction rate and a higher peak value. For the high-temperature reaction, the first-stage reaction

Figure 9. Comparisons of in-cylinder n-heptane mass fraction distribution for air jet pressures of 6 and 9 MPa. 678


Article

Energy & Fuels

also results in the temperature reduction. For the case of 9 MPa, the low in-cylinder temperature also results from an additional reason for the insufficient combustion, which can be observed from Figure 13.

Figure 11. Effects of air jet pressure and nonair jet on heat-release rate.

shows a high peak due to the premixed combustion in the local region, while the second-stage reaction presents a low peak and a long duration, indicating that the combustion process is slower than that in the first-stage reaction. This trend occurs because the high-pressure air has lower temperature compared to the mixture in the cylinder, and the rapid mixing inhibits the chemical reaction rate. Comparatively, an increased jet pressure leads to a more advanced and lower peak value in the first-stage reaction. The advanced peak value occurs because the mixture is compressed faster by the high-pressure air jet, which makes it reach the autoignition conditions in a shorter duration. The lower peak value occurs because less fuel is burned due to the rapid mixing process. Meanwhile, the increased jet pressure results in a lower peak value and a longer combustion process in the second-stage reaction. As the mixing rate has a significant impact on the combustion process, the inhibition effect on the second-stage reaction is more evident. In the 9 MPa case, the curve shows the lowest peak value and the longest combustion duration, which can also explain the fact of the excessively decreased peak pressure in Figure 10. For the overall characteristic, the shape of the high-temperature reaction curve is similar to that of the conventional diesel diffusion combustion with a long trail. However, high-pressure air JCCI is well-premixed combustion for the whole combustion process. 5.1.3. Effects of Air Jet Pressure on In-Cylinder Mean Temperature. Figure 12 shows the in-cylinder temperature

Figure 13. Effects of air jet pressure on combustion efficiency.

5.1.4. Effects of Air Jet Pressure on Combustion Efficiency. Because of the rapid mixing of the high-pressure air and the original mixture, the in-cylinder mixture in the local region might be too lean to burn sufficiently; therefore, the combustion efficiency should be investigated under different jet pressures. Figure 13 shows the n-heptane mass fraction and the heat-release percentage of total input energy. The combustion efficiency is mainly affected by the second-stage reaction, and the increased jet pressure results in more overlean mixture and lower local temperature due to the overmixing issue. For the case of 9 MPa, the final n-heptane mass fraction is ∼0.1%, and the heat-release percentage in total is ∼75%, which illustrates that a spot of fuel is still unburned. It can be concluded that the combustion efficiency can be improved by decreasing the air jet pressure, while the sufficient jet pressure need to be employed to ensure that the autoignition occurs in the local region. 5.1.5. Effects of Air Jet Pressure on CA10, CA50, and CA10−CA90. In Figure 14, CA10, CA50, and CA10−CA90 are defined as the start of combustion (SOC), combustion phasing, and burning duration, respectively. Because of the insufficient combustion issue, as shown in Figure 13, CA10−CA75 are presented in the 9 MPa case. For an increased air jet pressure, Figure 14 displays a slightly advanced SOC, a retarded

Figure 12. Effects of air jet pressure on in-cylinder mean temperature.

variations with the different air jet pressures. It can be found that there are two fluctuations near TDC. The first fluctuation is caused by the high-pressure air jet with low temperature and the low-temperature reaction in the chamber. The second fluctuation is induced by the lack of the high-temperature reaction during the downward movement of the piston. The increased jet pressure shows a lower and retarded peak temperature. This is because the longer combustion process produces a lower in-cylinder pressure and temperature. Meanwhile, the expansion of compressed high-pressure air

Figure 14. Effects of air jet pressure on CA10, CA50, and CA10− CA90. 679


Article

Energy & Fuels

seen in the 600 K case. Overall, the combustion in the 600 K case shows a larger autoignition region and high-temperature region. This trend indicates that using a higher jet temperature is beneficial for accelerating the combustion process. The in-cylinder stratification of n-heptane influenced by the air jet temperature can be observed in Figure 17. At TDC, the low-temperature reactions have occurred, which can be observed from Figure 20. The n-heptane distribution in the 600 K case shows a lower mass fraction, illustrating that more low-temperature reactions occur before TDC. From 3 to 10 °CA ATDC, the n-heptane mass fraction reduces gradually, and it decreases faster in the 600 K case. This trend indicates that a higher jet temperature is beneficial for the chemical reactions. At 10 °CA ATDC, there is still a certain amount of unburned fuel in the 300 K case, resulting from the longer combustion process. 5.2.2. Effects of Air Jet Temperature on In-Cylinder Pressure and Heat-Release Rate. The in-cylinder pressure traces under different jet temperatures are shown in Figure 18. A higher and slightly advanced peak pressure can be observed with a higher jet temperature due to the faster combustion process and the higher combustion efficiency. The higher air jet temperature also results in the higher pressure-rising rate. The peak pressure of the 600 K case increases by ∼2.2 MPa compared to that of the 300 K case. This increase indicates that a higher jet temperature is beneficial for promoting the incylinder pressure. The pressure rising after TDC significantly leads to the improvement of thermal efficiency. However, the limitation of knocking should be considered as well while promoting the in-cylinder pressure. Figure 19 shows the in-cylinder heat-release rates with the different jet temperatures. With a higher jet temperature, a slightly advanced and higher peak can be found in the lowtemperature reaction. This phenomenon indicates that the jet temperature has a slight effect on the low-temperature reaction, unlike the effects of the different air jet pressure cases. For the high-temperature reactions, two-stage reaction can also be found for each case. The same reasons have been discussed in the section 5.1.2. For the higher jet temperature, the first-stage reaction displays an advanced and a higher peak value. This trend occurs because the higher jet temperature allows premixed combustion in a larger region, as seen from the timing of 5 °CA ATDC in Figure 16. The second-stage reaction also presents a higher peak value and a shorter trail, because the inhibition of chemical reactions becomes weaker with higher jet temperature. For the single high peak of heat-release rate in PCCI combustion, the combustion typically leads to a high

combustion phasing, and a longer burning duration. All the CA10s occur between 2 °CA BTDC and TDC, which indicates that the SOC could be controlled near TDC at a jet pressure range from 6 to 9 MPa and a fixed jet timing of 7 °CA BTDC. It is worth noting that the CA50s and CA10−CA90s show a reversed trend to the CA10s. The reasons have been discussed in section 5.1.2. Because there is a slight difference among the CA10s, the air jet pressure can be adjusted to control the combustion phasing and burning duration under various engine loads and speeds. 5.2. Effects of High-Pressure Air Jet Temperature on the In-Cylinder Combustion Characteristics. To investigate the effects of the air jet temperature on the in-cylinder combustion characteristics, a variety of air jet temperatures from 300 to 600 K were simulated with a fixed air jet timing of 5 °CA BTDC, an air jet duration of 10 °CA, and an air jet pressure of 7 MPa. Total air jet mass varies with the air jet temperatures, which can be seen from Figure 15.

Figure 15. Total in-cylinder mass variations with four air jet temperatures.

5.2.1. In-Cylinder Temperature and n-Heptane Mass Fraction Distributions. Figure 16 shows the results of cutplanes colored by the in-cylinder temperature distributions for the air jet temperatures of 300 and 600 K. At TDC, the highpressure air flows along the piston head and the bottom surface of the cylinder head and mixes with the original mixture, and the temperature distribution presents an apparent stratification with a higher temperature in the upper region. Comparatively, the 600 K case displays a larger high-temperature region due to the higher jet temperature. At 5 °CA ATDC, with the continuous compression of the high-pressure air, part of the original mixture satisfies the autoignition conditions, leading to the initial combustion. The 600 K case presents a larger autoignition region relative to the 300 K case. At 10 and 15 °CA BTDC, a larger region with high temperature can also be

Figure 16. Comparisons of in-cylinder temperature distribution for air jet temperatures of 300 and 600 K. 680


Article

Energy & Fuels

Figure 17. Comparisons of in-cylinder n-heptane mass fraction distribution for air jet temperatures of 300 and 600 K.

Figure 18. Effects of air jet temperature on in-cylinder pressure.

Figure 20. Effects of air jet temperature on in-cylinder mean temperature.

Figure 19. Effects of air jet temperature on heat-release rate.

pressure-rising rate at high loads, resulting in knocking issues. Unlike PCCI combustion, high-pressure air JCCI combustion with two peaks of heat-release rate can significantly reduce pressure-rising rate and extend the load limitation. 5.2.3. Effects of Air Jet Temperature on In-Cylinder Mean Temperature. The increased air jet temperature can significantly lead to a higher in-cylinder temperature as shown in Figure 20. There are also two fluctuations for each case, as discussed in section 5.1.3. A higher jet temperature results in the autoignition in a larger region in the first-stage reaction and also accelerates the second-stage reaction for the hightemperature reaction. Meanwhile, the higher jet temperature leads to a higher combustion efficiency, which can be seen in Figure 21. These factors produce a higher in-cylinder pressure with the increased jet temperature, resulting in higher incylinder mean temperature. 5.2.4. Effects of Air Jet Temperature on Combustion Efficiency. Figure 21 shows the n-heptane mass fraction and heat-release percentage of n-heptane under the different air jet temperatures. With the increased jet temperature, the final nheptane mass fraction is reduced, and the heat-release percentage is increased progressively, which indicates that

Figure 21. Effects of air jet temperature on combustion efficiency.

increasing the air jet temperature is beneficial for improving the combustion efficiency. The main reason is because the higher air jet temperature accelerates the second-stage reaction in the high-temperature reaction as shown in Figure 19. 5.2.5. Effects of Air Jet Temperature on CA10, CA50, and CA10−CA90. Unlike the effects of the different air jet pressure cases, the three traces of CA10, CA50, and CA10−CA90 under different air jet temperatures present similar trends as shown in Figure 22. The CA10−CA78 period represents the burning duration of the 300 K due to the insufficient combustion. The SOC and the combustion phasing are advanced, and the burning duration is reduced progressively with the increased jet temperature. All the CA10s take place between TDC and 2 °CA ATDC, which demonstrates that the jet temperature has a slight influence on the SOC. However, the increased jet temperature significantly advances the combustion phasing and 681


Article

Energy & Fuels

Figure 22. Effects of air jet temperature on CA10, CA50, and CA10− CA90.

shortens the burning duration. The main reason is because the higher mixing temperature accelerates the high-temperature reaction rate in both the first-stage and second-stage reactions.

6. CONCLUSION A novel method called high-pressure air JCCI was proposed in this study. The compound thermodynamic cycle with lowcompression ratio, high pressure-rise rate, and high-expansion ratio was used to implement the combustion mode in HPE. The effects of high-pressure air jet pressure and temperature on combustion characteristics in the high-pressure air JCCI combustion mode have been studied by a 3D CFD model coupled with reduced n-heptane chemical kinetics. According to the numerical results of the jet pressure sweep from 6 to 9 MPa and the jet temperature sweep from 300 to 600 K, the following conclusions can be obtained. The in-cylinder mixture with low temperature and pressure is ignited by the high-pressure air jet compression. Thus, the combustion phasing can be controlled directly and precisely. High-pressure air jet leads to an intensified low-temperature reaction due to rapid temperature-rising rate and a two-stage, high-temperature reaction due to the autoignition in the local region and the subsequent combustion in the chamber. Using a lower air jet pressure can produce a higher and retarded peak pressure, a higher heat release peak value, and a shorter burning duration. Considering the thermal efficiency and combustion efficiency, the air jet pressure should be designed as low as possible in the circumstance that the autoignition can occur. Meanwhile, a higher air jet temperature can obtain a higher and advanced peak pressure. The higher air jet temperature significantly improves the high-temperature reaction in both stages, leading to a shorter combustion duration.

■

■

CA10 = 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 HPE = hybrid pneumatic engine HSDI = high-speed direct injection IEGR = internal exhaust gas recirculation IVC = intake valve closing JCCI = jet controlled compression ignition NOx = nitrogen oxides PCCI = premixed charge compression ignition PM = particulate matter SOI = start of injection SOC = start of combustion TDC = top dead center VCR = variable compression ratio VVT = variable valve timing

REFERENCES

(1) Hu, G. Prospect on diesel combustion research. China J. Dalian Inst. Technol. 1982, 4, 71−80. (2) Guodong, H. New strategy on diesel combustion development. SAE Tech. Pap. Ser. 1990; SAE Technical Paper 00442.10.4271/ 900442 (3) Long, W.; Ohtsuka, H.; Obokata, T. Characterization of conical spray flow for diesel engine by means of laser doppler methods.(PDA measurement of droplet size distribution). JSME Int. J., Ser. B 1996, 39 (3), 554−561. (4) Leng, X.; Feng, L.; Tian, J.; Du, B.; Long, W.; Tian, H. A study of the mixture formation process for a third-generation conical spray applied in HCCI diesel combustion. Fuel 2010, 89 (2), 392−398. (5) Aleiferis, P. G.; Charalambides, A. G.; Hardalupas, Y.; Taylor, A.; Urata, Y. Modelling and experiments of HCCI engine combustion with charge stratification and internal EGR. SAE Tech. Pap. Ser. 2005.10.4271/2005-01-3725 (6) García, M. T.; Jiménez-Espadafor Aguilar, F. J.; Sánchez Lencero, T. s. Combustion characteristics, emissions and heat release rate analysis of a homogeneous charge compression ignition engine with exhaust gas recirculation fuelled with diesel. Energy Fuels 2009, 23 (5), 2396−2404. (7) Park, Y.; Bae, C. Influence of EGR and pilot injection on PCCI combustion in a single-cylinder diesel engine. SAE Tech. Pap. Ser. 2011; SAE Technical Paper 2011-01-1823.10.4271/2011-01-1823 (8) Wos, P.; Balawender, K.; Jakubowski, M.; Kuszewski, H.; Lejda, K.; Ustrzycki, A. Design of affordable multi-cylinder variable compression ratio (VCR) engine for advanced combustion research purposes. SAE Tech. Pap. Ser. 2012; SAE Technical Paper 2012-010414.10.4271/2012-01-0414 (9) Kadota, M.; Ishikawa, S.; Yamamoto, K.; Kato, M.; Kawajiri, S. Advanced control system of variable compression ratio (VCR) engine with dual piston mechanism. SAE Tech. Pap. Ser. 2009; SAE Technical Paper 2009-01-1063.10.4271/2009-01-1063 (10) Peng, Z.; Jia, M. Full engine cycle CFD investigation of effects of variable intake valve closing on diesel PCCI combustion and emissions. Energy Fuels 2009, 23 (12), 5855−5864. (11) Murata, Y.; Kusaka, J.; Daisho, Y.; Kawano, D.; Suzuki, H.; Ishii, H.; Goto, Y. Miller-PCCI combustion in an HSDI diesel engine with VVT. SAE Tech. Pap. Ser. 2008; SAE Technical Paper 2008-010644.10.4271/2008-01-0644

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work is gratefully acknowledged by the National Natural Science Foundation of China (Grant No. 51076024). ABBREVIATIONS ATDC = after top dead center BTDC = before top dead center CFD = computational fluid dynamics 682


Article

Energy & Fuels (12) Kiplimo, R.; Tomita, E.; Kawahara, N.; Yokobe, S. Effects of spray impingement, injection parameters, and EGR on the combustion and emission characteristics of a PCCI diesel engine. Appl. Therm. Eng. 2012, 37, 165−175. (13) Torregrosa, A.; Broatch, A.; García, A.; Mónico, L. Sensitivity of combustion noise and NO x and soot emissions to pilot injection in PCCI Diesel engines. Appl. Energy 2013, 104, 149−157. (14) Schechter, M. M. New cycles for automobile engines. SAE Tech. Pap. Ser. 1999; SAE Technical Paper 1999-01-0623.10.4271/1999-010623 (15) Schechter, M. M. Regenerative compression brakingA low cost alternative to electric hybrids. SAE Tech. Pap. Ser. 2000; SAE Technical Paper 2000-01-1025.10.4271/2000-01-1025 (16) Dimitrova, Z.; Maréchal, F. Gasoline hybrid pneumatic engine for efficient vehicle powertrain hybridization. Appl. Energy 2015, 151, 168−177. (17) Bao, R.; Stobart, R. Using pneumatic hybrid technology to reduce fuel consumption and eliminate turbo-lag. SAE Tech. Pap. Ser. 2013; SAE Technical Paper 2013-01-1452.10.4271/2013-01-1452 (18) Bao, R.; Stobart, R. Study on Optimization of Regenerative Braking Control Strategy in Heavy-Duty Diesel Engine City Bus using Pneumatic Hybrid Technology. SAE Tech. Pap. Ser. 2014; SAE Technical Paper 2014-01-1807.10.4271/2014-01-1807 (19) Lee, C.-Y.; Zhao, H.; Ma, T. Analysis of a cost effective air hybrid concept. SAE Tech. Pap. Ser. 2009; SAE Technical Paper 200901-1111.10.4271/2009-01-1111 (20) Phillips, F.; Gilbert, I.; Pirault, J.-P.; Megel, M. Scuderi split cycle research engine: Overview, architecture and operation. SAE Tech. Pap. Ser. 2011; SAE Technical Paper 2011-01-0403.10.4271/2011-010403 (21) Branyon, D.; Simpson, D. Miller cycle application to the scuderi split cycle engine (by downsizing the compressor cylinder). SAE Tech. Pap. Ser. 2012; SAE Technical Paper 2012-01-0419.10.4271/2012-010419 (22) Meldolesi, R.; Badain, N. Scuderi Split Cycle Engine: Air Hybrid Vehicle Powertrain Simulation Study. SAE Tech. Pap. Ser. 2012; SAE Technical Paper 2012-01-1013.10.4271/2012-01-1013 (23) Fazeli, A.; Khajepour, A.; Devaud, C. A novel compression strategy for air hybrid engines. Appl. Energy 2011, 88 (9), 2955−2966. (24) Loeper, P.; Ra, Y.; Adams, C.; Foster, D. E.; Ghandhi, J.; Andrie, M.; Krieger, R.; Durrett, R. Experimental investigation of light-medium load operating sensitivity in a gasoline compression ignition (GCI) light-duty diesel engine. SAE Tech. Pap. Ser. 2013; SAE Technical Paper 2013-01-0896.10.4271/2013-01-0896 (25) Loeper, P.; Ra, Y.; Foster, D. E.; Ghandhi, J. Experimental and computational assessment of inlet swirl effects on a gasoline compression ignition (GCI) light-duty diesel engine. SAE Tech. Pap. Ser. 2014; SAE Technical Paper 2014-01-1299.10.4271/2014-01-1299 (26) Long, W.; Meng, X. Study of diesel premixed combustion based on a novel thermodynamic cycle. (1) Simulation analysis of high pressure air jet controlled compression ignition. China J. Eng. Thermophys. 2014, 35 (5), 1020−1025. (27) Long, W. Internal combustion engines. China Patent 200910010658.X. (28) Long, W.; Leng, X. Internal combustion engine with air supply system. China Patent 200910308415.4. (29) Long, W.; Leng, X.; Feng, L.; Sheng, K. Injection strategies for internal combustion-air hybrid engines. China Patent 200910012278.X. (30) Zhang, Q.; Long, W.; Tian, J.; Wang, Y.; Meng, X. Experimental and numerical study of jet controlled compression ignition on combustion phasing control in diesel premixed compression ignition systems. Energies 2014, 7 (7), 4519−4531. (31) Hanjalić, K.; Popovac, M.; Hadžiabdić, M. A robust near-wall elliptic-relaxation eddy-viscosity turbulence model for CFD. Int. J. Heat Fluid Flow 2004, 25 (6), 1047−1051. (32) Ricart, L. M.; Reltz, R. D.; Dec, J. E. Comparisons of diesel spray liquid penetration and vapor fuel distributions with in-cylinder optical measurements. J. Eng. Gas Turbines Power 2000, 122 (4), 588−595.

(33) Nordin, P. Complex chemistry modeling of diesel spray combustion. Ph.D. Thesis, Chalmers University of Technology, 2001. (34) Naber, J.; Reitz, R. D. Modeling engine spray/wall impingement. SAE Tech. Pap. Ser. 1988; SAE Technical Paper 880107.10.4271/880107 (35) Han, Z.; Reitz, R. D. A temperature wall function formulation for variable-density turbulent flows with application to engine convective heat transfer modeling. Int. J. Heat Mass Transfer 1997, 40 (3), 613−625. (36) Patel, A.; Kong, S. C.; Reitz, R. D. Development and validation of a reduced reaction mechanism for HCCI engine simulations. SAE Tech. Pap. Ser. 2004; SAE Technical Paper 2004-01-0558.10.4271/ 2004-01-0558 (37) Lee, S. S. Investigation of two low emissions strategies for diesel engines: Premixed charge compression ignition(PCCI) and stoichiometric combustion. Ph.D. Thesis, University of WisconsinMadison, 2006.

683


Investigation of Effects of Air Jet Pressure and ... - ACS Publications

Recommend Documents