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Effect of hydrogen addition on the combustion and emission of a diesel free-piston engine
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Effect of hydrogen addition on the combustion and emission of a diesel free-piston engine Chenheng Yuan*,1, Cuijie Han 1, Yang Liu, Yituan He, Yiming Shao, Xiaochun Jian College of Traffic & Transportation, Chongqing Jiaotong University, Chongqing, 400074, China
article info
abstract
Article history:
The free-piston engine (FPE) is a new crankless engine, which operates with variable
Received 21 November 2017
compression ratio, flexible fuel applicability and low pollution potential. A numerical
Received in revised form
model which couples with dynamic, combustion and gas exchange was established and
5 May 2018
verified by experiment to simulate the effects of different hydrogen addition on the com-
Accepted 7 May 2018
bustion and emission of a diesel FPE. Results indicate that a small amount of hydrogen
Available online xxx
addition has a little effect on the combustion process of the FPE. However, when the ratio of hydrogen addition (RH2) is more than 0.1, the RH2 gives a positive effect on the peak in-
Keywords:
cylinder gas pressure, temperature, and nitric oxide emission of the FPE, while soot
Free-piston engine
emission decreases with the increase of hydrogen addition. Moreover, the larger RH2 in-
Hydrogen addition
duces a longer ignition delay, shorter rapid combustion period, weaker post-combustion
Combustion
effect, greater heat release rate, and earlier peak heat release rate for the FPE. Neverthe-
Emission
less, the released heat in rapid combustion period is significantly enhanced by the increase
Heat release
of RH2. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In the context of severe energy shortage and serious environmental pollution, the conventional engine with low efficiency and high emission has been unable to meet the strong demand of energy saving and emission reduction [1,2]. The free-piston engine (FPE) comes into being under this background [3]. The new engine abandons the crank connecting rod mechanism of conventional engines, thus reducing the friction loss, improving the thermal efficiency, and moving with a variable compression ratio, which provides flexible fuel applicability and possibility of new clean combustion modes [4e6]. These potential advantages of FPE bring broad
application fields. It can be used not only as a driving system for hybrid electric vehicles or electric vehicles, but also as an auxiliary power for electromechanical device. In recent years, the FPE has attracted considerable research interests, and a great deal of research work has been done by scholars and research institutions around the world. Kosaka et al. made a significant contribution to the development of a single-piston FPE [7e9], including the fundamental characteristics of the FPE, the control system for generator and the control method of linear generator to improve the efficiency and stability. Roskilly [10] mainly used a computational fluid dynamics model to investigate the gas motion, combustion process and the formation mechanism of nitrogen oxides of a diesel FPE. They found that the FPE produces a stronger radial
* Corresponding author. E-mail address: [email protected] (C. Yuan). 1 Co-first author. Chenheng Yuan and Cuijie Han contributed equally to this work. https://doi.org/10.1016/j.ijhydene.2018.05.038 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Yuan C, et al., Effect of hydrogen addition on the combustion and emission of a diesel free-piston engine, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.038
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flow near the top dead center (TDC) than a conventional engine, but it has little effect on the combustion process. Lim et al. [11] studied the effects of key parameters such as equivalent ratio, ignition time and spring stiffness on the operation characteristics and performance of a spark-ignition FPE by means of numerical simulation. The results showed that reducing the ignition advance angle or increasing the spring stiffness can significantly improve the dynamic of piston, electric power output and the engine performance. Kim et al. [12] mainly studied the influence of air flow rate and alternator load on the operation characteristics of a liquefied petroleum gas FPE. Their results showed that piston dynamics are the main factors affecting indicated mean effective pressure (IMEP) when the conductance changes. They also found that the motion stroke and frequency are negatively related to the air flow rate, but the IMEP and electric output increase as the air flow rate increases. Yuan et al. [13] studied the combustion characteristics of a two-stroke FPE by a coupling simulation. They analyzed the influence of special piston motion on the combustion process of the FPE and compared the simulation results with a conventional engine. The results showed that the FPE has a faster piston motion and longer combustion duration than those of the conventional engine. Meanwhile, they found that the maximum in-cylinder pressure and temperature of FPE are lower than those of the conventional engine. The emission characteristics of FPE have been studied, some researches indicated that the FPE has the advantages of variable compression ratio, wide adaptability of fuel, and less nitric oxide (NO) emission, but some deficiencies were also found, such as high soot emission [14]. As is known to all, hydrogen fuel has the advantages of low ignition energy, fast flame propagation speed, large lean operation limit, and low hydrocarbon (HC) and carbon monoxide (CO) emissions, which is considered as an ideal alternative fuel for internal combustion engines [15,16]. There are many studies on hydrogen engines at present. Park et al. [17] investigated the effects of lean fuel and exhaust gas recirculation (EGR) on the combustion and emission performance of a heavy-duty engine with natural gas-hydrogen dual fuels. The results showed that the engine can reduce the emission and improve the efficiency by expanding the lean operation limit. Du et al. [18] studied the effect of addition of hydrogen and exhaust gas recirculation on combustion characteristics of a gasoline engine. The results found that with increase of hydrogen addition, the NO emission increases. Barrios et al. [19] experimentally studied the influence of hydrogen addition on combustion characteristics and particle number size distribution emissions of a diesel engine. Through the research they indicated that with hydrogen injection, the number of nucleation mode particles decreases in direct proportion to the total particle number, and the carbon dioxide (CO2) emissions decrease when hydrogen is injected. Jhang et al. [20] investigated experimentally the effect of hydrogen addition on the emissions of a heavy-duty diesel engine under constant speed from the low to high engine load. Their experimental results showed that the brake thermal efficiency increases with an increasing amount of hydrogen, and the hydrogen addition can reduce the emissions of CO2 and CO. At
the high operation load, the reduction in emissions was the most significant. From the above researches, it is known that adding hydrogen fuel to natural gas engines or diesel engines can improve the thermal efficiency and economy, meanwhile reduce some harmful emissions. While the FPE is a special power engine, which abandons the conventional crankconnecting rod mechanism, making the law of piston movement is different from that of the conventional engine. Some researches indicated that the FPE has higher movement speed and acceleration in late compression stroke, which brings about different ignition, fuel spray, evaporation, air-fuel mixing, heat transfer, and combustion characteristics for the FPE, compared with conventional engines [21e24]. So adding some hydrogen to the diesel FPE may have a particular effect on combustion process and emission characteristics due to the special piston motion law and variable compression ratio. However, there are few researches on hydrogen addition of the FPE. In this study, some hydrogen fuel is added to a diesel FPE, and the influence of different hydrogen addition ratios on the combustion and emission characteristics of the FPE was investigated and revealed.
Fundamental and method FPE prototype The FPE prototype presented for this study is shown in Fig. 1, which adopts a dual piston opposed structure. The FPE is composed of two free piston combustion cylinders arranged horizontally, with a linear generator arranged in the middle, and the piston rod assembly is coaxially connected with the mover of the linear generator. The engine uses a two-stroke, compression ignition work cycle. The gas exchange of the FPE is regulated by a looping scavenging way. The diesel fuel is injected directly into the cylinder at the appropriate time, and the hydrogen fuel is premixed with air. In starting process, the linear generator is used as an electric motor to initialize the engine. After starting, the motor is immediately converted into a generator. The converted electric energy can be stored or utilized directly. The main parameters of this FPE are listed in Table 1, and more information can be seen in elsewhere [22,24,25].
Simulation method Unlike conventional engines, the FPE abandons the crank-link mechanism, so the piston is “free”. Although the piston
Fig. 1 e The basic structure of the FPE. Please cite this article in press as: Yuan C, et al., Effect of hydrogen addition on the combustion and emission of a diesel free-piston engine, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.038
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Table 1 e Main parameters of the FPE. Engine parameters Engine bore Nominal effective stroke length Nominal stroke length/mm Compression ratio Mass of the mover Volume of the bowl Generator load Internal coil inductance Internal coil resistance Exhaust port height Scavenging port height Port overlapping distance
Value 60 mm 58 mm 94 variable 5.4 kg 5.65 106 m3 290 N.s/m 10.2 mH 5.4 U 25 mm 15 mm 13 mm
motion of FPE is no longer restricted by the mechanical mechanism, it is still affected by the in-cylinder gas force, friction force and generator force acting on the piston. Meanwhile the piston movement has a direct impact on intake and exhaust, compression, as well as combustion process. Therefore, there is a strong coupling relationship between the piston motion, gas exchange, and combustion process in the FPE. The simulation method generally applied to conventional engines is no longer suitable for the FPE. In this study, a comprehensive model which couples with combustion, gas exchange and piston motion was established and iteratively calculated to simulate the combustion and emission of the FPE with different hydrogen addition [14,26]. Specific modeling and iterative calculation method are shown in Fig. 2. The procedures of the iterative simulation are as follows: (1) The dynamic model was established and simulated in an initial condition to obtain the motion performance; (2) The gas exchange model was established by using the piston motion, and then it was calculated to determine the gas exchange performances, including the in-cylinder gas
3
temperature, pressure, EGR, and turbulent kinetic energy, etc.; (3) The combustion model was established and simulated based on the piston motion and the gas exchange results. (4) Using these results to update boundary conditions next iteration, and the dynamic model, gas exchange model, combustion model were re-established and simulated using new boundary. (5) The step (4) was operated repetitively until convergence.
Modeling and boundary Dynamic model The piston dynamic of FPE is determined by the resultant force acting on it, mainly including on the gas force in left and right cylinders, friction force, scavenging gas force and electromagnetic force generated by the alternator. It can be described by the Newton's second law and has been established detailedly in previous publication [13]. m
d2 x ¼ Fp þ Fe þ Ff þ Fs dt2
(1)
where, the m represents the mass of piston assembly; x is the displacement; Fp is the in-cylinder gas force; Fe stands for the alternator force; Ff denotes the friction force; and Fs is the scavenging gas force.
Gas exchange model Gas exchange of engines is aim to ensure that the exhaust gas can be driven out fully, meanwhile reduce the loss of fresh air as much as possible. The gas exchange system of the FPE was modeled using four separate regions, including exhaust duct, scavenging transfer duct, cylinder and a plenum box, which has been developed in research [27]. This research used it to perform the gas exchange simulation, but it would be updated using the results of the last iteration.
Combustion model Although the FPE has the special structure and operating principle, it still belongs to a compression ignition diesel
Fig. 2 e Iterative calculation method.
Fig. 3 e Computational mesh of combustion.
Please cite this article in press as: Yuan C, et al., Effect of hydrogen addition on the combustion and emission of a diesel free-piston engine, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.038
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Table 2 e Hydrogen scheme of intake-air composition. Scheme The mass of injected diesel/mg The mass of H2/mg The mass fraction of H2/% Fuel-air equivalence ratio
RH2 ¼ 0
RH2 ¼ 0.1
RH2 ¼ 0.2
RH2 ¼ 0.3
RH2 ¼ 0.4
7.40 0 0 0.431
6.66 0.262 0.107 0.424
5.92 0.524 0.213 0.417
5.18 0.7862 0.319 0.406
4.44 1.048 0.430 0.404
engine, and has the same essential properties as conventional diesel engines. Therefore, many combustion models suitable for conventional engines can also be used for FPE combustion simulation. In order to obtain the combustion characteristics of the FPE with hydrogen addition, AVL_Fire simulation software was used to simulate the combustion process in the cylinder. First of all, according to the combustion chamber structure of the FPE prototype, the moving mesh of combustion model was established, as shown in Fig. 3. The total number of model grids in the entire combustion chamber was 70686, and all of them were hexahedron cells. Hydrogen-mixed combustion belongs to the turbulent combustion, and turbulence plays an important role in the heat and mass transfer in combustion process, while the chemical reaction rate is affected by the temperature and the concentration of the reaction mechanism itself. So the interaction between the turbulence and the chemical reaction is the primary problem that the combustion model needs to solve. Based on the characteristics of engine turbulence combustion, ECFM-3Z model of the extended coherent flame model was selected as the combustion model [28], which was developed by the Groupement Scientifique Moteurs consortium specifically for both compression ignition and sparking ignition combustion. This model is based on a flame surface density transport equation and a mixing model that can describe inhomogeneous turbulent premixed and diffusion combustion. Besides, it is fully coupled to the spray model and enables stratified combustion modeling including EGR effects and NO formation. Before combustion, the fuel would undergo a series of physical changes, such as evaporation, breakup, turbulence, collision polymerization as well as interaction with wall in the cylinder. The k-zeta-f model with wide application, good accuracy and stability was selected as turbulence simulation. The heat and mass transfer of evaporation processes were described by the Dukowicz model [29], which assumed a uniform droplet temperature, and the rate of droplet temperature change was determined by the heat balance, which stated that the heat convection from the gas to the droplet either heats up the droplet or supplies heat for vaporization. The break-up process of fuel droplet was described by the Wave model [30,31], which considers that the droplet starts to break up when the amplitude of the unstable wave is greater than the critical value. The individual turbulent eddies of fuel droplets in cylinder were described by Gosman & Ioannidis model, which models the effect of turbulence on the spray particles by adding a fluctuating velocity [32]. The spray wall interaction was modeled by the Walljet 1 model [33]. Meanwhile, the Extended Zeldovich Mechanism was used to describe the NO formation process [34,35], and the Lund Flamelet model was used to anticipate the soot formation of FPE [36].
Table 3 e Initial boundary conditions. Initial conditions Intake pressure Intake temperature Ambient pressure Ambient temperature Initial EGR Injection position Number of nozzle holes Nozzle diameter Angle of spray Piston head temperature Cylinder liner temperature Cylinder head temperature
Value 1.2 bar 295 K 1 bar 295 K 0 56 mm 4 0.13 mm 160 500 K 400 K 450 K
Operation condition In order to study the effect of different hydrogen addition on the combustion and emission of the FPE, the equal heating value substitution method was used to set up the amount of hydrogen, which views that the equal heating value of diesel fuel was replaced with equal heating value of hydrogen fuel to ensure that the total heating value of the fuel in the cylinder remains constant. The rate of hydrogen addition (RH2) is defined as follows: RH2 ¼
mH2 HuH2 100% mH2 HuH2 þ mdiesel Hudiesel
(2)
where, mH2 represents the mass of hydrogen; mdiesel denotes the mass of in-cylinder diesel fuel; HuH2 and Hudiesel are the
Fig. 4 e Convergent results of piston motion.
Please cite this article in press as: Yuan C, et al., Effect of hydrogen addition on the combustion and emission of a diesel free-piston engine, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.038
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Table 4 e Iterative boundaries for combustion simulation. Conditions Speed/(r/min) Compression ratio Pressure/bar Temperature/K Turbulent energy/m2/s2 EGR Injection time/ ECA
RH2 ¼ 0
RH2 ¼ 0.1
RH2 ¼ 0.2
RH2 ¼ 0.3
RH2 ¼ 0.4
1834 15.29 1.16 315.8 27.6 0.0463 167.9
1823 15.01 1.16 314.5 27.9 0.0454 168.1
1857 15.71 1.16 313.6 27.4 0.0461 167.6
1896 15.95 1.15 313.2 26.9 0.0469 167.3
1954 16.07 1.14 312.7 26.7 0.0474 167.1
low heating values of hydrogen fuel and diesel fuel respectively. Because the physical and chemical properties of hydrogen are different from those of diesel fuel [2,15,16], so the amount of hydrogen fuel is changed in different RH2 conditions. The specific scheme is listed in Table 2. Besides, the initial boundary conditions are the same for the different hydrogen addition ratios to ensure the fairness of the contrast. The specific parameters of initial conditions are shown in Table 3.
Iterative boundary According to the iterative calculation method mentioned above in Fig. 2, the accurate piston motion and final iteration results under the condition of different RH2 conditions were obtained, as shown in Fig. 4 and Table 4. It is clear that the piston displacement changes accordingly when the RH2 changes. The more hydrogen addition makes the amplitude of the piston motion greater. Moreover, due to the difference of piston motion under different RH2 conditions, the injection timing and engine speed also change, which are different from those of the conventional engines. In order to ensure the consistency of the simulation condition, the same piston position is selected for fuel injection, and the corresponding injection timings are listed in Table 4.
Fig. 5 e Modeling validation.
Experimental validation In order to verify the accuracy of the modeling and simulation methods, a corresponding experiment was carried out under the condition that RH2 is zero. The detailed experimental prototype, experimental procedure and method could be found in our previous work [37]. Fig. 5 shows the comparison between the simulation value and the experimental value of the in-cylinder gas pressure. It can be seen that the simulation result is basically the same as the measured value, and the maximum error between the two values is very little, indicating that the model can accurately reflect the combustion conditions in the cylinder. Therefore, the model and method were validated to be used for the description of combustion and heat release in the FPE.
Results and discussion The combustion process of the FPE with hydrogen addition is both characteristic of the ignition engine and the compression ignition engine. When the piston approaches near the TDC, the diesel fuel would be injected and combust on the gas condition of high temperature and high pressure. At the same time, the premixed hydrogen fuel would be ignited by sparks which produced by the spontaneous combustion of diesel fuel. Because of the fast combustion rate of hydrogen, the
Fig. 6 e In-cylinder gas pressures.
Please cite this article in press as: Yuan C, et al., Effect of hydrogen addition on the combustion and emission of a diesel free-piston engine, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.038
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delay increases, resulting in the peak pressure and temperature increases. These reasons together result in the results showed in Figs. 6 and 7. When RH2 is 0.1, the mass of hydrogen is only 0.262 mg, which is too few to improve the combustion process in the cylinder. However, although the increase of hydrogen would reduce the supply of diesel fuel, the diesel fuel injected into the cylinder is enough to ignite the hydrogen. A lot of generated hydrogen flame spreads rapidly, which promotes the diesel diffusion combustion process and improves the perfection level of the combustion process. Moreover, enlarged compression ratios and retarded injection time also enhance the in-cylinder gas pressure and temperature gradually, and the moments when they reach peak are advanced.
Emission production Fig. 7 e In-cylinder gas temperatures. flame propagation velocity of premixed hydrogen combustion is faster than that of diesel diffusion combustion. Therefore, compared with pure diesel, the diffusion combustion stage of diesel would change greatly.
Combustion characteristics The variations of in-cylinder gas pressure and temperature of the FPE with different hydrogen additions are plotted in Fig. 6 and Fig. 7 respectively. It can be seen that the change trend of the in-cylinder gas temperatures is similar with that of incylinder gas pressures. Overall, when the RH2 is 0.1, the incylinder gas pressure is comparable to that of pure diesel. However, when RH2 is greater than 0.1, the increase of RH2 gives a positive effect on the peak values of in-cylinder gas pressure and temperature. Besides, it can also be found that the in-cylinder gas pressure curves are steeper when RH2 increases, namely the pressure rise rate also increases with the increase of RH2. Moreover, the moment when the pressure and temperature in the cylinder reach peak are advanced slightly accordingly. On the one hand, when the high-pressure diesel fuel was injected into the cylinder, it would combust spontaneously and then ignite the premixed hydrogen fuel rapidly. As we all know, hydrogen has the advantage of fast flame propagation velocity, so the diffusion combustion rate of diesel is accelerated, which significantly accelerates the combustion of overall fuel, and the chemical energy contained in the fuel is released more quickly [2,15,16]. On the other hand, in order to ensure the total heating value of the fuel in the cylinder keeps constant, increasing the hydrogen would inevitably reduce the mass of diesel fuel, which may deteriorate the combustion quality. Besides, unlike conventional engines, the piston motion and boundary condition of the FPE change with variable RH2, although their initial boundary conditions are the same. As shown in Table 4, when the RH2 increases, the compression ratio also increases accordingly, so the in-cylinder gas pressure and temperature increase correspondingly when the piston reaches the TDC. In addition, enlarging injection advance angles also prolongs the ignition delay, so the amount of combustible mixture prepared during ignition
Adding some hydrogen to the diesel FPE has an impact on evaporation, breakup, collision and wall interaction process of diesel fuel, which changes the combustion process in the cylinder. Moreover, the pressure, temperature and oxygen concentration change accordingly due to the combustion of hydrogen, which eventually leads to the different emission performances under the conditions of variable RH2 compared with the condition of pure diesel. In diesel engines, the emissions of CO and HC are very little. In addition, because of the high diffusivity of hydrogen fuel, the mixture of diesel and air would be more uniform, and the combustion condition in the cylinder would be further improved. As a result, the emissions of CO and HC are further reduced. So this study mainly focused on the NO and soot production.
NO production Fig. 8 demonstrates the NO performance of the FPE with different RH2 conditions. Clearly, with the increase of RH2, the NO production increases gradually, and when the RH2 comes up to 0.4, the NO is most, which increased by 7 times than that of pure diesel. The formation of NO depends on three key factors, which are the oxygen concentration in the cylinder, high temperature and duration of high temperature. For the FPE, there is sufficient oxygen in the cylinder, so temperature
Fig. 8 e NO mass fractions.
Please cite this article in press as: Yuan C, et al., Effect of hydrogen addition on the combustion and emission of a diesel free-piston engine, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.038
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Fig. 9 e Formation of NO.
is a key factor which determines the formation of NO. As can be seen from Fig. 7, the in-cylinder gas temperature and its maximum value gradually increase correspondingly with the increase of RH2. Besides, although the presence of hydrogen fuel improves the overall combustion rate in the cylinder, the diesel fuel still continues being injected after the complete combustion of the hydrogen, so the reaction time of the high temperature combustion is not shortened, both of which contribute to the formation of NO. As a result, it is can be found a positive correlation between the NO formation and RH2. The specific formation process of NO can be seen from Fig. 9. In the study, the diesel fuel injection takes place at around 167 ECA. Because of the rapid change of the piston speed near the TDC, the turbulent motion in the cylinder is intensified and the mixing of fuel and air is more uniform. When the piston approaches the TDC, the temperature and pressure are high enough, a large number of hydrogen-air mixtures are ignited by diesel and the generated hydrogen flame spreads rapidly, which accelerates the mixing rate of
Fig. 10 e Soot mass fractions.
diesel and air as well as promotes the combustion process of diesel, eventually leading to a rapid increase in the temperature around the TDC. So the formation of NO occurs near the 180 ECA. When the RH2 increases, the compression ratio of the FPE also increases accordingly, so the in-cylinder gas temperature increases more obviously. Therefore, the NO formation occurs earlier and the formation rate is faster. Moreover, it can also be found that the position of NO formation is closer to cylinder wall as the RH2 increases. The most important reason of this phenomenon is that, as more hydrogen fuel is added to the cylinder, faster piston speed could make the flame propagates quickly toward the cylinder wall, resulting in faster combustion rate in this region, so there are more NO near the cylinder wall.
Soot production Fig. 10 shows the effect of RH2 on soot formation of the FPE. It can be seen that the RH2 gives a negative effect on the soot production of the FPE. When the RH2 is 0.4, the soot of FPE decreases by 24 times than that of pure diesel. Moreover, the formation process of soot in Fig. 11 shows that the soot forms earlier in the larger RH2 condition and it is oxidized to lower level. The reason for this phenomenon may be the following aspects. Firstly, hydrogen has the advantages of fast combustion rate, good diffusion performance, so adding hydrogen fuel to the FPE can promote diesel combustion more fully, eventually leading to a decrease in soot. Secondly, in order to ensure that the total heating value in the cylinder keeps constant, as the RH2 increases, the mass of diesel reduces accordingly, resulting in a decrease in hydrocarbon fuel. Thirdly, after adding hydrogen to the FPE, the hydrocarbons are oxidized easily due to the higher temperature, so the final production of soot is reduced. Moreover, as described above, when RH2 increases, the motion frequency of the FPE also increases, which makes the mixture more uniform. Furthermore, the increased compression ratio causes the in-cylinder gas temperature and pressure to rise, which is also beneficial to the mixing of fuel and air.
Please cite this article in press as: Yuan C, et al., Effect of hydrogen addition on the combustion and emission of a diesel free-piston engine, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.038
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Fig. 11 e Formation of soot.
Heat release
In addition, it can also be found that soot is produced earlier than NO. This is because when the hydrogen fuel is ignited by diesel fuel, the hydrogen starts to combust instantly, leading to an increase in the temperature. At the same time the combustion of hydrogen depletes the oxygen around the diesel fuel, which forms a relatively high temperature and oxygen-poor environment around the diesel fuel, prompting the formation time of soot to be advanced, so there appears the above phenomenon. Moreover, it can also be found that the soot formation area is mainly concentrated in the piston bowl and cylinder wall. This is mainly due to the fact that the fuel in the cylinder can not be rapidly diffused under the pure diesel operation, so that the fuel concentration at the piston bowl is high, resulting in more soot emission. As hydrogen addition increases, the diffusion process of fuel is accelerated, resulting in more homogeneous mixture in the cylinder, so the soot mass fraction reduces.
The above differences in combustion and emission performance are mainly attributed to the fact that hydrogen addition affects the combustion process in the cylinder, which can be reflected by the combustion heat release rate in Fig. 12. It is clear that when the RH2 is 0.1, the peak value of the heat release rate in the cylinder is almost equal to that of the pure diesel. However, with the RH2 beyond 0.1, it can be seen that the peak heat release rates is larger and comes earlier in larger RH2 condition. As shown in Fig. 13, the hydrogen addition can reduce simultaneously the combustion duration, and the decreasing level is proportional to the mass of hydrogen added, indicating that hydrogen addition have the enhancing effect on the diffusion combustion process. When the RH2 is 0.4, the combustion duration is 19.4 ECA, a 59% reduction compared to the diesel operation.
Fig. 12 e Heat release rate.
Fig. 13 e Combustion process.
Please cite this article in press as: Yuan C, et al., Effect of hydrogen addition on the combustion and emission of a diesel free-piston engine, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.038
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Table 5 e Heat release performances. Heat release performances Released heat in rapid combustion/J Released heat in post combustion/J
RH2 ¼ 0
RH2 ¼ 0.1
RH2 ¼ 0.2
RH2 ¼ 0.3
RH2 ¼ 0.4
172.1 108.8
174.0 102.18
182.9 89.0
186.8 76.5
193.9 61.3
In order to deeply understand the influence of hydrogen addition on the combustion and heat release process in the FPE, the combustion process is divided into three parts: ignition delay stage, rapid combustion period and postcombustion period. The ignition delay stage is the period between the stating of fuel injection and the starting of combustion. During this stage, the fuel injected into the cylinder would undergo a series of complex physical and chemical processes, including evaporation, diffusion and mixing process. As can be seen from Fig. 13, as the RH2 increases, the ignition delay increases correspondingly. The duration of ignition delay mainly depends on the mixture quality of fuel and air in the cylinder. As the RH2 increases, the mass of hydrogen added into the cylinder increases while the mass of diesel fuel injected into the cylinder gradually decreases, resulting in a lean mixture of diesel and air, which prolongs the ignition delay period and retards the heat release process. Besides, the different RH2 conditions make that the compression ratio and motion frequency of FPE are also changed, eventually leading to different residual exhaust gas coefficients in the cylinder. It can be seen from the final iterative calculation results that the proportion of the residual exhaust gas in the cylinder increases as the RH2 increases, which also leads to the increase of ignition delay period. The second stage is the rapid combustion period, which lasts from the starting of combustion to the appearance of peak pressure. Fig. 13 indicates the rapid combustion period decreases with the increase of RH2. During the rapid combustion period, the formed mixture in the ignition delay stage combusts rapidly. As mentioned above, the larger RH2 leads to longer ignition delay duration for the FPE, so the mixing quality of mixture is better. The in-cylinder gas pressure and temperature increase more due to the faster flame propagation rate of hydrogen. Besides, the increased compression ratio also enhances the in-cylinder gas temperature and pressure at the end of the compression process, which leads to faster combustion heat release rate, thus enhancing the chemical reactivity of the fuel. As a result, the duration of rapid combustion period in larger RH2 condition was shorter, and the peak pressure and temperature as well as heat release are higher. Moreover, as can be seen from Table 5, the released heat during the rapid combustion period is 193.9 J in the RH2 of 0.4 condition, which is 13% higher than that of the pure diesel operation, indicating that the hydrogen addition can improve the combustion and heat release process. The post-combustion effect is weaker in higher RH2 condition, while the post-combustion lasts shorter and releases less heat. There are two main reasons for this result. Firstly, the faster flame propagation rate of hydrogen accelerates the combustion rate of overall fuel in the cylinder, resulting in less fuel left in the post-combustion period, which shortens the post-combustion duration. Furthermore, the increase of hydrogen content speeds up the movement of the piston, and
the in-cylinder gas temperature drops quickly after TDC, as shown in Fig. 7, which is not conducive to the post combustion process. Therefore, the increase of RH2 means the reduction of the released heat during the post-combustion stage. Besides, when RH2 is 0.4, the released heat has 61.3 J, which is 43.6% lower than that of pure diesel operation.
Discussion Unlike conventional engines, the FPE abandons the crank-link mechanism, so it has the advantages of variable compression ratio, flexible fuel flexibility and so on. Hydrogen fuel is considered as an ideal alternative fuel for internal combustion engines due to the advantages of good emission characteristics and high thermal efficiency [2,15]. Therefore, after adding different proportions of hydrogen to the FPE through intake ports, the different piston displacements and boundary conditions would be obtained by multiple iterations. On the one hand, hydrogen can accelerate the combustion speed of mixture in the cylinder, on the other hand, the compression ratio, movement frequency and residual exhaust gas coefficient that vary with hydrogen content also make the mixing and combustion processes change. These factors have different effects on the combustion and emissions of the FPE.
Conclusion In this study, a coupling model was established and validated to evaluate the effect of adding a small amount of hydrogen fuel on the diesel FPE, and the influence of hydrogen addition on the combustion and emission characteristics of the diesel FPE was investigated. The conclusions are concluded as follows: 1. When RH2 is 0.1, it has only mild effect on the in-cylinder gas pressure and combustion process. However, when RH2 is more than 0.1, both the peak pressure and temperature increase with the increase of RH2, and the peak pressure and temperature in the cylinder appear earlier slightly. 2. Compared with the pure diesel operation, the addition of hydrogen can make the NO emission increase gradually, and the NO production achieves the most when the RH2 comes up to 0.4. However, there is a negative correlation between the soot emission and hydrogen addition ratio. When the RH2 is 0.4, the soot of FPE decreases by 24 times than that of pure diesel. 3. The larger RH2 induces a longer ignition delay, shorter rapid combustion period, weaker post-combustion effect, greater heat release rates, and earlier peak heat release rates for the
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FPE. However, the released heat in rapid combustion period rises significantly with the increase of RH2. 4. Adding hydrogen in diesel FPE not only increases combustion speed, but also speeds up engine operation and enlargers compression ratio. These factors together further affect the mixing, combustion and emission characteristics of the FPE.
Acknowledgements This research was sponsored by the Scientific Research Development Program of Chongqing Jiaotong University (Grant number: 15JDKJC-A011) and the Chongqing Graduate Education Innovation Fund Project (Grant number: CYS17198). We express sincere gratitude to the sponsors.
Nomenclature FPE TDC IMEP NO HC CO CO2 RH2 ECA EGR
free-piston engine top dead center indicated mean effective pressure nitric oxide hydrocarbon carbon monoxide carbon dioxide rate of hydrogen addition equivalent crank angle exhaust gas recirculation
references
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Effect of hydrogen addition on the combustion and emission of a diesel free-piston engine Chenheng Yuan*,1, Cuijie Han 1, Yang Liu, Yituan He, Yiming Shao, Xiaochun Jian College of Traffic & Transportation, Chongqing Jiaotong University, Chongqing, 400074, China
article info
abstract
Article history:
The free-piston engine (FPE) is a new crankless engine, which operates with variable
Received 21 November 2017
compression ratio, flexible fuel applicability and low pollution potential. A numerical
Received in revised form
model which couples with dynamic, combustion and gas exchange was established and
5 May 2018
verified by experiment to simulate the effects of different hydrogen addition on the com-
Accepted 7 May 2018
bustion and emission of a diesel FPE. Results indicate that a small amount of hydrogen
Available online xxx
addition has a little effect on the combustion process of the FPE. However, when the ratio of hydrogen addition (RH2) is more than 0.1, the RH2 gives a positive effect on the peak in-
Keywords:
cylinder gas pressure, temperature, and nitric oxide emission of the FPE, while soot
Free-piston engine
emission decreases with the increase of hydrogen addition. Moreover, the larger RH2 in-
Hydrogen addition
duces a longer ignition delay, shorter rapid combustion period, weaker post-combustion
Combustion
effect, greater heat release rate, and earlier peak heat release rate for the FPE. Neverthe-
Emission
less, the released heat in rapid combustion period is significantly enhanced by the increase
Heat release
of RH2. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In the context of severe energy shortage and serious environmental pollution, the conventional engine with low efficiency and high emission has been unable to meet the strong demand of energy saving and emission reduction [1,2]. The free-piston engine (FPE) comes into being under this background [3]. The new engine abandons the crank connecting rod mechanism of conventional engines, thus reducing the friction loss, improving the thermal efficiency, and moving with a variable compression ratio, which provides flexible fuel applicability and possibility of new clean combustion modes [4e6]. These potential advantages of FPE bring broad
application fields. It can be used not only as a driving system for hybrid electric vehicles or electric vehicles, but also as an auxiliary power for electromechanical device. In recent years, the FPE has attracted considerable research interests, and a great deal of research work has been done by scholars and research institutions around the world. Kosaka et al. made a significant contribution to the development of a single-piston FPE [7e9], including the fundamental characteristics of the FPE, the control system for generator and the control method of linear generator to improve the efficiency and stability. Roskilly [10] mainly used a computational fluid dynamics model to investigate the gas motion, combustion process and the formation mechanism of nitrogen oxides of a diesel FPE. They found that the FPE produces a stronger radial
* Corresponding author. E-mail address: [email protected] (C. Yuan). 1 Co-first author. Chenheng Yuan and Cuijie Han contributed equally to this work. https://doi.org/10.1016/j.ijhydene.2018.05.038 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Yuan C, et al., Effect of hydrogen addition on the combustion and emission of a diesel free-piston engine, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.038
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flow near the top dead center (TDC) than a conventional engine, but it has little effect on the combustion process. Lim et al. [11] studied the effects of key parameters such as equivalent ratio, ignition time and spring stiffness on the operation characteristics and performance of a spark-ignition FPE by means of numerical simulation. The results showed that reducing the ignition advance angle or increasing the spring stiffness can significantly improve the dynamic of piston, electric power output and the engine performance. Kim et al. [12] mainly studied the influence of air flow rate and alternator load on the operation characteristics of a liquefied petroleum gas FPE. Their results showed that piston dynamics are the main factors affecting indicated mean effective pressure (IMEP) when the conductance changes. They also found that the motion stroke and frequency are negatively related to the air flow rate, but the IMEP and electric output increase as the air flow rate increases. Yuan et al. [13] studied the combustion characteristics of a two-stroke FPE by a coupling simulation. They analyzed the influence of special piston motion on the combustion process of the FPE and compared the simulation results with a conventional engine. The results showed that the FPE has a faster piston motion and longer combustion duration than those of the conventional engine. Meanwhile, they found that the maximum in-cylinder pressure and temperature of FPE are lower than those of the conventional engine. The emission characteristics of FPE have been studied, some researches indicated that the FPE has the advantages of variable compression ratio, wide adaptability of fuel, and less nitric oxide (NO) emission, but some deficiencies were also found, such as high soot emission [14]. As is known to all, hydrogen fuel has the advantages of low ignition energy, fast flame propagation speed, large lean operation limit, and low hydrocarbon (HC) and carbon monoxide (CO) emissions, which is considered as an ideal alternative fuel for internal combustion engines [15,16]. There are many studies on hydrogen engines at present. Park et al. [17] investigated the effects of lean fuel and exhaust gas recirculation (EGR) on the combustion and emission performance of a heavy-duty engine with natural gas-hydrogen dual fuels. The results showed that the engine can reduce the emission and improve the efficiency by expanding the lean operation limit. Du et al. [18] studied the effect of addition of hydrogen and exhaust gas recirculation on combustion characteristics of a gasoline engine. The results found that with increase of hydrogen addition, the NO emission increases. Barrios et al. [19] experimentally studied the influence of hydrogen addition on combustion characteristics and particle number size distribution emissions of a diesel engine. Through the research they indicated that with hydrogen injection, the number of nucleation mode particles decreases in direct proportion to the total particle number, and the carbon dioxide (CO2) emissions decrease when hydrogen is injected. Jhang et al. [20] investigated experimentally the effect of hydrogen addition on the emissions of a heavy-duty diesel engine under constant speed from the low to high engine load. Their experimental results showed that the brake thermal efficiency increases with an increasing amount of hydrogen, and the hydrogen addition can reduce the emissions of CO2 and CO. At
the high operation load, the reduction in emissions was the most significant. From the above researches, it is known that adding hydrogen fuel to natural gas engines or diesel engines can improve the thermal efficiency and economy, meanwhile reduce some harmful emissions. While the FPE is a special power engine, which abandons the conventional crankconnecting rod mechanism, making the law of piston movement is different from that of the conventional engine. Some researches indicated that the FPE has higher movement speed and acceleration in late compression stroke, which brings about different ignition, fuel spray, evaporation, air-fuel mixing, heat transfer, and combustion characteristics for the FPE, compared with conventional engines [21e24]. So adding some hydrogen to the diesel FPE may have a particular effect on combustion process and emission characteristics due to the special piston motion law and variable compression ratio. However, there are few researches on hydrogen addition of the FPE. In this study, some hydrogen fuel is added to a diesel FPE, and the influence of different hydrogen addition ratios on the combustion and emission characteristics of the FPE was investigated and revealed.
Fundamental and method FPE prototype The FPE prototype presented for this study is shown in Fig. 1, which adopts a dual piston opposed structure. The FPE is composed of two free piston combustion cylinders arranged horizontally, with a linear generator arranged in the middle, and the piston rod assembly is coaxially connected with the mover of the linear generator. The engine uses a two-stroke, compression ignition work cycle. The gas exchange of the FPE is regulated by a looping scavenging way. The diesel fuel is injected directly into the cylinder at the appropriate time, and the hydrogen fuel is premixed with air. In starting process, the linear generator is used as an electric motor to initialize the engine. After starting, the motor is immediately converted into a generator. The converted electric energy can be stored or utilized directly. The main parameters of this FPE are listed in Table 1, and more information can be seen in elsewhere [22,24,25].
Simulation method Unlike conventional engines, the FPE abandons the crank-link mechanism, so the piston is “free”. Although the piston
Fig. 1 e The basic structure of the FPE. Please cite this article in press as: Yuan C, et al., Effect of hydrogen addition on the combustion and emission of a diesel free-piston engine, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.038
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Table 1 e Main parameters of the FPE. Engine parameters Engine bore Nominal effective stroke length Nominal stroke length/mm Compression ratio Mass of the mover Volume of the bowl Generator load Internal coil inductance Internal coil resistance Exhaust port height Scavenging port height Port overlapping distance
Value 60 mm 58 mm 94 variable 5.4 kg 5.65 106 m3 290 N.s/m 10.2 mH 5.4 U 25 mm 15 mm 13 mm
motion of FPE is no longer restricted by the mechanical mechanism, it is still affected by the in-cylinder gas force, friction force and generator force acting on the piston. Meanwhile the piston movement has a direct impact on intake and exhaust, compression, as well as combustion process. Therefore, there is a strong coupling relationship between the piston motion, gas exchange, and combustion process in the FPE. The simulation method generally applied to conventional engines is no longer suitable for the FPE. In this study, a comprehensive model which couples with combustion, gas exchange and piston motion was established and iteratively calculated to simulate the combustion and emission of the FPE with different hydrogen addition [14,26]. Specific modeling and iterative calculation method are shown in Fig. 2. The procedures of the iterative simulation are as follows: (1) The dynamic model was established and simulated in an initial condition to obtain the motion performance; (2) The gas exchange model was established by using the piston motion, and then it was calculated to determine the gas exchange performances, including the in-cylinder gas
3
temperature, pressure, EGR, and turbulent kinetic energy, etc.; (3) The combustion model was established and simulated based on the piston motion and the gas exchange results. (4) Using these results to update boundary conditions next iteration, and the dynamic model, gas exchange model, combustion model were re-established and simulated using new boundary. (5) The step (4) was operated repetitively until convergence.
Modeling and boundary Dynamic model The piston dynamic of FPE is determined by the resultant force acting on it, mainly including on the gas force in left and right cylinders, friction force, scavenging gas force and electromagnetic force generated by the alternator. It can be described by the Newton's second law and has been established detailedly in previous publication [13]. m
d2 x ¼ Fp þ Fe þ Ff þ Fs dt2
(1)
where, the m represents the mass of piston assembly; x is the displacement; Fp is the in-cylinder gas force; Fe stands for the alternator force; Ff denotes the friction force; and Fs is the scavenging gas force.
Gas exchange model Gas exchange of engines is aim to ensure that the exhaust gas can be driven out fully, meanwhile reduce the loss of fresh air as much as possible. The gas exchange system of the FPE was modeled using four separate regions, including exhaust duct, scavenging transfer duct, cylinder and a plenum box, which has been developed in research [27]. This research used it to perform the gas exchange simulation, but it would be updated using the results of the last iteration.
Combustion model Although the FPE has the special structure and operating principle, it still belongs to a compression ignition diesel
Fig. 2 e Iterative calculation method.
Fig. 3 e Computational mesh of combustion.
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Table 2 e Hydrogen scheme of intake-air composition. Scheme The mass of injected diesel/mg The mass of H2/mg The mass fraction of H2/% Fuel-air equivalence ratio
RH2 ¼ 0
RH2 ¼ 0.1
RH2 ¼ 0.2
RH2 ¼ 0.3
RH2 ¼ 0.4
7.40 0 0 0.431
6.66 0.262 0.107 0.424
5.92 0.524 0.213 0.417
5.18 0.7862 0.319 0.406
4.44 1.048 0.430 0.404
engine, and has the same essential properties as conventional diesel engines. Therefore, many combustion models suitable for conventional engines can also be used for FPE combustion simulation. In order to obtain the combustion characteristics of the FPE with hydrogen addition, AVL_Fire simulation software was used to simulate the combustion process in the cylinder. First of all, according to the combustion chamber structure of the FPE prototype, the moving mesh of combustion model was established, as shown in Fig. 3. The total number of model grids in the entire combustion chamber was 70686, and all of them were hexahedron cells. Hydrogen-mixed combustion belongs to the turbulent combustion, and turbulence plays an important role in the heat and mass transfer in combustion process, while the chemical reaction rate is affected by the temperature and the concentration of the reaction mechanism itself. So the interaction between the turbulence and the chemical reaction is the primary problem that the combustion model needs to solve. Based on the characteristics of engine turbulence combustion, ECFM-3Z model of the extended coherent flame model was selected as the combustion model [28], which was developed by the Groupement Scientifique Moteurs consortium specifically for both compression ignition and sparking ignition combustion. This model is based on a flame surface density transport equation and a mixing model that can describe inhomogeneous turbulent premixed and diffusion combustion. Besides, it is fully coupled to the spray model and enables stratified combustion modeling including EGR effects and NO formation. Before combustion, the fuel would undergo a series of physical changes, such as evaporation, breakup, turbulence, collision polymerization as well as interaction with wall in the cylinder. The k-zeta-f model with wide application, good accuracy and stability was selected as turbulence simulation. The heat and mass transfer of evaporation processes were described by the Dukowicz model [29], which assumed a uniform droplet temperature, and the rate of droplet temperature change was determined by the heat balance, which stated that the heat convection from the gas to the droplet either heats up the droplet or supplies heat for vaporization. The break-up process of fuel droplet was described by the Wave model [30,31], which considers that the droplet starts to break up when the amplitude of the unstable wave is greater than the critical value. The individual turbulent eddies of fuel droplets in cylinder were described by Gosman & Ioannidis model, which models the effect of turbulence on the spray particles by adding a fluctuating velocity [32]. The spray wall interaction was modeled by the Walljet 1 model [33]. Meanwhile, the Extended Zeldovich Mechanism was used to describe the NO formation process [34,35], and the Lund Flamelet model was used to anticipate the soot formation of FPE [36].
Table 3 e Initial boundary conditions. Initial conditions Intake pressure Intake temperature Ambient pressure Ambient temperature Initial EGR Injection position Number of nozzle holes Nozzle diameter Angle of spray Piston head temperature Cylinder liner temperature Cylinder head temperature
Value 1.2 bar 295 K 1 bar 295 K 0 56 mm 4 0.13 mm 160 500 K 400 K 450 K
Operation condition In order to study the effect of different hydrogen addition on the combustion and emission of the FPE, the equal heating value substitution method was used to set up the amount of hydrogen, which views that the equal heating value of diesel fuel was replaced with equal heating value of hydrogen fuel to ensure that the total heating value of the fuel in the cylinder remains constant. The rate of hydrogen addition (RH2) is defined as follows: RH2 ¼
mH2 HuH2 100% mH2 HuH2 þ mdiesel Hudiesel
(2)
where, mH2 represents the mass of hydrogen; mdiesel denotes the mass of in-cylinder diesel fuel; HuH2 and Hudiesel are the
Fig. 4 e Convergent results of piston motion.
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Table 4 e Iterative boundaries for combustion simulation. Conditions Speed/(r/min) Compression ratio Pressure/bar Temperature/K Turbulent energy/m2/s2 EGR Injection time/ ECA
RH2 ¼ 0
RH2 ¼ 0.1
RH2 ¼ 0.2
RH2 ¼ 0.3
RH2 ¼ 0.4
1834 15.29 1.16 315.8 27.6 0.0463 167.9
1823 15.01 1.16 314.5 27.9 0.0454 168.1
1857 15.71 1.16 313.6 27.4 0.0461 167.6
1896 15.95 1.15 313.2 26.9 0.0469 167.3
1954 16.07 1.14 312.7 26.7 0.0474 167.1
low heating values of hydrogen fuel and diesel fuel respectively. Because the physical and chemical properties of hydrogen are different from those of diesel fuel [2,15,16], so the amount of hydrogen fuel is changed in different RH2 conditions. The specific scheme is listed in Table 2. Besides, the initial boundary conditions are the same for the different hydrogen addition ratios to ensure the fairness of the contrast. The specific parameters of initial conditions are shown in Table 3.
Iterative boundary According to the iterative calculation method mentioned above in Fig. 2, the accurate piston motion and final iteration results under the condition of different RH2 conditions were obtained, as shown in Fig. 4 and Table 4. It is clear that the piston displacement changes accordingly when the RH2 changes. The more hydrogen addition makes the amplitude of the piston motion greater. Moreover, due to the difference of piston motion under different RH2 conditions, the injection timing and engine speed also change, which are different from those of the conventional engines. In order to ensure the consistency of the simulation condition, the same piston position is selected for fuel injection, and the corresponding injection timings are listed in Table 4.
Fig. 5 e Modeling validation.
Experimental validation In order to verify the accuracy of the modeling and simulation methods, a corresponding experiment was carried out under the condition that RH2 is zero. The detailed experimental prototype, experimental procedure and method could be found in our previous work [37]. Fig. 5 shows the comparison between the simulation value and the experimental value of the in-cylinder gas pressure. It can be seen that the simulation result is basically the same as the measured value, and the maximum error between the two values is very little, indicating that the model can accurately reflect the combustion conditions in the cylinder. Therefore, the model and method were validated to be used for the description of combustion and heat release in the FPE.
Results and discussion The combustion process of the FPE with hydrogen addition is both characteristic of the ignition engine and the compression ignition engine. When the piston approaches near the TDC, the diesel fuel would be injected and combust on the gas condition of high temperature and high pressure. At the same time, the premixed hydrogen fuel would be ignited by sparks which produced by the spontaneous combustion of diesel fuel. Because of the fast combustion rate of hydrogen, the
Fig. 6 e In-cylinder gas pressures.
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delay increases, resulting in the peak pressure and temperature increases. These reasons together result in the results showed in Figs. 6 and 7. When RH2 is 0.1, the mass of hydrogen is only 0.262 mg, which is too few to improve the combustion process in the cylinder. However, although the increase of hydrogen would reduce the supply of diesel fuel, the diesel fuel injected into the cylinder is enough to ignite the hydrogen. A lot of generated hydrogen flame spreads rapidly, which promotes the diesel diffusion combustion process and improves the perfection level of the combustion process. Moreover, enlarged compression ratios and retarded injection time also enhance the in-cylinder gas pressure and temperature gradually, and the moments when they reach peak are advanced.
Emission production Fig. 7 e In-cylinder gas temperatures. flame propagation velocity of premixed hydrogen combustion is faster than that of diesel diffusion combustion. Therefore, compared with pure diesel, the diffusion combustion stage of diesel would change greatly.
Combustion characteristics The variations of in-cylinder gas pressure and temperature of the FPE with different hydrogen additions are plotted in Fig. 6 and Fig. 7 respectively. It can be seen that the change trend of the in-cylinder gas temperatures is similar with that of incylinder gas pressures. Overall, when the RH2 is 0.1, the incylinder gas pressure is comparable to that of pure diesel. However, when RH2 is greater than 0.1, the increase of RH2 gives a positive effect on the peak values of in-cylinder gas pressure and temperature. Besides, it can also be found that the in-cylinder gas pressure curves are steeper when RH2 increases, namely the pressure rise rate also increases with the increase of RH2. Moreover, the moment when the pressure and temperature in the cylinder reach peak are advanced slightly accordingly. On the one hand, when the high-pressure diesel fuel was injected into the cylinder, it would combust spontaneously and then ignite the premixed hydrogen fuel rapidly. As we all know, hydrogen has the advantage of fast flame propagation velocity, so the diffusion combustion rate of diesel is accelerated, which significantly accelerates the combustion of overall fuel, and the chemical energy contained in the fuel is released more quickly [2,15,16]. On the other hand, in order to ensure the total heating value of the fuel in the cylinder keeps constant, increasing the hydrogen would inevitably reduce the mass of diesel fuel, which may deteriorate the combustion quality. Besides, unlike conventional engines, the piston motion and boundary condition of the FPE change with variable RH2, although their initial boundary conditions are the same. As shown in Table 4, when the RH2 increases, the compression ratio also increases accordingly, so the in-cylinder gas pressure and temperature increase correspondingly when the piston reaches the TDC. In addition, enlarging injection advance angles also prolongs the ignition delay, so the amount of combustible mixture prepared during ignition
Adding some hydrogen to the diesel FPE has an impact on evaporation, breakup, collision and wall interaction process of diesel fuel, which changes the combustion process in the cylinder. Moreover, the pressure, temperature and oxygen concentration change accordingly due to the combustion of hydrogen, which eventually leads to the different emission performances under the conditions of variable RH2 compared with the condition of pure diesel. In diesel engines, the emissions of CO and HC are very little. In addition, because of the high diffusivity of hydrogen fuel, the mixture of diesel and air would be more uniform, and the combustion condition in the cylinder would be further improved. As a result, the emissions of CO and HC are further reduced. So this study mainly focused on the NO and soot production.
NO production Fig. 8 demonstrates the NO performance of the FPE with different RH2 conditions. Clearly, with the increase of RH2, the NO production increases gradually, and when the RH2 comes up to 0.4, the NO is most, which increased by 7 times than that of pure diesel. The formation of NO depends on three key factors, which are the oxygen concentration in the cylinder, high temperature and duration of high temperature. For the FPE, there is sufficient oxygen in the cylinder, so temperature
Fig. 8 e NO mass fractions.
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Fig. 9 e Formation of NO.
is a key factor which determines the formation of NO. As can be seen from Fig. 7, the in-cylinder gas temperature and its maximum value gradually increase correspondingly with the increase of RH2. Besides, although the presence of hydrogen fuel improves the overall combustion rate in the cylinder, the diesel fuel still continues being injected after the complete combustion of the hydrogen, so the reaction time of the high temperature combustion is not shortened, both of which contribute to the formation of NO. As a result, it is can be found a positive correlation between the NO formation and RH2. The specific formation process of NO can be seen from Fig. 9. In the study, the diesel fuel injection takes place at around 167 ECA. Because of the rapid change of the piston speed near the TDC, the turbulent motion in the cylinder is intensified and the mixing of fuel and air is more uniform. When the piston approaches the TDC, the temperature and pressure are high enough, a large number of hydrogen-air mixtures are ignited by diesel and the generated hydrogen flame spreads rapidly, which accelerates the mixing rate of
Fig. 10 e Soot mass fractions.
diesel and air as well as promotes the combustion process of diesel, eventually leading to a rapid increase in the temperature around the TDC. So the formation of NO occurs near the 180 ECA. When the RH2 increases, the compression ratio of the FPE also increases accordingly, so the in-cylinder gas temperature increases more obviously. Therefore, the NO formation occurs earlier and the formation rate is faster. Moreover, it can also be found that the position of NO formation is closer to cylinder wall as the RH2 increases. The most important reason of this phenomenon is that, as more hydrogen fuel is added to the cylinder, faster piston speed could make the flame propagates quickly toward the cylinder wall, resulting in faster combustion rate in this region, so there are more NO near the cylinder wall.
Soot production Fig. 10 shows the effect of RH2 on soot formation of the FPE. It can be seen that the RH2 gives a negative effect on the soot production of the FPE. When the RH2 is 0.4, the soot of FPE decreases by 24 times than that of pure diesel. Moreover, the formation process of soot in Fig. 11 shows that the soot forms earlier in the larger RH2 condition and it is oxidized to lower level. The reason for this phenomenon may be the following aspects. Firstly, hydrogen has the advantages of fast combustion rate, good diffusion performance, so adding hydrogen fuel to the FPE can promote diesel combustion more fully, eventually leading to a decrease in soot. Secondly, in order to ensure that the total heating value in the cylinder keeps constant, as the RH2 increases, the mass of diesel reduces accordingly, resulting in a decrease in hydrocarbon fuel. Thirdly, after adding hydrogen to the FPE, the hydrocarbons are oxidized easily due to the higher temperature, so the final production of soot is reduced. Moreover, as described above, when RH2 increases, the motion frequency of the FPE also increases, which makes the mixture more uniform. Furthermore, the increased compression ratio causes the in-cylinder gas temperature and pressure to rise, which is also beneficial to the mixing of fuel and air.
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Fig. 11 e Formation of soot.
Heat release
In addition, it can also be found that soot is produced earlier than NO. This is because when the hydrogen fuel is ignited by diesel fuel, the hydrogen starts to combust instantly, leading to an increase in the temperature. At the same time the combustion of hydrogen depletes the oxygen around the diesel fuel, which forms a relatively high temperature and oxygen-poor environment around the diesel fuel, prompting the formation time of soot to be advanced, so there appears the above phenomenon. Moreover, it can also be found that the soot formation area is mainly concentrated in the piston bowl and cylinder wall. This is mainly due to the fact that the fuel in the cylinder can not be rapidly diffused under the pure diesel operation, so that the fuel concentration at the piston bowl is high, resulting in more soot emission. As hydrogen addition increases, the diffusion process of fuel is accelerated, resulting in more homogeneous mixture in the cylinder, so the soot mass fraction reduces.
The above differences in combustion and emission performance are mainly attributed to the fact that hydrogen addition affects the combustion process in the cylinder, which can be reflected by the combustion heat release rate in Fig. 12. It is clear that when the RH2 is 0.1, the peak value of the heat release rate in the cylinder is almost equal to that of the pure diesel. However, with the RH2 beyond 0.1, it can be seen that the peak heat release rates is larger and comes earlier in larger RH2 condition. As shown in Fig. 13, the hydrogen addition can reduce simultaneously the combustion duration, and the decreasing level is proportional to the mass of hydrogen added, indicating that hydrogen addition have the enhancing effect on the diffusion combustion process. When the RH2 is 0.4, the combustion duration is 19.4 ECA, a 59% reduction compared to the diesel operation.
Fig. 12 e Heat release rate.
Fig. 13 e Combustion process.
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Table 5 e Heat release performances. Heat release performances Released heat in rapid combustion/J Released heat in post combustion/J
RH2 ¼ 0
RH2 ¼ 0.1
RH2 ¼ 0.2
RH2 ¼ 0.3
RH2 ¼ 0.4
172.1 108.8
174.0 102.18
182.9 89.0
186.8 76.5
193.9 61.3
In order to deeply understand the influence of hydrogen addition on the combustion and heat release process in the FPE, the combustion process is divided into three parts: ignition delay stage, rapid combustion period and postcombustion period. The ignition delay stage is the period between the stating of fuel injection and the starting of combustion. During this stage, the fuel injected into the cylinder would undergo a series of complex physical and chemical processes, including evaporation, diffusion and mixing process. As can be seen from Fig. 13, as the RH2 increases, the ignition delay increases correspondingly. The duration of ignition delay mainly depends on the mixture quality of fuel and air in the cylinder. As the RH2 increases, the mass of hydrogen added into the cylinder increases while the mass of diesel fuel injected into the cylinder gradually decreases, resulting in a lean mixture of diesel and air, which prolongs the ignition delay period and retards the heat release process. Besides, the different RH2 conditions make that the compression ratio and motion frequency of FPE are also changed, eventually leading to different residual exhaust gas coefficients in the cylinder. It can be seen from the final iterative calculation results that the proportion of the residual exhaust gas in the cylinder increases as the RH2 increases, which also leads to the increase of ignition delay period. The second stage is the rapid combustion period, which lasts from the starting of combustion to the appearance of peak pressure. Fig. 13 indicates the rapid combustion period decreases with the increase of RH2. During the rapid combustion period, the formed mixture in the ignition delay stage combusts rapidly. As mentioned above, the larger RH2 leads to longer ignition delay duration for the FPE, so the mixing quality of mixture is better. The in-cylinder gas pressure and temperature increase more due to the faster flame propagation rate of hydrogen. Besides, the increased compression ratio also enhances the in-cylinder gas temperature and pressure at the end of the compression process, which leads to faster combustion heat release rate, thus enhancing the chemical reactivity of the fuel. As a result, the duration of rapid combustion period in larger RH2 condition was shorter, and the peak pressure and temperature as well as heat release are higher. Moreover, as can be seen from Table 5, the released heat during the rapid combustion period is 193.9 J in the RH2 of 0.4 condition, which is 13% higher than that of the pure diesel operation, indicating that the hydrogen addition can improve the combustion and heat release process. The post-combustion effect is weaker in higher RH2 condition, while the post-combustion lasts shorter and releases less heat. There are two main reasons for this result. Firstly, the faster flame propagation rate of hydrogen accelerates the combustion rate of overall fuel in the cylinder, resulting in less fuel left in the post-combustion period, which shortens the post-combustion duration. Furthermore, the increase of hydrogen content speeds up the movement of the piston, and
the in-cylinder gas temperature drops quickly after TDC, as shown in Fig. 7, which is not conducive to the post combustion process. Therefore, the increase of RH2 means the reduction of the released heat during the post-combustion stage. Besides, when RH2 is 0.4, the released heat has 61.3 J, which is 43.6% lower than that of pure diesel operation.
Discussion Unlike conventional engines, the FPE abandons the crank-link mechanism, so it has the advantages of variable compression ratio, flexible fuel flexibility and so on. Hydrogen fuel is considered as an ideal alternative fuel for internal combustion engines due to the advantages of good emission characteristics and high thermal efficiency [2,15]. Therefore, after adding different proportions of hydrogen to the FPE through intake ports, the different piston displacements and boundary conditions would be obtained by multiple iterations. On the one hand, hydrogen can accelerate the combustion speed of mixture in the cylinder, on the other hand, the compression ratio, movement frequency and residual exhaust gas coefficient that vary with hydrogen content also make the mixing and combustion processes change. These factors have different effects on the combustion and emissions of the FPE.
Conclusion In this study, a coupling model was established and validated to evaluate the effect of adding a small amount of hydrogen fuel on the diesel FPE, and the influence of hydrogen addition on the combustion and emission characteristics of the diesel FPE was investigated. The conclusions are concluded as follows: 1. When RH2 is 0.1, it has only mild effect on the in-cylinder gas pressure and combustion process. However, when RH2 is more than 0.1, both the peak pressure and temperature increase with the increase of RH2, and the peak pressure and temperature in the cylinder appear earlier slightly. 2. Compared with the pure diesel operation, the addition of hydrogen can make the NO emission increase gradually, and the NO production achieves the most when the RH2 comes up to 0.4. However, there is a negative correlation between the soot emission and hydrogen addition ratio. When the RH2 is 0.4, the soot of FPE decreases by 24 times than that of pure diesel. 3. The larger RH2 induces a longer ignition delay, shorter rapid combustion period, weaker post-combustion effect, greater heat release rates, and earlier peak heat release rates for the
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FPE. However, the released heat in rapid combustion period rises significantly with the increase of RH2. 4. Adding hydrogen in diesel FPE not only increases combustion speed, but also speeds up engine operation and enlargers compression ratio. These factors together further affect the mixing, combustion and emission characteristics of the FPE.
Acknowledgements This research was sponsored by the Scientific Research Development Program of Chongqing Jiaotong University (Grant number: 15JDKJC-A011) and the Chongqing Graduate Education Innovation Fund Project (Grant number: CYS17198). We express sincere gratitude to the sponsors.
Nomenclature FPE TDC IMEP NO HC CO CO2 RH2 ECA EGR
free-piston engine top dead center indicated mean effective pressure nitric oxide hydrocarbon carbon monoxide carbon dioxide rate of hydrogen addition equivalent crank angle exhaust gas recirculation
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