Potential for Reducing Emissions in Reactivity-Controlled

Article pubs.acs.org/EF

Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Potential for Reducing Emissions in Reactivity-Controlled Compression Ignition Engines by Fueling Syngas and Diesel Zhen Xu,† Ming Jia,*,† Guangfu Xu,† Yaopeng Li,† Liang Zhao,† Leilei Xu,‡ and Xingcai Lu‡ †

Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, Dalian University of Technology, Dalian, P.R. China ‡ Key Laboratory for Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai, P.R. China ABSTRACT: A syngas/diesel dual-fuel reactivity-controlled compression ignition (RCCI) engine was numerically investigated by an improved multidimensional model coupled with a reduced chemical mechanism. In the test RCCI engine, the syngas was premixed with air in the intake manifold, while the diesel was directly injected into the cylinder well before top dead center (TDC). The effect of the syngas composition, the premixed ratio of the syngas, the initial in-cylinder temperature at intake valve closing (IVC), and the hydrogen (H2) proportion in the syngas on the RCCI combustion and emission characteristics were investigated. The results indicate that the utilization of the syngas/diesel dual-fuel strategy in the RCCI engine with lean and premixed combustion is capable of simultaneously reducing the emissions of nitrogen oxides (NOx) and soot. Compared with the gasoline/diesel RCCI combustion, the combustion characteristics of the syngas/diesel RCCI is much more complicated due to the complex composition of syngas, which plays an important role in the ignition and combustion processes. The H2 in the syngas inhibits the autoignition of the RCCI combustion and significantly affects the heat release process, while the inclusion of carbon monoxide (CO) in the syngas is beneficial to mitigate the rapid combustion rate of H2. Consistently, the addition of the inert gases (e.g., N2 and CO2) decreases the global heat release rate and ringing intensity, whereas excessive inert gases in the syngas lead to incomplete combustion and low fuel efficiency. In this study, the optimal solution with the syngas premixed ratio of 60% and the H2 volume fraction of 75% in the syngas can achieve RCCI combustion with both high fuel efficiency and low emissions. ignition first occurred in the in-cylinder region containing the highest concentration of the high-reactivity n-heptane, which increased the local temperature and initiated a reaction zone. The ignition and flame propagation proceeded from the high- to low-reactivity regions in the combustion chamber. For the cases with the lower premixed ratio, the higher n-heptane concentration provided a larger degree of flame propagation. In contrast, for the cases with the higher premixed ratio, the lower n-heptane concentration resulted in the more spontaneous ignition fronts. The physical and chemical properties of the fuel and the in-cylinder reactivity gradient were responsible for controlling the combustion event. Recently, Li et al.5 realized the methane/diesel dual-fuel RCCI combustion with methanol provided in the intake manifold. The results indicated that the fuel reactivity and equivalence ratio distributions in the cylinder are significantly affected by the methane fraction and the start of injection (SOI) of diesel. The engine with the low NOx and soot emissions and high fuel efficiency can be realized by the methanol/diesel RCCI combustion. Zoldak et al.6 numerically studied a heavy-duty RCCI engine fueled with compressed nature gas (CNG) and diesel. The results showed that the RCCI strategy had the potential for the reduction of 17.5% NOx, 78% soot, and 24% fuel consumption compared to the conventional diesel combustion at the same air−fuel ratio and exhaust gas recirculation (EGR) rate.

1. INTRODUCTION As the global environment and energy crisis are becoming serious, the internal combustion engine is under the pressure of further reduction of emissions and fuel consumption. To reduce the nitrogen oxides (NOx) and soot emissions while maintaining high fuel efficiency, the advanced combustion strategy of premixed low-temperature combustion (LTC) has been proposed, such as homogeneous charge compression ignition (HCCI), premixed charge compression ignition (PCCI), and reactivitycontrolled compression ignition (RCCI). In RCCI, a low-reactivity fuel is premixed with the air via the port fuel injection, while a high-reactivity fuel is directly injected in the cylinder during the early compression stroke.1 Through the dual-fuel, partially premixed combustion in RCCI, the combustion phasing can be more flexibly controlled, and the operating range can be considerably expanded compared to HCCI and PCCI. The engine research center (ERC) at the University of WisconsinMadison is in a leading position in the research of the RCCI engine.2 Splitter et al.3 compared the gasoline/diesel and ethanol/diesel dual-fuel RCCI combustion in a heavy-duty engine. It was found that the ethanol/diesel operation can meet the 2010 heavy-duty emission standards of the U.S. Environmental Protection Agency (EPA) for NOx and soot at all the test conditions, and the maximum gross indicated the thermal efficiency was as high as 56%. Based on direct numerical simulation (DNS), Bhagatwala et al.4 studied the characteristics of the autoignition and flame propagation in an n-heptane/ iso-octane RCCI engine. It was indicated that the spontaneous © XXXX American Chemical Society

Received: October 24, 2017 Revised: January 5, 2018 Published: February 6, 2018 A

DOI: 10.1021/acs.energyfuels.7b03265 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

experiment on the syngas/diesel engine under different loads was performed by Rinaldini et al.11 The results indicated that the employment of syngas benefits the improvement of fuel consumption and the combustion quality compared to the original diesel engine. Furthermore, Rahnama et al.20 investigated the effect of adding hydrogen, nitrogen, and syngas on the combustion and exhaust emissions of a natural gas/diesel RCCI engine. The results showed that the introduction of hydrogen and syngas as an additive is able to improve the combustion process of the engine and reduce the HC and CO emissions at low loads. However, with the addition of syngas, the excessively high peak pressure rise rate and the high ringing intensity were observed at mid loads. It can be found from the previous studies that, although the influence of syngas on the combustion and emission characteristics of CI engines has been investigated, the optimal solution for the engine operation with syngas as fuel has not been well understood.10 In addition, the investigations on the influence of the syngas compositions were mainly performed in the conventional dual-fuel engines, in which the diesel injection timing is around the TDC for igniting the syngas and the diffusion combustion dominates the combustion process. Thus, further investigation of the syngas/diesel RCCI combustion is required in order to understand the influence of the fuel composition and reactivity on the engine combustion and emission characteristics. In this study, multidimensional simulations were employed to explore the performance of the syngas/diesel dual-fuel RCCI engine. The influence of the syngas composition, the premixed ratio of syngas, the initial in-cylinder temperature at intake valve closing (IVC), and the proportion of H2/CO in the syngas on the RCCI combustion was discussed in detail.

As an alternative fuel, syngas is promising for internal combustion engines. Syngas is composed of hydrogen (H2), carbon monoxide (CO), and very often some carbon dioxide (CO2), methane (CH4), and nitrogen (N2). The complex components in the syngas are primarily due to its extensive sources. The main sources of the syngas include the catalytic oxidation of liquid hydrocarbon fuels, coal gasification, methane reformation, and biomass gasification.7,8 The primary combustible gases in the syngas are H2 and CO, and the inclusion of H2 leads to the increased combustion temperature and heat release rate, which further promote the formation of the NOx emissions in spark ignition (SI) engines.9 By decreasing the combustion temperature, the lean-burn combustion with compression ignition (CI) is beneficial to the reduction of NOx emissions for the application of syngas in engines. However, the low reactivity of syngas is not in favor of the ignition in CI engines. The injection of a high-reactivity fuel in the cylinder has potential to ignite the syngas. Most of the previous studies on CI combustion with syngas as fuel focused on the syngas/diesel dual fuel. Especially, with the recent advances in the production technique of syngas and the demand of clean energy, the syngas/ diesel CI engine has attracted increasing attention.10,11 Garnier et al.12 studied the application of syngas as the primary fuel in a single-cylinder direct-injection diesel engine. By investigating the substitution of diesel with syngas on the combustion characteristics and engine performance, it was found that the fraction of diesel should be minimized, while the substitution by syngas should be maximized in wide engine load and speed ranges. Furthermore, Boehman and Corre13 discussed the influence of the syngas addition on the combustion and emissions in a diesel engine. Based upon the results of the hydrogen-assisted CI combustion, a modest amount of syngas addition in combination with the advanced injection timing of diesel was able to achieve the high-efficiency, low-temperature combustion. Using H2 and CO as the surrogate of syngas, Bika et al.14 investigated the influence of H2 proportion on the syngas combustion and emissions in a diesel CI engine. The results showed that the increased H2 fraction in the syngas improved fuel efficiency, although the addition of syngas led to lower fuel efficiency relative to the baseline diesel engine because of more unburned gaseous fuel. Meanwhile, the NOx emissions increased with more diesel substitution at various H2/CO proportions, and the CO emissions were mainly caused by the CO in the syngas. Recently, Shan et al.15 and Wang et al.16 performed an experiment on the syngas/diesel CI engine with two biomass syngases containing a large amount of inert gases (N2 and CO2) to study the effect of H2/CO ratio and EGR rate on the combustion and emission characteristics. It was found that higher H2/CO ratio improved the indicated thermal efficiency but led to the increase of NOx emissions. The combustion with both high efficiency and low emissions in the syngas/diesel CI engine can be achieved through optimizing the H2/CO ratio, EGR rate, and injection strategy. Consistently, Sahoo et al.17,18 found that the thermal efficiency of the syngas/diesel dual-fuel CI engine is lower than that of the diesel engine. It was suggested that the syngas/diesel engine should be optimized by adjusting the syngas energy release as close as possible to top dead center (TDC) and advancing the injection timing of the diesel fuel. Recently, Yamasaki et al.19 realized the HCCI combustion of syngas. In order to achieve the compression ignition of syngas, the piston was modified, and the compression ratio was increased from 9.5 to 24.0. The results indicated that the volume ratio of H2 to CO determined the ignition timing and combustion duration. In addition, the

2. NUMERICAL MODELS AND VALIDATIONS 2.1. Numerical Models. In this study, the KIVA-3V code was used to perform the simulations of RCCI combustion, which has been supplemented by several enhanced physical and chemical submodels. The Re-Normalization Group (RNG) k−ε model21 was utilized as the turbulence model. The hybrid Kelvin Helmholtz−Rayleigh Taylor (KH-RT) model22 was adopted as the droplet breakup model. In addition, a new spray/wall interaction model proposed by Zhang et al.23 was employed to reproduce the size and velocity of the splashed droplets, which improved the predicted HC and CO emissions in RCCI combustion. Moreover, the Han and Reitz wall heat transfer model24 with the improvement by distinguishing the heat transfer in the laminar and turbulent boundary layer25 was introduced. In order to couple the detailed chemistry for the fuels, the CHEMKIN solver was integrated with the KIVA-3V code. The nonchemical processes of the diesel were modeled by the C12H26 Cummins model,26 while the chemical kinetics of the diesel was simulated by a skeletal n-heptane oxidation mechanism27 with 56 species and 176 reactions due to the similar ignition characteristics of n-heptane with that of diesel. The skeletal n-heptane mechanism was constructed based on a decoupling methodology, in which a detailed H2/CO/C1 oxidation mechanism28,29 was used as the “core” and a simplified model for the C4−C7 species was introduced to predict the ignition and oxidation behaviors of n-heptane. Thus, the n-heptane mechanism is capable of reproducing the ignition and combustion characteristics of both n-heptane and syngas. Furthermore, the extended Zeldovich NOx mechanism was employed to predict the emissions of NOx. For soot simulation, an updated phenomenological soot model30 was applied. B


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Energy & Fuels 2.2. Model Validations. In order to prove the reliability of the above computational model, the simulation results were validated with the experimental data of the syngas/diesel and H2/ diesel dual-fuel engine according to the work of Shan et al.15 and Saravanan et al.,31 respectively. The test engines were modified from a four-cylinder15 and a single-cylinder diesel engine,31 and the dual-fuel strategy was realized by the introduction of the incylinder direct injection of diesel and the port injection of the gas fuel. The detailed specifications of the two engines are listed in Table 1. Under the test loads in the experiments, the indicated

exhaust processes were not covered in the simulation, and the incylinder mixture at intake valve closing (IVC) was assumed to be uniform with the premixed syngas/air mixture. For the syngas/diesel engine of Shan et al.,15 Figure 2 compares the in-cylinder pressure predicted by the computational model and the measurements under different CO2/syngas ratios for the two syngases. It can be seen that the overall ignition and combustion behaviors of diesel/syngas are satisfactorily reproduced by the model. Further comparisons of the emissions of NOx and CO as a function CO2/syngas volume ratio between the predictions and the measurements are shown in Figure 3. As can be observed for both the syngases, the NOx emissions decrease, while the CO emissions increase at higher CO2 ratio. The trend of NOx and CO emissions with the variation of the CO2/syngas volume ratio is captured by the computational model reasonably well. Further validations of the in-cylinder pressure and exhaust emissions for the H2/diesel (corresponding to 100% H2 in the syngas tested in this study) engine from Saravanan et al.31 are shown in Figure 4. As can be seen, the predictions are in good agreement with the measurements. Moreover, the predicted maximum pressure rise rates also agree well with the measurements, which indicate that the computational model is capable of satisfactorily reproducing the RI characteristics of the test engine. It is worth noting that, for all the cases modeled herein, the submodels and the constants in the model were kept unchanged, and only the operating conditions were varied in accordance with the experimental data. The satisfactory agreements between the simulations and the measurements on the in-cylinder pressure and emissions prove the fidelity of the present model. Therefore, the computational model was employed to further study the combustion and emission characteristics of the syngas/diesel RCCI engine.

Table 1. Engine Specifications (a) Shan et al.15 bore × stroke (mm) connecting rod length (mm) number of valves intake valve closing (IVC) (°CA ATDC) exhaust valve opening (EVO) (°CA ATDC) displacement (L) compression ratio nozzle hole number nozzle hole diameter (mm) spray included angle (deg) pilot/main injection timing (°CA BTDC)

(b) Saravanan et al.31

113 × 130 216.0 4 −144.5

80 × 110 230.1 4 −145

112.5

145

1.325 18 7 0.24 150 60/10

0.553 16.5 3 0.20 120 23 (single injection)

mean effective pressures (IMEPs) were around 0.6 MPa15 and 1.0 MPa.31 The instantaneous injection rate and injection duration were measured by the monoinjection qualifier (EFS8246) by Shan et al.15 for the engine tested in this study, which were used for the simulation. Meanwhile, the syngas/diesel energy ratio was maintained at 50/50%,15 and the H2/diesel energy ratio was 6.3/93.7%.31 The detailed compositions of the two test syngases from Shan et al.15 are listed in Table 2, and more about the experimental configuration can be found in refs 15 and 31.

3. RESULTS AND DISCUSSIONS In order to reveal the potential of syngas for RCCI combustion, the comprehensive fuel supply strategies with different syngas compositions and various premixed ratios of syngas are investigated in this section. This study especially focuses on the lowto-medium load and medium speed, thus no EGR is introduced in order to improve the fuel efficiency. It was indicated by Ma33 and Splitter34 that the RCCI combustion with the single injection of diesel was capable of providing high thermal efficiency and low emissions, thus a single injection of diesel with the injection timing well before TDC is employed in this study for simplicity. The total fuel energy imported to the cylinder per engine cycle was fixed at 1500 J by varying the energy fraction of syngas and diesel. The premixed ratio (Pr) is defined as the energy ratio of the premixed syngas, and it is calculated as

Table 2. Compositions of the Test Syngases15 component

syngas A

syngas B

H2 (% vol.) CO (% vol.) CH4 (% vol.) N2 (% vol.)

5 40 5 50

15 30 5 50

The computational mesh used for the simulation shown in Figure 1 was generated by ANSYS ICEM CFD. It was found in

Pr =

Es ms × Hs = Es + Ed (ms × Hs) + (md × Hd)

(1)

where ms is the mass of syngas; md is the mass of diesel provided by the fuel system; and Hs and Hd represent the lower heating value of syngas and diesel, respectively. More detailed operating conditions are listed in Table 3. 3.1. Influence of Syngas Composition on RCCI Combustion. The components in a representative syngas can be divided into the combustible gases and the incombustible gases. The main components of the combustible gases are H2, CO, and a bit of CH4, while the incombustible gases consist of CO2 and N2. Due to the differences in the syngas production approaches and the variation of the operating conditions, the composition of

Figure 1. Computational mesh at TDC.

the previous study that the substitution of the whole cylinder with a sector can satisfactorily reproduce the in-cylinder pressure, heat release rate, and exhaust emissions,32 and thus a 51.43° (1/7th of the cylinder) mesh and a 120° (1/3th of the cylinder) mesh were employed in this study to save the computational time according to the number of the nozzle hole. The intake and C


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Figure 2. Comparisons of the in-cylinder pressure between simulations and measurements.

Figure 3. Comparisons of the emissions of CO and NOx between simulations and measurements.

Figure 4. Comparisons of the in-cylinder pressure and exhaust emissions between simulations and measurements of the H2/diesel engine.

combustion and emission characteristics, eight typical syngases available at present are investigated in this section. The detailed components of the test syngases are listed in Table 4 in order of

the syngas varies significantly, which further changes the physical and chemical properties of the syngas. In order to understand the impact of the syngas composition on the syngas/diesel RCCI D


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ignition characteristics can be affected by the heat absorption capacity and chemical reactivity of the syngas.38 From the in-cylinder temperature distribution at 5 °CA BTDC before CA10 shown in Figure 6, it can be observed that the temperature in the center region of the cylinder is similar for syngases 4, 6, and 8 due to the close specific heat ratio of the three syngases (see Figure 7a). It can also be found from Figure 6 that the temperature at the piston bowl lip is considerably higher than that of the other nearwall regions. The high-temperature region is caused by the lowtemperature diesel oxidation at the piston bowl lip with the accumulation of the diesel vapor due to the strategy of the advanced diesel injection. As can be found from Figures 4 and 5, the lowtemperature heat release of syngas 4 is more evident than that of syngases 6 and 8, which is mainly due to the variation of the chemical reactivity of the syngas/diesel mixture for the three syngases. Previous studies39 indicate that H2 and CO inhibit the autoignition of diesel because the active OH free radicals required by the diesel low-temperature oxidation are consumed, and the inhibition effect of H2 is more pronounced than that of CO at

Table 3. Engine Operating Conditions engine speed (rev/min)

1500

IMEP (MPa) initial in-cylinder pressure at IVC (bar) initial in-cylinder temperature at IVC (K) diesel injection pressure (bar) diesel injection timing (°CA ATDC)

0.5 1.7 330−400 800 −50

the lower heating value (LHV) of the syngas. For the cases tested in this section, the initial temperature at IVC and the premixed ratio of the syngas are fixed at 360 K and 60%, respectively. The other operating parameters can be found in Table 3. As shown in Figure 5, the combustion phasing of RCCI combustion fueled with different syngases are significantly sensitive to the syngas composition. For the syngases consisting only of H2 and CO (i.e., syngases 4, 6, and 8 shown in Figure 5a), the 10% heat release point (CA10) retards with the increased H2 fraction. In this study, CA10 represents the ignition timing and can be reflected by the heat release traces shown in Figure 5. The Table 4. Compositions of Typical Syngases syngas

H2 (%)

CO (%)

CO2 (%)

CH4 (%)

N2 (%)

LHV (MJ/kg)

ref

syngas 1 syngas 2 syngas 3 syngas 4 syngas 5 syngas 6 syngas 7 syngas 8

10 31.9 50 50 69.5 66.6 70.4 75

25 14.8 20 50 19.9 33.3 13.6 25

4 5.3 5 − − − 8.6 −

12 5.4 20 − 9.9 − 7.4 −

49 42.6 5 − 0.7 − − −

4.71 8.04 12.26 17.35 19.28 23.72 27.99 29.35

12 35 36 14 37 37 37 14

Figure 5. Evolutions of the in-cylinder pressure and heat release rate for different syngases.

Figure 6. In-cylinder temperature distributions for different syngases at 5 °CA BTDC. E


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Figure 7. Evolutions of the specific heat ratio for different syngases.

except for syngas 1. In contrast, due to the higher fraction of H2 and polyatomic molecule gas in syngas 3, the more significant inhibition effect from H2 and the lower specific heat ratio result in the latest CA50 among various syngases. Moreover, with the addition of CO2 and N2 in the syngas, the specific heat ratio and the chemical reactivity of the premixed syngas/air mixture reduce, which lead to the inhibition of the autoignition of the diesel. Compared with N2, CO2 has a more significant effect on the heat release process,18 and the lower specific heat ratio of CO2 reduces the energy quality in the expansion stroke, which further leads to the lower thermal efficiency. As shown in Figure 5, the peak of the heat release rate decreases, and the combustion duration extends with the addition of CO2 and N2 from syngas 8 to syngas 5 in the major heat release process. However, Hagos et al.42 found that the flammability limit of the syngas containing N2 and CO2 was narrowed, and the addition of H2 was able to broaden the flammability limit. Thus, excessive addition of the incombustible gases and the reduction of H2 fraction in syngas 1 cause the incomplete combustion (see Figure 5b). The addition of N2 and CO2 has a negative effect on the combustion efficiency of the dual-fuel engine, which is consistent with the conclusion from Wagemakers and Leermakers.43 From the above results, it can be concluded that the composition of the syngas determines the combustion characteristics of the RCCI engine. Generally, the proportion of CH4 in the syngas is too low to directly affect the engine performance, while the inert gases (i.e., N2 and CO2) in the syngas usually play a negative role in the combustion efficiency. In contrast, the syngases containing only H2 and CO are more prominent on the combustion efficiency. In order to deeply understand the influence of the syngas composition on the RCCI combustion and emissions, the representative syngas composed of H2 and CO is further investigated in the following sections. 3.2. Effect of the Premixed Ratio and the Initial Temperature of the Syngas on RCCI Combustion. As indicated by Bhagatwala et al.,4 the autoignition and combustion processes on RCCI engines are significantly affected by the premixed ratio. At the lower premixed ratio, the higher diesel concentration provides a larger degree of flame propagation, whereas the lower diesel concentration at the higher premixed ratio results in more spontaneous ignition fronts. In order to investigate the effects of the premixed ratio of the syngas (Pr) and the initial temperature at IVC (Tivc) on the RCCI combustion, syngas 8 with the volume ratio of H2/CO = 75/25 is chosen as the test syngas in this section. For the test cases, the premixed ratio is swept from 10% to 70%, and the initial temperature is swept from

temperatures below 1000 K. Thus, for the syngas containing only combustible gases, increasing H2 fraction from 50% in syngas 4 to 75% in syngas 8 results in the reduced low-temperature heat release rate of diesel (see Figure 4a). In RCCI combustion with diesel as the high-reactivity fuel, the low-temperature heat release of diesel determines the ignition timing.39 The heat released by diesel ignites the combustible gases in the syngas around TDC and leads to the peak of the heat release rate in the combustion stage, as shown in Figure 5a, and the in-cylinder pressure reaches its peak at the same time. With the increasing concentration of H2 from syngas 4 to syngas 8, the combustion duration shortens, and the peak heat release rate increases owing to the faster combustion rate and flame speed of H2 than those of CO.40 Meanwhile, because the combustion temperature of H2 is much higher than that of CO, the combustion efficiency is improved with the increased H2 fraction in the syngas. The effect of H2 on the combustion characteristics is similar to the results of Mohamed and Ramesh.41 Due to the complicated dilution, thermal, and chemical effects, the inclusion of the incombustible gases and methane in the syngas considerably affects the ignition and combustion of the syngas/diesel RCCI engine. It can be easily found in Figure 6 that the compressed temperature in the cylinder is considerably different for syngases 1, 2, 3, 5, and 7. This is primarily due to the difference in the specific heat ratio of the syngas/air mixture, as shown in Figure 7b. Moreover, the chemical inhabitation effect of the syngas on the diesel ignition varies from different syngases. Besides the inhibition effects of H2 and CO on the autoignition of the diesel as mentioned above, CH4 in the syngas also transforms the active OH radical to stable H2O2 in the low-temperature oxidation.40 Thus, the activity of the diesel ignition reaction is dramatically decreased with the addition of H2, CO, and CH4 in the syngas. Overall, because of the combined influence of the heat absorption capacity and the chemical inhibition from the syngas, the ignition timing of RCCI combustion is determined by the composition of the syngas, as shown in Figure 5b. For the syngases investigated in this study, the specific heat ratio of the syngas plays a significant role in the autoignition of RCCI combustion. It can be found from Figure 5b that, for syngas 5 with the largest fraction of H2 and the smallest fraction of the polyatomic molecule gas (i.e., CO2 and CH4), although the low-temperature heat release is the latest and lowest due to the inhibition effect of H2 on the ignition of diesel, the highest specific heat ratio of syngas 5 advances the ignition time. Thus, the start of the ignition of the syngas 5 is the earliest compared to the other syngases F


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Figure 8. Evolution of the in-cylinder pressure and HRR at different initial temperatures and premix ratios.

330 to 390 K. As the initial in-cylinder pressures are kept constant for the cases with different initial temperatures, the global equivalence ratio is in the range of 0.22 to 0.26. The computational results indicate that the slight variation of the global equivalence on the engine performance can be neglected. Figure 8(a) shows the RCCI combustion characteristics under various initial temperatures at the premixed ratio of 60%. It can be found that the initial temperature plays an important role in the combustion phasing and the heat release process. At a higher initial temperature, the combustion rate of diesel is enhanced, which further leads to the higher heat release rate of the syngas. When the initial temperature is higher than 360 K, the combustion phasing of CA50 is excessively ahead of TDC, and the peak of the heat release rate significantly rises, leading to the diminished combustion duration. However, the advanced combustion phasing results in the increase of the compression work and the heat transfer from the cylinder wall, which decrease IMEP. Meanwhile, at advanced combustion phasing, the high in-cylinder pressure rise rate results in excessively high ringing intensity (RI), which will be discussed below. On the other hand, once the initial temperature is as low as 340 K, the considerably retarded CA50 results in the incomplete combustion of the syngas with very low heat release rate. Thus, a reasonable range of the initial temperature is required for RCCI to achieve the optimal combustion process. The influence of the premixed ratio of the syngas on the incylinder pressure and heat release rate at the initial temperature of 360 K is shown in Figure 8(b). With more premixed syngas and less injected diesel, the combustion process transits from the diesel PCCI combustion to the syngas HCCI combustion with considerable variation of the combustion characteristics. As the premixed ratio of the syngas increases, the ignition timing is retarded significantly owing to the decreased fuel reactivity. The efficient combustion of the syngas can be realized with the mild pressure rise rate when the premixed ratio is higher than 40%. In short, by adjusting the premixed ratio in RCCI, the ignition and heat release processes of diesel/syngas can be flexibly controlled. The combined influence of the initial temperature and the premixed ratio on CA50 is shown in Figure 9. It can be found that the ignition timing retards linearly with both decreasing the initial temperature and increasing the premixed ratio. The excessively high premixed ratio and low initial temperature lead to the misfire of the fuel/air mixture. Moreover, the dependence of CA50 on the initial temperature and the premixed ratio is nearly identical. This is beneficial for the control of the combustion

Figure 9. CA50 distributions at different initial temperatures and premix ratios.

phasing and further optimization of the combustion and emissions, which will be discussed below. To understand the influence of the initial temperature and the premixed ratio on fuel efficiency, the equivalent indicated specific fuel consumption (EISFC) is introduced in this study, which has been widely applied to evaluate the fuel economy in dual-fuel engines.44 EISFC is calculated as EISFC =

Md × Hd + Ms × Hs Hd × Wi

(2)

EVO

Wi =

∫IVC

p × dV

(3)

where Md is the diesel mass injected into the cylinder; Ms is the syngas mass premixed in the intake port; Hd and Hs are the lower heating value of diesel and syngas, respectively; and Wi is the indicated work from IVC to EVO, which is calculated by the integral of the in-cylinder pressure and volume based equation 3. Figure 10 shows the distribution of EISFC under various initial temperatures and premixed ratios. In the region with the low initial temperature, the incomplete combustion of diesel/syngas leads to high EISFC. With the increasing initial temperature, the combustion efficiency is improved. However, excessively high initial temperature results in CA50 before TDC and subsequently the increased compression work and heat transfer losses, thus EISFC deteriorates. It is consistent with the finding from Dempsey et al.45 that the EISFC of RCCI is closely relevant to CA50. It can be found from Figure 10 that, for the achievement of the optimal EISFC, the high initial temperature is required with an increase of the premixed ratio, and the lowest initial temperature is 340 K. G


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where γ is the specific ratio; (dP/dt)max is the maximum pressure rise rate; Pmax and Tmax are the maximum in-cylinder pressure and temperature; and R is the ideal gas constant. The variations of RI with the initial temperature and the premixed ratio are shown in Figure 12. It can be found that the

Figure 10. EISFC distributions under various initial temperatures and premix ratios.

As shown in Figure 10, the increase of the premixed ratio benefits the improvement of EISFC, which is primarily due to the following three reasons. First, with the increasing premixed ratio, the liquid diesel adhered on the piston bowl, resulting from the decrease in advanced injection of diesel, which can be seen from the unburned liquid fuel shown in Figure 11; thus, the

Figure 12. Ringing intensity distributions under various initial temperatures and premix ratios.

operating region with the RI lower than 5 MW/m2 is mainly limited in the premixed ratio range of 20−70% and the initial temperature range of 330−370 K. For the region with the premixed ratio less than 30%, the effect of the premixed ratio on RI is more significant than that of the initial temperature, whereas the influence of the premixed ratio weakens in the region with the higher premixed ratio. With further increase of the premixed ratio, the primary fuel for RCCI combustion migrates from the injected diesel to the premixed syngas, and higher initial temperature is required to obtain the high-efficiency combustion of syngas. However, the excessively high initial temperature results in the undesirably high RI. With both the restriction of RI and combustion efficiency, the range of the initial temperature is narrowed at the higher premixed ratio. Meanwhile, the sensitivity of RI to the initial temperature is the lowest when the premixed ratio is between 40% and 50%, which is the optimal range of the premixed ratio with the acceptable RI. From the NOx emissions depicted in Figure 13, it can be observed that NOx is highly sensitive to the initial temperature

Figure 11. Unburned liquid fuel ratio under different premix ratios.

combustion efficiency increases. It is interesting that the unburned liquid fuel slightly increases at the higher initial charge temperature under some conditions, which is mainly due to the decreased oxygen concentration in the local region of the cylinder. However, the influence of the intake temperature on the fraction of the unburned liquid fuel is rather small, and the variation of the global equivalence ratio on the combustion process can be neglected for the cases tested in this study. Second, the larger proportion of H2 with more syngas accelerates the heat release rate, and the nearly constant-volume combustion improves EISFC. Third, the retarded CA50 with the increased premixed syngas reduces the negative compression work before TDC and the heat transfer losses, which also contributes to the better EISFC. Overall, the optimal premixed ratio is higher than 30% for the achievement of the EISFC less than 200 g/kWh. Due to the high combustion rate of H2, the engine knock becomes a serious problem with the utilization of syngas.46 Ringing intensity (RI) has been widely used to quantitatively evaluate the knock in the engine with LTC.47 The definition of RI is as follows

Figure 13. NOx distributions under different initial temperatures and premix ratios.

and the premixed ratio when the premixed ratio is less than 40%. In this region, the formation of the NOx emissions mainly results from the local high-temperature combustion of diesel, and the increased initial temperature leads to the advanced ignition timing. This leads to the extended residence time of the in-cylinder mixture

2

Ringing Intensity ≈

1 (0.05·(dP /dt )max ) · · γRTmax 2γ Pmax (4) H


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Energy & Fuels in the high-temperature conditions, inducing the increased NOx emissions. With the addition of syngas, CA50 is retarded, and the lean combustion of the premixed syngas/air mixture plays a more important role in the combustion process, which reduces the maximum combustion temperature, resulting in the reduced NOx emissions. Especially, when the premixed ratio is larger than 50%, the NOx emissions are nearly independent of the initial temperature and maintain very low level. Therefore, it can be concluded that the lean combustion of the premixed syngas is capable of effectively inhibiting the formation of NOx in a wide range of the initial temperature. The variation of the soot emissions at different initial temperatures and premixed ratios is exhibited in Figure 14.

Figure 15. Evolutions of the in-cylinder pressure and HRR under various H2 volume fractions.

H2 than CO is responsible for the faster heat release rate.50 Therefore, as soon as H2 in the lean premixed charge is ignited by the diesel fuel, rapid combustion occurs. Furthermore, the higher H2 fraction is helpful for the improvement of the combustion efficiency at the lean combustion mode, which can be observed from the considerably increased in-cylinder pressures in the expansion stroke in Figure 15. From the CA50 distributions shown in Figure 16, the effect of the H2 fraction and the initial temperature on CA50 can be Figure 14. Soot distributions under different initial temperatures and premix ratios.

It can be found that the addition of syngas can effectively reduce the soot emissions. The components of H2 and CO rather than the macromolecular hydrocarbons in the syngas are beneficial to the formation of the soot precursors and subsequently the reduction of soot emissions. On the other hand, the rich-fuel region is reduced with the increased premixed ratio, which is favorable to reduce the soot emissions. As can be seen from Figure 14, when the premixed ratio is larger than 50%, the soot emissions are considerably reduced. Overall, under the restriction of EISFC less than 200 g/kWh, RI less than 5 MW/m2, and NOx less than 0.4 g/kWh, the satisfactory region for diesel/syngas RCCI operation is 340 K < Tivc < 380 K and 30% < Pr < 70%. 3.3. Effect of H2 Proportion in the Syngas on RCCI Combustion. As indicated in previous studies, the ratio of H2 and CO in the syngas significantly affects the combustion and emissions of the syngas engines due to the considerable difference in the chemical characteristics between H2 and CO.48 In this section, the effect of the H2 proportion in the syngas on the RCCI combustion and emission is investigated under various initial temperatures. For the cases tested herein, the premixed ratio is kept at 60% because of the optimal engine performance in the combustion and emissions, as indicated in the previous section. It can be seen from Figure 15 that the combustion phasing and the heat release rate are extremely sensitive to the variation of the H2 fraction. The inhibition effect of the syngas on the diesel autoignition is enhanced by the increased H2 fraction in the syngas, leading to the retarded ignition timing of the diesel fuel. However, the peak of the heat release rate is larger, and the combustion duration is shorter with the increased proportion of H2, which was also experimentally observed by Karagöz et al.49 The wider flammability limit and the higher combustion rate of

Figure 16. CA50 distributions under different initial temperatures and H2 volume fractions.

well illustrated under the combined influences of the initial temperature, the inhibition effect of H2 and CO on the diesel autoignition, and the enhancement of H2 on the heat release rate. As mentioned above, the ignition timing of diesel monotonically retards with the increasing H2 proportion due to the inhibition effect of H2 on the low-temperature oxidation of diesel. However, the combustion duration of CA10−CA50 is shortened with the increased H2 fraction resulting from the high combustion rate of H2 (see Figure 17). Therefore, as depicted in Figure 16 CA50 is nearly insensitive to the H2 fraction and is dominated by the initial temperature. Figure 18 illustrates the EISFC distribution under various initial temperatures and H2 proportions. As indicated in Section 3.2, the combustion efficiency and CA50 determine the EISFC of RCCI combustion. In the incomplete combustion region where the combustion efficiency is less than 90%, both the initial temperature and the H2 proportion dramatically affect EISFC due to the variation in the combustion efficiency. As a result, the emissions of HC and CO in the incomplete combustion region are very high, as can be seen from Figure 19. The increase of the I


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Energy & Fuels

Figure 17. Combustion phasings under different initial temperatures and H2 volume fractions. Figure 20. RI distributions under different initial temperatures and H2 volume fractions.

distribution can be divided into three regions based on the combustion characteristics. Low RI is achieved in region A with the low H2 proportion or the low initial temperature due to the incomplete combustion of the syngas with the combustion efficiency of less than 90%. With the increasing combustion efficiency in region B with the high initial temperature at low H2 fraction or the low initial temperature at the high H2 fraction, the moderate heat release rate of the syngas can avoid the seriously high RI, and the boundary of the initial temperature with acceptable RI varies with the H2 fraction. However, with the further increased initial temperature and H2 proportion in region C, the high HRR and the advanced CA50 lead to the excessively high RI of more than 5 MW/m2, resulting in serious engine knock. The NOx and soot emissions are further demonstrated in Figure 21. It is evident that the NOx emissions are nearly independent of the H2 proportion because the high-temperature regions in the cylinder are dominated by the local diesel combustion rather than the premixed H2 combustion, which can be observed from the distribution of the in-cylinder temperature in Figure 22(a) and 22(b) for the cases with the initial temperature of 370 K. Consistently, Bika et al.14 experimentally observed that the NOx emissions nearly kept constant under various H2 proportion in the syngas. In contrast, by directly varying the combustion temperature and the ignition timing, the initial temperature determines the NOx emissions. As shown in Figure 22(b) and 22(c), the local high-temperature regions are more significant and expanded with the increasing initial temperature, which promotes the formation of NOx. The overall soot emissions are controlled in an acceptable level as shown in Figure 21(b) because the soot formation is effectively reduced by the premix

Figure 18. EISFC distributions under different initial temperatures and H2 volume fractions.

H2 fraction in the syngas is favorable for the reduction of HC and CO emissions due to its wide flammability and fast combustion rate, which is consistent with the study of Bhaduri et al. in a HCCI engine with the biomass-derived syngas as fuel51 and the investigation of Karagöz et al. in a CI engine with the hydrogen and methane addition.52 With the increase of the initial temperature and the H2 volume fraction, EISFC and the emissions of HC and CO are effectively reduced owing to the improved combustion efficiency and the optimal CA50. However, when the initial temperature is beyond 370 K, the improvement of EISFC becomes insignificant with the higher initial temperature because CA50 is well before TDC. The addition of H2 is beneficial to improve the combustion efficiency and EISFC, but the rapid combustion rate resulted from the larger H2 fraction leading to the increased peak of HRR.53 Figure 20 shows the RI distribution under various initial temperatures and H2 volume fractions. As can be seen, the RI

Figure 19. HC and CO emissions under different initial temperatures and H2 volume fractions. J


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Energy & Fuels

Figure 21. NOx and soot variations with different initial temperatures and H2 volume fractions.

Figure 22. In-cylinder temperature distributions for three cases with different initial temperatures and H2 volume fractions.

combustion of the syngas. Under the test operating conditions, the final soot emissions are dominantly controlled by the process of the soot oxidation. For the cases with higher H2 volume fraction, lower initial temperature is required, resulting in slower soot oxidation rate. Thus, the effect of the initial temperature on soot emissions is more obvious at higher H2 volume fraction. By summarizing the optimal operating region for the diesel/ syngas RCCI combustion in Figure 21, it can be found that the initial temperature and the H2 volume fraction should be optimized simultaneously; i.e., high initial temperature is required at low H2 volume fraction, while low initial temperature should be employed at high H2 volume fraction.

4. SUMMARIZATION The overall influences of the initial temperature at IVC, the premixed ratio of the syngas, and the H2 volume fraction in the syngas on CA50, EISFC, RI, and emissions are summarized in Figures 23−25. According to the above results, the case with the initial temperature of 360 K, the premixed ratio of 60%, and the H2 volume fraction of 75% are chosen as the baseline case. The variation of the initial temperature, the premixed ratio, and the H2 fraction relative to the baseline case are calculated as follows Variation =

V i − V 0i i i |V max − V min |

Figure 23. CA50 and RI with the changes of the three operating parameters.

symbols. Except for the H2 volume fraction, the variations of the initial temperature and the premixed ratio significantly affect CA50, which is closely associated with RI (see Figure 23(b)). To realize RI lower than 5 MW/m2, the lower initial temperature, smaller H2 proportion, and optimal premixed ratio are required. From the variations of EISFC, HC, and CO emissions with the three parameters shown in Figure 24, it can be found that the overall trends of EISFC, HC, and CO emissions are identical, which generally decrease with the higher H2 fraction, premixed ratio, and initial temperature. To avoid the high EISFC (EISFC > 220 g/kWh) represented by the hollow symbols in Figure 24(a), a H2 volume fraction higher than 37.5%, a initial temperature larger than 350 K, and a premixed ratio higher than 20% should be employed. Furthermore, it is worth nothing that, when the premixed ratio exceeds 50% (−14.3% variation in Figure 24), the CO emissions begin to increase. This is due to the fact that the injected diesel is

(5)

where Vi and Vi0 are respectively the ith operating parameter in the test case and the baseline case; Vimax and Vimin are, respectively, the maximum and minimum value of the ith operating parameter, as listed in Table 5. Thus, the magnitude of the three operating parameters can be represented by the nondimensional parameter “variation”. Figure 23(a) shows the effect of the three operating parameters on CA50 and RI, in which the operating parameters with high RI (RI > 5 MW/m2) are represented by the hollow K


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Energy & Fuels

are limited in a narrow range (see the solid symbols in Figure 25). Especially, the critical RI of 5 MW/m2 considerably restricts the increase of the premixed ratio and the H2 volume fraction.

5. CONCLUSIONS The present study investigates the combustion process of the syngas/diesel RCCI engine by using a multidimensional model. Comparing with the conventional syngas/diesel dual-fuel engines, the inhibition effect of the syngas on the diesel ignition is more significant, and the NOx emissions can be controlled in a satisfactory level by increasing the premixed ratio. Furthermore, the influences of the syngas composition, the premixed ratio of the syngas, the initial temperature at IVC, and the H2 proportion in the syngas on the combustion and emission characteristics are discussed in detail. The conclusions are revealed as follows. (1) The components of the syngas play a significant role in the RCCI combustion process. The H2 fraction in the syngas considerably affects the combustion phasing due to its obvious inhibition effect on the diesel ignition. The addition of CO is beneficial to mitigate the rapid combustion of H2, but excessive CO in the syngas leads to incomplete combustion. The inclusion of the inert gases (i.e., N2 and CO2) decreases the combustion rate and intensity, whereas high level of the inert gases leads to the reduced fuel efficiency. (2) The lean premix combustion of the syngas benefits the decrease of the in-cylinder temperature, which subsequently reduces the NOx emissions. The high premixed ratio of the syngas is favorable for the decrease of the soot emissions owing to the exclusion of the macromolecule hydrocarbon fuel in the syngas and the premixed syngas combustion. (3) The combustion phasing is simultaneously affected by the initial temperature and the premixed ratio. In order to realize the high efficiency and clean combustion, the high initial temperature and premixed ratio are preferable. However, under the restriction of RI, the initial temperature constricted in a narrow range, and RI is less sensitive to the initial temperature in the region with the premixed ratio between 40% and 50%. (4) The H2 volume fraction in the syngas significantly affects the combustion efficiency, and the fast combustion rate of H2 is helpful for the improvement of fuel efficiency. However, the large H2 fraction leads to the extremely high pressure rise rate and in-cylinder pressure, resulting in the increased RI. By optimizing the H2 and CO fractions in the syngas, high fuel efficiency and low emissions can be achieved simultaneously. (5) At the operation tested in this study, the optimal diesel/syngas RCCI combustion can be realized with the initial temperature of 360 K, the premixed ratio of 60%, and the H2 volume fraction of 75%.

Figure 24. EISFC, HC, and CO emissions with the changes of the three operating parameters.

Figure 25. NOx and soot emissions with the changes of the three operating parameters.

Table 5. Range of the Operating Parameters parameter

range

initial temperature premixed ratio H2 volume fraction

330−400 K 10−80% 0−100%

■

not enough to reach the piston surface at high premixed ratio. The low combustion temperature near the cylinder wall leads to the incomplete combustion of CO. By optimizing the injection angle of the diesel spray,54 the combustion region of diesel can be effectively controlled, which is able to achieve low CO emissions. From the NOx emissions depicted in Figure 25(a), it can be observed that the premixed ratio plays a much more dominant role than the H2 proportion and the initial temperature. For soot emission exhibited in Figure 25(b), the premixed ratio also plays an important role. Soot emissions monotonically reduce with the increased premixed ratio, initial temperature, and H2 fraction, and the considerably low soot emissions at the low initial temperatures are primarily due to the incomplete combustion. Overall, under the combined restrictions of EISFC and RI, the operating parameters

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-411-84706722. Fax: +86-411-84706722. E-mail: [email protected]. ORCID

Ming Jia: 0000-0003-2544-0259 Xingcai Lu: 0000-0003-3548-6058 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 91641117 and 51406027). L


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Energy & Fuels

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Potential for Reducing Emissions in Reactivity-Controlled

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