Effects of EGR Dilution on Combustion and Emission

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Effects of EGR Dilution on Combustion and Emission Performance of a Compression Ignition Engine Fueled with Dimethyl Carbonate and 2‑Ethylhexyl Nitrate Additive Mingzhang Pan,† Weiwei Qian,† Rong Huang,† Xiaorong Zhou,† Haozhong Huang,*,† Xuezhi Pan,‡ and Zhibo Ban‡ †

College of Mechanical Engineering, Guangxi University, Nanning 530004, China Guangxi Yuchai Machinery Company, Limited, YuLin 537005, China

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‡

ABSTRACT: The combination dimethyl carbonate (DMC)/diesel-blended fuels and the exhaust gas recirculation (EGR) can decrease nitrogen oxide (NOX) and soot emissions simultaneously emitted from the compression ignition engine. Nevertheless, the low cetane number of DMC/diesel mixtures at low loads results in a delayed combustion phase. The combustion phase has a significant influence on the engine emissions and combustion performance. Therefore, 2-ethylhexyl nitrate (EHN) must be added as cetane improver to fuel blends to ensure that the DMC/diesel mixtures have suitable combustion performance. In this paper, a four-cylinder diesel engine was used to investigate combustion and emissions performance. The five test fuels included diesel (D100) and a mixture of 20% DMC with 80% diesel (DMC20). In addition, EHN was added to DMC20 at ratios of 0.5%, 1%, and 2%. The results showed that the DMC20 increased the maximum heat release rate (MHRR), ignition delay (ID), and maximum pressure rise rate (MPRR) and reduced the soot emissions, nucleation, and accumulation mode particles. However, the brake-specific fuel consumption (BSFC) increased, and the brake thermal efficiency (BTE) decreased. Furthermore, when using the EGR, the NOX emission significantly decreased. When adding EHN to DMC20, the ID was shortened, the combustion phase advanced, and the MPRR decreased. The BTE first decreased and then increased with increasing EHN proportion. In general, use of the EGR coupled with DMC and EHN simultaneously decreased the NOX−soot emissions, and use of 0.5% and 1% EHN in combination with 20−30% EGR led to better engine emissions and combustion performance. chain.6 However, biofuels such as alcohols, aldehydes, and ethers have different oxygen contents, and the addition of biofuels to diesel has a different inhibitory effect on PM emissions. Blanquart et al.7 introduced a chemical mechanism of various hydrocarbons for high-temperature combustion. They concluded that acetylene, propyne, propene, and butadiene played important roles in the generation of soot. The polycyclic aromatic hydrocarbons produced by the decomposition of these substances promoted the growth of particles. Moreover, Westbrook et al.8 developed a chemical kinetic mechanism of the oxidation of n-heptane, isooctane, and their blends and reported that adding molecular oxygen to isomerized alkylperoxy radicals and alkyl could control the intermediate products and oxidation rate. Alexandrino et al.9 conducted a modeling and experimental research on the dimethoxymethane (DMM) pyrolysis at atmospheric pressure with a tubular flow reactor. They confirmed that the capacity of DMM to form soot was lower than that of other oxygenates. Chen et al.10 studied the effect of polyoxymethylene dimethyl ether (PODE) additive on the PM emissions of a diesel engine and concluded that the addition of PODE to diesel fuel could obviously reduce the PM emissions, and the PM emissions further reduced with increasing PODE blending ratio. Thus,

1. INTRODUCTION In the face of increasingly stringent emission regulations, engineers of internal combustion engines (ICEs) are moving further toward zero-emissions objectives. Researchers are constantly working on decreasing the emissions emitted from ICEs with various new combustion methods, such as lowtemperature combustion (LTC), reactivity controlled compression ignition (RCCI), and homogeneous charge compression ignition (HCCI).1−3 While focusing on emissions, researchers have continued their studies of the improvement of the ICE performance, in particular, the significant reduction of harmful emissions without degrading the engine performance. The addition of bio-oxygen fuels as diesel additive is a technology with great potential in the respective studies. Diesel engines adopt the liquid-phase fuel spray diffusion combustion mode in which the fuel and air are mixed unevenly, and local high-temperature flames and fuel overconcentration zones occur in the combustion cylinder. These inevitably lead to a certain amount of particulate matter (PM) and nitrogen oxide (NOX) generation.4 This trade-off relationship has always been the main dilemma in the control of diesel emissions.5 The emergence of molecular oxygen in biofuels provides a more efficient and convenient method for solving the problem of fuel overconcentration zones in diesel engines. Furthermore, oxygenated fuels are renewable and exhibit high energy density, which alleviates the dependence of nonrenewable resources and improves the energy supply © XXXX American Chemical Society

Received: May 11, 2019 Revised: July 19, 2019

A

DOI: 10.1021/acs.energyfuels.9b01494 Energy Fuels XXXX, XXX, XXX−XXX

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the emissions performance of an optically accessible diesel engine fueled with oxygenated blends. They found that the combination of LLFC and EGR could completely suppress soot and control the NOX emissions simultaneously. However, different concentrations of EGR have different effects on the diesel engine emissions and combustion performance. Plamondon et al.26 concluded that 15% EGR produced better results than 5% EGR. However, the effects of different EGR concentrations combined with DMC are unknown. The EGR causes a dilution of oxygen concentration, which decreases the DMC mitigation ability in the local fuel overconcentration zones. Therefore, the reduction of PM−NOX emissions with the simultaneous use of DMC and EGR must be investigated. The EGR contains gas (CO2 and water vapor) with high specific heat capacity (SHC). The introduction of a certain amount of EGR can increase the SHC of the mixtures, thereby decreasing the ignition temperature and increasing the ignition delay (ID).27,28 Furthermore, DMC has a lower cetane number (CN) of 35.5 than that of the diesel fuel. Using DMC in combination with EGR results in a significant increase in ID, which might increase the maximum in-cylinder pressure (MICP), maximum heat release rate (MHRR), and maximum pressure rise rate (MPRR).15 Zhang et al.29 confirmed that a higher MPRR led to a higher engine noise, which required higher intensity engines and affects the normal life of people. Ileri30 concluded that NOX was formed by hydrocarbon flame front radicals. Furthermore, the longer the premixing time, the larger was the amount of produced NOX. The lower CN resulted in a longer fuel premixing phase and a faster burning rate, which increased the NOX emissions. At low loads, a longer retardation period caused a delay in the exothermic period, which resulted in a deterioration of combustion and even misfires. To increase the low CNs of some oxygenated fuels, researchers have proposed to add cetane improver to the fuel. Hanson et al.31 analyzed the effects of 2-ethylhexyl nitrate (EHN) additive on the RCCI combustion mode at low loads. They concluded that the addition of 3.5% EHN to blended fuels maintained a 54% BTE with PM and NOX emissions below United States Environmental Protection Agency 2010. McCormick et al.32 confirmed that EHN could be added as an additive to improve the CN of the mixture. After adding 0.5% and 1% EHN, the NOX emissions were reduced by 2.03% and 2.77%, respectively. Suppes et al.33 studied the effectiveness of cetane enhancer in improving the fuel ignition performance stability. According to their results, the ID decreased with increasing EHN proportion. Zhang et al.29 found that adding 2% EHN to 2,5-dimethylfuran/diesel mixtures led to a consistent MPRR with respect to that of diesel and to 80% less soot. Therefore, EHN can reduce the ID, improve the combustion environment, and hopefully reduce NOX emissions. However, there exist few reports on using DMC/EHN blends as a diesel additive. Few researchers have explored the effects of oxygen-enriched additives combined with EHN on diesel combustion and emissions, in particular, PM emissions. To obtain a better combustion stability and maintain low soot and NOX emissions and to solve the problem of the increased ID caused by the mixing of diesel/DMC dual fuels, we investigated the addition of EHN to DMC/diesel blends to improve the fuel performance of the blended fuels and to solve the problem of the prolonged ID. On the basis of the previously presented reports, oxygen-rich fuel DMC was combined with the EGR to simultaneously decrease the PM−NOX emissions. To reduce the negative

the use of oxygen-rich fuels can reduce PM emissions to a greater extent. Dimethyl carbonate (DMC) is a green, oxygen-rich (up to 53.3% oxygen content), and nonpolluting fuel.11 Its special structure contains an oxygen atom between carbon−carbon bonds and less carbon (39.9%). Moreover, DMC is in full contact with oxygen during combustion, which reduces the possibility of an incomplete combustion and formation of CO2.12 In addition, DMC is insoluble in water and does not pose a hazard to groundwater resources. As an intermediate product, it is easy to prepare and can be formed through the conversion of alkanes and esters.13 Owing to its good properties, researchers have conducted various studies on the spray effect, laminar burning rate, chemical reaction kinetics, combustion performance, and emissions. Kocis et al.14 investigated the impacts of the DMC structure on soot production in an optically accessible diesel engine. The results showed that adding DMC to diesel fuel could reduce the peaks of soot during the combustion process and the exhaust soot emissions. Xiao et al.15 discovered that the spray effect of DMC was better than that of the pure diesel and that the atomized particles were smaller and more evenly distributed. Bardin et al.16 believed that the combustion mechanism of DMC could be further investigated and verified by measuring the laminar burning rate of combustible mixtures. In addition, the laminar burning velocities of DMC at 298, 318, 338, and 358 K were studied experimentally. Glaude et al.17 investigated the chemical kinetic mechanism of DMC and compared the results with the results of DMC measurements in convective nonpremixed flames, thereby confirming the consistency of the predicted and measured results. Zhang et al.18 explored the impact of DMC additive on the emissions and performance in a diesel engine and reported that adding the DMC/diesel mixtures increased the brake thermal efficiency (BTE) and decreased the PM emissions. Murayama et al.19 concluded that the amount of flue gas decreased approximately linearly with increasing oxygen content in the combustion through the soot reduction mechanism of DMC. Cheung et al.20 used different proportions of Euro V diesel fuel to explore the effect of DMC on the PM emissions of diesel engines. According to the results, the geometric mean diameter and number concentration of PM decreased with increasing amount of DMC additive. Huang et al.21 found that the duration of the combustion decreased with increasing DMC proportion. All previously mentioned studies prove that DMC as an additive can reduce PM emissions and improve the combustion performance. Nevertheless, using fuel-bound oxygen results in an increase in NOX emissions, in particular, for oxygen-rich fuels. Chen et al.22 confirmed that the NOX emissions increased with increasing proportion of oxygenated fuels. The easy-to-operate exhaust gas recirculation (EGR) is considered to be one of the main strategies for reducing NOX emissions. By introducing EGR into the intake air, the maximum in-cylinder temperature (MICT) decreased, which reduces the NOX emissions. Shen et al.23 concluded that both high- and low-pressure-cooled EGR could prolong the combustion duration and cause a reduction in the combustion temperature, thereby reducing the NOX and CO emissions. Other researchers have found that introduction of EGR into biodiesel blends improved the NOX−soot emissions balance without affecting the combustion and performance characteristics.24 Gehmlich et al.25 investigated the effects of the leaner lifted-flame combustion (LLFC) on B

DOI: 10.1021/acs.energyfuels.9b01494 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels effects caused by DMC and EGR, EHN was introduced in the experiments. The experiment was conducted to study the influence of a simultaneous reduction of the PM and NOX emissions and to ensure a good combustion performance of the ICEs. The results of the study provide a better comprehension of the engine emission characteristics and combustion performance with EHN additive in DMC blends and the EGR technology. The results can be applied in practical applications of commercial diesel engines.

Table 3. Experimental Engine Basic Parameters

2. EXPERIMENTAL FACILITY AND STEPS

engine type

four-stroke compression ignition

no. of cylinders cylinder diameter (mm) stroke (mm) compression ratio displacement (L) no. of valves max torque (N m) rated power (kW)/speed (r/min)

4 85 88.1 16.5 1.99 16 286 100/4000

2.1. Research Equipment and Fuels. Table 1 lists the properties of the diesel, DMC, and EHN. Table 2 lists the blended-

BTE =

Table 1. Physicochemical Properties of Diesel, DMC, and EHN properties molecular formula cetane number oxygen content (wt %) density (g/mL@20 °C) low heat value (MJ/kg) boiling point (°C) self-ignition temperature (°C) latent heat vaporization (KJ/kg) kinematic viscosity (mm2/s@40 °C)

diesela

DMCa

EHNb

C12−C25 54 0 0.826 42.8 221 254 250−290 3.52

C3H6O3 35.5 53.3 1.06 13.5 90 195 369 5.6

C8H17NO3

P (mdiesel + mDMC)LHVmix

(2)

where P is power (kW) and mdiesel and mDMC are the diesel mass (kg/ s) and the DMC mass (kg/s) in the blended fuel, respectively. The LHVmix is the low heat value of blended fuel. 2.3. Operating Conditions and Methods. During the test, the PowerLink eddy-current dynamometer was used to maintain the engine speed at 1400 ± 1 rpm. The cylinder pressure sensor Kistler 6052CU20 was used to test the in-cylinder pressure (ICP) per cycle in the diesel engine with an error of 0.01 MPa. The data was collected over an average of 200 cylinder cycles to ensure data accuracy. The intake air cooling temperature was 30 ± 1 °C. The cooling water temperature and oil temperature were maintained at 85 ± 2 °C by the heat exchanger. The INCA software was used to maintain a diesel load of 0.6 MPa by varying the amount of injected fuel. Furthermore, the INCA software maintained the injection timing at −3 °CA ATDC, and the injection pressure was at 120 MPa. The PM and soot emissions were measured with the Cambustion DMS500MKll and AVL 415SE, respectively. In addition, a HORIBA MEXA 7100DEGR was used to test the emissions of gases such as CO, HC, and NOX. The experimental conditions are listed in Table 4.

27.4 0.963

1.8

a

Source: ref 34. bSource: ref 35.

fuel properties. The mixing ratio of the test fuels was selected as follows: 100% commercial diesel (denoted as D100), a mixture of 20% DMC and 80% diesel (v/v) (denoted as DMC20), 99.5% DMC20 and 0.5% EHN (v/v) (denoted as DMC20 + 0.5%EHN), 99% DMC20 and 1% EHN (v/v) (denoted as DMC20 + 1%EHN), and 98% DMC20 and 2% EHN (v/v) (denoted as DMC20 + 2% EHN). The experimental equipment was a four-cylinder diesel engine. Table 3 provides the main basic parameters of the engine. Figure 1 shows the schematic diagram of the engine test. 2.2. EGR Setup. In the experiment, control of the EGR concentration has a great importance for the generation of NOX− soot emissions. Therefore, the EGR concentration must be controlled precisely. The turbocharged exhaust gas is mixed with fresh air at the intake port position and enters the combustion chamber afterward. The EGR concentration is defined as the concentrations of CO2 in the intake and exhaust36 ÄÅ ÉÑ ÅÅ (CO ) Ñ 2 intake − (CO2 )air Ñ Å ÑÑ × 100% Å EGR% = ÅÅ Ñ ÅÅÇ (CO2 )exhaust − (CO2 )air ÑÑÑÖ (1)

3. RESULTS AND DISCUSSION 3.1. Effects of Various EGR Concentrations on Combustion Performance with Different Fuels. Figure 2 presents the ICP and heat release rate (HRR) curves for different EGR concentrations. At the same EGR concentrations, compared with pure diesel, the MICPs of the mixtures were higher. For EGR concentrations below 20%, the MHRR of DMC20 was higher than that of diesel and DMC20 exhibited the longest ID of all test fuels. Adding EHN to the DMC/diesel blended fuels resulted in a decrease in the MHRRs of blended fuels. Moreover, the MHRRs of the blends further decreased with increasing proportion of EHN additive. For all EGR concentrations, the high−low MHRRs of blended fuels were arranged as follows: DMC20 > DMC20 + 0.5% EHN > DMC20 + 1%EHN > DMC20 + 2%EHN. This is because when adding EHN to DMC20 the CN of the mixture increased, the ID became shorter, and the starting point of the ignition advanced. Therefore, the MHRRs of the test fuels decreased. The results were in agreement with those of the previous study.33 For the same fuel, the MICPs of the test fuels gradually decreased with increasing EGR. This was because the

where (CO2)intake is the CO2 concentration in the intake pipe, (CO2)exhaust is the CO2 concentration in the exhaust pipe, and (CO2)air is the CO2 concentration in the air. In addition, the heat generated by the mixed fuels has a obvious effect on the BTE. Therefore, BTE can determined by the following equation

Table 2. Properties of Blended Fuels properties

DMC20

DMC20 + 0.5%EHN

DMC20 + 1%EHN

DMC20 + 2%EHN

cetane number density (g/mL@20 °C) low heat value (MJ/kg) kinematic viscosity (mm2/s@40 °C)

47.3a 0.873 36.96a 3.92

56.8a 0.873 36.88a 3.91

61.4a 0.874 36.72a 3.88

67.1a 0.875 36.25a 3.82

a

Blended fuels properties are tested by the College of Chemistry and Chemical Engineering, Guangxi University, Nanning, China. C

DOI: 10.1021/acs.energyfuels.9b01494 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Schematic of the experimental engine configuration.

the combustion phase, and prolonged the ID. Furthermore, DMC20 had the highest burning rate among all test fuels. As the EHN blending ratio increased, the combustion rates of the mixed fuels decreased. The burned mass fraction of DMC20 gradually increased with respect to those of the other fuels with increasing crank angle. Further observations revealed that the addition of EHN to DMC20 could advance the IDs of the mixed fuels, decrease the proportion of premixed combustion (POPC), and decrease the MHRR. The increase in the EGR concentration prolonged the ID, and the angle of the crankshaft corresponding to the intersection of different fuels at one point increased. In addition, after adding 2% EHN to DMC20, the burned mass fraction of DMC20 + 2%EHN was similar to that of diesel fuel. Higher pressure rise rates can cause problems such as an engine knock. Figure 4 presents the pressure rise rates of different fuels for different EGR concentrations. At the same EGR concentration, compared with pure diesel, the MPRR of DMC20 was higher, and it decreased with increasing EHN blending ratio. The main reason was the CN of DMC20 was smaller than that of diesel fuel and the POPC increased after the mixed fuel was injected into the engine combustion chamber, thereby resulting in an increase in the MPRR. However, when EHN was added to DMC20, the MPRR decreased and the ID was shortened. The MPRR of DMC20 + 0.5%EHN was approximately that of D100. For the same fuel,

Table 4. Operating Conditions of Engine item

parameters

speed (rpm) BMEP (MPa) injection pressure (MPa) injections injection timing (°CA ATDC) EGR ratio (%) intake temperature (°C) coolant temperature (°C)

1400 0.6 120 single −3 0%, 10% 20%, 30%, 40% 30 ± 1 85 ± 2

inert gas in the EGR increased the SHC of the blended fuel, the heat absorbed by the mixed fuel increased, and the MICP decreased. The burned mass fraction is an important parameter for judging the fuel combustion performance. Khoa et al.37 concluded that the combustion duration influences the engine power, effective release energy, and emissions. The curves of the burned mass fractions of different fuels for various EGR concentrations are shown in Figure 3. As the EGR concentration increased, the IDs of test fuels gradually increased and the angle of the crankshaft corresponding to the intersection of different fuels at one point increased. The reason was that the combustion temperature and ICP were reduced, which delayed the heat release starting point, lagged D

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Figure 2. In-cylinder pressure and heat release rate of different fuels for various EGR concentrations.

Figure 4. Pressure rise rate of different fuels for various EGR concentrations.

Figure 3. Burned mass fraction of different fuels for various EGR concentrations.

concentration was reduced, which resulted in a decrease in the combustion reaction rate. However, owing to the increasing EGR concentration, the resulting thermal effect promoted the combustion reaction and increased the MPRR.

the MPRR first decreased and then increased with increasing EGR. The decrease in the MPRR occurred mainly because the inert gas concentration increased and the combustible mixture E

DOI: 10.1021/acs.energyfuels.9b01494 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels The BSFC is an important parameter of economic performance of ICEs. Figure 5 presents a BSFC graph of

Figure 6. Brake thermal efficiency of different fuels for various EGR concentrations. Figure 5. Brake-specific fuel consumption of different fuels for various EGR concentrations.

Figure 7 shows the MICT plot of the test fuels for different EGR concentrations. As EGR concentrations increased, the

different fuels and various EGR concentrations. The BSFC first increased and then decreased with increasing EGR concentrations. The reason was that under small EGR concentrations the inert gas in EGR resulted in an increase in the SHC of the mixtures, which absorbed more heat. In addition, the incylinder temperature decreased, which was not conducive to the ignition of engines, and the BSFC increased. When EGR > 20%, the heat carried by the EGR improved the ignition temperature and reduced the BSFC. When EGR > 30%, the excessive EGR diluted the oxygen concentration in the mixtures and the thermal effect of EGR was weakened. Therefore, the BSFC increased or remained unchanged. At the same EGR concentrations, the use of DMC increased the BSFC significantly, and the BSFC of DMC20 increased by approximately 18% when compared with that of diesel. The reason was that compared with diesel, the LHV of DMC was lower. Consequently, the fuel amount of DMC20 was higher than that of diesel at the same output power. The result was in line with certain research reports. Canakci et al.38 concluded that the performances of biodiesel/diesel mixtures were similar to that of diesel and reported an increasing BSFC. Asokan et al.39 found that the higher biodiesel BSFC (with respect to that of diesel) was the result of a combination of the fuel density, viscosity, and LHV. Adding EHN to DMC/diesel blends led to an increase in the BSFC, and the BSFC further increased with increasing EHN concentration. The BTE is an important parameter for characterizing the performance of ICEs. The BTE is related to the air−fuel ratio, compression ratio, combustion process, and fuel performance.40 The impacts of EGR concentration and the test fuels on the BTE are shown in Figure 6. As the EGR concentrations increased, the BTE first decreased and then increased and finally decreased or remained unchanged. This was because the BTE is a function of BSFC and increased as BSFC decreased. According to the figure, 20−30% EGR concentrations promoted an increase in the BTE. When EGR = 30%, the BTEs of D100, DMC20, DMC20 + 0.5%EHN, DMC20 + 1% EHN, and DMC20 + 2%EHN were 31.60%, 30.74%, 30.40%, 30.52%, and 30.80%, respectively. In addition, the BTE of EHN blends increased with increasing EHN blending ratio. When the blending ratio of EHN was 2%, the BTE of DMC20 + 2%EHN was close to that of DMC20 under all operating conditions.

Figure 7. Maximum in-cylinder temperature of different fuels for various EGR concentrations.

MICT increased. Owing to the EGR introduction, the blended fuels were diluted by the inert gas, the ID was prolonged, and the proportion of diffusion combustion (PODC) increased, which increased the combustion temperature. At the same EGR concentrations, the MICT of DMC20 was lower than that of diesel. This was because compared with diesel fuel, the LHV of the DMC was lower and led to a smaller LHV of the DMC20. Although the fuel consumption was the higher, the unit heat release of DMC was small and the MICT of DMC20 reduced. Moreover, adding EHN to the DMC/diesel blends led to a lower MICT when compared with that of DMC20. 3.2. Effects of Various EGR Concentrations on Gaseous Emissions with Different Fuels. The generation of NOX is determined by the in-cylinder temperature, the oxygen concentration, and the excess air ratio.41 Yilmaz et al.42 observed that the NOX emissions dramatically increase over 1800 K of in-cylinder temperature and longer residence time of the mixtures at high temperatures and oxygen-rich area. The NOX emissions of test fuels for various EGR concentrations are shown in Figure 8. The NOX emissions of the test fuels decreased with increasing EGR. The main reason was that the use of EGR diluted the oxygen concentration, the excess air ratio decreased, the NOX generation was suppressed, and the NOX emission decreased. Moreover, the NOX emission of the DMC20 was higher than that of D100. This was mainly because the use of DMC provided enough oxygen molecules to F

DOI: 10.1021/acs.energyfuels.9b01494 Energy Fuels XXXX, XXX, XXX−XXX

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in the DMC promoted the oxidation of CO into CO2. Therefore, adding DMC to pure diesel reduced the CO emissions. Moreover, the addition of EHN to DMC20 resulted in an increase in CO emissions. This was mainly because the MICT of the mixtures decreased when adding EHN to DMC20 (Figure 7). The formation of hydrocarbon (HC) is mainly due to the oxidation of the fuel or the formation of an incomplete combustion caused by the spraying of combustion chamber walls. 43 As shown in Figure 10, at the same EGR

Figure 8. NOX emissions of different fuels for various EGR concentrations

promote the formation of chemical reactions. Although the incylinder combustion temperature was lower owing to the latent heat vaporization of DMC, sufficient oxygen molecules provided a better generated condition for NOX. Therefore, adding DMC to pure diesel led to an increase in NOX emissions. Compared with that of DMC20, the use of EHN had less impact on the NO X emissions. For EGR concentrations of 0−30%, the NOX emissions of the mixed fuel increased by approximately 25% compared with that of diesel. It was noted that when EGR was 40%, the NOX emissions of the mixed fuels approximated that of D100. Accordingly, the higher the EGR concentrations, the better was the inhibitory effect on the formation of NOX. Carbon monoxide (CO) is mainly generated from incomplete combustion of fuel. The influences of the fuel properties and EGR concentration on the CO emissions are shown in Figure 9. The CO emissions of the test fuels

Figure 10. HC emissions of different fuels for various EGR concentrations

concentrations, the oxygen content played an important role in the HC emissions. Compared with diesel fuel, the HC emission of DMC20 was significantly lower. In particular, when EGR = 0%, the HC emissions of diesel and DMC20 were 0.06 and 0.05 g/kWh, respectively. Thus, the HC emission of DMC20 was 16.7% lower than that of diesel. The reason was similar to that of CO emissions. In addition, the use of EHN to the DMC/diesel blends decreased the HC emissions. The higher the EHN blending ratio, the higher was the reduction in the HC emissions. The main reason was the use of EHN increased the CN and shortened the IDs, which accelerated the combustion process. 3.3. Effect of Various EGR Concentrations on soot and PM Emissions with Different Fuels. The generation of soot emissions is mainly connected with the in-cylinder oxygen concentration and combustion temperature. Figure 11 shows the soot emissions of the test fuels for various EGR

Figure 9. CO emissions of different fuels for various EGR concentrations.

increased with increasing EGR concentration. The reason was that as the EGR increased, the exhaust gas content increased, which resulted in a decrease in oxygen proportion. In addition, the introduction of EGR lowered the combustion temperature. Therefore, the CO emissions increased with increasing EGR concentrations. At the same EGR concentrations, compared with pure diesel, the CO emission of the DMC20 was lower. The main reasons were that, on one hand, the fuel and air were mixed more sufficient owing to the longer ID when compared with that of pure diesel; on the other hand, the oxygen content

Figure 11. Soot emissions of different fuels for various EGR concentrations. G

DOI: 10.1021/acs.energyfuels.9b01494 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 12. Particle size distribution of different fuels for various EGR concentrations.

Figure 13. Total particle number and mass concentrations of different fuels for various EGR concentrations.

concentrations. The soot emissions of the test fuels increased with increasing EGR concentrations. The reasons were, on one hand, the MICT significantly increased as the EGR increased. On the other hand, the intake oxygen concentration decreased, which resulted in an increase in the soot emissions. The addition of DMC to diesel markedly decreased the soot emission. The reason was the oxygen content of 53.3% in DMC structure, which led to an acceleration of the combustion process. In addition, the CN of DMC was lower than that of diesel, the POPC increased, and the PODC decreased, which led to a decrease in the soot emissions. When EGR < 20%, on adding a small concentration of EHN to the DMC/diesel mixtures, the soot emissions of the EHN blends were lower than that of DMC20. However, when EGR > 20%, as the EHN blending ratio increased, the soot emissions of the

EHN mixtures were higher than that of DMC20. The reason was that the small concentration of EHN accelerated the combustion process and decreased the soot emissions. Nevertheless, when the EHN blending ratio further increased, the engine was dominated by the diffusion combustion, which led to an increase in the soot emissions. Therefore, the combination of small EGR concentrations and small EHN proportion led to lower soot emissions with respect to that of DMC20. On the basis of the size of PM, particles with sizes below 50 nm are defined as nucleation mode particles (NMP), and particles with sizes between 50 and 1000 nm are defined as accumulation mode particles (AMP). The NMP are formed by HC compounds and sulfates generated during the cooling and exhaust processes. The AMP are mainly carbonaceous soot H

DOI: 10.1021/acs.energyfuels.9b01494 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels agglomerates generated during the engine combustion process.44 Figure 12 shows the influences of EGR concentrations and the fuel properties on particle side distributions (PSD) of the three fuels. It can be seen that the PSDs of the test fuels exhibited bimodal distributions, which were characterized by the NMP and AMP. As the EGR concentrations increased, the NMP decreased and the AMP increased. This was mainly because the excess air ratio decreased, the engine combustion started to deteriorate, the HC and soot emissions increased (Figures 10 and 11), and the oxidation of the large-size particles decreased with increasing EGR concentrations. Moreover, the amount of exhaust gas increased and included kinds of HC compounds, primary carbon particles, and sulfates, which accelerated the generation of AMP. Adding DMC to diesel led to a reduction of the AMP. The main reason was that compared with diesel fuel, DMC has lower CN and higher oxygen content, which resulted in the burning of more fuels in the premixed combustion stage. This led to a better combustion process and a decrease in the AMP concentrations.45 Further observations revealed that, compared with pure diesel, adding EHN to the DMC/diesel blends led to a reduction of AMP concentrations. The AMP concentrations increased with increasing EHN proportion. The main reason was that when adding EHN to DMC mixtures the engine combustion process was improved, the unburned HC and soot emissions decreased, and the ability of primary carbon particles to adsorb soot emissions weakened. Figure 13a presents the total particle number concentrations (TPNC) with respect to the EGR concentrations for the test fuels. The TPNC of diesel first decreased and then increased. When the EGR concentrations were 0−30%, the decrease in the NMP caused the TPNC to decrease as the EGR concentrations increased. When the EGR concentration further increased, the increase in the AMP led to an increase in the TPNC. This was because the introduction of EGR increased the retardation period and the mixing effect of fuel and oxygen improved. Nevertheless, when the EGR concentration was excessively high, the oxygen concentration decreased and the AMP significantly increased. This led to an increase in the TPNC. In addition, the use of DMC and EHN reduced the TPNC, which is attributed to the lower PM concentrations of the mixed fuels. Simultaneously, less PM was formed, and there were no sulfur and aromatic components in the DMC and EHN structures.46 Total particle mass concentrations (TPMC) as a function of the EGR concentrations of the test fuels are shown in Figure 13b. It can be seen that the TPMC of pure diesel first decreased and then increased, which in line with the trend of the TPNC. When EGR = 20%, the TPMC of D100 was the lowest among under all of the operating conditions. The TPMC was mainly determined by the large particles; the larger the particles, the higher was the TPMC.47 Furthermore, the higher AMP concentrations resulted in larger TPMC. Therefore, when EGR = 40%, the TPMCs of the test fuels were maximal. The TPMCs of D100, DMC20, DMC20 + 0.5% EHN, DMC20 + 1%EHN, and DMC20 + 2%EHN were 0.019, 0.004, 0.006, 0.009, and 0.012 μg/cm3, respectively. The NOX−soot relationship of the test fuels for various EGR concentrations is shown in Figure 14. The NOX emissions decreased dramatically with increasing EGR, in particular, when EGR concentrations were 20−40%. Moreover, as the EGR concentration increased, the soot emissions of diesel fuel increased significantly, thereby exhibiting a traditional trade-off

Figure 14. NOX and soot emissions relationship of different fuels for various EGR.

relationship. For mixed fuels, the soot emissions remained lower with increasing EGR concentration. According to the formation mechanism of NOX, the NOX generation was determined by the intake oxygen concentration, in-cylinder temperature, and high-temperature duration. When EGR concentrations were 20−40%, DMC20, DMC20 + 0.5% EHN, and DMC20 + 1%EHN exhibited better trade-off relationships than diesel.

4. CONCLUSIONS In this study, the influences of EHN additive and DMC/diesel blended fuels on the emissions and combustion performance were studied in a turbocharged diesel engine with various EGR concentrations. The emission characteristics, combustion performance and PM size distribution, number concentrations, and mass concentrations of the diesel engine were analyzed thoroughly. The following conclusions can be drawn. (1) The ID of DMC20 was longer than that of diesel. After adding EHN to the DMC/diesel blends, the ID was significantly shortened. With increasing EHN proportion, the combustion starting point gradually advanced. Moreover, the retardation period of the mixed fuel increased with increasing EGR. (2) The use of DMC delayed the combustion starting point, thereby leading to an increase in the MICP and HRR. Moreover, the use of EHN reduced the MPRR of the blended fuels. (3) The BSFC of mixed fuel was higher than that of pure diesel because of the lower LHV of the mixed fuel. Moreover, the BSFC increased after the addition of EHN to DMC20. The use of EHN reduced the BTE of DMC20. However, the BTE increased with increasing EHN proportion. The MICT of the mixed fuel was smaller than that of diesel. Furthermore, the MICT decreased with increasing EHN proportion. (4) The addition of DMC to diesel reduced the PM and soot emissions. However, the NOX emissions of the DMC blends increased. The combination of DMC and 40% EGR improved the soot−NOX trade-off relationship. I

DOI: 10.1021/acs.energyfuels.9b01494 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

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(5) The use of 0.5% and 1% EHN in combination with 20− 30% EGR resulted in better engine emissions and performance than that of pure diesel.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Haozhong Huang: 0000-0001-7181-3840 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (51865002). NOMENCLATURE DMC = dimethyl carbonate BMEP = brake mean effective pressure MPRR = maximum pressure rise rate HC = hydrocarbon NOX = nitrogen oxide BTE = brake thermal efficiency ICP = in-cylinder pressure LHV = low heat value ICE = internal combustion engine ID = ignition delay AMP = accumulation mode particles PSD = particle side distribution TPMC = total particle mass concentrations PODC = proportion of diffusion combustion EHN = 2-ethylhexyl nitrate EGR = exhaust gas recirculation BSFC = brake specific fuel consumption MHRR = maximum heat release rate CO = carbon monoxide PM = particulate matter HRR = heat release rate ATDC = after top dead center MICT = maximum in-cylinder temperature CN = cetane number MICP = maximum in-cylinder pressure NMP = nucleation mode particles TPNC = total particle number concentrations SHC = specific heat capacity POPC = proportion of premixed combustion

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DOI: 10.1021/acs.energyfuels.9b01494 Energy Fuels XXXX, XXX, XXX−XXX