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Effect of n-pentanol additive on compression-ignition engine performance and particulate emission laws
Fuel 267 (2020) 117201
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Full Length Article
Effect of n-pentanol additive on compression-ignition engine performance and particulate emission laws
T
Haozhong Huang, Xiaoyu Guo, Rong Huang, Han Lei, Yajuan Chen, Te Wang, Sai Wang, ⁎ Mingzhang Pan College of Mechanical Engineering, Guangxi University, Nanning 530004, China
ARTICLE INFO
ABSTRACT
Keywords: n-Pentanol EGR Biodiesel Combustion performance Particle emission characteristic
Biodiesel has a high viscosity, which limits its application to engines. However, n-pentanol has low viscosity and high volatility, which is considered a promising additive. This study examined the influence of n-pentanol/ biodiesel/diesel blends on the performance and particulate emissions of a diesel engine at different loads, under the condition of coupling exhaust gas recirculation (EGR) (0–30%). The three test fuels included pure diesel (D100); 80% diesel and 20% biodiesel blended fuel (BD20); 64% diesel, 16% biodiesel, and 20% n-pentanol blended fuel (BDP20). The results indicated that the influence of EGR on the NOx emissions between fuels was greater than that of the physicochemical properties of the fuels. For the same EGR rate, the heat release rate (HRR), peak in-cylinder pressure (IP), and brake thermal efficiency (BTE) increased with the engine load increased. After adding n-pentanol to BD20, the HRR increased. The peak IP and BTE values of BDP20 were similar to those of D100 and BD20. With the addition of n-pentanol, the total particulate number concentration (TPNC), soot, and total particulate mass concentration (TPMC) emissions decreased. Furthermore, the trade-off relationship between TPMC and NOx was improved, and the geometric mean diameter (GMD) of the particles decreased.
1. Introduction As a critical power source, the diesel engine is widely used in transportation and industry, owing to its high thermal efficiency, large torque, and good stability [1]. However, the automobile industry is developing rapidly; the demand for energy is increasing, which could accelerate the emergence of an energy shortage crisis [2]. At the same time, diesel engines produce many harmful substances in the process of operation that seriously affect human health [3]. Therefore, reducing the harmful emissions in the exhaust gas, especially NOx and particulate emissions, which is still the main challenge facing the engine industry [4]. To reduce the harmful emissions of the engine, many advance combustion modes and post-processing methods have been researched, such as, low-temperature combustion (LTC) [5], homogeneous charge compression-ignition [6], selective catalytic reduction [7], and diesel particulate filter [8]. Moreover, to solve the energy problem, researchers have worked on developing diesel alternative fuels, including, alcohol fuels [9], ether fuels [10], and biodiesel [11], to alleviate the above-mentioned difficulties. LTC has characteristics of high efficiency and low emissions; in
⁎
particular, it can efficiency reduce NOx emissions [12]. LTC refers to a combustion temperature in the cylinder below 2100 K, which is not conducive to the generation of carbon smoke and NOx. Thus, LTC is achieved by using EGR, a low compression ratio, and delayed injection [13]. An increasing number of researchers are interested in using a controllable EGR rate to achieve LTC. Verma et al. [14] studied the influence of compression ratio, EGR, and EGR temperature on engine performance and emission characteristics. They found that, owing to the effects of EGR, the engine efficiency increase, and NOx emissions are decreased. Huang et al. [15] and He et al. [16] researched the influence of EGR on engine emission characteristics. The results showed that NOx decreased with the EGR rate increase, and TPNC initially decreased and then increased. As can be seen from the literature, LTC can be achieved by adopting EGR technology, which can reduce NOx emissions. Biodiesel is a new type of alternative fuel which has a wide range of sources. It can be produced from non-edible oil, waste vegetable oil, animal fat and algae and other materials, moreover, biodiesel can be degraded and can be made to mix well with diesel [17]. Biodiesel has a high ignition point and cetane number (CN), and the aromatic
Corresponding author. E-mail address: [email protected] (M. Pan).
https://doi.org/10.1016/j.fuel.2020.117201 Received 26 December 2019; Received in revised form 21 January 2020; Accepted 23 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.
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Nomenclature BTE BSFC BD20 BDP20 CN D100 GMD HRR
HCAs IP ID LCAs LTC PM PSD TPNC TPMC
brake thermal efficiency brake specific fuel consumption 20% biodiesel + 80% pure diesel 20% n-pentanol + 16% biodiesel + 64% pure diesel cetane number pure diesel geometric mean diameter heat release rate
compounds and sulfur elements contained in the fuel itself are low, consequently producing less harmful emissions during combustion than diesel [18]. In addition, the large amounts of oxygen contained in biodiesel can improve the combustion quality in the engine cylinder, and facilitate better application to the engine [19]. Teoh et al. [20] and How et al. [21] researched the influence of biodiesel additives on common rail supercharged engine emission characteristics and performance. The results showed that the IP and HRR decreased, but the CO, soot, and particle emissions were reduced. Jiao et al. [22] simulated the effects of diesel/biodiesel on engine emissions at different altitudes. They found that at the same altitude, the particle emissions were less than those of pure diesel, but the NOx emissions were increased. Rajak et al. [23] researched the influence of spiral-microalgae diesel/biodiesel on engine emissions, and the results showed that the CO, hydrocarbon, and soot emissions decreased. From the above literature, it is known that biodiesel can reduce the emissions of harmful substances to a certain extent. However, owing to the high molecular weight and viscosity of biodiesel, the fuel easily adhere to the fuel injector and piston ring, resulting in coking of the fuel injector and adhesion of the piston ring [24,25]. Because of the poor atomization of biodiesel in the cylinder, which is not conducive to mixing with air, the incomplete combustion of fuel is intensified, resulting in the decreases of HRR and IP [26,27]. Therefore, it is needed to add a substance to a biodiesel/ diesel blend that increases the atomization ability and oxygen-content, and decreases the viscosity of the blend. As another clean alternative fuel, alcohol fuels have lower viscosity and higher oxygen content, and can be added to biodiesel to improve the properties of the blended fuel [28,29]. The chemical and physical properties of alcohol fuels will change with the change in the number of carbon atoms in their chemical structure and the arrangement of carbon atoms [30]. The number of carbon atoms increase can improve the CN of the fuel, shorten the ignition delay (ID) period, and improve the volatility and ignitability of the fuel [31,32]. According to the number of carbon atoms in alcohols, they can be divided into high-carbon alcohols (HCAs) and low-carbon alcohols (LCAs). As compared with LCAs, HCAs have higher calorific value and a lower latent heat of vaporization; simultaneously, they show better mixing and lubricity abilities, and exhibit lower corrosiveness to the engine [33,34]. Among them, n-pentanol is the most representative high-carbon alcohol [35]. As compared with ethanol and n-butanol alcohol fuels, n-pentanol is better mixed with diesel and biodiesel [36,37]. The addition of n-pentanol can improve the physical and chemical properties, increase the oxygen concentration, reduce the viscosity, and reduce the pollutant emissions of the engine, especially those of particulate matter (PM), which has great potential [38,39]. The research on n-pentanol additive has attracted the attention of many researchers [40]. Li et al. [41] researched the effect of pentanol on single-cylinder DI diesel engine emissions and combustion, founding that the addition of pentanol increased the HRR while the soot and particles emissions decreased. Wei et al. [42] studied the influence of diesel/n-pentanol mixture on the particulate emissions of a naturallyaspirated DI engine, and found that the addition of n-pentanol could reduce the mass concentration and number concentration of PM. Zhang
high-carbon alcohols in-cylinder pressure ignition delay low-carbon alcohols low-temperature combustion particulate matter particle size distribution total particulate number concentration total particulate mass concentration
et al. [43] studied the influence of a ultra-low sulfur diesel/pentanol mixture on the mass and number of PM and the composition of carbon PM in a DI diesel engine, They discovered that the masses of PM and primary carbon emissions decreased, and the emissions of volatile and solid particles decreased. Zhu et al. [44] found that adding n-pentanol to biodiesel can effectively reduce the mass and quantity concentrations of particles, and they can improve the BTE of an engine. Yilmaz et al. [45,46] found that adding n-pentanol to biodiesel can effectively reduce the emissions and improved the BTE. The above literature shows that npentanol can reduce harmful emissions and improve engine performance. It is a cleaner alternative fuel and can be well blended with biodiesel and diesel. The above mentioned studies indicate that biodiesel and n-pentanol have a better emission reduction capacity when they are used in engines. However, the viscosity of biodiesel is too high, which is not conducive to atomization combustion. In contrast, the viscosity of npentanol is low, and the oxygen-content is high. Thus, it can better mix with biodiesel and improve the properties of the mixed fuel. In the case of coupled EGR, the influences of the n-pentanol/biodiesel/diesel blend on diesel engine performance and particulate emissions, especially on particle number, particle size distribution (PSD), and particle mass, are not clear. Therefore, the purpose of this study is to explore the relationships between n-pentanol/biodiesel/diesel blends and diesel engine PM emissions and performance under two loads and different EGR rates. This work is of great significance and the results of this study can be used for reference in the study of diesel alternative fuels and the development of commercial diesel engines. 2. Experimental details 2.1. Test engine The engine used in the experiment is a 4-cylinder supercharged intercooled engine with a displacement of 2.0 L. The main parameters of this engine are shown in Table 1, and the structure of the experiment is shown in Fig. 1. In the experiment, the injection parameters are set using the Inca software and are controlled by an open ECU. The pressure in the engine cylinder is measured by the piezoelectric pressure sensor (Kistler 6052cu20). The uncertainty of the sensor is 0.1%, the measurement period is 200 s, and the sampling interval of the crank angle is 1°. In the experiment, the turbocharger is adjusted and an Table 1 Technical specifications of the engine.
2
Item
Value
Number of valves Number of cylinders Displacement (L) Compression ratio Cylinder diameter (mm) Stroke (mm) Maximum toque (N·m) Rated power (kW)/speed (r/min)
16 4 1.99 16.5 85 88.1 286 100/4000
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Fig. 1. The structure of the experiment.
external compressor is used to supply air, to ensure the stability of the intake pressure. The other main test instruments in the test bench include an eddy current dynamometer, AVL415se smoke meter, Dewetron combustion analyzer, MEXA 7100degr exhaust analyzer (produced by Horiba Company), and DMS500mkii engine particle analyzer.
analysis, and the specific calculation is shown in the following formula (3):
µC = 1
In this experiment, the pure diesel (D100) is used as a base fuel. The biodiesel used is soybean biodiesel, and the n-pentanol is purchased from the corresponding chemical companies. Three types of experimental fuels are used in this experiment: D100, biodiesel/diesel (BD20, with diesel volume at 80% and biodiesel volume at 20%), and n-pentanol/biodiesel/diesel (BDP20, with n-pentanol volume at 20%, biodiesel volume at 16%, and diesel volume at 64%). The physical and chemical properties of the experimental fuels are listed in Table 2. The basic physical and chemical properties of the blended fuel are obtained by corresponding calculation [47], and as shown in Table 3.
2.4. Test condition To maintain the nature of the blended fuel, all fuel is allocated and used on the same day. Before the experiment, the main testing instruments are preheated. In the experiment, the diesel engine speed is kept at 1400r/min (rpm), the intake air temperature is controlled at 30 ± 2℃, the temperature of cooling water is kept at 83 ± 3℃, the intake air pressure is maintained at 0.12 MPa, and the fuel injectiontiming is 3°CA BTDC. The effects of the fuels on engine particulate emissions and performance are tested under two loads of 0.8 MPa and 1.0 MPa, and different EGR rates (0–30%). To reduce the error of the experiment, the experiments of the all fuel were conducted on the same day. Simultaneously, each experimental point was repeated three times.
2.3. Definition of combustion parameters The brake specific fuel consumption (BSFC) of test fuels can be calculated by the following Eq. (1) [49]:
BSFCblends LHVblends LHVdiesel
(1)
Table 2 Physical and chemical properties of the test fuels.
The excess air coefficient (φ) is the ratio of the actual air quality required for the complete combustion of the fuel to the theoretical air quality required. It can be calculated by Eq. (2) [50]:
=
Mair Qfuel
(VD + vd )/(VDAstoic + vdastoic )
(3)
Among, msoot , mCO and mTHC are emissions of Soot, CO and THC; HuC , HuCO and HuTHC are the low calorific value of Soot, CO and THC; mf is the cycle fuel injection quantity and Huf is the low calorific value of the fuel.
2.2. Experimental fuel
BSFCequivalent =
msoot × HuC + mCO × HuCO + mTHC × HuTHC × 100% mf × Huf
(2)
In the formula, Qfuel and Mair are the actual mass flow of the fuel and air, respectively; V and v represent the volume fractions of diesel oil and additives in the mixed fuel, respectively; D and d indicate the densities of the diesel and additives, respectively; and Astoic and astoic represent the stoichiometric air fuel ratios of the diesel and additives, respectively. The combustion efficiency is mainly calculated by energy balance
Properties
Diesela
n-pentanolb
Biodieselc
Octane number Chemical formula Density (g/mL@15 °C) Cetane number Kinematic viscosity (mm2/s) at 40 °C Low heating value (MJ/kg) Oxygen content (wt%) Boiling point (°C)
– C12–C25 0.825 54 3.196 42.8 0 180–360
74–82 C5H11OH 0.815 20 2.89 35.06 18.15 138
– R-COO-R0.890 56 5.249 37.14 11 180–370
a b c
3
Source: ASTM D975. Source: Ref. [42]. Source: Ref. [48].
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After each change in the EGR rate, there is a delay, until the engine runs stably for 60 s, before starting the test.
Table 3 Main properties of blend fuels. Properties
D100
BD20
BDP20
Density (g/mL@15 °C) Cetane number Kinematic viscosity (mm2/s) at 40 °C Low heating value (MJ/kg)
0.825 54 3.196 42.8
0.823 54.4 3.529 41.67
0.8364 47.52 3.391 40.35
3. Results and analysis 3.1. Combustion performance Fig. 2 shows the IP and HRR for three test fuels under different loads
(a) EGR rate = 0%
(b) EGR rate = 15%
(c) EGR rate = 30% Fig. 2. IP and HRR of fuel under different loads and EGR rates conditions. 4
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and EGR rates. It could be easily revealed from the figure, the peak IP decreased, and the HRR increased with an increase in the EGR rate; for the same EGR rate, with the engine loads increased, the peak IP and HRR increased. The main reason was that with the EGR rate increase; the fuel ID period was prolonged, and a more homogeneous mixture was formed in the cylinder to promote fuel combustion; therefore, the HRR was increased. However, from the effects of the EGR, the ID was
prolonged, and the combustion volume increased, causing the IP to decrease with the increased in the EGR rate. When the engine load increased, the fuel injection quantity increased; thus, the HRR and the IP were higher than that with a medium load (0.8 MPa). Fig. 2 shows that, the addition of n-pentanol could improve the engine combustion process, as the peak HRR of the engine increased. This main reason was that n-pentanol had lower CN than other fuels, the ID of BDP20 was
(a) Brake thermal efficiency
(b) Combustion efficiency
(c) Brake specific fuel consumption
(d)Excess air coefficient
(e) Ignition delay
(f) Combustion duration
Fig. 3. Fuel combustion characteristics under different loads and EGR.
5
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slightly longer than that of D100 and BD20, and BDP20 had enough time for a complete mix with air to reduce the ratio of diffusion combustion. Moreover, owing to the high oxygen concentration characteristic of n-pentanol (which could promote the combustion), the influence of EGR decreased; thus, the peak HRR of BDP20 was higher than that of D100 and BDP20. The BTE values of the three fuels at medium and high loads, and at different EGR rates are depicted in Fig. 3a. It was known from Fig. 3a that the BTE of the engine increased with the load increase, but BTE decreased with the EGR rate increase. The main reason for this was that when the load increased, the combustion efficiency and combustion temperature in the cylinder increased (see Fig. 3b), and the mechanical efficiency increased; this was beneficial for improving the BTE. However, when the EGR rate increased, the oxygen concentration in the cylinder decreased, and the excess air coefficient decreased (see Fig. 3d), restraining the complete combustion of fuel. Thus, the BTE decreased with increase in EGR. Owing to the lower heating values of npentanol and biodiesel compared with that of diesel (see Table 2), the BTE values of BD20 and BDP20 were lower than those of D100. Therefore, to obtain the same power, more biodiesel and n-pentanol must be consumed, the BSFC values of BDP20 and BD20 were higher than those of D100 (see Fig. 3c). ID is an important parameter, and the influence of alcohol fuel on the ID is also proved by relevant research [51]. The ID of the fuel at different EGR rates and loads are depicted in Fig. 3e. It could be seen that the ID increased with EGR rates increase. This was due to the reduction of combustion temperature in the cylinder when the introduction of EGR, so the ID was extended. At the same EGR rate, the ID increased with the load increase; because the temperature in the cylinder was increased, which was conducive to shorten the ID period. As could be seen that the addition of n-pentanol can prolong the ID which is effected by the low CN and high latent heat of vaporization; a longer ID of BDP20 which promoted the uniform mixing of fuel and air, and the high oxygen content of n-pentanol could also accelerate the combustion speed; therefore, BDP20 had the shortest combustion duration (see Fig. 3f).
evidently shows that the emission of NOx decreased when the EGR rates increased, but the emission of soot increased. This was because in the introduction of EGR technology, several exhaust gases entered into the cylinder. The oxygen-content and combustion temperature in the cylinder were reduced, and the generation of NOx was inhibited; accordingly, the emission of NOx was reduced. However, the decreased oxygen-content in the cylinder could inhibit the complete combustion of fuel, leading to increased soot emissions. The difference in NOx emission between the fuels in the figure show that the effects of EGR technology on reducing NOx were greater than those originating from the nature of the fuel itself; moreover, soot emissions were greatly affected by the fuel. As shown in the figure, the addition of n-pentanol could decrease soot emission, owing to the high oxygen-content of npentanol which could promote the oxidation of soot particles and improve combustion. 3.3. Particulate emission characteristics 3.3.1. Particle size distribution concentration Fig. 5 shows the PSD concentrations of the three fuels at different loads and EGR rates. The particles contained in engine exhaust can be divided into those in a nucleation state (< 50 nm) and those in an accumulation state (> 50 nm), according to the apparent diameter of the particles [52]. As indicated by the figure, with the EGR rate increase, the PSD concentration under two loads increased, and showed a bimodal distribution, mainly concentrated on the accumulated particles. At the same EGR rate, the PSD concentration of the engine at a high load (1.0 MPa) was higher than that of the engine at a medium load (0.8 MPa). This was because owing to the effects of EGR technology, the oxygen concentration in the cylinder was reduced, incomplete combustion was aggravated, several carbon black materials could not be burned, and large-scale soot particles increased. Under a high load, the quantity of fuel-injection was too high, which made the fuel concentration area increase in the cylinder; meanwhile, owing to the influence of EGR technology, the oxygen concentration in the cylinder decreased, and the complete combustion of fuel was inhibited. Thus, the PSD concentration was greater than that with the medium load. As shown in Fig. 5, when n-pentanol was added to the mixed fuel, the total PSD concentration of the fuel decreased, and the concentration of oversized particles was lower than those in the other two fuels. The
3.2. Emission characteristics of soot and NOx The emission characteristics for soot and NOx of the three fuels under different loads and EGR rates are presented in Fig. 4. The figure
(a) NOx
(b) soot
Fig. 4. Emission characteristics of NOx and soot under different EGR rates and loads.
6
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(a) EGR rate= 0%
(b) EGR rate = 15%
(c) EGR rate = 30% Fig. 5. PSD concentration under different loads and EGR rates.
main reason was that n-pentanol has the characteristics of high oxygencontent; the oxygen atoms of n-pentanol form an oxygen-containing active substance in the combustion process, which can improve the oxygen poor area caused by EGR, improve the combustion quality, and reduce the PSD concentration. However, under the conditions of a high EGR rate (30%) and high load, owing to the decrease in the oxygencontent and the influence of too much fuel-injection, incomplete combustion was aggravated, and the PSD concentration was increased.
was because under a high load, the amount of fuel-injection quantity by the engine increased; simultaneously, under the influence of EGR, the over-concentrated fuel area in the cylinder increased, and the oxygencontent was insufficient. Therefore, the combustion deteriorated, and the particle emissions increased. Fig. 6 shows that adding n-pentanol to an engine at high and medium loads can reduce the TPNC; particularly, when the EGR rate was between 5 and 20%, the TPNC emissions of the three fuels could be ranked as D100 > BD20 > BDP20. Fig. 6b and c demonstrate that the concentrations of the nucleation mode number and accumulation mode number for BDP20 were smaller than those for the other two fuels. This was because the volatility of n-pentanol was high. Thus, the mixing rate of fuel oil and air was increased, the local rich fuel area was reduced, and the combustion was more complete. Moreover, and the oxygencontent of the fuel itself was high, which could promote the oxidation of carbon particles. Accordingly, the TPNC was reduced.
3.3.2. Total particle number concentration Fig. 6a shows the TPNCs of the three fuels at different loads and EGR rates. It could be seen from the figure, under the condition of a 0.8 MPa load, the TPNC of the three fuels initially decreased and then increased with the EGR rate increase. In particular, when the EGR rate was 5%-20%, the TPNC hierarchy between the three fuels was D100 > BD20 > BDP20. As the dilution and hot melt affected the EGR, the ID period was prolonged, more uniform combustible mixture was formed and combustion was promoted; thus, the TPNC was reduced. However, an excessive EGR rate would reduce the oxygen concentration and cause combustion deterioration in the cylinder, and leading to an increase in the TPNC. At a high load (1.0 MPa), the TPNC values of the three fuels increased with an increase in the EGR rate, and the TPNC at this load was higher than that at the medium load. This
3.3.3. Total particulate mass concentration Fig. 7 shows the TPMC, nucleated particle mass concentration, and accumulated particle mass concentration under different loads and EGR rates. Fig. 7a shows that under the condition of a 0.8 MPa load, the TPMC of D100 initially decreased and then increased with an increased in EGR, whereas the TPMC of BDP20 and BD20 increased with an 7
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(a) TPNC
(b) Nucleation mode number concentration
(c)Accumulation mode number concentration
Fig. 6. TPNC, nucleation and accumulation particle number concentration under different EGR rates and loads.
increased in the EGR rate; However, under a load of 1.0 MPa, the TPMC of the three fuels increased with an increased in EGR, and the TPMC values were higher than those at a 0.8 MPa load. This was because at 0.8 MPa, during the process of the EGR increase, the ID period of D100 was prolonged, and a more uniform mixture could be formed in the early stage. This promoted combustion, thereby reducing the TPNC (see Fig. 6a). In particular, the concentration of nucleation particles was reduced (see Fig. 6b), causing a reduction in the TPMC of D100 in the early stage. However, owing to the influence of EGR, the number of accumulated particles increased, and the particle size increased (see Fig. 5); thus, the TPMC increased. Nevertheless, the TPMC values of BD20 and BDP20 were lower than those of D100. Fig. 7b and c show that the mass concentrations of nucleated and accumulated particles of BD20 and BDP20 increased with the increased of EGR; therefore, the mass concentrations of particles increased with EGR. Under the load of 1.0 MPa, the TPNC of the three fuels was higher (see Fig. 6a); in particular, the accumulated particle number was higher than that with the medium load; thus, the TPMC was higher than that of the medium load. As shown in Fig. 7, the TPMC of BDP20 was lower than that of the other two fuels. This was because the addition of n-pentanol increased
the oxygen-content and reduced the viscosity of the blended fuel, promoted the complete combustion of the fuel, and reduced the number of particles (see Fig. 6). Therefore, the TPMC of BDP20 was lower than that of the other two fuels, especially under a load of 0.8 MPa, where the TPMC was the lowest. 3.3.4. Relationship between total particulate number concentration and total particulate mass concentration and NOx The relationships between the TPMC and TPNC of the three test fuels and NOx emissions under different EGR rates and loads are shown in Fig. 8. Fig. 8 shows that in the direction of increasing EGR, the NOx emissions of the three fuels continuously decreased; the TPNC and the TPMC of the fuel increased, indicating that the TPNC and TPMC have a trade-off relationship with NOx emissions. However, as the figure indicates, the addition of n-pentanol to the blended fuel could effectively reduce the TPNC and TPMC (see Figs. 6a and 7a), and the NOx emissions of BDP20 were similar to those of the other two test fuels (see Fig. 4a). Thus, the addition of n-pentanol could improve the trade-off relationship between the TPNC, TPMC, and NOx. 8
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(a) TPMC
(b) Nucleation mode mass concentration
(c) Accumulation mode mass concentration
Fig. 7. TPMC, nucleation, and accumulation mode mass concentration under different loads and EGR rates.
3.3.5. Particle number concentration in different particle size range Fig. 9 shows the particle number concentration, proportion of small particles (< 25 nm), and total particle number in various modes (particles < 25 nm, between 25 and 50 nm, between 50 and 100 nm, and between 100 and 1000 nm) under different EGR rates and loads. Under medium-load conditions (0.8 MPa), with the EGR rate increased, the concentration of nucleated particles of the three fuels initially decreased and then increased, and the proportion of small particles kept decreasing, whereas the concentration of accumulated particles increased. The main reason was the effects of EGR; the ID of the fuels was prolonged, the premix ratio increased and promoted the combustion, and the HRR was increased (see Fig. 2). This promoted the oxidation of nucleated particles; however, with the EGR rate increased, the content of exhaust gas entering the cylinder was also increased, which inhibited the combustion of fuel. This resulted in the increase in the particle number, also with an increase in the collision and adsorption between particles. This, in turn, resulted in several accumulated particles, therefore, a decrease in the proportion of small particles. Under highload conditions (1.0 MPa), the figure demonstrates that the
concentrations of nucleated particle numbers and accumulated particle numbers increased with an increase in EGR, and the proportion of small particles decreased with the increased EGR; however, the proportion was lower than that with the medium load. The main reason was that owing to the increase in the load, the fuel-injection quantity in the cylinder increased, which released higher heat (see Fig. 2) and promoted the oxidation of nucleated particles. However, owing to the influence of EGR, the oxygen concentration in the cylinder decreased, the fuel combustion was insufficient, and the fuel rich area in the cylinder increased; thus, the particle number increased (especially the particle size range between 50 and 100 nm and 100–1000 nm). The proportion of small particles decreased because the nucleated particle size was in the range of 25–50 nm. As shown in Fig. 9, when n-pentanol was added to the mixed fuel, the TPNC was lower than that of other test fuels, but the proportion of small particles was higher than other test fuels. The main reason was the oxygen-content of n-pentanol can promote the oxidation of accumulated particles, developing in the direction of smaller particle sizes. Therefore, the concentration of the accumulated particles was lower 9
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Fig. 8. Relation between NOx and TPNC and TPMC under different EGR rates and loads.
than in those in the other two fuels (see Fig. 6c). The particle size of nucleated particles was concentrated in the range of 25–50 nm, and the particle number concentration of the small particles experienced slight variation. However, as the TPNC was lower than that in the other two fuels (see Fig. 6a), the proportion of small particles was higher than proportion in the other two fuels.
combustion deterioration was more serious than that at a medium load, and more particles were generated. In addition, the fuel HRR increased, increasing the kinetic energy of the particles, and causing the collisions between particles to become more frequent. Thus, the particle size was larger than at a medium load (see Fig. 5); accordingly, the GMD at a high load was greater than that at a medium load. As shown in Fig. 10a, when n-pentanol was added to the mixed fuel, the GMD of the fuel was smaller than that of the other two test fuels. The main reason was the high oxygen-content, high volatility and low viscosity of n-pentanol could mix evenly with air, which promoted combustion, thereby decreasing the number of particles emitted (see Fig. 6). Thus, the probability of collision between particles was lower than that in D100 and BD20, and the GMD of BDP20 was smaller. Fig. 10b and c shows the relationship between the GMD of particles and the TPNC under different EGR rates and loads. The figure shows that the GMD of particles increases with an increase in the TPNC, which indicates that the TPNC and the GMD are affected by each other. Fig. 10b and c demonstrate that a larger number of particles (especially the particle size) corresponds to a larger GMD. This was because the larger the total number of particles were, the greater the probability of particle adsorption and collision with each other was. A larger number of accumulated particles corresponded to a larger particle size; thus, the GMD would increase.
3.3.6. Relationship between geometric mean diameter of particles and total particulate number concentration Fig. 10a shows the GMD of particles for the three fuels at different loads and EGR rates. It was known from the figure that the GMD of particles increases with an increase in the EGR rate. The GMD of particles under a 1.0 MPa load was larger than that under a 0.8 MPa load. This was because in the process of increasing the EGR, too much exhaust gas entered the cylinder. Therefore, the excess air coefficient decreased (see Fig. 3d), the oxygen concentration decreased, and the combustion in the cylinder deteriorated, resulting in the formation of several particles. The adsorption between particles increased, the number of accumulated particles increased (see Fig. 6c), and the particle size increased; thus, the GMD of the particles increased. When the engine was at the 1.0 MPa load, the fuel-injection quantity was greater more than that at the 0.8 MPa load, and the fuel enrichment area in the cylinder increased. Meanwhile, owing to the effects of EGR, the 10
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(a) 0.8MPa
(b) 1.0MPa Fig. 9. Particle number concentration and proportion of small particles in different particle size ranges under different EGR rates and loads.
4. Conclusion
EGR rates and loads; by adding n-pentanol into the mixed fuel, the TPNC and the TPMC decreased, and the trade-off relationship between the TPNC, TPMC, and NOx was alleviated. 4. With the increase in EGR rates and loads, the number concentrations of particles in various modes (< 25 nm, 25–50 nm, 50–100 nm, 100–1000 nm) increased, but the proportion of small particles (< 25 nm) decreased. The addition of n-pentanol could reduce the concentration of large size particles and improve the particle emissions of each mode. 5. Adding n-pentanol to the fuel mixture could reduce the GMD of particles and improve the relationship between the TPNC and GMD of the particles. 6. In this work, the addition of n-pentanol could reduce the emissions of carbon particles and improved the combustion quality of the engine; the conclusions and phenomena of this experiment can be proved for analyzing the effect of adding n-pentanol on PM through chemical reaction, and also provide support for analyzing the change of nanostructure of carbonaceous particles.
The effects of n-pentanol additive on particulate emission characteristics and combustion performance, and especially on particulate emission laws, were studied under different EGR rates and two loads (0.8 MPa and 1.0 MPa). The main conclusions are as follows. 1. With the EGR rate increased the engine HRR increased, but the IP and BTE decreased. At the same EGR rate, increasing the engine load improved the HRR, IP and BTE, but the BSFC decreased. The addition of n-pentanol could increase the BSFC and ID, but it could further improve the combustion efficiency and decreased combustion duration of the diesel engine, and the IP and the BTE values were similar to those of the other two fuels. 2. Using EGR technology, the NOx emissions could be effectively reduced, and there was no significant difference in NOx emissions between the two loads and the fuel. The addition of n-pentanol could effectively reduce the emissions of soot and the PSD concentration and had slight effect on the emissions of NOx. 3. The TPNC and TPMC of the engine increased with an increase in the 11
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(a) GMD
(b)
(c)
Fig. 10. GMD of particles under different EGR rates and loads and relationship with the TPNC.
CRediT authorship contribution statement
References
Haozhong Huang: Conceptualization, Methodology, Supervision, Resources. Xiaoyu Guo: Writing - original draft, Formal analysis, Investigation. Rong Huang: Data curation. Han Lei: Investigation. Yajuan Chen: Investigation. Te Wang: Writing - review & editing. Sai Wang: Validation. Mingzhang Pan: Project administration.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work is supported by the National Natural Science Foundation of China (51966001, 51865002). 12
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Full Length Article
Effect of n-pentanol additive on compression-ignition engine performance and particulate emission laws
T
Haozhong Huang, Xiaoyu Guo, Rong Huang, Han Lei, Yajuan Chen, Te Wang, Sai Wang, ⁎ Mingzhang Pan College of Mechanical Engineering, Guangxi University, Nanning 530004, China
ARTICLE INFO
ABSTRACT
Keywords: n-Pentanol EGR Biodiesel Combustion performance Particle emission characteristic
Biodiesel has a high viscosity, which limits its application to engines. However, n-pentanol has low viscosity and high volatility, which is considered a promising additive. This study examined the influence of n-pentanol/ biodiesel/diesel blends on the performance and particulate emissions of a diesel engine at different loads, under the condition of coupling exhaust gas recirculation (EGR) (0–30%). The three test fuels included pure diesel (D100); 80% diesel and 20% biodiesel blended fuel (BD20); 64% diesel, 16% biodiesel, and 20% n-pentanol blended fuel (BDP20). The results indicated that the influence of EGR on the NOx emissions between fuels was greater than that of the physicochemical properties of the fuels. For the same EGR rate, the heat release rate (HRR), peak in-cylinder pressure (IP), and brake thermal efficiency (BTE) increased with the engine load increased. After adding n-pentanol to BD20, the HRR increased. The peak IP and BTE values of BDP20 were similar to those of D100 and BD20. With the addition of n-pentanol, the total particulate number concentration (TPNC), soot, and total particulate mass concentration (TPMC) emissions decreased. Furthermore, the trade-off relationship between TPMC and NOx was improved, and the geometric mean diameter (GMD) of the particles decreased.
1. Introduction As a critical power source, the diesel engine is widely used in transportation and industry, owing to its high thermal efficiency, large torque, and good stability [1]. However, the automobile industry is developing rapidly; the demand for energy is increasing, which could accelerate the emergence of an energy shortage crisis [2]. At the same time, diesel engines produce many harmful substances in the process of operation that seriously affect human health [3]. Therefore, reducing the harmful emissions in the exhaust gas, especially NOx and particulate emissions, which is still the main challenge facing the engine industry [4]. To reduce the harmful emissions of the engine, many advance combustion modes and post-processing methods have been researched, such as, low-temperature combustion (LTC) [5], homogeneous charge compression-ignition [6], selective catalytic reduction [7], and diesel particulate filter [8]. Moreover, to solve the energy problem, researchers have worked on developing diesel alternative fuels, including, alcohol fuels [9], ether fuels [10], and biodiesel [11], to alleviate the above-mentioned difficulties. LTC has characteristics of high efficiency and low emissions; in
⁎
particular, it can efficiency reduce NOx emissions [12]. LTC refers to a combustion temperature in the cylinder below 2100 K, which is not conducive to the generation of carbon smoke and NOx. Thus, LTC is achieved by using EGR, a low compression ratio, and delayed injection [13]. An increasing number of researchers are interested in using a controllable EGR rate to achieve LTC. Verma et al. [14] studied the influence of compression ratio, EGR, and EGR temperature on engine performance and emission characteristics. They found that, owing to the effects of EGR, the engine efficiency increase, and NOx emissions are decreased. Huang et al. [15] and He et al. [16] researched the influence of EGR on engine emission characteristics. The results showed that NOx decreased with the EGR rate increase, and TPNC initially decreased and then increased. As can be seen from the literature, LTC can be achieved by adopting EGR technology, which can reduce NOx emissions. Biodiesel is a new type of alternative fuel which has a wide range of sources. It can be produced from non-edible oil, waste vegetable oil, animal fat and algae and other materials, moreover, biodiesel can be degraded and can be made to mix well with diesel [17]. Biodiesel has a high ignition point and cetane number (CN), and the aromatic
Corresponding author. E-mail address: [email protected] (M. Pan).
https://doi.org/10.1016/j.fuel.2020.117201 Received 26 December 2019; Received in revised form 21 January 2020; Accepted 23 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.
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Nomenclature BTE BSFC BD20 BDP20 CN D100 GMD HRR
HCAs IP ID LCAs LTC PM PSD TPNC TPMC
brake thermal efficiency brake specific fuel consumption 20% biodiesel + 80% pure diesel 20% n-pentanol + 16% biodiesel + 64% pure diesel cetane number pure diesel geometric mean diameter heat release rate
compounds and sulfur elements contained in the fuel itself are low, consequently producing less harmful emissions during combustion than diesel [18]. In addition, the large amounts of oxygen contained in biodiesel can improve the combustion quality in the engine cylinder, and facilitate better application to the engine [19]. Teoh et al. [20] and How et al. [21] researched the influence of biodiesel additives on common rail supercharged engine emission characteristics and performance. The results showed that the IP and HRR decreased, but the CO, soot, and particle emissions were reduced. Jiao et al. [22] simulated the effects of diesel/biodiesel on engine emissions at different altitudes. They found that at the same altitude, the particle emissions were less than those of pure diesel, but the NOx emissions were increased. Rajak et al. [23] researched the influence of spiral-microalgae diesel/biodiesel on engine emissions, and the results showed that the CO, hydrocarbon, and soot emissions decreased. From the above literature, it is known that biodiesel can reduce the emissions of harmful substances to a certain extent. However, owing to the high molecular weight and viscosity of biodiesel, the fuel easily adhere to the fuel injector and piston ring, resulting in coking of the fuel injector and adhesion of the piston ring [24,25]. Because of the poor atomization of biodiesel in the cylinder, which is not conducive to mixing with air, the incomplete combustion of fuel is intensified, resulting in the decreases of HRR and IP [26,27]. Therefore, it is needed to add a substance to a biodiesel/ diesel blend that increases the atomization ability and oxygen-content, and decreases the viscosity of the blend. As another clean alternative fuel, alcohol fuels have lower viscosity and higher oxygen content, and can be added to biodiesel to improve the properties of the blended fuel [28,29]. The chemical and physical properties of alcohol fuels will change with the change in the number of carbon atoms in their chemical structure and the arrangement of carbon atoms [30]. The number of carbon atoms increase can improve the CN of the fuel, shorten the ignition delay (ID) period, and improve the volatility and ignitability of the fuel [31,32]. According to the number of carbon atoms in alcohols, they can be divided into high-carbon alcohols (HCAs) and low-carbon alcohols (LCAs). As compared with LCAs, HCAs have higher calorific value and a lower latent heat of vaporization; simultaneously, they show better mixing and lubricity abilities, and exhibit lower corrosiveness to the engine [33,34]. Among them, n-pentanol is the most representative high-carbon alcohol [35]. As compared with ethanol and n-butanol alcohol fuels, n-pentanol is better mixed with diesel and biodiesel [36,37]. The addition of n-pentanol can improve the physical and chemical properties, increase the oxygen concentration, reduce the viscosity, and reduce the pollutant emissions of the engine, especially those of particulate matter (PM), which has great potential [38,39]. The research on n-pentanol additive has attracted the attention of many researchers [40]. Li et al. [41] researched the effect of pentanol on single-cylinder DI diesel engine emissions and combustion, founding that the addition of pentanol increased the HRR while the soot and particles emissions decreased. Wei et al. [42] studied the influence of diesel/n-pentanol mixture on the particulate emissions of a naturallyaspirated DI engine, and found that the addition of n-pentanol could reduce the mass concentration and number concentration of PM. Zhang
high-carbon alcohols in-cylinder pressure ignition delay low-carbon alcohols low-temperature combustion particulate matter particle size distribution total particulate number concentration total particulate mass concentration
et al. [43] studied the influence of a ultra-low sulfur diesel/pentanol mixture on the mass and number of PM and the composition of carbon PM in a DI diesel engine, They discovered that the masses of PM and primary carbon emissions decreased, and the emissions of volatile and solid particles decreased. Zhu et al. [44] found that adding n-pentanol to biodiesel can effectively reduce the mass and quantity concentrations of particles, and they can improve the BTE of an engine. Yilmaz et al. [45,46] found that adding n-pentanol to biodiesel can effectively reduce the emissions and improved the BTE. The above literature shows that npentanol can reduce harmful emissions and improve engine performance. It is a cleaner alternative fuel and can be well blended with biodiesel and diesel. The above mentioned studies indicate that biodiesel and n-pentanol have a better emission reduction capacity when they are used in engines. However, the viscosity of biodiesel is too high, which is not conducive to atomization combustion. In contrast, the viscosity of npentanol is low, and the oxygen-content is high. Thus, it can better mix with biodiesel and improve the properties of the mixed fuel. In the case of coupled EGR, the influences of the n-pentanol/biodiesel/diesel blend on diesel engine performance and particulate emissions, especially on particle number, particle size distribution (PSD), and particle mass, are not clear. Therefore, the purpose of this study is to explore the relationships between n-pentanol/biodiesel/diesel blends and diesel engine PM emissions and performance under two loads and different EGR rates. This work is of great significance and the results of this study can be used for reference in the study of diesel alternative fuels and the development of commercial diesel engines. 2. Experimental details 2.1. Test engine The engine used in the experiment is a 4-cylinder supercharged intercooled engine with a displacement of 2.0 L. The main parameters of this engine are shown in Table 1, and the structure of the experiment is shown in Fig. 1. In the experiment, the injection parameters are set using the Inca software and are controlled by an open ECU. The pressure in the engine cylinder is measured by the piezoelectric pressure sensor (Kistler 6052cu20). The uncertainty of the sensor is 0.1%, the measurement period is 200 s, and the sampling interval of the crank angle is 1°. In the experiment, the turbocharger is adjusted and an Table 1 Technical specifications of the engine.
2
Item
Value
Number of valves Number of cylinders Displacement (L) Compression ratio Cylinder diameter (mm) Stroke (mm) Maximum toque (N·m) Rated power (kW)/speed (r/min)
16 4 1.99 16.5 85 88.1 286 100/4000
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Fig. 1. The structure of the experiment.
external compressor is used to supply air, to ensure the stability of the intake pressure. The other main test instruments in the test bench include an eddy current dynamometer, AVL415se smoke meter, Dewetron combustion analyzer, MEXA 7100degr exhaust analyzer (produced by Horiba Company), and DMS500mkii engine particle analyzer.
analysis, and the specific calculation is shown in the following formula (3):
µC = 1
In this experiment, the pure diesel (D100) is used as a base fuel. The biodiesel used is soybean biodiesel, and the n-pentanol is purchased from the corresponding chemical companies. Three types of experimental fuels are used in this experiment: D100, biodiesel/diesel (BD20, with diesel volume at 80% and biodiesel volume at 20%), and n-pentanol/biodiesel/diesel (BDP20, with n-pentanol volume at 20%, biodiesel volume at 16%, and diesel volume at 64%). The physical and chemical properties of the experimental fuels are listed in Table 2. The basic physical and chemical properties of the blended fuel are obtained by corresponding calculation [47], and as shown in Table 3.
2.4. Test condition To maintain the nature of the blended fuel, all fuel is allocated and used on the same day. Before the experiment, the main testing instruments are preheated. In the experiment, the diesel engine speed is kept at 1400r/min (rpm), the intake air temperature is controlled at 30 ± 2℃, the temperature of cooling water is kept at 83 ± 3℃, the intake air pressure is maintained at 0.12 MPa, and the fuel injectiontiming is 3°CA BTDC. The effects of the fuels on engine particulate emissions and performance are tested under two loads of 0.8 MPa and 1.0 MPa, and different EGR rates (0–30%). To reduce the error of the experiment, the experiments of the all fuel were conducted on the same day. Simultaneously, each experimental point was repeated three times.
2.3. Definition of combustion parameters The brake specific fuel consumption (BSFC) of test fuels can be calculated by the following Eq. (1) [49]:
BSFCblends LHVblends LHVdiesel
(1)
Table 2 Physical and chemical properties of the test fuels.
The excess air coefficient (φ) is the ratio of the actual air quality required for the complete combustion of the fuel to the theoretical air quality required. It can be calculated by Eq. (2) [50]:
=
Mair Qfuel
(VD + vd )/(VDAstoic + vdastoic )
(3)
Among, msoot , mCO and mTHC are emissions of Soot, CO and THC; HuC , HuCO and HuTHC are the low calorific value of Soot, CO and THC; mf is the cycle fuel injection quantity and Huf is the low calorific value of the fuel.
2.2. Experimental fuel
BSFCequivalent =
msoot × HuC + mCO × HuCO + mTHC × HuTHC × 100% mf × Huf
(2)
In the formula, Qfuel and Mair are the actual mass flow of the fuel and air, respectively; V and v represent the volume fractions of diesel oil and additives in the mixed fuel, respectively; D and d indicate the densities of the diesel and additives, respectively; and Astoic and astoic represent the stoichiometric air fuel ratios of the diesel and additives, respectively. The combustion efficiency is mainly calculated by energy balance
Properties
Diesela
n-pentanolb
Biodieselc
Octane number Chemical formula Density (g/mL@15 °C) Cetane number Kinematic viscosity (mm2/s) at 40 °C Low heating value (MJ/kg) Oxygen content (wt%) Boiling point (°C)
– C12–C25 0.825 54 3.196 42.8 0 180–360
74–82 C5H11OH 0.815 20 2.89 35.06 18.15 138
– R-COO-R0.890 56 5.249 37.14 11 180–370
a b c
3
Source: ASTM D975. Source: Ref. [42]. Source: Ref. [48].
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After each change in the EGR rate, there is a delay, until the engine runs stably for 60 s, before starting the test.
Table 3 Main properties of blend fuels. Properties
D100
BD20
BDP20
Density (g/mL@15 °C) Cetane number Kinematic viscosity (mm2/s) at 40 °C Low heating value (MJ/kg)
0.825 54 3.196 42.8
0.823 54.4 3.529 41.67
0.8364 47.52 3.391 40.35
3. Results and analysis 3.1. Combustion performance Fig. 2 shows the IP and HRR for three test fuels under different loads
(a) EGR rate = 0%
(b) EGR rate = 15%
(c) EGR rate = 30% Fig. 2. IP and HRR of fuel under different loads and EGR rates conditions. 4
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and EGR rates. It could be easily revealed from the figure, the peak IP decreased, and the HRR increased with an increase in the EGR rate; for the same EGR rate, with the engine loads increased, the peak IP and HRR increased. The main reason was that with the EGR rate increase; the fuel ID period was prolonged, and a more homogeneous mixture was formed in the cylinder to promote fuel combustion; therefore, the HRR was increased. However, from the effects of the EGR, the ID was
prolonged, and the combustion volume increased, causing the IP to decrease with the increased in the EGR rate. When the engine load increased, the fuel injection quantity increased; thus, the HRR and the IP were higher than that with a medium load (0.8 MPa). Fig. 2 shows that, the addition of n-pentanol could improve the engine combustion process, as the peak HRR of the engine increased. This main reason was that n-pentanol had lower CN than other fuels, the ID of BDP20 was
(a) Brake thermal efficiency
(b) Combustion efficiency
(c) Brake specific fuel consumption
(d)Excess air coefficient
(e) Ignition delay
(f) Combustion duration
Fig. 3. Fuel combustion characteristics under different loads and EGR.
5
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slightly longer than that of D100 and BD20, and BDP20 had enough time for a complete mix with air to reduce the ratio of diffusion combustion. Moreover, owing to the high oxygen concentration characteristic of n-pentanol (which could promote the combustion), the influence of EGR decreased; thus, the peak HRR of BDP20 was higher than that of D100 and BDP20. The BTE values of the three fuels at medium and high loads, and at different EGR rates are depicted in Fig. 3a. It was known from Fig. 3a that the BTE of the engine increased with the load increase, but BTE decreased with the EGR rate increase. The main reason for this was that when the load increased, the combustion efficiency and combustion temperature in the cylinder increased (see Fig. 3b), and the mechanical efficiency increased; this was beneficial for improving the BTE. However, when the EGR rate increased, the oxygen concentration in the cylinder decreased, and the excess air coefficient decreased (see Fig. 3d), restraining the complete combustion of fuel. Thus, the BTE decreased with increase in EGR. Owing to the lower heating values of npentanol and biodiesel compared with that of diesel (see Table 2), the BTE values of BD20 and BDP20 were lower than those of D100. Therefore, to obtain the same power, more biodiesel and n-pentanol must be consumed, the BSFC values of BDP20 and BD20 were higher than those of D100 (see Fig. 3c). ID is an important parameter, and the influence of alcohol fuel on the ID is also proved by relevant research [51]. The ID of the fuel at different EGR rates and loads are depicted in Fig. 3e. It could be seen that the ID increased with EGR rates increase. This was due to the reduction of combustion temperature in the cylinder when the introduction of EGR, so the ID was extended. At the same EGR rate, the ID increased with the load increase; because the temperature in the cylinder was increased, which was conducive to shorten the ID period. As could be seen that the addition of n-pentanol can prolong the ID which is effected by the low CN and high latent heat of vaporization; a longer ID of BDP20 which promoted the uniform mixing of fuel and air, and the high oxygen content of n-pentanol could also accelerate the combustion speed; therefore, BDP20 had the shortest combustion duration (see Fig. 3f).
evidently shows that the emission of NOx decreased when the EGR rates increased, but the emission of soot increased. This was because in the introduction of EGR technology, several exhaust gases entered into the cylinder. The oxygen-content and combustion temperature in the cylinder were reduced, and the generation of NOx was inhibited; accordingly, the emission of NOx was reduced. However, the decreased oxygen-content in the cylinder could inhibit the complete combustion of fuel, leading to increased soot emissions. The difference in NOx emission between the fuels in the figure show that the effects of EGR technology on reducing NOx were greater than those originating from the nature of the fuel itself; moreover, soot emissions were greatly affected by the fuel. As shown in the figure, the addition of n-pentanol could decrease soot emission, owing to the high oxygen-content of npentanol which could promote the oxidation of soot particles and improve combustion. 3.3. Particulate emission characteristics 3.3.1. Particle size distribution concentration Fig. 5 shows the PSD concentrations of the three fuels at different loads and EGR rates. The particles contained in engine exhaust can be divided into those in a nucleation state (< 50 nm) and those in an accumulation state (> 50 nm), according to the apparent diameter of the particles [52]. As indicated by the figure, with the EGR rate increase, the PSD concentration under two loads increased, and showed a bimodal distribution, mainly concentrated on the accumulated particles. At the same EGR rate, the PSD concentration of the engine at a high load (1.0 MPa) was higher than that of the engine at a medium load (0.8 MPa). This was because owing to the effects of EGR technology, the oxygen concentration in the cylinder was reduced, incomplete combustion was aggravated, several carbon black materials could not be burned, and large-scale soot particles increased. Under a high load, the quantity of fuel-injection was too high, which made the fuel concentration area increase in the cylinder; meanwhile, owing to the influence of EGR technology, the oxygen concentration in the cylinder decreased, and the complete combustion of fuel was inhibited. Thus, the PSD concentration was greater than that with the medium load. As shown in Fig. 5, when n-pentanol was added to the mixed fuel, the total PSD concentration of the fuel decreased, and the concentration of oversized particles was lower than those in the other two fuels. The
3.2. Emission characteristics of soot and NOx The emission characteristics for soot and NOx of the three fuels under different loads and EGR rates are presented in Fig. 4. The figure
(a) NOx
(b) soot
Fig. 4. Emission characteristics of NOx and soot under different EGR rates and loads.
6
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(a) EGR rate= 0%
(b) EGR rate = 15%
(c) EGR rate = 30% Fig. 5. PSD concentration under different loads and EGR rates.
main reason was that n-pentanol has the characteristics of high oxygencontent; the oxygen atoms of n-pentanol form an oxygen-containing active substance in the combustion process, which can improve the oxygen poor area caused by EGR, improve the combustion quality, and reduce the PSD concentration. However, under the conditions of a high EGR rate (30%) and high load, owing to the decrease in the oxygencontent and the influence of too much fuel-injection, incomplete combustion was aggravated, and the PSD concentration was increased.
was because under a high load, the amount of fuel-injection quantity by the engine increased; simultaneously, under the influence of EGR, the over-concentrated fuel area in the cylinder increased, and the oxygencontent was insufficient. Therefore, the combustion deteriorated, and the particle emissions increased. Fig. 6 shows that adding n-pentanol to an engine at high and medium loads can reduce the TPNC; particularly, when the EGR rate was between 5 and 20%, the TPNC emissions of the three fuels could be ranked as D100 > BD20 > BDP20. Fig. 6b and c demonstrate that the concentrations of the nucleation mode number and accumulation mode number for BDP20 were smaller than those for the other two fuels. This was because the volatility of n-pentanol was high. Thus, the mixing rate of fuel oil and air was increased, the local rich fuel area was reduced, and the combustion was more complete. Moreover, and the oxygencontent of the fuel itself was high, which could promote the oxidation of carbon particles. Accordingly, the TPNC was reduced.
3.3.2. Total particle number concentration Fig. 6a shows the TPNCs of the three fuels at different loads and EGR rates. It could be seen from the figure, under the condition of a 0.8 MPa load, the TPNC of the three fuels initially decreased and then increased with the EGR rate increase. In particular, when the EGR rate was 5%-20%, the TPNC hierarchy between the three fuels was D100 > BD20 > BDP20. As the dilution and hot melt affected the EGR, the ID period was prolonged, more uniform combustible mixture was formed and combustion was promoted; thus, the TPNC was reduced. However, an excessive EGR rate would reduce the oxygen concentration and cause combustion deterioration in the cylinder, and leading to an increase in the TPNC. At a high load (1.0 MPa), the TPNC values of the three fuels increased with an increase in the EGR rate, and the TPNC at this load was higher than that at the medium load. This
3.3.3. Total particulate mass concentration Fig. 7 shows the TPMC, nucleated particle mass concentration, and accumulated particle mass concentration under different loads and EGR rates. Fig. 7a shows that under the condition of a 0.8 MPa load, the TPMC of D100 initially decreased and then increased with an increased in EGR, whereas the TPMC of BDP20 and BD20 increased with an 7
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(a) TPNC
(b) Nucleation mode number concentration
(c)Accumulation mode number concentration
Fig. 6. TPNC, nucleation and accumulation particle number concentration under different EGR rates and loads.
increased in the EGR rate; However, under a load of 1.0 MPa, the TPMC of the three fuels increased with an increased in EGR, and the TPMC values were higher than those at a 0.8 MPa load. This was because at 0.8 MPa, during the process of the EGR increase, the ID period of D100 was prolonged, and a more uniform mixture could be formed in the early stage. This promoted combustion, thereby reducing the TPNC (see Fig. 6a). In particular, the concentration of nucleation particles was reduced (see Fig. 6b), causing a reduction in the TPMC of D100 in the early stage. However, owing to the influence of EGR, the number of accumulated particles increased, and the particle size increased (see Fig. 5); thus, the TPMC increased. Nevertheless, the TPMC values of BD20 and BDP20 were lower than those of D100. Fig. 7b and c show that the mass concentrations of nucleated and accumulated particles of BD20 and BDP20 increased with the increased of EGR; therefore, the mass concentrations of particles increased with EGR. Under the load of 1.0 MPa, the TPNC of the three fuels was higher (see Fig. 6a); in particular, the accumulated particle number was higher than that with the medium load; thus, the TPMC was higher than that of the medium load. As shown in Fig. 7, the TPMC of BDP20 was lower than that of the other two fuels. This was because the addition of n-pentanol increased
the oxygen-content and reduced the viscosity of the blended fuel, promoted the complete combustion of the fuel, and reduced the number of particles (see Fig. 6). Therefore, the TPMC of BDP20 was lower than that of the other two fuels, especially under a load of 0.8 MPa, where the TPMC was the lowest. 3.3.4. Relationship between total particulate number concentration and total particulate mass concentration and NOx The relationships between the TPMC and TPNC of the three test fuels and NOx emissions under different EGR rates and loads are shown in Fig. 8. Fig. 8 shows that in the direction of increasing EGR, the NOx emissions of the three fuels continuously decreased; the TPNC and the TPMC of the fuel increased, indicating that the TPNC and TPMC have a trade-off relationship with NOx emissions. However, as the figure indicates, the addition of n-pentanol to the blended fuel could effectively reduce the TPNC and TPMC (see Figs. 6a and 7a), and the NOx emissions of BDP20 were similar to those of the other two test fuels (see Fig. 4a). Thus, the addition of n-pentanol could improve the trade-off relationship between the TPNC, TPMC, and NOx. 8
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(a) TPMC
(b) Nucleation mode mass concentration
(c) Accumulation mode mass concentration
Fig. 7. TPMC, nucleation, and accumulation mode mass concentration under different loads and EGR rates.
3.3.5. Particle number concentration in different particle size range Fig. 9 shows the particle number concentration, proportion of small particles (< 25 nm), and total particle number in various modes (particles < 25 nm, between 25 and 50 nm, between 50 and 100 nm, and between 100 and 1000 nm) under different EGR rates and loads. Under medium-load conditions (0.8 MPa), with the EGR rate increased, the concentration of nucleated particles of the three fuels initially decreased and then increased, and the proportion of small particles kept decreasing, whereas the concentration of accumulated particles increased. The main reason was the effects of EGR; the ID of the fuels was prolonged, the premix ratio increased and promoted the combustion, and the HRR was increased (see Fig. 2). This promoted the oxidation of nucleated particles; however, with the EGR rate increased, the content of exhaust gas entering the cylinder was also increased, which inhibited the combustion of fuel. This resulted in the increase in the particle number, also with an increase in the collision and adsorption between particles. This, in turn, resulted in several accumulated particles, therefore, a decrease in the proportion of small particles. Under highload conditions (1.0 MPa), the figure demonstrates that the
concentrations of nucleated particle numbers and accumulated particle numbers increased with an increase in EGR, and the proportion of small particles decreased with the increased EGR; however, the proportion was lower than that with the medium load. The main reason was that owing to the increase in the load, the fuel-injection quantity in the cylinder increased, which released higher heat (see Fig. 2) and promoted the oxidation of nucleated particles. However, owing to the influence of EGR, the oxygen concentration in the cylinder decreased, the fuel combustion was insufficient, and the fuel rich area in the cylinder increased; thus, the particle number increased (especially the particle size range between 50 and 100 nm and 100–1000 nm). The proportion of small particles decreased because the nucleated particle size was in the range of 25–50 nm. As shown in Fig. 9, when n-pentanol was added to the mixed fuel, the TPNC was lower than that of other test fuels, but the proportion of small particles was higher than other test fuels. The main reason was the oxygen-content of n-pentanol can promote the oxidation of accumulated particles, developing in the direction of smaller particle sizes. Therefore, the concentration of the accumulated particles was lower 9
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Fig. 8. Relation between NOx and TPNC and TPMC under different EGR rates and loads.
than in those in the other two fuels (see Fig. 6c). The particle size of nucleated particles was concentrated in the range of 25–50 nm, and the particle number concentration of the small particles experienced slight variation. However, as the TPNC was lower than that in the other two fuels (see Fig. 6a), the proportion of small particles was higher than proportion in the other two fuels.
combustion deterioration was more serious than that at a medium load, and more particles were generated. In addition, the fuel HRR increased, increasing the kinetic energy of the particles, and causing the collisions between particles to become more frequent. Thus, the particle size was larger than at a medium load (see Fig. 5); accordingly, the GMD at a high load was greater than that at a medium load. As shown in Fig. 10a, when n-pentanol was added to the mixed fuel, the GMD of the fuel was smaller than that of the other two test fuels. The main reason was the high oxygen-content, high volatility and low viscosity of n-pentanol could mix evenly with air, which promoted combustion, thereby decreasing the number of particles emitted (see Fig. 6). Thus, the probability of collision between particles was lower than that in D100 and BD20, and the GMD of BDP20 was smaller. Fig. 10b and c shows the relationship between the GMD of particles and the TPNC under different EGR rates and loads. The figure shows that the GMD of particles increases with an increase in the TPNC, which indicates that the TPNC and the GMD are affected by each other. Fig. 10b and c demonstrate that a larger number of particles (especially the particle size) corresponds to a larger GMD. This was because the larger the total number of particles were, the greater the probability of particle adsorption and collision with each other was. A larger number of accumulated particles corresponded to a larger particle size; thus, the GMD would increase.
3.3.6. Relationship between geometric mean diameter of particles and total particulate number concentration Fig. 10a shows the GMD of particles for the three fuels at different loads and EGR rates. It was known from the figure that the GMD of particles increases with an increase in the EGR rate. The GMD of particles under a 1.0 MPa load was larger than that under a 0.8 MPa load. This was because in the process of increasing the EGR, too much exhaust gas entered the cylinder. Therefore, the excess air coefficient decreased (see Fig. 3d), the oxygen concentration decreased, and the combustion in the cylinder deteriorated, resulting in the formation of several particles. The adsorption between particles increased, the number of accumulated particles increased (see Fig. 6c), and the particle size increased; thus, the GMD of the particles increased. When the engine was at the 1.0 MPa load, the fuel-injection quantity was greater more than that at the 0.8 MPa load, and the fuel enrichment area in the cylinder increased. Meanwhile, owing to the effects of EGR, the 10
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(a) 0.8MPa
(b) 1.0MPa Fig. 9. Particle number concentration and proportion of small particles in different particle size ranges under different EGR rates and loads.
4. Conclusion
EGR rates and loads; by adding n-pentanol into the mixed fuel, the TPNC and the TPMC decreased, and the trade-off relationship between the TPNC, TPMC, and NOx was alleviated. 4. With the increase in EGR rates and loads, the number concentrations of particles in various modes (< 25 nm, 25–50 nm, 50–100 nm, 100–1000 nm) increased, but the proportion of small particles (< 25 nm) decreased. The addition of n-pentanol could reduce the concentration of large size particles and improve the particle emissions of each mode. 5. Adding n-pentanol to the fuel mixture could reduce the GMD of particles and improve the relationship between the TPNC and GMD of the particles. 6. In this work, the addition of n-pentanol could reduce the emissions of carbon particles and improved the combustion quality of the engine; the conclusions and phenomena of this experiment can be proved for analyzing the effect of adding n-pentanol on PM through chemical reaction, and also provide support for analyzing the change of nanostructure of carbonaceous particles.
The effects of n-pentanol additive on particulate emission characteristics and combustion performance, and especially on particulate emission laws, were studied under different EGR rates and two loads (0.8 MPa and 1.0 MPa). The main conclusions are as follows. 1. With the EGR rate increased the engine HRR increased, but the IP and BTE decreased. At the same EGR rate, increasing the engine load improved the HRR, IP and BTE, but the BSFC decreased. The addition of n-pentanol could increase the BSFC and ID, but it could further improve the combustion efficiency and decreased combustion duration of the diesel engine, and the IP and the BTE values were similar to those of the other two fuels. 2. Using EGR technology, the NOx emissions could be effectively reduced, and there was no significant difference in NOx emissions between the two loads and the fuel. The addition of n-pentanol could effectively reduce the emissions of soot and the PSD concentration and had slight effect on the emissions of NOx. 3. The TPNC and TPMC of the engine increased with an increase in the 11
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(a) GMD
(b)
(c)
Fig. 10. GMD of particles under different EGR rates and loads and relationship with the TPNC.
CRediT authorship contribution statement
References
Haozhong Huang: Conceptualization, Methodology, Supervision, Resources. Xiaoyu Guo: Writing - original draft, Formal analysis, Investigation. Rong Huang: Data curation. Han Lei: Investigation. Yajuan Chen: Investigation. Te Wang: Writing - review & editing. Sai Wang: Validation. Mingzhang Pan: Project administration.
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