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HC and CO emissions reduction by early injection strategy in a bioethanol blended diesel-fueled engine with a narrow angle injection system
Applied Energy 107 (2013) 81–88
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
HC and CO emissions reduction by early injection strategy in a bioethanol blended diesel-fueled engine with a narrow angle injection system Su Han Park a, Seung Hyun Yoon b, Chang Sik Lee c,⇑ a b c
Time Resolved Research Group, X-ray Science Division, Advanced Photon Sources, Argonne National Laboratory, 9700 S Cass Ave., Lemont, IL 60439, USA Division of Automotive Engineering, Yeungnam College of Science & Technology, 170 Hyeonchung-ro, Nam-gu, Daegu 705-703, Republic of Korea School of Mechanical Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea
h i g h l i g h t s " Combustion and emissions features were investigated in diesel engine with narrow-angle injector. " The premixed combustion duration in narrow angle injector increased with bioethanol. " The investigations of variation in the peak combustion pressure in the narrow angle injector is low. " At early injection, narrow angle injector reduced CO/HC emissions in bioethanol fueled engine.
a r t i c l e
i n f o
Article history: Received 4 December 2012 Received in revised form 31 January 2013 Accepted 2 February 2013 Available online 8 March 2013 Keywords: Narrow angle injector Diesel–bioethanol blended fuels Hydrocarbon Carbon monoxide Early injection combustion strategy
a b s t r a c t The main purpose of this study was to investigate how a narrow angle injector affects the combustion and exhaust emissions characteristics in a single-cylinder diesel engine fueled by diesel–bioethanol blends. This study focused on reducing HC and CO emissions in the exhaust emissions by the bioethanol blending of diesel. A narrow angle injector with an injection angle of 70° was used and compared with a conventional angle injector having a 156° injection angle. The bioethanol was blended with the conventional diesel up to 30% with 5% biodiesel. Experiments revealed that, in a narrow angle injector, the premixed combustion duration increased with bioethanol contents unlike the similar value of conventional injector. The premixed combustion phasing decreased with the increase of bioethanol in both injectors. The variation in the peak combustion pressure of the narrow angle injector was smaller than that of a conventional injector. In addition, the narrow angle injector induced a higher indicated mean effective pressure (IMEP) and a shorter ignition delay compared to the conventional injector. In terms of exhaust emissions characteristics, the low and stable ISHC and ISCO emissions can be achieved through the application of narrow angle injector to the diesel–bioethanol blends combustion. By the early injection combustion strategy, ISHC and ISCO emissions are significantly reduced. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction For the last few decades, energy consumption has increased geometrically, and environmental destruction has become a serious social issue. The fossil fuels predominantly used in internal combustion engines are the main culprits of climate change and environmental pollution, which has motivated vehicle manufacturers and research groups to work toward the design of highly efficient combustion engines and ultra-low emission vehicles. In addition, as restrictions on fuel consumption and carbon dioxide
⇑ Corresponding author. Tel.: +82 2 2220 0427; fax: +82 2 2281 5286. E-mail address: [email protected] (C.S. Lee). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.02.015
(CO2) gradually become more stringent, diesel engines with high thermal efficiency are becoming more desirable. However, the large quantities of nitrogen oxides (NOx) and particulate matter (PM) emitted from diesel engines are serious obstacles to the widespread use of diesel engines in the automobile market. Recently, these problems have been improved by combustion optimization strategies, after-treatment devices, and the use of biofuels. Combustion optimization strategies including premixed combustion methods such as homogeneous charge compression ignition (HCCI), premixed charge compression ignition (PCCI) and partial premixed compression ignition (PPCI) have been investigated in order to simultaneously reduce NOx and PM (soot) emissions through the formation of a more uniform mixture in the combustion chamber [1–3]. As after-treatment methods, the diesel
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Nomenclature ATDC BTDC CA10 CA50 COV CO D100 DE10 DE20 DE30
after top dead center before top dead center crank angle at a 10% of the cumulative heat release crank angle at a 50% of the cumulative heat release coefficient of variation carbon monoxide pure diesel fuel blended fuel of 10% bioethanol + 5% biodiesel + 85% diesel blended fuel of 20% bioethanol + 5% biodiesel + 75% diesel blended fuel of 30% bioethanol + 5% biodiesel + 65% diesel
particulate filter (DPF), selective catalytic reduction (SCR), and lean-NOx trap (LNT) have been widely studied and applied to diesel engines for an effective reduction of PM and NOx emissions [4–6]. Diesel oxidation catalysts (DOCs) have generally been used for the reduction of hydrocarbon (HC) and carbon monoxide (CO) emissions. In addition, combined after-treatment devices (e.g., LNT + SCR, SCR + DPF) were also investigated [7–9] for cost reasons in order to significantly reduce emissions. The use of alternative fuels derived from biomass in diesel engines is more cost-effective method to satisfy stronger emission regulations than the use of after-treatment devices. Biodiesel and dimethyl ether (DME) are representative alternative fuels that can be used in a compression ignition diesel engine because they have higher cetane numbers and oxygen content compared to conventional diesel fuel. According to previous research, biodiesel fueled engines showed low NOx, soot, HC, and CO emissions due to low heating values and oxygen levels [10–13]. However, the atomization performance of biodiesel is poor because it has high viscosity. Therefore, biodiesel blended with other low viscosity fuels (diesel or ethanol) are mainly used in diesel engines [14,15]. On the other hand, DME showed a soot-free combustion characteristic because it has no direct connection between carbon and carbon in its molecular structure [16]. In addition, DME can reduce wall wetting and allows a premixed charge compression ignition due to its high vapor pressure [17]. However, as DME has very low viscosity and exists in the gas phase under atmospheric conditions, the fuel injection system should be modified. In this study, a bioethanol with oxygen was used in a diesel engine to improve its combustion performance and exhaust emission characteristics. Investigations on bioethanol blended diesel fuels in a compression ignition diesel engine have been conducted by many research groups [18–22]. Recently, Pidol et al. [23] reported that the application of ethanol–biodiesel–diesel blended fuels resulted in a combined reduction in smoke levels and NOx emissions in addition to extending the low temperature combustion operating range and improving maximum power output. Hulwan and Joshi [24] reported that ethanol blended diesel-fueled engines showed improved thermal efficiency and reduced smoke opacity, while CO emissions drastically increased. Many investigations indicated that bioethanol blended diesel fueled engines have a longer ignition delay, lower NOx and soot emissions, and higher HC emissions, and that CO emissions depend on the engine operating conditions. In a previous study [25], we reported that bioethanol blending effects clearly appeared under early injection conditions in the premixed combustion region. In that study, NOx emissions from the combustion of bioethanol blended diesel fuels decreased under early injection conditions due to the cooling effect of bioethanol. Meanwhile, HC emissions significantly increased
HC IMEP ISmfuel NOx Pinj Pmax SOE TDC
hydrocarbon indicated mean effective pressure (MPa) indicated specificinjection quantity (mg) nitrogen oxides injection pressure (MPa) peak combustion pressure in combustion chamber (MPa) start of energizing (degree of crank angle) top dead center
with early injection condition because the premixing period prior to ignition increased, and the injected spray continuously developed toward the cylinder wall. Thus, most of the fuel adsorbed to the cylinder wall was exhausted as HC emissions rather than being ignited during the combustion process. These findings led us to hypothesize that the injection angle should be limited in order to avoid wall wetting of the injected fuel, which is why we used an injector with a narrow injection angle in the present study. The purpose of this study was to investigate the effect of a narrow angle injector on exhaust emissions. The HC and CO emissions characteristics were investigated in the premixed combustion region in a diesel engine operated with bioethanol–diesel blends. For the realization of the premixed combustion region, the injection timings were changed from BTDC 20° to BTDC 50° and their influence, in addition to the effect of using a narrow angle injector (70° of injection angle), on combustion of the bioethanol blended diesel fuel was assessed in order to identify optimal combustion conditions in order to reduce emissions. 2. Experimental setup and procedure 2.1. Test engine system and operating procedure Fig. 1 shows a schematic of the test engine with the emission analyzer and engine operating systems. The single cylinder diesel engine used in this work was has a displacement volume of approximately 373 cc. The bore and stroke of the test single
Fig. 1. Schematic of the single-cylinder diesel engine system.
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S.H. Park et al. / Applied Energy 107 (2013) 81–88 Table 1 Specifications of the single-cylinder diesel engine.
Table 2 Properties of each pure fuel.
Item
Specification
Engine type Number of cylinder Bore stroke Displacement volume Fuel injection system Valve type Compression ratio
Direct injection diesel engine 1 75.0 mm 84.5 mm 373.3 cc Bosch common rail DOHC 4 valves 17.8
Injector
6 0.128 mm 156°
Number of hole Hole diameter Spray angle
cylinder engine are 75 mm and 84.5 mm, respectively, and its compression ratio is 17.8. The detailed engine specifications are listed in Table 1. The exhaust emissions from a single cylinder diesel engine were measured and analyzed using an HC, CO, and NOx analyzer (MEXA-554JKNOx, Horiba), and a soot analyzer (415-S, AVL). The CO and HC emissions were measured with non-dispersive infrared rays, and the NOx emissions were measured using a chemical method. The soot analyzer is a filter-type smoke meter used for measuring soot content. The combustion pressure in the cylinder was measured with a piezo-electric pressure transducer (6052A80, Kistler) coupled to a charge amplifier (5011B, Kistler). The combustion pressure data were acquired from a DAQ board (PCI-MIO-16E-1, National Instruments) at a sampling interval of 0.1° crank angle (CA) to ensure accurate ignition timing and phasing of heat release over 300 cycles. The obtained in-cylinder combustion pressure data for the crank angle were averaged in order to remove the effect of cycleto-cycle variations using the detailed positions of the crank and cam. An optical cam position sensor (BF4RE, Autonics) and a rotary encoder (HYRE-A-1800, Hanyoung) with a resolution of 1800 pulses per 360° were installed at the cam and crank shafts. Each signal was synchronized to the TDC signal. An injector driver (TDA-3300H, TEMS) synchronized with a crank angle sensor was used to control the injection timing and injection mass. The test engine was operated at an engine speed of 1200 rpm and an injection pressure of 120 MPa. The injected fuel mass was fixed at 10 mg in order to represent the middle engine load condition. 2.2. Test fuels Three blended fuels and conventional diesel fuel were used in this study. Bioethanol for blends has 99.9% purity and the volumetric blending ratios of bioethanol are 10%, 20%, and 30%. All blended fuels have 5% biodiesel fuel in order to prevent phase separation between the diesel and bioethanol [26–28]. The biodiesel fuel used in this study was derived from soybean oil. Table 2 lists the detailed properties of the diesel, bioethanol, and biodiesel fuels used in this study. 2.3. Test injectors and spray targeting for injection timings In this study, two solenoid-type injectors with different injection angles were used to investigate the effect of a narrow angle injector on emissions. The injection angles of the test injectors are 156° (conventional) and 70° (narrow), and the nozzle hole diameter of both injectors is 0.126 mm. The test engine has a reentrant type piston bowl as shown in Fig. 2a. Fig. 2b shows the spray targeting position of two injectors according to the injection timing. As shown in Fig. 2b, at advanced injection timings, the injected spray of the conventional injector target reaches the cylinder wall, while the injected spray from the narrow angle injector
Formula Molecular weight Density (kg/m3 @ 15 °C) Kinematic viscosity (mm2/s @ 15 °C) Surface tension (mN/m @ 15 °C) Flash point (°C) Auto-ignition temperature (°C) Lower heating value (MJ/kg) Cetane number Vapor pressure (kPa @ 38 °C) Stoichiometric air/fuel ratio Latent heat of vaporization (kJ/kg) Oxygen contents (%) Distillation (°C) IBP 10 mass% 50 mass% 90 mass% FBP
Diesel
Biodiesel
Bioethanol
C12H26– C14H30 170–198 824.8 4.54
C19H34O2
C2H5OH
294 883 7.16
46.07 795 1.65
26.9 52 300–340 43 >50 0.34 14.7 370 –
31.1 100–170 – 36–38 48–65 – 13.8 330 11
21.7 13 425 27 <15 17 8.96 921.1 34.8
77.025 138.825 265.325 350.225 516.85
313.7 332.725 346.925 351.075 514.5
– – – – –
is distributed into the piston bowl. On the other side, the spray from the narrow angle injector targets the piston surface at retarded injection timings (around TDC). It cannot achieve a uniform mixture of fuel and air. Therefore, the effect for the above phenomena was experimentally measured and analyzed for advanced injection timings from BTDC 20° to BTDC 50°. 3. Results and discussion Fig. 3 compares the combustion characteristics of diesel– bioethanol blends and pure diesel using conventional and narrow angle diesel injectors. The engine speed, injection pressure, and injection quantity were fixed at 1200 rpm, 120 MPa, and 10 mg, respectively. Combustion characteristics were represented by combustion pressure curves, premixed combustion duration (CA10– CA50), and premixed combustion phasing (CA50). In this study, CA10 and CA50 refer to the crank angle for the 10% and 50% level of total accumulated heat release, respectively. In a comparison of conventional and narrow angle injectors applied to combustion, as shown in Fig. 3a and b, the ignition in the narrow angle injector was found to occur slightly earlier than in the conventional injector (refer to Fig. 6). The ascending gradient of combustion pressure in the narrow angle injector became steeper than that in the conventional injector. Except in the case of DE30, the timing approached the specific combustion pressure in the narrow angle injector was faster than that in the conventional injector. These results can be explained by the difference in the ignition delay and the formed mixture. The figures to the right of the combustion pressure curves in Fig. 3 illustrated the premixed combustion phasing (CA50) and the premixed combustion duration (CA10–CA50) for discussing the combustion characteristics according to the bioethanol blends in the conventional and narrow angle injectors. As shown in Fig. 3, the CA50 in both injectors decreased with the increase of bioethanol content because the decrease of cetane number retarded the ignition toward top dead center (TDC). At the same injection timing, the CA50 of the narrow angle injector was advanced about 19.8% for the conventional injector. For the narrow angle injector at BTDC 25° injection timing, an increase in bioethanol to 30% retarded the CA50 by about 7.5° of the crank angle (57% for the CA50 of D100), while at BTDC 45°, it retarded the CA50 by about 10.6° of the crank angle (60.6% for the CA50 of D100). On the other hand, the premixed combustion
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Fig. 2. Schematic and picture of the re-entrant type piston used in this study (a) and injection direction and spray targeting for the various injection timings (b).
in a conventional injector increased with the decrease of bioethanol. This result can be explained by the relatively low homogeneity of the air/fuel mixture, which was caused by wall-wetting of the injected spray and the increase in bioethanol with low cetane number and low heating value under low ambient density condition (SOE; BTDC 25°). In general, the wall-wetting makes locally fuel-rich region near the cylinder wall and piston surface. It affects the formation of inhomogeneity mixture. Whereas, in the case of the narrow angle injector, the amount of spray impinged onto the cylinder wall and piston surface was less than that with the conventional injector because of the small injection angle. Hence, although the cetane number and heating value decreased, the formation of a relatively uniform air/fuel mixture is possible during the ignition delay. For these reasons, the premixed combustion duration was similar regardless of bioethanol content in diesel– bioethanol blends. On the other hand, Fig. 3c showed that the advance of injection timing using the narrow angle injector increased the premixed combustion duration, and retarded the premixed combustion phasing. The combustion characteristics are presented quantitatively in Figs. 4–6 by the coefficient of variation of the maximum combustion pressure ðCOVPmax Þ, the indicated mean effective pressure (IMEP), and ignition delay. Fig. 4 compares COVPmax for various injection timings using conventional and narrow angle injector cases. The COVPmax is related to the combustion stability of the index showing the variation in peak combustion pressure. As shown in Fig. 4, the COVPmax of the narrow angle injector was generally lower than that of the conventional injector, and this characteristic was more apparent as the injection timings advanced, although the increase in bioethanol induced a slight increase in the ratio of COVPmax of the narrow angle injector compared to that of the conventional injector. These results can be explained by Fig. 3, which shows the reduction in wall-wetting of the injected spray and more uniform air/fuel mixture formation during ignition delay, which caused a decrease in the in-homogeneity of the equivalence ratio. This result means that more stable combustion characteristics in diesel–bioethanol blends fueled engine can be achieved using early injection strategy and narrow angle injector. Fig. 5 compares the IMEP of conventional and narrow angle injectors by injection timing and start of combustion (SOC) timing. A higher IMEP was obtained by using a narrow angle injector at 5–22%, compared to the conventional injector. In the injection
timings of BTDC 20°–BTDC 50°, the IMEPs of the narrow angle injector of each test fuel (D100, DE10, DE20, and DE30) averaged 0.34 MPa, 0.37 MPa, 0.39 MPa, and 0.41 MPa, respectively, while those of the conventional injector averaged 0.29 MPa, 0.30 MPa, 0.36 MPa and 0.38 MPa, respectively. In addition, for the same test fuel, the variation in IMEP by injection timing in the conventional injector was about 0.094 MPa, which was approximately 230% higher than with the narrow angle injector (IMEP variation in the narrow angle injector 0.041 MPa). These results indicate that the reduction in IMEP due to the advancing the injection timing and the application of bioethanol blended diesel fuels did not occur when the narrow angle injector was applied with an early injection combustion strategy. On the other hand, Fig. 5b shows the IMEP characteristics for SOC timings. As shown in the figure, the IMEP for the narrow angle injector generally followed a uniform trend line with a first order equation (solid line in the figure), regardless of fuel type. However, each test fuel followed a separate trend curve with a second order equation (dotted line in the figure) for the conventional injector. The trend curve moved toward the upper-right according to the increase of bioethanol blends and the retardation of SOC. This means that a higher IMEP appeared when the SOC was moved to TDC. The retarded SOC–TDC induced a decrease in negative work due to the combustion reaction before TDC and the slight increase in the combustion pressure of each expansion stroke, consequently increasing the IMEP. We can also confirm that there was relatively small variation in IMEP when the narrow angle injector was applied to the combustion of diesel–bioethanol blends. A comparison of the ignition delay in the narrow angle injector and the conventional injector is shown in Fig. 6, which indicates that the ignition delay in the narrow angle injector was slightly shorter than that in the conventional injector. Under early injection conditions, the spray from the conventional injector with a 156° injection angle was generally injected toward the cylinder wall, not toward the piston head bowl. In addition, the injected spray rapidly developed and propagated in the cylinder because the temperature and pressure in the combustion cylinder were very low, thus the density of ambient gas was low. Consequently, the spray impinged to the cylinder wall before vaporization and ignition. The impinged fuel adsorbed to the cylinder wall, and the droplets formed a locally fuel-rich region in the combustion cylinder. Hence, the formation of a uniform air/fuel mixture is relatively
S.H. Park et al. / Applied Energy 107 (2013) 81–88
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Fig. 3. Combustion pressure curves of bioethanol blended diesel fuels in conventional and narrow angle injectors (Pinj = 120 MPa, mfuel = 10 mg, Engine speed: 1200 rpm).
difficult. However, the narrow angle injector with 70° decreased the adsorption of the injected spray on the cylinder wall. Therefore, most injected spray fuel forms a more uniform air/fuel mixture. Under the same combustion conditions (temperature and pressure), the narrow injection angle induced faster ignition than the conventional injection angle. Fig. 7 shows the ISHC emission characteristics for injection timings and SOC timings in conventional and narrow angle injectors. From previous research, we know the main sources of HC emission is injection timing, mixing time between fuel and air,
and premixed combustion phasing (CA50) [29]. Specifically, in the early injection cases, it is well known that HC emission results from the liquid fuel film formed by spray impingement and overmixing [30,31], wall-wetting, insufficient oxygen, and the residual fuel in the sac volume [32]. As shown in Fig. 7, the less ISHC emissions generally exhausted at narrow angle injector than conventional angle injector. In addition, the variation width for bioethanol blending in the narrow angle injector was about 38% smaller than that in the conventional injector (narrow angle injector: 0.25 g/kW h, conventional injector: 0.65 g/kW h).
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50 P inj =120MPa, m fuel =10mg
Pinj=120MPa, mfuel=10mg, Engine speed 1200rpm D100
DE10
DE20
Conv. Narr.
DE30
Ignition delay (degree)
Pmax,narrow
COVPmax (Narrow angle) COV
/ COVPmaxCOV (Conventional angle) Pmax,conv
3
2
1
0
-40
-30
-20
-10
D100 DE10 DE20 DE30
40
30
20
10
0
0 -55
Start of energizing (deg. ATDC)
-50
-45
-40
-35
-30
-25
-20
-15
Injection timing (deg. ATDC)
Fig. 4. Comparison of the COV (coefficient of variation) of peak combustion pressure in narrow angle and conventional injectors.
Fig. 6. Ignition delay characteristics of bioethanol blended diesel fuels in conventional and narrow angle injectors (Pinj = 120 MPa, mfuel = 10 mg, Engine speed: 1200 rpm).
0.6 Pinj=120MPa, mfuel=10mg Conv. Narr. D100 DE10 DE20 DE30
1.2 Pinj=120MPa, mfuel=10mg
0.3
0.2 -55
Conv. Narr. D100 DE10 DE20 DE30
0.9
0.4
ISHC (g/kWh)
IMEP (MPa)
0.5
-50
-45
-40
-35
-30
-25
-20
0.6
0.3
-15
Injection timing (deg. ATDC) (a) IMEP for injection timings 0.0 -55
-50
0.6 Pinj=120MPa, mfuel=10mg
-35
-30
-25
1.2
Conv. Narr. D100 DE10 DE20 DE30
0.9
0.3
-20
-15
-10
-5
0
-15
Pinj=120MPa, mfuel=10mg
0.4
0.2 -25
-20
(a) ISHC for injection timings
D100 DE10 DE20 DE30
ISHC (g/kWh)
IMEP (MPa)
-40
Injection timing (deg. ATDC)
Conv. Narr.
0.5
-45
5
0.6 Narrow angle injector
Conventional injector
0.3
Start of combustion (deg. ATDC) (b) IMEP for start of combustion Fig. 5. IMEP characteristics in conventional and narrow angle injectors (Pinj = 120 MPa, mfuel = 10 mg, Engine speed: 1200 rpm).
0.0 -25
-20
-15
-10
-5
0
5
Start of combustion (deg. ATDC) (b) ISHC for start of combustion
For detailed analysis of HC emissions under early injection conditions, two early injection regions were chosen, regions A and B as indicated in the figure (region A: BTDC 35°, region B: BTDC 25°). In region B, the HC emissions with the narrow angle and conventional injectors were either similar or less with the narrow angle injector. However, in region A, the HC emissions with the narrow angle injector were about 20% (standard of raw emission data) lower than that with the conventional angle injector Table 3. In addition, for the same test fuel, the HC emissions of the conventional angle
Fig. 7. Comparison of ISHC characteristics by injection timings and SOC (start of combustion) in conventional and narrow angle injectors (Pinj = 120 MPa, mfuel = 10 mg, Engine speed: 1200 rpm).
injector tended to increase as the injection timing was advanced, while the narrow angle injector showed generally uniform or small variations in HC emissions. These results can be attributed to the reduction in the wall-film on the cylinder wall and on the piston
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S.H. Park et al. / Applied Energy 107 (2013) 81–88 Table 3 Raw data for HC emission with conventional and narrow angle injectors. Unit:ppm
Conventional injector
Narrow angle injector
Relative valueA
(a) BTDC 35° (region A) D100 DE10 DE20 DE30 Mean
133 123 120 151 –
104 101 101 116 –
0.782 0.821 0.842 0.768 0.803
(b) BTDC 25° (region B) D100 DE10 DE20 DE30 Mean
56 70 59 111 –
72 71 84 86 –
1.28 1.01 1.42 0.77 1.12
#
RelativeA means the ratio of HC emissions with a narrow angle injector compared to a conventional injector, and was calculated by the following equation: A Relative ¼ ðHC emissions in narrow angle injectorÞ=ðHC emissions in conventional injectorÞ.
80 Pinj=120MPa, mfuel=10mg Conv. Narr. D100 DE10 DE20 DE30
ISCO (g/kWh)
60
40
Conventional injector 51.7g/kWh
Narrow angle injector 12.0g/kWh
20
0 -50
-45
-40
-35
-30
-25
-20
Injection timing (deg. ATDC) (a) ISCO for injection timings 100 Pinj=120MPa, mfuel=10mg Conv. Narr. D100 DE10 DE20 DE30
ISCO (g/kWh)
80
60
40 Conventional injector
20
0 -25
Narrow angle injector
-20
-15
-10
-5
0
5
Start of combustion (deg. ATDC) (b) ISCO for start of combustion Fig. 8. Comparison of ISCO characteristics by injection timings and SOC (start of combustion) in conventional and narrow angle injectors (Pinj = 120 MPa, mfuel = 10 mg, Engine speed: 1200 rpm).
head after spray impingement, which is one of the main sources of HC emissions, when using a narrow angle injector. The HC emission reduction is related to the provision of sufficient time for mixing of air and fuel by advancing the injection timing. Fig. 7b shows the ISHC emissions according to the start of combustion timing. ISHC emissions with the narrow angle injector generally were lower than those with a conventional injector over a
wide range of combustion start times. In addition, the variation width for SOC and bioethanol blending in the narrow angle injector was smaller than that in the conventional injector. Fig. 8 shows the ISCO emissions characteristics for injection timings and SOC timings. Use of the narrow angle injector with early injection combustion resulted in minimal variation in ISCO emissions compared to when a conventional injector was used (narrow angle injector: 12.0 g/kW h, conventional injector: 51.7 g/kW h). CO emissions are mainly generated from the partial oxidation of locally rich mixture and incomplete combustion [33]. CO emissions are an intermediate product of the oxidation of hydrocarbons in the combustion chamber [34]. As shown in Fig. 8a, the ISCO emission from the narrow angle injector at regions A and B were about 60% and 20% that of the emissions from the conventional injector combustion in the same regions, respectively. Reducing CO emissions is an obstacle to creating a clean diesel combustion vehicle when using bioethanol and early injection timing. Bioethanol blended diesel-fueled combustion engines exhaust more CO than does a pure diesel engine [24,35]. However, in this investigation, we confirmed that CO emissions can be significantly reduced by applying a narrow angle injector and early injection combustion strategy. CO emissions and equivalence ratio (locally or globally) are closely related [29,36], in that an increase in the equivalence ratio induces an increase in CO emissions. The low CO emissions in Fig. 8 can be explained by the reduction in the locally high equivalence ratio region because the early and narrow angle injection resulted in sufficient mixing time, enabling the formation of uniform mixture of air and fuel. In addition, the relatively short ignition delay, short premixed burn duration, and advanced combustion phasing resulted in low CO emissions (refer to Fig. 3) [29]. Fig. 8b shows the ISCO emissions characteristics of both injectors with conventional and narrow angles at various SOC timings. The superior CO emission characteristics of the narrow angle injector during conventional injector combustion are apparent. At test conditions of this study, the ISCO emissions of pure diesel and bioethanol blended diesel fuels was below 18 g/ kW h with narrow angle injector combustion.
4. Conclusions This study describes how a narrow angle injector affects the combustion and exhaust emission characteristics under an early injection combustion strategy. Experiments were conducted using a single-cylinder diesel engine fueled with diesel–bioethanol blended fuels. Based on the results, we can conclude the following:
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1. The premixed combustion phasing decreased with the increase of bioethanol in both conventional and narrow angle injectors. At the same injection timing, the narrow angle injector showed the increase of the premixed combustion duration with the increase of bioethanol, while the conventional injector showed almost uniform values regardless of bioethanol contents. The advance of the injection timing in the narrow angle injector induced the slight increase of the premixed combustion duration and the advance of the premixed combustion phasing. 2. In the combustion using the narrow angle injector, the variation in the peak combustion pressure is smaller than the combustion using the conventional injector. In addition, it showed a higher IMEP and shorter ignition delay characteristics. 3. When the narrow angle injector applied to the combustion with diesel–bioethanol blends, the low and stable ISHC/ISCO emissions can be achieved. In addition, by the early injection combustion strategy, ISHC and ISCO emissions are significantly reduced. 4. According to above investigation results, more stable combustion and higher IMEP can be achieved through the use of the narrow angle injector in diesel–bioethanol blends fueled combustion. Especially, the combination of early injection strategy and narrow angle injector induced the reduction of ISHC and ISCO emissions, which is the weakness in diesel–bioethanol blends fueled combustion.
Acknowledgements This work was supported by the Second Brain Korea 21 Project and was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MEST) (No. 2012007015). References [1] Yao M, Zheng Z, Liu H. Progress and recent trends in homogeneous charge compression ignition (HCCI) engines. Prog Energy Combust 2009;35:398–437. [2] Kim YJ, Kim KB, Lee KH. Effect of a 2-stage strategy on the combustion and flame characteristic in a PCCI engine. Int J Auto Tech-Kor 2011;12:639–44. [3] Masuda Y, Chen Z. Combustion and emission characteristics of a PCI engine fueled with ethanol–diesel blends. SAE tech paper 2009, SAE 2009-01-1854. [4] Hsieh MF, Wang J. Sliding-mode observer for urea-selective catalytic reduction (SCR) mid-catalyst ammonia concentration estimation. Int J Auto Tech-Kor 2011;12:321–9. [5] Stratakis GA, Psarianos DL, Stamatelos AM. Experimental investigation of the pressure drop in porous ceramic diesel particulate filters. Proc Inst Mech Eng D-J Auto 2002;216:773–84. [6] Benramdhane S, Millet CN, Jeudy E, Lavy J, Blasin-Aube V, Daturi M. Diesel lean NOx-trap thermal aging and performance evolution characterization. Oil Gas Sci Technol 2011;66:845–53. [7] He Y, Brown DB, Lu S, Paratore MJ, Li J. Opportunities and challenges for blended 2-way SCR/DPF aftertreatment technologies. SAE tech paper 2009, SAE 2009-01-0274. [8] Theis JR, Dearth M, McCabe R. LNT + SCR catalyst systems optimized for NOx conversion on diesel applications. SAE tech paper 2011, SAE 2011-01-0305. [9] Parks J, Prikhodko V. Ammonia production and utilization in a hybrid LNT+SCR system. SAE tech paper 2009, SAE 2009-01-2739. [10] Sinha S, Agarwal AK. Performance evaluation of a biodiesel fuelled transport diesel engine. SAE tech paper 2005, SAE 2005-01-1730. [11] Canakci M, Erdil A, Arcaklioglu E. Performance and exhaust emission of a biodiesel engine. Appl Energy 2006;83:594–605.
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Contents lists available at SciVerse ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
HC and CO emissions reduction by early injection strategy in a bioethanol blended diesel-fueled engine with a narrow angle injection system Su Han Park a, Seung Hyun Yoon b, Chang Sik Lee c,⇑ a b c
Time Resolved Research Group, X-ray Science Division, Advanced Photon Sources, Argonne National Laboratory, 9700 S Cass Ave., Lemont, IL 60439, USA Division of Automotive Engineering, Yeungnam College of Science & Technology, 170 Hyeonchung-ro, Nam-gu, Daegu 705-703, Republic of Korea School of Mechanical Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea
h i g h l i g h t s " Combustion and emissions features were investigated in diesel engine with narrow-angle injector. " The premixed combustion duration in narrow angle injector increased with bioethanol. " The investigations of variation in the peak combustion pressure in the narrow angle injector is low. " At early injection, narrow angle injector reduced CO/HC emissions in bioethanol fueled engine.
a r t i c l e
i n f o
Article history: Received 4 December 2012 Received in revised form 31 January 2013 Accepted 2 February 2013 Available online 8 March 2013 Keywords: Narrow angle injector Diesel–bioethanol blended fuels Hydrocarbon Carbon monoxide Early injection combustion strategy
a b s t r a c t The main purpose of this study was to investigate how a narrow angle injector affects the combustion and exhaust emissions characteristics in a single-cylinder diesel engine fueled by diesel–bioethanol blends. This study focused on reducing HC and CO emissions in the exhaust emissions by the bioethanol blending of diesel. A narrow angle injector with an injection angle of 70° was used and compared with a conventional angle injector having a 156° injection angle. The bioethanol was blended with the conventional diesel up to 30% with 5% biodiesel. Experiments revealed that, in a narrow angle injector, the premixed combustion duration increased with bioethanol contents unlike the similar value of conventional injector. The premixed combustion phasing decreased with the increase of bioethanol in both injectors. The variation in the peak combustion pressure of the narrow angle injector was smaller than that of a conventional injector. In addition, the narrow angle injector induced a higher indicated mean effective pressure (IMEP) and a shorter ignition delay compared to the conventional injector. In terms of exhaust emissions characteristics, the low and stable ISHC and ISCO emissions can be achieved through the application of narrow angle injector to the diesel–bioethanol blends combustion. By the early injection combustion strategy, ISHC and ISCO emissions are significantly reduced. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction For the last few decades, energy consumption has increased geometrically, and environmental destruction has become a serious social issue. The fossil fuels predominantly used in internal combustion engines are the main culprits of climate change and environmental pollution, which has motivated vehicle manufacturers and research groups to work toward the design of highly efficient combustion engines and ultra-low emission vehicles. In addition, as restrictions on fuel consumption and carbon dioxide
⇑ Corresponding author. Tel.: +82 2 2220 0427; fax: +82 2 2281 5286. E-mail address: [email protected] (C.S. Lee). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.02.015
(CO2) gradually become more stringent, diesel engines with high thermal efficiency are becoming more desirable. However, the large quantities of nitrogen oxides (NOx) and particulate matter (PM) emitted from diesel engines are serious obstacles to the widespread use of diesel engines in the automobile market. Recently, these problems have been improved by combustion optimization strategies, after-treatment devices, and the use of biofuels. Combustion optimization strategies including premixed combustion methods such as homogeneous charge compression ignition (HCCI), premixed charge compression ignition (PCCI) and partial premixed compression ignition (PPCI) have been investigated in order to simultaneously reduce NOx and PM (soot) emissions through the formation of a more uniform mixture in the combustion chamber [1–3]. As after-treatment methods, the diesel
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Nomenclature ATDC BTDC CA10 CA50 COV CO D100 DE10 DE20 DE30
after top dead center before top dead center crank angle at a 10% of the cumulative heat release crank angle at a 50% of the cumulative heat release coefficient of variation carbon monoxide pure diesel fuel blended fuel of 10% bioethanol + 5% biodiesel + 85% diesel blended fuel of 20% bioethanol + 5% biodiesel + 75% diesel blended fuel of 30% bioethanol + 5% biodiesel + 65% diesel
particulate filter (DPF), selective catalytic reduction (SCR), and lean-NOx trap (LNT) have been widely studied and applied to diesel engines for an effective reduction of PM and NOx emissions [4–6]. Diesel oxidation catalysts (DOCs) have generally been used for the reduction of hydrocarbon (HC) and carbon monoxide (CO) emissions. In addition, combined after-treatment devices (e.g., LNT + SCR, SCR + DPF) were also investigated [7–9] for cost reasons in order to significantly reduce emissions. The use of alternative fuels derived from biomass in diesel engines is more cost-effective method to satisfy stronger emission regulations than the use of after-treatment devices. Biodiesel and dimethyl ether (DME) are representative alternative fuels that can be used in a compression ignition diesel engine because they have higher cetane numbers and oxygen content compared to conventional diesel fuel. According to previous research, biodiesel fueled engines showed low NOx, soot, HC, and CO emissions due to low heating values and oxygen levels [10–13]. However, the atomization performance of biodiesel is poor because it has high viscosity. Therefore, biodiesel blended with other low viscosity fuels (diesel or ethanol) are mainly used in diesel engines [14,15]. On the other hand, DME showed a soot-free combustion characteristic because it has no direct connection between carbon and carbon in its molecular structure [16]. In addition, DME can reduce wall wetting and allows a premixed charge compression ignition due to its high vapor pressure [17]. However, as DME has very low viscosity and exists in the gas phase under atmospheric conditions, the fuel injection system should be modified. In this study, a bioethanol with oxygen was used in a diesel engine to improve its combustion performance and exhaust emission characteristics. Investigations on bioethanol blended diesel fuels in a compression ignition diesel engine have been conducted by many research groups [18–22]. Recently, Pidol et al. [23] reported that the application of ethanol–biodiesel–diesel blended fuels resulted in a combined reduction in smoke levels and NOx emissions in addition to extending the low temperature combustion operating range and improving maximum power output. Hulwan and Joshi [24] reported that ethanol blended diesel-fueled engines showed improved thermal efficiency and reduced smoke opacity, while CO emissions drastically increased. Many investigations indicated that bioethanol blended diesel fueled engines have a longer ignition delay, lower NOx and soot emissions, and higher HC emissions, and that CO emissions depend on the engine operating conditions. In a previous study [25], we reported that bioethanol blending effects clearly appeared under early injection conditions in the premixed combustion region. In that study, NOx emissions from the combustion of bioethanol blended diesel fuels decreased under early injection conditions due to the cooling effect of bioethanol. Meanwhile, HC emissions significantly increased
HC IMEP ISmfuel NOx Pinj Pmax SOE TDC
hydrocarbon indicated mean effective pressure (MPa) indicated specificinjection quantity (mg) nitrogen oxides injection pressure (MPa) peak combustion pressure in combustion chamber (MPa) start of energizing (degree of crank angle) top dead center
with early injection condition because the premixing period prior to ignition increased, and the injected spray continuously developed toward the cylinder wall. Thus, most of the fuel adsorbed to the cylinder wall was exhausted as HC emissions rather than being ignited during the combustion process. These findings led us to hypothesize that the injection angle should be limited in order to avoid wall wetting of the injected fuel, which is why we used an injector with a narrow injection angle in the present study. The purpose of this study was to investigate the effect of a narrow angle injector on exhaust emissions. The HC and CO emissions characteristics were investigated in the premixed combustion region in a diesel engine operated with bioethanol–diesel blends. For the realization of the premixed combustion region, the injection timings were changed from BTDC 20° to BTDC 50° and their influence, in addition to the effect of using a narrow angle injector (70° of injection angle), on combustion of the bioethanol blended diesel fuel was assessed in order to identify optimal combustion conditions in order to reduce emissions. 2. Experimental setup and procedure 2.1. Test engine system and operating procedure Fig. 1 shows a schematic of the test engine with the emission analyzer and engine operating systems. The single cylinder diesel engine used in this work was has a displacement volume of approximately 373 cc. The bore and stroke of the test single
Fig. 1. Schematic of the single-cylinder diesel engine system.
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S.H. Park et al. / Applied Energy 107 (2013) 81–88 Table 1 Specifications of the single-cylinder diesel engine.
Table 2 Properties of each pure fuel.
Item
Specification
Engine type Number of cylinder Bore stroke Displacement volume Fuel injection system Valve type Compression ratio
Direct injection diesel engine 1 75.0 mm 84.5 mm 373.3 cc Bosch common rail DOHC 4 valves 17.8
Injector
6 0.128 mm 156°
Number of hole Hole diameter Spray angle
cylinder engine are 75 mm and 84.5 mm, respectively, and its compression ratio is 17.8. The detailed engine specifications are listed in Table 1. The exhaust emissions from a single cylinder diesel engine were measured and analyzed using an HC, CO, and NOx analyzer (MEXA-554JKNOx, Horiba), and a soot analyzer (415-S, AVL). The CO and HC emissions were measured with non-dispersive infrared rays, and the NOx emissions were measured using a chemical method. The soot analyzer is a filter-type smoke meter used for measuring soot content. The combustion pressure in the cylinder was measured with a piezo-electric pressure transducer (6052A80, Kistler) coupled to a charge amplifier (5011B, Kistler). The combustion pressure data were acquired from a DAQ board (PCI-MIO-16E-1, National Instruments) at a sampling interval of 0.1° crank angle (CA) to ensure accurate ignition timing and phasing of heat release over 300 cycles. The obtained in-cylinder combustion pressure data for the crank angle were averaged in order to remove the effect of cycleto-cycle variations using the detailed positions of the crank and cam. An optical cam position sensor (BF4RE, Autonics) and a rotary encoder (HYRE-A-1800, Hanyoung) with a resolution of 1800 pulses per 360° were installed at the cam and crank shafts. Each signal was synchronized to the TDC signal. An injector driver (TDA-3300H, TEMS) synchronized with a crank angle sensor was used to control the injection timing and injection mass. The test engine was operated at an engine speed of 1200 rpm and an injection pressure of 120 MPa. The injected fuel mass was fixed at 10 mg in order to represent the middle engine load condition. 2.2. Test fuels Three blended fuels and conventional diesel fuel were used in this study. Bioethanol for blends has 99.9% purity and the volumetric blending ratios of bioethanol are 10%, 20%, and 30%. All blended fuels have 5% biodiesel fuel in order to prevent phase separation between the diesel and bioethanol [26–28]. The biodiesel fuel used in this study was derived from soybean oil. Table 2 lists the detailed properties of the diesel, bioethanol, and biodiesel fuels used in this study. 2.3. Test injectors and spray targeting for injection timings In this study, two solenoid-type injectors with different injection angles were used to investigate the effect of a narrow angle injector on emissions. The injection angles of the test injectors are 156° (conventional) and 70° (narrow), and the nozzle hole diameter of both injectors is 0.126 mm. The test engine has a reentrant type piston bowl as shown in Fig. 2a. Fig. 2b shows the spray targeting position of two injectors according to the injection timing. As shown in Fig. 2b, at advanced injection timings, the injected spray of the conventional injector target reaches the cylinder wall, while the injected spray from the narrow angle injector
Formula Molecular weight Density (kg/m3 @ 15 °C) Kinematic viscosity (mm2/s @ 15 °C) Surface tension (mN/m @ 15 °C) Flash point (°C) Auto-ignition temperature (°C) Lower heating value (MJ/kg) Cetane number Vapor pressure (kPa @ 38 °C) Stoichiometric air/fuel ratio Latent heat of vaporization (kJ/kg) Oxygen contents (%) Distillation (°C) IBP 10 mass% 50 mass% 90 mass% FBP
Diesel
Biodiesel
Bioethanol
C12H26– C14H30 170–198 824.8 4.54
C19H34O2
C2H5OH
294 883 7.16
46.07 795 1.65
26.9 52 300–340 43 >50 0.34 14.7 370 –
31.1 100–170 – 36–38 48–65 – 13.8 330 11
21.7 13 425 27 <15 17 8.96 921.1 34.8
77.025 138.825 265.325 350.225 516.85
313.7 332.725 346.925 351.075 514.5
– – – – –
is distributed into the piston bowl. On the other side, the spray from the narrow angle injector targets the piston surface at retarded injection timings (around TDC). It cannot achieve a uniform mixture of fuel and air. Therefore, the effect for the above phenomena was experimentally measured and analyzed for advanced injection timings from BTDC 20° to BTDC 50°. 3. Results and discussion Fig. 3 compares the combustion characteristics of diesel– bioethanol blends and pure diesel using conventional and narrow angle diesel injectors. The engine speed, injection pressure, and injection quantity were fixed at 1200 rpm, 120 MPa, and 10 mg, respectively. Combustion characteristics were represented by combustion pressure curves, premixed combustion duration (CA10– CA50), and premixed combustion phasing (CA50). In this study, CA10 and CA50 refer to the crank angle for the 10% and 50% level of total accumulated heat release, respectively. In a comparison of conventional and narrow angle injectors applied to combustion, as shown in Fig. 3a and b, the ignition in the narrow angle injector was found to occur slightly earlier than in the conventional injector (refer to Fig. 6). The ascending gradient of combustion pressure in the narrow angle injector became steeper than that in the conventional injector. Except in the case of DE30, the timing approached the specific combustion pressure in the narrow angle injector was faster than that in the conventional injector. These results can be explained by the difference in the ignition delay and the formed mixture. The figures to the right of the combustion pressure curves in Fig. 3 illustrated the premixed combustion phasing (CA50) and the premixed combustion duration (CA10–CA50) for discussing the combustion characteristics according to the bioethanol blends in the conventional and narrow angle injectors. As shown in Fig. 3, the CA50 in both injectors decreased with the increase of bioethanol content because the decrease of cetane number retarded the ignition toward top dead center (TDC). At the same injection timing, the CA50 of the narrow angle injector was advanced about 19.8% for the conventional injector. For the narrow angle injector at BTDC 25° injection timing, an increase in bioethanol to 30% retarded the CA50 by about 7.5° of the crank angle (57% for the CA50 of D100), while at BTDC 45°, it retarded the CA50 by about 10.6° of the crank angle (60.6% for the CA50 of D100). On the other hand, the premixed combustion
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Fig. 2. Schematic and picture of the re-entrant type piston used in this study (a) and injection direction and spray targeting for the various injection timings (b).
in a conventional injector increased with the decrease of bioethanol. This result can be explained by the relatively low homogeneity of the air/fuel mixture, which was caused by wall-wetting of the injected spray and the increase in bioethanol with low cetane number and low heating value under low ambient density condition (SOE; BTDC 25°). In general, the wall-wetting makes locally fuel-rich region near the cylinder wall and piston surface. It affects the formation of inhomogeneity mixture. Whereas, in the case of the narrow angle injector, the amount of spray impinged onto the cylinder wall and piston surface was less than that with the conventional injector because of the small injection angle. Hence, although the cetane number and heating value decreased, the formation of a relatively uniform air/fuel mixture is possible during the ignition delay. For these reasons, the premixed combustion duration was similar regardless of bioethanol content in diesel– bioethanol blends. On the other hand, Fig. 3c showed that the advance of injection timing using the narrow angle injector increased the premixed combustion duration, and retarded the premixed combustion phasing. The combustion characteristics are presented quantitatively in Figs. 4–6 by the coefficient of variation of the maximum combustion pressure ðCOVPmax Þ, the indicated mean effective pressure (IMEP), and ignition delay. Fig. 4 compares COVPmax for various injection timings using conventional and narrow angle injector cases. The COVPmax is related to the combustion stability of the index showing the variation in peak combustion pressure. As shown in Fig. 4, the COVPmax of the narrow angle injector was generally lower than that of the conventional injector, and this characteristic was more apparent as the injection timings advanced, although the increase in bioethanol induced a slight increase in the ratio of COVPmax of the narrow angle injector compared to that of the conventional injector. These results can be explained by Fig. 3, which shows the reduction in wall-wetting of the injected spray and more uniform air/fuel mixture formation during ignition delay, which caused a decrease in the in-homogeneity of the equivalence ratio. This result means that more stable combustion characteristics in diesel–bioethanol blends fueled engine can be achieved using early injection strategy and narrow angle injector. Fig. 5 compares the IMEP of conventional and narrow angle injectors by injection timing and start of combustion (SOC) timing. A higher IMEP was obtained by using a narrow angle injector at 5–22%, compared to the conventional injector. In the injection
timings of BTDC 20°–BTDC 50°, the IMEPs of the narrow angle injector of each test fuel (D100, DE10, DE20, and DE30) averaged 0.34 MPa, 0.37 MPa, 0.39 MPa, and 0.41 MPa, respectively, while those of the conventional injector averaged 0.29 MPa, 0.30 MPa, 0.36 MPa and 0.38 MPa, respectively. In addition, for the same test fuel, the variation in IMEP by injection timing in the conventional injector was about 0.094 MPa, which was approximately 230% higher than with the narrow angle injector (IMEP variation in the narrow angle injector 0.041 MPa). These results indicate that the reduction in IMEP due to the advancing the injection timing and the application of bioethanol blended diesel fuels did not occur when the narrow angle injector was applied with an early injection combustion strategy. On the other hand, Fig. 5b shows the IMEP characteristics for SOC timings. As shown in the figure, the IMEP for the narrow angle injector generally followed a uniform trend line with a first order equation (solid line in the figure), regardless of fuel type. However, each test fuel followed a separate trend curve with a second order equation (dotted line in the figure) for the conventional injector. The trend curve moved toward the upper-right according to the increase of bioethanol blends and the retardation of SOC. This means that a higher IMEP appeared when the SOC was moved to TDC. The retarded SOC–TDC induced a decrease in negative work due to the combustion reaction before TDC and the slight increase in the combustion pressure of each expansion stroke, consequently increasing the IMEP. We can also confirm that there was relatively small variation in IMEP when the narrow angle injector was applied to the combustion of diesel–bioethanol blends. A comparison of the ignition delay in the narrow angle injector and the conventional injector is shown in Fig. 6, which indicates that the ignition delay in the narrow angle injector was slightly shorter than that in the conventional injector. Under early injection conditions, the spray from the conventional injector with a 156° injection angle was generally injected toward the cylinder wall, not toward the piston head bowl. In addition, the injected spray rapidly developed and propagated in the cylinder because the temperature and pressure in the combustion cylinder were very low, thus the density of ambient gas was low. Consequently, the spray impinged to the cylinder wall before vaporization and ignition. The impinged fuel adsorbed to the cylinder wall, and the droplets formed a locally fuel-rich region in the combustion cylinder. Hence, the formation of a uniform air/fuel mixture is relatively
S.H. Park et al. / Applied Energy 107 (2013) 81–88
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Fig. 3. Combustion pressure curves of bioethanol blended diesel fuels in conventional and narrow angle injectors (Pinj = 120 MPa, mfuel = 10 mg, Engine speed: 1200 rpm).
difficult. However, the narrow angle injector with 70° decreased the adsorption of the injected spray on the cylinder wall. Therefore, most injected spray fuel forms a more uniform air/fuel mixture. Under the same combustion conditions (temperature and pressure), the narrow injection angle induced faster ignition than the conventional injection angle. Fig. 7 shows the ISHC emission characteristics for injection timings and SOC timings in conventional and narrow angle injectors. From previous research, we know the main sources of HC emission is injection timing, mixing time between fuel and air,
and premixed combustion phasing (CA50) [29]. Specifically, in the early injection cases, it is well known that HC emission results from the liquid fuel film formed by spray impingement and overmixing [30,31], wall-wetting, insufficient oxygen, and the residual fuel in the sac volume [32]. As shown in Fig. 7, the less ISHC emissions generally exhausted at narrow angle injector than conventional angle injector. In addition, the variation width for bioethanol blending in the narrow angle injector was about 38% smaller than that in the conventional injector (narrow angle injector: 0.25 g/kW h, conventional injector: 0.65 g/kW h).
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50 P inj =120MPa, m fuel =10mg
Pinj=120MPa, mfuel=10mg, Engine speed 1200rpm D100
DE10
DE20
Conv. Narr.
DE30
Ignition delay (degree)
Pmax,narrow
COVPmax (Narrow angle) COV
/ COVPmaxCOV (Conventional angle) Pmax,conv
3
2
1
0
-40
-30
-20
-10
D100 DE10 DE20 DE30
40
30
20
10
0
0 -55
Start of energizing (deg. ATDC)
-50
-45
-40
-35
-30
-25
-20
-15
Injection timing (deg. ATDC)
Fig. 4. Comparison of the COV (coefficient of variation) of peak combustion pressure in narrow angle and conventional injectors.
Fig. 6. Ignition delay characteristics of bioethanol blended diesel fuels in conventional and narrow angle injectors (Pinj = 120 MPa, mfuel = 10 mg, Engine speed: 1200 rpm).
0.6 Pinj=120MPa, mfuel=10mg Conv. Narr. D100 DE10 DE20 DE30
1.2 Pinj=120MPa, mfuel=10mg
0.3
0.2 -55
Conv. Narr. D100 DE10 DE20 DE30
0.9
0.4
ISHC (g/kWh)
IMEP (MPa)
0.5
-50
-45
-40
-35
-30
-25
-20
0.6
0.3
-15
Injection timing (deg. ATDC) (a) IMEP for injection timings 0.0 -55
-50
0.6 Pinj=120MPa, mfuel=10mg
-35
-30
-25
1.2
Conv. Narr. D100 DE10 DE20 DE30
0.9
0.3
-20
-15
-10
-5
0
-15
Pinj=120MPa, mfuel=10mg
0.4
0.2 -25
-20
(a) ISHC for injection timings
D100 DE10 DE20 DE30
ISHC (g/kWh)
IMEP (MPa)
-40
Injection timing (deg. ATDC)
Conv. Narr.
0.5
-45
5
0.6 Narrow angle injector
Conventional injector
0.3
Start of combustion (deg. ATDC) (b) IMEP for start of combustion Fig. 5. IMEP characteristics in conventional and narrow angle injectors (Pinj = 120 MPa, mfuel = 10 mg, Engine speed: 1200 rpm).
0.0 -25
-20
-15
-10
-5
0
5
Start of combustion (deg. ATDC) (b) ISHC for start of combustion
For detailed analysis of HC emissions under early injection conditions, two early injection regions were chosen, regions A and B as indicated in the figure (region A: BTDC 35°, region B: BTDC 25°). In region B, the HC emissions with the narrow angle and conventional injectors were either similar or less with the narrow angle injector. However, in region A, the HC emissions with the narrow angle injector were about 20% (standard of raw emission data) lower than that with the conventional angle injector Table 3. In addition, for the same test fuel, the HC emissions of the conventional angle
Fig. 7. Comparison of ISHC characteristics by injection timings and SOC (start of combustion) in conventional and narrow angle injectors (Pinj = 120 MPa, mfuel = 10 mg, Engine speed: 1200 rpm).
injector tended to increase as the injection timing was advanced, while the narrow angle injector showed generally uniform or small variations in HC emissions. These results can be attributed to the reduction in the wall-film on the cylinder wall and on the piston
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S.H. Park et al. / Applied Energy 107 (2013) 81–88 Table 3 Raw data for HC emission with conventional and narrow angle injectors. Unit:ppm
Conventional injector
Narrow angle injector
Relative valueA
(a) BTDC 35° (region A) D100 DE10 DE20 DE30 Mean
133 123 120 151 –
104 101 101 116 –
0.782 0.821 0.842 0.768 0.803
(b) BTDC 25° (region B) D100 DE10 DE20 DE30 Mean
56 70 59 111 –
72 71 84 86 –
1.28 1.01 1.42 0.77 1.12
#
RelativeA means the ratio of HC emissions with a narrow angle injector compared to a conventional injector, and was calculated by the following equation: A Relative ¼ ðHC emissions in narrow angle injectorÞ=ðHC emissions in conventional injectorÞ.
80 Pinj=120MPa, mfuel=10mg Conv. Narr. D100 DE10 DE20 DE30
ISCO (g/kWh)
60
40
Conventional injector 51.7g/kWh
Narrow angle injector 12.0g/kWh
20
0 -50
-45
-40
-35
-30
-25
-20
Injection timing (deg. ATDC) (a) ISCO for injection timings 100 Pinj=120MPa, mfuel=10mg Conv. Narr. D100 DE10 DE20 DE30
ISCO (g/kWh)
80
60
40 Conventional injector
20
0 -25
Narrow angle injector
-20
-15
-10
-5
0
5
Start of combustion (deg. ATDC) (b) ISCO for start of combustion Fig. 8. Comparison of ISCO characteristics by injection timings and SOC (start of combustion) in conventional and narrow angle injectors (Pinj = 120 MPa, mfuel = 10 mg, Engine speed: 1200 rpm).
head after spray impingement, which is one of the main sources of HC emissions, when using a narrow angle injector. The HC emission reduction is related to the provision of sufficient time for mixing of air and fuel by advancing the injection timing. Fig. 7b shows the ISHC emissions according to the start of combustion timing. ISHC emissions with the narrow angle injector generally were lower than those with a conventional injector over a
wide range of combustion start times. In addition, the variation width for SOC and bioethanol blending in the narrow angle injector was smaller than that in the conventional injector. Fig. 8 shows the ISCO emissions characteristics for injection timings and SOC timings. Use of the narrow angle injector with early injection combustion resulted in minimal variation in ISCO emissions compared to when a conventional injector was used (narrow angle injector: 12.0 g/kW h, conventional injector: 51.7 g/kW h). CO emissions are mainly generated from the partial oxidation of locally rich mixture and incomplete combustion [33]. CO emissions are an intermediate product of the oxidation of hydrocarbons in the combustion chamber [34]. As shown in Fig. 8a, the ISCO emission from the narrow angle injector at regions A and B were about 60% and 20% that of the emissions from the conventional injector combustion in the same regions, respectively. Reducing CO emissions is an obstacle to creating a clean diesel combustion vehicle when using bioethanol and early injection timing. Bioethanol blended diesel-fueled combustion engines exhaust more CO than does a pure diesel engine [24,35]. However, in this investigation, we confirmed that CO emissions can be significantly reduced by applying a narrow angle injector and early injection combustion strategy. CO emissions and equivalence ratio (locally or globally) are closely related [29,36], in that an increase in the equivalence ratio induces an increase in CO emissions. The low CO emissions in Fig. 8 can be explained by the reduction in the locally high equivalence ratio region because the early and narrow angle injection resulted in sufficient mixing time, enabling the formation of uniform mixture of air and fuel. In addition, the relatively short ignition delay, short premixed burn duration, and advanced combustion phasing resulted in low CO emissions (refer to Fig. 3) [29]. Fig. 8b shows the ISCO emissions characteristics of both injectors with conventional and narrow angles at various SOC timings. The superior CO emission characteristics of the narrow angle injector during conventional injector combustion are apparent. At test conditions of this study, the ISCO emissions of pure diesel and bioethanol blended diesel fuels was below 18 g/ kW h with narrow angle injector combustion.
4. Conclusions This study describes how a narrow angle injector affects the combustion and exhaust emission characteristics under an early injection combustion strategy. Experiments were conducted using a single-cylinder diesel engine fueled with diesel–bioethanol blended fuels. Based on the results, we can conclude the following:
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1. The premixed combustion phasing decreased with the increase of bioethanol in both conventional and narrow angle injectors. At the same injection timing, the narrow angle injector showed the increase of the premixed combustion duration with the increase of bioethanol, while the conventional injector showed almost uniform values regardless of bioethanol contents. The advance of the injection timing in the narrow angle injector induced the slight increase of the premixed combustion duration and the advance of the premixed combustion phasing. 2. In the combustion using the narrow angle injector, the variation in the peak combustion pressure is smaller than the combustion using the conventional injector. In addition, it showed a higher IMEP and shorter ignition delay characteristics. 3. When the narrow angle injector applied to the combustion with diesel–bioethanol blends, the low and stable ISHC/ISCO emissions can be achieved. In addition, by the early injection combustion strategy, ISHC and ISCO emissions are significantly reduced. 4. According to above investigation results, more stable combustion and higher IMEP can be achieved through the use of the narrow angle injector in diesel–bioethanol blends fueled combustion. Especially, the combination of early injection strategy and narrow angle injector induced the reduction of ISHC and ISCO emissions, which is the weakness in diesel–bioethanol blends fueled combustion.
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