- Email: [email protected]
Effect of undiluted bioethanol on combustion and emissions reduction in a SI engine at various charge air conditions
Fuel 97 (2012) 887–890
Contents lists available at SciVerse ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Short communication
Effect of undiluted bioethanol on combustion and emissions reduction in a SI engine at various charge air conditions Seung Hyun Yoon a,1, Chang Sik Lee b,⇑ a b
Department of Mechanical Engineering – Engineering Mechanics, Michigan Technological University, 917 R.L. Smith Building, 1400 Townsend Drive, Houghton, MI 49931, USA Department of Mechanical Engineering, Hanyang Univeristy, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea
a r t i c l e
i n f o
Article history: Received 27 October 2011 Received in revised form 30 January 2012 Accepted 2 February 2012 Available online 15 February 2012 Keywords: Bioethanol Alternative fuel Spark ignition engine Combustion characteristics Intake air temperature
a b s t r a c t This paper investigates the effects of neat bioethanol combustion on the performance and emission reduction characteristics of a spark ignition (SI) engine at various air temperature conditions. The experiments were carried out for different intake air temperatures and various operating conditions, and the results were compared to those for conventional gasoline fuel. The investigated results show that as intake ambient air temperatures is decreased, the intake flow rates is increased by the increased density of the intake air. The ethanol fuel has a higher volumetric efficiency than gasoline for all engine speeds and ambient temperature conditions. The cylinder pressures are increased with the improvement of volumetric efficiency due to the decrease of intake temperature. In addition, ethanol combustion creates higher combustion pressures than that of gasoline due to the high latent heat evaporation rate and low boiling point. The coefficients of variation of maximum pressure show slightly decreasing trends as the ambient air temperature increases. The concentration of NOx emissions tends to increase proportionally with the increase of ambient air temperature and engine speed for all test conditions. However, the HC, and CO emissions from ethanol combustion are improved than those of gasoline combustion. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Bioethanol is renewable, biodegradable, and environmentally friendly alternative fuel, because it can be produced from agricultural products and scrapped resources. The road transport network accounts for 23% of total greenhouse gas and through the use of bioethanol fuel, these emissions will be reduced. Because of these numerous benefits, bioethanol, and ethanol–gasoline blends are widely used and investigated as alternative fuels in automotive vehicles [1–3]. The combustion of ethanol in spark ignition (SI) engine can be promoted the brake thermal efficiency by increasing the compression ratio without knock phenomena and higher latent heat of vaporization. Al-Hasan [4] investigated the effect of gasoline–ethanol blends on the combustion characteristics, and showed that the combustion of blends increased brake power performance and brake thermal efficiencies. In addition, the concentrations of CO and HC emissions decreased. Also, Hsieh et al. [5] studied the performance and pollutant emissions of a SI engine using ethanol–gasoline blending ratios from 0% to 30% and showed that the torque performance and brake specific fuel consumption (BSFC) of engines increase slightly when using ethanol–gasoline ⇑ Corresponding author. Tel.: +82 2 2220 0427; fax: +82 2 2281 5286. 1
E-mail addresses: [email protected] (S.H. Yoon), [email protected] (C.S. Lee). Tel.: +1 906 487 2817; fax: +1 906 487 2822.
0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2012.02.001
blended fuels, and that CO and HC emissions decrease due to the lean effect in the mixture caused by the ethanol. However, the applications of ethanol fuel to automobile engines are limited due to cold start problems that result from modifications to engine designs and fuel supply systems. The high boiling point (78 °C) of ethanol also causes vaporization difficulties in cold conditions, which leads to cold start problems. For this reason, unmodified engines use fuels with lower ethanol content such as gasohol, which contains 10% ethanol volume fraction [6–9]. Although there are researches on the combustion and emission characteristics of bioethanol fuel at the low temperature condition, it is necessary to more investigate the detailed reduction of exhaust emissions and combustion characteristics of the bio-ethanol fueled engine. The objective of this study is to investigate the effect of variation of intake air temperature on the combustion and exhaust emission characteristics in a SI engine fueled with undiluted bioethanol. In order to analyze the combustion characteristics of neat bioethanol and conventional gasoline fuel, the combustion pressure and the rate of heat release (ROHR) are evaluated under changing operating conditions such as intake air temperature and engine speed. In addition, to compare the engine performance, combustion stability, the brake torque, and the concentrations of NOx, HC, and CO emissions are also investigated under various engine test conditions.
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Table 1 Properties of test fuels.
Chemical formula Molecular weight (kg/kmol) Density (kg/m3 at 20 °C) Oxygen (wt%) Octane number (RON) Boiling point (°C) Latent heat of vaporization (kJ/kg) Auto-ignition temperature (°C) A/F ratio (by volume) Lower heating value (MJ/kg)
MBT timing, WOT Gasoline CnH1.87n 114.15 732 0 86–94 25–230 289 257 14.7 43.8
Ethanol C2H5OH 46.07 792 35 105–108 78.5 854 423 9.0 26.7
o
Volumetric efficiency (%)
Properties
75 Tamb C 10 30 50
70
Ethanol Gasoline
65
60
55 1500
2000
2. Experimental
3. Results and discussion Fig. 1 shows the effects of intake air temperatures and engine speeds on the volumetric efficiency and combustion characteristics according to the test fuels. During the intake process, the high
3000
3500
Engine speed (rpm)
(a) Volumetric efficiency 7
Cylinder pressure (MPa)
6
120
MBT timing, Engine speed : 2500rpm, WOT MBT timing, WOT
100
o
Tamb C Ethanol Gasoline 10 30 50
5
80
4 60 3 40 2 Pressure
20
1
Rate of heat release (J/deg.)
ROHR
0 -40
0 -30
-20
-10
0
10
20
30
40
50
60
Crank angle (deg. ATDC)
(b) Combustion pressure and heat release rate 550 MBT timing, WOT o
Tamb C Ethanol Gasoline 10 30 50
500
BSFC (g/kWh)
In this study, pure bioethanol with an anhydrous ethanol purity ratio of 99.9% that meets the ASTM D 5798 standard and unleaded gasoline without any additives, which satisfies the ASTM standard specification for gasoline fuel were used as the test fuels. The physical properties of the bioethanol and gasoline are listed in Table 1. In this work, the test engine is a port fuel injection four-cylinder SI gasoline engine with a sweep volume of 1591 cm3. It has a bore of 77.0 mm, a stroke of 85.44 mm, and a compression ratio of 10.5. The rated maximum power and maximum torque are 89 kW at 6200 rpm and 152.88 N m at 4200 rpm, respectively. The experimental apparatus consists of an engine control system, an exhaust emissions analyzer, a dynamometer, and the bioethanol test engine. The injection duration is adjusted by a closed-loop lambda control to investigate the effect of ethanol fuel. In order to investigate the effect of the intake air temperature, the engine is equipped with an air heat exchanger installed upstream from the throttle valve. The heat exchanger can be operated with a hot and cold water bath. In order to get a designed intake temperature range of 10–50 °C, the ambient air conditions were adjusted by cooling and a heating device. The cylinder pressure is measured by a piezo-electric pressure sensor (spark plug, Type 6117BFD17 and sensor, Type 6052B1, Kistler), and is coupled with a charge amplifier and data acquisition board. The in-cylinder pressure for crank angle are averaged to reduce the effect of cycle-to-cycle variations. In this case, in-cylinder pressures and other characteristics values were averaged over the duration of the 300 engine cycles. The concentrations of the pollutants are monitored with a NOx analyzer (BCL-511, Yanaco), and a non-dispersive infrared type of HC and CO analyzer (MEXA-554JK, Horiba). Various engine speed conditions ranging from 1500 rpm to 3500 rpm in steps of 500 rpm are controlled by the EC dynamometer. The test engine load is fixed at 100% (wide open throttle, WOT), the injection strategy (such as injection duration) based on fuel quantities is modified to maintain a value for lambda (k) of around 0.9 for the application of gasoline and ethanol to the test engine. The fuel injection strategy of injectors was controlled by electronic control unit. With this method, the engine load can be adjusted by opening the throttle valve to 100% (full load), and the brake torque corresponding to each operating condition is measured by the control system of the EC dynamometer. To assess the MBT timing of each test fuel, the spark ignition timing is varied by using the ECU control from top dead center (TDC) to before top dead center (BTDC) 40° crank angle (CA) with a step of 5°.
2500
450
400 300
250 1500
2000
2500
3000
3500
Engine speed (rpm)
(c) Brake specific fuel consumption Fig. 1. Volumetric efficiency, combustion characteristics and brake consumption with intake air temperature at 2500 rpm.
latent heat of evaporation for ethanol caused a decrease in the specific volume of air–fuel mixture at higher engine speeds, so a greater air–fuel charge flowed into the combustion chamber and increased the volumetric efficiency during the intake process [10,11]. In SI engines, the variation of intake air temperature affects the density of the inducted fresh air in the engine. As illustrated in Fig. 1a, the ethanol has a higher volumetric efficiency than gasoline for all engine speeds and intake temperature conditions. In
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S.H. Yoon, C.S. Lee / Fuel 97 (2012) 887–890
10
COV of maximum pressure
9
MBT timing WOT
o
Tamb C Ethanol Gasoline 10 30 50
8 7 6 5 4 3 2 1 1500
2000
2500
3000
3500
Engine speed (rpm)
(a) Coefficient of variable of maximum pressure 55 MBT timing, WOT
50
Brake power (kW)
45
o
Tamb C Ethanol Gasoline 10 30 50
40 35 30 25 20 15 1500
2000
2500
3000
3500
Engine speed (rpm)
(b) Brake power 1.15 MBT timing, WOT
1.10 1.05
BMEP (MPa)
particular, ethanol at an air charge temperature of 10 °C shows higher volumetric efficiency than gasoline. These results indicate that reducing the intake air temperature and ethanol fuel promoted the intake air and volumetric efficiency for low engine speeds. However, the volumetric efficiency decreases as engine speeds exceed 3000 rpm because the intake flow rates are reduced. Fig. 1b shows a comparison of the combustion pressure and rate of heat release (ROHR) for ethanol and gasoline at engine speeds of 2500 rpm. The ignition timing is maintained at a MBT and also at wide open throttle (WOT). The peak combustion pressures and rates of heat release increased considerably as the intake air temperature decreased due to the increased air flow rate, which also means larger amount of injected fuel. In particular, ethanol shows better combustion performance than gasoline at all test conditions, because ethanol fuel has a high evaporation rate about three times higher than gasoline fuel. Also, the high latent heat of evaporation and low boiling point cause a decrease in the air–fuel mixture temperature, so a greater air–fuel charge flows into the combustion chamber during the intake process. In addition, ethanol contains about 35% oxygen, and for the same amount of induced-air, more ethanol is needed to satisfy the stoichiometric air–fuel ratio, which is about 9 to 1 for ethanol. The oxygen content in ethanol plays an important role at higher engine speeds where the available time is insufficient to form a stabilized mixture. These effects produce a superior combustion performance [12]. In Fig. 2a, to investigate the effects of intake temperature on the combustion stability, the parameters of combustion variation were studied by analyzing the coefficients of variation of peak combustion pressure (COVpmax). The COVpmax during 300 consecutive cycles for the test fuels at various engine speeds were investigated under WOT conditions. The COVpmax shows the decreasing trends as the intake air temperature is increased up to 50 °C, and also reduced as the increase of engine speed. This is primarily because increasing the intake air temperature and engine speed generally promoted fuel evaporation and forms a well-premixed air–fuel mixture before ignition occurred, which lead to stable cycle-to-cycle variation. Ethanol combustion is achieved fairly stable combustion at lower intake temperatures compared to gasoline combustion, because of oxygen content and high octane value. At the engine speed of 2500 rpm, the values of COVpmax for gasoline are 6.65 (10 °C), 5.54 (30 °C), and 4.92 (50 °C), and the values for ethanol are 5.71 (10 °C), 4.34 (30 °C), and 3.99 (50 °C). Considering these results, the operating range and limit of engine drive can be widened by using ethanol. Fig. 2b shows the effects of intake air temperature on the brake power at a constant air–fuel excess ratio and WOT conditions. As the engine speed is increased, the brake power of the test fuels is increased. As illustrated in Fig. 2c, the BMEP of ethanol for each temperature condition is increased by approximately 5% in all test ranges. In addition, the maximum value of BMEP for ethanol is 1.052 MPa at 10 °C, which is about 3% higher than that of gasoline. The decrease of volumetric efficiency by the increase of intake air temperature resulted in the lower BMEP [11]. Moreover, the difference in BMEP between ethanol and gasoline is maximized according to the decrease of temperature and increase of engine speed. These higher performance outputs for the ethanol are also caused by the higher oxygen content of ethanol fuel [13]. As shown in Fig. 3a, the concentration of NOx emissions tends to increase with the increase of intake air temperature and engine speed for all test conditions. However, NOx emissions were slightly less for ethanol than for gasoline under most test conditions. In addition, the lowest level of NOx emissions could be achieved using ethanol with 10 °C of intake air, which is about 25% below the level measured using gasoline fuel for all engine speed ranges. These lower levels of NOx emissions for ethanol and its blends are primarily due to the low combustion temperature with the high heat
o
Tamb C Ethanol Gasoline 10 30 50
1.00 0.95 0.90 0.85 0.80 0.75
1500
2000
2500
3000
3500
Engine speed (rpm)
(c) BMEP Fig. 2. Coefficient of variation of maximum pressure, brake power and BMEP variations with intake air temperature.
of vaporization and high heat capacity of ethanol. Fig. 3b and c shows the concentrations of hydrocarbon (HC) and carbon monoxide (CO) emissions for various intake air temperatures and engine speeds. As shown in this figure, the concentrations of HC emissions for ethanol are lower than those of gasoline for various engine speeds. Furthermore, the CO emissions are also decreased when using ethanol, and are about 20% below the levels measured with gasoline. The reason for the reduced concentrations of HC and CO
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BSNOx (g/kWh)
20
4. Conclusions
MBT timing, WOT o
Tamb C Ethanol Gasoline
0.9
To analyze the effects of intake air conditions on ethanol fueled engine, the combustion performance, pressure stability and emissions reduction characteristics were investigated in a four-cylinder SI engine. As the intake air temperatures are decreased, the ethanol fuel has a higher volumetric efficiency than gasoline at all engine speed and various intake air conditions. At the constant engine speed, the cylinder pressures are increased with the improvement of volumetric efficiency due to the decrease of intake temperature. In addition, the ethanol fuel showed the higher combustion pressure than that of gasoline. The coefficient of variation of maximum pressure decreased slightly as the intake air temperature increased at all engine speeds. The concentration of NOx emissions tends to increase with the increase of intake air temperature and engine speed for all test conditions. However, the NOx emissions of ethanol were slightly less than those of gasoline. The HC emissions for ethanol are lower than those of gasoline under various engine speed conditions. Furthermore, the CO emissions also decreased for ethanol fuel, and are about 20% below the levels measured for gasoline fuel. However, the decrease of intake air temperature has an effect on the increase of HC and CO emissions for both test fuels.
0.6
Acknowledgments
15
10 30 50
10
5
0
1500
2000
2500
3000
3500
Engine speed (rpm)
(a) NOx emissions 1.5
MBT timing, WOT o
Tamb C Ethanol Gasoline
BSHC (g/kWh)
1.2
10 30 50
This work was supported in part by the Second Brain Korea 21 Project. Also, this work was supported by the National research Foundation of Korea (NRF) grant funded by Korea government (MEST) (No. 2011-0025295).
0.3
0.0
1500
2000
2500
3000
3500
Engine speed (rpm)
References
(b) HC emission 25
BSCO (g/kWh)
20
MBT timing, WOT o Tamb C Ethanol Gasoline 10 30 50
15
10
5
0
1500
2000
2500
3000
3500
Engine speed (rpm)
(c) CO emission Fig. 3. Concentrations of NOx, HC and CO emissions with intake air temperature.
emissions is that ethanol fuel contains higher oxygen content in the fuel and leads to more stable and complete combustion. However, the decrease of intake air temperature has significant effects on HC and CO emissions. This may be due to the decreased evaporation and mixing of fuel under low temperature. Therefore, low temperature combustion depresses the hydrocarbon oxidization and creates higher CO emissions.
[1] Francisco AM, Ahmad RG. Performance and exhaust emissions of a singlecylinder utility engine using ethanol fuel. SAE technical paper; 2006 [2006-320078]. [2] Marriott CD, Wiles MA, Gwidt JM, Parrish SE. Development of a naturally aspirated spark ignition direct-injection flex-fuel engine. SAE technical paper; 2008[2008-01-0319]. [3] Yoon SH, Park SH, Lee CS. Experimental investigation on the fuel properties of biodiesel and its blends at various temperatures. Energy Fuel 2008;22:652–6. [4] Al-Hasan M. Effect of ethanol–unleaded gasoline blends on engine performance and exhaust emission. Energy Convers Manage 2003;44(9): 1547–61. [5] Hsieh WD, Chen RH, Wu TL, Lin TH. Engine performance and pollutant emission of an SI engine using ethanol–gasoline blended fuels. Atmos Environ 2002;36:403–10. [6] Wu CW, Chen RH, Pu JY, Lin TH. The influence of air–fuel ratio on engine performance and pollutant emission of an SI engine using ethanol–gasolineblended fuels. Atmos Environ 2004;38:7093–100. [7] Yuksel F, Yuksel B. The use of ethanol–gasoline blend as a fuel in an SI engine. Renew Energy 2004;29:1181–91. [8] Jeuland N, Montagne X, Gautrot X. Potentiality of ethanol as a fuel for dedicated engine. Oil Gas Sci Technol 2004;59:559–70. [9] Topgul T, Yucesu HS, Cinar C, Koca A. The effect of ethanol–unleaded gasoline blends and ignition timing on engine performance and exhaust emissions. Renew Energy 2006;31:2534–42. [10] Takashi T, Yohei H, Shintaro U, Takashi K. High concentration ethanol effect on SI engine cold startability. SAE technical paper;2007 [2007-01-2036]. [11] Morgan MA, Wai KC, Thomas K. Effect of air temperature and humidity on gasoline HCCI operating in the negative-valve-overlap mode. SAE technical paper; 2007 [2007-01-0221]. [12] Yoon SH, Ha SY, Roh HG, Lee CS. Effect of bioethanol as an alternative fuel on the emissions reduction characteristics and combustion stability in a spark ignition engine. PI Mech Eng D-J Aut 2009;223:941–51. [13] Al-Farayedhi AA, Al-Dawood AM, Gandhidasan P. Experimental investigation of SI engine performance using oxygenated fuel. J Eng Gas Turb Power 2004;126:178–91.
Contents lists available at SciVerse ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Short communication
Effect of undiluted bioethanol on combustion and emissions reduction in a SI engine at various charge air conditions Seung Hyun Yoon a,1, Chang Sik Lee b,⇑ a b
Department of Mechanical Engineering – Engineering Mechanics, Michigan Technological University, 917 R.L. Smith Building, 1400 Townsend Drive, Houghton, MI 49931, USA Department of Mechanical Engineering, Hanyang Univeristy, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea
a r t i c l e
i n f o
Article history: Received 27 October 2011 Received in revised form 30 January 2012 Accepted 2 February 2012 Available online 15 February 2012 Keywords: Bioethanol Alternative fuel Spark ignition engine Combustion characteristics Intake air temperature
a b s t r a c t This paper investigates the effects of neat bioethanol combustion on the performance and emission reduction characteristics of a spark ignition (SI) engine at various air temperature conditions. The experiments were carried out for different intake air temperatures and various operating conditions, and the results were compared to those for conventional gasoline fuel. The investigated results show that as intake ambient air temperatures is decreased, the intake flow rates is increased by the increased density of the intake air. The ethanol fuel has a higher volumetric efficiency than gasoline for all engine speeds and ambient temperature conditions. The cylinder pressures are increased with the improvement of volumetric efficiency due to the decrease of intake temperature. In addition, ethanol combustion creates higher combustion pressures than that of gasoline due to the high latent heat evaporation rate and low boiling point. The coefficients of variation of maximum pressure show slightly decreasing trends as the ambient air temperature increases. The concentration of NOx emissions tends to increase proportionally with the increase of ambient air temperature and engine speed for all test conditions. However, the HC, and CO emissions from ethanol combustion are improved than those of gasoline combustion. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Bioethanol is renewable, biodegradable, and environmentally friendly alternative fuel, because it can be produced from agricultural products and scrapped resources. The road transport network accounts for 23% of total greenhouse gas and through the use of bioethanol fuel, these emissions will be reduced. Because of these numerous benefits, bioethanol, and ethanol–gasoline blends are widely used and investigated as alternative fuels in automotive vehicles [1–3]. The combustion of ethanol in spark ignition (SI) engine can be promoted the brake thermal efficiency by increasing the compression ratio without knock phenomena and higher latent heat of vaporization. Al-Hasan [4] investigated the effect of gasoline–ethanol blends on the combustion characteristics, and showed that the combustion of blends increased brake power performance and brake thermal efficiencies. In addition, the concentrations of CO and HC emissions decreased. Also, Hsieh et al. [5] studied the performance and pollutant emissions of a SI engine using ethanol–gasoline blending ratios from 0% to 30% and showed that the torque performance and brake specific fuel consumption (BSFC) of engines increase slightly when using ethanol–gasoline ⇑ Corresponding author. Tel.: +82 2 2220 0427; fax: +82 2 2281 5286. 1
E-mail addresses: [email protected] (S.H. Yoon), [email protected] (C.S. Lee). Tel.: +1 906 487 2817; fax: +1 906 487 2822.
0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2012.02.001
blended fuels, and that CO and HC emissions decrease due to the lean effect in the mixture caused by the ethanol. However, the applications of ethanol fuel to automobile engines are limited due to cold start problems that result from modifications to engine designs and fuel supply systems. The high boiling point (78 °C) of ethanol also causes vaporization difficulties in cold conditions, which leads to cold start problems. For this reason, unmodified engines use fuels with lower ethanol content such as gasohol, which contains 10% ethanol volume fraction [6–9]. Although there are researches on the combustion and emission characteristics of bioethanol fuel at the low temperature condition, it is necessary to more investigate the detailed reduction of exhaust emissions and combustion characteristics of the bio-ethanol fueled engine. The objective of this study is to investigate the effect of variation of intake air temperature on the combustion and exhaust emission characteristics in a SI engine fueled with undiluted bioethanol. In order to analyze the combustion characteristics of neat bioethanol and conventional gasoline fuel, the combustion pressure and the rate of heat release (ROHR) are evaluated under changing operating conditions such as intake air temperature and engine speed. In addition, to compare the engine performance, combustion stability, the brake torque, and the concentrations of NOx, HC, and CO emissions are also investigated under various engine test conditions.
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S.H. Yoon, C.S. Lee / Fuel 97 (2012) 887–890
Table 1 Properties of test fuels.
Chemical formula Molecular weight (kg/kmol) Density (kg/m3 at 20 °C) Oxygen (wt%) Octane number (RON) Boiling point (°C) Latent heat of vaporization (kJ/kg) Auto-ignition temperature (°C) A/F ratio (by volume) Lower heating value (MJ/kg)
MBT timing, WOT Gasoline CnH1.87n 114.15 732 0 86–94 25–230 289 257 14.7 43.8
Ethanol C2H5OH 46.07 792 35 105–108 78.5 854 423 9.0 26.7
o
Volumetric efficiency (%)
Properties
75 Tamb C 10 30 50
70
Ethanol Gasoline
65
60
55 1500
2000
2. Experimental
3. Results and discussion Fig. 1 shows the effects of intake air temperatures and engine speeds on the volumetric efficiency and combustion characteristics according to the test fuels. During the intake process, the high
3000
3500
Engine speed (rpm)
(a) Volumetric efficiency 7
Cylinder pressure (MPa)
6
120
MBT timing, Engine speed : 2500rpm, WOT MBT timing, WOT
100
o
Tamb C Ethanol Gasoline 10 30 50
5
80
4 60 3 40 2 Pressure
20
1
Rate of heat release (J/deg.)
ROHR
0 -40
0 -30
-20
-10
0
10
20
30
40
50
60
Crank angle (deg. ATDC)
(b) Combustion pressure and heat release rate 550 MBT timing, WOT o
Tamb C Ethanol Gasoline 10 30 50
500
BSFC (g/kWh)
In this study, pure bioethanol with an anhydrous ethanol purity ratio of 99.9% that meets the ASTM D 5798 standard and unleaded gasoline without any additives, which satisfies the ASTM standard specification for gasoline fuel were used as the test fuels. The physical properties of the bioethanol and gasoline are listed in Table 1. In this work, the test engine is a port fuel injection four-cylinder SI gasoline engine with a sweep volume of 1591 cm3. It has a bore of 77.0 mm, a stroke of 85.44 mm, and a compression ratio of 10.5. The rated maximum power and maximum torque are 89 kW at 6200 rpm and 152.88 N m at 4200 rpm, respectively. The experimental apparatus consists of an engine control system, an exhaust emissions analyzer, a dynamometer, and the bioethanol test engine. The injection duration is adjusted by a closed-loop lambda control to investigate the effect of ethanol fuel. In order to investigate the effect of the intake air temperature, the engine is equipped with an air heat exchanger installed upstream from the throttle valve. The heat exchanger can be operated with a hot and cold water bath. In order to get a designed intake temperature range of 10–50 °C, the ambient air conditions were adjusted by cooling and a heating device. The cylinder pressure is measured by a piezo-electric pressure sensor (spark plug, Type 6117BFD17 and sensor, Type 6052B1, Kistler), and is coupled with a charge amplifier and data acquisition board. The in-cylinder pressure for crank angle are averaged to reduce the effect of cycle-to-cycle variations. In this case, in-cylinder pressures and other characteristics values were averaged over the duration of the 300 engine cycles. The concentrations of the pollutants are monitored with a NOx analyzer (BCL-511, Yanaco), and a non-dispersive infrared type of HC and CO analyzer (MEXA-554JK, Horiba). Various engine speed conditions ranging from 1500 rpm to 3500 rpm in steps of 500 rpm are controlled by the EC dynamometer. The test engine load is fixed at 100% (wide open throttle, WOT), the injection strategy (such as injection duration) based on fuel quantities is modified to maintain a value for lambda (k) of around 0.9 for the application of gasoline and ethanol to the test engine. The fuel injection strategy of injectors was controlled by electronic control unit. With this method, the engine load can be adjusted by opening the throttle valve to 100% (full load), and the brake torque corresponding to each operating condition is measured by the control system of the EC dynamometer. To assess the MBT timing of each test fuel, the spark ignition timing is varied by using the ECU control from top dead center (TDC) to before top dead center (BTDC) 40° crank angle (CA) with a step of 5°.
2500
450
400 300
250 1500
2000
2500
3000
3500
Engine speed (rpm)
(c) Brake specific fuel consumption Fig. 1. Volumetric efficiency, combustion characteristics and brake consumption with intake air temperature at 2500 rpm.
latent heat of evaporation for ethanol caused a decrease in the specific volume of air–fuel mixture at higher engine speeds, so a greater air–fuel charge flowed into the combustion chamber and increased the volumetric efficiency during the intake process [10,11]. In SI engines, the variation of intake air temperature affects the density of the inducted fresh air in the engine. As illustrated in Fig. 1a, the ethanol has a higher volumetric efficiency than gasoline for all engine speeds and intake temperature conditions. In
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S.H. Yoon, C.S. Lee / Fuel 97 (2012) 887–890
10
COV of maximum pressure
9
MBT timing WOT
o
Tamb C Ethanol Gasoline 10 30 50
8 7 6 5 4 3 2 1 1500
2000
2500
3000
3500
Engine speed (rpm)
(a) Coefficient of variable of maximum pressure 55 MBT timing, WOT
50
Brake power (kW)
45
o
Tamb C Ethanol Gasoline 10 30 50
40 35 30 25 20 15 1500
2000
2500
3000
3500
Engine speed (rpm)
(b) Brake power 1.15 MBT timing, WOT
1.10 1.05
BMEP (MPa)
particular, ethanol at an air charge temperature of 10 °C shows higher volumetric efficiency than gasoline. These results indicate that reducing the intake air temperature and ethanol fuel promoted the intake air and volumetric efficiency for low engine speeds. However, the volumetric efficiency decreases as engine speeds exceed 3000 rpm because the intake flow rates are reduced. Fig. 1b shows a comparison of the combustion pressure and rate of heat release (ROHR) for ethanol and gasoline at engine speeds of 2500 rpm. The ignition timing is maintained at a MBT and also at wide open throttle (WOT). The peak combustion pressures and rates of heat release increased considerably as the intake air temperature decreased due to the increased air flow rate, which also means larger amount of injected fuel. In particular, ethanol shows better combustion performance than gasoline at all test conditions, because ethanol fuel has a high evaporation rate about three times higher than gasoline fuel. Also, the high latent heat of evaporation and low boiling point cause a decrease in the air–fuel mixture temperature, so a greater air–fuel charge flows into the combustion chamber during the intake process. In addition, ethanol contains about 35% oxygen, and for the same amount of induced-air, more ethanol is needed to satisfy the stoichiometric air–fuel ratio, which is about 9 to 1 for ethanol. The oxygen content in ethanol plays an important role at higher engine speeds where the available time is insufficient to form a stabilized mixture. These effects produce a superior combustion performance [12]. In Fig. 2a, to investigate the effects of intake temperature on the combustion stability, the parameters of combustion variation were studied by analyzing the coefficients of variation of peak combustion pressure (COVpmax). The COVpmax during 300 consecutive cycles for the test fuels at various engine speeds were investigated under WOT conditions. The COVpmax shows the decreasing trends as the intake air temperature is increased up to 50 °C, and also reduced as the increase of engine speed. This is primarily because increasing the intake air temperature and engine speed generally promoted fuel evaporation and forms a well-premixed air–fuel mixture before ignition occurred, which lead to stable cycle-to-cycle variation. Ethanol combustion is achieved fairly stable combustion at lower intake temperatures compared to gasoline combustion, because of oxygen content and high octane value. At the engine speed of 2500 rpm, the values of COVpmax for gasoline are 6.65 (10 °C), 5.54 (30 °C), and 4.92 (50 °C), and the values for ethanol are 5.71 (10 °C), 4.34 (30 °C), and 3.99 (50 °C). Considering these results, the operating range and limit of engine drive can be widened by using ethanol. Fig. 2b shows the effects of intake air temperature on the brake power at a constant air–fuel excess ratio and WOT conditions. As the engine speed is increased, the brake power of the test fuels is increased. As illustrated in Fig. 2c, the BMEP of ethanol for each temperature condition is increased by approximately 5% in all test ranges. In addition, the maximum value of BMEP for ethanol is 1.052 MPa at 10 °C, which is about 3% higher than that of gasoline. The decrease of volumetric efficiency by the increase of intake air temperature resulted in the lower BMEP [11]. Moreover, the difference in BMEP between ethanol and gasoline is maximized according to the decrease of temperature and increase of engine speed. These higher performance outputs for the ethanol are also caused by the higher oxygen content of ethanol fuel [13]. As shown in Fig. 3a, the concentration of NOx emissions tends to increase with the increase of intake air temperature and engine speed for all test conditions. However, NOx emissions were slightly less for ethanol than for gasoline under most test conditions. In addition, the lowest level of NOx emissions could be achieved using ethanol with 10 °C of intake air, which is about 25% below the level measured using gasoline fuel for all engine speed ranges. These lower levels of NOx emissions for ethanol and its blends are primarily due to the low combustion temperature with the high heat
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(c) BMEP Fig. 2. Coefficient of variation of maximum pressure, brake power and BMEP variations with intake air temperature.
of vaporization and high heat capacity of ethanol. Fig. 3b and c shows the concentrations of hydrocarbon (HC) and carbon monoxide (CO) emissions for various intake air temperatures and engine speeds. As shown in this figure, the concentrations of HC emissions for ethanol are lower than those of gasoline for various engine speeds. Furthermore, the CO emissions are also decreased when using ethanol, and are about 20% below the levels measured with gasoline. The reason for the reduced concentrations of HC and CO
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S.H. Yoon, C.S. Lee / Fuel 97 (2012) 887–890
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4. Conclusions
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To analyze the effects of intake air conditions on ethanol fueled engine, the combustion performance, pressure stability and emissions reduction characteristics were investigated in a four-cylinder SI engine. As the intake air temperatures are decreased, the ethanol fuel has a higher volumetric efficiency than gasoline at all engine speed and various intake air conditions. At the constant engine speed, the cylinder pressures are increased with the improvement of volumetric efficiency due to the decrease of intake temperature. In addition, the ethanol fuel showed the higher combustion pressure than that of gasoline. The coefficient of variation of maximum pressure decreased slightly as the intake air temperature increased at all engine speeds. The concentration of NOx emissions tends to increase with the increase of intake air temperature and engine speed for all test conditions. However, the NOx emissions of ethanol were slightly less than those of gasoline. The HC emissions for ethanol are lower than those of gasoline under various engine speed conditions. Furthermore, the CO emissions also decreased for ethanol fuel, and are about 20% below the levels measured for gasoline fuel. However, the decrease of intake air temperature has an effect on the increase of HC and CO emissions for both test fuels.
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Acknowledgments
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This work was supported in part by the Second Brain Korea 21 Project. Also, this work was supported by the National research Foundation of Korea (NRF) grant funded by Korea government (MEST) (No. 2011-0025295).
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References
(b) HC emission 25
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MBT timing, WOT o Tamb C Ethanol Gasoline 10 30 50
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(c) CO emission Fig. 3. Concentrations of NOx, HC and CO emissions with intake air temperature.
emissions is that ethanol fuel contains higher oxygen content in the fuel and leads to more stable and complete combustion. However, the decrease of intake air temperature has significant effects on HC and CO emissions. This may be due to the decreased evaporation and mixing of fuel under low temperature. Therefore, low temperature combustion depresses the hydrocarbon oxidization and creates higher CO emissions.
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