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Effects of the sulfur content of liquefied petroleum gas on regulated and unregulated emissions from liquefied petroleum gas vehicle
Fuel 137 (2014) 328–334
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Effects of the sulfur content of liquefied petroleum gas on regulated and unregulated emissions from liquefied petroleum gas vehicle C.P. Cho, O.S. Kwon, Y.J. Lee ⇑ Korea Institute of Energy Research, Energy Efficiency Department, 152, Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea
h i g h l i g h t s Sulfur content effects in LPG on the performance of LPG vehicle were investigated. Regulated and unregulated time-resolved emissions increased during cold start phase. Most of the mass emissions increased as the sulfur content of the LPG increased. However, Fuel economy was not affected by the sulfur content of the LPG fuel.
a r t i c l e
i n f o
Article history: Received 23 September 2013 Received in revised form 27 July 2014 Accepted 29 July 2014 Available online 20 August 2014 Keywords: LPG vehicle Sulfur content Chassis dynamometer test Regulated emission Unregulated emission
a b s t r a c t A more stringent standard for sulfur in liquefied petroleum gas (LPG) is currently being considered by the Ministry of Environment of Korea. Therefore, we investigated the characteristics of exhaust emissions from liquid phase LPG injection passenger car depending on the sulfur content of the LPG. Using a chassis dynamometer test with exhaust gas analyzers and Fourier-transform infrared spectroscopy, regulated emissions of carbon monoxide, non-methane hydrocarbons, and nitrates were investigated, as well as unregulated emissions of CH4, N2O, NH3, and SO2. Vehicle tests were carried out using Federal Test Procedure 75 (FTP-75) and Highway Fuel Economy Test (HWFET) cycles and time-resolved exhaust emissions and mass emissions were analyzed. The concentrations of regulated and unregulated time-resolved emissions increased during the cold start phase (Phase 1) of the FTP-75 cycle, in which the catalyst does not reach the light-off temperature. In the FTP-75 and HWFET cycles, most of the regulated and unregulated mass emissions increased as the sulfur content of the LPG increased. The SO2 concentration in the emissions increased in proportion to the sulfur content in the LPG, particularly in the high-speed portion of the FTP-75 cycle. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Liquefied petroleum gas (LPG or autogas) is a byproduct of the crude oil refinery process and mainly consists of propane (C3H8) and butane (C4H10). The composition of LPG for vehicle use varies by country; the standards for LPG in Korea are shown in Table 1. In Korea, the main component of LPG as a vehicular fuel is butane, and propane is added at 15–35 mol% to improve the ability to start the vehicle in the winter. Except during this season, the propane content is restricted to < 10 mol%. The sulfur content of LPG is limited to < 40 ppm. LPG is known as a clean fuel, because it contains less carbon than gasoline. LPG vehicles typically produce lower CO2 emissions than gasoline vehicles, as well as reductions of up to 40% and 60% ⇑ Corresponding author. Tel.: +82 42 860 3334; fax: +82 42 860 3335. E-mail address: [email protected] (Y.J. Lee). http://dx.doi.org/10.1016/j.fuel.2014.07.090 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
for hydrocarbon (HC) and carbon monoxide (CO) emissions, respectively [1]. In addition, the concentration of nanoparticles emitted from LPG engines is much lower than that from gasoline engines [2]. Recently, many countries have promoted the use of LPG as an environmentally beneficial and economical alternative fuel. Currently, about 2.5 million LPG vehicles, representing 13% of the registered vehicles, are being driven in Korea due to the relatively low price of LPG fuel. About 80% of these vehicles are passenger cars. Liquid phase LPG injection (LPLi) and gas phase LPG injection (LPGi) type LPG vehicles have been produced in the Korean market. Mixer type LPG vehicles having low performance are no longer produced. Because the LPLi vehicles inject the liquid phase LPG into the engine intake port, it is possible to control the fuel flow rate precisely and the volumetric efficiency of the engine is increased by reducing the intake air temperature due to vaporization of the
329
C.P. Cho et al. / Fuel 137 (2014) 328–334 Table 1 Automotive LPG fuel standards in Korea.
a
Table 2 Specifications of the test vehicle.
Property
Until December 31 2008
After January 1 2009
Sulfur content (wt ppm) Vapor pressure (40 °C, MPa) Density (15 °C, kg/m3) Copper strip corrosion (1 h at 40 °C) Residue on evaporation of 100 mL (mL) Propane content (mole %) November 1–March 31 April 1–Oct 31
100 max 1.27 max 500–620 1 max 0.05 max
40 max 1.27 max 500–620 1 max 0.05 max
15–40
15–40 (35)a
10 max
10 max
Effective on December 1 2010.
LPG. Therefore, fuel economy and power are greater and the exhaust emissions of the LPLi vehicles are lower than those of conventional mixer type LPG and LPGi vehicles [3]. In LPGi vehicles, the liquid phase LPG in the evaporator is vaporized and the gas is injected into the engine intake port. The performance of LPGi vehicles is lower than that of LPLi vehicles, because their volume efficiency is lower due to superheating of the LPG gas in the evaporator and because the fuel flow rate is less controlled than in an LPLi engine [4]. LPG contains a small amount of sulfur after refining from crude oil, and this sulfur concentration is increased by odorants that contain sulfur. Sulfur poisons the three-way catalyst of LPG and gasoline vehicles, which reduces the conversion efficiency of the catalyst [5,6] and results in increased hazardous emissions [7,8]. SO2 also contaminates the exhaust oxygen sensor, which interferes with precise feedback controlling the air to fuel ratio. Alliance and Association of International Automobile Manufacturers (AIAM) have reported that emissions of HC, CO, and NOx were reduced by more than 20% in low and ultralow emissions vehicles when the sulfur content in gasoline decreased from 100 to 30 ppm [9]. The Ministry of Environment in Korea has gradually reduced the allowable sulfur content in the quality standards for vehicular fuel. In 2009, the allowable sulfur content in LPG was lowered to < 40 ppm from < 100 ppm. This limit, however, is still much higher than 10 ppm, which is the sulfur content limit for gasoline and diesel [10]. Therefore, a more stringent standard is currently being considered by the Ministry of Environment. In the future, lean burn and/or LPG direct injection (LPDI) engines are expected to be introduced to improve vehicle fuel economy. Ultra-low sulfur fuel may be required to improve the durability of the DeNOx catalyst used in these new vehicles, because this catalyst is much more sensitive to sulfur than the three-way catalyst [11]. In this study, regulated and unregulated exhaust emissions from an LPLi vehicle were investigated for LPG fuels with sulfur contents of 4 ppm (sulfur free level) and 40 ppm. Emissions analyzed included regulated gases such as CO, non-methane hydrocarbons (NMHC), and NOx and unregulated gases such as CH4, N2O, NH3, and SO2. Among these gases, CH4 and N2O are greenhouse gases whose global warming potentials are 21 and 310 times greater than that of CO2, respectively [12]. Ammonia is a toxic gas that directly affects human health and is also a secondary particulate matter precursor, along with SO2 and NOx. 2. Experimental setup 2.1. Test vehicle and test cycles An LPLi type LPG passenger car was tested with the Federal Test Procedure 75 (FTP-75) cycle and the Highway Fuel Economy Test (HWFET) cycle. The specifications of the test vehicle are shown in Table 2.
Fuel injection type Displacement Model year Odometer Catalyst type Manufacturer Model
LPLi 1998 cc 2010 7300 km Three-way catalyst Kia K5
The FTP-75 cycle is based on the urban dynamometer driving schedule (UDDS) with an average speed of 34.12 km/h. It consists of three phases: in Phase 1, a cold start cycle that simulates driving after a cold start runs for 5.78 km in 505 s; in Phase 2, a stabilized cycle simulates driving in a stable state and runs for 6.29 km in 865 s; and in Phase 3, a hot start cycle, in which the engine is stopped and started again after Phase 2, consists of the same speed profile as Phase 1. The HWFET cycle is a highway driving cycle whose running distance and average speed are 16.45 km and 77.7 km/h, respectively. To ensure that the test vehicle was in a normal state, a preliminary test was carried out using the FTP-75 cycle, which is the emissions certification test cycle in Korea. The test vehicle’s emissions met the Ultra Low Emissions Vehicles (ULEV) standard for LPG vehicles in Korea, as shown in Table 3. 2.2. Test fuels As noted in Section 1, the composition of propane and butane in LPG depends on the season, in accordance with the quality standards for LPG in Korea. In this study, LPG fuels with 4 and 40 ppm sulfur were used as test fuels; the propane and butane contents were held constant to exclude the effects of compositional change on exhaust emissions. The properties of the LPG fuels tested are shown in Table 4. The test fuels were stored in their original 85-L containers. When a test fuel was replaced with another, the remaining fuel in the fuel line of the engine was flushed out by driving Phases 1 and 2 of the FTP-75 cycle. 2.3. Test equipments and emissions analysis A 48-inch single-roll chassis dynamometer (ECDM-48L; MAHAAIP), a gaseous emissions analyzer (MEXA-9200; Horiba), and a full-flow constant volume sampling exhaust gas analyzer system (CVS-9100; Horiba) were used to measure emissions of CO2, NMHC, CO, NOx, and CH4. Simultaneously, unregulated emissions (N2O, SO2, NH3) were measured by Fourier-transform infrared (FTIR) spectroscopy, as shown in Table 5. The experimental setup is shown schematically in Fig. 1. 3. Results and discussions 3.1. Time-resolved emissions Fig. 2 shows the catalyst-out emissions concentrations for regulated and unregulated emissions of the test vehicle measured on a real-time basis during the FTP-75 cycle using LPG fuel with a sulfur content of 40 ppm. In general, engine-out CO from a spark-ignited engine is produced mainly from incomplete combustion under rich mixture conditions; engine-out NMHC is produced both during non-combustion and incomplete combustion. Generally, these gases are reduced by the three-way catalyst when the engine is operated under a stoichiometric air-to-fuel ratio. However, the concentrations of CO and NMHC significantly increased
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Table 3 Preliminary test results of regulated emissions for the test vehicle during the FTP-75 cycle.
Standard Test results
CO (g/km)
NMHC (g/km)
NOx (g/km)
CO2 (g/km)
Fuel efficiency (km/L)
1.06 max 0.22
0.025 max 0.006
0.031 max 0.004
– 189.6
– 9.3
Table 4 Properties of test fuels. Properties
LPG 1
LPG 2
Component (mol%) C3 C4 1,3-butadiene (mol%) Vapor pressure @ 40 °C (MPa) Density @ 15 °C (kg/m3) Sulfur content (wt ppm)
5 95 Non-detected 0.39 576 4
5 95 Non-detected 0.39 576 40
Table 5 Specifications of Fourier-transform infrared spectroscopy. Model Spectral resolution Reference laser Detector Sample gas flow rate Gas cell temperature Scan rate
AVL SESAM 2030 with MKS bench 0.5 cm 1 Helium–neon LN2-cooled mercury cadmium telluride (MCT) 10 L/min 191 °C 5 Hz
immediately upon the initial cold start of Phase 1 (Fig. 2a). This occurred because the three-way catalyst did not reach the lightoff temperature and also because the fuel was enriched to improve the cold start ability of the engine. In contrast, the concentrations of these gases decreased at the hot start in Phase 3, which has the same speed profile as that of Phase 1, because of the warmed-up catalyst. NOx emissions consists of NO and NO2. Most NOx produced during combustion is generally NO; the FTIR analysis confirmed that
NO made up more than 99% of the NOx. Little NOx is typically produced in a rich mixture because of a deficiency of oxygen; greater amounts are produced in lean mixtures around the excess air ratio of 1.1 due to an abundance of oxygen. Exhaust gas recirculation and three-way catalysts are used to reduce the NOx. The concentration of NOx was low in the initial cold start region of Phase 1 (Fig. 2b), because the engine was operated with a richer mixture to improve the cold start ability, as noted above [13]. N2O is formed as an intermediate product when NO is reduced by catalysts and is produced at lower temperatures before the catalysts are activated. As a result, a large amount of N2O was produced in the initial cold start region of Phase 1 (Fig. 2b). Ammonia emitted from vehicles is produced from reaction of NO with H2. It has been reported that NH3 emissions are higher in vehicles with three-way catalysts with higher conversion efficiencies [14]. Emissions of CO and NH3 produced during the principal reaction step of hydrocarbon combustion increase under the same rich fuel conditions. NH3 emissions, therefore, are strongly correlated with CO emissions [15]. CH4, a greenhouse gas, was strongly correlated with NMHC, which is in the same hydrocarbon family. The concentration of CH4 was high in regions with high concentrations of NMHC (Fig. 2b). Fig. 3 shows the time resolved catalyst-out SO2 emissions during the FTP-75 cycle using LPG fuels with sulfur contents of 4 and 40 ppm. The concentration of SO2 was highest in the highspeed regions during Phases 1 and 3 and was also higher for the LPG fuel with higher sulfur content. SO2 is the main sulfur compound exhausted from an internal combustion engine and undergoes various transformations based on the temperature and composition of the exhaust gas stream. In the presence of a
Fig. 1. Schematic diagram of the vehicle test system.
C.P. Cho et al. / Fuel 137 (2014) 328–334
three-way catalyst, sulfide ions (S2 ) can be formed under fuel-rich conditions; conversely, under fuel-lean conditions, S species are formed in the gas phase and undergo chemisorption onto the surface of the catalyst support [5,6]. Such sulfur poisoning deactivates the catalyst, thus reducing its conversion efficiency. In contrast, studies have shown that SO2 is discharged in higher concentrations during a high acceleration run [16], consistent with the findings of this study. 3.2. Mass emissions Figs. 4–6 show the regulated exhaust emissions of NMHC, CO, and NOx measured during the FTP-75 and HWFET cycles using LPG fuel with sulfur contents of 4 and 40 ppm. Figs. 7–10 show
331
the unregulated exhaust emissions of CH4, NH3, N2O, and SO2 under the same conditions. Although the sulfur contents of the test LPG fuels were fairly low, variations in the sulfur contents produced marked effects on the regulated and unregulated emissions. Emissions of most compounds had a tendency to increase as the sulfur content increased. With an increase in the sulfur content from 4 to 40 ppm, NMHC emissions increased by 24% in the FTP-75 cycle and 35% in the HWFET cycle. CO emissions in the FTP-75 cycle significantly increased in Phase 1 during the cold start, while the overall weighted concentration did not increase significantly. Although concentrations of CO were low, they were 58% higher during the HWFET cycle with the higher sulfur content. NOx emissions were 14% higher in the FTP-75 cycle and 4% higher in the HWFET
(a) Concentrations of regulated exhaust gases
(b) Concentrations of unregulated exhaust gases Fig. 2. Time-resolved emissions concentrations for 40-ppm sulfur fuel.
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Fig. 3. Time-resolved SO2 concentrations for 4-ppm and 40-ppm sulfur fuels.
Fig. 4. NMHC emissions for the FTP-75 cycle and HWFET cycle.
Fig. 6. NOx emissions for the FTP-75 cycle and HWFET cycle.
Fig. 5. CO emissions for the FTP-75 cycle and HWFET cycle.
Fig. 7. CH4 emissions for the FTP-75 cycle and HWFET cycle.
C.P. Cho et al. / Fuel 137 (2014) 328–334
333
Fig. 8. NH3 emissions for the FTP-75 cycle and HWFET cycle. Fig. 11. Fuel economy for the FTP-75 cycle and HWFET cycle.
Fig. 9. N2O emissions for the FTP-75 cycle and HWFET cycle.
SO2 emissions were higher during the FTP-75 cycle with the higher sulfur content and this was also the case in the HWFET cycle (Fig. 7). SO2 emissions were particularly higher in Phase 1 and Phase 3, in which the concentration of SO2 was high, as discussed in Section 3.1. The three-way catalyst uses Pt (platinum)/Pd (palladium)/Rh (rhodium) and the effects of sulfur on each component of the three-way catalyst vary. In terms of all three components, the NMHC reduction efficiency decreases due to sulfur; however, Pd can have a negative effect on CO and NOx reduction depending on the conditions of the exhaust stream [7]. Therefore, the reduction characteristics of each component can vary depending on the proportion of the metals in the three-way catalyst. The higher sulfur level in the fuel produced higher SO2 in the discharge gas in addition to the sulfur poisoning effect. Fig. 11 shows the fuel economy measured during the FTP-75 and HWFET cycles using LPG with sulfur contents of 4 and 40 ppm. Fuel economy was not affected by the sulfur content in the LPG fuel. 4. Conclusions
Fig. 10. SO2 emissions for the FTP-75 cycle and HWFET cycle.
cycle with the higher sulfur content. Non-regulated emissions also increased at the higher sulfur content. CH4 was 69% higher during the FTP-75 cycle and 9 times higher during the HWFET cycle, whereas NH3 was 47% higher during the FTP-75 cycle and 3 times higher during the HWFET cycle. Although the concentration of N2O was less than 1 mg/km, it was 17% higher in the FTP-75 cycle with the higher sulfur content, but showed little change in the HWFET cycle when fully warmed up and at a high speed.
The objective of this research was to verify the effects of varying sulfur content in LPG fuel on the fuel economy and exhaust emissions of an LPG vehicle. Regulated emissions of CO, NMHC, and NOx and unregulated emissions of CH4, N2O, NH3, and SO2 were analyzed using an LPLi type passenger car and LPG fuels with sulfur contents of 4 and 40 ppm. Time-resolved analysis of the exhaust emissions from the test vehicle demonstrated that the concentrations of regulated emissions such as NMHC and CO greatly increased at the initial start phase of the FTP-75 cycle; unregulated emissions such as CH4, NH3, and N2O also showed a similar tendency. Time-resolved SO2 emissions, the major exhaust gas containing sulfur, were relatively high in the high-speed regions of the FTP-75 cycle and were highest for the LPG fuel with the higher sulfur content. During the FTP-75 and HWFET cycles, most of the regulated and unregulated mass emissions were higher for the higher sulfur LPG. The effects of the sulfur content on NMHC, CO, and NOx emissions were strongest in the cold start phase (Phase 1) of the FTP-75 cycle, during which large amounts of pollutants were emitted. In the HWFET cycle, during which relatively low levels of pollutants were emitted (compared to the FTP-75 cycle), the concentrations of gaseous exhaust emissions were also higher with higher sulfur content. The concentration of SO2 emitted was higher for the higher
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sulfur LPG. Fuel economy was not affected by the sulfur content of the LPG fuel. Acknowledgements This research was supported by the Korean Auto-Oil Program under the auspices of the Ministry of Environment, the Korea Automobile Manufacturers Association, the Korea Petroleum Association, the Korea LPG Association, the Korean Association for Natural Gas Vehicles, and the Korea City Gas Association. References [1] Snelgrove D, Dupont P, Bonetto R. An investigation into the influence of LPG (autogas) composition on the exhaust emissions and fuel consumption of 3 bifueled Renault vehicles. SAE Paper 961170; 1996. [2] Myung C, Lee H, Choi K, Lee Y, Park S. Effects of gasoline, diesel, LPG and lowcarbon fuels and various certification modes on nanoparticle emission characteristics in light-duty vehicles. Int J Automot Technol 2009;10(5):537–44. [3] Kwak H, Myung C, Park S. Experimental investigations on the time resolved THC emission characteristics of liquid phases of LPG injection (LPLi) engine during a cold start. Fuel 2007;86:1475–82. [4] Masi M. Experimental analysis on a spark ignition petrol engine fuelled with LPG. Energy 2012;41:252–60.
[5] Yao H, Stepien H, Gandhi H. The effects of SO2 on the oxidation of hydrocarbons and carbon monoxide over Pt/c-Al2O3 catalysts. J Catal 1981;67:213–36. [6] Ansell G, Golunski S, Hatcher H, Rajaram R. Effects of SO2 on the alkane activity of three-way catalysts. Catal Lett 1991;11:183–90. [7] Beck D, Krueger M, Monroe D. The impact of sulfur on three-way catalysts: storage and removal. SAE Paper 910844; 1991. [8] Thoss J, Rieck J, Bennett C. The impact of fuel sulfur levels on FTP emissions— effects on PGM catalyst types. SAE Paper 970737; 1997. [9] ACEA. Worldwide fuel charter. 4th ed. Brussels: European Automobile Manufacturers Association; 2006. [10] MOE. Enforcement decrees on the Clean Air Conservation Act. Seoul: Ministry of Environment; 2012. [11] Automotive News Europe. Mitsubishi re-engineers GDI engine for Europe. May 12; 1997, p. 13. [12] IPCC. Working Group I contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change 2007. Geneva: IPCC; 2007. [13] Heywood J. Internal combustion engines fundamentals. New York: McGraw Hill; 1998. [14] Gandhi H, Shelef M. Effects of sulphur on noble metal automotive catalysts. Appl Catal 1991;77:175–86. [15] Livingston C, Rieger P, Winer A. Ammonia emissions from a representative inuse fleet of light and medium-duty vehicles in the California South Coast Air Basin. Atmos Environ 2009;43:3326–33. [16] Maricq MM, Chase RE, Xu N, Podsiadlik DH. The effects of the catalytic converter and the fuel sulfur level on motor vehicle particulate matter emissions: gasoline vehicles. Environ Sci Technol 2002;36:276–82.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Effects of the sulfur content of liquefied petroleum gas on regulated and unregulated emissions from liquefied petroleum gas vehicle C.P. Cho, O.S. Kwon, Y.J. Lee ⇑ Korea Institute of Energy Research, Energy Efficiency Department, 152, Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea
h i g h l i g h t s Sulfur content effects in LPG on the performance of LPG vehicle were investigated. Regulated and unregulated time-resolved emissions increased during cold start phase. Most of the mass emissions increased as the sulfur content of the LPG increased. However, Fuel economy was not affected by the sulfur content of the LPG fuel.
a r t i c l e
i n f o
Article history: Received 23 September 2013 Received in revised form 27 July 2014 Accepted 29 July 2014 Available online 20 August 2014 Keywords: LPG vehicle Sulfur content Chassis dynamometer test Regulated emission Unregulated emission
a b s t r a c t A more stringent standard for sulfur in liquefied petroleum gas (LPG) is currently being considered by the Ministry of Environment of Korea. Therefore, we investigated the characteristics of exhaust emissions from liquid phase LPG injection passenger car depending on the sulfur content of the LPG. Using a chassis dynamometer test with exhaust gas analyzers and Fourier-transform infrared spectroscopy, regulated emissions of carbon monoxide, non-methane hydrocarbons, and nitrates were investigated, as well as unregulated emissions of CH4, N2O, NH3, and SO2. Vehicle tests were carried out using Federal Test Procedure 75 (FTP-75) and Highway Fuel Economy Test (HWFET) cycles and time-resolved exhaust emissions and mass emissions were analyzed. The concentrations of regulated and unregulated time-resolved emissions increased during the cold start phase (Phase 1) of the FTP-75 cycle, in which the catalyst does not reach the light-off temperature. In the FTP-75 and HWFET cycles, most of the regulated and unregulated mass emissions increased as the sulfur content of the LPG increased. The SO2 concentration in the emissions increased in proportion to the sulfur content in the LPG, particularly in the high-speed portion of the FTP-75 cycle. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Liquefied petroleum gas (LPG or autogas) is a byproduct of the crude oil refinery process and mainly consists of propane (C3H8) and butane (C4H10). The composition of LPG for vehicle use varies by country; the standards for LPG in Korea are shown in Table 1. In Korea, the main component of LPG as a vehicular fuel is butane, and propane is added at 15–35 mol% to improve the ability to start the vehicle in the winter. Except during this season, the propane content is restricted to < 10 mol%. The sulfur content of LPG is limited to < 40 ppm. LPG is known as a clean fuel, because it contains less carbon than gasoline. LPG vehicles typically produce lower CO2 emissions than gasoline vehicles, as well as reductions of up to 40% and 60% ⇑ Corresponding author. Tel.: +82 42 860 3334; fax: +82 42 860 3335. E-mail address: [email protected] (Y.J. Lee). http://dx.doi.org/10.1016/j.fuel.2014.07.090 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
for hydrocarbon (HC) and carbon monoxide (CO) emissions, respectively [1]. In addition, the concentration of nanoparticles emitted from LPG engines is much lower than that from gasoline engines [2]. Recently, many countries have promoted the use of LPG as an environmentally beneficial and economical alternative fuel. Currently, about 2.5 million LPG vehicles, representing 13% of the registered vehicles, are being driven in Korea due to the relatively low price of LPG fuel. About 80% of these vehicles are passenger cars. Liquid phase LPG injection (LPLi) and gas phase LPG injection (LPGi) type LPG vehicles have been produced in the Korean market. Mixer type LPG vehicles having low performance are no longer produced. Because the LPLi vehicles inject the liquid phase LPG into the engine intake port, it is possible to control the fuel flow rate precisely and the volumetric efficiency of the engine is increased by reducing the intake air temperature due to vaporization of the
329
C.P. Cho et al. / Fuel 137 (2014) 328–334 Table 1 Automotive LPG fuel standards in Korea.
a
Table 2 Specifications of the test vehicle.
Property
Until December 31 2008
After January 1 2009
Sulfur content (wt ppm) Vapor pressure (40 °C, MPa) Density (15 °C, kg/m3) Copper strip corrosion (1 h at 40 °C) Residue on evaporation of 100 mL (mL) Propane content (mole %) November 1–March 31 April 1–Oct 31
100 max 1.27 max 500–620 1 max 0.05 max
40 max 1.27 max 500–620 1 max 0.05 max
15–40
15–40 (35)a
10 max
10 max
Effective on December 1 2010.
LPG. Therefore, fuel economy and power are greater and the exhaust emissions of the LPLi vehicles are lower than those of conventional mixer type LPG and LPGi vehicles [3]. In LPGi vehicles, the liquid phase LPG in the evaporator is vaporized and the gas is injected into the engine intake port. The performance of LPGi vehicles is lower than that of LPLi vehicles, because their volume efficiency is lower due to superheating of the LPG gas in the evaporator and because the fuel flow rate is less controlled than in an LPLi engine [4]. LPG contains a small amount of sulfur after refining from crude oil, and this sulfur concentration is increased by odorants that contain sulfur. Sulfur poisons the three-way catalyst of LPG and gasoline vehicles, which reduces the conversion efficiency of the catalyst [5,6] and results in increased hazardous emissions [7,8]. SO2 also contaminates the exhaust oxygen sensor, which interferes with precise feedback controlling the air to fuel ratio. Alliance and Association of International Automobile Manufacturers (AIAM) have reported that emissions of HC, CO, and NOx were reduced by more than 20% in low and ultralow emissions vehicles when the sulfur content in gasoline decreased from 100 to 30 ppm [9]. The Ministry of Environment in Korea has gradually reduced the allowable sulfur content in the quality standards for vehicular fuel. In 2009, the allowable sulfur content in LPG was lowered to < 40 ppm from < 100 ppm. This limit, however, is still much higher than 10 ppm, which is the sulfur content limit for gasoline and diesel [10]. Therefore, a more stringent standard is currently being considered by the Ministry of Environment. In the future, lean burn and/or LPG direct injection (LPDI) engines are expected to be introduced to improve vehicle fuel economy. Ultra-low sulfur fuel may be required to improve the durability of the DeNOx catalyst used in these new vehicles, because this catalyst is much more sensitive to sulfur than the three-way catalyst [11]. In this study, regulated and unregulated exhaust emissions from an LPLi vehicle were investigated for LPG fuels with sulfur contents of 4 ppm (sulfur free level) and 40 ppm. Emissions analyzed included regulated gases such as CO, non-methane hydrocarbons (NMHC), and NOx and unregulated gases such as CH4, N2O, NH3, and SO2. Among these gases, CH4 and N2O are greenhouse gases whose global warming potentials are 21 and 310 times greater than that of CO2, respectively [12]. Ammonia is a toxic gas that directly affects human health and is also a secondary particulate matter precursor, along with SO2 and NOx. 2. Experimental setup 2.1. Test vehicle and test cycles An LPLi type LPG passenger car was tested with the Federal Test Procedure 75 (FTP-75) cycle and the Highway Fuel Economy Test (HWFET) cycle. The specifications of the test vehicle are shown in Table 2.
Fuel injection type Displacement Model year Odometer Catalyst type Manufacturer Model
LPLi 1998 cc 2010 7300 km Three-way catalyst Kia K5
The FTP-75 cycle is based on the urban dynamometer driving schedule (UDDS) with an average speed of 34.12 km/h. It consists of three phases: in Phase 1, a cold start cycle that simulates driving after a cold start runs for 5.78 km in 505 s; in Phase 2, a stabilized cycle simulates driving in a stable state and runs for 6.29 km in 865 s; and in Phase 3, a hot start cycle, in which the engine is stopped and started again after Phase 2, consists of the same speed profile as Phase 1. The HWFET cycle is a highway driving cycle whose running distance and average speed are 16.45 km and 77.7 km/h, respectively. To ensure that the test vehicle was in a normal state, a preliminary test was carried out using the FTP-75 cycle, which is the emissions certification test cycle in Korea. The test vehicle’s emissions met the Ultra Low Emissions Vehicles (ULEV) standard for LPG vehicles in Korea, as shown in Table 3. 2.2. Test fuels As noted in Section 1, the composition of propane and butane in LPG depends on the season, in accordance with the quality standards for LPG in Korea. In this study, LPG fuels with 4 and 40 ppm sulfur were used as test fuels; the propane and butane contents were held constant to exclude the effects of compositional change on exhaust emissions. The properties of the LPG fuels tested are shown in Table 4. The test fuels were stored in their original 85-L containers. When a test fuel was replaced with another, the remaining fuel in the fuel line of the engine was flushed out by driving Phases 1 and 2 of the FTP-75 cycle. 2.3. Test equipments and emissions analysis A 48-inch single-roll chassis dynamometer (ECDM-48L; MAHAAIP), a gaseous emissions analyzer (MEXA-9200; Horiba), and a full-flow constant volume sampling exhaust gas analyzer system (CVS-9100; Horiba) were used to measure emissions of CO2, NMHC, CO, NOx, and CH4. Simultaneously, unregulated emissions (N2O, SO2, NH3) were measured by Fourier-transform infrared (FTIR) spectroscopy, as shown in Table 5. The experimental setup is shown schematically in Fig. 1. 3. Results and discussions 3.1. Time-resolved emissions Fig. 2 shows the catalyst-out emissions concentrations for regulated and unregulated emissions of the test vehicle measured on a real-time basis during the FTP-75 cycle using LPG fuel with a sulfur content of 40 ppm. In general, engine-out CO from a spark-ignited engine is produced mainly from incomplete combustion under rich mixture conditions; engine-out NMHC is produced both during non-combustion and incomplete combustion. Generally, these gases are reduced by the three-way catalyst when the engine is operated under a stoichiometric air-to-fuel ratio. However, the concentrations of CO and NMHC significantly increased
330
C.P. Cho et al. / Fuel 137 (2014) 328–334
Table 3 Preliminary test results of regulated emissions for the test vehicle during the FTP-75 cycle.
Standard Test results
CO (g/km)
NMHC (g/km)
NOx (g/km)
CO2 (g/km)
Fuel efficiency (km/L)
1.06 max 0.22
0.025 max 0.006
0.031 max 0.004
– 189.6
– 9.3
Table 4 Properties of test fuels. Properties
LPG 1
LPG 2
Component (mol%) C3 C4 1,3-butadiene (mol%) Vapor pressure @ 40 °C (MPa) Density @ 15 °C (kg/m3) Sulfur content (wt ppm)
5 95 Non-detected 0.39 576 4
5 95 Non-detected 0.39 576 40
Table 5 Specifications of Fourier-transform infrared spectroscopy. Model Spectral resolution Reference laser Detector Sample gas flow rate Gas cell temperature Scan rate
AVL SESAM 2030 with MKS bench 0.5 cm 1 Helium–neon LN2-cooled mercury cadmium telluride (MCT) 10 L/min 191 °C 5 Hz
immediately upon the initial cold start of Phase 1 (Fig. 2a). This occurred because the three-way catalyst did not reach the lightoff temperature and also because the fuel was enriched to improve the cold start ability of the engine. In contrast, the concentrations of these gases decreased at the hot start in Phase 3, which has the same speed profile as that of Phase 1, because of the warmed-up catalyst. NOx emissions consists of NO and NO2. Most NOx produced during combustion is generally NO; the FTIR analysis confirmed that
NO made up more than 99% of the NOx. Little NOx is typically produced in a rich mixture because of a deficiency of oxygen; greater amounts are produced in lean mixtures around the excess air ratio of 1.1 due to an abundance of oxygen. Exhaust gas recirculation and three-way catalysts are used to reduce the NOx. The concentration of NOx was low in the initial cold start region of Phase 1 (Fig. 2b), because the engine was operated with a richer mixture to improve the cold start ability, as noted above [13]. N2O is formed as an intermediate product when NO is reduced by catalysts and is produced at lower temperatures before the catalysts are activated. As a result, a large amount of N2O was produced in the initial cold start region of Phase 1 (Fig. 2b). Ammonia emitted from vehicles is produced from reaction of NO with H2. It has been reported that NH3 emissions are higher in vehicles with three-way catalysts with higher conversion efficiencies [14]. Emissions of CO and NH3 produced during the principal reaction step of hydrocarbon combustion increase under the same rich fuel conditions. NH3 emissions, therefore, are strongly correlated with CO emissions [15]. CH4, a greenhouse gas, was strongly correlated with NMHC, which is in the same hydrocarbon family. The concentration of CH4 was high in regions with high concentrations of NMHC (Fig. 2b). Fig. 3 shows the time resolved catalyst-out SO2 emissions during the FTP-75 cycle using LPG fuels with sulfur contents of 4 and 40 ppm. The concentration of SO2 was highest in the highspeed regions during Phases 1 and 3 and was also higher for the LPG fuel with higher sulfur content. SO2 is the main sulfur compound exhausted from an internal combustion engine and undergoes various transformations based on the temperature and composition of the exhaust gas stream. In the presence of a
Fig. 1. Schematic diagram of the vehicle test system.
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three-way catalyst, sulfide ions (S2 ) can be formed under fuel-rich conditions; conversely, under fuel-lean conditions, S species are formed in the gas phase and undergo chemisorption onto the surface of the catalyst support [5,6]. Such sulfur poisoning deactivates the catalyst, thus reducing its conversion efficiency. In contrast, studies have shown that SO2 is discharged in higher concentrations during a high acceleration run [16], consistent with the findings of this study. 3.2. Mass emissions Figs. 4–6 show the regulated exhaust emissions of NMHC, CO, and NOx measured during the FTP-75 and HWFET cycles using LPG fuel with sulfur contents of 4 and 40 ppm. Figs. 7–10 show
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the unregulated exhaust emissions of CH4, NH3, N2O, and SO2 under the same conditions. Although the sulfur contents of the test LPG fuels were fairly low, variations in the sulfur contents produced marked effects on the regulated and unregulated emissions. Emissions of most compounds had a tendency to increase as the sulfur content increased. With an increase in the sulfur content from 4 to 40 ppm, NMHC emissions increased by 24% in the FTP-75 cycle and 35% in the HWFET cycle. CO emissions in the FTP-75 cycle significantly increased in Phase 1 during the cold start, while the overall weighted concentration did not increase significantly. Although concentrations of CO were low, they were 58% higher during the HWFET cycle with the higher sulfur content. NOx emissions were 14% higher in the FTP-75 cycle and 4% higher in the HWFET
(a) Concentrations of regulated exhaust gases
(b) Concentrations of unregulated exhaust gases Fig. 2. Time-resolved emissions concentrations for 40-ppm sulfur fuel.
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Fig. 3. Time-resolved SO2 concentrations for 4-ppm and 40-ppm sulfur fuels.
Fig. 4. NMHC emissions for the FTP-75 cycle and HWFET cycle.
Fig. 6. NOx emissions for the FTP-75 cycle and HWFET cycle.
Fig. 5. CO emissions for the FTP-75 cycle and HWFET cycle.
Fig. 7. CH4 emissions for the FTP-75 cycle and HWFET cycle.
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Fig. 8. NH3 emissions for the FTP-75 cycle and HWFET cycle. Fig. 11. Fuel economy for the FTP-75 cycle and HWFET cycle.
Fig. 9. N2O emissions for the FTP-75 cycle and HWFET cycle.
SO2 emissions were higher during the FTP-75 cycle with the higher sulfur content and this was also the case in the HWFET cycle (Fig. 7). SO2 emissions were particularly higher in Phase 1 and Phase 3, in which the concentration of SO2 was high, as discussed in Section 3.1. The three-way catalyst uses Pt (platinum)/Pd (palladium)/Rh (rhodium) and the effects of sulfur on each component of the three-way catalyst vary. In terms of all three components, the NMHC reduction efficiency decreases due to sulfur; however, Pd can have a negative effect on CO and NOx reduction depending on the conditions of the exhaust stream [7]. Therefore, the reduction characteristics of each component can vary depending on the proportion of the metals in the three-way catalyst. The higher sulfur level in the fuel produced higher SO2 in the discharge gas in addition to the sulfur poisoning effect. Fig. 11 shows the fuel economy measured during the FTP-75 and HWFET cycles using LPG with sulfur contents of 4 and 40 ppm. Fuel economy was not affected by the sulfur content in the LPG fuel. 4. Conclusions
Fig. 10. SO2 emissions for the FTP-75 cycle and HWFET cycle.
cycle with the higher sulfur content. Non-regulated emissions also increased at the higher sulfur content. CH4 was 69% higher during the FTP-75 cycle and 9 times higher during the HWFET cycle, whereas NH3 was 47% higher during the FTP-75 cycle and 3 times higher during the HWFET cycle. Although the concentration of N2O was less than 1 mg/km, it was 17% higher in the FTP-75 cycle with the higher sulfur content, but showed little change in the HWFET cycle when fully warmed up and at a high speed.
The objective of this research was to verify the effects of varying sulfur content in LPG fuel on the fuel economy and exhaust emissions of an LPG vehicle. Regulated emissions of CO, NMHC, and NOx and unregulated emissions of CH4, N2O, NH3, and SO2 were analyzed using an LPLi type passenger car and LPG fuels with sulfur contents of 4 and 40 ppm. Time-resolved analysis of the exhaust emissions from the test vehicle demonstrated that the concentrations of regulated emissions such as NMHC and CO greatly increased at the initial start phase of the FTP-75 cycle; unregulated emissions such as CH4, NH3, and N2O also showed a similar tendency. Time-resolved SO2 emissions, the major exhaust gas containing sulfur, were relatively high in the high-speed regions of the FTP-75 cycle and were highest for the LPG fuel with the higher sulfur content. During the FTP-75 and HWFET cycles, most of the regulated and unregulated mass emissions were higher for the higher sulfur LPG. The effects of the sulfur content on NMHC, CO, and NOx emissions were strongest in the cold start phase (Phase 1) of the FTP-75 cycle, during which large amounts of pollutants were emitted. In the HWFET cycle, during which relatively low levels of pollutants were emitted (compared to the FTP-75 cycle), the concentrations of gaseous exhaust emissions were also higher with higher sulfur content. The concentration of SO2 emitted was higher for the higher
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sulfur LPG. Fuel economy was not affected by the sulfur content of the LPG fuel. Acknowledgements This research was supported by the Korean Auto-Oil Program under the auspices of the Ministry of Environment, the Korea Automobile Manufacturers Association, the Korea Petroleum Association, the Korea LPG Association, the Korean Association for Natural Gas Vehicles, and the Korea City Gas Association. References [1] Snelgrove D, Dupont P, Bonetto R. An investigation into the influence of LPG (autogas) composition on the exhaust emissions and fuel consumption of 3 bifueled Renault vehicles. SAE Paper 961170; 1996. [2] Myung C, Lee H, Choi K, Lee Y, Park S. Effects of gasoline, diesel, LPG and lowcarbon fuels and various certification modes on nanoparticle emission characteristics in light-duty vehicles. Int J Automot Technol 2009;10(5):537–44. [3] Kwak H, Myung C, Park S. Experimental investigations on the time resolved THC emission characteristics of liquid phases of LPG injection (LPLi) engine during a cold start. Fuel 2007;86:1475–82. [4] Masi M. Experimental analysis on a spark ignition petrol engine fuelled with LPG. Energy 2012;41:252–60.
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