Energy Fuels 2009, 23, 4363–4369 Published on Web 08/13/2009
: DOI:10.1021/ef9004708
Effect of Biofuels on Nanoparticle Emissions from Spark- and Compression-ignited Single-cylinder Engines with Same Exhaust Displacement Volume Jinwook Lee Department of Mechanical Engineering, Soongsil University, 511 Sangdo-Dong, Dongjak-Gu, Seoul, Republic of Korea
Rishin Patel, A Schonborn, and Nicos Ladommatos Department of Mechanical Engineering, University College London (UCL), London, United Kingdom
Choongsik Bae* Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-Dong, Yuseong-Gu, Daejeon, Republic of Korea Received May 15, 2009. Revised Manuscript Received July 17, 2009
Nanosized particles emitted from automotive engines continue to attract concern because of their adverse health effects and their impact on the environment. Automotive engines are a major source of fine and ultrafine particles emitted into the atmosphere. Through stricter emission regulations and the introduction of advanced technologies, the specific particulate mass emissions (in g/km and g/kWh) from internal combustion engines have decreased by about 1 order of magnitude since the 1980s. However, the number concentration of nanoparticles (No./m3) emitted from internal combustion engines may continue to increase considerably and has recently attracted the attention of the Particle Measurement Programme (PMP). This program is intended to evaluate engine nanoparticle measurement systems for use in future emission regulations. In the study reported in this paper, two latest-generation engines, one spark-ignited and the other compression-ignited, were used for a comparison of the particulate emission characteristics, including number density. Both engines were single-cylinder with the same displacement volume of 500 cm3, and neither included after-treatment traps or catalytic converters. Test fuels used for the study were: gasoline and E85 (mixture of 85% ethanol and 15% gasoline, sometimes called gasohol) for the spark-ignited engine having a compression ratio of 10; and ultralow sulfur diesel (ULSD) and BD100 (100% biodiesel, i.e. soybean methyl ester) for the compression-ignited engine having a compression ratio of 15. A fast-response particle spectrometer (DMS500) with heated sample line was used for continuous measurement of the particle size and number distribution in the size range of 5-1000 nm (aerodynamic diameter). The experimental results showed that particle number peaked within the range of 10-300 nm under all engine operating conditions, regardless of engine combustion type. An observed shift toward larger particle size with increasing engine load could be explained by particle coagulation. The effect of the different fuels on nanoparticle size distributions was dependent on the engine type (spark-ignition or compression-ignition). penetrating the blood and brain barriers, and can induce cancer.1,2 Most of engine particles having a size less than 1 μm in diameter originate from the incomplete combustion of the fuel and from the engine lubricant. The particulate has an organic composition and contains diverse hazardous substances.3,4 The major ingredients are carbons clusters, soluble organic fraction (SOF), sulfates, and inorganic species such as sulfur dioxide. The SOF adheres to the particulates under certain exhaust gas temperatures. The SOF has an H/C ratio of approximately 1.5 and consists of unburned hydrocarbons, partially oxidized hydrocarbons, and polycyclic aromatic
1. Introduction Automotive engines represent a major source of environmental pollution and greenhouse gases, contributing significantly to global CO2 emissions. Among the regulated emissions from spark ignition and compression ignition engines, particulate matter (PM) is continuing to attract the attention of researchers and regulators because of their toxic health effects on humans and other living organisms. The World Health Organisation (WHO) has classified these particles as being hazardous to human health because of their toxicity. In particular, it is known that the nanosized particles can be transported into human organ systems, via the lungs,
(2) Donaldson, K.; Liu, X. Y.; MacNee, W. J. Aerosol Sci. 1998, 29, 553–560. (3) Kasper, M.; Matter, U.; Burtscher, H. NanoMet: On-line characterization of nanoparticle size and composition. SAE Paper 200101-1998; 2001. (4) Abdul-Khalek, l. S; Kittelson, D. B. SAE Trans. J. Fuels Lubricants 1998, 107 (4), 1647–1660.
*To whom correspondence should be addressed. Telephone: þ82-42350-3044. Fax: þ82-42-350-5023. E-mail:
[email protected]. (1) Understanding the Health Effects of Components of the Particulate Matter Mix: Progress and Next Steps; Health Effects Institute (HEI): USA, 2002. r 2009 American Chemical Society
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: DOI:10.1021/ef9004708
Lee et al.
Table 1. Particle and Gas Phase Processes and Factors Affecting the Particle Size Distributions6 step engine
exhaust system
after-treatment devices
dilution tunnel
atmospheric transport and transformation
particle and gas phase processes
factors affecting particles
• particle formation by carbon nucleation • oxidation • agglomeration
• • • •
• oxidation • agglomeration • thermophoresis
• exhaust temperature • particle concentration • exhaust system design
• physical removal of particles • chemical reactions of gas phase species
• ceramic particle trap • oxidation catalytic converter
• • • • •
• dilution ratio • vapor pressures • relative humidity and water vapor concentration • temperature • time
adsorption and condensation nucleation gas-to-particle conversion chemical reactions partitioning of hc species between particle and vapor phase
• photochemical reactions • particle surface reactions • gas-to-particle conversions
hydrocarbons (PAH) having 2 or more benzene or cyclopentane rings. In terms of size, the particulates can be grouped into the nucleation mode with particle sizes of less than 50 nm and into the accumulation mode with particle sizes in the 100-250 nm diameter range.5 Between their formation within the engine and their dispersion into the atmosphere, particulates undergo a number of transformation processes affecting their size and chemical composition. These processes are summarized in Table 1,6 and they include gas/solid phase conversions and photochemical reactions in the atmosphere. The measurement of the amount of PM emitted by combustion engines has relied on weight (gravimetric) methods that record the total weight of particles collected on a sample filter using procedures prescribed by standard regulations. However, in recent years there has been increased interest in the measurement of both particle number density and size distribution. This reflects awareness that smaller particles, such as those within the nucleation range, may be more harmful as they can penetrate deep into the lungs and overcome the body’s defenses. Because these nanoparticles contribute little to the total PM mass concentration, procedures for the measurement of number density and size distribution are attracting increasing attention in automotive research.7,8 These concerns over particle size and number density have been amplified by recent studies9-12 which show that most particles emitted from spark-ignited combustion engines
engine design-fuel injection/mixing operating conditions fuel state of maintenance
• gas species concentration • solar radiation • time
(SICE) are in the nanosized particle range (smaller than 50 nm diameter). These studies also showed that the number concentration of nanoparticles emitted by SICEs can approach that of compression-ignited combustion engines (CICE). A significant factor affecting particle size distribution and number concentration is the conditions of exhaust dilution, investigated by a number of studies.13-15 However, as yet a universally agreed sampling and dilution system have not been clearly defined. This may lead to significant difficulties in comparison of results from different studies because gas-toparticle conversion takes place during the dilution process. Therefore, more work is needed to establish consistent, welldefined sampling and dilution procedures for particle measurements in the nanosized range. The past decade has seen a rapid rise in interest in biomassbased renewable fuels as alternatives to conventional gasoline and diesel fuels. The biofuels with the largest present market potential in Europe currently seem to be biodiesel and bioethanol.16,17 Methyl esters of vegetable oils (e.g., methyl ester of soybean oil), known as biodiesel, are receiving increasing attention as an alternative fuel for diesel engines. Biodiesel is nontoxic, biodegradable, and a renewable fuel with potential to reduce some of the engine exhaust emissions. Several previous studies18-21 (13) Luders, H.; Kruger, M.; Stommel, P.; Luers B. SAE Trans. J. Fuels Lubricant 1998, 107(4), 527-534. (14) Abdul-Khalek, l. S.; Kittelson, D. B.; Brear F. SAE Trans. J. Fuels Lubricants 1999, 108(4), 563-571. (15) Wei, Q.; Kittelson, D. B.; Watts, W. F. SAE Trans. J. Fuels Lubricants 2001, 110(4), 247-258. (16) Verhaeven, E.; Pelkmans, L.; Govaerts, L.; Lamers, R.; Theunissen, F. Results of Demonstration and Evaluation Projects of Biodiesel From Rapeseed and Used Frying Oil on Light- and HeavyDuty Vehicles. SAE Paper 2005-01-2201; 2005. (17) Morita, K. JSAE Rev. 2003, 24, 1–5. (18) Choi, C. Y.; Bower, G. R.; Reitz, R. D. SAE Trans. J. Engines 1997, 106(3), 388-407. (19) Chang, D. Y.; Van Gerpen, J. H. Determination of particulate and unburned hydrocarbon emissions from diesel engines fueled with biodiesel. SAE Paper 982527; 1998. (20) Akasaka, Y.; Suzuki, T.; Sakurai, Y. Exhaust emissions of a DI diesel engine fueled with blends of biodiesel and low sulfur diesel fuel. SAE Paper 972998; 1997. (21) Krahl, J.; Munack, A.; Schrorder, O.; Stein, H.; Baxnger, J. SAE Trans. J. Fuels Lubricants 2003, 112(4), 2447-2455.
(5) Pagan, J. SAE Trans. J. Fuels Lubricants 1999, 108(4), 557-562. (6) Lee, J. W.; Jeong, Y. I.; Jung, M. W.; Cha, K. O.; Kwon, S. I.; Kim, J. C.; Park, S. S. Int. J. Automot. Technol. 2008, 9, 397–403. (7) Kittelson, D. B. J. Aerosol Sci. 1998, 29, 575–588. (8) Andrew, G. E.; Clarke, A. G.; Rojas, N. Y.; Sale, T.; Gregory, D. Diesel particle size distribution: The conversion of particle number size distribution to mass distribution. SAE Paper 2001-01-1946; 2001. (9) Kayes, D.; Hochgreb, S. Environ. Sci. Technol. 1993, 33, 3957–3967. (10) Maricq, M. M.; Podsiadlik, D. H.; Brehob, D. D.; Haghgooie, M. Particulate Emissions from a Direct-injection spark ignition engine. SAE paper 1999-01-1530; 1999. (11) Hall, D. E.; Dickens, C. J. SAE Trans. J. Fuels Lubricants 1999, 108(4), 1603-1613. (12) Graskow, B. R.; Kittelson, D. B.; Ahmadi, M. R.; Morris, J. E. SAE Trans. J. Fuels Lubricants 1999, 108(4), 602-609.
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have focused on the exhaust emissions with biodiesel and biodiesel/fossil diesel blends, and several of them noted a higher NOx emission with biodiesel blends. Some of the studies that compared particulate matter size distributions for biodiesel and conventional diesel fuel did not optimize the engine performance for the respective fuels. Ethanol is a biobased renewable and oxygenated fuel that has been the subject of several studies22-26 reporting improvement in engine performance and reductions in emissions of carbon monoxide and hydrocarbons. However, several of these studies were carried out on gasoline and diesel engines with combustion systems and control technologies optimized for conventional fuels. In summary, a large volume of research has been carried out concerning (1) the effect of nanosized particles on health, (2) the composition of particulates and the formation and oxidation processes in the internal combustion engine, (3) the particle size distribution and the relation of number concentration with total PM mass, (4) particle emissions from SICE, (5) particulate sampling and dilution systems, and (6) the effect of biofuels on NOx and particle emissions. Although there are many detailed studies on either SICE or CICE, comparisons between these different engine types with respect to particulate analysis with different fuels are scarce. The study reported here first investigated the effects of alternative biofuels on nanoparticle number emission (5-1000 nm diameter) and, second, compared quantitatively particle emissions from SICE and CICE. The SICE and CICE used for the investigations were both single cylinder and had the same displacement of 500 cm3. Both had the latest generation of combustion chamber designs and were both equipped with direct fuel injection systems. The comparisons between the two engines were made with conventional hydrocarbon fuels, including gasoline and diesel, as well as with alternative biofuels, including E85 (mixture of 85% ethanol and 15% gasoline, sometimes called gasohol) and BD100 (100% biodiesel, i.e. soybean methyl ester). It was considered necessary to keep the same exhaust displacement volume in both engines in an attempt to keep the particle residence time in the exhaust systems closer.
Table 2. Specifications of the Two Single-cylinder Engines with Same Displacement Volume
Table 3. Physical and Chemical Properties of Gasoline, E85, ULSD, and BD100 Fuels properties
gasoline
chemical formula carbon (wt %) hydrogen (wt %) oxygen (wt %) Density (kg/L at 20 °C) octane number cetane number Stoi. A/F ratio LHV (MJ/kg) autoignition temp. (°C)
CnH1.87n 85 15 0 0.732 95 14.7 43.8 257
E85 57 13.5 30 0.783 105 9.86 29.2
ULSD
BD100
CnH1.8n 86 14 0 0.84
methyl ester 77 12 11 0.88
40-50 14.6 42.5 250
55 12.3 37.3 178
torque) ignition timing was used for both PFI and DI engine operation. The 500 cm3 single-cylinder direct-injection diesel engine was constructed using the cylinder head, piston assembly, linear and common-rail fuel injection system (maximum 1300 bar injection pressure) of a production 2.0 L (4-cylinder) automotive diesel engine (Ford Puma), all assembled onto a Ricardo Hydra engine test base. The compression ratio for the spark-ignited engine was 10:1 and that for the diesel engine was 15:1, and both engines were naturally aspirated. Specifications for the above two engines are also shown in Table 2. The fuels used for this study are shown in Table 3 and they were standard unleaded 95RON gasoline and E85 for SICE, and ULSD and BD100 for CICE. The ethanol concentration in the E85 blend was on a volume basis and represented an oxygenate/gasoline blend to meet clean fuel requirements.27 The ULSD (Ultralow sulfur diesel) fuel used had less than 10 ppm sulfur content. The BD100 fuel contained ester molecules produced by trans-esterification reaction of soybean oil with monohydric alkyl alcohol. Particle size distribution between 5 and 1000 nm and number density were measured with a DMS (differential mobility spectrometer) 500 system. The measurement principle
2. Experimental Apparatus and Test Procedure The single-cylinder spark-ignited engine had a displacement of 500 cm3, four valves, pent-roof combustion chamber design, and was equipped with two different fuelling systems, port fuel injection (PFI) and spray-guided direct injection (DI) type. The PFI six-hole injector was mounted on the siamesed bifurcated intake port so that three fuel streams were directed on the back of each inlet valve. The injector was supplied with fuel at 3.5 bar, with closed-valve start of fuel injection timing at TDC compression. The DI six-hole injector was centrally mounted in the combustion chamber and was supplied with fuel at 120 bar. MBT (minimum spark advanced for best (22) Dodge, L. G.; Shouse, K.; Grogan, J.; Leone, D. M.; Whitney, K. A.; Merritt, P. M. SAE Trans. J. Fuels Lubricants 1998, 107(4), 459-469. (23) Ahmed, I. Oxygenated diesel: Emissions and performance characteristics of ethanol- diesel blends in CI engines. SAE Paper 2001-012475; 2001. (24) Sandquist, H.; Karlsson, M.; Denbratt, I. Influence of ethanol content in gasoline on speciated emissions from a direct-injection stratified charge SI engine. SAE Paper 2001-01-1206; 2001. (25) Malhotra, R. K.; Raje, N. R. Bio-fuels as blending components for gasoline and diesel fuels. SAE Paper 2003-26-0011; 2003. (26) Al-Hasan, M. Energy Convers. Manage. 2003, 44, 1547–1561.
(27) Poulopoulos, S. G.; Philippopoulos, C. J. Trans. ASME 2003, 125, 344–350.
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Figure 1. Schematic diagram of the DMS500 classifier column and charger.28,29
Figure 3. Fuel injection pressure (bar) map for the CRDI CICE.
Figure 2. DMS Dynamic particle spectrum for successive engine cycles (2500 rpm, IMEP 6.3 bar, ULSD) (vertical scale shows particle number and horizontal scale is particle size in nm). Table 4. Basic Experimental Conditions (a) Engine Speed: 1500 rpm (IMEP 4 bar) engine type injection type injection pressure (bar) SICE CICE
PFI DI CRDI
3.5 120 500
Figure 4. In-cylinder combustion pressure (ULSD) in a compression-ignition engine.
injection timing
ratios. The second stage dilution ratio was set to 10:1 for the SICE and 30:1 for the CICE experiments so as to maintain the particle concentration within the dynamic range of the DMS 500 (see Figure 2). Each of the two 0.5 L displacement single-cylinder, naturally aspirated engines was coupled to a dynamometer and were run under steady-state conditions. In-cylinder gas pressure transducers enabled monitoring of the IMEP, which was set to the values shown in Table 4. The SICE engine could be operated with either PFI or DI fuel injection, and the CICE engine was equipped with a high-pressure common rail directinjection (CRDI) system. Investigations were carried out at two engine speeds of 1500 and 2500 rpm and at two loads corresponding to 4 and 6.3 bar IMEP. The temperature of the naturally aspirated intake air was controlled at 25 ( 3 °C, while the coolant temperature was maintained at 80 ( 2 °C. For the SICE experiments a stoichiometric AFR (air-fuel ratio) was maintained using an AFR analyzer in conjunction with an exhaust oxygen sensor, and the fuel flow supplied to the engine was measured with a gravimetric fuel balance. The two PFI injection timings used were set at TDCcompression (PFI_IVC), whereby fuel was injected after the intake valve had closed and at 45 CAD ATDCintake (PFI_IVO), whereby the intake-valve was open. For direct injection, fuel injection timing was varied from 370 to 420 CAD during the intake
360 TDCcompression 400 BTDC 4.5 BTDC
(b) Engine Speed: 2500 rpm (IMEP 6.3 bar) engine type injection type injection pressure (bar) SICE CICE
PFI DI CRDI
3.5 120 1000
injection timing 360 TDCcompression 380 BTDC 14 BTDC
of the DMS system is based on mobility differences of particles having different sizes/charges and subjected to electrostatic and flow resistive forces (see Figure 1). A cyclone separator received the exhaust gas sample from the engine and removed particles having a diameter over diameter 1000 nm. Downstream of the cyclone a stainless steel restrictor reduced the sample pressure from near atmospheric to about 250 mb absolute. This restictor was cleaned frequently during the experiments. The sample of exhaust gas then passed through a corona discharge charger and into the classifier column. The system has two dilution stages. The first stage was set to a constant dilution ratio of 4:1 and used dry air at 100 °C in order to prevent the condensation of moisture and volatile matter found in the exhaust gas. The second dilution stage used a cavity-rotating disk diluter to achieve various dilution 4366
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: DOI:10.1021/ef9004708
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Figure 6. Specific fuel consumption between ULSD and BD100 fuels used in CICE.
Figure 7. Effect of injection type on particle size spectral density for different engine speed (gasoline fuel).
Figure 5. Effect of injection type and timing on specific fuel consumption with gasoline and E85 fuels.
of course due to the lower heating value of ethanol. The specific fuel consumptions with ULSD and BD100 fuels at the same operating conditions in the CICE are shown in Figure 6. As expected, the biodiesel fuel had a higher specific consumption mainly due to its lower heating value. Particle emissions for the SICE are shown in Figure 7 for the two injection types (PFI and DI with gasoline) and two combinations of engine speeds and loads. Most particles were within the size range of 5-200 nm. Within this size range, the particle number density with DI was generally much higher than with PFI regardless of engine operating conditions. DI operation with gasoline in the SICE produced accumulation mode particles having maximum concentration at about 100 nm. The formation of these particles in the DI engine is likely to relate to rich mixture regions arising from imperfect mixing of the stoichiometrically proportioned fuel and air streams supplied to the engine. It is expected that these particulates would contain a high proportion of elemental carbon. With PFI, most particles appeared to be under about 10 nm in size. The injection of the stoichiometrically metered PFI gasoline spray onto the back of the inlet valves is expected to have produced a more homogenized stoichiometric mixture than that obtained with DI operation. It is possible that these PFI particulates, having sizes below 10 nm, were composed mostly of volatile material.
stroke while the A/F was maintained always at the stoichiometric level. For the CICE experiments the fuel injection pressure was set according to the engine manufacturer’s map as shown in Figure 3. The injection timing was adjusted to maintain the location of the cylinder gas pressure peak between 5 and 10 CAD ATDCcompression (COV was less than 2%), as shown in Figure 4. Particle and regulated emissions were sampled 250 mm downstream of the exhaust manifold. Hydrocarbon (HC) and NOx emissions were measured continuously in the undiluted exhaust gas with an exhaust gas analyzer (Horiba MEXA 9100HEGR). HC was sampled through heated lines at a temperature of 190 °C. 3. Results and Discussion Figure 5 shows the specific fuel consumption for the SICE engine running on gasoline and E85 fuels at four engineoperating conditions and two types of injection (port fuel injection and direct injection). As mentioned above, stoichiometric mixture and MBT spark timing were maintained throughout. DI had a slightly better fuel consumption with E85 than PFI. The lower gasoline specific fuel consumption is (28) DMS500 User Manual, Version 2.0.; Cambustion Ltd. (29) Symondsa, J.; Reavella, K. S.; Olfertb, J. S.; Campbella, B. W.; Swiftb, S. J. J. Aerosol Sci. 2007, 38, 52–68.
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Figure 10. Comparison of mean particle diameter and total particle number concentration with different engine operating conditions between gasoline and E85 fuels.
Figure 8. Comparison of particle size spectral density with different DI engine operating conditions between gasoline and E85 fuels.
Figure 9. Comparison of particle size spectral density with different engine operating conditions between ULSD and BD100 fuels.
Figure 11. Comparison of mean particle diameter and total particle number concentration with different engine operating conditions between ULSD and BD100 fuels.
Figure 8 shows the effect of E85 fuel on particle size distribution at different engine speed and load conditions with DI. With the E85 fuel, particle number density decreased considerably compared with gasoline. Possible reasons for the lower particle density in the case of E85 could include: the 30% oxygen bound in the ethanol molecule helping particle oxidation; molecules in gasoline (e.g., isomers, alkenes, and aromatic) which are known to produce more soot particles; and a lower proportion of the less-volatile gasoline fractions that might have produced localized rich mixture regions. Particle emissions from the CICE with the ULSD and BD100 fuels are shown in Figure 9. The BD100 fuel generally produced considerably lower particle density in the exhaust. However, the particles with BD100 fuel tended to be of smaller size and more numerous than those from ULSD in the
