Article pubs.acs.org/EF
Effects of Ethanol on In-Cylinder and Exhaust Gas Particulate Emissions of a Gasoline Direct Injection Spark Ignition Engine Mohammad Fatouraie,*,† Margaret S. Wooldridge,† Benjamin R. Petersen,‡ and Steven T. Wooldridge‡ †
Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States Research and Advanced Engineering, Ford Motor Company, Dearborn, Michigan 48124, United States
‡
S Supporting Information *
ABSTRACT: The effects of ethanol on reducing the particulate emissions of a direct injection spark ignition (DISI) engine were investigated using a single-cylinder, optically accessible engine. Neat anhydrous ethanol was compared with a baseline fuel of reference grade gasoline. A high speed camera was used to record crank-angle resolved in-cylinder images of the fuel spray, combustion, and thermal radiation from the soot formed for the engine operating conditions studied. Particulate emissions in the engine exhaust gas were also measured using opacity measurements (i.e., using a smoke meter). All experiments were conducted at the same load conditions with a net indicated mean effective pressure of IMEPnet ≈ 5.5 bar and an intake manifold absolute pressure of 76 kPa. The engine speed was fixed at 1500 rpm, and the fuel injection duration was controlled to achieve stoichiometric combustion. Spark timing was adjusted to target combustion phasing (CA50) of 8° aTDC. The effects of engine coolant temperature and fuel injection timing on fuel spray characteristics and soot formation were studied for each fuel. The imaging data indicate that soot formation is a strong function of liquid fuel impingement on the piston surface, consistent with previous in-cylinder imaging studies of soot formation in DISI engines using different engine hardware and imaging orientation. A quantitative metric was applied to the imaging data of this work, and the in-cylinder soot formation based on the imaging data were in good agreement with the engine-out smoke measurements, indicating the optical imaging results are representative of engine-out particulate emissions. Higher coolant temperatures and later fuel injection timing significantly mitigated the incylinder soot emissions for both fuels by altering the fuel spray interactions with the piston surface and cylinder liner. As expected based on previous studies, ethanol systematically produced less soot than gasoline for each operating condition. Features of the fuel spray roll-up were identified as important indicators of pool fires on the piston surface for the earliest fuel injection timings.
1. INTRODUCTION The attractive features of higher thermal efficiency, lower fuel consumption, and lower CO2 emissions of direct injection spark ignition (DISI) engines have increased the DISI market share.1 The technology is considered one of the pathways to meet corporate average fleet emission (CAFE) targets; however, particulate matter (PM) emissions of DISI engines compared with port fuel injection (PFI) engines can be exacerbated by the possibility of higher surface wetting of combustion chamber surfaces by the DI fuel spray. Studies have shown that PM emissions of DISI engines can be higher than the PM emissions of port fuel injected gasoline2−4 if, for example, calibration parameters such as start-of-injection timing lead to excessive piston wetting. Previous studies of DISI engines have shown that piston and wall wetting by the liquid fuel spray can lead to pool fires that become significant sources of PM.5,6 In general, soot formation is a complex function of the fuel properties and the in-cylinder combustion physics which affect the local in-cylinder equivalence ratio and temperature.7 Further understanding and characterization of the underlying physical and chemical mechanisms of PM formation in DI combustion systems is critical to achieving clean emissions. Ethanol has the potential to suppress soot formation in engines,2,8,9 and some methods to produce ethanol are making the fuel more attractive as a sustainable transportation fuel that may reduce life cycle carbon emissions and not compete with © 2015 American Chemical Society
food crops. Ethanol has been blended with gasoline in the United States and Europe to increase the biofuel share of the energy portfolio and to reduce dependence on crude oil. In countries such as Brazil, with significant biofuel production infrastructure, ethanol is being used in vehicles as a neat fuel, i.e. without blending with gasoline. Some important thermophysical properties of ethanol differ from the properties of gasoline, impacting engine performance. Ethanol has a higher laminar flame speed compared with isooctane10,11 which results in shorter combustion duration and therefore higher thermodynamic efficiency.12,13 Caton13 also indicated that the less complex chemical structure of ethanol compared with iso-octane results in lower exergy destruction. Lower NOx and unburned hydrocarbon (UHC) emissions have been demonstrated using ethanol as well,14 outcomes which are attributed to lower combustion temperatures and a lower boiling point compared with gasoline. Ethanol has been demonstrated to favor reaction chemistry which is intrinsically less likely to produce PM. Kasper et al.15 investigated differences in ethanol versus hydrocarbon combustion chemistry and observed a strong ability of ethanol to suppress the formation of benzene as well as some higher aromatic species, species considered precursors or building Received: December 9, 2014 Revised: April 2, 2015 Published: April 16, 2015 3399
DOI: 10.1021/ef502758y Energy Fuels 2015, 29, 3399−3412
Article
Energy & Fuels blocks for PM. Barrientos et al.16 measured the sooting tendency of a range of fuels and fuel blends and showed that ethanol leads to a decrease in the sooting tendency of ethanol/ gasoline blends. High content ethanol fuel (E85) has also been demonstrated to reduce PM number density and PM mass.2,9 Of further note, however, is that particulate emissions of engines operating on ethanol blends are also highly dependent on the gasoline blendstock. Outside of the beneficial chemical characteristics of ethanol with regard to particulate formation, it is possible for ethanol blends to have a higher propensity of PM formation if, for example, the blendstock is formulated such that it contains higher concentrations of low volatility, aromatic hydrocarbons.17 The fuel properties affecting charge preparation, in particular spray breakup, atomization, and vaporization, play important roles on fuel impingement on combustion chamber surfaces and on thermal and compositional charge stratification. The higher kinematic viscosity of ethanol affects the turbulence induced by the spray and spray breakup. The lower heating value of ethanol results in a larger volume of ethanol injected in each cycle compared with gasoline to develop equivalent power. These properties coupled with the significantly higher enthalpy of vaporization and lower boiling point of ethanol affect the spray pattern, spray tip penetration, and mixing,5,18,19 which in turn affect the liquid fuel spray impingement on the cylinder wall and piston surfaces, which affect the likelihood of sooting in DISI engines. Experiments to capture time-resolved in-cylinder imaging of the fuel spray properties and the corresponding soot formation have been conducted for gasoline6,20,21 and bioethanol.22−25 These studies identified pool fires of fuel on piston surfaces as a primary source of soot in DISI engines using side mounted23,26 and centrally mounted fuel injectors.6 Mittal et al.27 used a pulsed copper vapor pulsed laser to characterize the spray impingement of E85, E50, and gasoline in a centrally mounted direct injection engine. They showed that all of the fuels exhibited similar piston impingement. For the centrally mounted injectors, the fuel impingement intensity was strongly correlated to the spray cone angle. Aleiferis et al.28 investigated the spray development of gasoline and ethanol in a direct injection engine and observed few differences between the sprays at cold coolant conditions, which was supported by their observations in a constant volume chamber.29 However, for side mounted injectors, the rebound of the fuel off the piston was expected to be a more significant factor. Catapano et al.22 studied the effects of engine speed on PM emissions of ethanol gasoline blends. They measured the number concentration and size distribution of the particles produced by ethanol−gasoline blends with the same end of injection (EOI) using a differential mobility spectrometer. They observed a linear decrease in both the particle size and number concentration as the ethanol percentage in the blend increased for the cases with no significant fuel impingement at 2000 rpm. But as the engine speed was increased to 4000 rpm, the longer injection durations for ethanol blends resulted in more impingement with the piston, and the time available for vaporization was decreased, which increased the particle number and sizes for the ethanol−gasoline blends. This trend has also been reported by Di Iorio et al.30 and Lee et al.31 for the ethanol−gasoline blends. Catapano et al.22 also observed that while some soot was formed during combustion away from the chamber walls, the soot formation was dominated by piston wetting as the flame
front reached the wetted surfaces. They reported increasing ethanol in the blend decreased soot emissions systematically at a 2000 rpm full load, but not at 4000 rpm, in which the ethanol blends actually increased soot emissions while keeping the same EOI. Their results indicate the formation and vaporization of the fuel film dominates soot formation despite the chemical tendency of ethanol to produce less soot than gasoline. Piston wetting can increase with ethanol versus gasoline due to the lower energy content of the ethanol, which requires the injection duration of the blends to be longer to achieve the same stoichiometric condition. Further, if the EOI is kept fixed, a more advanced start of injection (SOI) is required when the piston is closer to the injector, which increases the fuel impingement with the piston, increasing the risk of creating high sooting conditions. Few studies have imaged the combustion chamber through transparent cylinder liners with a metal piston, as opposed to the piston view imaging through a transparent window in the piston crown. As observed in this study, piston-view imaging of the combustion chamber through an optical piston can dramatically change the soot producing characteristics of the engine due to the differences in the thermal conductivity and surface−fuel interactions between metal and optical pistons. Side-view imaging of the combustion chamber also provides important spatial information which is used in this work to study the cycle-to-cycle variability in the soot formed and the spray features of the fuels. This study builds on the previous work reported in Fatouraie et al.,5 where an optically accessible single cylinder engine was used to acquire crank-angle-resolved imaging data of the fuel spray and PM formation in the engine. The previous work considered gasoline/ethanol blends and identified pool fires on the piston crown as the critical source of in-cylinder soot formation. The effects of fuel injection timing and engine coolant temperature on the fuel spray and soot formation were evaluated in the current work. Engine-out smoke measurements were also performed to identify links between the in-cylinder PM imaging and the PM engine-out emissions measurements.
2. EXPERIMENTAL APPROACH An optically accessible single cylinder engine with a Bowditch piston design was used in this study. The engine specifications are summarized in Table 1. A schematic of the experimental setup is
Table 1. Single Cylinder Engine Specifications displacement [cm3] bore [mm] stroke [mm] nominal compression ratio number of valves intake valve opening (IVO) [° aTDC] intake valve closing (IVC) [° aTDC] exhaust valve opening (EVO) [° bTDC] exhaust valve closing (EVC) [° aTDC]
506 89.0 81.4 9.4 4 11 247 186 38
shown in Figure 1. The cylinder head was a modified design based on a modern direct injection gasoline engine featuring four valves with dual overhead camshafts and a side-mounted fuel injector and a centrally mounted spark plug. The standard metal cylinder liner for the engine was 140 mm in height. Side-view imaging was achieved in this study by replacing the metal cylinder liner with a two-piece liner where the upper portion of the cylinder liner was a fused silica transparent section 25 mm in height and the bottom portion was a metal liner 3400
DOI: 10.1021/ef502758y Energy Fuels 2015, 29, 3399−3412
Article
Energy & Fuels
alcohol USP 200 proof) with a purity ≥99.9% was used and designated as E100. The fuel properties are shown in Table 2. The fueling system was purged twice with the new fuel before each set of tests with a new fuel.
Table 2. Fuel Propertiesa
Figure 1. Schematic of the optical engine setup.
a
115 mm in height. An aluminum piston with a low ring pack was used to eliminate the interaction between the piston rings and the optical liner. The experiments were performed at fixed valve timing set by the independent variable camshaft timing system. The event times are provided in Table 1. Cylinder pressure was measured using a piezoelectric transducer (Kistler 6052A) and charge amplifier (Kistler 5010B). Absolute manifold pressure was measured using a Druck PMP-2060 transducer and the intake pressure was measured with a Kistler 4045A2 transducer and a Kistler 4618 amplifier. A hydraulic dynamometer (Micro-Dyn 35) equipped with an automatic control system was used to maintain the desired engine speed by driving the engine or absorbing net power output, as required. The experiments were conducted at a constant engine speed of 1500 rpm. Stoichiometric combustion was targeted for the experiments, and the stoichiometry was controlled by adjusting the fuel injection duration (specifically the fuel injector driver pulse width) at a fixed intake manifold absolute pressure of 76 kPa and a fixed fuel rail pressure of 100 bar. The spark timing was adjusted to maintain a fixed combustion phasing of CA50 ≈ 8° aTDC. The following conditions were used in the study. The test cell temperature and humidity were controlled to constant conditions of 21 °C and 30% relative humidity. The intake temperature (at the intake plenum) for all of the cases was fixed at 22 ± 1 °C. The fuel/air equivalence ratio was measured based on the oxygen concentration in the engine-out exhaust emissions using a lambda meter (ETAS LA4) with a broadband lambda sensor (Bosch LSU 4.9). The lambda meter settings were changed for each fuel using the appropriate C/O and C/ H ratios. The engine was not equipped with any exhaust aftertreatment devices, and the engine-out emissions (CO, UHC, CO2, and NOx) were measured using an automotive emissions analyzer (Horiba MEXA-584L). The reported UHC emissions represent the hexane equivalent values. The engine-out particulate matter (PM) emissions were measured using an opacity meter (AVL 415 smoke meter) with a 6 s sample duration (sample volume of 1080 cm3), and a filter smoke number (FSN) was calculated based on the blackening index of the filter. Exhaust plenum temperature significantly affected the smoke meter reading. The sensitivity of paper blackening to exhaust temperature is well-known.32 Therefore, the exhaust plenum temperature was controlled to ∼80 ± 2.5 °C for these experiments, which was the quasi-steady temperature during continuous firing. The exhaust plenum was heated to this temperature by operating the engine at a lean condition and retarding injection timing (to suppress sooting) until the exhaust plenum was stable at 80 °C. The operating conditions were then switched to the actual test conditions. Indolene, a reference grade gasoline (EPA Tier II EEE, analyzed at 86.46% by weight carbon,
