Article pubs.acs.org/EF
Combustion and Emissions Characterization of Biodiesel Blends in a City-Car Engine Giancarlo Chiatti,† Ornella Chiavola,*,† Fulvio Palmieri,† and Stefano Albertini‡ †
Engineering Department, “Roma Tre” University, via Della Vasca Navale, 79, 00146 Rome, Italy DP Lubrificanti srl, via della Meccanica, 16, 04011 Aprilia, Latina, Italy
‡
ABSTRACT: Increasing attention has been devoted to the use of biodiesel fuel in internal combustion diesel engine due to its positive attributes as compared to the other types of fuel: e.g., being a renewable source, non-petroleum-based, with lower carbon monoxide, hydrocarbon, and particulate matter emissions. This work investigates the performance and the air emission of a small displacement engine fueled with blends of distilled biodiesel (from this point forward biodiesel only) and ultralow-sulfur diesel fuel up to 40% by volume. The considered engine plays a leading role in city cars and urban vehicles; the urban congestion and the antipollution regulations for urban vehicles make this kind of engine very attractive in the near future, especially if it will be fueled with biodiesel blends for their potential of reducing the pollutant emissions in urban areas. The first part of this work aims at comparing the results obtained with biodiesel blends and those determined using diesel fuel as a reference under various operating regimes without any modification to the injection process. The impact of biodiesel blend fuels on the engine’s power, specific fuel consumption, and emissions is analyzed. The second part of this work aims at investigating the influence of a variation of the injection parameters on the performance and emissions of the engine using biodiesel blends (20% and 40%). Five engine operation modes are considered, in which the engine is tested with split injection (preinjection and main injection) and various preinjection and main injection timings and durations.
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INTRODUCTION The continuous tightening of regulations on engine exhaust emissions has led to new solutions capable of reducing nitrogen oxides (NOx) and particulate matter (PM) and also limiting fuel consumption and related costs. It is well-known that performance and emission characteristics are strongly governed by injector design that has a key role in the atomization and spray development.1,2 A number of researchers have investigated the effects of the injection parameter settings on diesel engine performance and emission characteristics.3−6 According to these authors, NOx emissions are affected by injection timing and pressure. The increase of pressure and the advanced injection timing cause the gas temperatures to rise during the combustion process. A more homogeneous mixture is therefore realized, with an increase of NOx emissions due to the kinetic constants and oxygen concentration of the Zeldovich mechanism. Soot formation is enhanced by heterogeneous fuel/air mixtures and fuel droplets size. The increase of injection pressure and the advanced injection timing determines a decrease of soot emission.7 In order to reduce harmful diesel engine emissions, considerable attention has been devoted to the investigation of the impact of fuel composition on diesel emissions. Numerous types of fuels have been alternatively proposed, and many studies have demonstrated the suitability of renewable nature fuels made from agricultural products to be used in diesel engines. The key advantages of biodiesel fuel include reduction in the undesirable exhaust emissions (NOx and PM) and, more in general, a lower petroleum dependence. However, the use of the biodiesel fuels is limited by several problems, e.g. the high viscosity of vegetable oils and their © 2014 American Chemical Society
derivatives (methyl or ethyl esters) and also the component degradation after a long engine operation. To overcome these problems, blends of vegetable oils with the normal diesel fuel are used in engines without any modification of the system’s hardware. Recent literature years have demonstrated that physical properties, chemical composition, and the structure of fuel deeply affect the injection process, the fuel ignition, the combustion process evolution, and the exhaust emissions.8−10 A number of researchers have investigated the performance and emissions characteristics of vegetable oils and biodiesel blends when fueling diesel engines. Numerous studies have been performed to better understand the particulate formation and oxidation process11−13 and to analyze in detail the effect of oxygenated fuel blends in the mechanism of soot emission: e.g., the reduction due to either a suppression of the soot formation mechanism or an enhancement of the particulate oxidation, or to a combined effect. The experimental works have demonstrated that it is a very challenging task to simultaneously reduce NOx and PM emissions. Literature7,14−16 has shown that oxygenated fuels reduce PM emissions as a consequence of both the oxygen content in the fuel and also the chemical structure of the fuel molecule. The reduction of such an emission mostly comes along with the increase of NOx emissions. In addition, the experimental activity has proven that the specific engine configuration and the engine testing (fuel injection process) is responsible for somehow contradictory conclusions concerning NOx, CO and unburned HC emissions.14,15,17−19 This may be due to some factors such as Received: February 20, 2014 Revised: July 3, 2014 Published: July 6, 2014 5076
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the biodiesel feedstock, blend ratios, and the fuel injection system which have a large influence on the characteristics of the injection process and then on the spray pattern.14,20 Performance and emissions of an engine fueled with either petroleum or biodiesel fuels are significantly influenced by the design of the engine and the operating speed and load values.14,21 The main aim of the present work is to analyze the performance and emission characteristics of a small displacement, a direct injection “common rail” diesel engine, a light and compact engine that has a leading role in city cars and urban vehicles being a light and compact system. Whereas in the available literature, most of the researches addressed the multicylinders diesel engine of large displacement;7,22−27 only some works have investigated the light duty engines, designed for agricultural purpose and mainly tested for a fixed value of the engine speed.28−30 The urban congestion and the antipollution regulations on urban vehicles make this kind of engine very attractive in the next future, especially if it will be fueled with biodiesel blends for their potential of reducing the pollutant emissions in urban areas. This work aims at providing additional insight into the investigation of the effect on both performance and emission during an experimentation of blends in which a second generation biodiesel (advanced biofuel) is used. This biodiesel was obtained starting from a mixture of used cooking oil (UCO), a source not suitable for human consumption. Due to its poor quality, UCO requires additional steps to become suitable with respect to a product obtained from the refined vegetable oil. The employment of UCO is able to guarantee substantial benefits concerning environmental and social impacts in comparison to biofuels from food crops (first generation of biodiesel). Since the behavior of the engine may not be necessarily linear or consistently scaling with respect to the blend ratio between biodiesel and petroleum diesel,23 the experimentation was performed by fueling the engine with biodiesel and petroleum diesel in blends of various percentages. The main aims of the study where (1) to analyze the influence of the biodiesel fuel blends on the engine performance and emissions as compared to low-sulfur diesel (ULSD) fuel when the injection process is not being modified and (2) to establish the effect of the fuel injection strategy (injection quantities and timings) on the performance and emissions when the engine is operated with biodiesel blends.
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Table 1. Engine Specifications engine type cylinders displacement bore stroke compression ratio maximum power maximum torque
four stroke, direct injection, naturally aspirated, water-cooled 2 440 cm3 68 mm 60.6 mm 20:1 8.5 kW at 4400 rpm 25 N m at 2000 rpm
Figure 1. Experimental setup.
Pressure sensors were used to measure the instantaneous pressure in the cylinder (piezoelectric transducer AVL GU13P) and in the intake (piezoresistive transducers Kistler 4007BS5F) and exhaust systems (piezoelectric water-cooled transducers AVL QC43D). The in-line injection pressure was measured through a high-pressure piezoelectric transducer (Kistler 4067A2000). Thermocouples K were used for temperature measurements along the intake and exhaust systems. The engine exhaust emissions (CO, CO2, HC, O2, and NOx expressed as NO equivalent) were characterized by means of Bosch BEA352. AVL particle counter (APC) and AVL micro soot sensors were used to measure the nonvolatile particle number concentration and the soot concentration in the engine exhaust gas, respectively. All of the signals were simultaneously acquired by National Instruments data acquisition devices type 6110 (for the analogical signals) and 6533 (for the optical angle encoder signals). During the tests, the sampling rate was varied according to the engine speed value in order to guarantee a fixed angular resolution of 0.125 crank angle degree (cad). The data acquisition was managed by means of a custom program developed by the authors26 in the LabVIEW10 environment.
EXPERIMENTAL METHODOLOGY
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Test Apparatus. The experimentation was performed on a twocylinder water-cooled diesel engine (Lombardini LDW442CRS) equipped with a common rail fuel injection system. The injection pump delivers fuel with a maximum pressure of up to 80 MPa. The electronically controlled injector has 5 holes, the diameter of each being equal to 0.123 mm. The injection parameters (strategy, timing, and duration of each shot) were controlled by means of a fully opened electronic control unit (ECU) connected to a personal computer. The engine, whose main technical data are summarized in Table 1, was connected to an asynchronous motor (SIEMENS 1PH7; nominal torque, 360 N m; power, 70 kW; control system, SIEMENS SINAMICS S120). Figure 1 shows the complete engine setup. Torque measures were carried out by means of HBM T12. The engine speed and the crank angle position were determined by an optical encoder AVL 364C. An AVL gravimetric balance was used to supply the fuel and to measure the fuel consumption.
TEST FUELS AND MEASUREMENTS In the experimentation, three biodiesel blends were investigated: (1) B10 (90% ULSD and 10% biodiesel, by volume), (2) B20 (80% ULSD and 20% biodiesel, by volume), and (3) B40 (60% ULSD and 40% biodiesel, by volume). Initially, it was determined the highest value of the blend ratio of biodiesel to ULSD that could be experimentally investigated without the need of modification to the engine hardware: the value was found to be B40, due to degradation of the rubber hoses/seals in the engine fuel system. Since the aim of this work was to investigate the potential use of biodiesel blends in a small displacement diesel engine in agreement with the commercial purpose of the engine, the experimentation was performed with a biodiesel percentage lower than 40% by volume. This allowed the investigated blends to be ready for use in actual engine 5077
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arrangements with no additional analysis on the materials reliability. In this work, a second generation biodiesel was used, which was obtained starting from a mixture of waste oil (used cooking oil, UCO) and byproducts from refining of vegetable oils (acid oils). Due to their poor quality, the materials require some treatments to become similar to a product obtained from refined vegetable oils (first generation biodiesel). Indeed, UCO cleaning was applied to remove solid particulates and water-soluble substances: for instance, a metallic sieve, mesh 18, was used to remove particulates coming from food frying. Then, in a stirred vessel, the UCO was heated to 80 °C by steam, using an external heat exchanger, and added with 7% by weight of water. The blend was kept mixed for 1 h before being clarified in a two-stepwise separation process. A first stage self-cleaning disk separator (able to periodically discharge the separated solids at full speed) was used to remove 90% of the water containing the water-soluble matter and solids that escaped through the first sieve; a second stage disk separator machine was used to remove the water left over. Then, physical deacidifcation was also needed to remove organic free acidity (FFA) due to the product deterioration as a consequence of the use in food cooking. Column operation under high-vacuum conditions was used to strip out the FFA, by means of steam countercurrent injected at the bottom of the column. The used steam was then collected and condensed at the top of the column, whereas the deacidified UCO was recovered at the bottom. For acid oils, having average acidity above 50% by weight, the removal of FFA occurs through the chemical process of esterification with glycerol. Then, in a stirred reactor, acids oil and glicerine are recombined back to triglycerides using a Tytan-based catalyst to shift the equilibrium to the product side. The neutralized products can then be converted via a standard transesterification process in which the triglycerides are converted into methyl esters, with the addition of methanol in the presence of an alkaline catalyst, freeing the glycerin linked to them. The resulting raw biodiesel, coming from poor raw material, was distilled in order to comply with the reference specifications of biodiesel (EN 14214). The properties of the biodiesel and ULSD are listed in Table 2 (data were obtained according to EN ISO 3675 (density), EN
Table 3. Biodiesel Composition
biodiesel
ULSD
density (kg/m at 15 °C} viscosity (cSt at 40 °C) lower heating value (MJ/kg) cetane no.
877 4.4 37.1 56
830 2.5 43.1 52
3
biodiesel
carbon hydrogen oxygen sulfur
0.812 0.065 0.117 0.006
The differences in the chemical and thermal−physical properties of the fuels influence spray development, droplet atomization, and injector dynamics, leading to a difference in the combustion process development; combustion efficiency and emissions are therefore affected. Before each new fuel was tested, sufficient time was given to consume the remaining fuel in the fuel supply system. For each blend, the experiments were performed by imposing values of speed and load selected in order to cover the engine’s complete operative range. In the real conditions of application, the engine is mainly used in microcars, small commercial and leisure vehicles. Furthermore, it is equipped with a continuously variable transmission (CVT), which is able to guarantee an operation with improved fuel economy and low exhaust emission, better than the conventional transmission, in which the ratio changes in stages by shifting gears. The experimental tests were performed with the aim to analyze the following: (1) the effects of the biodiesel fuel blends on the engine performance and emissions compared to ULSD fuel; (2) to establish the effect of the fuel injection strategy (injection quantities and timings) on the performance and emissions when the engine was operated with biodiesel blends. A two-shot injection pattern was implemented for each engine operating condition investigated; the timing and phasing of the shots were adjusted at each operating condition. Moreover, although ECU in the standard configuration delivers a fixed quantity (equal to 1 mm3/stroke) of preinjected fuel regardless of the engine operative conditions, variations of the quantity of fuel delivered during each shot were considered. The results presented in the next section have been obtained during tests that were started only after temperatures of oil and coolant reached normal operating conditions (engine warm up); besides, data were collected only after the engine had reached nominally stationary conditions. All of the acquired signals were averaged on 25 cycles, in order to attenuate the presence of the engine cyclic irregularities.
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RESULTS AND DISCUSSION The engine torque, power, fuel consumption, and exhaust gases emission were investigated on the engine using diesel fuel and the biodiesel blends previously mentioned. Figures 2 and 3 show the variation of engine torque and power with engine speed, respectively. As it can be observed, the available engine torque does not differ significantly when ULSD was replaced by B10; this was expected since commercial diesel fuel may already have biodiesel content up to 7% by volume (depending on the season), content which does not appreciably affect the engine performance. Such behavior is in agreement with similar data found in literature.20 As a result of the close agreement between the engine torque output and the engine fueled with ULSD and B10, as illustrated in Figure 2, only the results from the B20 and B40 experiments are discussed in the remainder of the work.
Table 2. Biodiesel and ULSD Fuel Properties properties
mass fraction
ISO 3104 (viscosity), DIN 51900 1-2-3 (lower heating value), and EN ISO 5165 (cetane number); its chemical composition is given in Table 3 (data were obtained on the basis of gas chromatography, EN 14103). Biodiesel fuel offers the advantage of having minimal sulfur content and a higher lubricity; however, it is characterized by a lower calorific value and a higher viscosity. Due to the latter property, usually small blend ratios are used in the diesel fuel. 5078
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Figure 4. Variation of brake specific fuel consumption with engine speed for 100% load: black diamonds, diesel fuel; blue squares, B20; red triangles, B40.
Figure 2. Variation of engine torque with engine speed for 100% load: black diamonds, diesel fuel; yellow asterisks, B10; blue squares, B20; red triangles, B40; green circles, diesel fuel at 80% load.
should be considered as being responsible for the lower increase of BSFC. Figure 5 presents the brake thermal efficiency (BTE) trends obtained at full load conditions by dividing the power delivered
Figure 3. Variation of engine power with engine speed for 100% load: black diamonds, diesel fuel; blue squares, B20; red triangles, B40.
The engine torque values related to B40 were lower at all engine speeds. This was expected, since biodiesel fuel has a lower heating value than ULSD. In the plot, the partial load (80% of full load) characteristic of diesel fuel is also reported. Such a value was calculated based on the available torque at full throttle opening (full load condition). The points in the curve represent the operative conditions in which the engine was tested with biodiesel blends. Indeed, all data here reported are related to such a load condition (80% of full load evaluated by using diesel fuel) that allowed to impose on the engine the same load values as all the tested fuels. The decrease of the engine power as compared to the value of the diesel fuel is mainly due to the lower energy content of the biodiesel fuel blends. It is possible to observe a reduced power decrease at low speeds, while at high speeds the reduction of power slightly increases as compared to ULSD. In order to quantify the fuel efficiency, the brake specific fuel consumption (BSFC) was computed. Figure 4 shows the data obtained during tests on engine running with diesel, B20 and B40 at 100% load condition. The plot indicates that the fuel consumption increases with the biodiesel content in the blends; the average increase in brake specific fuel consumption over all engine speeds is 3.9% and 7.1%, respectively. Similar results have been obtained by previous research16,21 which suggested that this could be attributed to the reduction of the energy content in the biodiesel as compared to diesel fuel. Since the reduction of caloric values of biodiesel blends as compared to those of the diesel is larger than the amount of brake specific fuel consumption increase, some improvements in the combustion due to the oxygenated nature of biodiesel
Figure 5. Variation of brake thermal efficiency with engine speed for 100% load: black diamonds, diesel fuel; blue squares, B20; red triangles, B40.
at the crankshaft by the power provided by the fuel consumed. Even if the trends are not correlated, the differences in the averaged values of B20 and B40 decrease only by about 1% as compared to diesel. The engine exhaust temperature is an important indicator of the cylinder combustion temperature and represents a key parameter in the analysis of the exhaust emissions, especially of NOx. The literature presents contradictory results in this field, since some authors claim that biodiesel exhibits higher combustion temperature than diesel fuel,19,31 while other authors21,32 report opposite findings. The obtained exhaust temperature trends for biodiesel blends as compared to ULSD are shown in Figure 6 (the transducer was placed just downstream of the first two-branches junction, that connects the cylinders to the exhaust duct). For all of the fuels, the exhaust gas temperature rises with the increase of engine speed. It was observed that the increase in biodiesel blend ratio is correlated to the reduction in exhaust temperature. The reason for the reduction in the exhaust temperature may be due to the lower heating value of biodiesel, which reduces the amount of total energy released, hence lowering the peak cylinder temperature and then the exhaust temperature. The NOx emissions produced by biodiesel blends and diesel fuel are shown in Figure 7 (note that NOx is expressed as NO 5079
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Figure 8. CO emission: black diamonds, diesel fuel; blue squares, B20; red triangles, B40.
Figure 6. Variation of exhaust temperature with engine speed for 100% load: black diamonds, diesel fuel; blue squares, B20; red triangles, B40.
the fuel/air mixing process, thus lowering the CO emissions, according to most of the literature reviewed.7,34,35 The variation in the HC emission of the engine with biodiesel blends as compared to ULSD is shown in Figure 9. The plot highlights the reduction with blends with regard to baseline diesel fuel, according to the data by previous reports.7,34−36
equivalent). The concentration of NOx in the exhaust was observed to reduce with increasing engine speed, in agreement with previous results.30
Figure 7. NOx emission: black diamonds, diesel fuel; blue squares, B20; red triangles, B40. Figure 9. HC emission: black diamonds, diesel fuel; blue squares, B20; red, triangles, B40.
This behavior is primarily due to the increase in gas flow motion within the cylinder under higher engine speeds, which leads to a faster mixing between air and fuel, and a shorten ignition delay. The reaction time of each engine cycle is reduced at higher speed so that the residence time of the high gas temperature within the cylinder is shortened. This leads to a lower NOx emissions under higher engine speed,30 in spite of the temperature trends shown in Figure 6. About the NOx concentration obtained with B20 and B40 fuel blends as compared to ULSD, it should be noted that the values are related to mean values, averaged on 25 cycles. Even if the differences are not so much remarkable, the plot highlights the impact in increasing NOx emissions when the content of biodiesel in the blend increases (the increment was more pronounced for the B40 fuel at engine speeds greater than 3000 rpm). The increase of NOx is explained by various reasons such as the difference in the flame temperatures, duration of premixed and diffusion burn regimes caused by difference in spray properties, ignition delay caused by oxygen content, and fuel chemistry that affects the nitric oxide formation.14,17,27,33 Figure 8 shows the variation of CO emissions versus engine speed. As Figure 8 shows, the CO emissions of the engine with the blends decrease for all of the tested engine speeds. This may be attributed to the oxygen content of the blends that enhances
Figure 10 shows the variation of CO2 emission with the engine speed.
Figure 10. CO2 emission: black diamonds, diesel fuel; blue squares, B20; red, triangles, B40.
The experimental data show that CO2 emissions have the same trends and values for ULSD and biodiesel blends. It is reported in literature that CO2 emissions for biodiesel blends are higher compared to diesel fuel.37 This is attributed to the higher density of biodiesel which increases the overall mass 5080
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authors report an increased number of nanoparticles (Dp < 50 nm) and a reduced number of emitted ultrafine (Dp < 100 nm) and fine particles (Dp < 2.5 μm) when biodiesel blends are employed in comparison with ULSD. The data shown in Figure 12 are the number of particles with a size range of 23 nm to 2.5 μm; they are cumulative values that account for the balance between the reduction and the increase of numbers of emitted particles in the complete size range. With B40, the particulate matter has a further decrease. Such a trend is in accordance with results reported in literature.24 Particulate emission consists of two parts: an insoluble fraction and a soluble organic fraction. It has been observed that, in some cases,14,33,39 the use of biodiesel results in an increase of the soluble organic fraction as a consequence of the lower volatility of the unburned hydrocarbons that favorably condense and adsorb on the particulate surface. Some researchers have found that the employment of biodiesel blends causes the emission of more particles of smaller dimensions.24,38,40 It can be explained by the oxygen content of biodiesel fuel, which favors the completeness of the combustion process in the region with fuel-rich diffusion flames.33 The more complete combustion promotes the oxidation of the already formed soot and inhibits the soot growth. The smaller dimensions of PM emissions have a harmful effect due to the longer residence time in atmospheric suspension, and thus higher probability of inhalation; their higher specific surface make them capable of adsorbing organic compounds; besides, they have lower filterability in traps and filters, thus reducing the efficiency of after treatment systems.14 The plot of Figure 13 shows the variation of soot concentration in the exhaust with the engine speed for diesel fuel and biodiesel blends.
of fuel under complete combustion; the more complete combustion with biodiesel blends may have also contributed.37 However, some studies refer to an opposite trend in CO2 emission,15,21 as a consequence of the presence of oxygen atoms and the lower carbon to hydrogen ratio of biodiesel fuel. Figure 11 presents the O2 content in the exhaust for various engine regimes. It can be seen that the variation of oxygen
Figure 11. O2 content: black diamonds, diesel fuel; blue squares, B20; red, triangles, B40.
concentration with the engine speed has a slight reliance on the percentage of biodiesel in the blend. For what concerns the particulate emissions, many researchers have shown the significant influence of fuel type and engine conditions on soot properties. The structure of soot depends upon its formation conditions (temperature and residence time); the average spherule and aggregates size generally increase with the overall engine equivalence ratio at various engine speeds and loads.33,38 Particulate matter is currently regulated in terms of grams per kilometer or grams per kilowatt-hour; number-based regulation for particulates is under discussion for introduction in the near future. In the following figures both the particle number and the mass concentration trends are shown. Figure 12 shows the variation of nonvolatile particle number concentration (PNC) in the engine exhaust system with the
Figure 13. Soot concentration in the exhaust (where, for example, 1.2E+01 represents 1.2 × 10): black diamonds, diesel fuel; blue squares, B20; red, triangles, B40.
All trends are characterized by an increase of emission as the engine speed increases, according to results reported in literature.33 When the engine speed increases, both periods of the air-mixing and combustion process are shortened. These result in a less uniform mixture and more incomplete combustion in terms of overrich and overlean mixtures and then in a higher particle concentration. A significant reduction in weight of particulate is observed for all biodiesel blends. The significant decrease in PM emissions with increasing biodiesel fuel fraction is almost universally reported in literature.3,14,15,24 There are various reasons to explain such a trend; for instance (1) the oxygen content of a biodiesel molecule which enables a more complete combustion,
Figure 12. Particle number concentration (where, for example, 4.0E +07 represents 4.0 × 107): black diamonds, diesel fuel; blue squares, B20; red, triangles, B40.
engine speed. All values were normalized with respect to the available power. It can be noted that the employment of B20 slightly reduces the number of particles as compared to the baseline fuel. Some literature states that it is difficult to evaluate the effect of biodiesel blends on particle size distribution since this is deeply affected by engine type and operating conditions and dilution needed prior to sampling.14,24 The majority of 5081
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even in the regions of the combustion chamber with fuel-rich diffusion flames, and promotes the oxidation of the already formed soot;5,11,14,23,33 (2) the different structures of soot particles between biodiesel and diesel fuels, which may favor the oxidation of soot from biodiesel;14 and (3) the nil sulfur content which prevents sulfate formation, which is a significant component of typical diesel PM.14,15 Figure 14 shows the soot emissions against NOx trend. It can be observed the reduction of particulate emissions as the
Figure 15. Diagram of injection operation modes (A−E).
number inside each rectangle stands for the injected fuel quantity expressed as mm3/stroke). In order to compare the effect of different injection strategies on the combustion process, the in-cylinder pressure signals and the rate of heat release (ROHR) trends were investigated. Figure 16 shows the gas pressure traces inside the cylinder during the crank angle interval corresponding to the
Figure 14. Soot concentration in the exhaust versus NOx trend (where, for example, 1.2E+01 represents 1.2 × 10): black diamonds, diesel fuel; blue squares, B20; red, triangles, B40.
concentration of biodiesel in the fuel blend increases. For each biodiesel blend, the reduction of PM is attained along with a corresponding slight increase in NOx, according to the wellknown trade-off between NOx and PM. Aimed at evaluating the potential solutions able to decrease significantly particulate emissions without increasing NOx emissions, variations of the injection parameters were considered. A programmable electronic control module (ECM), connected to a PC, was used to manage the injection settings; five operation modes (named “mode A” to “mode E”) were performed, in which the injection timing and duration were varied, whereas the total amount of delivered fuel was maintained unchanged. The engine modes A and B differ for what concerns the preinjection and main injection duration, while SOI timings remain unchanged. The engine modes A and C differ only for the preinjected fuel timing. In engine operation mode D, advances of both preinjection and main injection timings were imposed as compared to mode A. The data related to mode E were characterized by a delay of the main injection process with respect to mode A. In Table 4, all data are reported related to B20, engine condition 3300 rpm, and 80% load (the values are those reported by the ECM, according to the data imposed by a specific software used to manage the injection process). The diagram of Figure 15 highlights the differences for each mode (two rectangles are shown, one for preinjection and one for main injection; the
Figure 16. In-cylinder gas pressure at 3300 rpm, 80% load for B20 (where, for example, 1.0E+02 represents 1.0 × 102): black line, mode A; brown line, mode B; blue line, mode C; red line, mode D; green line, mode E.
combustion process of B20; Figure 17 presents the corresponding traces obtained by fueling the engine with B40. As the in-cylinder pressure traces highlight, the injection parameter setting is responsible for the variation in the engine performance. Although the same amount of fuel was delivered during the injection process (according to the gravimetric fuel flow measurement), the available torque changes as follows: (1) modes B and D have a moderate torque increase as compared to mode A; (2) the available torque during operation mode C does not differ significantly from mode A; (3) mode E is
Table 4. Engine Operating Conditions at 3300 rpm and 80% Load, B20 and B40 engine mode B20
B40
injection data
A
B
C
D
E
A
B
C
D
E
3
1 9.9 22.1 9.8
2 8.9 22.3 9.9
1 10 24.7 9.9
1 10 24 11.8
1 9.8 21.7 6.8
1 9.8 22 9.7
2 8.8 22.9 9.9
1 9.8 25.4 9.9
1 9.8 24.5 11.8
1 9.9 22.7 6.8
Qpre (mm /stroke) Qmain (mm3/stroke) SOIpre (cad) SOImain (cad)
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Figure 19. Rates of heat release at 3300 rpm, 80% load for B40: black line, mode A; brown line, mode B; blue line, mode C; red line, mode D; green line, mode E.
Figure 17. In-cylinder gas pressure at 3300 rpm, 80% load for B40 (where, for example, 1.0E+02 represents 1.0 × 102): black line, mode A; brown line, mode B; blue line, mode C; red line, mode D; green line, mode E.
mode C main injection with regard to mode D is observable in the maximum heat release rate phase. In mode E, the long time between the end of preinjection and the beginning of main-injection process is responsible for a delay of the maximum heat release rate phase as compared to the other modes. Figures 20 and 21 present the pollution emissions for NOx and PM, respectively (in terms of nonvolatile particle number
characterized by a lower value of torque with respect to mode A. A very important parameter that can be derived from the incylinder pressure data is the rate of heat release; this provides information to help identify the combustion characteristics of a fuel and therefore the analysis of overall engine performance and emissions. It was computed according to the following equation:41 dp dHR w γ dHR 1 dV = V + p + dα γ − 1 dα γ − 1 dα dα
where γ is the specific heat ratio, α the crank angle, p the incylinder pressure, and V the in-cylinder volume. The third term on the right-hand side represents the instantaneous heat loss on the cylinder wall computed by the Woschni model. Figures 18 and 19 show the variation of the heat release rate obtained with B20 and B40, at 80% of load, 3000 rpm, respectively. Figure 20. NOx emission: black, diesel; blue, B20; red, B40.
Figure 18. Rates of heat release at 3300 rpm, 80% load for B20: black line, mode A; brown line, mode B; blue line, mode C; red line, mode D; green line, mode E.
Figure 21. Particle number concentration: black, diesel; blue, B20; red, B40.
The combustion development exhibits two-stage ignition and a low-temperature heat release followed by a main combustion phase. The timings, durations, and amplitudes of each one are closely related to the injection strategy. Mode B is characterized by the fast burning rate of preinjected fuel, which causes a decrease a ignition delay of the main injection as compared to mode A (the modes have the same injection timings). The shift in SOIpre timing of modes C and D with regard to mode A leads to an advance of the initial increase of the heat release caused by the preinjected fuel combustion; the delay in
concentration, PNC) obtained with the engine running under the five operation modes. All values were normalized with respect to the available power. Figure 20 illustrates the dependency of NOx on the incylinder temperature as influenced by the injection parameters. The emission increases for modes B and D result from the longer period of high temperatures in comparison with mode A; during mode C, NOx emission does not differ from mode A; during mode E, NOx emission reaches the lowest values. 5083
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The increase of particulate matter emission during mode B was probably due to the higher quantity of preinjected fuel and lower main-phase injected fuel quantity that causes a less effective reduction of emission as compared to mode A. A benefit in PNC emission can be observed for modes C and D, whereas, for mode E, the particulate matter does not differ from mode A.
ACKNOWLEDGMENTS
We acknowledge the fundamental contribution of AVL which provided the instrumentation for the particle matter measurements (AVL Particle Counter and AVL Micro Soot Sensor) used during the research activity.
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ABBREVIATIONS CVT = continuously variable transmission; FFA = free fatty acids; HR = heat release; p = in-cylinder pressure; PM = particulate matter; PNC = nonvolatile particle number concentration; Q = delivered fuel quantity; ROHR = rate of heat release; SOI = start of injection; UCO = used cooking oil; ULSD = low-sulfur diesel fuel; V = cylinder volume; α = crank angle; γ = specific heat ratio; subscript main = main injection process; subscript pre = preinjection process; subscript w = wall
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CONCLUSION Blends of a second generation biodiesel with ultralow-sulfur diesel fuel up to 40% by volume were tested in a small displacement diesel engine that has a leading role in city cars and urban vehicles. The objective of the experimentation was to investigate the role of the biodiesel fuel blends as compared to ULSD fuel on the characteristics of performance and exhaust air emissions with no modification to the injection process and when a variation of the injection strategy (injection quantities and timings) is imposed on the engine. The investigation was performed in the real operative field of the engine which is coupled to a continuously variable transmission (CVT). The comparison between the results obtained under various operating regimes and with no modification of the injection process confirm the findings provided by literature also in the case of a small-size engine and provides a quantification of the produced emissions. The engine performance (available torque, power, and fuel consumption) under biodiesel blended fuel was very similar for B10 and B20, while B40 suffered for the lower calorific value of the mixture. The employment of B20 slightly reduced the PNC as compared to the reference fuel. With B40, the particulate matter emission had a further decrease. The use of biodiesel blends has an inconvenient effect in NOx emissions with both B20 and B40 as compared to ULSD. CO emission decreases due to the oxygen content of the blends that enhances the fuel/air mixing process. Biodiesel blends allow a reduction of HC emission. The variation of the injection strategy (quantities and timings) affects performance and specific emissions and may be summarized as follows: (1) The increase of fuel delivered during the preinjection (and the reduction of main-injected mass) is responsible for increases in NOx and particle number concentrations. (2) The advance of only preinjected fuel timing allows one to reduce the particle number concentration; when an advance of main-injected fuel timing is also considered, NOx emission increases. (3) A delay of main-injected fuel timing is responsible for the reduction of NOx, while particle number concentration remains unchanged. Based on these results, the employment of this kind of engine in urban congestion area may be considered very attractive in the near future, for its potential of reducing the pollutant emissions when fueled with biodiesel blends.
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The authors declare no competing financial interest. 5084
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