Front. Energy 2011, 5(1): 104–114 DOI 10.1007/s11708-011-0138-x
RESEARCH ARTICLE
Ni ZHANG, Zuohua HUANG, Xiangang WANG, Bin ZHENG
Combustion and emission characteristics of a turbo-charged common rail diesel engine fuelled with diesel-biodiesel-DEE blends
© Higher Education Press and Springer-Verlag Berlin Heidelberg 2011
Abstract The combustion and emission characteristics of a turbo-charged, common rail diesel engine fuelled with diesel-biodiesel-DEE blends were investigated. The study reports that the brake-specific fuel consumption of dieselbiodiesel-DEE blends increases with increase of oxygenated fuel fractions in the blends. Brake thermal efficiency shows little variation when operating on different dieselbiodiesel-DEE blends. At a low load, the NOx emission of the diesel-biodiesel-DEE blends exhibits little variation in comparison with the biodiesel fraction. The NOx emission slightly increases with increase in the biodiesel fraction in diesel-biodiesel-DEE blends at medium load. However, the NOx emission increases remarkably with increase of the biodiesel fraction at high load. Particle mass concentration decreases significantly with increase of the oxygenated-fuels fraction at all engine speeds and loads; particle number concentration decreases remarkably with increase of the oxygenated-fuels fraction. HC and CO emissions decrease with increasing oxygenated-fuels fraction in these blends. Keywords Combustion, particulate emissions, dieselbiodiesel-DEE blend, diesel engine
1
Introduction
Biodiesel, produced from vegetable and animal oils by transesterification, is an attractive alternative to diesel fuels. Biodiesel use has been reported to reduce pollutant emissions, such as HC, CO, and PM [1]; however, NOx emissions would increase with biodiesel utilization [1–3]. Received September 10, 2010; accepted December 7, 2010
✉
Ni ZHANG, Zuohua HUANG ( ), Xiangang WANG, Bin ZHENG State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China E-mail:
[email protected]
Different approaches have been tested to decrease NOx emission for biodiesel operations, wherein an approach is to add additives with high cetane number to biodiesel [1,4,5]. Diethyl ether (DEE) has a high cetane number (approximates to 125), and it has been used as a cold-start assistance in the diesel engine [6,7]. DEE is a liquid in the normal atmospheric environment and can be effectively mixed with diesel and/or biodiesel in any proportion. Bailey et al. [6] reviewed DEE as a renewable alternative fuel and presented an investigation of diesel-DEE blends in the diesel engine. Kapilan et al. [8] investigated the performance and emission of the engine fuelled with diesel-DEE blends and reported that a 5% DEE addition could produce higher NOx at medium and high loads. They attributed this phenomenon to the higher combustion temperature resulting from the 5% DEE addition. Kumar et al. [9] investigated an engine fuelled with vegetable oil and discovered that NOx emission decreased with DEE addition. Iranmanesh et al. [10] improved the combustion and emission of a diesel engine that was fuelled with biodiesel by using DEE as a cetane number improver. A combination of the premixed DEE and diesel injection on exhaust emissions was studied by Cinar et al. [11], and they reported that the introduction of premixed DEE could decrease NOx emissions as compared with diesel combustion. The utilization of DEE as a cetane number improver in diesel-biodiesel blends appears to be a promising method to reduce NOx emissions. Previous studies have indicated that PM has an impact on human beings, the environment and engine durability [12]. An appreciable reduction in PM was achieved by using biodiesel in the diesel engine [13–15]. Current regulations do not include restrictions of particulate size distribution; however, they are likely to be covered it in more stringent engine-exhaust regulations in the near future. Previously, studies have indicated that the particle number and size were more harmful to human beings in comparison with particulate mass [12,16]. Tsolakis [17]
Ni ZHANG et al. Combustion and emission characteristics of a turbo-charged common rail diesel engine
examined EGR effects on particulate size distribution on RME biodiesel and reported that EGR had minimal effect on particulate distributions. Lapuerta et al. [18] studied the particulate emission of a diesel engine that was fuelled with biodiesel prepared from cooking oil; they found that the mean particle size decreased in biodiesel, but the number of small particles did not increase significantly. Lin et al. [19] studied the particle size distribution of a diesel engine that was fuelled with palm-biodiesel blends and reported an increased number of particles with low aerodynamic diameters in biodiesel and biodiesel-diesel blends, in comparison with that of diesel. Zhu et al. [20] examined the influence of methanol-biodiesel blends on particle distribution in a direct injection diesel engine and reported that the use of methanol-biodiesel blends could reduce the number concentration of all sizes, as compared with biodiesel. Park et al. [21] studied nanoparticle emissions of biodiesel-diesel blends and reported that both the total number and total mass were reduced when blending biodiesel; however, the number of fine particles was observed to have increased. Di et al. [22] studied the particulate number concentration of a diesel engine fuelled with cooking oil and observed more nanoparticulates in biodiesel operation. Kim and Choi [23] investigated naonoparticles from a CRDI engine which operated on biodiesel-diesel blends and found that biodiesel operation would produce finer particles. Heikkilä et al. [24] ran alternative fuels on a heavy-duty engine and analyzed nanoparticle emissions to demonstrate that different fuel characteristics result in different core-particle-formation mechanisms. Fontaras et al. [25] applied biodiesel to modern passenger cars and detected a reduction in nonvolatile particle numbers. Although previous studies have revealed particle and NOx emissions of diesel engines that operate on biodiesel and/or biodiesel-diesel blends, minimal information was reported for combustion and emission characteristics when engines, especially turbo-charged, high-pressure common rail injection diesel engines, were operated on dieselbiodiesel-DEE blends. Thus, the effects of the addition of DEE on engine combustion and emissions are worth investigating. This article aims to study combustion and emission characteristics of a turbo-charged, high-pressure common rail diesel engine that operates on dieselbiodiesel-DEE blends, and the effect of DEE addition on combustion and emissions.
2
Experimental setup and procedures
A four-cylinder, four-stroke common rail, water-cooled, turbo-diesel engine was used, and its specifications are listed in Table 1. Figure 1 illustrates the schematic diagram of the experimental setup. The entire experimental setup comprises an engine system, a controlling system and an exhaust-gas-measuring system. The engine is connected to
Fig. 1 Table 1
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Schematic diagram of the experimental setup
Specifications of the engine
Item
Parameter
Type
In line 4-cylinder
Injection system
Common rail
Compression ratio
17.2∶1 –1
703 (3600)
Rated torque/(N$m) (speed/(r$min )) 1
Maximum torque/(N.m) (speed/(r$min – ))
2255 (1600–2600)
Stroke/mm
102
Bore/mm
93
Displacement/L
2.771
Type of injectors
BOSCH CR1P2
Type of common rail Maximum injection pressure/MPa Injection mode
BOSCH LWR 135 Split injection Main-injection
an eddy-current dynamometer. The engine speed and load are controlled by an engine-testing control system (Powerlink FC2000) with an accuracy of 1 r/min and 0.01 N$m. A pressure sensor (Kistler 6055C) was used to measure the cylinder pressure of the engine with 0.1-crank-angle degree resolutions. A digital data-acquisition system (Yokogawa DL750 Scopecorder) was used to record the pressure. The exhaust-gas temperature was measured by a thermoelectric couple, while the fuel flow rate was recorded by an electronic weighting apparatus with an accuracy of up to 0.1 g. The concentration of NOx was recorded by a Horiba NOx analyzer (MEXA-720 NOx) with a measuring accuracy of 30 10–6; the HC and CO emissions were recorded by a Horiba exhaust-gas analyzer (MEXA-554JA) with an accuracy of 2 10–6 and 0.01%. The real-time particle size distribution with a size range of
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0.007–10 μm and a filter stage was measured by an ELPI (Dekati ELPI TM Electrical Low Pressure Impactor). The operating principle of the ELPI could be divided into three major parts: particle charging in a unipolar corona charger, size classification in a cascade impactor and electrical detection with sensitive electrometers. Particles were first charged in the corona charger and, then, entered a cascade low-pressure impactor with electrically insulated collection stages. These particles were measured at different stages based on their aerodynamic diameter. Finally, the particle-carried charge was measured by multi-channel electrometers. Van Gulijk et al. [26] studied the restriction for the ELPI in diesel particulate measurements and demonstrated that ELPI suffered overloading. Therefore, the exhaust gas was diluted prior to sampling in this experiment. Two dilutors were used in these experiments, and their operation principle was based on an ejection-type dilution. Dilution air was conveyed to the diluter and caused sample flow into the diluter. The dilution air and sample air were mixed in the ejector cavity and then diffused in consequence of a homogeneous dilution of gases and particles. The dilution ratios were 8.43 and 8.45 in the system, respectively. The dilution method and ratio were kept constant for all tests, in order to prevent changes to the particle distribution [27]. The first diluter was heated Table 2
to 200°C, in case of nucleation and condensation of volatile materials. Table 2 lists the specifications of the ELPI and the dilutors. Fuels used in this study are diesel, biodiesel and DEE. The biodiesel is developed from soybean oil. The DEE is of anhydrous grade, with a purity of 99.7%. Six blends were prepared and include D100, D78B20E2, D56B40E4, D34B60E6, D12B80E8 and B90E10. Here, D78B20E2 implies that this blend contained 78% of diesel, 20% of biodiesel and 2% of DEE on basis of volume percentage. These fuels were mixed without separation in quiescence. Fuel specifications are presented in Table 3. Engine speed was set at 1600 and 2600 r/min, and the load at 25%, 50% and 75% of full load, respectively. Prior to taking measurements, the engine and exhaust gas analyzers were warmed.
3
Results and discussion
3.1
Engine performances
Brake-specific fuel consumption (BSFC), brake mean effective pressure (BMEP) and brake thermal efficiency (BTE) are calculated from experimental data, based on the
Specifications of ELPI
Item
Parameter
Stage
D50%/m
Particle size range (with filter stage)/ m
0.007–10
13
10
12
12
6.8
10
11
4.4
Φ 65 300
10
2.5
H570 W400 D230
9
1.6
Collection plate diameter/mm
25
8
1
Unit weight/kg
35
7
0.65
Lowest stage pressure/kPa
10
6
0.4
Operating temperature/°C
5–40
5
0.26
Operating humidity/%
0–90
4
0.17
1
3
0.108
7 m /h at 10 kPa
2
0.06
1
0.03
Number of size classes –1
Sample flow rate (L$min ) Impactor dimensions/(mm mm) ELPI dimensions/(mm mm mm)
Time resolution/Hz 3
Pump requirements
Table 3
Specifications of fuels 4.269 Density / (kg$m–3)
Viscosity / (mm2$s–1)
Oxygen /%
Carbon /%
Hydrogen /%
Cetane number
T50 / °C
T90 / °C
Latent heat of vaporization /(kJ$kg–1)
Low heating value /(MJ$kg–1)
Diesel
853.8
3.223
–
86.70
12.71
45
~327
~346
290
42.4
Biodiesel
881.4
4.269
10.97
77.10
11.81
51.5
~336
~340
–
37.4
DEE
713.4
0.3266
21.6
64.9
13.5
> 125
–
–
351.16
33.9
Fuel
Ni ZHANG et al. Combustion and emission characteristics of a turbo-charged common rail diesel engine
Internal Combustion Engine [28]. The engine load is 25%, 50% and 75% of full load, and corresponds with BMEP of 0.255, 0.510, and 0.765 MPa, respectively. Figure 2 demonstrates variations of brake-specific fuel consumption with biodiesel fraction at different engine speeds and loads. BSFC decreases with increase of BMEP at two engine speeds, and this is consistent with a diesel engine that is fuelled with diesel. For a given engine speed and load, the BSFC increases almost linearly with increase of the biodiesel fraction in the diesel-biodiesel-DEE blends, and this conforms with many previous reports [1,2,4,22]. The decrease in the heating value of oxygenated fuel contributes to the increase of the BSFC since more fuel has to be injected to obtain the same power output. Figure 3 illustrates the BTE versus biodiesel fraction at different engine speeds and loads. The BTE increases as engine load increases from 0.255 to 0.765 MPa at different
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speeds. For a given engine speed and load, the BTE shows little variation with the biodiesel fraction. Although the evaporation of DEE reduces the cylinder gas temperature, the oxygen enrichment enhances combustion. The combined effect of these two factors results in slight variation of the BTE when operating on different DEE fractions in diesel-biodiesel-DEE blends. Figure 3 indicates that the lower heat values of biodiesel and DEE, as compared with diesel, do not adversely affect the BTE of blends. Further, the higher fuel consumption of the biodiesel-diesel-DEE blends is caused by a lower heating value rather than loss of thermal efficiency. 3.2
Engine emissions
Figure 4 shows NOx emission versus biodiesel fraction in the diesel-biodiesel-DEE blends. The cylinder pressure
Fig. 2 BSFC versus biodiesel percentage at different speeds and loads (a) When engine speed is 1600 r/min; (b) when engine speed is 2600 r/min
Fig. 3 BTE versus biodiesel percentage at different speeds and loads (a) When engine speed is 1600 r/min; (b) when engine speed is 2600 r/min
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Fig. 4 NOx versus biodiesel percentage at different speeds and loads
and net heat-release rate versus crank angle are plotted in Fig. 5. The NOx emission increases with the increase of engine load, and this increase is more remarkable at high load. At a low load, the NOx emission of the biodieseldiesel-DEE blends exhibits minimal variation versus the biodiesel fraction. This reveals that NOx is hardly influenced by the type of fuels used. The oxygen in the fuel does not contribute to NOx emission. The NOx emission slightly increases with increase of biodiesel fraction in the diesel-biodiesel-DEE blends at medium load. This is attributed to combustion enhancement as oxygenated fuels are blended, leading to an increased cylinder gas temperature and increased NOx formation. The NOx emission increases markedly with increase in the biodiesel fraction in the diesel-biodiesel-DEE blends at high load. In addition to the combustion enhancement, the increase of the easily evaporated DEE can generate larger amounts of premixed mixture in the combustion chamber. The combustion of this premixed DEE contributes to increased NOx emission, and the contribution becomes obvious at high engine loads. The NOx emission result obtained in this study is comparable with the result reported by Adi et al. [29]. Eckerle et al. [30] demonstrated that the NOx emission was more serious on advanced engines operating on biodiesel. As mentioned earlier, DEE has an outstanding cetane number, which can reduce ignition delay and shorten the premixed combustion phase. This, in turn, reduces NOx emission (Fig. 5 (a)). However, fast combustion of DEE increases the cylinder temperature, which consequently enhances the evaporation and atomization of biodiesel [31,32]. Meanwhile, blending DEE with biodiesel reduces the viscosity of fuel blends, which enhances the spray atomization and mixing with the surrounding air [32,33]. The latter factors might predominate at medium and high loads, and this could cause an increase in NOx emission with increase in the biodiesel
fraction. Different fuels demonstrate a similar tendency of cylinder pressure and NHRR. Thus, in the present study, only cylinder pressures and NHRRs of diesel and D78B20E2 are illustrated. Figure 5 indicates that there are very minor differences between the NHRR of diesel and D78B20E2 curves although fuel properties are different. At low load, blending of oxygenated fuels results in low cylinder pressure and decreases early heat release. At medium and high loads, the cylinder pressure of D78B20E2 is lower than that of diesel in the initial stage and becomes higher at later stages. The diesel-biodieselDEE blends generate a higher heat-release rate in the main combustion phase. Biodiesel is an attractive alternative fuel to reduce the concentration of particulate matter in the diesel engine. Similar results are obtained when operating on the biodiesel-diesel-DEE blends in this turbo-charged, common rail diesel engine, as demonstrated in Fig. 6. For the same fuel and load, an increase in engine speed decreases the total particle mass concentration. The increase in biodiesel fraction decreases the particle mass concentration at all speeds and loads. At a low engine load, the maximum reduction in particle mass concentration is achieved for the B90E10 fuel blend (with a reduction of 85.1% at 1600 r/ min and 78.6% at 2600 r/min compared with those of diesel fuel). At a medium load, the maximum reduction in particle mass concentration is achieved for the B90E10 fuel blend (with a reduction of 94.4% at 1600 r/min and 85.6% at 2600 r/min compared with those of diesel fuel). At a high load, the maximum reduction in particle mass concentration is achieved for the B90E10 fuel blend, (with a reduction of 97.1% at 1600 r/min and 92.1% at 2600 r/ min compared with those of diesel fuel). Results of this study indicate that blending oxygenated fuel with diesel generates a dramatic reduction in particle mass concentration. The influence of the following factors can explain this
Ni ZHANG et al. Combustion and emission characteristics of a turbo-charged common rail diesel engine
Fig. 5
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Cylinder pressure and NHRR versus crank angle at different speeds and loads
phenomenon. Tree and Svensson [34] believed that PM was formed from unburned fuel, and, in the presence of sufficient oxygen, the precursors of PM would be oxidized. Biodiesel and DEE contain high oxygen, and the oxygen in the fuel can enhance the combustion and suppress the formation of soot precursors. Kittelson [12] reported that the particles are mainly constituted of solid carbon and volatile organic and sulfur compounds. Biodiesel contains less number of aromatics and sulfur while DEE does not contain aromatics and sulfur. Thus the formation of particulate matters can be suppressed [35]. In addition, biodiesel creates more reactive surface chemistry particles, which can enhance the oxidation of particles [36]. In recent times, great concern has been evinced with regard to nanoparticle diameter range. Small- or fine-sized particles are considered to be more harmful than big particles [12]. Figure 7 presents the particle size distribution at different engine speeds and loads. Figure 8 illustrates total number concentrations at different engine
speeds and loads. From Fig. 7, it can be seen that the particle size-distribution patterns are in the shape of a unimodel. Minimal variation in distribution pattern is presented at different fuels and engine-operation conditions. The majority of particles exist in the second stage. Increasing engine speed can decrease the number of particles. At an engine speed of 1600 r/min, the particle number concentration demonstrates a low value when particle aerodynamic diameter is larger than 0.40 μm; however, the particle number concentration shows a low value when particle aerodynamic diameter is larger than 0.26 μm at an engine speed of 2600 r/min. This implies that the engine emits lesser number of large particles at high speed as compared with emission at low speed. Particle number concentration decreases with the increase of oxygenated fuels in the diesel-biodiesel-DEE blends. This is attributed to the oxygen enrichment and lesser aromatics and sulfur content. Compared with diesel operation, the maximum reduction in particle number
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Fig. 6 Total mass concentration versus biodiesel percentage at different speeds and loads
concentration is achieved at high load for the B90E10 blend. The reduction is 94.6% at 1600 r/min; while, for 2600 r/min, the reduction is 88.0%. An approximately linear reduction in total number concentration with the increase of oxygenated fuel fraction is demonstrated. This suggests that an effective reduction in total number concentration can be realized with the introduction of oxygenated fuels in diesel engine. Except for diesel fuel, a decrease of total number concentration for different fuels is presented with the increase of engine load in Fig. 8. Although more particles are produced at high load, the coagulation rate is enhanced by high temperature and pressure [37]. This may result in a decrease in particle number concentration at a high load. The maximum reduction in total number concentration is demonstrated for B90E10, which are 61.6% and 75.1% at medium and high loads as compared with that at low load. Figures 9 and 10 plot HC and CO emissions versus biodiesel fraction in the diesel-biodiesel-DEE blends. HC
emission occurs mainly when the fuel in the engine is only partially combusted. Incomplete combustion, which occurs at low air-to-fuel ratios in the engine, causes CO emission. HC and CO decrease with the increase of biodiesel fraction in the blends. For a specific fuel, HC and CO emissions decrease with the increase of engine load. The decrease in HC and CO for oxygenated fuels results from the oxygen enrichment and decrease in the fuel-rich zone in spray [1,2,13,22]. Enhancement in HC and CO oxidation and high-temperature combustion leads to decreased exhaust HC and CO emissions with increase in the engine load. This is favorable for application in engine-emissions reduction when mixed with oxygenated fuels because all emissions decrease, with the exception of NOx at medium and high loads. However, NOx can be decreased by introducing EGR in the diesel engine when oxygenated fuel is utilized. These results indicate that the effect of premixed DEE on NOx emission is a combination of blending ratio, engine operating condition and engine type.
Ni ZHANG et al. Combustion and emission characteristics of a turbo-charged common rail diesel engine
Fig. 7 DN/DlogDp versus Dp at different speeds and loads
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Fig. 9 HC emissions versus biodiesel percentage at different speeds and loads
Fig. 10 CO emissions versus biodiesel percentage at different speeds and loads
4
Fig. 8 Total number concentration versus biodiesel percentage at different speeds and loads
Conclusions
Combustion and emission characteristics of a turbocharged, common rail diesel engine fuelled with dieselbiodiesel-DEE blends were investigated in this study. The primary conclusions are as follows: 1) Brake-specific fuel consumption of biodiesel-dieselDEE blends increases with the increase of oxygenated-fuel fraction in these blends. Brake thermal efficiency exhibits minimal variation when operating on different biodieseldiesel-DEE blends. 2) At low load, NOx emission of diesel-biodiesel-DEE blends demonstrates slight variation when compared with the biodiesel fraction. NOx emission slightly increases with increasing biodiesel fraction in the diesel-biodiesel-DEE blends fraction at medium load. NOx emission increases remarkably with the increase of biodiesel fraction in the diesel-biodiesel-DEE blends at high load. 3) Particle mass and number concentration decreases significantly with the increase of oxygenated fuel fraction
Ni ZHANG et al. Combustion and emission characteristics of a turbo-charged common rail diesel engine
in the diesel-biodiesel-DEE blends at all engine speeds and loads. 4) HC and CO emissions decrease with the increase of oxygenated fuel fraction in the blends. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 50821064).
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