iso-butanol blends in a diesel engine

iso-butanol blends in a diesel engine

Energy Conversion and Management 149 (2017) 381–391 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www...

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Energy Conversion and Management 149 (2017) 381–391

Contents lists available at ScienceDirect

Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Combustion performance and pollutant emissions analysis using diesel/gasoline/iso-butanol blends in a diesel engine Mingrui Wei, Song Li, Helin Xiao, Guanlun Guo ⇑ Hubei Key Laboratory of Advanced Technology for Automotive Components, Wuhan University of Technology, Wuhan 430070, China Hubei Collaborative Innovation Center for Automotive Components Technology, Wuhan University of Technology, Wuhan 430070, China

a r t i c l e

i n f o

Article history: Received 10 May 2017 Received in revised form 20 June 2017 Accepted 17 July 2017

Keywords: Diesel engine Diesel/gasoline/iso-butanol Combustion characteristics Gaseous pollutants emissions Particulate matter

a b s t r a c t In this study, the effects of diesel/gasoline/iso-butanol blends, including pure diesel (D100), diesel (70%)/gasoline (30%) (D70G30, by mass), diesel (70%)/iso-butanol (30%) (D70B30) and diesel (70%)/gasoline (15%)/iso-butanol (15%) (D70G15B15), on combustion and exhaust pollutant emissions characteristics in a four-cylinder diesel engine were experimentally investigated under various engine load conditions with a constant speed of 1800 rpm. The results indicated that D70G30, D70G15B15 and D70B30 delayed the ignition timing and shortened the combustion duration compared to D100. Additionally, CA50 was retarded when engine fuelled with D70G30, D70G15B15 and D70B30 at low engine load conditions, but it was advanced at medium and high engine loads. The maximum pressure rise rates (MPRRs) of D70G30, D70G15B15 and D70B30 were increased compared with D100 except for at engine load of 0.13 MPa BMEP (brake mean effective pressure). Meanwhile, D70G15B15 and D70B30 produced higher brake specific fuel consumption (BSFC) than that of D100. The effects of diesel blend with gasoline or iso-butanol on exhaust pollutant emissions were varied with loads. CO emissions were increased obviously and NOx emissions were decreased under low engine loads. However, CO emissions were decreased and NOx emissions were slightly increased under the medium and high engine load conditions. However, D70G30, D70G15B15 and D70B30 leaded to higher HC emissions than D100 regardless the variation of engine load. Moreover, the particulate matter (PM) (diameter, number and mass concentrations) emissions by using D70G30, D70G15B15 and D70B30 were significantly reduced compared to D100, but more small-size particles were produced. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Recently, the great challenges in the use of compression ignition (CI) engines are to reduce the reliance on conventional petroleum fuels and exhaust emissions. While the current available technologies for reducing engine emissions are nearly close to the limits of statute and those limits are expected to be more stringent in the near future [1]. As such, many investigations have been carried out on using alternative and renewable fuels to reduce engine emissions without modifying the engine. Recently, alcohols have been emerging as a promising alternative fuel for CI engines, because they can be produced from renewable resources and blended into diesel allowing the fuel to have a more complete combustion with the combustion efficiency increased and pollutant emissions reduced [2–6]. The widely stud⇑ Corresponding author at: Wuhan University of Technology, 122 Luoshi Road, Wuhan, Hubei 430070, China. E-mail address: [email protected] (G. Guo). http://dx.doi.org/10.1016/j.enconman.2017.07.038 0196-8904/Ó 2017 Elsevier Ltd. All rights reserved.

ied representatives of alcohol fuels in the world are methanol and ethanol, but their applications are limited by low energy density, large energy consumption and high volatility [7]. Iso-butanol, as one of butanol isomers, is a well-known alcohol fuel has widely been paid attention, and the following properties of iso-butanol make it a suitable for the alternative fuel of CI engines [8–10]: (1) Iso-butanol is a renewable and biomass fuel, which can be produced from agricultural crops. (2) Compare to methanol and ethanol, iso-butanol can be easily mixed with gasoline or diesel and no phase separation for a long period due to its low water affinity characteristic. (3) As shown in Table 1, the latent heat of vaporization, specific gravity and stoichiometric air/fuel ratio of iso-butanol are closer to pure diesel than those of methanol and ethanol. (3) The higher net heating value of iso-butanol is an important advantage in engine performance and fuel economy compared with methanol and ethanol. (4) With about 21.5% oxygen content and a hydroxyl group, iso-butanol can significantly improve the process of engine combustion. (5) Among four butanol fuels (i.e. nbutanol, sec-butanol, iso-butanol and tert-butanol), iso-butanol

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Table 1 Properties of the diesel, gasoline, methanol ethanol and iso-butanol fuel [11–15]. Parameters

Diesel

Gasoline

Methanol

Ethanol

Iso-butanol

Chemical formula Cetane number Boiling point (°C) Lower heating value (MJ/kg) Heat of evaporation (kJ/kg) Auto-ignition temperature (°C) Stoichiometric A/F ratio Density at 20 °C (kg/cm3) Carbon content (mass%) Hydrogen content (mass%) Oxygen content (mass%) C/H

C12–C25 52.1 180–360 42.5 250–290 180–220 14.3 826 86.7 12.7 – 6.8

C8H15 <15 25–215 42.9 223.2 420 14.7 744.6 86 14 – 6.1

CH3OH 2 65 20.08 918.4 363 6.4 786.6 37.5 12.5 50.0 2.98

C2H5OH 11 79 26.83 1162.6 385 9 790.9 52.1 13.1 34.7 4.0

C4H9OH <15 107.9 33.3 474.3 415.6 11.1 802 64.9 13.5 21.5 4.8

produces lowest smoke emissions with similar engine efficiency and gaseous emissions as compared to other isomer fuels in the use of diesel additive. Many researches found that iso-butanol fuel could be blended with diesel and used in CI engines. Karabektas et al. [16] investigated the performance and emissions of a naturally-aspirated, single-cylinder diesel engine using diesel/iso-butanol blends at different engine speeds. They found that the brake power of the engine decreased and the brake specific fuel consumption (BSFC) increased. In addition, nitrogen oxides (NOx) and carbon monoxide (CO) emissions decreased as the iso-butanol blending ratio increased, while the unburned hydrocarbons (HC) emissions increased. Ozsezan et al. [17] studied the effects of three isobutanol/diesel blends (5%, 10%, and 15%) in a CI engine and they got similar results of NOx, HC and CO emissions as Ref. [16]. Meanwhile, they found that the smoke emissions were reduced with isobutanol fraction increased in the blends. Gu et al. [18] studied the individual effects of diesel/n-butanol and diesel/iso-butanol blends on combustion and emissions in a six-cylinder diesel engine at the high and medium loads. They concluded that lower smoke and NOx emissions, and an acceptable fuel economy can be achieved combining with a low exhaust gas recirculation (EGR) and appropriate retarded injection timing. Recently, Zheng et al. [10] investigated the combustion and emissions characteristics of four butanol isomers (n-butanol, sec-butanol, iso-butanol and tert-butanol) added into diesel respectively in a single-cylinder diesel engine under different EGR rates. Their results indicated that the diesel/ iso-butanol blend has the longest ignition delay and the lowest smoke emissions compared to the blends of other three butanol isomers. Moreover, regulated gaseous emissions are not significantly affected by the addition of butanol isomers. Kumar et al. [9,19,20] investigated the effects of diesel/iso-butanol blend fuels on combustion performance and emissions of a single-cylinder diesel engine. Their results showed that the CO, NOx and smoke opacity emissions decreased and the HC emission increased with increasing the iso-butanol content under the entire engine load conditions. As is well known, the combustion performance and pollutant emissions of diesel engines are governed by the mixture quality of the injected fuel and the intake air. A homogeneous fuel/air mixture improves the combustion efficiency, and also reduces the emissions of an engine [21]. Gasoline has low cetane number and is an ideal option to be blended with diesel for longer ignition delay, thus the in-cylinder fuel/air mixing process can be sufficiently improved to reduce the local fuel-rich regions [22,23]. In addition, gasoline is easier to be vaporized and potentially favorable to reduce the locally rich region, if blended with diesel. Both diesel and gasoline are easily available and have been used as practical fuels in CI and spark ignition (SI) engines respectively for many decades.

Blending gasoline in the base fuel of diesel has also been investigated in CI engines. Hanson et al. [24] studied the combustion and emissions of an engine using diesel direct injection in combination with gasoline port fuel injection and found that gasoline injection delays the combustion phasing, which in turn reduces heat transfer, fuel consumption, and emissions of NOx and PM. Kalghatgi [25] investigated the performance of running diesel, 84 RON and 95 RON in a single-cylinder diesel engine and found that gasoline fuel is very beneficial for adjusting the premixed combustion phasing compared with diesel fuel due to its long ignition delay. Subsequently, Kalghatgi [26] compared the performance and emissions of an Euro VI multi-cylinder diesel engine fuelled with gasoline and diesel respectively, and the comparisons show that even with lower injection pressure and lower EGR levels, gasoline yields similar level of NOx emission, much lower smoke, lower the values of maximum pressure rise and BSFC. In addition, by using gasoline extends the ignition delay time because of reduced chemical reactivity of the premixed charge [27]. In addition, Zhong et al. [28] experimentally investigated the HCCI engine combustion and emissions using neat gasoline and diesel/gasoline blends and found that it produced much less harmful emissions during the combustion of blended fuels than pure gasoline combustion, especially HC and NOx emissions. Li et al. [29] numerically assessed the effects of diesel/gasoline blends on the combustion and emissions characteristics of a conventional diesel engine and found that the blend fuels show a better behavior on emissions at medium and high engine load conditions. It was reported that [30] low emissions and high efficiency can hardly be both achieved using dual fuel (such as diesel/gasoline) for CI engines, so in order to utilize the advantages of different type of fuels on engine combustion, ternary fuel was investigated on engine recently [1,30–35]. Huang et al. [34,35] studied the combustion and emission performance of a diesel engine using diesel/gasoline/n-butanol blends under low temperature combustion. They found that diesel/gasoline/n-butanol blends are suitable as alternative fuels for CI engine. But Gu et al. [17] and Pan et al. [36] found that diesel/iso-butanol blend can provide longer ignition delay, higher pressure peak and higher premixed heat release rate, as well as lower smoke emissions, than those of diesel/nbutanol blend. So iso-butanol may be a more applicable additive for diesel/gasoline blends than n-buthanol. However, limited information is currently available about the effects of diesel/gasoline/ iso-butanol blends on combustion and emissions characteristics of a diesel engine. It is known that particulate matter (PM) emissions from diesel engine have become a growing concern as it may adversely affect human health and the environment, but reducing the PM emissions has long been a severe challenge [37]. Generally, PM emissions in engine exhaust are classified into the nucleation mode (NM, Dp <50 nm) and the accumulation mode (AM, 50 nm

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Table 2 Engine specifications. Type of engine Bore/mm Stroke/mm Compression ratio Displacement/cc Rated power/KW Rated speed/rpm Type of ignition Method of starting Injection time/ °CA ATDC

Four-cylinder four-stroke 96 103 17.5 2982 85 3200 Compression ignition Electric start 7.5

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2. Experimental setup and strategies 2.1. Engine specifications and instrumentations The experimental engine employed for the present study was a four-cylinder, four-stroke, turbocharged, water-cooling, commonrail diesel engine, with specifications listed in Table 2 and the experimental layout is shown in Fig. 1. This engine was controlled by an eddy current dynamometer, which adjusted the torque output and kept the engine speed constant. An Electronic Control Unit (ECU) was employed to control and monitor the injection timing, injection pressure and injection quantity. A fuel consumption meter (FCD-M) with a gravity scale was used to measure the fuel flow rate. The cylinder pressure traces were detected by a Kistler 6025 C pressure sensor installed in the combustion chamber. The pressure signals were conveyed to a charge amplifier (Kistler 5108A1003) and then to a CB-466 burning analyzer. The burning pressures were recorded for 100 cycles at an interval of 0.25 crank angle (CA). The intake-air temperature and pressure were controlled by an air conditioning system and a supernumerary compressor, respectively. The heat release rate (HRR) was calculated using the measured pressure data according to the first law of thermodynamics and the perfect gas equation of state, the equations were shown in the study of Wei et al. [44]. Gaseous pollutants (HC, CO and NOx) and PM were sampled from the engine exhaust manifold and measured by AVL gas analyzer and DMS500 MKII fast particulate spectrometer (Cambustion Ltd.), respectively. All emissions were measured during steadystate engine operation. DMS500 has a fully-integrated two-stage dilution system and classified the particles by the electrical mobility diameter from 5 to 1000 nm in 22 classes. The first diluter worked at the sampling point to prevent particles condensation and aggregation, and a rotating disc offered a high-ratio second diluter that allowed to sample a very wide content range of aerosols. The content of the detected particles was auto-corrected for the applied dilution. The primary dilution was set to be 5 throughout the engine operation, and change the secondary dilution factor according to the user interface, which ensured the particle concentration was within the operation range of the spectrometer. PM emissions were measured in terms of the particle size distribution (PSD), the count mean diameter (CMD), the number and mass concentrations of NM, AM and total particle.

Fig. 1. Research engine experimental layout.

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M. Wei et al. / Energy Conversion and Management 149 (2017) 381–391 Table 3 Uncertainties of the acquired quantities. Measurement

Uncertainty (%)

Torque Fuel flow meter Air flow meter In-cylinder pressure Excess air ratio BSFC BTE HC CO NOx PM

±1.0 ±1.0 ±0.5 ±0.1 ±0.1 ±1.93 ±1.72 ±0.2 ±0.2 ±0.2 ±0.1

2.2. Test fuels and experimental procedures Iso-butanol, conventional diesel and gasoline were used in this study, with properties shown in Table 1. To investigate the effects of diesel/gasoline/iso-butanol blends on combustion and emissions performance, pure diesel (denoted as D100) was prepared as based fuel and three blends were prepared and denoted as D70G30 (70% diesel; 30% gasoline, mass basis), D70B30 (70% diesel; 30% isobutanol) and D70G15B15 (70% diesel; 15% gasoline; 15% isobutanol) for evaluation. The engine speed was kept at a constant speed of 1800 ± 5 rpm, the injection timing was fixed at 7.5 crank angle (CA) before top dead center (TDC) and the EGR valve was kept closed for all test cases. Experiments were carried out at 10%, 30%, 50%, 70% and 90% engine loads, corresponding to brake mean effective pressures (BMEP) of 0.13, 0.38, 0.63, 0.88 and 1.13 MPa, respectively. To ensure the reproducibility and reliability of all measured data, the engine was first heated and maintained at steady state for several minutes at each test condition, and the coolant was kept at 85 ± 1 °C, the lubricating oil at 87 ± 2 °C, and the intake air temperature and pressure precisely at 15 ± 0.5 °C and 110 ± 0.3 kPa. Then in-cylinder pressure, emissions and control parameters were recorded for off-line analysis. After switching to a new blend, the engine was allowed to run for 15 min before data collection, to ensure the new fuel was not contaminated by the remnants of the old fuel. Each measurement was repeated 20 times to guarantee the reliability, and the uncertainties of the main measurements were summarized in Table 3. 3. Results and discussion 3.1. Combustion performance The in-cylinder pressures and corresponding heat release rates for four blend fuels at 0.13, 0.63 and 1.13 MPa BMEP are presented in Fig. 2. At 0.13 MPa BMEP, as shown in Fig. 2(a), D70G30, D70G15B15 and G70B30 retard the combustion ignition time, produce higher heat release rates but lower combustion pressure peaks than D100. The ignition delay time is determined by the cetane number, latent heat of vaporization and auto-ignition temperature of injected fuel in a diesel engine. Compared with diesel fuel, gasoline or iso-butanol has lower cetane number, higher latent heat of vaporization and auto-ignition temperature, resulting in the autoignition resistance characteristics of D70G30, D70G15B15 and D70B30 are enhanced. The lower in-cylinder pressure peaks for D70G30, D70G15B15 and G70B30 than that of D100 is due to the enhanced ignition delay, and the combustion mainly occurring far away from the engine TDC in the expansion stroke. D70B30 has the longest ignition time period and the highest peak value of heat release rate due to the cetane number of iso-butanol is lower than that of diesel and gasoline.

Fig. 2. Variations of in-cylinder pressure and corresponding heat release rate for four fuels at the engine loads of (a) 0.13 MPa BMEP, (b) 0.63 MPa BMEP and (c) 1.13 MPa BMEP.

At 0.63 and 1.13 MPa BMEP, shown in Fig. 2(b)–(c), it is observed that the combustion starting points of D70G30, D70G15B15 and D70B30 are also delayed compared to D100, while these three fuels produce higher peaks of heat release rate and incylinder pressure. As stated before, the combustion start points of D70G30, D70B30 and D70G15B15 are delayed on account of the lower cetane number, the higher latent heat of vaporization and

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the higher autoignition temperature of gasoline or iso-butanol with respect to pure diesel. The delayed ignition time of D70G30, D70G15B15 and D70B30 lead to the premixed combustion proportion increased, which resulted in higher values of the peak heat release rate and the peak in-cylinder combustion pressure. D70B30 has the latest combustion start point and the most abundant oxygen content, which could explain why it produce the highest peak heat release rate and in-cylinder combustion pressure. Fig. 3 displays the variations of the ignition delay, the combustion duration, CA50, the maximum pressure rise rate (MPRR), the excess air coefficient, the BSFC and the brake thermal efficiency (BTE) with respect to D100, D70G30, D70G15B15 and D70G30 under different engine loads. Fig. 3(a) shows the ignition delay (defined as the difference in crank angle position between fuel injection timing and 10% of total heat is released) during the combustion of D100, D70G30, D70G15B15 and D70G30. As mentioned before, because of lower cetane number and higher auto-ignition temperature of gasoline, the ignition delay period of D70G30 is longer with respect to D100. While D70G15B15 and D70B30 have much longer ignition delay periods than D100 and D70G30 on account of the much lower cetane number and the much greater latent heat of vaporization of iso-butanol. It is worth to notice that the difference between the ignition delay periods of the four fuels are changed dramatically at light loads but no distinct change at heavy loads,

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indicating that the effects of fuel properties on the combustion process are weakened at heavy loads. Fig. 3(b) shows the combustion duration (defined as the difference in crank angle position between 10% and 90% of the total heat release) during the combustion of the four fuels. The combustion duration changed in an opposite trend compared to the ignition delay (Fig. 2(a)), because an extended ignition delay can promote the fuel/air mixing process and thus accelerate the premixed burning. Meanwhile, gasoline and iso-butanol have lower viscosities and better volatilities than that of diesel, so blended fuels are atomized and volatized more easily in the cylinder, which can promote uniform fuel/air premixing and accelerate the combustion speed. The shorter combustion duration of D70B30 than D70G30 could be mainly attributed to the longer ignition delay and the higher oxygen content [45]. Fig. 3(c) shows the variations of CA50 (the crank angle at which 50% of the total heat is released) during the combustion of the four fuels at different engine loads. At the 0.13 and 0.38 MPa BMEP engine loads, CA50 is delayed with gasoline or iso-butanol addition, in a descending order of CA50 advance, D100 > D70G30 > D70G15B15 > D70B30. This variation trend can be explained by the drastically prolonged ignition delay for D70G30, D70G15B15 and D70B30 than D100 (see Fig. 2(a)). At the medium and heavy loads (0.63, 0.88 and 1.13 MPa BMEP), CA50 is advanced with gasoline or iso-butanol addition. This trend

Fig. 3. Variations of (a) the ignition delay, (b) the combustion duration, (c) CA50, (d) MPRR, (e) excess air coefficient, (f) BSFC and (g) BTE for four blends at different engine loads.

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Fig. 3 (continued)

could be explained as follows: as the engine load increases, the incylinder temperature is increased and the disparities of the ignition delay periods of D100 and other blends are shortened. In addition, the combustion duration is decreased after the addition of gasoline or iso-butanol. Fig. 3(d) displays the variation of MPRRs as a function of the engine loads. It shows that, at 0.13 MPa BMEP engine load, the MPRRs for D70G15B15 and D70B30 decrease compared with that of D100, which is due to the start of combustion for D70G15B15 and D70B30 are further retarded far away from TDC, leading to the lower MPRR compare to D100. Above 0.13 MPa BMEP load, the MPRR values of D70G30, D70G15B15 and D70B30 are higher than that of D100. This can be explained by the longer ignition delay period and the larger premixed combustion proportion by the addition of gasoline and iso-butanol. It is worth to notice that the MPRR value of D70B30 is higher than that of D70G30, which due to the longer ignition delay and the higher oxygen content for D70B30. Fig. 3(e) indicates the variation of excess air coefficients for the test fuels under different engine loads. It can be observed that, the excess air coefficients decrease for all test fuels as the engine load increases. With the engine load increases, the injected fuel mass increases and the in-cylinder oxygen concentration decreases relatively resulting in the excess air coefficient decreased. D70G30 has similar excess air coefficients with D100 due to the stoichiometric air/fuel ratio of gasoline is close to diesel. Iso-butanol is an oxygenated fuel (with about 21.5% oxygen content), so with

iso-butanol addition into diesel, the excess air coefficients are increased slightly. Fig. 3(f) displays the variation of BSFCs versus the engine load for different fuels. As observed, the BSFCs of D70G15B15 and D70B30 are slightly higher than that of D100. This is mainly due to the lower energy content of iso-butanol (33.3 MJ/kg) as compared to diesel (42.50 MJ/kg) and gasoline (42.90 MJ/kg). Masses of D70B30 and D70G15B15 yield are slightly higher with the same output power than those of D100 and D70G30, resulting in slightly higher BSFC values. Fig. 3(g) presents the variation of BTEs during combustion of the four fuels at different engine loads. It can be observed that D70G30 produce slightly higher BTE values compared to D100, in addition, the thermal efficiencies are further improved by adding isobutanol into diesel in spite of decrease in the heating value of the blend fuels, which can be explained that the addition of gasoline or iso-butanol can provide additional fuel lubricity, reduce fuel viscosity, improve atomization, and provide more oxygen contents for improving the in-cylinder combustion process in converting fuel chemical energy into useful engine work. 3.2. HC and CO emissions characteristics Fig. 4 illustrates the change of the unburned HC and CO emissions for all fuels. It can be observed from Fig. 4(a), relative to D100, the HC emissions from D70G30, D70G15B15 and D70B30 are higher over the whole engine load ranges and the increments

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fuels are higher with respect to D100 and lower at medium and heavy loads. In comparison with D100, at 0.13 and 0.38 MPa BMEP engine load conditions, the CO emissions are increased by a mean of 16.67% for D70G30, 58.33% for D70G15B15 and 191.67% for D70B30, at above 0.38 MPa BMEP engine loads, mean reductions in CO emission of 53.33% for D70G30, 56.67% for D70G15B15 and 83.33% for D70B30 are observed. Han et al. [49] pointed out that the CO emissions are mainly influenced by the global equivalence-ratio. At low engine loads (0.13 and 0.38 MPa BMEP), although the excess air/fuel ratio is larger for D70G30, D70G15B15 and D70B30 than that of D100, overlong ignition delay cause the start point of combustion far away from TDC, resulting in uncomplete combustion, especially for D70B30. As engine load increase, the higher in-cylinder temperature, the better fuel volatility and the longer ignition delays improve the combustion of D70G30, D70G15B15 and D70B30. Compared to D70G30, D70B30 has a longer ignition delay, lower C/H, lower stoichiometric air/fuel ratio and higher oxygen content, which can promote more complete combustion and then reduce the CO emissions. 3.3. PM emissions characteristics Fig. 5 illustrates the particle size distributions (PSDs) of the test fuels under 0.38 MPa and 0.88 MPa BMEP engine loads. As can be seen from Fig. 5, both PSD curves of four fuels show only unimodal lognormal distribution. The PSD peak values of D70G30,

Fig. 4. Variations of HC and CO emissions for four fuels at different engine loads.

are found to be lower at medium and heavy loads. Compared to D100, D70G30, D70G15B15 and D70B30 produce average increases in HC of about 31.04%, 73.95% and 140.47%, respectively. It was reported that fuel with lower cetane number increased HC emissions [46,47]. Although the molecular oxygen inside D70B30 promote complete combustion, more fuel vapor is entrained into the crevice region for D70B30 due to the longer ignition delay compared to other fuels, which cause the HC emissions to be higher than D100, similar observation have also been reported in Ref. [48]. In addition, the higher HC emissions of D70G30, D70G15B15 and D70B30 can also be ascribed to the lower densities and viscosities of those fuels [47], the fuel with lower density and viscosity was reported to produce smaller size droplets in the process of spray and atomization. These smaller fuel droplets reach to the closer place of the cylinder walls results in quenching effect due to leaner fuel/air mixture and also lead to an increase in unburned fuel emission. The higher HC emissions for diesel/isobutanol blends compared to diesel/gasoline blend at any given engine load due to the lower octane and higher heat of evaporation of iso-butanol than gasoline. With the engine load increase, the shorter ignition difference between D100 and other blend fuels can ease this phenomenon. The change in CO emissions for D100, D70G30, D70G15B15 and D70B30 are shown in Fig. 4(b). It is interesting to notice that at light loads (0.13 and 0.38 MPa BMEP) the CO emissions for blend

Fig. 5. Particle size distributions for four fuels at the engine loads of (a) 0.38 MPa BMEP and (b) 0.88 MPa BMEP.

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D70G15B15 and D70B30 are lower than that of D100, it can also be observed that gasoline and iso-butanol addition make the peak of PSD move to smaller size particles: the number concentration of particles size larger than 13 nm decreased but smaller particles increased. The primary reasons for the reduction of larger particles (Dp > 13 nm) are listed below. The blend fuels with the addition of iso-butanol have a longer ignition delay time, resulting in a more thorough fuel/air mixture. Meanwhile iso-butanol addition can increase the amount of oxygen atoms during the combustion process, decrease local fuel-air equivalence ratios, leading to the oxidation of soot precursors (such as acetylene, propargyl and aromatic hydrocarbons) and soot particles and therefore results in a decrease of larger particles [50–52]. Meanwhile, the lower C/ O in oxygenated fuel can reduce the carbon concentration in the process of soot formation. D70G30 has longer ignition delay for improving the fuel/air mixture. In addition, the better volatility of gasoline can improve the atomization during D70G30 injection [53]. However, without oxygen content in gasoline and shorter ignition delay results in soot emissions from D70G30 being higher than those from D70B30. The lower number of larger particles emissions after gasoline and iso-butanol addition can also be attributed to the lower or absence of sulfur and aromatic content in the gasoline and iso-butanol [54]. The possible reasons for numerous particles that size smaller than 13 nm increased with the addition of gasoline and isobutanol are listed as follows: (1) Smaller size particles formation can be enhanced by the better fuel/air mixing and fuel atomization [55,56]. (2) Gasoline and iso-butanol could produce higher HC

emissions (see Fig. 4(a)) results in increase the number of soluble organic fraction (SOF) and smaller soot particles [57]. (3) Gasoline or iso-butanol addition can reduce the number of larger particles, as stated previously, so the surface area of solid soot particles is lessened, leading to weaken the ability of SOF condensation and adsorption on the particles, which can relieve the suppression and strengthen the homogeneous nucleation, thus promoting the formation of small particles [52,58]. (4) As previously mentioned that the BSFC increase with iso-butanol addition due to its lower heat value (see Fig. 3(f)), more blends fuel combustion may promote more soot particles nucleation and produce more small size particles. (5) With gasoline or iso-butanol added into diesel fuel lead to suppress the soot formation. However, the processes of coagulations and agglomerations of the small particles will be slowed down simultaneously, which can increase the number of small particles relatively [59]. Fig. 6 displays the number and mass concentrations (NC and MC) of NM and AM particles of the four fuels under different engine loads. As can be seen from Fig. 6, as the engine load increases, the NC and MC of NM and AM particles show the tendency of increase for four fuels. Fig. 7 shows the particle count mean diameters (CMDs) of NM and AM particles for the four fuels at all operating engine loads. As shown in Fig. 7, as the engine load

Fig. 6. Number and mass concentrations (NC and MC) of (a) nucleation mode (NM) and (b) accumulation mode (AM) particles for four fuels at different engine loads.

Fig. 7. Count mean diameters (CMDs) of (a) nucleation mode (NM) and (b) accumulation mode (AM) particles for four fuels at different engine loads.

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increases, the CMDs of NM particles for D100 and D70G30 increase slightly, while the CMDs of D70G15G15 and D70G30 shows no regular changes. As the engine load increases, the CMDs of AM particles for D100 continuous increases, but the CMDs of AM particles for blend fuels first remains almost unchanged at about 40 nm, then increases rapidly. At a constant engine load, the NC, MC and CMDs of NM and AM particles of D70B30 are the smallest, followed by D70G15B15 and D70G30, and those parameters of D100 are the greatest.

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Under low engine load conditions, the mixture of fuel and air is lean and generate relatively low combustion temperature, resulting in the formation of less carbonaceous particles. As the engine load increases, a larger amount of fuel mass are injected into cylinder, the in-cylinder excess air coefficient decreases gradually (see Fig. 3(e)), which will increases the regions of local oxygen deficit, together with the higher pressure and temperature of the engine combustion chamber, promoted the fuel burned in the diffusion mode, which induced and accelerated the soot nucleation process [60] and enhances the aggregation and coagulation of primary carbon granules, finally leading to an increase in the CMDs, NC and MC of particles. Moreover, Tsolakis [61] thought that the increased PM formation at high engine load is attributed to the decline of soot oxidation at low oxygen content. As stated previously, the PSD peaks of D70G30, D70G15B15 and D70B30 are lower than that of D100, so the NC and MC decreased after gasoline and isobutanol addition. The reduction in the NC of particles can weaken the processes of coagulation and agglomeration among particles, which can decrease the CMD values. 3.4. The trade-off relationship between NOx and PM The most harmful pollutants to human health and the environment for diesel engine are NOx and PM. In this study, the NC, MC and CMD of total particle (denote as TNC, TMC and TCMD respectively) are calculated according to the measurement results of PSDs. The variations in NOx vs TNC, TMC and TCMD for the test fuels are shown in Fig. 8. As shown in Fig. 8, at 0.13 MPa BMEP engine load, it is obviously shown that the NOx and PM emissions can be reduced simultaneously using D70G30, D70G15B15 and D70B30 compared to D100, which may be attributed to the low temperature combustion. Above engine load of 0.13 MPa BMEP, D70G30, D70G15B15 and D70B30 have lower TNC, TMC and TCMD with slightly increases in NOx emissions than D100. The slightly increases in combustion temperatures, which can be seen from the exhaust temperatures for the test fuels (as shown in Fig. 9), may can explain the effects on NOx emission using four fuels. Based on the the tradeoff relationship between NOx and PM results, it can be suggested that both blend fuels can be used as alternative fuel for CI engine. In order to better compare the NOx emissions of the four fuels, NOx emissions from different fuels at different engine loads are depicted in Fig. 10 as well. The test results show that the blend fuels increase NOx by a mean of 2.03–6.73% compared to D100 at all engine loads.

Fig. 8. Variations in NOx vs. (a) TNC, (b) TCMD and (c) TMC at different loads.

Fig. 9. Variations of exhaust gas temperature for the test fuels at different engine.

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longer ignition delay. CO emissions are increased by a mean of 16.67% for D70G30, 58.33% for D70G15B15 and 191.67% for D70B30 with respect to D100 at low loads of 0.13-0.38 MPa BMEP due to combustion deteriorate. Above 0.38 MPa BMEP engine load, mean reductions in CO emission of 53.33% for D70G30, 56.67% for D70G15B15 and 83.33% for D70B30 are observed due to the higher excess air coefficients of blend fuels. (5) Compared to D100, both blend fuels can reduce the TCMD, TNC and TMC over all engine loads, but increase the NOx emissions slightly at above 0.13 MPa BMEP engine load. The blend fuels increase NOx by a mean of 2.03–6.73% under the entire engine loads. The inherent trade-off between NOx and PM can be eliminated at engine load of 0.13 MPa BMEP using D70G30, D70G15B15 and D70B30.

Acknowledgments Fig. 10. Variations of NOx emission for four fuels at different engine loads.

4. Conclusions This study experimentally investigated the combustion performance and emissions characteristics of a four-cylinder diesel engine fueled with four fuels including pure diesel, diesel/gasoline blend, diesel/iso-butanol blend and diesel/gasoline/iso-butanol blend under various engine loads (0.13, 0.38, 0.63, 0.88 and 1.13 MPa BMEP). The main conclusions can be summarized as follows. (1) At 0.13 MPa BMEP engine load, the peak values of the incylinder combustion pressures of D70G30, D70G15B15 and D70B30 were lower than that of D100, the lowest being D70B30, which can be due to the combustion occurred far away TDC for those blend fuels. At medium and high engine loads, the peak values of the in-cylinder combustion pressures and heat release rates of blend fuels were higher than those of D100, the highest being D70B30, which can be due to the high in-cylinder temperature, lower octane number, higher auto-combustion temperature and higher latent heat of vaporization of gasoline and iso-butanol. (2) At engine loads of 0.13–0.38 MPa BMEP, in a descending order of CA50 advance, D100>D70G30>D70G15B15>D70 B30, while at engine loads of 0.88–1.13 MPa BMEP, it shows the opposite trend. Gasoline addition can increase the MPRR at all engine loads. However, at engine load of 0.13 MPa BMEP, iso-butanol addition decrease the MPRR, while the MPRR increased gradually with the increasing the mass fraction of iso-butanol at high engine loads of 0.38–1.13 MPa BMEP. D70G30 has similar excess air coefficient and BSFC values with D100. D70G15B15 and D70B30 have higher excess air coefficient and BSFC values compared to D100, due to the oxygen content in fuels. (3) PSDs are mainly decided by the nucleation mode under different engine loads. With the addition of gasoline or isobutanol, the peaks of PSDs decreased and the position of the peaks move to smaller size particle, leading to the number of small size particles (<13nm) increased, and the number of large particles declined. The peak values of PSDs were the highest for pure diesel, followed by D70G30, D70G15B15 and D70B30. With the addition of gasoline or iso-butanol, CMD, NC and MC of NM and AM particles are decreased. (4) Compared to D100, D70G30, D70G15B15 and D70B30 produce average increases in HC emissions of about 31.04%, 73.95% and 140.47% respectively, which mainly due to their

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