Effect of glycerol ethoxylate as an ignition improver on injection and combustion characteristics of hydrous ethanol under CI engine condition

Energy Conversion and Management 98 (2015) 282–289

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Effect of glycerol ethoxylate as an ignition improver on injection and combustion characteristics of hydrous ethanol under CI engine condition R. Munsin a, Y. Laoonual a,⇑, S. Jugjai a, M. Matsuki b, H. Kosaka b a Combustion and Engines Research Laboratory (CERL), Department of Mechanical Engineering, Faculty of Engineering, King Mongkut’s University of Technology Thonburi, 126 Pracha Uthit Road, Bang Mod, Thung Khru, Bangkok 10140, Thailand b Advanced Thermo-Fluid Dynamics Laboratory, Department of Mechanical and Aerospace Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan

a r t i c l e

i n f o

Article history: Received 31 January 2015 Accepted 30 March 2015 Available online 15 April 2015 Keywords: Ethanol Ignition improvers Injection characteristics Combustion characteristics Two-color method

a b s t r a c t This paper investigates the effects of glycerol ethoxylate as an ignition improver on injection and combustion characteristics of hydrous ethanol under a CI engine condition. Injection characteristics were investigated by an in-house injection rate measurement device based on the Zeuch method, while spray combustion has been performed in the rapid compression and expansion machine (RCEM). The CI engine condition indicated as density, pressure and temperature of compressed synthetic gas, consisting of 80% argon and 20% oxygen, at fuel injection timing in RCEM of 21 kg/m3, 4.4 MPa and 900 K, respectively. This condition is equivalent to the isentropic compression of air of the actual CI engine with compression ratio of 22. Hydrous ethanol without ignition improver (Eh95) and the ethanol dedicated for heavy duty vehicles (ED95: composed of hydrous ethanol with the commercial additive for ED95) are reference fuels representing low and high quality ethanol fuel for CI engines, respectively. All test fuels are injected at constant heat input. The results indicate that the additional ignition improvers change injection characteristics, i.e. injection delay, injection rate and discharge coefficient of hydrous ethanol. The maximum injection rates at fully opened needle for the ethanol dedicated for heavy duty vehicles (ED95) and hydrous ethanol with 5% glycerol ethoxylate (5%GE) are lower than that of hydrous ethanol without ignition improver (Eh95) by approximately 10%. Additional injection duration is required for ED95 and 5%GE to maintain a constant energy input. Ignition delay and heat release rate of Eh95 are significantly improved by ignition improvers. The effect of ignition improvers on flame temperature is minor. KL factor, which is approximately proportional to soot amount in the optical path, and the measured soot emission, are affected by these improvers. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Due to global warming issues and public health concerns, the worldwide emission regulations for internal combustion engines have become more stringent. The possibility of simultaneous reduction of exhaust and greenhouse gas emissions, the improvement of thermal efficiency and use of sustainable energy can be achieved by the use of ethanol, considered as carbon–neutral and a renewable energy source. Ethanol can be used as fuel not only in spark ignition engines, but also in compression ignition (CI) engines which obtain higher thermal efficiency. Alternative ethanol fuelled CI engine buses have been used for public transportation in Sweden since 1984 [1] ⇑ Corresponding author. Tel.: +66 2470 9273; fax: +66 2470 9111. E-mail address: [email protected] (Y. Laoonual). http://dx.doi.org/10.1016/j.enconman.2015.03.116 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.

facilitating the development and years of continual improvement of ethanol buses and trucks. From 2006 to 2009, a European project named BioEthanol for Sustainable Transport (BEST) promoted the use of city buses equipped with commercial ethanol CI engines fuelled by the ethanol dedicated for heavy duty vehicles (ED95: composed of 95% hydrous ethanol and 5% commercial additives by volume). The project demonstrated that ethanol can be substituted for a significant percentage of the fossil fuels used for transport in Europe [2,3] and Brazil [4,5]. In 2010–2011, a similar demonstration project was launched for public transportation in Thailand, further contributing to the development of sustainable fuels [6,7]. Ethanol fuelled CI engines showed satisfactory performance [1–7] and promising reduction of particulate matter and NOx [8–11] compared with a conventional diesel engine.

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Nomenclature SOE SOI ASOI dm/dt mf mth

qf K A DP dQ/dt V P C1 Pb

start of energizing start of injection after start of injection injection rate measured mass flow rate theoretical mass flow rate fuel density bulk modulus of fuel area of orifice outlet pressure difference heat release rate in-cylinder volume in-cylinder pressure the first Planck’s constant back pressure

It is known that using ethanol in CI engines produces poor ignition due to difficulties in simultaneously achieving suitable concentration and temperature for self-ignition [12]. To utilize ethanol in CI engines, various techniques can be used, i.e. blending [13,14,18], dual fuel technique [15–18], spark assisted device [3,18], and using ignition improvers [6,7,18]. Ecklund et al. [18] concluded that for short-term displacement of diesel shortages, the ethanol–diesel solution and emulsion could best be employed. However, these techniques are limited by the separation of fuel and small percentages of diesel replacement (less than 25%). In the moderate length shortages of diesel, dual fuel techniques (including fumigation and pilot injection) are better, and it is possible to switch back to dedicated diesel engines. The ethanol percentage for fumigation and pilot injection techniques can be used up to 50% and 90%, respectively. For total substitution of diesel, the use of a spark assisted device, or ignition improvers for ethanol, are required. Ignition improvers are a more practical solution, with minor engine modifications, e.g. increasing the compression ratio and injector nozzle hole, required to achieve viable engine output. Ignition improvers must be miscible with ethanol. The effective ignition improvers should be thermally unstable, readily creating free radicals, which accelerate the thermal decomposition and chain branching reactions of the main fuel and stimulating autoignition [9]. A number of ignition improvers were tested in unmodified diesel engines [8,19,20]. The results concluded that nitrate-based additives have a strong effect on the ignition of alcohol. However, the use of nitrate-based improvers has some disadvantages, including the potential for increased corrosion, possibility of explosion, toxicity, wearing, and NOx emissions [21]. More recently, a commercial ethanol fuel (ED95) developed by SEKAB Biofuels & Chemical AB, has been used for ethanol buses and heavy duty diesel engines. Current additive for ED95 contains the components to act as lubricant, corrosion inhibitor and ignition improver, i.e. Beraid 3555 (the main component being glycerol ethoxylate) [22]. Few results of the effects of glycerol ethoxylate on the combustion of ethanol have been investigated [23,24]. Lif and Svennberg [23] showed that the ignition of hydrous ethanol fuel for CI engines is essentially improved by glycerol ethoxylate. Our recent work [24] showed that the ignition delays for the commercial ethanol fuel (ED95) and hydrous ethanol with 5% glycerol ethoxylate are lower than 1 ms, which is considered acceptable for real diesel engines over a wide range of operating conditions. The emissions from the ethanol CI engines should not be considerably affected by the additives. This is one of the requirements for an additive of ethanol used in CI engines [19]. It is expected that

c Pinj

e k K L

a T Ta I(k,T) Tg C2

specific heat ratio injection pressure emissivity wavelength absorption coefficient path length an empirical constant equal to 1.39 in the visible spectrum true temperature of the flame apparent temperature of the flame monochromatic radiant intensity gas temperature the second Planck’s constant

the addition of ignition improvers could change fuel properties, i.e. fuel viscosity and density, leading to changes in injection characteristics (injection delay, injection rate and discharge coefficient) [25], spray development [26] and combustion characteristics (ignition characteristic, heat release rate, flame temperature and KL factor) [27,28]. However, the previous works using ethanol in CI engines [23,24,27] do not include the effect of additives on injection and combustion of ethanol in CI engines which is a crucial issue for the use of ethanol in the CI engine. Therefore, the objective of this study is to investigate the effect of glycerol ethoxylate and current commercial additive for ethanol on injection characteristics (i.e. injection delay, injection rate and discharge coefficient) and combustion characteristics (i.e. ignition delay, heat release rate, flame temperature, KL factor and soot emission). The results will be relevant to alternative CI engine design and provide extended data and analysis on the injection and combustion characteristics of ethanol. 2. Methodology 2.1. Injection characteristics The injection characteristics, including injection rate, injection delay and discharge coefficient are studied. Injection rate measurements are performed by the Zeuch method. The test fuel is injected

Average fuel injection rate

SOI

Stabilized zone Injection Transitional zone SOE delay (needle opening)

Transitional zone (needle closing)

Fig. 1. Typical fuel injection rate profile of ethanol at Pinj = 90 MPa injected by single hole injector using for common rail injection system.

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into a constant volume chamber filled with test fuel at a certain pressure. The chamber pressure is increased in proportion to the injected fuel. Using bulk modulus (K) of fuel and the mass conservation, injection rate can be estimated by Eq. (1) [29,30].

mf ¼ dm=dt ¼ qf ðV=KÞðdP=dtÞ

ð1Þ

Bulk modulus (K) of fuel that is the fuel resistance to compression is defined as the ratio of the pressure change to the relative decrease of the volume, shown in Eq. (2) [30].

K ¼ dP=ðdV=VÞ

ð2Þ

Fig. 1 shows a typical fuel injection rate profile of ethanol obtained by using Eq. (1). The injection rate curve can be divided into four stages, i.e. injection delay, needle opening (transitional zone), stabilized zone and needle closing (transitional zone). The injection delay is the period between the start of energizing (SOE), i.e. supplying voltage for injector, and the start of injection (SOI) where injection rate recovers from the negative value, which is obtained from the change of small volume due to needle lift. Discharge coefficient (Cd) is defined as the ratio of the measured mass flow rate to the theoretical mass flow rate computed from the Bernoulli equation as shown in Eq. (3) [25].

C d ¼ mf =mth ¼ mf =ðAð2DPqf Þ1=2 Þ

ð3Þ

2.2. Combustion characteristics Combustion characteristics, including ignition delay, heat release rate, flame temperature and KL factor, are studied. Ignition delay is defined as the time interval between the start of injection (SOI) and the start of combustion (SOC) [24]. Heat release rate can be determined from the first law of thermodynamics [31] shown in Eq. (4).

dQ =dt ¼ ðc=ðc  1ÞÞPðdV=dtÞ þ ð1=ðc  1ÞÞVðdP=dtÞ

ð4Þ

Flame temperature and KL factor (optical thickness of soot) were measured by the two-color method. The principle of the two-color method is the measurement of the thermal radiation from soot particle at two wavelengths on the emission spectrum [32,33]. The monochromatic radiant intensity from the non-black body of the flame can be expressed in Eq. (5).

Iðk;TÞ ¼ ek ðC 1 =ðpk5 ÞÞexpðC 2 =ðkTÞÞ

ð5Þ

As with the monochromatic radiant intensity, it can also be expressed in terms of the apparent temperature, Ta, as shown in Eq. (6).

Iðk;TÞ ¼ ðC 1 =ðpk5 ÞÞexpðC 2 =ðkT a ÞÞ

ð6Þ

The modified equation for monochromatic emissivity of a soot cloud is given by Eq. (7)

ek ¼ 1  expðKL=ka Þ

ð7Þ

The monochromatic radiant intensity from Eq. (5) and Eq. (6) is equal. Replacing ek in the right-hand side of Eq. (5), it is obtained:

KL ¼ ka lnð1  expððC2=kÞðð1=T a Þ  ð1=TÞÞÞ

ð8Þ

KL, representing soot concentration, can be assumed to be independent of wavelength for the small variations encountered when using this method. Therefore, the measurement of two radiations at two distinct wavelengths enables solution for the KL and T, based on Eq. (8).

3. Experiment 3.1. Experimental setup A schematic diagram of the injection rate measurement system is shown in Fig. 2(a). It consists of a common rail injection system, hand pump, constant volume chamber, pneumatic cylinder and data acquisition. Before the injection test, the bulk modulus of test fuel is calibrated by fuel compression driven by the pneumatic cylinder. Then, the bulk modulus of test fuel is calculated by Eq. (2). To find the injection rate, the test fuel is filled into the constant volume chamber by a hand pump until the desired pressure, where it is relevant to the initial pressure in combustion chamber (4.4 MPa), is reached. This is measured by a static pressure transducer. Then test fuel is injected, resulting in a steep pressure rise measured by a dynamic pressure transducer. The injection rate (mf) and discharge coefficient (Cd) can be calculated by using Eqs. (1) and (3), respectively. The combustion experiments were carried out in a rapid compression and expansion machine (RCEM), as shown in Fig. 2(b). The RCEM has a 100 mm bore with stroke and clearance height of 150 mm and 20 mm, respectively, and compression ratio of 8.5 [34]. The RCEM with compression ratio of 8.5 and the flat piston is used, because the volume of combustion chamber at TDC should be enough to achieve the simple impinging spray flame and reduce the effect of side wall on the spray. Even though the compression ratio of RCEM was not so high compared to the actual engine, this RCEM can simulate the condition of CI engine at compression ratio as high as 22. Synthetic gas that represents the air for combustion, consisting of 80% argon and 20% oxygen, is prepared in a mixing tank at 180 °C, and then entrained into the cylinder. Argon is used instead of nitrogen as argon has a higher specific heat ratio. The synthetic gas is compressed by piston from BDC to TDC within 30 ms. The piston is then held motionless at TDC for 150 ms to provide constant volume conditions. Fuel is injected into the cylinder at a constant pressure of 90 MPa via a single hole nozzle with a diameter of 0.24 mm. Gas pressure inside the combustion chamber is measured by a piezoelectric pressure transducer (Kistler, 6125C01). The heat release rate is analyzed by using Eq. (4). The spray flame was imaged by a high speed color camera (NAC Memrecam GX-1), at 10,000 frames per second and exposure time of 5 ls capturing an image of 476  464 pixels. The high speed camera was fitted with a micro-Nikkor 55 mm f/2.8 lens using aperture f/4 and an infrared filter (SIGMA KOKI-HAF-50S-15H). This filter transmits only visible light to allow the calculation of flame temperature and KL factor using the two-color method Eq. (8). The visible light can be converted by the CCD detector in the high speed color camera into three color bands, i.e. red, green and blue. Any two of the three color bands can then be used for the calculation of temperature and KL factor. Two wavelengths that used in this work are 495 nm (the blue band) and 575 nm (the red band), because these wavelengths have spectral response with insignificant overlap region. The relationship between the photomultiplier output and apparent temperature is calibrated by a blackbody furnace. Soot emission is measured by a smoke meter (SOKKEN, model GSM3).

3.2. Experimental conditions Table 1 shows the compositions of test fuels. To focus on the effects of ignition improver on combustion characteristics, the important parameters that affect ignition and combustion, i.e. incylinder gas temperature and pressure at time of fuel injection, nozzle orifice, oxygen concentration [35] and injection pressure

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Common rail controller Pulse generator Injector Hand pump

Monitor Static pressure transducer

Common rail system

Fuel tank

Dynamic pressure Chamber transducer

Pneumatic cylinder N2

Oscilloscope Plunger

(a) Injection rate measurement system Fuel tank Common rail system

Common rail controller Trigger signal

Injector

Computer Camera

Charge amplifier

Computer

RCEM

Pressure transducer

Displacement sensor

Ar

Computer

O2

Oscilloscope Mixing tank

(b) Rapid compression and expansion machine (RCEM) for combustion study Fig. 2. Schematic diagram of experiments.

Table 1 Test fuel compositions.

Table 3 Experimental conditions.

Compositions, %wt

Eh95

ED95

5%GE

Surrounding conditions

Value

Unit

Ethanol Water Lauric acid Commercial additive for ED95 Glycerol ethoxylate

94.00 5.00 1.00 – –

84.36 4.44 – 11.20 –

89.30 4.75 0.95 – 5.00

Temperature: Tg Pressure: Pg Density: qg

900 4.4 21

K MPa kg/m3

Injection conditions Single hole nozzle diameter: / Injection pressure: Pinj Equivalence ratio: U

0.24 90 0.14–0.16

mm MPa –

Table 2 Properties of ethanol and glycerol ethoxylate. Properties

Ethanol [12]

Glycerol ethoxylate [38,39]

Formula Oxygen content (% by weight) Viscosity (cP) Density (g/cc at 20 °C) LHV (MJ/kg) Cetane number Latent heat of vaporization (kJ/kg) Boiling temperature (°C) Autoignition temperature (°C)

C2H5OH 34.80 1.19@20 °C 0.785 26.80 <5.0 854.8 78 422

C5H14O5 48.78 364.20@21 °C 1.138 25.26 n.a. n.a. 200 n.a.

n.a.: not available.

Test fuels

Injection amounts (mg)

Injection duration (ms)

Energy input (J)

Eh95 ED95 5%GE

36.68 36.77 36.99

3.05 3.09 3.30

937 937 937

[35,36], were fixed. Glycerol ethoxylate and commercial additive for ED95 (the trade name is Additive ED95 101) are used as the ignition improvers. The three fuels considered in this study are hydrous ethanol without ignition improver (Eh95); the commercial ethanol fuel (ED95) consisting of hydrous ethanol and the commercial additive for ED95; and hydrous ethanol containing 5% by weight of glycerol ethoxylate (5%GE). The composition of ED95 in Table 1 is shown in mass fraction. With this mass composition,

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16 Eh95 ED95 5%GE

Fuel injection rate , mg/ms

14 12 10 8 6 4 2 0

0

1

2

3

4

5

6

Time after start of energizing, ms

(a) Fuel injection rate measurement

Discharge coefficient (Cd)

0.8

injection delay. In the stabilized zone where the needle is fully opened, the injection rate curve of Eh95 is the highest, while ED95 and 5%GE are lower than that of Eh95 by approximately 10%, and there is little difference between the two. The longer injection duration for ED95 and 5%GE is obtained to maintain the required constant heat input of 937 J as shown in Table 3. Fig. 3(b) shows the corresponding discharge coefficient (Cd). Discharge coefficient for Eh95, ED95 and 5%GE are 0.72, 0.68 and 0.66, respectively. The cause of decreased Cd for ED95 and 5%GE is their higher viscosity. In addition, the increasing theoretical mass flow rate of ED95 and 5%GE due to the increased densities is higher than their increasing measured mass flow rates. In summary, the longer injection delay, smaller injection rate and smaller discharge coefficient of hydrous ethanol are clearly obtained when ignition improvers are added. The change of injection will affect the combustion characteristics of ethanol fuels. Therefore, it is necessary to investigate the effect of ignition improvers on the combustion characteristics of test fuel as shown in the next section. 4.2. Combustion characteristics

0.7

0.6

0.5 Eh95

ED95

5%GE

(b) Corresponding discharge coefficient (Cd) of test fuels Fig. 3. Fuel injection characteristics at Pinj = 90 MPa and Pb = 4.4 MPa.

ED95 becomes 95% hydrous ethanol and 5% commercial additives by volume. For Eh95, 1% by weight of lauric acid was added to hydrous ethanol to prevent injection system lubrication failure [37]. However, lauric acid is not added to ED95, because it already contains a lubricant in the commercial additive. Eh95 and ED95 are used to represent ethanol fuel of a low and high ignition quality fuel for CI engines. Table 2 shows properties of ethanol [12] and additive [38,39]. Table 3 shows the experimental conditions for the combustion study. The surrounding condition was kept constant at mixture gas density, pressure and temperature of 21 kg/m3, 4.4 MPa and 900 K, respectively, and was used consistently for all experiments. These in-cylinder gas conditions represent isentropic compression of air from ambient conditions at compression ratios as high as 22. The results of injection and combustion were averaged from 10 tests to analyze uncertainty and repeatability. The heat input of injected fuels was kept constant at 937 J by varying the amount of injected fuel.

Fig. 4 shows the average ignition delay of test fuels. The results show that the ignition improvers significantly improve ignition quality of hydrous ethanol, because the ignition improvers are thermally unstable, initiating free radicals that might accelerate the reactions leading to auto-ignition of the fuel [41]. In addition, the number of attack sites for creation of radicals (i.e. chain length) has increased with increasing molecular weight (by adding the additive) yielding a shorter ignition delay [31]. Ignition delays for Eh95, ED95 and 5%GE, are 1.33 ms, 0.91 ms and 0.79 ms, respectively. Although the ignition delay of ED95 (0.912 ms) is longer than that of diesel about of 75% [24], it is considered acceptable for diesel engines over a wide range of operating conditions for low-compression-ratio DI diesel engines (0.6–3.0 ms), high-compression-ratio and turbocharged DI diesel engines (0.4–1.0 ms) and IDI diesel engines (0.6–1.5 ms) [31]. From the experimental results, the pressure and heat release rate of 5%GE are nearly identical to those of ED95, while Eh95 shows a different results as shown in Fig. 5(a) and (b). Eh95 has the most rapid pressure rise (in Fig. 5(a)) and highest heat release rate (in Fig. 5(b)), because the longest ignition delay of Eh95 allows more time for the better fuel–air mixing leading to a strong premixed combustion, the steep pressure rise and the large heat release during the premixed combustion phase. The strong premixed combustion could increase the possibility of engine wear and combustion noise in CI engines, commonly referred to as diesel knocking [31]. However, heat released during the premixed combustion phase can be reduced approximately 55% by using ED95 and 5%GE with a shorter ignition delay and smaller pressure rise. The shorter ignition delays of ED95 and 5%GE result in the earlier and longer mixing-controlled combustion phase as shown in

2.0

4.1. Injection characteristics Fig. 3(a) shows injection rate of test fuels. The square profile of injection rate can be observed for three fuels. At the same start of energizing time, injection delays for Eh95, ED95 and 5%GE are 0.78, 0.83 and 0.85 ms, respectively. The difference in injection delay is caused by differing fuel viscosities [25,40]. ED95 and 5%GE require additional time to raise the injector needle due to the higher viscous forces resisting needle movement, resulting in the longer

Ignition delay, ms

4. Results and discussion 1.5 1.0 0.5 0.0

Eh95

ED95

5%GE

Fig. 4. Ignition delay of test fuels at Pinj = 90 MPa and Tg = 900 K.

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6.5

(a)

Eh95 ED95 5%GE

1900

Flame temperature, K

Pressure, MPa

6.0 5.5

Eh95 ED95 5%GE

5.0 4.5 4.0

1850 1800 1750 1700 1650

Injection signal

Heat release rate, J/ms

(b) 0

400

5

10

15

20

Time after start of injection, ms Fig. 7. The average flame temperature profile of test fuels at Pinj = 90 MPa and Tg = 900 K.

200

0

Start of mixing-controlled combustion 0

1

2

3

4

5

6

Time after start of injection, ms Fig. 5. (a) Pressure and (b) heat release rate of test fuels at Pinj = 90 MPa and Tg = 900 K.

Fig. 5(b). It is expected that the change of combustion will affect the emissions, especially NOx and soot that are highly produced from CI engines. For more quantitative analysis inside the spray combustion, the two-color method was used to investigate the flame temperature and KL factor that interpret as an indication of NOx and soot formation, respectively [42]. Fig. 6 shows the examples of typical flame temperature images obtained by the two-color method. The flame shapes of test fuels during combustion are similar. 5%GE shows the highest local temperature (up to 2000 K or higher), especially during 2–3 ms ASOI, while ED95 and Eh95 do not have the local temperature up to 2000 K. The locally high temperature of 5%GE combustion seems to be resulted from its higher viscosity that has an influence on the injection characteristics, as shown in Fig. 3, leading to the lower level of fuel–air mixing [42]. The existence of the locally high temperature region in the flame of 5%GE could contribute to a higher value of NOx. Unfortunately, NOx could not be measured

1.5 ms

2.0 ms

2.5 ms

in these experiments because no nitrogen presented in the synthetic gas and test fuels. Fig.7 shows the average flame temperature obtained from the average of temperatures in the flame area of ten experiments. The average flame temperature can be obtained by summing the flame temperature of each pixel in the flame area and dividing by the total number of pixels contained inside the flame area. In Fig. 7, two peaks of temperature distribution could be observed as same with the other work [43], which might be caused by the variation of localized combustion. The initial flames temperatures of Eh95, ED95 and 5%GE in Fig. 7 correspond to the start of the mixing-controlled combustion in Fig. 5(b) at 2.1, 1.5 and 1.5 ms ASOI, respectively. After the flame appearance, the flame temperatures of test fuels rise rapidly and are above 1800 K for several milliseconds. Then the flame temperature slowly decreases. These results suggest that the NOx formation for test fuels is possibly produced at high temperature above 1643 K [44] during the mixing-controlled combustion phase. The highest maximum flame temperature is obtained by 5%GE, while Eh95 has the lowest and shifts toward later time that affected from the longest ignition delay resulting in the latest start of mixing-controlled combustion as discussed above. However, the average flame temperature in the premixed combustion phase is difficult to obtain, because the premixed combustion phase is almost invisible flame and has very low luminosity from the low amount of soot particles.

3 ms

5 ms

10 ms

0 cm

Eh95

5 10

0 cm

ED95

5 10 0 cm

5%GE

5 10 Fig. 6. Examples of typical flame temperature images after start of injection at Pinj = 90 MPa and Tg = 900 K.

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1.5 ms

2.0 ms

2.5 ms

3 ms

5 ms

10 ms

0 cm

Eh95

5 10

0 cm

ED95

5 10 0 cm

5%GE

5 10 Fig. 8. KL factor images of test fuels at Pinj = 90 MPa and Tg = 900 K.

1.4

3

Eh95 ED95 5%GE

1.2

Soot, mg/m3

1.0

-4

KL (x10 )

2

1

0.8 0.6 0.4

0

0.2

Injection signal 0.0 0

5

10

15

Eh95

20

Time after start of injection, ms Fig. 9. Temporal variation of KL factor at Pinj = 90 MPa and Tg = 900 K.

Fig. 8 depicts the KL factor contour. It can be observed that at 2.5 ms ASOI, the higher soot particles of Eh95 are mainly formed at the center of the spray flame tip where it is a region of vortex (rotating flow) [45]. Then the soot particles are forced aside to the spray periphery (at 3 ms ASOI) and re-entrained into the flame center by vortex (at 5 ms ASOI). This characteristic is also shown in case of ED95 and 5%GE. However, ED95 produces higher value of KL factor than Eh95 and 5%GE, because the variation of localized combustion [43]. Fig. 9 shows the temporal variation of the average KL factor that gives the overall quantitative soot information in the flame area obtained from ten experiments. The average KL factor can be obtained in the same way with the average flame temperature. The soot formation of ED95 and 5%GE, which is characterized by the rise of KL factor up to the peak value [46], are earlier, and their maximum values increase as compared to that of Eh95. These results suggest that the soot formation of ethanol is increased by the ignition improvers. In addition, these results also support that the soot formation is depended on the start of ignition (ignition delay) as expected, because the additional ignition improvers can shorten ignition delay, resulting in the increase of mixing-controlled combustion phase. For all fuels, it can be observed that after the end of injection, the soot formations of test fuels still continue for several milliseconds, but there is no significantly different in the start of soot oxidation where it is a time of the start of the decreasing KL [46]. The oxidation rate of ED95 is similar to that

ED95

5%GE

Fig. 10. Soot concentrations at Pinj = 90 MPa and Tg = 900 K.

of Eh95, while the oxidation rate of 5%GE is slower. The reason of the slower oxidation rate of 5%GE cannot be concluded from this work, and a further study is required. The KL levels of test fuels at latter time in Fig. 9 show a good relation to the measured soot concentration as shown in Fig. 10. The measured soot concentrations of test fuels are in the range of 0.61–1.02 mg/m3 with high variation. However, the order of magnitude of soot concentration from ethanol combustion is much lower than those of diesel combustion (in the range of 10–300 mg/m3) [47], because the oxygen content of ethanol contribute a more complete combustion.

5. Conclusion The effect of the ignition improvers, i.e. glycerol ethoxylate and the commercial additive for ED95, on the injection and combustion characteristics of hydrous ethanol was investigated under CI engine condition. Three test fuels injected at the constant heat input (937 J) are hydrous ethanol without ignition improver (Eh95), the ethanol dedicated for heavy duty vehicles (ED95) and hydrous ethanol with 5% glycerol ethoxylate (5%GE). The main conclusions are that: The ignition improvers change not only the injection delay, but also the maximum injection rate and discharge coefficient of hydrous ethanol. Injection delays for hydrous ethanol are lengthened by commercial additive for ED95 and 5% glycerol ethoxylate by 0.08 and 0.1 ms, respectively. The injection rates of ED95 and 5%GE are lower than that of Eh95 by approximately 10%.

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Discharge coefficients for hydrous ethanol are decreased by commercial additive for ED95 and 5% glycerol ethoxylate by 6% and 8%, respectively. The additional injection duration is required to maintain the same heat input for ED95 and 5%GE. The commercial additive for ED95 and 5% glycerol ethoxylate can improve not only the ignition delay, but also the heat released in the premixed combustion phase of ethanol which could reduce the possibility of the strong premixed combustion and combustion noise for the CI engine. Glycerol ethoxylate at 5% and the commercial additive for ED95 show a minor effect on the highest average flame temperature of hydrous ethanol, but the local flame temperature are increased. The highest KL factor and soot concentration are sensitive to these improvers. The soot formation of ED95 and 5%GE are earlier and greater than that of Eh95. This is depended on the start of ignition. Acknowledgments The financial support of Ronnachart Munsin and Sumrerng Jugjai from Office of Higher Education Commission and The Thailand Research Fund through the Royal Golden Jubilee Ph.D. Program (Grant No. PHD/0083/2551) is gratefully acknowledged. The research fellowship of Yossapong Laoonual in Japan from Hitachi Scholarship Foundation is also highly appreciated. In addition, this work was supported by the Higher Education Research Promotion and National Research University Project of Thailand, Office of the Higher Education Commission. The authors also acknowledge Mr. Norihiko Takahashi and a technical staff of Tokyo Institute of Technology for their help during the experimental work. References [1] Egeback KE. Experiences from the use of ethanol for heavy duty compression ignition engines. SAE Paper 931630. SAE Brazil: Sao Paulo; 1993. [2] Birath K. Experiences from introduction of ethanol buses and ethanol fuel stations. Technical report no. D 2.1 and D 2.2. Stockholm; June 2007. [3] Fenton P, Carlsson H. Bioethanol for sustainable transport results and recommendations from the European BEST Project. Lenanders Grafiska AB: Kalmar; 2010. p. 66–72. [4] Moreira J, Apolinário S, Coelho S, Velázquez S, Melo EH, Elmadjian PH. Bioethanol for sustainable transport – BEST project. SAE Paper 2008-36-0216. SAE Brazil: Sao Paulo. 2008. [5] Velázquez S, Apolinário SM, Melo EH, Elmajian PHB, Janssen R, Hofer A. Report on experiences of ethanol buses and fuel stations in São Paulo. Final report. Report no D2.07. São Paulo; December 2009. [6] Chollacoop N, Saisirirat P, Sukkasi S, Tongroon M, Fukuda T, Fukuda A, et al. Potential of greenhouse gas emission reduction in Thai road transport by ethanol bus technology. Appl Energy 2013;102:112–23. [7] Laoonual Y. Ethanol fuel technology for substitution of diesel. Automotive navigator magazine. Thailand Automotive Institute; July/September 2013. p. 26; October/December 2013. p. 28. [8] Hardenberg HO, Schaefer AJ. The use of ethanol as a fuel for compression ignition engines. SAE Paper 811211. SAE International: Warrendale; 1981. [9] Nord K, Haupt D, Ahlvik P, Egebäck KE. Particulate emissions from an ethanol fueled heavy-duty diesel engine equipped with EGR, catalyst and DPF. SAE Paper 2004-01-1987. SAE International: Toulouse; 2004. [10] Saitoh H, Uchida K. Effect of hot EGR on combustion and emission characteristics in a diesel type alcohol engine. In: Proceeding of 24th CIMAC world congress on combustion engine technology. Kyoto, Japan; June 7–11, 2004. [11] Park SH, Youn IM, Lee CS. Influence of ethanol blends on the combustion performance and exhaust emission characteristics of a four-cylinder diesel engine at various engine loads and injection timings. Fuel 2011;90(2):748–55. [12] Saitoh H, Uchida K. Difference of spray mixture formation between gas-oil and ethanol in the constant volume electrical heating chamber. SAE Paper 200701-3617. SAE International: California; 2007. [13] Kannan D, Pachamuthu S, Nabi MN, Hustad JE, Løvås T. Theoretical and experimental investigation of diesel engine performance, combustion and emissions analysis fuelled with the blends of ethanol, diesel and Jatropha methyl ester. Energy Convers Manage 2012;53(1):322–31. [14] Labeckas G, Slavinskas S, Mazˇeika M. The effect of ethanol–diesel–biodiesel blends on combustion, performance and emissions of a direct injection diesel engine. Energy Convers Manage 2014;79:698–720. [15] Morsy MH. Assessment of a direct injection diesel engine fumigated with ethanol/water mixtures. Energy Convers Manage 2015;94:406–14.

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