The effect of triacetin as a fuel additive to waste cooking biodiesel on engine performance and exhaust emissions

Fuel 182 (2016) 640–649

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Full Length Article

The effect of triacetin as a fuel additive to waste cooking biodiesel on engine performance and exhaust emissions Ali Zare a,⇑, Md Nurun Nabi a,b, Timothy A. Bodisco c, Farhad M. Hossain a, M.M. Rahman b, Zoran D. Ristovski a,b, Richard J. Brown a a b c

Biofuel Engine Research Facility, Queensland University of Technology (QUT), QLD 4000, Australia International Laboratory for Air Quality and Health, Queensland University of Technology (QUT), QLD 4000, Australia School of Engineering, Deakin University, Waurn Ponds, Victoria 3217, Australia

h i g h l i g h t s
Oxygen ratio was used instead of the equivalence ratio.
Oxygen ratio decreases with engine load, but increases with engine speed.
IMEP, BMEP, friction power, CO2, HC, PM and PN decreased with oxygenated fuels.
BSFC, BTE and NOx increased with oxygenated fuels.
Accumulation mode count median diameter decreased with oxygenated fuels.

a r t i c l e

i n f o

Article history: Received 8 March 2016 Received in revised form 4 June 2016 Accepted 7 June 2016 Available online 11 June 2016 Keywords: Oxygen Oxygen ratio Blow-by Driving cycle Friction

a b s t r a c t This study investigates the effect of oxygenated fuels on engine performance and exhaust emission under a custom cycle using a fully instrumented 6-cylinder turbocharged diesel engine with a common rail injection system. A range of oxygenated fuels based on waste cooking biodiesel with triacetin as an oxygenated additive were studied. The oxygen ratio was used instead of the equivalence ratio, or air to fuel ratio, to better explain the phenomena observed during combustion. It was found that the increased oxygen ratio was associated with an increase in the friction mean effective pressure, brake specific fuel consumption, CO, HC and PN. On the other hand, mechanical efficiency, brake thermal efficiency, CO2, NOx and PM decreased with oxygen ratio. Increasing the oxygen content of the fuel was associated with a decrease in indicated power, brake power, indicated mean effective pressure, brake mean effective pressure, friction power, blow-by, CO2, CO (at higher loads), HC, PM and PN. On the other hand, the brake specific fuel consumption, brake thermal efficiency and NOx increased by using the oxygenated fuels. Also, by increasing the oxygen content, the accumulation mode count median diameter moved toward the smaller particle sizes. In addition to the oxygen content of fuel, the other physical and chemical properties of the fuels were used to interpret the behavior of the engine. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction High fuel prices, global warming, environmental degradation and adverse health effects of fossil fuels have been a topic for a significant amount of recent engine research in the last decade [1]. The European Union issued a directive to offset fossil fuel usage with renewable biofuels by 10% by 2020 (EU Directive 2009/28/ EC). Fuel specific solutions to this issue range from finding new sources of fuel or adding additives to fossil fuels. ⇑ Corresponding author. E-mail address: [email protected] (A. Zare). http://dx.doi.org/10.1016/j.fuel.2016.06.039 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.

Recently several types of biofuel have been introduced by researchers and industry. Waste cooking oil has received attention due to its low price, close properties to diesel and global availability [2]. Disposal issues including dumping waste cooking oil into waterways could be another reason to use it as a fuel [3]. The current literature shows some advantages and disadvantages for the use of waste cooking biodiesel [4]. A study reported that using waste olive oil decreased CO up to 58.9% and CO2 by up to 8.6%; however, NO2 and BSFC (brake specific fuel consumption) increased [5]. Similarly, another study also reported an increase in NOx, with a subsequent decrease in PM [4].

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The oxygen-bond in biofuels is a significant factor that makes them different from conventional petro-diesel. Most biofuels, such as those from animal fats, vegetable oils and waste cooking oil, contain long-chain alkyl esters which have two oxygen atoms per molecule. However, the fuel oxygen content is derived from the fatty acid ester profile such as unsaturation level and carbon chain length [6]. In terms of emission reduction, the oxygen content of the fuel has been reported to be a major factor compared to the other properties [7–9]. In this regard, a low volume of fuel additives with high oxygen content could play an import role boosting the emission reduction. As a highly oxygenated additive to fuel, triacetin [C9H14O6] (a triester of glycerol acetic acid) could be used in combustion process [10]. Glycerol is a byproduct of the biodiesel transesterification process and is therefore readily available, especially given that the production of waste glycerol will increase proportionally by increasing the production of biofuels. Utilisation of this costeffective feedstock as a fuel is possible, however, the physical and chemical properties of direct glycerol have limited its usage for this purpose [11]. Accordingly, triacetin as the product of acetylation process of glycerol and acetic acid could be used instead. A thorough search could not identify any publication in the literature investigating the effect of triacetin as a fuel additive with waste cooking biodiesel. However, a recent study showed that blending triacetin with biofuels increased the oxygen content, density and kinematic viscosity; however the cetane number and heating value of the blend reduced [10]. This paper intends to study engine performance and exhaust emission using a range of fuels with 0–14.23% oxygen content, based on waste cooking biodiesel as the primary fuel and triacetin as a highly oxygenated additive. 2. Experimental facilities 2.1. Engine specification In this experimental study a 6-cylinder turbocharged aftercooled diesel engine with a common rail injection system was used. An electronically controlled water brake dynamometer was coupled with this engine to control the steady-state and transient load on the engine. Regarding the indicated parameters, a piezoelectric transducer (Kistler 6053CC60) with a simultaneous analog-to-digital converter (Data Translation DT9832) were utilised to collect the in-cylinder data, crank angle and engine speed data were collected by a crank angle encoder set (Kistler type 2614). In addition, a blow-by sensor was utilised to measure the exhaust flow from the engine crankcase. For more specific information about the engine and experimental facilities readers can refer to Ref. [12]. Table 1 shows the test engine specification. 2.2. Exhaust sampling and test setup Exhaust emissions were sampled from the exhaust manifold via a 0.5 m long stainless steel tube. Apart from a fraction which was sent to the gas analysers via a copper tube fitted with a HEPA filter, the rest passed through the dilution tunnel and then to the particle measuring systems. Fig. 1 shows the schematic diagram of the test setup. 2.3. Fuel selection Table 1 shows the set of fuels used in this study. Except neat diesel (D100) and D60B35T5, the blends were made based on the waste cooking biodiesel (B) as the primary fuel and triacetin (T)

Table 1 Test engine specification. Model Cylinders Capacity Bore  stroke Maximum power Maximum torque Compression ratio Aspiration Fuel injection Dynamometer type Emission standard

Cummins ISBe220 31 6 in-line 5.9 L 102  120 (mm) 162 kW @ 2000 rpm 820 Nm @ 1500 rpm 17.3:1 Turbocharged High pressure common rail Electronically controlled water brake dynamometer Euro III

as an additive. The first row of this table shows the 6 different fuels used in this study classified by the portion of each fuel in the final fuel. For example, T8B92 stands for 8% (by volume) of triacetin added to 92% (by volume) of waste cooking biodiesel. The miscibility and stability of blends were tested at the room temperature for 96 h and no phase separation was observed. The oxygen content of the selected fuels ranged from 0 to 14.23%. Increasing the oxygen content of the fuel decreases the lower heating value (LHV), which in turn negatively affects the engine power. In addition, the kinematic viscosity of the fuel, which is related to the degree of unsaturation, increases with the fuel oxygen. This can adversely affect the atomisation of the fuel spray and evaporation characteristics of the fuel during combustion [1]. The values for the blends listed in Table 2 are estimated based on pure substance compositions. For more information about triacetin and waste cooking biodiesel, the reader can refer to Ref. [10] and a recent publication from our research group [13]. 2.4. Design of experiment Driving cycles which show the driving pattern are not only used to model the exhaust emission, but also to evaluate fuel consumption and engine performance. Driving cycles are classified based on the driving pattern. The first category which is composed of different quasi steady-state modes of speed and load is called ‘‘modal” or ‘‘polygonal”, and the second one, real world driving cycles are based on actual driving data [14]. In the literature, many investigations used real world driving cycles such as US Federal Test Procedure (FTP) transient cycle [1]. These driving cycles contain a very large amount of transient fluctuations with rapid changes which limit the fundamental investigations on transient response. Hence a modal controllable driving cycle with a gentle mode change may help a more fundamental investigation in both transient response and steady-state response [15]. To date, the number of investigations using controllable transient conditions are limited [1,15,16]. It is preferable to use existing modal driving cycle from the literature, however no suitable example could be found to achieve the research goals of the investigation. Hence, a custom cycle based on modal driving cycle schedule with some controllable transient modes was designed to evaluate the engine performance and exhaust emission during steady-state and transient discrete modes of operation. Since the engine used in this study has a Euro III emission certification, to design this custom cycle, speed and torque were selected from European Stationary Cycle (ESC) driving cycle pattern, which was a legislated test cycle for heavy-duty engines in the Euro III legislation. Unlike the ESC, which has a sharp change between steady-state modes, different controllable transient ramps were added between the steady-state modes instead of sharp changes. The concept was based on the Supplemental Emissions Test (SET) introduced in the

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Fig. 1. Schematic diagram of test set-up.

Table 2 Fuel properties. Fuel

D100

B100

T100

D60B35T5

T4B96

T8B92

T10B90

O (wt%) C (wt%) H (wt%) Density@15 °C (g/cc) HHV (MJ/kg) LHV (MJ/kg) kV@40 °C (mm2/s) Cetane number

0 85.1 14.8 0.84 44.79 41.77 2.64 53.3

10.93 76.93 12.21 0.87 39.9 37.2 4.82 58.6

44.00 49.53 6.42 1.159 18.08 16.78 7.83 15

6.02 80.46 13.47 0.866 41.74 38.92 3.66 53.24

12.25 75.81 11.97 0.882 39.02 36.38 4.94 56.86

13.57 74.73 11.74 0.893 38.15 35.57 5.06 55.11

14.23 74.19 11.63 0.898 37.72 35.16 5.12 54.24

2200 75

2000 1800

50 1600 1400

25

1200 0 0

200

400

600

800

Engine Speed (rpm)

2400

1000 1000

Time (s) Fig. 2. Custom cycle designed for this study.

A ¼ nlo þ 0:25ðnhi  nlo Þ B ¼ nlo þ 0:50ðnhi  nlo Þ

Load (%)

100

Load (%)

US EPA 2004 emission standards. A reason behind adding the ramped modes in the custom cycle was to study the engine performance and exhaust emission under the discrete modes of transient operation such as load increase and acceleration, however this is not in the scope of this paper. In addition, compared to ESC, this custom cycle has better control of mode changes, hence the engine will stabilise faster than the ESC during the steady state sections. The custom cycle uses 3 engine speeds and during each speed there are 5 loads involved. Fig. 2 shows the custom cycle designed for this study. The three speeds in this cycle are based on the ESC; A, B, and C are calculated from Eq. (1) with units of rpm. The high speed, nhi, is the highest speed in which 70% of the maximum power occurs, and nlo is the lowest speed in which 50% of maximum power occurs. These two speeds can be achieved by mapping the engine power and speed as follow:

ð1Þ

C ¼ nlo þ 0:75ðnhi  nlo Þ; 2.5. Error analysis In order to minimise the faulty measurement from emission instrumentation at low redundancy, three particle measuring instruments and two gas analysers were utilised. For each fuel, the test was repeated at least two times and the repeatability of the results were confirmed by calculating the standard deviation and coefficient of variation. Also, the results showed the same trend with another cycle, ESC, which had similar operating modes.

The data presented in this paper is the averaged value of the measured exhaust emissions and engine performance parameters at different modes of the custom cycle where the speed and torque are constant. Since in the custom cycle there is a load increase transient mode before the start of each steady-state mode, the response signal at each mode has some fluctuations in the designed steady-state part before becoming stable. This could be due to turbocharger lag. In order to assure that in data analysis the response signals for the steady-state component were averaged over the actual steady-state part, the settling time (ts) with 2% threshold was utilised to calculate the time that the response signal needs to get stable. Hence, the response signals were

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averaged after ts. Settling time is the required time for the response signal to reach and stay within a range of 2% of the final value [17]. 3. Result and discussion 3.1. Oxygen ratio The nearness of the air and fuel mixture during combustion to its stoichiometric condition affects the engine performance and exhaust emission characteristics [18]. The air-fuel equivalence ratio, k, is a widely used parameter to quantify the mixture by using the actual and stoichiometric AFR (air to fuel ratio), but AFR only shows the ratio of the intake air and fuel, and it cannot show the real amount of oxygen when the fuel is oxygenated, while the presence of oxygen molecules during combustion can be a significant factor in the analysis of emission and engine performance. Hence, when fuels are oxygenated, OR (oxygen ratio), which is a more appropriate measure of stoichiometry, can be used instead of equivalence ratio [18]. The OR is defined as the ratio of total oxygen atoms in the mixture to the total required oxygen atoms for the stoichiometric combustion [6]. The OR is defined by Eq. (2), where ‘‘Oxygenfuel” and ‘‘Oxygenair” are the masses of oxygen within the fuel and the intake air, respectively. The ‘‘Stoichiometric oxygen requirement” is the theoretical amount of oxygen required for stoichiometric combustion.

OR ¼ ðOxygenfuel þ Oxygenair Þ=ðStoichiometric oxygen requirementÞ

ð2Þ

Fig. 3(a) and (b), respectively show k and oxygen concentration in the exhaust with respect to the oxygen ratio for the tested fuels at all the modes of the custom test. In all the figures corresponding to the OR, the 6 fuels are differentiated with different colors,1 three engine speeds are shown with three different shapes, and 4 engine loads are displayed with 4 shape sizes (the higher the load, the bigger the shape size). In general, k and OR decrease with engine load, but increases with engine speed, causing higher load and lower speed both lead to a higher rate of injected fuel compared to the rate of intake air. As illustrated in Fig. 3(a), k and OR has a linear correlation with the R2 (coefficient of determination) of one. The difference is that in OR the fuel oxygen content is considered. It also shows that the oxygenated fuels have higher k and OR which means that the combustion is leaner when the fuel is oxygenated. This leaner combustion can be confirmed by Fig. 3(b) where oxygen concentration in the exhaust gas increases by increasing the oxygen content of the fuel. For example, compared to the oxygenated fuels, D100 emits the lowest amount of oxygen at different modes of the custom cycle. The variation of this parameter for different oxygen contents compared to D100 increases with load, but decreases with engine speed. For example, the highest variations are around 4% higher than D100 related to the two blends with 13.57% and 14.23% oxygen content occurs at 1472 rpm high loads, while the lowest variations are around 0.2% higher than D100 related to the blend with 6% oxygen content occur at 1865 and 2257 rpm low load. The regression analysis between the exhaust oxygen and the OR shows a high value of R2 for each trendline confirming a strong correlation between these two parameters. This correlation, which is demonstrated by a second order polynomial regression line, shows that the exhaust oxygen concentration decreases by increasing the engine load and/or by decreasing the engine speed. Since OR is a more relevant parameter to characterise oxygenated fuels it is used to present the data throughout the remainder of this paper. 1 For interpretation of color in Figs. 3–10, the reader is referred to the web version of this article.

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3.2. Engine performance 3.2.1. Power and mean effective pressure Brake mean effective pressure (BMEP) can be defined as the amount of work produced per unit displacement volume of an engine, while indicated mean effective pressure (IMEP) is the equivalent uniform pressure throughout the power stroke to produce the same amount work that can be produced by the actual varying pressure during the stroke. Fig. 4(a) shows the IMEP and BMEP variation of the oxygenated fuels compared to D100. In the BMEP calculation, brake power and engine speed are the main parameters, hence at a constant engine speed BMEP and brake power have the same variation trend. This is also true for IMEP and indicated power. As can be seen by increasing the oxygen content of the fuel, all these parameters decrease. For example, the blend with 14.23% oxygen content (T10B90) has the largest drop compared to D100 at all three speeds. According to the trendlines, it can be observed that, compared to BMEP (dashed trendline), IMEP has a higher R2 at all three speeds which means IMEP has a stronger linear correlation with the fuel oxygen content. Also it can be seen that at high speed (2257 rpm) this linear trend is stronger than the other two speeds. It should be mentioned that the variation of IMEP/BMEP with oxygen content was restricted by the fact that a constant load percentage for different fuels had different brake powers, however Fig. 4(a) shows the variation under full engine load with a wide open throttle. The heating value, which is the amount of heat energy released by burning a unit quantity of the fuel, could be the reason of the variation [19]. This value decreases by increasing the oxygen content of the fuels [20] as shown in Table 2. The effect of heating value on BSFC is also discussed in Section 3.2.3. 3.2.2. Friction and mechanical efficiency Fig. 4(b) shows the friction mean effective pressure (FMEP) which is the difference between IMEP and BMEP. FMEP is the engine friction losses which arise from mechanical friction and accessory work such as fuel pump, oil pump and water pump. It can be observed that FMEP decreases with engine load, but it increases with engine speed. At most of the custom cycle, D100 showed a higher FMEP compared to B100. Adding triacetin to BD100 slightly increased the FMEP in most of the cycle modes. In addition to FMEP, friction power which is the difference between indicated power and brake power, is another parameter that relates to the engine friction losses. Fig. 5(a) illustrates that the mean value of the friction power over the custom cycle decreases by using the oxygenated fuel. As can be seen, the D100 has the highest value compared to the other tested fuels. It also can be observed that the blend of waste cooking biodiesel and triacetin with 12.25% fuel oxygen has a slightly higher value compared to BD100. However, the variations with the three blends of waste cooking biodiesel and triacetin compared to BD100 are less than 1%. In addition to the engine operating condition, the variation in FMEP and friction power are affected by some other factors such as engine oil and fuel properties. The reduction in friction losses due to the better lubricity of the biodiesel has been reported in the literature [13,21–23]. A recent study reported that the higher lubricity of biodiesel caused a reduction in friction losses in a diesel engine with common rail injection system [21]. Another study also showed that the brake power improved due to the better lubricity of biodiesel which reduced the fiction losses in the common rail fuel pump [22]. The better lubricity of the biodiesel, regardless of feedstock, is reported to be originated from the inherent lubrication properties from fatty acid esters [24,25]. It was shown that the better lubricity of biodiesel leads to the reduction in wear, friction in the high pressure fuel pump and friction on the cylinder liner [26–28].

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Load% (Shape Size): 100 (9) - 75 (7) - 50 (5) - 25 (3) Speed (rpm): 1472 ( ) 1865 (O) 2257(x) 0% O2 6.02% O2 10.93% O2 12.25% O2 13.57% O2 14.23% O2 16

Exhaust oxygen (%)

Equivalence ratio, λ

7.5 6.5

R² = 1

5.5 4.5 3.5 2.5 2.5

3.5

4.5

5.5

6.5

14

R² = 0.98 R² = 0.97 R² = 0.98 R² = 0.98 R² = 0.97 R² = 0.97

12 10 8 6 4 2.5

7.5

3.5

4.5

5.5

OR

OR

(a)

(b)

6.5

7.5

Fig. 3. (a) Equivalence ratio (k) vs. OR and (b) exhaust oxygen vs. OR at different modes of the custom cycle.

0

0.15

FMEP (MPa)

Variation of BMEP and IMEP (%)

Load% (Shape Size): 100 (9) - 75 (7) - 50 (5) - 25 (3) Speed (rpm): 1472 ( ) 1865 (O) 2257(x) 0% O2 6.02% O2 10.93% O2 12.25% O2 13.57% O2 14.23% O2

-5

R² = 0.99 R² = 0.98 R² = 0.96 R² = 0.95 R² = 0.91 IMEP (Large symbol) — R² = 0.88 BMEP (Small symbol) - -

-10 -15 -20

0.12 0.09 0.06 0.03

-25 0

5

10

0

15

2.5

3.5

4.5

Fuel oxygen content (%)

5.5

6.5

7.5

OR

(a)

(b)

Fig. 4. (a) IP/IMEP and BP/BMEP variation vs. fuel oxygen content at three different speeds full load and (b) FMEP vs. OR at all modes of the custom cycle.

Load% (Shape Size): 100 (9) - 75 (7) - 50 (5) - 25 (3) Speed (rpm): 1472 ( ) 1865 (O) 2257(x) 0% O2 6.02% O2 10.93% O2 12.25% O2 13.57% O2 14.23% O2 100

57

95

55 18.5

53

17.5

51 49

16.5

47

15.5

45 0

3

6

9

12

15

Fuel oxygen content (%)

(a)

ME (%)

19.5

59

Blow-by (L/min)

Friction power (kW)

20.5

R² = 0.93 R² = 0.97 R² = 0.97 R² = 0.97 R² = 0.98 R² = 0.96

90 85 80 75 70 65 2.5

3.5

4.5

5.5

6.5

7.5

OR

(b)

Fig. 5. (a) Mean friction power and blow-by vs. oxygen content over the custom cycle and (b) ME vs. OR at all modes of the custom cycle.

The higher density, viscosity and surface tension of the oxygenated fuel compared to diesel lead to a larger droplet size with a higher spray penetration into the cylinder [13]. This can cause a higher fuel impingement of the biodiesel upon surfaces acting as a lubricant agent leading to improve the lubricity and reduce the friction losses in the cylinder. This could be another reason of lower friction losses with the oxygenated fuels compared to diesel. However, higher viscosity increases the shear in hydrodynamic lubrication of the piston and the piston rings, which can increase friction losses in some cases. This could be the reason of a slight increase in FMEP with the triacetin blends compared to BD100, as the triacetin has a higher viscosity than the waste cooking

biodiesel, as shown in Table 2. However, the friction losses variation could result from a number of mechanisms reinforcing or weakening one another under different conditions. The viscosity of the engine lubricating oil which plays an important role in friction reduction can be reduced by the fuel dilution [29]. This can occur due to blow-by which is when the leaked unburned fuel and exhaust gas pass the piston rings form cylinder into crankcase. This dilution over time reduces the viscosity of the lubricating oil which can potentially reduce the friction losses. Fig. 5(a) also shows that increasing the oxygen content of fuel is associated with the decreased blow-by rate. This could be due to the viscosity of the tested fuel as it increases by increasing the fuel

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oxygen content (shown in Table 2). For example, the T10B90 with the highest viscosity and oxygen content showed the lowest blowby rate. It can be seen that adding triacetin to the waste cooking biodiesel caused a decrease in blow-by rate, as triacetin has a higher viscosity compared to the BD100. Mechanical efficiency (ME) which is shown in Fig. 5(b) is another parameter related to the friction losses. On the contrary to FMEP, ME decreases with engine load and increases with engine speed (ME decreases with OR). Regarding the variation of ME vs. OR, the R2 of each regression line shows that there is a strong linear correlation between these two parameters. The variation of ME for the tested fuels is lower than 3% at most of the modes. 3.2.3. Brake specific fuel consumption and thermal efficiency Brake specific fuel consumption (BSFC) is defined as the mass of fuel consumed per unit of mechanical energy produced by an engine in kW h. This parameter can be used to show the fuel efficiency and the effect of fuel on the engine performance. Fig. 6(a) shows that BSFC increases with OR. A linear regression line shows the correlation between BSFC and OR. The figure also shows that D100 with 0% oxygen content has the lowest BSFC at all the modes of the custom cycle and by increasing the fuel oxygen content, BSFC increases. Compared to D100, the variation of this parameter increases with the fuel oxygen content. For example, the highest variations are related to the blend with 14.23% oxygen content at all the modes. This behavior was expected due to the lower calorific value of the oxygenated fuel, as to produce the same power more amount of fuel with lower calorific value must be consumed. The brake thermal efficiency (BTE) which is shown in Fig. 6(b) is the ratio of actual brake work produced by the engine to the chemical energy released during combustion. It shows how efficiently the chemical energy of the fuel is transformed into useful work. Fig. 6(b) shows that the BTE increases with engine load, but decreases with engine speed, which means engine has a better BTE at lower OR. A linear regression line shows the correlation between BTE and OR. In general, BD100 with 10.93% oxygen content has a higher BTE compared to D100 and other fuels in this study, however the highest variation is 3% and at most of the modes this variation is less than 2%. Adding triacetin to B100 decreases the BTE. This could be due to the higher viscosity, density, and lower heating value of the T100 compared to B100. 3.3. Emissions 3.3.1. Carbon dioxide and carbon monoxide (CO) CO2 formation is directly dependent on fuel consumption. As illustrated in Fig. 7(a), CO2 concentrations decreases by increasing the OR. CO2 concentration increases by increasing the load and/or by decreasing the engine speed. The coefficient of determination, R2, of each second order polynomial regression line shows a strong correlation between these two parameters. In addition, CO2 concentration decreases when the fuel is oxygenated. The reason is due to the lower fuel burning rate (not BSFC) of the oxygenated fuels at different loading conditions. According to the same figure, D100 has the highest value at all the modes. The highest variations are around 2.5% lower than that of D100 at 1472 rpm high loads for the B100 and its three blends with triacetin, however, the most of the variations are less than 1.5%. The formation of CO in CI engines highly depends on AFR. Insufficient oxygen in CI engine combustion is the main cause of CO formation. As it can be seen in Fig. 7(b), the higher indicated specific CO emissions correspond to the higher OR at quarter load in which this emission increases by increasing the oxygen content of fuel. This can be due to the high AFR and lean combustion in which the presence of oxygen makes the combustion leaner. This can be

645

seen in Fig. 3(b) which shows the oxygen concentration in the exhaust gas. Except at quarter load, the indicated specific CO emission of the oxygenated fuels is lower than that of D100 at the other modes. This can be due to the lower AFR at higher loads in which the oxygen in the fuels helps with the combustion to be more close to the stoichiometric condition. 3.3.2. Hydrocarbons Another incomplete combustion product which is related to the over rich/lean air fuel mixture is hydrocarbon (HC) emissions. From the literature, using biofuels can decrease HC formation due to the oxygen content of fuel and absence of aromatics, both promote more complete combustion [1]. As can be seen in Fig. 8 (a), the higher indicated specific HC emissions correspond to the higher OR. The coefficient of determination, R2, the regression line for each fuel shows a linear correlation between the emitted HC and OR. D100 with 0% oxygen has the highest value at all the modes and the other fuel with 6.02% oxygen content which contains diesel in the blend stands below the D100 at all the modes. As it can be seen at all the modes of the custom cycle, B100 with the highest cetane number and 11% oxygen content has the lowest emitted HC. Using this fuel can reduce the HC emission. Compared to D100, the HC emission reduction with this fuel is between 51% and 64% at different modes of the custom cycle. There is an increase in exhaust HC emission by adding triacetin to waste cooking biodiesel. The reason is that triacetin has a low cetane number and adding triacetin to waste cooking biodiesel decreases the cetane number of the blend, hence the emitted HC increases. It should be mentioned that the higher cetane number of the oxygenated fuels in this study leads to a reduction in ignition delay time in which HC mainly forms. Hence the higher the cetane number, the lower the HC emission. Also the higher viscosity and density of the triacetin blends, compared to waste cooking biodiesel, can cause an improper mixing with the air which all leads to higher HC formation. In addition, higher AFR of D100 at lower loads and over leaning combustion can lead to more unburned fuel during combustion and higher HC emission. This shows that the above reasons have more effect than the fuel oxygen content on HC formation, compared to PM formation in which the oxygen content is the main reason, shown in Section 3.3.4. 3.3.3. Nitrogen oxides NOx formation highly depends on temperature, residence time under high temperature, oxygen presence, fuel properties and injection-related factors [1]. The majority of published works in the literature have reported that using oxygenated fuels leads to higher NOx formation due to reasons such as higher bulk modulus, cetane number, density and viscosity of the biofuels [1]. Fig. 8(b) show the indicated specific NOx for the tested fuels under the custom cycle in respect to OR. It shows that NOx emission decreases with OR. It means that NOx formation is highly correlated to the engine load as it increases at higher loads in which the combustion temperature is higher, while a decreasing trend can be observed with an increase in engine speed. In addition, in Fig. 8(b) a moderate increase in NOx formation can be seen by increasing the oxygen content of fuel. Except three quarter load modes, in all the other modes using pure B100 or the other three blends of waste cooking biodiesel and triacetin leads to an increase in NOx emission, compared to D100. The highest variation is at 1865 rpm full load in which the NOx emission from the oxygenated fuels is 19–31% higher than that of D100. At 1865 and 2257 rpm quarter load the maximum NOx variation compared to D100 is around 5%, while at 1472 rpm quarter load this variation is around 10%.

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350

40

310

38

270

R² = 0.79 R² = 0.95 R² = 0.85 R² = 0.87 R² = 0.92 R² = 0.94

230 190 150 2.5

3.5

4.5

5.5

6.5

7.5

BTE (%)

BSFC (g/kWh)

Load% (Shape Size): 100 (9) - 75 (7) - 50 (5) - 25 (3) Speed (rpm): 1472 ( ) 1865 (O) 2257(x) 0% O2 6.02% O2 10.93% O2 12.25% O2 13.57% O2 14.23% O2 R² = 0.77 R² = 0.92 R² = 0.83 R² = 0.85 R² = 0.89 R² = 0.92

36 34 32 30 28 2.5

3.5

4.5

5.5

OR

OR

(a)

(b)

6.5

7.5

Fig. 6. (a) BSFC vs. OR and (b) BTE vs. OR at all modes of the custom cycle.

13.5

R² = 0.98 R² = 0.97 R² = 0.98 R² = 0.97 R² = 0.97 R² = 0.98

CO2 (%)

11.5 9.5 7.5 5.5 3.5 2.5

3.5

4.5

5.5

6.5

7.5

Indicated specific CO (g/kWh)

Load% (Shape Size): 100 (9) - 75 (7) - 50 (5) - 25 (3) Speed (rpm): 1472 ( ) 1865 (O) 2257(x) 0% O2 6.02% O2 10.93% O2 12.25% O2 13.57% O2 14.23% O2 4 3 2 1 0 2.5

3.5

4.5

5.5

OR

OR

(a)

(b)

6.5

7.5

Fig. 7. (a) CO2 concentration vs. OR and (b) indicated specific CO vs. OR at all modes of the custom cycle.

12

R² = 0.98 R² = 0.97 R² = 0.86

10 8 6 4

R² = 0.87 R² = 0.80 R² = 0.92

2 0 2.5

3.5

4.5

5.5

6.5

7.5

Indicated specific NOx (g/kWh)

Indicated specific HC (g/kWh)

Load% (Shape Size): 100 (9) - 75 (7) - 50 (5) - 25 (3) Speed (rpm): 1472 ( ) 1865 (O) 2257(x) 0% O2 6.02% O2 10.93% O2 12.25% O2 13.57% O2 14.23% O2 6 5.5 5 4.5 4 3.5 3 2.5 2.5

3.5

4.5

5.5

OR

OR

(a)

(b)

6.5

7.5

Fig. 8. (a) Indicated specific HC vs. OR and (b) indicated specific NOx vs. OR at all modes of the custom cycle.

Compared to D100, the local conditions during the combustion of the oxygenated fuels are closer to the stoichiometric condition due to the presence of oxygen in fuel. This leads to a better and more complete premixing during the ignition delay, hence a higher fraction of heat release occurs during the premixed phase of combustion, and consequently, it leads to higher in-cylinder temperature and longer residence times at high temperature. This mechanism would be expected to lead to a higher NOx formation [1,30,31]. The increased NOx with biodiesel results from a number of mechanisms affecting each other positively or negatively under different conditions. Ref. [30] analysed different proposed hypotheses to interpret the NOx increase with biodiesels, and reported that the premixing level plays a more important role than

the fuel molecular structure. It also found that the proximity of the air fuel mixture to the stoichiometric condition during ignition and in the standing premixed auto-ignition zone near the flame lift-off length can better explain the NOx increase with biodiesels [30]. Adiabatic flame temperature could be another parameter that can affect the NOx formation [31]. It has been reported that because of the double bonds in the molecules of biodiesels, the adiabatic flame temperature of biodiesel is higher than the D100. Hence it could be a reason for higher NOx formation [31,32]. Some researchers found that the adiabatic flame temperature is affected by the incylinder radiative heat transfer from particle formation. The same as this study (Section 3.3.4), it has been frequently reported that the oxygenated fuels, compared to neat diesel, has a lower amount of

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PM formation which leads to a lower radiative heat transfer, higher flame temperature and higher NOx formation [30,31]. 3.3.4. Particulate matter In general, particulate matter is a liquid and solid mixture suspended in a gas. In compression ignition engines, the injected fuel will be mixed with the oxidant in the combustion chamber; consequently, a great degree of heterogeneity characterises the combustion process. This process is defined as diffusion flame combustion, which is the main cause of PM emission [33]. Particulate matter is a complex pollutant that is not a chemically well-defined substance in terms of formation and composition. Particulate matter formation depends on various factors such as fuel type, engine load, engine speed, after-treatment devices, dilution level after being emitted and engine maintenance. Fig. 9(a) shows the indicated specific PM for the tested fuels under the custom cycle. As can be seen, PM emission has lower values at higher OR where the engine load decreases. This trend is similar to another study from our research group in which the indicated specific PM emission decreased by increasing the OR [6]. In addition, Fig. 9(a) also shows that by increasing the oxygen content of fuel, PM decreases gradually. For example D100, with 0% oxygen content, emits the highest amount of PM in all of the modes, while the lowest emitted PM corresponds to the blend with the highest oxygen content, 14.23%. From the literature, most studies showed that oxygenated fuels have the potential to reduce PM. A recent publication showed that the chemical composition of the fuel, particularly the fuel oxygen content, has more effect on PM emission reduction than the fuel physical properties such as viscosity and cetane number [7]. Similarly, some other investigations showed that the reduction of PM is related to the oxygen content of fuel. The more oxygen content, the more PM reduction [1,34,35].

Generally, PM formation takes place in the fuel-rich zone under high temperature decomposition during combustion. It mainly happens in the fuel spray fuel core region. Apart from the type of fuel, the presence of oxygen helps the soot oxidation process. In addition to the intake air, in oxygenated fuels, the local fuel-rich zone within the core region of the sprayed fuel is reduced, preventing higher PM formation. Hence, the higher the oxygen content of the fuel, the less PM formation. There are some investigations in the literature regarding the PM emission over various driving cycles. For example, the fuel oxygen effect on PM emission reduction during the FTP transient cycle [36], and during the NEDC cycle [37]. Similar to the literature, Fig. 9(a) shows that the mean value of PM formation over the custom cycle decreases dramatically by increasing the oxygen content of the fuel. For example, using the blend with 14.23% oxygen content can reduce the PM mean value by 90%. A second order polynomial regression line with R2 = 0.99 in the figure shows that there is a strong correlation between these two parameters. 3.3.5. Particle number and particle size distribution Recently, in terms of air quality, particle number (PN) emissions from engines are gaining more attention. The reason could be the toxicity of particles which increases by decreasing the particle size [38]. As a consequence the EU included PN in Euro 6 and EURO 5b emission standards for heavy-duty and light-duty vehicles. Fig. 9(b) shows the indicated specific PN for the tested fuels under the custom cycle. The PN emission increases by increasing the engine speed, while it decreases by increasing the engine load. In this study, the trend of PN formation corresponding to the OR for different engine speeds is similar to another study from our research group which used fractionated palm oil [6]. Regarding the effect of fuel oxygen content, the variation of PN emission is

1.5E+15

0.2

Indicated specific PN (g/kWh)

Indicated specific PM (g/kWh)

Load% (Shape Size): 100 (9) - 75 (7) - 50 (5) - 25 (3) Speed (rpm): 1472 ( ) 1865 (O) 2257(x) 0% O2 6.02% O2 10.93% O2 12.25% O2 13.57% O2 14.23% O2 Mean value over custom cycle

1.2E+15

0.15 0.1 0.05 0 2.5

3.5

4.5

5.5

6.5

9E+14 6E+14 3E+14 0 2.5

7.5

3.5

4.5

5.5

OR

OR

(a)

(b)

6.5

7.5

Fig. 9. (a) Indicated specific PM vs. OR (subdiagram illustrates the mean value of PM emission over the custom cycle) and (b) indicated specific PN vs. OR at all modes of the custom cycle.

dN/dlogDp (#/cm3)

dN/dlogDp

(#/cm3)

0% O2 6.02% O2 10.93% O2 12.25% O2 13.57% O2 14.23% O2 2.E+07 1.E+07 8.E+06 4.E+06 0.E+00 1

10

100

Particle size (nm)

(a)

1000

2.E+07 1.E+07 8.E+06 4.E+06 0.E+00 1

10

100

Particle size (nm)

(b)

Fig. 10. PN size distribution for oxygenated fuels under full load at (a) 1865 rpm and (b) 2257 rpm.

1000

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similar to PM emission. As it can be seen in Fig. 9(b) that oxygenated fuels emits lower PN compared to D100 with 0% oxygen content. For example, at 25% load and 2257 rpm D100 has the highest amount and the blend with 14.23% oxygen content has the lowest amount. Between these points PN decreases by increasing the fuel oxygen. A recent publication attempted to explained the PN emission reduction corresponding to the fuel oxygen content [39]. Similar to PM, PN reduction is due to the presence of oxygen. It helps with the oxidation of the already formed soot and also promotes the combustion in the fuel-rich diffusion-flame region and within the core region of the sprayed fuel. Lack of aromatic hydrocarbons which leads to less soot formation could be another reason for decrease in PN. These reasons also lead to a change in particle size from larger to smaller particles since the oxygen content of fuel causes the higher soot oxidation rate after diffusion combustion and the higher combustion temperature [15]. There are two main categories for the particle size distribution, nucleation and accumulation modes. In the nucleation mode the particles diameter sizes are between 3 and 30 nm, which only contains a few percent of the total particle mass. These small particles consist of sulfate and Soluble Organic Fraction (SOF) are mainly formed during dilution and cooling. On the other hand, in the accumulation mode, the agglomerated soot and adsorbed materials, which mainly have the diameter size of 30–1000 nm, form the main portion of the total particle mass [15]. Fig. 10(a) and (b) shows the particle size distribution for the tested fuels under full load at two engine speeds; 1865 and 2257 rpm. As illustrated, by increasing the oxygen content, the accumulation mode count median diameter (the peaks in the PN size distribution graph in the accumulation part) moves toward the smaller particle sizes. It can be observed that the accumulation mode peak size decreases from 75 to 56 nm (at 1865 rpm) and from 65 to 56 nm (at 2257). The fuel oxygen content was reported to be the reason of this reduction [15]. The other size distribution graphs for different modes of the custom cycle showed the same trend. Most of the previous investigations on exhaust emissions have been done on diesel-tuned engines and some disadvantages could root from calibration issues. A recalibration of the ECU and injection system might improve some of the disadvantages of using biofuels [1].

4. Conclusion This study investigated the effect of oxygenated fuels on engine performance and exhaust emission under a custom cycle using a modern six-cylinder turbocharged common-rail diesel engine. Regarding the tested fuels, the blends were based on waste cooking biodiesel and triacetin was used as an additive. The following conclusions were drawn: Since the tested fuels were oxygenated, the OR (oxygen ratio) was used instead of equivalence ratio. The OR decreases with engine load, but increases with engine speed. In terms of the engine performance parameters, by increasing the OR, there was an increase in FMEP and BSFC, while ME and BTE decreased. Increasing the oxygen content of the fuel decreased the IP, BP, IMEP and BEMP due to the lower calorific value of the oxygenated fuel. Compared to diesel, the range of decrease in the mentioned parameters were 5 (6.02% fuel oxygen blend) to 23% (14.23% fuel oxygen blend). The BD100 (10.93% oxygen content) showed the highest BTE, however at most of the modes the variation was less than 2%. In addition, the better lubricity and higher viscosity of the oxygenated fuels may have been factors to decrease friction power, which ranged from 6 to 20% lower than diesel. A

strong correlation was observed between fuel kinematic viscosity and blow-by. In terms of exhaust emission, by increasing the OR there was an increase in CO, HC and PN, while CO2, NOx and PM decreased. Compared to diesel, using the oxygenated fuel decreased CO2 up to 2.5%, CO (at higher loads) up to 82%, HC up to 64%, PM and PN both up to 90%, while it increased NOx up to 31%. Also, by increasing the oxygen content, the accumulation mode count median diameter moved toward the smaller particle sizes.

Acknowledgements This research supported under Australian Research Council’s Linkage Projects funding scheme (project number LP110200158). The author also would like to acknowledge the laboratory assistance from Mr. Noel Hartnett, DynoLog software development by Mr. Andrew Elder from DynoLog Dynamometer Pty Ltd, Peak3 Pty Ltd for helping with measuring instruments, Eco Tech Biodiesel (Dr. Doug Stuart) for the supply of waste cooking biodiesel and Dr. Meisam Babaie and Dr. Michael Cholette for their guide and support.

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