Combustion Characteristics of a Diesel Engine Fueled with Waste Cooking Oil and Toluene

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Fuel 303 (2021) 121284

Available online 24 June 2021

Full Length Article

Effect of toluene addition to waste cooking oil on combustion characteristics of a ci engine

Salih Ozer ¨

^a^,^*

, Mehmet Akçay

^a

, Erdinç Vural

^b

aMus Alparslan University, Faculty of Engineering and Architecture, Department of Mechanical Engineering, Mus, Turkey

bAydin Adnan Menderes University, Germencik Yamantürk Vocational School, Aydın, Turkey

A R T I C L E I N F O Keywords:

Diesel engine Toluene Waste cooking oil In-cylinder pressure Heat release

A B S T R A C T

In recent years, significant emphasis has been placed on biodiesel as a renewable and environmentally friendly fuel. However, biodiesel production costs are still higher than commercial diesel fuel and it requires a lot of time and effort. In this study, the usability of waste cooking oils (WCO) in diesel engine without converting to biodiesel was investigated. We blended WCO with diesel fuel at 25% (in vol. 25% waste oil +75% diesel fuel) and 50% (in vol. 50% waste oil +50% diesel fuel) ratios. In addition, 5%, 10% and 15% toluene were added to the waste cooking oil-diesel fuel blends to improve its fuel properties. A single-cylinder, direct injection, air cooled, diesel engine was used for engine experiments. The experiments were conducted with a constant engine speed (1800 rpm) and different engine powers (2 kW and 4 kW). The results show that toluene addition is effective on density, viscosity and calorific value and in-cylinder pressure values. The toluene addition increased the maximum in-cylinder pressure values. The maximum pressure point approached the top dead center (TDC).

Improvement in thermal efficiency and BSFC has occurred with the addition of toluene. With the exception of NOx emissions, other emissions were reduced by the toluene addition.

1. Introduction

In recent years, the rapid depletion of fossil fuels and the strict standards imposed on emissions caused by these fuel sources have increased the importance of renewable fuels [1]. In this context, research on non-petroleum, renewable, non-polluting and low-cost fuels has gained intensity. Vegetable oils have significant potential as alternative fuels in diesel engines. Using vegetable oils in diesel engines is not new. Rudolph Diesel tested the first diesel engine with diesel in the 1800 s using peanut oil. However, the high viscosity and density of vegetable oils limit their direct use of diesel engines [2]. Direct use of vegetable oils in diesel engines with no improvement causes some disadvantages, such as injector adhesion, poor atomisation, carbon deposits [3]. In reducing the viscosity of vegetable oils, various methods such as transesterification [4], mixing with diesel fuel [5] or alcohols [6] and pre- heating [7] are used. Among these methods, the transesterification method is a time-consuming and expensive process [8].

Waste cooking oil (WCO) is defined as a vegetable oil that is used for food cooking and is not suitable for reuse [9]. WCO is the lowest priced resource compared to edible/non-edible oils sources [10]. In addition, much of the WCO is dumped into rivers and empty spaces, causing

pollution. An effective way to dispose of the WCO is to replace it and use it as fuel in internal combustion engines [11]. It is possible to summarize the outstanding studies on the use of waste vegetable oils in diesel engines directly or by creating a mixture as follows. Sanli [5] investigated the effects of waste frying oil (WFO)-mineral diesel fuel (MDF) mixtures on engine performance, combustion and emissions in Common Rail DI- diesel engine. It achieved higher BSFC and lower BTE with WFO-MDF mixtures than MDF. They observed a decrease in the cylinder pressure values with WFO-MDF mixtures. While there would be no significant difference in injection timing, they found the amounts of fuel injected during pilot and main injection to be higher for WFO mixtures. WFO- MDF mixtures especially caused an increase in HC emissions compared to MDF. Kumar and Jaikumar [11] compared the performance, combustion and emission characteristics of a single-cylinder diesel engine using neat WCO, neat diesel and WCO emulsion as fuel.

Neat WCO exhibited higher smoke, HC and CO emissions compared to neat diesel. With the WCO emulsion, a significant reduction in all emissions has been achieved. They observed that the ignition delay was higher with neat WCO and WCO emulsion than neat diesel fuel.

Kalam et al. [8] palm (P5) and coconut oil (C5) were mixed at a rate of 5% as waste cooking oil to diesel fuel and studied the effect on performance and emissions a multi-cylinder diesel engine. Compared to

* Corresponding author.

E-mail address: [email protected] (S. Ozer). ¨

Contents lists available at ScienceDirect

Fuel

journal homepage: www.elsevier.com/locate/fuel

https://doi.org/10.1016/j.fuel.2021.121284

Received 9 March 2021; Received in revised form 15 May 2021; Accepted 14 June 2021

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diesel, there was a 1.2% and 0.7% reduction in brake power for the P5 and C5. Reduction in hydrocarbon (HC), CO, NOx and smoke opacity occurred with P5 and C5. Khalid et al. [12] investigated the effects of WCO mixed fuel on combustion properties and exhaust emissions.

Increased WCO mixture ratio resulted in decreased exhaust emissions (NOx, Smoke opacity, HC, CO, carbon dioxide (CO₂)) and exhaust temperature. Lalvani et al. [13] investigated the effect of a WCO mixture of 10, 20 and 30% mass into diesel fuel on engine performance and exhaust emissions in a single-cylinder four-stroke direct injection diesel engine. Because of the increase in the WCO mixture ratio, there was a decrease in engine power, HC and CO emissions, and an increase in CO2, NOx and smoke emissions. Corsini et al. [14] studied different proportions of diesel fuel waste frying oil (WCO) mixtures in the four- cylinder common rail diesel engine. The results got in the study were compared with the values got in using pure diesel and pure WCO. Engine torque and efficiency deteriorated with WCO, with the biggest difference being between neat diesel fuel and neat WCO.

In the literature research, it has been seen that if WCO is used directly or mixed into diesel fuel at certain rates, there is deterioration in engine performance and emissions. Qian et al. [15] investigated the effect of aromatic hydrocarbons (toluene, n-butylenzene, tetralin and 1-methyl- naphthalene) on the combustion performance and emissions of a diesel engine. They stated that the ignition delay increased with aromatic mixtures and a higher peak heat release rate was achieved, and that at high loads, there was an improvement in smoke emissions because of a longer ignition delay.

Therefore, in this study, it was aimed to prevent the adverse combustion conditions that occur with the use of WCO by adding toluene in certain proportions to the diesel fuel - WCO mixture, unlike the literature. The applicability of the toluene additive to the diesel fuel-WCO mixture in a diesel engine with no modifications has been investigated. Two different mixtures, DW25 and DW50, were got by mixing 25% and 50% WCO into diesel fuel by volume. DW25T5, DW25T10, DW25T15, DW50T5, DW50T10 and DW50T15 mixtures were got by adding toluene in 5, 10 and 15% volumetric ratios to both mixtures.

Each mixtures were tested on a single-cylinder CI engine and its performance, combustion and emission we also examined characteristics.

Experiments were carried out with two different engine powers: 2 kW and 4 kW and at a constant engine speed of 1800 rpm.

2. Materials and methods 2.1. Supply of waste oil

In this study, we aimed it to evaluate WCO as a fuel additive in a CI engine and thus to reduce both the cost of fuel and to minimize envi- ronmental pollution caused by waste oils. WCO used in the study, was supplied by the refectory of the Mus Alparslan University, Mus, Turkey.

We formed approximately 1500 L of WCO because of frying operations

in the refectory of Mus¸ Alparslan university.

We WCO, got from the food factory, was first left to rest for 24 h, allowing the sediment and unwanted particles in it to collapse to the bottom. The WCO was first passed through a coarse filter and then passed through a 0.45 µm filter (Whatman filter paper) under vacuum to remove the smallest particles that can be contained in it. In order to remove the moisture in the waste oil, we heated the waste oil to a temperature of 105 ^◦C and dried for 1 h.

2.2. The formation of fuel mixtures

In order to obtain fuels within the study, D100 (neat diesel), DW25 (75% diesel +25% WCO) and DW50 (50% diesel +50% WCO) fuels were obtained by mixing 25% and 50% waste oil into diesel fuel for the first time. DW25T5, DW25T10, DW25T15, DW50T5 DW50T10 and DW50T15 fuels were obtained by adding 5%, 10% and 15% toluene to these mixtures. Some properties of commercial diesel fuel, waste oil, toluene and mixtures used in the study’s scope are given in Table 1.

2.3. Engine test setup

We tested the resulting fuel mixtures in a single-cylinder, direct injection, air cooled diesel engine. We have made no changes to the experimental engine. Technical characteristics related to the experimental engine are given in Table 2.

An electric dynamometer with a maximum power of 26 kW and a torque absorption capacity of 80 Nm and a maximum speed of 5000 rpm (±50) was used to conduct engine experiments. A schematic view of the experimental assembly is given in Fig. 1. The exhaust emission values of the engine were measured using the Bosh Bea 350 brand gas analyzer.

Measurement ranges and measurement uncertainties of the devices used in the experiments are presented in Table 3. A K-type thermocouple was used to measure the exhaust gas temperature of the experimental Nomenclature

D100 diesel fuel

DW25 25% waste cooking oil with 75% diesel fuel in vol.

DW50 50% waste cooking oil with 50% diesel fuel in vol.

DW25T5 95% DW25 with 5% toluene in vol.

WCO waste cooking oil CI compression ignition

CP in-cylinder pressure (bar) HRR heat release rate (J/℃A) BTE break thermal efficiency (%)

BSFC break specific fuel consumption (kg/kWh) EGT exhaust gas temperature ℃

CO carbon monoxide NOX oxides of nitrogen TDC top dead centre BDC bottom dead centre BTDC before top dead centre rpm revolution per minute SOC start of combustion SOI start of injection

Table 1

Chemical and physical properties of fuels.

Fuels Density (kg/m³) at 25 ^◦C

Kinematic viscosity (cSt) at 40 ^◦C

Calorific value (Mj/

kg)

Flash point (^◦C)

Cetane number

D100 837 2.48 44.26 45 55

WCO 924.5 37.41 39.678 307 35

Toluene 827 0.6 40.6 9 4

DW25 865.8 10.8 43.02 109 49

DW50 883.7 19.6 41.88 165 44

DW25T5 856.2 9.4 42.99 104 48

DW25T10 854 7.1 42.78 92 45

DW25T15 850.3 6.3 41.6 89 43

DW50T5 871.0 17.3 41.8 167 43

DW50T10 870.3 15.6 41.3 155 40

DW50T15 862.6 13.2 41.01 148 37

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engine.

The cylinder pressure of the experimental engine was measured by an air cooled pressure sensor (Oprand, OPTD 32288GPA), while the air cooled pressure sensor (Kistler, 6052C). Crankshaft position was determined by FNC brand optical crank encoder. Fuel consumption was measured by mass using precision scales and stopwatch. The schematic picture of the experimental Assembly is given in Fig. 1.

2.4. Experimental procedures

The experimental work was carried out for two different engine powers, 2 kW and 4 kW, with a constant engine speed of 1800 rpm, where maximum engine torque was achieved. In-cylinder pressure values are taken for 0.1 ℃A. Each in-cylinder pressure chart is based on an average of 100 cycles. Heat release analysis was also performed using 100 cycles average cylinder pressure data. The rate of heat release rate (HRR) was calculated through an analytical model by applying the first law of thermodynamics, depends on pressure of cylinder and volume measurements for each ℃A. The formula for the calculation of the HRR is shown in Eq. (1) [16]:

dQn

dθ = γ γ− 1PdV

dθ+ 1 γ− 1VdP

dθ (1)

Here; ^dQ_dθⁿnet heat release rate (J/℃A), θ crank angle (℃A), γ is the specific heat ratio, V is the cylinder volume (m³), and P is the cylinder pressure (bar). The total uncertainties of the measurements performed in the experimental study were calculated according to the Kline and McClintock method [17,18]. The accuracy and total uncertainties of the measurements are given in Table 4.

3. Result and discussions

3.1. Engine performance characteristics

In this section, the effect of toluene additive on brake thermal efficiency (BTE), break specific fuel consumption (BSFC), and exhaust gas temperature on diesel-WCO mixtures has been presented and interpreted in graphs depending on engine power and toluene additive ratio.

Table 2

Technical characteristics of the experimental engine.

Make/Model Lombardini/3LD510

Engine type 4-Storke, DI-diesel engine, naturally aspirated Cylinder Number 1

Bore-stroke [mm] 85–90 Displacement [cm³] 510 Compressions ratio 17.5:1

Maximum power [kW] 9 kW @ 3000 rpm Maximum torque [Nm] 32.8 @ 1800 rpm Fuel injection timing

[℃A] 24 BTDC

Intake valve diameter

[mm] 35

Exhaust valve diameter

[mm] 31

Intake valve close 52^◦after BDC Exhaust valves open 52^◦before BDC Combustion chamber

geometry Hemispherical open type

Fuel injection system Mechanical pump, Stanadyne PFR1K70/32500, Housing with plunger and barrel assembly with Roller Tappet Assy., 3 Plunger Pump

Injection nozzle 0.24 [mm] *4 holes *160^o Nozzle Opening

pressure (bar) 200

Fig. 1. Schematic picture of the experimental assembly.

Table 3

Measurement range of gas analyzer and calculated uncertainties.

Component Measurement Range Resolution Accuracy %

CO (% vol.) 0–10.00 0.001 ±0.01

CO2 (% vol.) 0–18.00 0.01 ±0.05

HC (ppm) 0–9999 1 ±0.01

O2 (% vol.) 0–22.00 0.01 ±0.04

Lambda 0.50–9.99 0.001 ±0.0001

NO (ppm) 0–5000 ≤1 ±0.1

Smoke Opacity (%) 0–100 0.1 ±0.1

Table 4

Measurement accuracies and the uncertainties in the calculated results.

Measured Parameter Measurement Device Accuracy

Engine load Load cell, N ±0.6%

Engine speed Incremental encoder, rpm ±1%

Cylinder pressure Pressure sensor, bar ±0.5%

Fuel line pressure Pressure sensor, bar ±1%

Fuel mass Precision scale, g ±0.1%

Exhaust gas temperature Thermocouple, ^◦C ±1%

Time measurement Digital chronometer, s ±1%

Calculated results Uncertainty value

Power ±1.17%

BSEC ±1.54%

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a) Brake thermal efficiency

It is extremely important how much of the fuel taken into the cylinder turns into useful work. Fig. 2 shows the effect of fuel mixtures used on thermal efficiency. As can be seen from Fig. 2, BTE in general increased with increased engine load. The reason for the increase in BTE at high engine loads is due to the increase in the combustion temperature and pressure in the cylinder [19]. Thermal efficiency was reduced with the addition of WCO to diesel fuel for each engine load. Compared to diesel fuel, 2 kW and 4 kW engine powers showed a 0.6% and 1.5%

reduction in thermal efficiency with DW25 fuel, while with DW50 fuel, this ratio was obtained as 2.3% and 4.5%. High viscosity and density of the fuel prevent the formation of an ideal mixture in the cylinder and partially worsen combustion [20]. Therefore, brake thermal efficiency decreases. Similar results were obtained by Kumar and Jaikumar [11]

and Sanli [5].

A general increase in breake thermal efficiency occurred with the addition of toluene to DW25 and DW50 fuel, and the maximum increase for both fuels was achieved by the 15% toluene addition. The increase in maximum cylinder pressure caused by the toluene addition for DW25 and DW50 fuels was measured as 5% and 3% for at 2 kW engine power, and 2% and 1% at 4 kW engine power, respectively. This can be explained by improved fuel atomization and air–fuel mixture, which provides better combustion with improved viscosity of the mixture [19,21].

b) Break specific fuel consumption

The effect of toluene additive on BSFC in WCO-diesel fuel mixture is shown in Fig. 3. In diesel engines, the specific fuel consumption value can vary depending on the combustion efficiency of the fuels in the cylinder and the calorific value of the fuels. Studies report that fuel consumption values increase by using of low heating value fuels in general [22]. BSFC is the mass of fuel consumed to produce 1 kW of power. If biodiesel is used, it leads to an increase in Bsfc due to the need to consume more fuel to produce 1 kW of power [23]. Again, it is stated that besides the calorific value of the fuel, viscosity, density, and cetanes number can affect fuel consumption [24]. As can be seen from Fig. 3, according to diesel fuel DW25 and DW50 fuels and 2 kW engine power specific fuel consumption value increases by 27% and 40%, while the difference between calorific values varies by 2.3% and 5.6%.

Along with the addition of toluene to DW25 and DW50 fuels, BSFC tended to decrease proportionally. Compared to B25 and B50 fuels, the maximum reduction in BSFC was achieved by 21% and 8% respectively for 2 kW engine power with a 15% toluene addition, while for 4 kW

engine power it was achieved by 16% and 22%. The density and viscosity values of fuel mixtures are reduced by the addition of toluene (Table 1). As a result, the atomization of the fuel improves and the combustion efficiency increases by forming a more homogeneous mixture of air fuel. Therefore, BSFC values decrease due to the fact that more power can be obtained with less fuel. In the literature, it can be stated that combustion properties improve and BSFC decreases due to the decrease in viscosity and density with the chemical addition [25,26].

c) Exhaust gas temperature

In diesel engines, exhaust gas temperature (EGT) depends on the heating value of the fuels, such as viscosity and density, as well as the combustion efficiency in the cylinder [23]. The high viscosity value of the fuel causes lower exhaust gas temperature [3]. In addition, how much of the fuel mixtures taken into the cylinder are burned during the premixed-combustion phase is an important parameter affecting EGT [27]. Vegetable oils usually contain components with higher boiling points than diesel fuel. Because the components with a high boiling point cannot be adequately evaporated during the main combustion phase, the late combustion phase also continue to burn thus increases exhaust gas temperatures [28]. The effect of toluene additive on EGT for WCO-diesel fuel mixture is shown in Fig. 4. As can be seen from Fig. 4, the exhaust gas temperature decreased with the addition of WCO to

Fig. 2. Brake Thermal Efficiency changes.

Fig. 3. Break Specific Fuel Consumption changes.

Fig. 4.Exhaust Gas Temperature changes.

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diesel fuel. When Table 1 is examined, it is seen that the viscosity, density and calorific values of DW25 and DW50 fuel mixtures are lower than that of diesel fuel. It can be stated that this reduces the combustion efficiency of fuel mixtures and partially deteriorates the combustion in the cylinder, resulting in a decrease in their EGT.

Along with the addition of toluene to DW25 and DW50 fuels, an increase in EGT values occurred. With the addition of toluene to DW25 fuel, the average temperature increase in EGT was achieved as 376℃ and 458℃ for 2 kW and 4 kW engine power, respectively. With the addition of toluene to DW50 fuel, 2 kW and 4 kW were obtained for engine powers at 327℃ and 424℃ respectively. Additive to Toluene, fuel properties of mixtures (viscosity, density, calorific value, etc.) because it improves, combustion efficiency increases, the entire fuel mixture participates in combustion, extending the combustion time, and therefore the EGT increases.

3.2. Combustion characteristics

In this section, the effect of the WCO and toluene addition to the diesel fuel on the in-cylinder pressure and heat release rate is presented and interpreted in the form of graphics by the different engine powers.

a) In-cylinder pressure

In internal combustion engines, the explosion created on the piston by burning the fuel taken into the cylinder allows the work to be done.

The formation of maximum pressure in the cylinder varies according to parameters such as viscosity, density, number of cetanes and thermal value of the fuels used [29]. The in-cylinder pressure changes of each fuel used in the experimental study are shown in Fig. 5 depending on the crank angle. As can be seen from Fig. 5, the maximum in-cylinder

pressure values also increased with the increased engine power for each fuel. Along with the contribution of waste oil to diesel fuel (DW25, DW50), there was a decrease in the maximum values of in-cylinder pressure in all engine powers. Again, combustion was delayed by waste oil additive (up to 6 ^◦C). The higher viscosity and lower volatility of diesel-WCO mixtures leads to poor atomization and poor air–fuel mixture during the ignition delay time, resulting in a decrease in cylinder peak pressure [11]. High-viscosity fuels cannot be fully atomized in the cylinder and can worsen combustion, as well as affect the ignition delay time [30]. Also, the high cetane number of the fuel means a lower ignition delay. A high cetane number is also an important value that affects the premixed-combustion phase, the maximum pressure value, and the rate of heat dissipation [31].

With the addition of toluene to DW25 and DW50 fuels, an increase in the pressure values in the cylinder occurred. Sun et al. [32] in the simulation study, the maximum in-cylinder pressure value and ignition delay time were increased with the addition of toluene. Along with the contribution to toluene, the density and viscosity values of fuels improve. This can enable more regular mixing of fuels in the cylinder and atomization of fuel mixtures with air. In addition, the number of cetanes in fuel mixtures decreases. Although the number of cetanes decreased, it is believed that the combustion was partially suggested and the increase in in-cylinder pressure values was caused by the toluene flash point. More recent studies have obtained similar results in con- tributions with a high flash point [33]. When considering each fuel, the highest in-cylinder pressure value is achieved at 4 kW engine power, D100 fuel 62.26 bar, DW25 51.93 bar, DW25T15 56.66 bar, DW25T10 58.56 bar, DW25T15 60.2 bar.

The crankshaft angle at which maximum pressure occurs in diesel engines is one of the important parameters affecting engine power [27].

Parameters such as the time of spraying of the fuel, the viscosity and

Fig. 5.In-cylinder pressure changes.

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density of the fuel, as well as the number of cetanes of the fuel, the heat of evaporation, and temperature of ignition affect the moment when combustion is maximum and the place of formation of the maximum pressure can vary. In this case, the power and torque values obtained from the engine may also vary [34]. Fig. 6 shows the location of maximum in-cylinder pressure values and maximum in-cylinder pressure values.

It is seen that with the addition of waste oil to diesel fuel, the increased density, viscosity and the reduced cetane number and heating value together with the maximum pressure is formed. In addition, the ignition temperature of fuel mixtures increases, and the thermal energy required to burn fuel mixtures also increases. But with the addition of toluene, it was observed that the point at which the maximum pressure was formed closed the top dead center (TDC). In the B50 fuel mixture, the maximum pressure point compared to the D100 fuel is shifted to 2 ℃ A 2 kW engine power and 4 ℃A for 4 kW engine power. But with the 15% toluene addition, these values decrease to − 2 ℃A and 0 ℃A.

b) Heat release rate

The change in heat release rates for each fuel depending on the crank angle is shown in Fig. 7. A study from Fig. 7 showed that other fuel mixtures exhibit a lower rate of heat release than diesel fuel. Due to the increased viscosity for DW25 and DW50 fuels, this leads to poor air–fuel mixing and inefficient combustion occurs. This leads to less heat release during the premixed phase. Most of the combustion occurs in the expansion stroke, resulting in a lower heat release [35].

With the addition of toluene to DW25 and DW50 fuels, there has been some increase in the rate of heat release compared to the use of waste oil directly. With the toluene addition, heating value, viscosity and density values are partially improved according to the oil mixture. It is believed that this condition increases the rate of heat release [36]. In addition, it is observed that a second peak point is formed at the rate of heat release. It is believed that the second peak heat release occurs due to the sudden combustion of toluene with a low Flash point in the cylinder. Thus, the DW25T15 and DW5050T15 fuel mixtures form a second peak with early ignition and allow the burning of vegetable oil in diesel fuel.

Ignition delay in diesel engines increases the amount of accumulated fuel in the cylinder. In this case, the combustion process starts suddenly and allows the fuel to form in the premixed-combustion phase of a large section and increases the heat release value [37]. In the study, the cetane number and heating values of fuel mixtures decreases with the addition of waste oil to diesel fuel. The reduced heating value and the number of cetanes partially change the combustion, which explains the reduction of the heat release rate compared to the D100 fuel.

With the addition of toluene to the DW25 and DW50 fuel mixtures;

there is a slight increase in the maximum value of the heat release rate (Fig. 8). This can be explained by the fact that the flash point of toluene is low when compared to other fuels. Because with the addition of toluene, fuel mixtures take the combustion in the premixed-combustion phase earlier with an earlier combustion. In this way, the maximum heat release rate increases. Similar studies in the literature report that fuels with low flash points partially improve combustion in the premixed- combustion phase [38,39].

3.3. Emission characteristics

In this section, the effects of diesel-waste vegetable oil dual mixtures and diesel-waste vegetable oil-toluene triple mixtures on CI engine emissions (CO, NOx and smoke opacity emissions) were investigated and presented in graphs.

a) Carbon monoxide emissions

CO emissions are an emission product caused by incomplete combustion of fuels in cylinders. In general, it can be formed as a result of the fact that fuel particles do not meet enough oxygen in the cylinder [40].

The effect of toluene additive on CO emission in WCO-diesel fuel mixture is shown in Fig. 9. Although combustion is known to worsen with the addition of WCO to diesel fuel, there has been a decrease in CO emissions. This can also be expressed by the decrease in the total number of C in the fuel mixture. In some studies, it is reported that the decreasing number of C atoms causes a decrease in CO emissions [41]. In addition, the oxygen content of the mixture increases with the contribution of WCO to diesel fuel. Oxygen content leads to improved oxygen-fuel ratio in fuel-rich regions, combustion becoming more complete, reducing CO emissions [42]. With the addition of toluene, this amount of reduction increased even more. In similar studies, it is stated that such additives have a positive effect on CO emissions [43].

b) Nnitrogen oxides emissions

The change of NOx emissions for each fuel with the engine power is shown in Fig. 10. NOx emissions are an important emission value in diesel engines, and their reduction is important in terms of diesel engine emissions. It is determined that the chain structure of the fuels, the end- of-combustion temperature and the air/fuel ratio in the cylinder are effective in the formation of NOx emissions [44]. It is generally stated that long-chain, non-ideal air fuel ratio and low heating value fuels reduce NOx emissions [45].

With the addition of WCO to diesel fuel, NOx emissions decreased with reduced calorific value and long chain bond structure. But with the addition of toluene to the fuel mixture, NOx emissions increased again.

Fig. 6.(a) Maximum in-cylinder pressure value (b) location of maximum in-cylinder pressure value.

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With the addition of toluene, the calorific value of the fuel mixture increases and the mixture becomes enriched. It also has a toluene solvent structure. Such additives reduce the viscosity of the diesel +waste oil blend and provide better atomization of the mixtures. This situation causes the fuel in the cylinder to be mixed homogeneously. In similar studies with chemical solvents like toluene, it has been stated that additives increase the combustion temperature by acting as a catalyst and cause high NOx formation in the cylinder [26]. Although NOx emissions have increased with the addition of toluene, they are quite low compared to diesel fuel.

c) Smoke emissions

In internal combustion engines, smoke emissions arise as a product of incomplete combustion. Incomplete combustion of the fuel is stated to be due to lack of oxygen, incomplete mixing of air and fuel in the combustion chamber and low combustion temperature [46]. The effect of toluene in the WCO-diesel fuel mixture on smoke emissions is shown in Fig. 11.

As shown in Fig. 11, higher is emissions were achieved with DW25 and DW50 fuels compared to diesel fuel. The largest increase in smoke emissions was measured at 71 with DW50 fuel. Along with the addition of toluene to the DW25 and DW50 fuel mixtures, a reduction in smoke emission occurred, the maximum reduction was achieved by the addition of 15% toluene for both fuel mixtures and engine power. The maximum reduction in smoke emission compared to DW25 and DW50 Fig. 7. The variation of heat release rate with crank angle for 2 kW and 4 kW engine power.

Fig. 8. Maximum value of heat dissipation rate. Fig. 9.Variation of CO emissions changes.

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fuels was achieved by 21% and 29% for 2 kW engine power and 30% and 41% for 4 kW engine power, respectively. In particular, this emission values obtained with DW25T15 fuel decreased lower than the values obtained with D100 fuel. Insufficient combustion of WCO due to its high viscosity and poor volatility causes smoke emission [11]. The contribution of toluene to WCO is partly due to improved density and viscosity (see Table 1.) it is believed that smoke emissions are reduced with increased combustion efficiency. Similar results were obtained by Hazar et al. [3].

4. Conclusions

In this study, the effect of toluene addition to the WCO-diesel fuel blends on engine performance, combustion and emissions characteristics was investigated. Experiments were carried out at constant engine speed (1800 rpm) and different engine loads (2 kW and 4 kW). In- cylinder pressure, fuel consumption, heat rate and emissions (CO, NOx, and smoke opacity) was investigated.

•Compared to D100 fuel for each engine power, there was a decrease in thermal efficiency with DW25 and DW50 fuels. The addition of toluene to DW25 and DW50 fuel increased the thermal efficiency of the engine.

• DW25 and DW50 fuels showed higher BSFC than D100 fuel. BSFC was reduced with the addition of toluene to DW25 and DW50 fuels.

• Along with the waste oil addition to diesel fuel, there was a decrease in the maximum in-cylinder pressure values in all engine powers.

According to the D100 fuel, the point at which the maximum pressure is formed with the DW25 and DW50 fuels has moved away from the TDC. With the addition of toluene, it was observed that the maximum in-cylinder pressure values increased and that the point at which the maximum pressure was formed again approached the TDC.

• DW25 and DW50 fuels have a lower heat release rate than D100 fuels. By adding toluene to DW25 and DW50 fuels, there was an improvement in the rate of heat release, but overall the maximum heat dissipation rate remained at lower levels than D100.

• DW25 and DW50 fuels showed lower CO emissions compared to D100 fuel. Along with the toluene contribution, CO emissions were further reduced.

• NOx emissions have decreased along with the contribution of waste oil to diesel fuel. But with the contribution of toluene, NOx emissions increased again.

• There has been an increase in smoke emissions compared to DW25 and DW50 fuels and D100 fuels. With the addition of toluene to DW25 and DW50 fuels, smoke emissions decreased. The maximum reduction in smoke emissions was achieved by adding 15% toluene for both fuel mixture and engine power.

According to the results, it has been observed that similar results to pure diesel fuel are achieved by adding toluene to WCO-diesel mixtures.

Therefore, toluene-added WCO-diesel blends can be considered as an alternative fuel instead of pure diesel.

CRediT authorship contribution statement

Salih Ozer: ¨ Conceptualization, Methodology, Software, Writing - original draft, Writing - review & editing. Mehmet Akçay: Writing - original draft. Erdinç Vural: Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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