Impact of ZnO nanoparticles as additive on performance and emission characteristics of a diesel engine fueled with waste plastic oil

Abstract

Neat waste plastic oil (WPO) application as a fuel in engines reduces BTE and increases deleterious emissions of CO, UHC, NOx, and smoke due to the presence of insufficient oxygen and unbreakable hydrocarbon chains in WPO. Present investigation was performed to evaluate the impact of ZnO nanoparticles on the performance and emission characteristics of a diesel engine operated with the waste plastic oil (WPO20) blend. The objective of doping ZnO nanoparticles with WPO20 was to enhance the oxidation reaction and heat transfer rate between fuel droplets during combustion, which aids in completing the combustion. The sol-gel technique was adopted to successfully synthesize the ZnO nanoparticles using zinc acetate (Zn(CH₃CO₂)₂.2H₂O) and sodium hydroxide (NaOH) precursors. The structure and morphology of resulted particles were studied by XRD and FESEM tests. Both results indicate the stable formation of ZnO, and exhibit the crystallinity nature, spherical surface, and size consistency. The synthesized ZnO nanoparticles were infused in WPO20 blend in the amounts of 50, 100, and 150 ppm with the aid of the ultrasonication technique. Engine test was conducted with diesel fuel, WPO20 blend, and nano-infused fuels at a constant speed of 1500 rpm under various loads. The disparities in performance and emission characteristics were examined and compared with pure diesel fuel. The findings demonstrated that adding nanoparticles to WPO20 significantly lowers the smoke, CO, UHC, and NOx emissions and simultaneously improves the BTE and decreases the BSFC of the diesel engine. Optimum results were obtained for 100 ppm concentration of ZnO nanoparticles. Reduction of smoke by 11.86%, CO by 5.7%, UHC by 28%, and NOx by 14.93%, along with the enhancement of BTE by 2.47%, were noticed at maximum load with 100 ppm particles. Based on the test results, it is concluded that ZnO nanoparticles can be used as a suitable additive in WPO blends to improve the overall engine characteristics. Further scope of the present work is to study the effect of organic nanoparticles with WPO on engine behaviour, the detailed combustion of nanoparticles infused WPO, and the nanoparticles doped WPO on engine wear and corrosion.

Graphical abstract

Highlights

• •ZnO nanoparticles were successfully synthesized using the sol-gel approach.
• •ZnO infused WPO20 nano-fuel prepared through ultrasonication route.
• •WPO20ZnO100 fuel offered optimal output, and it augmented BTE by 2.47%.
• •WPO20ZnO100 nano-fuel reduced all pernicious emissions.

Nomenclature

ASTM

American society for testing and materials

BSFC

Brake specific fuel consumption

bTDC

Before top dead centre

BTE

Brake thermal efficiency

CA

Crank angle

cc

Cubic centimetre

CNG

Compressed natural gas

CO

Carbon monoxide

CO2

Carbon dioxide

DEE

Di-ethyl ether

DICI

Direct injection compression ignition

EGT

Exhaust gas temperature

FESEM

Field emission scanning electron microscopy

H2O

Water

HCNG

Hydrogen compressed natural gas

kHz

kilohertz

kW

Kilowatt

MEA

2-methoxy ethyl acetate

MT

Million tons

NaOH

Sodium hydroxide

nm

nanometre

NOx

Oxides of nitrogen

OH

Hydroxide

PD

Pure diesel

ppm

Parts per million

PPO

Plastic pyrolysis oil

RON88

Premium gasoline fuel

RON90

Pertalite gasoline fuel

rpm

Revolutions per minute

TiO2

Titanium dioxide

UHC

Unburned hydrocarbon

w.r.t.

With respect to

WPO

Waste plastic oil

WPO20

80% diesel +20% waste plastic oil

WPO20ZnO100

80% diesel +20% waste plastic oil +100 ppm ZnO nanoparticles

WPO20ZnO150

80% diesel +20% waste plastic oil +150 ppm ZnO nanoparticles

WPO20ZnO50

80% diesel +20% waste plastic oil +50 ppm ZnO nanoparticles

XRD

X-ray diffraction

ZnO

Zinc oxide

1. Introduction

India is the third biggest consumer of petroleum products after the United States and China [1]. Due to the dependence on petroleum products and rapid growth in population, the demand for energy was continuously increasing for the last few years, as a result of which the price of fuel in India is at its peak in Indian history. The Indian Prime Minister suggested increasing the use of renewable energy sources to reduce dependence on petroleum products. Therefore, this research is devoted to finding a potential alternative to diesel fuel for contribution to Indian renewable energy policy.

The use of plastic is rigorously increasing in domestic and industrial applications. Around 275 MT of plastics were generated globally in 2012, and had increased to 460 MT in 2019. If the plastic growth rate persists, plastic production will reach up to 1231 MT in 2060 [2]. Plastic use has become ingrained in the daily life of people, and most of the plastics are discarded after one use [3]. It was reported that around 50% of plastic wastes are recycled while the rest are dumped into the environment (land, sea, and burned in the open air), which is detrimental to ecosystem health and human well beings 4,5. Plastic made up of monomers such as propylene and ethylene are non-degradable, therefore plastic does not decompose; rather, it accumulates [6]. Just in India alone, around 5.6 million tons of plastic waste are generated per year. Though the incineration process has helped in recuperating some energy, it releases greenhouse gases that are ecologically detrimental. Disposing of discarded plastic has become very challenging from an environmental perspective. Since discarded plastic contains a long chain of hydrocarbons with high calorific value, boiling point, and aromatic content, it can be converted into high-quality oil referred to as waste plastic oil (WPO) by adopting of the pyrolysis technique, which breaks the long hydrocarbon chains of plastic into smaller chains [7,8]. The WPO can be treated as an excellent substitute for diesel fuel owing to its carbon chain range (C10 – C25), high miscibility with diesel, high calorific value, and low viscosity [9,10]. Thus, the conversion of plastic waste would be an unusual way to treat plastic waste and produce a promising alternative to diesel fuel.

Few researchers have used the WPO in diesel engines with no alteration and reported the performance and emission behaviour. Mangesh et al. [11] experimented with assessing the emission and efficiency behaviour of a diesel engine operated with 5, 10, and 15% polypropylene oil (PPO) blends. They witnessed that engine performance improves as the concentration of PPO increases in blends, but the deleterious emissions also increase. The appearance of a larger number of saturated carbon compounds and unsaturated double-bond compounds was challenging to combust due to the shortage of oxygen, which led to an increment in UHC by 90% and NOx by 50% at apex load for a 15% blend of PPO compared to diesel. Kumar et al. [9] tested the different blends of WPO in a 4-stroke diesel engine. Their findings discovered that the WPO20 blend shows the lowest BSFC and highest BTE among all the tested fuels. Also, NOx and UHC emissions were noticed to be lower at part load, while both emissions increased at high load. Subhaschandra et al. [12] evaluated the effect of the WPO20 blend on diesel engine characteristics under various speeds (1200, 1500, and 1800 rpm) and loads. The cylinder pressure (78.16 bar) and BTE (30.85%) for WPO20 were witnessed to be higher with 1800 rpm at 100% load conditions. Besides, NOx and smoke emissions were reliably decreased with 1500 rpm at full load. Khairil et al. [13] conducted an investigation to appraise the performance of WPO mixed RON88 and RON90 fuels in an otto engine. They found that the blend of RON90 gasoline and 30% WPO provides superior performance among other tested fuels thanks to the higher calorific value of both RON90 and WPO fuels. Singh et al. [7] pointed out that a higher proportion of WPO in the blend augments the BTE, heat release rate, cylinder pressure and ignition delay, declines the BSFC and enhances the emissions (CO, UHC, NOx, and CO2) as a consequence of the unavailability of sufficient oxygen during combustion that leads to partial combustion of fuel. They suggest utilizing WPO in the engine only up to 50% in the blend. Panda et al. [14] examined the emission and performance of a diesel engine loaded with WPO blends. It was concluded that BSFC was greater for every WPO blend corresponding to reference fuel, and they noticed the vibration when the engine was run with blends containing more than 50% WPO. Maithomklang et al. [15] also suggest not using WPO in high concentrations without any suitable additive due to combustion delay and higher exhaust emissions. According to available studies on WPO blends, engine characteristics have not upgraded significantly with regard to diesel. Although the chemical and physical properties of WPO are pretty similar to diesel, the heavier hydrocarbons chains presence provokes to produce higher UHC and CO emissions.

The above literature suggests that the combustion performance of WPO associated blends needs to be improved due to elevated exhaust emissions. The addition of effective oxygenated additives such as alcohols, ether, esters, or metal-oxide nanoparticles in WPO blends can lessen the aromatic content of WPO, which assists in overcoming performance and emission relevant issues. Kaewbuddee et al. [16] mixed 10% methyl ester derived from castor oil and palm oil separately, into WPO for testing in the engine. From the test results, although a decrement in UHC and NOx emissions was found, CO and smoke emissions had increased for both esters. In addition, BTE was also lower for both esters due to inferior calorific value to WPO. Bridjesh et al. [17] blended the 2-methoxy ethyl acetate (MEA) and di-ethyl ether (DEE) by 10% in WPO40 blend to operate the diesel engine. They observed that MEA as an additive plays a better role than DEE in the combustion chamber because the self-ignition point and boiling point of MEA are pretty close to diesel fuel, and both points aid in improving the fuel atomization and vaporization process. An escalation in BTE by 5.5% and abatement in BSFC by 1.6%, CO by 5.9%, UHC by 1.9.3%, and smoke by 1.6% were witnessed with MEA additive compared to DEE additive. However, the DEE additive produced much lower NOx than MEA, which perhaps ascribed to shorter ignition delay and cooling effect caused by the high latent heat of vaporization of DEE infused fuel.

Alcohols are superior in reducing smoke when mixed in the WPO blend; however, they are capable of lowering CO and UHC emissions as well. But due to the lower density, the miscibility of alcohols is very poor with WPO and diesel fuels. To eliminate the miscibility issue and improve the combustion performance, a recently developed approach of the application of nanoparticles as an additive was utilized in WPO blends through the ultrasonication method. Elkelawy et al. [18] reported that the high surface energy of nanoparticles enhances the oxidation process, increases the reaction rate, and shortens the ignition delay, which stimulates the combustion process, thereby reducing emissions and improving performance. Sathyamurthy et al. [19] used the TiO2 nanoparticles as an additive in the amount of 100, 150, and 200 ppm with neat WPO to assess the variation in engine outputs with respect to the reference fuel. An ascent in BTE by 3.44%, a descent in CO by 51.16%, and UHC by 24.27% were noticed. Praveenkumar et al. [20] also studied the impact of 100 ppm TiO2 nanoparticles with WPO40 blend on diesel engine behaviour. They found a 1.5% increment in BTE and a significant decrement in UHC, CO, and NOx emissions. Nanoparticles incorporation enhanced the evaporation rate and induced the combustion rate due to the oxygen molecules, which resulted in lower emissions and lower brake-specific energy consumption (BSEC).

Numerous researches related to the usage of biodiesel produced from jatropha [21], tamarind [22], algae [23], mixture of oils [24], animal fat [25], etc., as engine fuel to analyse the engine characteristics on various engine input parameters are available. However, research work on the use of WPO as an engine fuel is limited, and very minimal research work is available on nanoparticles applications on WPO. The reason for the limited use of WPO in the engine could be high noxious emission production due to an insufficient volume of oxygen (0.603%) and a larger proportion of C12–C20 impendence in WPO [10]. Besides, WPO production challenges such as inconsistent quality of feedstock, difficulty in plastic separation from waste, and the complicated process could be some of the liable factors for the limited use of WPO [26]. In the author's knowledge, no report is available on the usage of zinc oxide (ZnO) nanoparticles as an additive in the WPO blend that powered a diesel engine. Hence, there is an existence of a literature gap in studying the aspects of engine characteristics analysis on nanoparticles infused WPO blend, which offers uniqueness and novelty to the present work. The current investigation explores the impact of ZnO nanoparticles with WPO blend on engine behaviour. The intent of the proposed study is to assess the efficiency and emissions of a diesel engine filled with ZnO infused WPO fuel and compare the outcomes of the ZnO infused WPO fuel with the neat WPO blend and diesel fuel.

2. Material and methods

Waste plastic oil was procured online from Indiamart website, and Zinc acetate dihydrate (Zn(CH₃CO₂)₂.2H₂O) and sodium hydroxide (NaOH) chemicals were obtained from the university lab.

2.1. Synthesisation of ZnO nanoparticles

Sol-gel method was implemented to synthesize the ZnO nanoparticles using Zn(CH₃CO₂)₂.2H₂O and NaOH precursors. Firstly, 0.2 M solution of zinc acetate was formulated by dissolving 4.3 gm of Zn(CH3CO2)2.2H2O into 100 ml of deionized water. The subsequent solution was stirred at 600 rpm (room temperature) on heating and stirring element for 120 min. In the meantime, other 0.5 M NaOH solution was formulated by adding 2 gm of NaOH into 100 ml of deionized water. Then, 0.5 M NaOH solution was introduced dropwise to an agitating zinc acetate solution. It was witnessed that after the addition of NaOH the colour starts changing to white as the solution's pH level reaches up to 8, which is the indication of particles formation. The adding of the NaOH solution was continued until the pH value reached 9, and at this stage, the solution colour appeared milky white. The resulting solution was stirred at 60 °C for 120 min to complete the formation of ZnO residue. Then the obtained residue was filtered with permeable filter paper and desiccated overnight at 60 °C in a vacuum oven. The desiccated product was calcined in an electric furnace at 400 °C for 120 min. Eventually, the calcined substance was pulverized through a mortar pestle to form fine powder. Fig. 1 illustrates the complete procedure of ZnO synthesis.

Fig. 1.

Procedure of ZnO nanoparticles preparation.

2.2. Preparation of ZnO nano-fuel

Initially, ZnO nanoparticles were measured in adequate quantities of 50, 100, and 150 ppm using a precise weighing scale. These measured particles were dispersed with 20 ml of deionized water. Then the surfactant named “sodium dodecyl sulfate (NaC12H25SO4)” was added in an appropriate amount, which conceals the nanoparticles with extended-loop and tail and develops the degree of continuity at the interface of nanoparticles and fluid. Initially, the subsequent mixture was sonicated for steady blending in a bath sonicator for 60 min and followed by ultrasonication at 20 kHz frequency for 10 min to make the stable nano-fluid. The stability of nanofluid mainly relies on the concentration of surfactant because addition of surfactant forms the electrostatic charge called zeta potential, which creates the electrostatic repulsion force among particles to prevent the coagulation and coalescence and recompensate the Vander Walls attraction forces or reduces the surface tension. Then, the prepared zinc oxide nanofluids were introduced into the WPO20 fuel blend followed by heating (60 °C) and stirring (800 rpm) for 60 min using a hot plate magnetic stirrer to eliminate the vestiges of H₂O molecules. Lastly, a similar ultrasonication procedure was performed with subsequent aliquots to sustain the colloid stability of all ZnO-reformulated WPO20 blend. The ZnO nanoparticles infused fuels are abbreviated as WPO20ZnO50, WPO20ZnO100, and WPO20ZnO150. Fig. 2 depicts the extensive phases encompassed in nano-fuel preparation. Table 1 demonstrates the determined properties of the reformulated fuels.

Fig. 2.

Procedure of preparing ZnO-infused nano-fuel.

Table 1.

Physicochemical properties of fuels.

Notations	Density (g/cc)	Kinematic Viscosity (cst)	Cetane Index	Calorific Value (MJ/kg)	Flash point (° C)	Fire point (° C)
Diesel	0.840	2.58	46.0	43.0	71	75
WPO	0.790	3.04	40.0	42.2	42	45
WPO20	0.830	2.67	44.8	42.84	65	69
WPO20ZnO50	0.832	2.70	45.9	42.87	67	71
WPO20ZnO100	0.835	2.72	47.0	42.94	69	74
WPO20ZnO150	0.840	2.75	46.8	42.91	70	76
ASTM Standard	D 4052	D 445	D 4737	D 4809	D 2500	D 92

2.3. Experimental setup and uncertainty analysis

The summary of experimental setup used in this work is clearly mentioned in Table 2. The comprehensive narration of lab setup, equipment, and procedures was demonstrated in our previous works [27,28] on engine behaviour analysis using Fe₃O₄ nanoparticles infused methyl ester blends, and CNG and HCNG as secondary fuels.

Table 2.

Specification summary of the experimental setup.

Parameters	Specifications
Make & Model	Kirloskar & TV 1
Engine configuration	4-stroke, vertical 1-cylinder, totally enclosed, dual-fuel diesel engine
Cooling method	Water cooled
Combustion method	Direct injection
Nozzle orifice diameter	0.3 mm
Injection timing	23° CA bTDC
Injection pressure	200 bar
Connecting rod length	234 mm
Stroke length	110 mm
Crank radius	55 mm
Bore diameter	87.5 mm
Rated speed	1500 rpm
Rated power	3.5 kW
Swept volume	661 cc
Cavity volume	34.0 cc
Clearance volume	40.1 cc
Peak pressure	76 bar
Compression ratio	17.5:1

Pure diesel, WPO20, and nano-reformulated fuels were employed to power the DICI engine for evaluating the emissions and performance characteristics. The experimentation trials for all fuels were executed at 25%, 50%, 75%, and 100% loads with a 1500 rpm steady speed on the depicted experimental setup in Fig. 3.

Fig. 3.

Experimental setup.

In the beginning, the engine was operated with diesel fuel under various load conditions at 1500 rpm constant speed. The emissions and performance-related readings were noted at each load when the engine was in the steady state condition. After that, diesel fuel was replaced by ZnO nanoparticles infused fuels, and the data were recorded under identical conditions as for the diesel fuel. Exhaust emission readings were recorded by smoke meter and exhaust gas analyzer (EGA). The specifications of the smoke meter and EGA are presented in Table 3.

Table 3.

Specifications of smoke meter and exhaust gas analyzer (EGA).

Parameter	Range	Resolution
Smoke Meter (Model: SM-05, Brand: MARS)
Smoke	0–99.9%	0.1%
K Value	0–9.99 m-1	0.01 m-1
Zero and Span Drift	±0.1 m-1	–
Linearity	±0.1 m-1	–
Repeatability	±0.1 m-1	–
Exhaust Gas Analyzer (Model: MN-05, Brand: MARS)
CO	0–9.99% vol.	0.001% vol.
UHC	0–15000 ppm	1 ppm
CO2	0–20% vol.	0.01% vol.
NOx	0–5000 ppm	1 ppm
O2	0–25% vol.	0.1% vol.
Lambda	0.200–2.000%	0.001

Uncertainty analysis is a coordinated set of process followed to assess inaccuracies in experimental data. Inaccuracies are triggered by the errors in human calculations, mechanical and electronic components, device calibration, environmental factors, etc. This analysis is of excellent value for investigational studies since its outcome provides an idea relating to the correctness and repeatability of the offered findings to the readers. Therefore, the experimental percentage uncertainties for the present work were computed using equation (1) below [29];

$$\frac{U_y}{y}=\sqrt{\sum _{i=1}^n{\left(\frac{1}{y}\frac{\partial y}{\partial x_i}\right)}^2}$$ (1)

In the above equation (1), y signifies the specific parameter that relies on $x_i$ parameter, and $U_y$ represents the deviation or uncertainty in specific parameter (y).

The uncertainties of physical parameters and computed values were ±0.4, ±0.5, ±0.4, ±1.5, ±1.4, ±1.5, and ±0.9 for BSFC, BP, BTE, CO, UHC, NOx, and smoke respectively. The total uncertainty was enumerated using equation (2) which is given below [27];

Total uncertainty =

$$\sqrt{\begin{array}{c}{\mathrm{u}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{r}\mathrm{t}\mathrm{a}\mathrm{i}\mathrm{n}\mathrm{t}\mathrm{y}\%\mathrm{o}\mathrm{f}\{{\left(\mathrm{B}\mathrm{S}\mathrm{F}\mathrm{C}\right)}^2+\left(\mathrm{B}\mathrm{P}\right)}^2+{\left(\mathrm{B}\mathrm{T}\mathrm{E}\right)}^2+{\left(\mathrm{C}\mathrm{O}\right)}^2+{\left(\mathrm{U}\mathrm{H}\mathrm{C}\right)}^2+{\left(\mathrm{N}\mathrm{O}\mathrm{x}\right)}^2+{\left(\mathrm{S}\mathrm{m}\mathrm{o}\mathrm{k}\mathrm{e}\right)}^2\}\end{array}}$$ (2)

$$=\sqrt{{\left(0.4\right)}^2+{\left(0.5\right)}^2+{\left(0.4\right)}^2+{{\left(1.5\right)}^2+\left(1.4\right)}^2+{\left(1.5\right)}^2+{\left(0.9\right)}^2}=\pm 2.8\%$$

As per the previous research, the obtained total uncertainty value is under the acceptable range [30,31].

3. Results and discussion

3.1. Characterization of synthesized ZnO nanoparticles

The equilibrium between the condensation and hydrolysis reactions was the major cause of the obtained final product. Owing to heating, Zn(CH₃CO₂)₂.2H₂ O in the aliquots passes off a hydrolysis reaction, as a result in the formation of zinc and acetate ions. The profusion of electrons in the oxygen atom formed zinc hydroxide acetate in the presence of OH⁻ ions and H₂O, which was the hydrolysis reactions’ intermediate product. However, it was efficiently converted into ZnO at elevated temperature with extended periods of refluxing. The overall chemical reaction of the formation of ZnO nanoparticles is illustrated below;

Zn(CH₃CO₂)₂.2H₂O + 2NaOH →ZnO + 2NaCH₃CO₂ + H₂O

X-ray diffraction (XRD) technique in the 2θ range of 20–60 ͦ with 0.05 step size was performed to explore the phase structure or crystallinity as a function of 2θ angle. Fig. 4 demonstrates the X-ray diffraction pattern of the synthesized sample. XRD peaks for the synthesized ZnO nanoparticles were obtained at 2θ = 31.8 ͦ, 34.45 ͦ, 36.25 ͦ, 47.5 ͦ, and 56.7 ͦ assigned to the lattice planes of (100), (002), (101), (102), and (110) respectively. The most intense and sharp peak was noticed at 2θ = 36.25 along (101) orientation, which indicates that the synthesized ZnO was highly crystallized. No extra peaks were witnessed, which indicates the purity and single phase. The XRD peaks of the sample reported in the present work are quite close to the previous studies [32,33]. The ZnO crystallite size was calculated using equation (3) below [25]:

$$D=\frac{k.\lambda }{\beta .\mathit{cos}\theta }$$ (3)

Where, D represents the crystallite size in the above equation, k indicates the Scherrer constant and its value is taken as 0.9; the other parameters θ, β, and λ denote the Bragg angle (ͦ), full width at half maximum of the peak (radians), and X-ray wavelength, respectively. The average crystallite size of synthesized particles was computed employing the above equation (3) and was 45.89 nm.

Fig. 4.

XRD pattern of synthesized ZnO nanoparticles.

FESEM is an excellent tool to visualize the surface morphology and determine the chemical composition of synthesized particles. Fig. 5 illustrates the FESEM image of the particles at 70,000× magnification. It was clear from the result that the particles are of different sizes and shapes (spherical and irregular with wurtzite hexagonal phase). The result of this study is in agreement with the preceding researches [32,34]. The irregular shape that was formed might be caused by the agglomeration of crystals; however, the majority of the particles were in a spherical shape with purity and homogeneity.

Fig. 5.

FESEM image of ZnO particles.

3.2. Performance characteristics

BTE refers to the combustion-ability of a fuel, which signifies how efficiently a fuel's chemical energy is transformed into useful work output. Fig. 6 presents the disparities for all tested fuels under the loading range of 25% to 100%. At 100% load, the BTE value of the WPO20 blend (31.68%) is very close to the diesel fuel (31.75%). Higher viscosity and higher aromatic content in WPO fuel possibly are the significant factors in reducing the BTE because higher viscosity depreciates the atomization, which leads to a poor combustion rate [20], and higher aromatic content requires additional energy to break the hydrocarbons bond, which causes high heat loss and combustion delayed. As the ZnO nanoparticles were incorporated in WPO20, the BTE was improved by 1.1%, 2.47%, and 2.12% corresponding to 50, 100, and 150 ppm dosages of nanoparticles, respectively. This augmentation in BTE was attained attributed to the donation of oxygen molecules during combustion from the infused nanoparticles, which assists in breaking the saturated and unsaturated carbon compounds resulting in superior combustion [11]. Also, a higher thermal exchange process and high catalytic activity of nanoparticles ascribed to superior vaporization leads to improved atomization, reduced ignition delay and reduced heat loss, thus higher BTE was attained [25]. BTE results attained for the present research are supported by Gavhane et al. [32] report.

Fig. 6.

BTE vs. Load.

Fig. 7 displays the revulsion in BSFC against load for all the tested fuels. Like BTE, BSFC is also an important characteristic that expresses how much fuel is needed to engender the unit brake power output, although inferior BSFC is always preferable for all fuel types. As noticed in Fig. 7, the WPO20 blend shows a higher BSFC at all loads; thus, BSFC demonstrates the opposite trends in accordance with the BTE trends. Forasmuch, the BSFC of tested fuels at full load can line up in the ascending result order of WPO20ZnO100 (261.26 g/kW.h) < WPO20ZnO150 (269.01 g/kW.h) < WPO20ZnO50 (284.13 g/kW.h) < PD (292.55 g/kW.h) < WPO20 (297.12 g/kW.h). It is evident from the results that nanoparticles as additives decline the BSFC owing to the micro-explosion phenomenon causes triggering the secondary-atomization, which stimulates evaporation of fuel droplets. High evaporation leads to minimizing ignition delay by combusting excess fuel in the pre-mixed phase of the combustion reaction, resulting in produced additional power which reduces the BSFC [27,35,36]. 100 ppm infused WPO20 fuel provides the lowest BSFC. For a higher dosage (150 ppm) of ZnO nanoparticles, BSFC starts to increase because particles start to aggregate, which reduces the surface area-volume ratio resulting in a descent in the catalytic activity of nanoparticles [29]. A similar kind of reduction in BSFC with ZnO additive was reported by Dhahad and Chaichan [37].

Fig. 7.

BSFC vs. Load.

Fig. 8 depicts the disparity in exhaust gas temperature (EGT) for pure diesel, WPO20, and nanoparticles infused blend fuels against load. For pure diesel fuel, the EGT was changed from 165 °C at 25% load to 361 °C at maximum load, whereas it was varied from 190 °C to 461 °C for WPO20 blend, 185 °C to 445 °C for WPO20ZnO50, 173 °C to 415 °C for WPO20ZnO100, and 180 °C to 433 °C for WPO20ZnO150 nano fuel. The results obviously demonstrate that EGT accelerates as the load elevates for all the tested fuels. This might be attributed to the requirement of an appended volume of fuel by the engine to engender the supplementary power for bearing the additional loading. However, EGT also augmented as WPO fuel concentration was added in diesel fuel. The reasons of higher EGT for WPO20 blend could possibly be the highly viscous nature and inferior calorific value of WPO led to promote burning of additional fractions of fuel as a result in fuel burns during controlled combustion as well as goes through combustion in an aftermost part of the power stroke, that is the reason of combustion delayed. The above results reported for EGT are supported by the previous literature [7,14]. Infusion of nanoparticles in fuel declined the EGT because their existence shortened the ignition delay and invigorated the quality of combustion by improving the radiative and conductive heat and mass transfer properties of the fuel. On account of better combustion, maximum heat is liberated within the combustion chamber itself, causing a reduction in EGT [38]. However, for a higher dosage of ZnO, the EGT had slightly increased, possibly because of the retarded activity of nanoparticles as a consequence of an increment in viscosity value and deprived atomization of fuel droplets. Hence, the use of an adequate quantity of nanoparticles with fuel is beneficial as far as engine performance is concerned [39].

Fig. 8.

EGT vs load.

3.3. Emission characteristics

UHC emission usually forms when the fuel does not participate in the exothermic combustion reaction. Fig. 9 presents the variation of UHC emission against load for each sample of fuel. As witnessed in the figure, UHC increased for all samples of fuel with a raising load. This can be ascribed to the participation of an excess amount of fuel in combustion devoid of varying the air amount caused hefty fuel-air mixture at a higher load, thus, producing higher UHC emission [40,41]. UHC emission of WPO20 was higher by 14% than pure diesel fuel. High viscosity of WPO causes the generation of big droplets and lessened vapour pressure responsible for partial combustion, thus higher UHC attained [7]. Also, the presence of unbreakable unsaturated hydrocarbons in WPO is probably another reason of higher UHC [42]. UHC emission was significantly decreased by 12.28%, 28%, and 22.80% compared to WPO20 blend when 50, 100, and 150 ppm ZnO nanoparticles were added in WPO20 blend, respectively. This might be ascribed to the releasing of oxygen molecules from ZnO structure during combustion and large reactive surface of nanoparticles; both these factors aid in oxidizing the uncombusted fuel particles and enhance the evaporation rate resulting in reduction of rich fuel zones in combustion chamber, thus lower UHC generation [11,25]. Suhel et al. [43] reported similar UHC results for nanoparticles infused fuels.

Fig. 9.

UHC vs. Load.

Like UHC emission, CO emission generates due to the shortage of oxygen, deprived mixing, low combustion temperature, and locally rich mixture in the combustion chamber; because these parameters cause unfinished combustion [27]. As per attained results, CO emission of fuel samples can be arranged in the manner of WPO20 (0.37%) < PD (0.34%) < WPO20ZnO50 (0.35%) < WPO20ZnO150 (0.33%) < WPO20ZnO100 (0.32%). Fig. 10 depicts the dissimilarities in CO emissions against various loads for each sample. It is apparent from the above results, that the WPO20 blend produces higher CO compared to other fuels. The reason might be the additional amount of fuel consumption at a higher load. Besides, higher viscosity of WPO leads to form rich fuel zones in the combustion chamber caused by poor atomization, while aromatic content in WPO elevates the ignition delay and diminishes the combustion duration; both these factors tend to cause incomplete combustion, thus producing higher CO emission [7,42]. Reduction in CO emission was witnessed with the ZnO doped fuels. This might possibly be on account of the disintegration of ZnO into Zn and O atoms in the course of the combustion reaction. O atoms intensified the oxidation rate of fuel molecules in the reaction zone, while Zn atoms sped up the heat transfer among the flame front and uncombusted fuel molecules owing to good thermal conductivity resulting in enhanced combustion. As a result, uncombusted fuel molecules burned up in a shorter amount of time, thus decrease in CO formation [44]. Sachuthananthan et al. [45] study supports the CO results.

Fig. 10.

CO vs. Load.

NOx formation predominantly relies on the in-cylinder temperature, air-fuel mixture, residence time, and fuel structure. Fig. 11 shows the effect of NOx against load of each fuel. A surging trend in NOx emission was witnessed with regard to load for all fuels. NOx emission for WPO20 blend was witnessed to be higher by 22.4% compared to PD at full load. WPO has high carbon compounds that need excess oxygen and time to break down, resulting in higher in-cylinder temperature and delayed combustion, which causes higher NOx [9,11,46]. Introducing ZnO nanoparticles into WPO20 blend descends the NOx emission by promoting the vaporization process during combustion, which combusts fuel particles uniformly and reduces the heat loss [20,47] NOx emission was lower by 7.8% (WPO20ZnO50), 14.93% (WPO20ZnO100), and 13.12% (WPO20ZnO150) than neat WPO20 blend. Vali and Wani [44] reported similar NOx results with ZnO nanoparticles.

Fig. 11.

NOx vs. Load.

Smoke emission primarily develops in fuel-rich regions; therefore, its formation is more progressive as engine load increases. High reaction temperature, surplus oxygen, and adequate reaction time are the significant factors that cause smoke oxidation. Smoke particles are predominantly oxidized near the flame's surface, depending on the intensity of turbulence. The variation in smoke with respect to load is presented in Fig. 12. It is conspicuous from the graph that the WPO20 blend produced the highest smoke among all fuels. This may be ascribed to the unavailability of homogeneous air-fuel mixture, shorten the combustion period, and hasty flame propagation [14]. At full load, WPO20 blend demonstrated higher smoke by 11.32% in comparison to pure diesel. However, by the dispersion of ZnO nanoparticles in WPO20 blend, smoke decreased by 5.08%, 11.86%, and 10.16% corresponding to 50, 100, 150 ppm dosages of particles, respectively. This happened thanks to the nanoparticles' catalytic effect, which promotes combustion, shortens the ignition delay, and provides oxygen during the combustion reaction. The appearance of oxygen molecules (O₂) in the chemical structure of ZnO is discharged during a premixed phase of combustion reaction, leading to an escalation in the oxidation rate of soot particles, thus generating less smoke [29,48]. A similar smoke emission pattern was observed by Soudagar et al. [49] in their study of hybrid ZnO nano-fuel.

Fig. 12.

Smoke vs. Load.

3.4. Rate of change of engine characteristics

The overall effect of each type of fuel on engine emissions and performance characteristics can be demonstrated by measuring the rate of change in engine parameters with regard to load. Table 4 demonstrates the computed values of weighted mean and rate of change of the measured and calculated parameters of all the tested fuels. The weighted mean and rate of change of each engine parameter were calculated using equation (4) and equation (5) which are given below [22,50]:

$$\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{e}\mathrm{d}\mathrm{M}\mathrm{e}\mathrm{a}\mathrm{n}=\frac{X1*Y1+X2*Y2+..........+Xn*Yn}{X1+X2+.........+Xn}$$ (4)

$$RateofChange=\frac{Weightedmeanofeachfuel-WeightedmeanofPD}{WeightedmeanofPD}$$ (5)

Where, Xn represents the engine load and Yn represents the measured parameter value at corresponding load (Xn). Fig. 13 depicts the rate of increase and decrease of measured engine parameters for WPO20, WPO20ZnO50, WPO20ZnO100, and WPO20ZnO150 fuels corresponding to PD fuel. Fig. 14 demonstrates the rate of change of engine parameters in regard to the WPO20 blended fuel.

Table 4.

Weighted mean and rate of change of engine parameters for the tested fuels.

Parameters	Weighted mean of different tested fuels					Rate of change with respect to PD				Rate of change with respect to WPO20
	PD	WPO20	Fuel 1	Fuel 2	Fuel 3	WPO20	Fuel 1	Fuel 2	Fuel 3	PD	Fuel 1	Fuel 2	Fuel 3
BTE	26.44	26.31	27.27	28.57	28.31	−0.49	3.13	8.05	7.07	0.49	3.64	8.58	7.60
BSFC	327.48	331.86	317.93	295.94	303.52	1.33	−2.91	−9.63	−7.31	−1.31	−4.19	−10.82	−8.53
EGT	291.9	367.6	354	331.7	344.8	25.93	21.27	13.63	18.12	−20.59	−3.69	−9.76	−6.20
UHC	44	50	44.9	37.6	39.6	13.63	2.04	−14.54	−10	−12	−10.2	−24.8	−20.8
CO	0.305	0.335	0.311	0.283	0.296	9.83	1.96	−7.21	−2.95	−8.95	−7.16	−15.52	−11.64
NOx	522.3	658.5	600.3	543.8	561.2	26.07	14.93	4.11	7.44	−20.68	−8.83	−17.41	−14.77
Smoke	47.7	52.6	50.3	45.4	46.8	10.27	5.45	−4.82	−1.88	−9.31	−4.37	−13.68	−11.02

In the above table *Fuel 1 = WPO20ZnO50, *Fuel 2 = WPO20ZnO100, and *Fuel 3 = WPO20ZnO150.

Fig. 13.

Rate of change of engine parameters for tested fuels in regard to PD.

Fig. 14.

Rate of change of engine parameters for tested fuels in regard to WPO20 blend.

4. Conclusion

The present work emphasizes on the synthesis of ZnO nanoparticles and empirically explores the potential of synthesized nanoparticles in an engine as a combustion catalyst. The synthesized particles were infused with waste plastic oil blend in different concentrations and accordingly, performance and emission behaviour of all tested fuels were discussed in detail. The following key points are drawn from the obtained results:

• •ZnO nanoparticles were successfully synthesized by adopting the sol-gel technique using Zn(CH3CO2).2H2O and NaOH precursors. The XRD test revealed that particles are in crystallinity nature with high purity, whereas the FESEM result demonstrated that most of the particles' shapes are spherical.
• •Inclusion of ZnO nanoparticles into the WPO20 blend improves the fuel properties such as flash point, cetane index, and calorific value.
• •WPO20 blend offered very close BSFC and BTE values to PD fuel for each engine load. However, pernicious emissions such as NOx, smoke, UHC, and CO were considerably higher than diesel fuel.
• •Incorporation of ZnO nanoparticles in WPO20 blend improved the BTE and BSFC, and significantly diminished all emissions attributed to the complete combustion of fuel by reason of an improved air-fuel ratio and accelerated oxidation of fuel droplets.
• •Investigation also revealed that 100 ppm dose of ZnO nanoparticles as an additive offers the optimum results. Beyond 100 ppm dose of nanoparticles slightly reduced the BTE and increased all emissions compared to 100 ppm dose. Difficulties in nano-fuel stability occur for higher dosages of nanoparticles because of particle agglomeration.
• •WPO20ZnO100 fuel among all offered the optimum outcomes in terms of performance and emissions. This fuel could be a promising alternative of pure diesel.
• •Limitations of the usage of ZnO nanoparticles with WPO may be the stability and economic aspects.
• •Comprehensive research is required to analyse the impact of ZnO nanoparticles with WPO blend on the wear of engine components and corrosion of exhaust pipe.
• •Nanoparticles-infused fuel should be commercialized and employed in automotive, powerplants, domestic heating systems, renewable energy, and other sectors due to its high energy density, superior thermal exchanging nature, rapid oxidation, and effortless transportation. In addition, nanoparticles can be utilized to develop coolants and lubricants.

Author contribution statement

Ameer Suhel: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Wrote the paper.

Norwazan Abdul Rahim & Khairol Amali Bin Ahmad: Conceived and designed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data.

Mohd Rosdzimin Abdul Rahman & Noh Zainal Abidin: Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data.

Umrah Khan: Conceived and designed the experiments; Performed the experiments.

Yew Heng Teoh: Conceived and designed the experiments; Analyzed and interpreted the data.

Funding statement

This work was supported by National Defence University of Malaysia (NDUM) under 1A004-UPNM/2021/FA2386-21-1-4016 (International Grant) and Tabung Amanah PPPI (A0014).

Data availability statement

The data that has been used is confidential.

Declaration of interest’s statement

The authors declare no conflict of interest.