1. Introduction
For many decades, fossil fuels including petroleum, natural gas, and coal have been considered as the major energy resources globally. However, since these energy resources are non-renewable, they are likely to be depleted soon due to increasing demand resulting from rapid population growth and industrialization. In addition, the excessive use of fossil fuels has led to negative implications for the environment. As a result, emission regulations are increasingly being strengthened to mitigate environmental degradation. Therefore, the need for cleaner and economically viable renewable energy sources has led researchers to seek new sources [
1]. In this context, biodiesel produced from vegetable oils has been identified as a potential substitute for petroleum diesel in compression ignition engines [
2].
Vegetable oils range from edible oils such as soybean, rapeseed, sunflower, palm [
3], and coconut oil [
4,
5] to non-edible oils such as Karanja, Jatropha, Jojoba, Polanga, Mahua, rubber seed, cotton seed, tobacco, neem, linseed, and microalgae oil [
6]. Other non-edible oils reported in the literature include eucalyptus oil, tea tree oil and, orange oil [
7]. The use of non-edible oils as feedstock for biodiesel production has drawn greater research attention as it overcomes challenges related to food security and debate of food versus fuel. Furthermore, waste cooking oils are considered a cheaper biodiesel feedstock since the price of the oil is significantly lower compared with new oil from other sources [
8]. An additional benefit associated with the use of waste cooking oil is that its recycling as an energy resource presents the best means of disposal.
Waste cooking oils have higher viscosity compared to conventional diesel fuel, and hence cannot be used directly in the diesel engine. The higher viscosity is caused by their larger molecular mass and chemical structure [
9]. Transesterification process has been reported to be an effective method of viscosity reduction through the conversion of the waste cooking oil (WCO) to waste cooking oil methyl ester (WCOME) [
10,
11]. However, application of the resulting biodiesel in diesel engine leads to higher oxides of nitrogen (NO
x) emissions [
12]. Experimental investigations by García-martín et al. [
13] and Abu-Jrai et al. [
14] reported notable increase in NO
x emission with increasing quantities of waste cooking oil biodiesel in fuel blends with mineral diesel. Experimental measurements reported by Qasim et al. [
15] on diesel engine operation with waste canola oil methyl esters also reveal higher NO
x emission for biodiesel–diesel blends compared to neat diesel. Similar observations were made by Lin et al. [
16] in an experimental study of diesel engine performance with normal diesel, biodiesel/diesel blends, and neat biodiesel derived from waste cooking oil from restaurants.
In an attempt to address the challenges of increased NO
x emission from biodiesel-fueled compression ignition (CI) engines, recent studies have indicated that addition of certain nanoparticles to biodiesel has the potential to improve engine performance and lower exhaust emissions [
17]. The nanomaterials most commonly considered as engine fuel additives include metal-based elements/compounds such as Al
2O
3, CeO
2, TiO
2, FeCl
3, MnO, ZnO, CuO, Fe
3O
4, Fe, Ce, Bo, and Al [
18], as well as non-metal nano-materials such as graphite oxide (GO) and carbon nanotubes (CNTs) [
19]. Ashok et al. [
20] studied experimentally the effect of ZnO nanoparticles on combustion, performance, and emission characteristics of a twin cylinder CI engine operated with neat biodiesel fuel. They reported improvement in thermal efficiency by 4.7% and a reduction in NO
x emission by 12.6% at full load. Related studies by Nanthagopal et al. [
21] showed improved in-cylinder pressure and heat release rate with addition of ZnO and TiO
2 to biodiesel. A significant reduction in carbon monoxide (CO), unburned hydrocarbon (HC), and oxides of nitrogen (NO
x) emission were also reported.
Experimental investigations by Muthusamy et al. [
22] on the effect of Al
2O
3 nanoparticles blended pongamia methyl ester on diesel engine performance showed marginal increase in brake thermal efficiency and significant reduction in CO, HC, and smoke emissions while NO
x emission increased. Higher NO
x emission with addition of Al
2O
3 nanoparticles was attributed to the combined effect of the oxygen content in biodiesel and the catalytic effect of nanoparticles. The enhanced combustion process generated elevated cylinder peak temperatures, hence oxidizing more nitrogen into nitric oxide. Improved diesel engine performance with addition of Al
2O
3 nanoparticles to Jatropha biodiesel [
23] and Jojoba biodiesel [
24] have also been reported.
A comprehensive experimental investigation was conducted by Selvan et al. [
25] using Cerium Oxide (CeO
2) nanoparticles and carbon nanotubes as additives in Diesterol (diesel–biodiesel–ethanol) blends and significant improvement in engine performance was observed. The thermal efficiency increased by up to 7.5%, while unburned hydrocarbon and smoke emission was reduced by 7.2% and 47.6%, respectively, relative to fuel blend without nanoparticles. This was attributed to cerium oxide nanoparticles acting as an oxygen donating catalyst which provides oxygen for the oxidation of carbon monoxide while absorbing oxygen, causing the reduction of oxides of nitrogen. It has been reported that cerium oxide also aids in burning off carbon deposits within the engine cylinder, hence reducing unburned hydrocarbon (UHC) and smoke emissions. Khalife et al. [
26] studied diesel engine performance with emulsion fuel containing aqueous nano-CeO
2 additive in diesel–biodiesel blends and recorded improved combustion quality, where the brake specific fuel consumption (BSFC) was reduced by up to 16%, the brake thermal efficiency (BTE) improved by up to 23%, while CO, HC, and NO
x emissions were reduced by 51%, 45%, and 27%, respectively.
Studies involving investigation on non-metal based nano-materials such as graphite oxide nanoparticles as fuel additives have been conducted with different fuels, including diesel [
27,
28] and biodiesel [
29]. Carbon nanotubes have also been tested with diesohol (diesel + ethanol) [
30], biodiesel [
31,
32], and water-diesel emulsion fuel [
33]. Most of the studies have reported improved engine combustion, performance, and emission characteristics owing to the enhanced combustion process associated with the catalytic effect of the nano-additives.
More recent studies have focused on the use of a combination of different nanoparticles like cerium oxide (ceria)-zirconium dioxide nanoparticle (CeO
2-ZrO
2) [
34], carbon nanotubes-ceria (CNT-ceria), and samarium-doped ceria (SDC) [
35] as potential fuel additives for improved engine performance. Among oxides, ceria is considered one of the best hydrocarbon oxidation catalysts owing to the relative ease with which Ce can go from Ce
4+ to Ce
3+ [
35]. Mirzajanzadeh et al. [
36] reported a significant improvement in performance of a direct injection (DI) diesel engine fueled with diesel–biodiesel blends with addition of a hybrid nano-catalyst containing cerium oxide on amide-functionalized multiwall carbon nanotubes (MWCNT–CeO
2 catalyst). A significant overall improvement in engine performance was recorded for a fuel blend containing 20% biodiesel and 90 ppm of the catalyst. Engine torque and power improved by 4.91% and 7.89%, respectively, while NO
x, CO, HC, and soot was reduced by up to 18.9%, 38.8%, 71.4%, and 26.3%, respectively.
From the foregoing review, nano-additives have the potential for remarkable improvement of diesel engine performance with biodiesel and diesel–biodiesel fuel blends. It is also clearly seen that application of nanoparticles as additives in liquid fuel is an interesting concept which is yet to be fully explored. Therefore, the aim of the present study is to investigate the effect of iron-doped cerium oxide (Fe–CeO
2) on performance and emission characteristics of a CI engine fueled with biodiesel–diesel fuel blend. Several experimental investigations by different researchers have shown that cerium oxide nanoparticles have the potential to significantly improve engine performance and reduce exhaust emissions [
25,
36,
37]. Furthermore, cerium oxide doped with certain elements such as iron has been reported to display higher catalytic activity compared to pure cerium oxide, in different applications [
38]. Other studies have also reported a reduction in cerium oxide nanoparticles’ size with increasing iron content, suggesting higher activity due to the larger surface area [
39]. However, there is a scarcity of literature regarding the application of iron-doped cerium oxide in engine performance enhancement. The present study therefore seeks to investigate the performance of iron-doped cerium oxide as engine fuel additive.
From the literature, the optimum dosage of cerium oxide nanoparticles with biodiesel is reported as 90 ppm. Hence, in this research work, 90 ppm of iron-doped cerium oxide was added to biodiesel–diesel blend of 30% waste cooking oil methyl ester (denoted by B30) to investigate the combustion, performance, and emission characteristics of the diesel engine.