1. Introduction
The industrial growth and modernization of the world have progressively caused the depletion of fossil resources and serious environmental problems, which has led to a global interest in the search for alternative fuels that can be used in diesel engines. These new fuels must be characterized by coming from renewable resources, minimizing environmental impact, and being economically viable [
1,
2]. Biodiesel appears to be a promising solution, as it allows the continued use of existing technology of internal combustion engines, either purely or combined with diesel [
3]. Other advantages of using biodiesel are its lower sulfur and aromatic content, and higher flash point compared to diesel. These characteristics make biodiesel safer to handle, and biodegradable [
4]. In addition, its chemical composition contributes to a reduction in carbon monoxide, carbon dioxide, hydrocarbons, and soot emissions [
5].
Among the different products for the production of biodiesel, palm oil stands out due to its properties similar to diesel from regular petroleum [
6,
7]. These characteristics have caused the biodiesel derived from palm oil to be been widely investigated. However, the use of edible vegetable oils is related to environmental problems, such as deforestation, soil destruction, and consumption of much of the arable land [
8]. In addition, biofuels are criticized for the use of feedstocks that could also be used as food resources. Therefore, not all of them are economically profitable, as they can be considered to be food crops [
9]. Because of this, the selection of edible vegetable oils as a potential biodiesel feedstock cannot be considered a long-term option.
A researched alternative is the production of biodiesel through the use of residual palm oil, which can eliminate the problems associated with the use of edible oils and reduce pollution from this type of waste [
10]. In particular, the palm oil industry produces a large amount of waste in the liquid or solid form [
11]. It is reported that only 10% of the biomass produced in palm oil farms is converted for edible use; the remaining 90% is polluting waste material [
12,
13]. Among the by-products of palm, oil residues are residual palm oil, fatty acid residues, residual oil from the empty futon cluster, residual oil from the palm decanter cake, and industrial liquid waste [
14]. The use of palm oil residues, in addition to positively impacting the environment, have certain economic advantages. Studies show that the cost of production of palm oil residues is 20% to 30% lower when compared to refined vegetable oil [
15], the raw material is abundant and at a reduced price [
16].
Additionally, a large amount of waste from used cooking oil (UCO) is progressively generated by homes, restaurants, and food processing industries. This type of waste has been investigated for the production of biodiesel [
17,
18,
19]. However, the higher content of fatty acids, the percentage of water, and the lower calorific value compared to standard diesel have caused a reduction in the viability of using UCO as a raw material for the production of biodiesel [
14]. Due to the above, alternative forms are required for the use of UCO as a raw material in the generation of biodiesel, which would contribute to the reduction of pollution problems associated with the UCO.
Among the different types of residual cooking oils, sunflower oil has a relatively high energy density, which makes it a promising material for biofuel production [
20,
21]. Saifuddin and Boyce [
22] concluded that the properties of biodiesel produced from sunflower oil are within the ASTM (American Society for Testing and Materials) international standards. However, high viscosity remains a problem for its massive implementation. The production of biodiesel from blends of different types of raw material is one of the methods investigated to produce biodiesel with properties closer to conventional diesel.
Elkelawy et al. [
23] investigated several blends of biodiesel produced from sunflower oil and soybean oil. The results indicated a decrease in CO and HC emissions and an increase in fuel consumption. Gupta et al. [
24] investigated the optimization process for the production of biodiesel from blends of edible and inedible vegetable oils. The results show that the biodiesel obtained has good combustion properties that meet the ASTM standards. De Almeida et al. [
25] studied the production of biodiesel from blends of residues of fish oil, palm oil, and frying oil. It was concluded that fuel properties, such as viscosity and oxidation stability, show improvements. The research developed by Costa et al. [
26] showed that olive oil could be implemented to improve the properties of biodiesel produced from waste oils from the fishing industry.
The previous studies show the viability of the blends of different raw materials for the production of biodiesel, showing in some cases an improvement in the physical and chemical properties compared to the single use of a material. However, a large part of the biodiesel studies consisting of blends of different oils, do not show results from the analysis of this type of biodiesel in the combustion parameters and its relationship with polluting emissions.
The objective of this work is to evaluate the combustion, performance and emission characteristics of a diesel–biodiesel blend produced from two major polluting sources: residual sunflower oil and palm oil from agroindustry liquid waste because this two residues offer the highest production per hectare of crop and have characteristics relatively close to conventional diesel compared to other sources of raw material. Therefore, a combustion diagnostic model based on the combustion chamber pressure has been developed in order to study the effect of the biofuel produced in the combustion processes of a single-cylinder diesel engine of low displacement, which is widely used in areas not interconnected for energy production. In addition, the effect of the combustion process on yield and CO, CO2, NOx, and smoke opacity emissions are studied.
6. Conclusions
In the present study, an analysis of the combustion process, performance, and emissions of a single-cylinder engine was carried out, using biodiesel blends formed by the blend of palm oil and sunflower oil residues.
The results of the diagnostic model show that the pressure curves in the cylinder chamber decrease as the biodiesel content in the fuel increases. For the modes of operation considered, the maximum pressures were found in the range of 16–57.58 bar, 17–60.68 bar, 18.1–65 bar, 19.1–68 bar and 20–72.32 bar for the PB10SB5, PB5SB5, PB10, PB5, and diesel, respectively. This decrease in pressure was attributed to the lower calorific value of biodiesel compared to diesel. Similarly, the results showed a decrease in the rate of heat release for biodiesel blends compared to diesel for all tested modes of operation. It was observed that in the highest engine operating mode, the HRR curves showed a maximum of 1.14 J/deg, 1.18 J/deg, 1.22 J/deg, 1.26 J/deg and 1.30 J/deg for the PB10SB5, PB5SB5, PB10, PB5, and diesel, respectively. The higher viscosity and lower calorific value of biodiesel blends are considered to cause this behavior. However, the addition of a percentage of sunflower oil residues does not cause a large difference in maximum pressures, and heat release rates, when compared to biodiesel produced only with residual palm oil.
The higher viscosity of the biodiesel tested also causes a reduction in the combustion rate, decreasing the heat release of the combustion process. This fact is reflected in the accumulated heat release curves, which were larger for fuels with a higher biodiesel content. This effect is reflected in the thermal efficiency of biodiesel blends. It was observed that the maximum thermal efficiency was 29.4%, 30%, 30.6%, 31.2% and 31.8% for PB10SB5, PB5SB5, PB10, PB5, and diesel, respectively.
The addition of the percentage of residual sunflower oil in biodiesel caused an increase in BSFC. On average, the PB10SB5 and PB5SB5 biodiesel increased the BSFC by 15.5% compared to the PB10 and PB5 biodiesel, respectively.
The emissions analysis showed that PB10SB5 and PB5SB5 fuels show a reduction in CO, CO2, HC, and smoke opacity emissions of approximately 14–23%, 23.3–28.1%, 15.58–19.5% and 7.9–9.4% in comparison with pure diesel, respectively. The above-mentioned results are mainly attributed to the higher oxygen content in this type of biodiesel, contributing to a cleaner and complete combustion.
The temperature values in the combustion chamber showed that the addition of sunflower oil residues causes a rise in the maximum temperature. This temperature increase facilitated the production of NOx emissions. The results show that on average, the PB5SB5 and PB10SB5 increase NOx emissions by 8.3% compared to biodiesel blends of residual palm oil.
In general, biodiesel with the percentage of residual sunflower oil does not cause a significant change in the combustion process and engine performance, when compared to biodiesel that includes only residual palm oil. Despite the increase in NOx emissions, biodiesel blends with the addition of residual sunflower oil allow CO, CO2, HC, and smoke opacity emissions to be reduced. Therefore, biodiesel produced by mixing palm oil and sunflower oil residues could be used to replace up to 15% diesel, allowing the reduction of highly polluting waste and the production of a cleaner and more renewable fuel.