1. Introduction
As car ownership in China continues to rise rapidly, automobile exhaust emissions of harmful substances like CO, NO
x, and PM have become the primary source of urban air pollution, posing significant threats to human health. As we strive to enhance the quality of life and boost prosperity, one of the most significant challenges is meeting the ever-increasing demand for energy. Despite ongoing efforts to transition to alternative energy sources, it is apparent that fossil fuel-based internal combustion engines (ICEs) will remain the primary power source for energy and transportation in the foreseeable future. [
1,
2,
3]. However, as concerns about global environmental deterioration grow, the exhaust emissions from ICEs, such as carbon monoxide (CO), oxides of nitrogen (NO
x), carbon dioxide (CO
2), and hydrocarbons (HC) are increasingly subject to stringent regulations [
4,
5,
6]. In order to promote the use of alternative fuels and address environmental concerns, major advanced and emerging economies have carried out a series of regulations [
7,
8,
9]. As a green and environmentally friendly oxygenated fuel, biodiesel can effectively reduce the CO, NO
x, and PM emissions of diesel engines. In addition, it can curb the increasing trend of automobile emission pollution as soon as possible, and significantly improve the air quality of cities in China. Because of this, biodiesel has attracted more and more attention from scholars.
Biofuel is a newly developed fuel derived from biological materials that can exist in solid, liquid, and gaseous forms. As a promising renewable energy source, biodiesel offers excellent environmental protection characteristics such as good cold start and lubrication performance, reduced greenhouse gas emissions, and potential opportunities for sustainable economic growth [
10]. On the other hand, it is essential to consider the physicochemical properties of biodiesel, given its unique composition and properties. The higher kinematic viscosity of biodiesel improves the combustion quality of fuel [
11,
12], and the higher flash point makes the storage, treatment and transportation safer [13-15]. The cetane number (CN) influences the ignition quality of the fuel and is a measure of the ignition timing in the combustion chamber [
16]. Overall, biodiesel has higher CN than pure diesel. Higher CN usually indicates a shorter ignition delay and earlier combustion, which is conducive to the smooth operation of the engine [
17,
18]. The biodegradability and low toxicity of plant biofuels have rendered them a desirable and practical substitute for diesel fuel. [
19,
20].
Based on the above, biodiesel can play a significant role in reducing environmental pollution, leading countries worldwide to explore oils with suitable properties for producing biodiesel. Among the many options, the Tung tree is a valuable woody oil tree species in China [
21]. Known for its high seed oil content, it is considered one of the four major woody oil tree species. Tung oil, also known as "China wood oil" [
22], is extracted by pressing the seeds of the tung tree [
23]. China tung tree species not only have a variety, but also high yield, and good oil quality, the oil content of absolute dried tung seed is more than 50%, and the oil content of absolute dried tung kernel can be as high as 68%. In the context of the country's vigorous biomass energy development, biodiesel preparation from tung oil has great practical significance in today's increasing energy shortage. The high value and wide use of tung oil have attracted the attention of many countries. Its special chemical structure and active chemical properties have aroused the interest of many chemists who are committed to the study of tung oil chemistry.
The performance and emission characteristics of an engine fueled with biodiesel have been the subject of extensive research by scholars and experts alike. Researchers have investigated the combustion performance of various types of biodiesel fuels, seeking to identify potential benefits and limitations. Ahmad Muhsin Ithnin et al. [
24] studied the combustion performance and emission analysis of diesel engines fueled with low-grade diesel emulsified fuel. And they found that the W / D formed from low-grade diesel is a potential alternative fuel, which could result in greener exhaust emissions and reduced fuel use without worsening its performance. Osmano Souza Valente et al. [
25] investigated the fuel consumption and emissions of diesel generators fueled with soybean biodiesel and castor oil. The results showed that the specific fuel consumption increased with the increase of biodiesel content in the fuel. Compared with diesel, biodiesel blends showed higher carbon dioxide emissions at low loads and lower carbon dioxide emissions at high loads. HC emissions were usually higher. The research conducted by Özer Can [
26] examines the exhaust emissions, combustion characteristics, and performance of a diesel engine that utilizes blends of biodiesel derived from waste oil. The results indicated that with the increase of biodiesel, NO
x emissions increased by 8.7%, while smoke emissions decreased, and CO
2 emissions increased slightly.
The feasibility of biodiesel production from tung oil was studied by Ji-Yeon Park et al. [
27]. When methanol and tung oil were mixed at the optimum molar ratio, the acid value decreased. Despite the fact that eleostearic acid, the primary constituent of tung oil, resulted in low oxidation stability as determined by the Rancimat method, the cold filter plugging point (CFPP) was satisfactory. Qiong Shang et al. [
28] studied the chemical properties of tung oil biodiesel and its mixture with 0
# diesel were studied. The effect of transesterification temperature on the performance of tung oil-based biodiesel was studied. Biodiesel was produced by the transesterification of benzene oil with methanol. It was observed that the tung oil-based biodiesel exhibited a low CFPP of -19°C and a higher kinematic viscosity (KV) of 7.070 mm
2/s as per the property analysis. Moreover, an increase in acid value (AV), KV, and CFPP was noted with increasing storage time. Nevertheless, the stability of the tung oil-based biodiesel could be improved by blending it with 0
# diesel, and a storage time of one month did not affect the ability of B20 or lower blends to meet the ASTM D7467 specification. Additionally, these blends were found to be more stable compared to pure tung oil biodiesel.
There are many studies on the combustion and emissions performance of different types of biological diesel, as well as a certain study of the feasibility and chemical properties of tung oil-based biological diesel. However, there is a lack of research on the emission characteristics of tung oil-based biodiesel. Based on this, the performance and emission characteristics of an engine fueled with tung oil-based biodiesel were studied in this paper. In the present experiment, an analysis of those was investigated using three different portions of tung oil-based biodiesel (10%, 20% and 50%) blended with 0# diesel on a test engine. The test of speed characteristic, load characteristic, exhaust smoke and exhaust gas pollutant emission were conducted on the test ZS1115 GM diesel engine. The results were studied and analyzed, and the potential for future use of tung oil-based biodiesel was explored. It provides a data support for promoting and applying biodiesel, environmental protection, and emission reduction.
4. Conclusion
Biodiesel was prepared from tung oil by conventional transesterification. The major physical and chemical properties of biodiesels and their combined blends were tested by ASTM standard. The performance and emission characteristics of 10%, 20% and 50% tung oil-based biodiesel blends were studied on a single-cylinder direct injection diesel engine. Based on the above research, the following conclusions can be drawn on it:
1) In terms of economy and power performance,compared with the use of 0# diesel, the maximum power at full load is 16kW for B10, which is 1.9% higher than 0# diesel. In terms of torque, the B10 increased by almost 6.6%. But the B20 and B50 decreased by 2.4% and 1.2%, respectively. The fuel consumption rate of B50 increases the most, and it increases by nearly 5.3%. This is mostly due to the calorific value of biodiesel, cetane number and other parameters that are quite different from diesel.
2) From the analysis of exhaust emission of the blends, it has been found that the NOx emissions blends decrease as increasing the proportion of tung oil-based except B50. B10 has the most obvious effect on reducing CO emissions at different rated loads. B20 shows the best performance with the most significant reduction in HC emissions. Biodiesel fuel reduces the exhaust emission such as CO, HC, and NOx, the CO-specific emissions of B10 decreased by 42.86% at 75% load compared to 0# diesel, while that of B50 increased by 60% at 25% load. Compared to 0# diesel, NOX-specific emissions of tung oil-based biodiesel blends were reduced at all load conditions, except for B50. In addition, HC-specific emissions of tung oil-based biodiesel blends were all reduced, especially for B20 decreased by 22.15% at 10% load.
3) When burning biodiesel, the exhaust smoke of biodiesel is significantly reduced. B50 is reduced the most, reducing by nearly 41%. This is because biodiesel contains fewer aromatic hydrocarbons, and biodiesel is an oxygen-containing fuel during fuel combustion.
The results show that if tung oil-based biodiesel is burned on the engine, the fuel injection system should to be optimized to improve SFC and exhaust emissions. Further research could focus on enhancing the stability of the fuel, reducing NOx emissions and exploring uncontrolled emissions such as smoke and particulate matter.