## 1 Introduction  
Diesel engine is one of the main power sources for the transportation industry, but driving vehicles consume a lot of petroleum resources. Transportation has basically become the largest fuel consuming sector in many countries [1]. It is predicted that from 2010, the global transportation energy consumption will increase year by year at a growth rate of 2.2%, and by 2040, the total consumption will reach 139.5 quadrillion Btu [2], which will continue to exacerbate oil tensions. In addition to the large amount of oil consumption, the pollution problem caused by internal combustion engine (ICE) operation is also becoming very serious, due to emissions of carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), and particulate matter (PM) [3]. Researchers are striving to develop new energy vehicle technology, and at the same time emission standards are being tightened to further limit engine pollution. The latest Euro7/VII standards are on the agenda. In order to solve the increasingly serious oil shortage and environmental problems, the field of ICE technology is constantly evolving. New injection strategies, fuel injection systems, and combustion systems are constantly being developed and used in advanced diesel engines [4]. For example, [PERSON] et al. combined an advanced Euro VI combustion system and a common rail fuel injection system to successfully develop a dedicated 0.5 dm\({}^{3}\) single-cylinder engine and found that it has obvious advantages as concerns both smoke and noise emission control [5]. As technology continues to innovate, the search for suitable alternative energy sources has never stopped. For diesel engines, biofuel is one of the most interesting alternative fuels [6], among which biodiesel has attracted the most extensive attention of researchers [7; 8]. Biodiesel is usually extracted from various vegetable oils, animal fat, waste cooking oil, and algae by transesterification [9]. Its characteristics are most similar to diesel, and it is the most popular alternative fuel for diesel engines. Although the combustion quality of the engine is improved due to the oxygen contained in biodiesel,the high pollution in the production process deserves further reflection [10]. According to estimates, when the diesel engine works at full load, the specific environmental pollution cost and the total environmental pollution cost of biodiesel are higher than those of diesel [11]. Therefore, some scholars have tried to use vegetable oils (i.e., palm oil, soybean oil, rapeseed oil, and sunflower oil) directly as an alternative fuel for diesel [12]. Among them, palm oil is the most important petroleum product, accounting for more than one-third of the world's total output. Palm is a common oil crop with high oil yield, and its oil has good performance in productivity, marketability, and land use [13]. Palm oil is made by pressing palm fruits, which contain a large amount of gelatin and free fatty acids, and is the main raw material of palm oil biodiesel [14]. Similar to other vegetable oils, the substitution of palm oil after simple treatment can reduce the consumption of fossil energy and help improve the ratio of carbon neutrality [15]. Since there is no further processing cost, the cost of palm oil is usually low. However, palm oil contains a large amount of colloid and free fatty acids, which increase the viscosity of palm oil and affect the fluidity and spray state. Therefore, the pure palm oil cannot be directly used in engines [14]. In order to solve the main problem of high viscosity of vegetable oils, researchers usually adopt four methods: direct dilution, microemulsion, pyrolysis, and transesterification [16; 17]. Crude vegetable oil can be treated simply to meet the requirements of engine operation. [PERSON] et al. [18] mixed rapeseed oil and diesel in the proportions of 0%, 20%, and 50%, respectively, and heated the mixture to 100\({}^{\circ}\)C in the optimal conditions.