## 1 Introduction  
Energy crisis and environmental pollution have always been important issues of widespread concern around the world. With the rapid development of industry, the automobile ownership has remained high. Traditional internal combustion engines (ICE) mainly use fossil fuels as raw materials. According to incomplete statistics, the US recorded oil consumption of 19.4 Mbd in 2019. As Asia's biggest oil consumer, China's oil consumption grew 5.1% year-on-year from 13.37 Mbd to 14.05 Mbd. The U.S. alone accounts for approximately 19.76% of the world's total consumption. The large consumption of fossil fuels has caused environmental changes such as greenhouse effect and abnormal climate. Serious air pollution has had a serious impact on human health. It is reported that the premature death of 5 million people every year are mainly related to particulate pollution in the air [1]. Therefore, developing green and pollution-free clean fuels to replace traditional fossil fuels is of great significance to reduce fossil fuel dependence and environmental pollution. On the other hand, diesel engines with low fuel consumption, high efficiency characteristics, good economic property, low CO\({}_{2}\) emissions, and so on are widely used in medium/heavy vehicles, ships propulsion device, and the electric power facility [2, 3]. However, diesel engines emit more nitrogen oxides (NOx) and particulate matter (PM) than gasoline engine, which has posed severe environment problems and thus restrict its development [4, 5]. At present, diesel engines mainly reduce NOx and PM emissions through exhaust after-treatment devices such as Selective Catalytic Reduction (SCR) and Diesel Particulate Filters (DPFs), so as to meet increasingly strict emission specifications. Although these post-treatment devices have high emission reduction efficiency, they have many disadvantages, such as high price, regular maintenance, ammonia (NH3) slip, incomplete conversion of NOx, deposit formation, DPF regeneration, etc. [6, 7]. Using biodiesel, ethanol, and other biofuels to completely or partially replace diesel can not only reduce the emissions of harmful gases from diesel engines, but also be of great help to achieve global carbon neutrality in the future [8; 9]. Biodiesel, as an important alternative fuel for diesel engines, can be obtained from a variety of vegetable oils, waste cooking oil, animal fats, algae, and other raw materials through transesterification [10; 11]. It can be directly or indirectly (blended with diesel fuel) used in diesel engines without any modification. Compared with conventional diesel fuel, biodiesel is considered to be a reliable alternative fuel because it has higher lubricity, higher flash point, and better biodegradability. In addition, the oxygen-containing characteristics and high cetane number of biodiesel can not only improve the combustion characteristics of diesel engines, but also reduce the regulated emissions such as carbon monoxide (CO), unburned hydrocarbon (HC) and PM, as well as unregulated emissions such as volatile organic compounds (VOCs), and polycyclic aromatic hydrocarbons (PAHs) [12; 13; 14]. However, in addition to the advantages of biodiesel, biodiesel's high viscosity, low volatility, and poor cold flow performance at low temperatures will affect its combustion quality. High viscosity and low volatility will adversely affect fuel atomization, while poor cold flow performance will lead to fatty acid compounds solidification and crystal formation at low temperatures [15]. Other researchers also pointed out that adding biodiesel to diesel engines leads to the increase of NOx, and long-term use of biodiesel will also lead to diesel engine nozzle blockage, rubber ring corrosion, and other problems [10; 16]. As another common alternative fuel for ICE, ethanol has higher oxygen content than biodiesel. Moreover, the higher evaporation latent heat characteristics of ethanol can reduce the combustion temperature in the combustion chamber, which is beneficial to reduce NOx [17].