## 1 Introduction  
As clean and renewable energy, biodiesel has been proved to be an important alternative fuel for diesel engines, and has been widely used worldwide. This is because biodiesel has the following advantages: (1) a generally higher oxygen content, as biodiesel fuels obtained by transesterification contain about 11-15% oxygen by weight from the carboxyl group in their structure [1]. The oxygen carried by biodiesel improves the problem of local oxygen deficiency caused by an uneven distribution of small portions of fuel during diesel combustion, which leads to complete combustion to reduce the emissions of carbon monoxide (CO), hydrocarbons (HC), and smoke. In addition, the high oxygen content in biodiesel is beneficial to increasing combustion efficiency and reaction temperature, thereby shortening ignition delay and total combustion duration [2, 3]; (2) the cetane number of most biodiesel is between 45 and 65, which is higher than that of diesel (40-55). The cetane number is an important index for measuring the ignition performance of diesel engines. The higher the cetane number, the better the ignition performance, the shorter the ignition delay period, and the more uniform and stable combustion. In general, burning fuel with a high cetane number beneficially increases power output, reduces exhaust smoke and engine noise, and improves cold start characteristics. In contrast, low cetane fuel leads to difficult ignition and combustion, even misfire, a long ignition delay period, incomplete emissionscombustion, large heat loss, and more harmful exhaust emissions [1; 4]; (3) since biodiesel does not contain sulfur and aromatic compounds [5], adding biodiesel to diesel can greatly reduce the emissions of sulfur and aromatic compounds from diesel engines [6]; (4) superior flash point, better lubricity, and biodegradability [5; 7; 8]; although the flash point has no impact on combustion and emission characteristics, it can ensure the safety of storage and transportation. Moreover, the better lubrication properties can avoid excessive friction between moving parts, and the biodegradable properties provide numerous positive contributions to the ecological environment. Currently, most of the research on biodiesel in a diesel engine is focused on adding different biodiesel with different mixing proportions to diesel and then studying the impact of different blended fuels on the combustion, engine performance, and emissions of a single-cylinder diesel engine under different engine load conditions. [PERSON] [9] investigated the effects of mustard oil biodiesel fuel blends on the combustion, performance, and emissions of a single-cylinder direct injection (DI) diesel engine at different engine loads. Test results showed that the mustard oil biodiesel fuel blends could be used effectively in the DI diesel engine without modifications, and the CO and smoke emissions are significantly reduced with mustard oil biodiesel-diesel fuel blends compared to standard diesel fuel. Moreover, there is no significant difference in in-cylinder pressure between mustard oil biodiesel blends and diesel fuel. [PERSON] et al. [10] evaluated the effects of engine load and soybean biodiesel percentage on the performance of a single-cylinder DI diesel engine through experiments and a theoretical model. They showed that the theoretical results agree with the experimental data. And at a constant biodiesel percentage and compression ratio, the effective power increases continuously, the effective efficiency increases to the specified value, and then begins to decrease with the engine load increase. [PERSON] et al. [11] studied the effects of poppy oil biodiesel-diesel fuel blends on the combustion, performance, and emissions of a single-cylinder, four-stroke, naturally aspirated, direct injection diesel engine with a compression ratio of 18:1 under different engine loads. It was reported that the poppy oil biodiesel fuel could be used in a single diesel engine without any modification. [PERSON] et al.