## 1 Introduction
Ethanol is a low-cost alternative fuel with high oxygen content, which can be produced from sugarcane, corn, beet, molasses, cassava root, barley sugar, starch, cellulose, and other raw materials [18]. Its oxygen content is as high as 35%, and it has been used in ethanol diesel (E-diesel) fuel mixture. Compared with diesel fuel, adding ethanol to diesel fuel can significantly reduce the harmful substances such as CO and so emitted by diesel engine. As a highly oxygenated fuel, ethanol can be mixed with diesel (E-diesel) and used in diesel engines. Compared with fossil diesel, the use of ethanol in diesel can significantly reduce PM emissions and reduce toxic gases in exhaust emissions, such as sulfur oxides (SOx) [19]. At present, most of the research on ethanol in diesel engines is using biodiesel-ethanol-diesel (BED) ternary blended fuel. [PERSON] [20] compared the effects of different biodiesel-methanol-diesel (BMD) and BED blended fuels on the engine performance and emissions of a diesel engine. The results showed that although the addition of methanol and ethanol leads to the increase of brake specific fuel consumption (BSFC), CO, and HC emissions, it significantly reduces the NO emission. [PERSON] et al. [21] studied the engine performance, combustion, and emission characteristics of a single cylinder diesel engine at constant speed 1500 rpm and injection pressure 210 bar. Experiments show that compared with diesel at full load, for the blended fuels, the brake thermal efficiency (BTE) is reduced by about 2%, the BSFC is increased by 3%, the exhaust gas temperature (EGT) is increased by 3%, the CO\({}_{2}\) is increased by 0.86%, the HC is reduced by 12 ppm, and the CO and NOx are reduced by 0.029% and 8%, respectively. [PERSON] et al. [19], [PERSON] et al. [22], and [PERSON] et al. [23] investigated the application characteristics of BED ternary mixed fuel in diesel engines. Most research results show that the cooling effects and oxygen contents of alcohols are two of the most important factors on combustion and emissions from diesel engines. Through reviewing many previous studies, it is found that there are few studies on the direct use of diesel-ethanol blended fuels on diesel engines. At present, most of the raw materials for preparing biodiesel come from the second generation such as vegetable oils and waste cooking oils. As a result, it is more expensive than bioethanol. Because the raw materials for ethanol production are rich, including many kinds of peel chips and waste biomass, direct use of bioethanol in diesel engines can not only replace some expensive biodiesel, but also play an important role in energy conservation and emission reduction. In addition, it is of great significance to fill the gap in the direct application of bioethanol in diesel engines by focusing on the effects of different concentrations of bioethanol on the combustion and emission characteristics of diesel engines. In general, because bioethanol has a high octane number, it is often used as an antiknock additive in gasoline engines [24]. Since there are few studies on direct use of bioethanol in diesel engines at present, the main motivation of this study is to investigate the application characteristics of direct use of bioethanol in diesel engines and compare the effects of different mixing ratios on combustion, engine performance, and emission characteristics. Therefore, in this paper, combustion, engine performance, and emission characteristics of a four-cylinder common rail direct injection (CRDI) diesel engine running on diesel-ethanol (DE) blended fuels were investigated at a constant engine speed of 1500 rpm with various engine loads. Based on the fact that the combustion and emission characteristics of the diesel engine largely depend on the engine operating conditions and ethanol mixture ratio, this may have a positive and negative impact on the whole. Combining these two influencing factors, this paper focuses on evaluating the potential application of ethanol as an alternative fuel in diesel engines in the future.