## 1 Introduction
[12] performed experimental studies to investigate the influence of waste fish oil biodiesel on combustion and emission characteristics of a single-cylinder variable compression ratio (VCR) Ricardo E6 diesel engine. They concluded that waste fish oil biodiesel leads to higher in-cylinder pressure, shorter heat release rate duration, and lower CO and HC emissions than common diesel fuel. In addition, more stable combustion without large cycle-to-cycle variations could be reached by using the waste fish oil biodiesel blends. [PERSON] et al. [13] compared the effects of different types of blended fuels with diesel, palm, and jatropha biodiesel on the performance and emissions characteristics of a single cylinder DI diesel engine with varying loads. They observed lower biodiesel blends help reduce CO emissions, while higher biodiesel blends increase CO emissions due to a higher viscosity and density than diesel fuel. Although most researchers have reported that a variety of biodiesel can be well used in diesel engines without any modification, most commercial biodiesel is produced by transesterification. In the transesterification process, a large amount of methanol and catalyst must be consumed and washed continuously. This not only increases costs, but also produces water pollution and so on. [PERSON] et al. [14] also indicated that the high production cost is the major barrier to extensively using biodiesel as an alternative fuel to diesel engines. Therefore, directly using biodiesel raw materials such as vegetable oils blended with diesel fuel in the diesel engine is the simplest and most effective way to reduce production costs. [PERSON] et al. [15] investigated the effects of palm oil/diesel blends with antioxidants on the performance, combustion, and emission characteristics of a single-cylinder diesel engine at a constant speed of 1500 rpm for different load conditions. They reported the feasibility of using the palm oil/diesel blends with additives as an alternative fuel in diesel engines. By reviewing a large amount of literature on the direct use of vegetable oil in diesel engines, [PERSON] et al. [16] showed that the direct use of waste vegetable oil (WVO) as fuel is very attractive for reducing fossil fuels utilization and increasing sustainability. Moreover, the operation, when combined with high injection pressure, can improve the atomization of high viscosity fuel and make the mixture more completely combustible to reduce CO, HC, and soot emissions. On the other hand, with the continuous renewal of diesel engine fuel injection technology, high-pressure common rail system has been widely used in diesel engines. The use of high-pressure injection technology can overcome the negative effects (e.g., illiquid and atomizing difficulty) caused by the high viscosity of vegetable oil. Based on how few studies on the direct use of vegetable oil on four-cylinder common-rail diesel engines exist, this study mainly investigates the effects of crude palm oil (CPO)-diesel blended fuels on combustion, engine performance, and emission characteristics in a common rail direct injection (CRDI) diesel engine. Furthermore, the influences of various idling speeds, including 750 rpm, 1500 rpm, and 2250 rpm, on combustion and emission characteristics are also investigated. Here, the idling operational condition is defined as a condition in which the engine runs at a low load and a low-rated speed [17; 18]. This engine idling is necessary for most drivers. While resting in the cab, they keep the engine idling to maintain cab comfort and provide power to loads such as air conditioning, heating, and refrigerators [19]. It is reported that most long-haul trucks need to idle for 6 to 16 h a day [20]. When the engine is idling, the harmful exhaust emissions increase due to the low combustion temperature and pressure in the cylinder. Engine idling pollution has become one of the main factors of urban air pollution [17].