# Combustion and Emission Characteristics of a Diesel Engine with a Variable Injection Rate
## 1 Introduction  
Diesel engines, as the core components of power machines, have been widely applied in various fields. Worldwide, the need for a high-efficiency, low-emission diesel engine has increased [1]. The increasing depletion of petroleum resources and the worsening of environmental pollution have increased the significance of energy conservation and environmental protection in the development of modern high-power marine diesel engines [2]. New diesel engine technology must be developed to fulfill future energy needs and tightening regulations concerning emissions, such as new combustion strategies, ultrahigh-pressure injection, posttreatment systems, and residual heat recovery systems. However, posttreatment and residual heat recovery systems are structurally complex and very expensive to construct; thus, improvements in fuel-air mixing and combustion in cylinders have been adopted as effective strategies for controlling exhaust emissions [3, 4]. The injection pressure significantly affects the combustion of a diesel engine. Increased pressure can help to improve the quality of atomization while effectively decreasing nitrous oxide (NO\({}_{\lambda}\)) and particulate matter (PM) emissions [5]. Moreover, injection rate control is regarded as the most effective technology for reducing NO\({}_{\lambda}\) emissions, PM emissions, and combustion noise [6]. Therefore, efforts have been made to achieve an ideal injection rate as the requirements for injection pressure continuously increase. Nevertheless, the current common high-pressure rail system suffers from various technical defects. The components of this type of system always require high pressures, which leads to major mechanical stress and fuel leakage. The shape of the injected fuelis approximately rectangular and difficult to adjust [7; 8]. To obtain an ideal injection rate, the Denso company designed a throttling orifice and specific boot-shaped valves for the injector of the ECD-U2 high-pressure common rail system. In this manner, the injection rate can be adjusted by changing the movement speed in a needle valve while maintaining the same injection pressure [9]. In addition, the Bosch company flexibly adjusted the injection rate by changing the injection pressure in each injection process in the CRSN4 high-pressure common rail system [10]. Tsinghua University proposed regulations of the pilot injection rate for plunger discharge and solenoid valve control in an electrically controlled pump-valve-pipe-nozzle system and carried out research on the injection rate characteristics [11]. [PERSON] et al. [12] devised a double-staggered nozzle; then, the scholars calculated and evaluated the combustion and emission characteristics of the diesel engine on which the nozzle was installed. The researchers found that a double-staggered nozzle could achieve a greater flow area than a common nozzle, thereby increasing the injection rates [12]. In general, the injection rate in a high-pressure common rail system can be adjusted in different ways, e.g., altering the pressure difference on both sides of an orifice in the structural design, changing the injection pressure during the injection process, or using different orifice flow areas. If a high-pressure common rail system is fixed, the injection rate can be adjusted in a limited manner through structural changes. However, managing injection pressure control can serve as a potentially enhanced strategy for significantly controlling the injection rate. In recent years, the effects of injection pressure and rate on the performance of diesel engines have been studied worldwide. [PERSON] et al. [13] explored the influence of injection pressure on the combustion performance of a diesel engine. Their results revealed that an increase in injection pressure leads to an increase in cylinder pressure peak and a decrease in ignition delay time, but the effect diminishes after the injection pressure exceeds 140 MPa. [PERSON] et al.