# Study on the Common Rail Type Injector Nozzle Design Based on the Nozzle Flow Model
## 1 Introduction  
Dimethyl ether (DME) is a type of ether compound that combines one oxygen molecule and two methyl radicals. It is obtained by dehydration of methanol at low temperatures. DME is characterized by a relatively high cetane number and can be used as an alternative fuel for compression ignition engines. In addition, DME produces very little particulate matter because it has a high oxygen content and the absence of C-C (carbon to carbon) bonds in the molecular structure [1; 2]. It started receiving attention as an alternative fuel for diesel in the 1990s as it has the potential to utilize the existing LPG (liquefied petroleum gas) transport infrastructure owing to similar properties [3; 4; 5; 6].  
A number of studies have been conducted to replace diesel fuel with DME; they have aimed to optimize engine performance and offer applications in vehicles. In previous investigations [7; 8; 9], it has been demonstrated that NOx (nitrogen oxide) and PM (particulate matter) emissions and combustion noise from compression ignition engines were reduced by adopting DME compared to diesel. [PERSON] et al. [7] investigated the effects of injection pressure and injection rate on DME and diesel engines performance and emission characteristics. Exhaust gas characteristics were improved by varying the pilot injection period and needle lift. In addition, more research is being done on improving and controlling the fuel injection system to overcome the problem of differences in fuel properties and low heating value of the fuel itself. The viscosity of DME is lower than that of diesel fuel, causing leakage from the fuel supply system. Its lower lubricity characteristics can cause intensified surface wear of moving parts within the fuel injection system. Therefore, the maximum injection pressure of DME was found to be limited and lower than that of diesel [5].  
When DME is applied to conventional diesel common rail injectors, the speed of the needle lift is slower because DME has a relatively higher compressibility. Due to the large loss of pressure inthe nozzle seat, the speed at the exit of the nozzle hole is reduced. There is also a high possibility of pulsation between the fuel pump and the injector [6]. In order to compensate for the amount of heat generated, nozzle hole diameters are basically larger with the conventional diesel common rail injectors. DME (\(Q_{LHV}\)) was found to have a lower heating value of 27.6 (MJ/kg), 65% of that of the diesel; a larger amount of DME is needed to supply the combustion chamber and obtain the equivalent engine power. In addition, the minimum pass-through flow area of the needle seat should also be considered, since nozzle hole size increase alone does not provide the desired flow rate [10].  
In this study, the nozzle for the DME common rail injector with \(P_{max}=50\) (MPa) was designed as such to achieve the same power output as conventional diesel engines with \(P_{max}=1600\) (MPa) using a zero-dimensional nozzle flow model for the same injection duration. The flow condition of the nozzle exit affects the vaporization of fuel, directly related to the combustion efficiency and the engine power. The peak injection rate, the discharge coefficient, and the effective diameter of the nozzle exit were analyzed for injection pressure and nozzle geometry for diesel and DME. The peak injection rate based on the Bosch tube method was measured to validate the nozzle flow model. The new injector nozzle with \(\varnothing=0.245\) (mm), \(h=0.15\) (mm) and \(D_{st}=1.88\) (mm) for DME injection pressure of 50 (MPa) can obtain the same engine power as that of the diesel common rail system.