**1. Introduction**

The intake and exhaust system of a diesel engine is related to the performance of the engine [1]. In a four-stroke engine, the gas exchange process releases combustion gases at the end of the power stroke, bringing more air into the next intake stroke [2]. The volumetric efficiency of an engine appears in a form similar to torque and is affected by the mass flow rate. It may be affected by various variables such as engine speed, air–fuel ratio, compression ratio, intake and exhaust valve geometries, and intake and exhaust pipe length [3]. Diesel engines are designed to maximize engine performance at the commercial speed; if the engine was operating outside of the commercial speed, the volumetric efficiency decreases and a lot of environmental pollutants are discharged. However, research is required to improve the performance and emission of environmental pollutants during operation, and not just at the commercial speed [4–6].

Regulations on the emission of environmental pollutants from marine diesel engines have been strengthened. Greenhouse gases, sulfur oxides, and nitrogen oxides are regulated and more regulation will be tightened in the future [7]. Scrubber, exhaust gas recirculation (EGR), selective catalytic

reduction (SCR), etc., are used as devices to reduce the emission of environmental pollutants [8]. Gas flow analysis of the intake and exhaust system is required for the design of a diesel engine, such as when installing environmental pollution reduction devices and calculating the kinetic energy of exhaust gas acting on a turbine for turbocharger matching [9]. If only the components of the diesel engine are separated and the gas flow is analyzed, it is di fficult to determine the e ffect of the components on a diesel engine [10].

Gas flow analysis of the entire diesel engine system in three-dimensional (3D) format is ine fficient because it requires a high-resolution workstation and an enormous amount of time for the analysis [11–13]. It is indicative the fact that to carry out 3D gas flow analysis of the intake and exhaust gas exchange process of a single cylinder diesel engine without a combustion reaction using Ansys Fluent R15.0, a commercial flow analysis program, takes approximately 61 hours (based on a 16-core central processing unit) [14]. Therefore, the method of characteristics (MOC) approach has been used for one-dimensional (1D) format gas flow analysis with a fast calculation time and a low-resolution workstation [15].

The MOC is a method of calculating a pressure wave using Riemann variables called characteristic curves. It is a method with a fast calculation time and a high accuracy in a straight pipe, nozzle, and orifice. However, there is a disadvantage of low accuracy in complex geometries such as branches and bent pipes [16].

Diesel engines are designed according to the e ffects of reflected waves in the flow areas of intake and exhaust pipes, underscoring the need for numerical analysis based on reflected waves [17]. For 1D flow analysis using the MOC, the calculation includes the influence of reflected waves [18]. In this study, MOC was used to perform unsteady 1D gas flow analysis on a single cylinder diesel engine.

This 1D gas flow analysis was performed under the same boundary conditions as the experiment and the results were compared. The object of comparison was the average mass flow rate of the intake air and the intake and exhaust pipe pressure. The discharge coe fficient was calculated using the average mass flow rate of the intake air and was applied to the 1D gas flow analysis. The results of the intake and exhaust pipe pressure of the gas flow analysis applying the discharge coe fficient were compared with those of the experiment.

Previous studies using 1D gas flow analysis have focused on merits and verification. The purpose of the study is to analyze the errors in the 1D gas flow analysis caused by not realizing the shape as it is. By evaluating the validity of the results, we tried to determine the cause of the low accuracy of the 1D gas flow analysis, and to find a way to solve the problem in diesel engine intake and exhaust system.
