Proceedings of the ASME 2014 International Mechanical Engineering Congress and Exposition IMECE2014 November 14-20, 2014, Montreal, Quebec, Canada IMECE2014-36413 A STUDY OF THE SCAVENGING PROCESS IN A TWO-STROKE FREE PISTON LINEAR ENGINE USING CFD SIMULATION Nguyen Ba Hung Ocktaeck Lim Graduate school of Mechanical and Automotive Department of Mechanical and Automotive Engineering, University of Ulsan Engineering, University of Ulsan Ulsan, South Korea Ulsan, South Korea ABSTRACT [1,2]. The variation of compression ratio in the FPLE also It is widely known that scavenging process is of allows the engine to operate with various kinds of fuels and importance in the performance and thermal efficiency of two- homogeneous charge compression ignition (HCCI) [3,4]. For stroke engines. Unlike the conventional two-stroke engine, a the conventional two-stroke engines, the power output is two-stroke free piston linear engine can change compression dependent on the mass of air and fuel trapped inside the ratio due to free motion of a piston in a cylinder. The free combustion chamber prior to combustion. The trapped contents motion of the piston directly affects the gas exchange process are effected by the gas exchange process as well as scavenging or scavenging process of the free piston engine. In this study, a process. Further, in the combustion chamber of a two-stroke numerical simulation using computational fluid dynamics engine, a large amount of unburned HC emissions and residual (CFD) is conducted to examine the scavenging process taking gas can be produced if the scavenging process is inefficient [5]. place in a two-stroke free piston linear engine. The motion of It can be seen that the scavenging process in two-stroke the free piston is built first based on a dynamic model to define engines plays an important role for improving the power the motion profiles of the piston, and afterwards the piston’s output and the residual exhaust gas. Although two-stroke free motion profiles are imported into a commercial CFD software piston engines have no a crankshaft, the scavenging process in (Ansys Fluent v.14) to simulate the scavenging process. To the free piston engine is similar to conventional two-stroke provide information for this study, some key parameters such engines. For the two-stroke free piston engines, the scavenging as operating frequency of the piston, effective stroke length process is considered to be key to realizing the combustion and inlet pressure are changed to find out their effects on the efficiency and emissions potential of the engine [6]. Due to trapping efficiency. The simulation results show that the difficulties associated with the measurement techniques, CFD trapping efficiency has a maximum value of 83% at optimal (Computational Fluid Dynamics) is a very useful tool to operating frequency f=33Hz. Besides, by increasing effective analyze the scavenging process. Goldsborough and Blarigan stroke length and reducing inlet pressure, the trapping [6] presented a study for optimizing the scavenging system of a efficiency is increased two-stroke free piston engine using KIVA-3V code. Zhu et al. [7] presented a numerical simulation of gas exchange process in a two-stroke free piston engine using 3D CFD method. Yu et INTRODUCTION al. [8] used a commercial CFD software AVL FIRE to simulate the gas flow of a hydraulic free piston engine during the A two-stroke free piston linear engine (2S-FPLE) is scavenging process. considered to be a crankless internal combustion engine with In this paper, the scavenging process is numerically studied free motion of a piston in a cylinder. Unlike conventional using a commercial CFD software Ansys Fluent v.14. This engines with a crankshaft mechanism, the FPLE can optimize study is conducted on a two-stroke free piston linear engine the combustion process through variable compression ratios using loop scavenging type. The dimensional parameters are 1 Copyright © 2014 by ASME chosen based on a real FPLE. In order to simulate the connecting rod to move back and forth. This stage is called the scavenging process in the FPLE, this study is organized as firing mode. The movement of the connecting rod will follows. At first, the motion of the free piston is built based on generate current in the wires due to the changing magnetic a dynamic model to calculate the piston’s motion profiles. flux linked with the wire in the stator. Then, the piston’s motion profiles are used as input variables for CFD model to simulate the scavenging process. The motion SIMULATION SETUP profiles of the piston are adjusted depending on the operating Dynamic model frequency and piston stroke. In this study the key parameters Due to the fact that the piston motion profile of the FPLE is such as operating frequency, effective stroke length and inlet different from conventional engines, a dynamic model is pressure are changed to find out their effects on the trapping established to describe the piston motion. Fig. 2 shows a free efficiency, which relates directly to the output power of the body diagram in motoring mode. The motion of the piston engine. obeys Newton’s second law, and the piston acceleration is determined by: OPERATION OF THE FPLE The FPLE is a combination of two main components including 2 xd mFFFAPAPF (1) a free piston engine and a linear alternator, as shown in fig. 1. srslfrlm dt 2 Reed where, F is a motoring force provided by the linear alternator Valve Spring Coil Permanent m magnet (N), Pl, Pr is the pressure to the left and right of the cylinder Charge 2 (Pa), respectively, A is the area of the piston crown (m ), Ff is Flow SN friction force (N), m is reciprocating mass (kg), x is the 2 2 Intake port displacement of piston (m), d x/dt is the acceleration of piston Free piston (m/s2), while F and , F are respectively, the spring force to engine sl sr the left and right (N). These spring forces are calculated by: sl . xkF l (2) Spark plug sr . xkF r (3) Exhaust port N S where, k is the spring stiffness, Δxl and, Δxr are, respectively, the deformation of the left and right spring. Compressor Linear Alternator Fig. 1: The operating model of FPLE. Fsl Fsr Fm The free piston engine consists of a dual piston connected by a N S rod system. Spark plugs are arranged at each cylinder head to ignite the air/fuel mixture at the end of the compression Ff process. In order to increase intake pressure, the engine is designed with two compressors on each side of the engine, as shown in fig. 1. Each compressor also includes a dual piston, Pl A Pr A which is connected by a connecting rod system. A spring is arranged at each cylinder of the compressor as a damping device. In order to provide an intake mixture to the cylinder of the FPLE, a reed valve is arranged at each cylinder head of the N S compressor. The second component is the linear alternator with a permanent magnet mounted on the connecting rod as the translator. In the stator of the linear alternator, copper wires are wrapped around the back iron made of silicon steel. To start the engine, the linear alternator will operate to 0 x drive the free piston engine through connecting rod system. Fig. 2: Free body diagram of FPLE. This stage is called the motoring mode. After certain frequencies, spark plugs are activated, and the combustion The velocity and displacement of piston are calculated by process will occur alternatively at each cylinder, forcing the equations: 2 Copyright © 2014 by ASME dx dx 2 xd t 2 (4) dt dt 0 dt 2 xd t 2 Le dx dt 2 xx t (5) 0 dt 2 where x0 is the initial position of piston, which is the middle position (x0=0) of maximum stroke xm, as shown in fig. 3. 0 x -x 0 +x -x 0 +x m m m m Fig. 4: Geometry of the FPLE. Fig. 3: The engine model with the piston at the mid position. Parameters Values The dynamic model is calculated continuously by a program Bore (mm) 30 written in Fortran. However, the obtained result is only a time Effective stroke length L (mm) 18, 19, 20, 21 notation since the free piston engine does not have a e crankshaft. To get a crank angle notation, it is necessary to Width of exhaust port (mm) 20 convert from time to equivalent crank angle (ECA). Here, the Height of exhaust port (mm) 15 equivalent crank angle is defined by [6]: ECA=(t-t0).f.360, in Width of intake port (mm) 15 which t is time, f is operating frequency of the piston, t0 is Height of intake port (mm) 13 initial time. Reciprocating mass m (kg) 0.8 Compression ratio Variable CFD model Inlet pressure Pin (bar) 1.1, 1.2, 1.3, 1.4, 1.5, 1.6 The motion curves obtained from the dynamic model will be Inlet temperature T (K) 293 provided as an input for computational fluid dynamic (CFD) in model to calculate the trapping efficiency. In two-stroke free Tab. 1: Engine specifications and operating conditions piston engines, the trapping efficiency is defined as follows [6]: The geometry of the FPLE is imported into a commercial CFD Trapping efficiency: the ratio of the trapped fresh charge to the software Ansys Fluent v.14 to define the calculating surfaces total delivered fresh charge. and volumes as well as to generate mesh. After generating The trapping efficiency is calculated by a program written in mesh in the Ansys software, a picture of the engine mesh is Microsoft Visual C++ before importing into the Ansys shown in fig.