A study of operating parameters on the linear spark ignition engine q Ocktaeck Lim a,⇑ , Nguyen Ba Hung a , Seokyoung Oh a , Gangchul Kim b , Hanho Song c , Norimasa Iida d a School of Mechanical and Automotive Engineering, University of Ulsan, San 29, Mugeo2-dong, Nam-gu, Ulsan 680-749, South Korea b Korea Institute of Energy Research, 102 Gajeong-ro, Yuseong-gu, Daejeon 305-343, South Korea c Department of Mechanical and Aerospace Engineering, Seoul National University, South Korea d Department of System Design Engineering, Keio University, Yokohama, Japan highlights  An experimental and simulation study of a linear engine is conducted.  The effects of operating parameters on the generating power are investigated.  The air gap length has a significant influence on the generating power.  The generating power of the linear engine is optimized with the value of 111.3 W.  There are no problems for the linear engine after 100 h of durable test. article info Article history: Received 6 November 2014 Received in revised form 2 June 2015 Accepted 16 August 2015 Available online 12 September 2015 Keywords: Linear engine Free piston engine Linear alternator Spark timing Equivalence ratio Air gap abstract In this paper, we present our experiment and simulation study of a free piston linear engine based on operating conditions and structure of the linear engine for generating electric power. The free piston lin- ear engine includes a two-stroke free piston engine, linear generators, and compressors. In the experi- mental study, the effects of key parameters such as input caloric value, equivalence ratio, spark timing delay, electrical resistance, and air gap length on the piston dynamics and electric power output are investigated. Propane is used as a fuel in the free piston linear engine, and it is premixed with the air to make a homogeneous charge before go into the cylinder. The air and fuel mass flow rate are varied by a mass flow controller. The experimental results show that the maximum generating power is found with the value of 111 W at the input caloric value of 5.88 kJ/s, spark timing delay of 1.5 ms, equivalence ratio of 1.0, electric resistance of 30 X, and air gap length of 1.0 mm. In order to check the durability of the linear engine, a durable test is conducted during 100 h. The experimental results show that there are no problems for the linear engine after about one hundred hours of the durable test. Beside experimental study, a simulation study is conducted to predict operating behavior of the linear engine. In the simula- tion study, the two-stroke free piston linear engine is modeled and simulated through a combination of three mathematical models including a dynamic model, a linear alternator model and a thermodynamic model. These mathematical models are combined and solved by a program written in Fortran. Besides, the effects of key parameters such as reciprocating mass, spark timing and spring stiffness on the piston dynamics and electric power output of the linear engine are also investigated. The simulation results show that the simulation and experimental data are nearly similar at the same initial conditions. In addi- tion, a highest generating power of the linear engine can be easily found by optimizing the key parameters. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction The crisis of global warming and shortage of fossil fuels are being a motivation for the scientists as well as the institutes to develop new energy conversion devices and environmentally friendly fuels. One of the methods to solve the crises is using a free-piston linear engine (FPLE). A free-piston linear engine is con- sidered to be a crankless internal combustion engine with free motion of piston in cylinder. In terms of structure, the engine http://dx.doi.org/10.1016/j.apenergy.2015.08.035 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved. Abbreviations: FPLE, free piston linear engine; HCCI, homogeneous charge compression ignition; ECU, electronic control unit; DSP, digital signal processing; DAQ, data acquisition. q This article is based on a short proceedings paper in Energy Procedia Volume 161 (2014). It has been substantially modified and extended, and has been subject to the normal peer review and revision process of the journal. This paper is included in the Special Issue of ICAE2014 edited by Prof. J Yan, Prof. DJ Lee, Prof. SK Chou, and Prof. U Desideri. ⇑ Corresponding author. E-mail address: otlim@ulsan.ac.kr (O. Lim). Applied Energy 160 (2015) 746–760 Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy