Effect of exhaust gas recirculation on the cycle-to-cycle variations in a natural gas spark ignition engine Asok K. Sen a, * , Sudhir K. Ash b , Bin Huang c , Zuohua Huang c a Richard G. Lugar Center for Renewable Energy and Department of Mathematical Sciences, Indiana University, 402 N. Blackford Street, Indianapolis, IN 46202, USA b Department of Chemistry, B. B. College, Burdwan University, Asansol, India c State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China article info Article history: Received 13 December 2010 Accepted 15 March 2011 Available online 2 April 2011 Keywords: Natural gas Cycle-to-cycle variations Spark ignition engine Exhaust gas recirculation Wavelet analysis abstract This study investigates the effect of exhaust gas recirculation (EGR) on the cycle-to-cycle variations (CCV) in combustion in a natural gas spark ignition engine. The engine is operated at 2000 rpm and a stoi- chiometric fuel-air mixture is used. The EGR level is changed from 0% to 5%, 10%, 15%, and 20%. For each EGR level, a continuous wavelet transform is used to analyze the time series of the indicated mean effective pressure (IMEP) over 200 cycles. The dominant oscillatory modes of the CCV are identified and the engine cycles over which these modes may persist are delineated. The results reveal that the CCV of the IMEP occur on multiple timescales and exhibit complex dynamics. With no EGR, mainly high frequency intermittent fluctuations are observed. As the EGR level is increased, more persistent low frequency variations tend to develop. In addition, the spectral power increased with an increase in the EGR level. At the EGR level of 20%, the spectral power is found to increase significantly indicating that EGR has a pronounced effect on increasing the CCV. Knowledge of the dominant modes of variability may be useful to develop effective control strategies for reducing the CCV and improving engine performance in the presence of EGR. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Internal combustion engines running on conventional fuels such as gasoline or diesel often emit undesirable amounts of NOx, CO, and unburned hydrocarbons (UHC) into the atmosphere. In order to regulate environmental pollution, higher emission standards are being continually imposed on vehicles that are operated in the United States and many other countries around the world. To meet these emission standards, alternative fuels such as natural gas and biodiesel are currently being used [1e5]. The main constituent of natural gas is methane, which has the lowest carbon to hydrogen (C/H) ratio compared to other hydrocarbon fuels, making natural gas one of the cleanest fuels available today. Because of low C/H ratio of methane, a natural gas engine produces less CO and UHC emissions than a gasoline or a diesel engine [6e9]. In order to achieve low NOx emissions, a natural gas engine needs to be operated with ultra lean mixtures. However, the lean burn strategy reduces the thermal efficiency of the engine, and also tends to increase CO and UHC emissions. Therefore, depending on the air-fuel ratio in the combustible mixture, there is a trade-off between thermal efficiency and exhaust emissions. An alternate way of reducing NOx emissions is to retard spark timing, but this has also been found to decrease thermal efficiency and increase UHC emissions. In the 1970s, the three-way catalyst (TWC) tech- nology was developed to reduce exhaust emissions. In fact, the TWC technology can reduce NOx, CO and UHC emissions, but it works most efficiently with a stoichiometric mixture. When the engine operates with a near-stoichiometric mixture, the thermal efficiency is improved, but the in-cylinder temperature increases; consequently, the thermal stresses and knocking tendency increase [10]. NOx is formed in high concentrations when the in-cylinder temperature exceeds about 2500  F. The in-cylinder temperature can be reduced by recycling part of the exhaust gases into the engine cylinders. This process is referred to as exhaust gas recir- culation (EGR). There are two ways in which EGR can be imple- mented. External EGR is achieved by connecting a pipe from the exhaust to the intake manifold, with the flow of exhaust gases regulated by a control valve. Internal EGR is achieved when the valve timing is arranged so that there is some backflow into the combustion chamber from the exhaust, or all of the exhaust gases are not expelled from the combustion chamber during the exhaust * Corresponding author. Tel.: þ1 317 274 6922; fax: þ1 317 274 3460. E-mail address: asen@iupui.edu (A.K. Sen). Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.03.018 Applied Thermal Engineering 31 (2011) 2247e2253