Research paper Enhancing the heavy load performance of a gasoline engine converted for LPG use by modifying the ignition timings Barıs¸ Erkus¸ a, * , M. _ Ihsan Karamangil a , Ali Sürmen b a Uludag University, Faculty of Engineering, Dept. of Automotive Eng., Bursa, Turkey b Bursa Technical University, Faculty of Natural Sciences, Architecture and Engineering, Dept. of Mechanical Eng., Bursa, Turkey highlights  ICE performance is improved with optimised ignition timings without knock with LPG.  Advanced ignition timing improves the performance more for leaner mixtures of LPG.  For leaner mixtures at knock limit more control tool is needed to suppress the knock.  Advancing the ignition timing causes increase in HC and NO x emissions.  The effect of ignition timing on CO emissions is negligible. article info Article history: Received 6 November 2014 Accepted 7 March 2015 Available online 18 April 2015 Keywords: SI engine LPG ECU tuning Ignition timing Excess air coefficient abstract This paper presents the results of the experiments conducted on a spark-ignition (SI) engine fuelled with liquefied petroleum gas (LPG) by varying the ignition timing at the excess air coefficients of 1.0 and 1.3. Experiments were carried out at wide open throttle (WOT) position and at engine speed of 4300 rpm aiming to determine the lean operation performance of an engine when fuelled with LPG at full load. Performance parameters, namely brake power, brake-specific fuel consumption (BSFC), brake thermal efficiency and exhaust emissions such as unburned hydrocarbons (HC), carbon monoxide (CO) and ni- trogen oxides (NO x ), were studied. It was shown that advancing the ignition timing improved the per- formance of LPG-fuelled SI engine for excess air coefficients higher than 0.8. The highest brake power and the lowest BSFC were obtained with modified ignition timing at an excess air coefficient of 1.0. The lowest exhaust emissions were obtained with an excess air coefficient of 1.3. In general, advancing the ignition timings caused increase in HC and NO x emissions, while the effect of ignition timing on CO emissions was negligible. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction With the increasing consciousness of environmental protec- tion and energy conservation throughout the world, the research and development of motor vehicles that use clean alternative fuels has become an important subject [1,2]. Recently, LPG is widely accepted as an alternative fuel for vehicles [3] since LPG is a clean fuel and it has higher octane number and auto ignition tempera- ture, greater flame velocity and wider flammability limit than gasoline [4]. The ignition processes strongly affect the overall performance in spark ignition engines [5]. More power and higher temperatures are generated when the spark setting is configured to give maximum pressure and temperature at approximately 5e10  after top dead centre (ATDC) [6]. A gasoline engine converted to an LPG engine has to be run with optimised ignition timing maps to ach- ieve the best performance due to the different physical and chemical properties LPG and gasoline. In particular the dissimilar- ities in flame development and flame propagation periods require modifying the original ignition timing maps of gasoline engines. The maximum flame speed occurs at excess air coefficient close to 0.8 for gasoline-type fuels [6] and 0.9 for LPG [7], and any deviation in equivalence ratio from those values requires optimising the ignition timing to obtain the best performance. Power loss is a problem when using gaseous LPG compared to gasoline as a result * Corresponding author. Tel.: þ90 224 2941996; fax: þ90 224 2941903. E-mail addresses: berkus@uludag.edu.tr (B. Erkus¸ ), ihsan@uludag.edu.tr (M. _ I. Karamangil), ali.surmen@btu.edu.tr (A. Sürmen). Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng http://dx.doi.org/10.1016/j.applthermaleng.2015.03.076 1359-4311/© 2015 Elsevier Ltd. All rights reserved. Applied Thermal Engineering 85 (2015) 188e194