International Journal of Thermal Technologies E-ISSN 2277 – 4114 ©2016 INPRESSCO ® , All Rights Reserved Available at http://inpressco.com/category/ijtt/ Research Article 350| International Journal of Thermal Technologies, Vol.6, No.4 (Dec 2016) Experimental and simulation study to determine the effect octane number on performance in SI engine Mohammed Kadhim Allawi * and Riyadh Salman Radhi Technical Engineering College- Baghdad, Middle Technical University, Iraq Accepted 20 Nov 2016, Available online 01 Dec 2016, Vol.6, No.4 (Dec 2016) Abstract Experimental and Simulation investigations are carried out to study and compare the effect of using four cylinder, four-stroke spark ignition (SI) engine fuelled by four grades of gasoline used in Iraq, RON75, RON80, RON85 and RON92on the performance of SI engine at different engine speeds and loads". "Simulation combustion model Lotus Engine Simulation (LES) can predict the engine performance when compared with the experimental findings". "(LES) The program was used to study the effect of some parameters in experimental testing, this program gives the best performance of the engine at maximum brake power, and the same input data were fed to the program where they were taken from the data of experimental results". "Results show that the engine performances for both fuels are compatible, with marginal differences, under the tested operating conditions". Higher power and less specific fuel consumption are observed when fuels of octane number 92 are used compared with octane 75 blends. " Keywords: Gasoline, octane number, antiknock additive 1. Introduction 1 Downsizing the engine and reducing fuel consumption and emissions are the major goals in the field of internal combustion engines today (Anderson JE, et al, 2012), (Heywood JB,1988).The internal combustion engine, powering 90% of world vehicles, is the main driver in the transportation sector from which20% of total world energy is consumed. The engine performance, thermal efficiency and pollutant emissions have a significant impact extending to the environment (IEA Key World Energy Statistics, 2013). Researcher studied the effect of octane number of the fuel on the performance of the spark ignition engine. The study included the preparation of fuel with octane number (70, 75, 80, 85, and 90). The results show that the engine performances are increased step by step according to the increases of the octane number of fuel (Dr. Mohammed Hassan Aboud, 2006). also tested the influence of two octane gasoline fuels, which are RON91 and RON95, on engine performance using a low compression ratio engine (8.0:1). They noticed that RON91 gasoline produced 4.2–4.8% higher power and 5.6% lower BSFC than RON95 fuel. The results also showed that lower emissions were detected by using RON91 fuel with 5.7% and 3.4% of CO and HC respectively (Sayin C, et al, 2005). This system allows an increase of the compression ratio, which in turn reduces the specific fuel consumption. The antiknock *Corresponding author: Mohammed Kadhim Allawi quality of a fuel is usually quantified through either the Research Octane Number (RON) or the Motor Octane Number (MON). Higher values of these octane numbers reduced a better antiknock quality of the fuel (Kamil M , et al, 2013), (Abdullah NR, et al, 2014 ).When engine was fueled octane, engine performance parameters such as break thermal efficiency increases with increasing (RON) octane number while bsfc decreased. The results show that the concentration of exhaust emissions decreases with increases (RON) octane number (Mohammed Kadhim Allawi, 2016). The addition of oxygenates to gasoline offers many advantages, among which: more complete combustion and reduction of carbon monoxide emission, being a renewable energy source, increased octane number, and increased volatility (McNair H.M, et al, 2001) .Gasoline with higher octane number has numerous benefits including reduced exhaust emissions and engine noise, improved cold starting and engine durability (Brown S, et al, 1999). Octane requirement to avoid knocking is related to the engine type and engine operating conditions. Research octane number (RON) and motor octane number (MON) are measures of fuel anti-knocking performance. American cars use an octane scale derived from the average of both RON and MON (Saudi Aramco, 2008). Researcher showed that the engine performances of both fuels are comparable, with marginal differences, under the tested operating conditions, practically for engine speeds less than 3500 rpm. Higher power and less specific fuel consumption are observed when octane 91