Lean operation is an attractive operational condition; it is known as one of the methods to increase thermal efficiency, and to decrease exhaust emissions and fuel consumption. However, as the mixture leans, cyclic variations increase. Cyclic variations are usually attributed to the result of random fluctuations, excess air ratio and flow field due to the turbulent nature of the flow in the cylinder that limits the range of operating conditions of the spark ignition engine. Gaseous fuels as clean, economical and abundant fuels can improve the lean operating limits and decrease the cyclic variations. Therefore, the purpose of this research is to investigate the use of liquefied petroleum gas (LPG) as a fuel for spark ignition engine in terms of lean operation, and focuses on the cyclic variations and exhaust emissions. The results of this study showed that use of LPG decreased the coefficient of variation in the indicated mean effective pressure, and emission. (c) 2005 Elsevier Ltd. All rights reserved.