Diesel engines, especially for trucks and buses, cause many economical and ecological problems. Diesel exhaust emissions are a major source of pollution in most urban centers around the world. Furthermore, the price of crude oil continues to increase rapidly. The use of alternative fuels (liquified petroleum gas. LPG and compressed natural gas, CNG) and the optimization of combustion present effective solutions. Improving combustion is directly related to improving the intake aerodynamic movements which is influenced by the inlet system, especially the intake manifold. In this paper we have studied the geometry effects of two intake manifolds on the in-cylinder flows by two methods, numerically and experimentally. These two manifolds are mounted on a fully instrumented, six-cylinder, 13.8 I displacement, heavy duty, IVECO engine, installed at the authors' laboratory, which is used to power the urban bus diesel engines in Sfax. This engine was modified to bi-fuel spark ignition engine gasoline and gas fuelling. The 1st manifold presents an unspecified geometry whereas the 2nd presents an optimal filling geometry. A three-dimensional numerical modeling of the turbulent in-cylinder flow through the two manifolds was undertaken. The model is based on solving Navier-Stokes and energy equations in conjunction with the standard k-epsilon turbulence model, using the 3D CFD code FloWorks. This modeling made it possible to provide a fine knowledge of in-flow structures, in order to examine the adequate manifold. Experimental measurements are also carried out to validate this manifold by measuring the important engine performances. Brake power (BP), brake torque (BT) and brake thermal efficiency (BTE), are increased by 16%, 13.9%, and 12.5%, respectively, using optimal manifold. The brake specific fuel consumption (BSFC) is reduced by 28%. Simulation and experiments results confirmed the benefits of the optimized manifold geometry on the in-cylinder flow and engine performances. (C) 2011 Elsevier Ltd. All rights reserved.