Generally, the CI (compression ignition) engine that runs with biodiesel experiences lower performance and certain higher emissions than the one that runs with petro-diesel. For that reason, this research investigated the effect of guide vanes installed in front of the intake manifold of a CI engine run with biodiesel to enhance the in-cylinder airflow to promote the evaporation, diffusion, and combustion processes to eventually reduce those problems. This research began with optimizing the design of the guide vanes through the simulation method. However, the optimization was limited only to the number of guide vanes while vane height, angle, and length were kept constant. After analyzing the simulation results of in-cylinder airflow characteristics of TKE (turbulence kinetic energy), velocity, vorticity, and swirling strength of 10 guide vanes models of different vane numbers that varied in between 3 and 12, the model with 4 vanes was found to be the optimum one. After that, five guide vanes models with the number of vanes ranging between 3 and 7 were fabricated and tested one by one on a CI engine run with biodiesel along with a standard CI engine run with biodiesel and petro-diesel having no vanes. Based on the simulations and experimental results, 4 vanes were found to be the optimum number with vane angles of 35 degrees along with vane height and length of 0.2 and three times the radius of the intake runner, respectively. (C) 2014 Elsevier Ltd. All rights reserved.