The present work is dedicated to the study of air-natural gas mixing in a model intake manifold for Natural Gas Spark ignition engine. A configuration of continuous Natural Gas injection is considered. The fuel injection located upstream the intake valve is submitted to a time varying crossflow with high acceleration and deceleration levels. A physical analysis is developed and presented. Relevant new non-dimensional numbers are established for the engine situation. With this analysis, we compute a critical diameter of injection that characterizes the sensitivity of the jet to the pulsed transverse flow. Experiment is used to check these predictions.Schlieren technique is applied in a test section to obtain instantaneous images of the injection jet phased in the engine cycle. We visualize the global behavior of the jet and consequently the mixing in the intake manifold. The interaction between the jet and the crossflow is analyzed for different engine speeds (800 and 1400 rpm), injection diameters (2 and 7 mm) and air-fuel ratio (Φ= 0.5 and Φ=1). For different jet outlet velocities (3.5 to 150 m/s), we observe that acceleration phases of the crossflow, have fundamental effects on the behavior of the jet. For a low injection velocity in the acceleration phase, gas can be completely stratified near the upper wall of the duct. In the deceleration phase, a competition between drag and acceleration effects exists. This situation affects the mixing in the intake manifold and therefore in the cylinder. For high injection velocities, the jet is little sensitive to the unsteady effects, usually impacts on the lower wall of the duct and enhances the mixing. Injection velocities are therefore a fundamental parameter of mixing process control in natural gas engine.