Laser Wake Field Acceleration of relativistic electron bunches is a promising method to produce a large amount of energetic particles with table top equipment. One of the possible methods to inject particles in the appropriate acceleration phase of the wake behind the pulse takes advantage of the partial longitudinal breaking of the wake crests across a density downramp. In this paper results of 2.5D PIC simulations, showing the production of an electron bunch with reduced energy spread, are reported. Also, a possible method to produce the required plasma density transition by laser explosion of a suitable couple of thin foils is discussed.