The properties used to explode an object cannot be animated. When something explodes, it happens at a single point in time. This means that all the settings in the Particle Generator menu are used at once, at the beginning of the explosion.
(a) Stream Type box (b) Interactive Type box (c) Render Type box
To view the explosion as you make changes to your particle stream, change the Interactive Type box to Polygon as well. See Setting Particle Rendering Properties.
The following figure shows the schematic and the result of exploding a 3D object of an apple. The number of polygons is set to 200. The result shows frame 20.