above stated if the device is dimensioned for this purpose the bed will become fluidized in such a manner that the abrasive particles are carried to the top of the catalyst surfaces only at or near the maximum speed of the engine this representing generally maximum exhaust discharge when and if the catalyst is not reactivated adequately a greater abrasive action over a wider range of operation can be obtained by using a similar device of slightly smaller size the top screen 14 prevents loss of the abrasive particles into the tail pipe in the event of rapid acceleration or a backfire and thus assure an adequate supply of abrasive for the life of the device by suitable adjustment of the respective heights or depths of the abrasive bed the area of the bed and the height of the clearance space the catalyzing device will assure a movement of abrasive particles to the top of the device at times of maximum engine output without excessive accumulation of the particles against the top screen the device will be mounted in a vertical position to assure the optimum performance of the fluidized bed with adequate cleaning action on the catalytic surfaces although the term 34 screen 34 has been used.