of maximizing the system particulate capture efficiency minimizing pressure drop and minimizing no 2 emissions it is necessary to carefully select the system design parameters the selection of these design parameters will depend in part on the no x particulate ratio and the exhaust mass flow rate of the engine as discussed above the no x particulate ratio depends largely on the emissions regulations in place at the time the engine is manufactured and the mass flow rate is related to the displacement and power output of the engine to ensure sufficient regeneration of the particulate reduction devices while minimizing no 2 it is necessary to select the relative precious metal loadings of the catalyst coatings of each particulate reduction device so as to generate the appropriate amount of no 2 the required precious metal loading will depend in part on the residence time of no within the particulate reduction device which in turn depends on the mass flow rate of exhaust gas and the volume of the particulate reduction device the no residence time may be expressed as a space velocity defined as the volumetric flow rate of exhaust gases divided by the volume of the device per unit of time preferably to ensure regeneration without excessive no 2 emissions the ratio of the mass of no 2 to the mass of particulate matter in the exhaust stream should be between 3.