fig1 - 3 illustrate a cps valve 10 which comprises a housing 12 fabricated from a suitable plastic material and having an inlet nipple 14 and an outlet nipple 16 . when the cps valve is installed in an on - board fuel vapor recovery system of an automotive vehicle , inlet nipple 14 is placed in communication with a vapor collection canister 18 , and outlet nipple 16 is placed in communication with an intake manifold 20 of an internal combustion engine 22 that powers the automotive vehicle . cps valve 10 further comprises a solenoid 24 for operating a valve 26 that controls the flow through the cps valve between inlet nipple 14 and outlet nipple 16 . solenoid 24 comprises a length of wire wound on a plastic bobbin 28 between upper and lower bobbin flanges 29 to form an electromagnetic coil 30 . bobbin 28 comprises a tubular core 32 whose interior is lined by a circular cylindrical brass tube 34 that is pressed into , or otherwise secured with respect to , core 32 . a ferromagnetic pole piece 36 is disposed at the upper end of the solenoid and comprises a cylindrical stator 38 that is snugly fitted into approximately the upper half of tube 34 and a circular flange 40 at the upper end of stator 38 that extends radially outwardly to the o . d . of bobbin 28 . a cylindrical ferromagnetic shell 42 laterally encloses the solenoid , extending downwardly from flange 40 to wrap over the outer edge of a ferromagnetic ring 44 that is disposed against the lower bobbin flange 29 . the ends of the wire forming coil 30 are joined to upstanding electrical terminals 46 at one side of the upper end of the solenoid . the solenoid is retained within housing 12 by a plastic cover 48 that closes the open upper end of the housing above the solenoid and is secured therein by deforming the edge of the housing over and onto the cover &# 39 ; s perimeter . cover 48 includes provision for sealing the interior of the housing from the outside while allowing the distal ends of terminals 46 to pass through to the interior of a surround 50 that cooperates with the terminals to form a connector plug that mates with a complementary connector plug ( not shown ) for connecting the solenoid to an electrical control circuit ( not shown ) for operating the cps valve . valve 26 comprises a generally cylindrical ferromagnetic armature 52 and a generally circular non - metallic valve element 54 mounted on the lower end of armature 52 . the upper portion of armature 52 has a circular cylindrical outer wall that fits closely within tube 34 . valve element 54 has a circular lip 56 at the bottom that is shown in fig1 seated on a circular internal valve seat 58 that is provided internally of housing 12 at the end of an inlet passage opposite inlet nipple 14 . valve element 54 also has a circular shoulder that faces upwardly toward a circular groove in a portion of bobbin 28 that is spaced below lower flange 29 . one axial end of a helical bias spring 60 is disposed against that shoulder , and the other axial end of the spring is disposed in that groove , causing the spring to resiliently bias valve 26 to the position of fig1 showing valve 26 seated on seat 58 thereby closing cps valve 10 to flow between inlet nipple 14 and outlet nipple 16 . the upper end face of armature 52 confronts the lower end face of stator 38 within the interior of tube 34 across an air gap . the lower end face of the stator is flat except for a circular cylindrical protrusion 62 at the center . the upper end face of the armature is flat except for a circular cylindrical intrusion 64 at the center which is in alignment with protrusion 62 . the diameter of intrusion 64 is just slightly larger than that of protrusion 62 . when coil 30 is not being energized , stator 38 exerts no magnetic attraction force on armature 52 . when coil 30 is energized , stator 38 exerts magnetic attraction force on armature 52 , with the magnitude of the force being related to the duty cycle of the electric input to the coil . accordingly , valve 26 opens to an extent determined by the duty cycle input to the solenoid . the clearance between the circular cylindrical outer surface of armature 52 and the surrounding i . d . of tube 34 is made sufficiently small that a certain pneumatic damping of the armature motion occurs when the duty - cycling electric current in the solenoid commands armature movement . the fit of pole piece 36 to the solenoid closes the upper end of tube 34 . it is only via the close clearance between armature 52 and tube 34 that air can pass into and out of the air gap space between the confronting faces of the armature and stator as the armature moves , and this is what creates the pneumatic damping of the armature motion . the &# 34 ; stepped &# 34 ; relationship of the confronting surfaces of the armature and stator across the intervening air gap , provides a gap - force relationship that has a more gradual transition over the region where the flow characteristic is prone to irregularity , and this serves to substantially eradicate such undesired effects . the magnetic flux path for the flux issued by coil 30 comprises stator 38 , armature 52 , ring 44 , shell 42 , and flange 40 . because tube 34 is a relatively magnetically impermeable material , it has no effect on the magnetic circuit . when valve 26 unseats from seat 58 , purge flow from canister 18 passes into inlet nipple 14 , through the space between valve 26 and seat 58 , and from outlet nipple 16 to manifold 20 . fig4 presents a portion of another form of cps valve 10a which is like cps valve 10 except for the inclusion of a vacuum regulator 66 . vacuum regulator 66 functions to deliver to the upstream side of valve seat 58 vacuum which is regulated to a reasonably steady value despite wider variations in the actual manifold vacuum that is delivered to outlet nipple 16 . in cps valve 10a the passage extending into housing 12 from outlet nipple 16 is provided with a valve seat 68 . vacuum regulator 66 comprises a diaphragm - operated valve mechanism 70 comprising a diaphragm 72 which divides a lower portion of housing 12 into two chambers 74 and 76 on opposite sides of the diaphragm . a central region of the diaphragm carries a valve element 78 that coacts with valve seat 68 . the diaphragm is resiliently biased by means of a coil spring 80 such that valve element 78 is resiliently urged away from seat 68 . chamber 74 is communicated to manifold vacuum while chamber 76 is communicated to atmosphere . the pressure differential between actual manifold vacuum and atmosphere acting on the diaphragm will serve to position valve element 78 relative to seat 68 such that the vacuum in chamber 74 is regulated to a reasonably steady value despite fluctuations in actual manifold vacuum . this in turn maintains a steadier differential across the open valve 26 and the possibility for making the cps flow characteristic less sensitive to manifold vacuum variations over the regulation range . the regulation also benefits from the pintle shape of valve element 78 . fig5 is a representative graph plot of current flow in coil 30 in response to the application of a rectangular voltage waveform to the coil . fig6 is a representative graph plot of flow through cps valve 10 as a function of the duty cycle of the energizing input . fig7 is an enlargement of the lower range of the graph plot of fig6 showing how the irregularity that is characteristic of the earlier cps valve is alleviated by the invention . fig8 is a representative graph plot of flow through cps valve 10a as a function of intake manifold vacuum showing the improvement that is attained by the present invention over a prior vacuum regulated canister purge valve . while a presently preferred embodiment of the invention has been illustrated and described , it should be appreciated that principles are applicable to other embodiments .