fig1 displays a schematic illustration of a canister purge system 10 embodying principles of the invention . the system comprises a solenoid - actuated valve 12 and a vacuum - actuated valve 14 , both of which are normally closed . solenoid - actuated valve 12 comprises an inlet nipple 16 , an outlet nipple 18 , and a valve member 20 that controls the degree of restriction that the valve imposes on flow from inlet nipple 16 to outlet nipple 18 . a helical coil spring 22 biases valve member 20 to close the passageway between the inlet and outlet nipples . valve member 20 has an armature that is disposed within a solenoid 24 . solenoid 24 is electrically coupled with the engine ecu ( not shown ) by means of an electrical terminal plug 26 . the ecu delivers a pulse width modulated control signal to the solenoid for the purpose of selectively positioning valve member 20 within the valve against the bias force of spring 22 . at and below a certain minimum pulse width , the degree of energization of solenoid 24 is insufficient for valve member 20 to be displaced against the spring bias , and so the valve remains closed . as the pulse width increases above this certain minimum , valve member 20 is increasingly displaced to correspondingly decrease the degree of restriction between nipples 16 and 18 . the reference numeral 100 in fig2 designates a graph plot of flow , in liters per minutes , vs . percent duty cycle of energization for a representative valve 12 by itself . the graph plot is reasonably linear , but the maximum rate that can be flowed through the valve is limited to about thirty - six liters per minute . the improvement which is afforded by the present invention retains substantially the same flow vs . duty cycle characteristic up to about a 30 % duty cycle , but enables substantially greater purge flows for larger duty cycles . the flow vs . percent duty cycle for a representative system of the improvement is designated by the reference numeral 102 in fig3 . the maximum flow rate is now increased to over one hundred liters per minute , a very substantial amplification . the improvement afforded by the invention resides in the manner of association of valve 14 with valve 12 . valve 14 comprises an inlet nipple 28 , an outlet nipple 30 , and a diaphragm valve 32 that is positionable to open and close the passageway from inlet 28 to outlet 30 to flow in accordance with the magnitude of vacuum that is applied to the nipple of a control port 34 . a helical coil spring 36 bias diaphragm valve 32 to close the passageway between nipples 28 and 30 to flow . the delivery of a sufficiently high vacuum to control port 34 will cause the diaphragm valve to overcome the spring bias and allow flow from nipple 28 to nipple 30 . the cooperation between the two valves 12 and 14 is provided by connecting valve 12 in a first conduit portion 38 extending from the vapor collection canister to the engine intake manifold , by connecting valve 14 in a second conduit portion 40 also extending from the canister to the manifold , and by connecting nipple 34 via a third conduit portion 42 to a tap 44 into the first conduit portion 38 between nipple 16 and an orifice 46 , as shown . the system operates in the following manner . as valve 12 is increasingly opened up to about a forty percent duty cycle , increasing flow is permitted from the canister to the manifold while valve 14 remains closed . as the flow through the first conduit portion 38 thusly increases , the vacuum applied to control port 34 also increases . at the forty percent duty cycle applied to valve 12 , the vacuum at control port 34 is sufficiently large to cause valve 14 to begin to flow , and thereby create a second flow path from the canister to the manifold . progressively increasing the duty cycle of valve 12 beyond the forty percent level results in a flow characteristic like that presented by the corresponding segment of the graph plot 102 of fig3 . as can be seen , this is substantially greater than the corresponding segment of the graph plot 100 . accordingly , the invention provides acceptable control resolution over its full operating range , especially at low flow rates , and the capacity for high flow rates at high duty cycles of valve 12 . it can also be appreciated that the point at which valve 14 is allowed to open is calibratable by the selection of design parameters . fig4 illustrates a second embodiment in which a solenoid - actuated valve 12a , equivalent to solenoid - actuated valve 12 , and a vacuum - actuated valve 14a , equivalent to vacuum - actuated valve 14 , are integrated into a unitary assembly 10a . the equivalent of nipple 18 , conduit portions 38 and 40 , and nipple 30 is found in an internal tube 50a . the upper end of tube 50a as viewed in fig4 provides a seat for the diaphragm valve 32a of vacuum - actuated valve 14a , which is equivalent to the diaphragm valve 32 of vacuum - actuated valve 14 , while the lower end of the tube provides a seat for the valve member 20a of solenoid - actuated valve 12a , which is equivalent to the valve member 20 of solenoid - actuated valve 12 . the intake manifold is communicated to assembly 10a by means of a nipple 52a which extends to radially intercept tube 50a in the manner of a tee , as shown . the canister is communicated to assembly 10a by means of a nipple 54a . internally , nipple 54a sub - divides into a passageway 56a leading to the chamber space of vacuum - actuated valve 14a which contains a spring 36a , equivalent to spring 36 , that biases diaphragm valve 32a toward seating on tube 50a . passageway 56a contains an orifice disc 46a , providing an orifice equivalent to orifice 46 , and it also contains an orifice 58a between orifice disc 46a and vacuum - actuated valve 14a . thus passageway 56a is equivalent to the flow path defined by elements 46 , 44 , 42 , and 34 in the embodiment of fig1 . the equivalent of elements 38 and 16 from fig1 is found in a passageway 60a which tees into passageway 56a between orifice disc 46a and orifice 58a and extends to the seat side of valve member 20a . other elements of fig4 which are equivalent to corresponding elements of fig1 are identified by the same numerals but with the addition of the suffix a . the operation of assembly 10a is equivalent to the operation previously described for the first embodiment . the inclusion of orifice 58a is to damp vacuum changes so that transient fluttering of diaphragm valve 32a that might occur in response to sharp vacuum changes is attenuated , or even precluded . fig5 presents a third embodiment which is a system 10b , equivalent to the system 10 of fig1 but further including a pressure regulator 62b disposed between the intake manifold and the solenoid - actuated valve for the purpose of compensating for changes in intake manifold vacuum such that over the effective range of the pressure regulator the purge flow through the solenoid - actuated valve is rendered substantially unaffected by changes in intake manifold vacuum . those elements of the third embodiment that are equivalent to corresponding elements of the first embodiment are designated in fig5 by the same reference numeral used in fig . but with the inclusion of the letter b as a suffix . a detailed description of such elements of fig5 is therefore unnecessary . pressure regulator 62b comprises a first nipple 64b which connects to nipple 18b of solenoid - actuated valve 12b via a conduit 38b &# 39 ; and a second nipple 66b which connects via a conduit 38b &# 34 ; to intake manifold . within its interior the pressure regulator comprises a diaphragm valve 68b that divides the interior into two chambers . one chamber 70b is communicated to atmosphere ; the other chamber 72b is in communication with nipple 64b . a helical spring 74b disposed in chamber 72b biases diaphragm valve 68b away from a valve seat 76b which is at the end of an internal passageway leading from nipple 66b . the pressure regulator is constructed and arranged such that the effective opening between valve seat 76b and diaphragm valve 68b is set by the magnitude of intake manifold vacuum relative to atmospheric pressure to prevent changes in vacuum from having substantial influence on a purge flow that is set by solenoid - actuated valve 12b . fig5 also shows the inclusion of an orifice 78b between the canister and nipple 28b . orifice 78b is for the purpose of calibrating the flow rate through vacuum - actuated valve 14b at a particular set of conditions , and is really in the nature of a manufacturing convenience since a basic valve 14b can be fabricated and then calibrated by the use of a particular orifice size for orifice 78b . the same convenience can be incorporated into the other embodiments disclosed herein . fig6 shows a fourth embodiment 10c in which a solenoid - actuated valve 12c , equivalent to solenoid - actuated valve 12b , a vacuum - actuated valve 14c , equivalent to vacuum - actuated valve 14b , and a pressure regulator 62c , equivalent to pressure regulator 62b , are integrated into a unitary assembly 10c . those elements of fig6 which are equivalent to corresponding elements of the fig5 embodiment are identified by the same base numerals , but with the suffix b changed to the suffix c . a detailed description of such elements is unnecessary and will not be given in the interests on conciseness . a nipple 80c communicates assembly 10c to intake manifold , and a nipple 82c communicates the assembly to canister . interior of assembly 10c , nipple 82c sub - divides into a passageway 84c leading to vacuum - actuated valve 14c , equivalent to the flow path through orifice 78b and nipple 28b of fig5 and to a passageway 86c that leads to both the seat side of solenoid - actuated valve 12c and the chamber of vacuum - actuated valve 14c that contains spring 36c . an internal passageway 88c extends from solenoid - actuated valve 12c to pressure regulator 62c and is equivalent to the flow path that is provided by elements 18b , 38b &# 39 ;, and 64b in the embodiment of fig5 . assembly 10c functions in equivalent manner to the embodiment of fig5 . fig7 and 7a depict representative performance of an assembly such as that of fig4 . there are five plots of purge flow vs . duty cycle of a pulse width modulated signal applied to the solenoid - actuated valve for each of five different levels of manifold vacuum . each plot has a distinctive dual - slope character wherein the lesser slope represents the low flow rate purging accomplished by the solenoid - actuated valve and the greater slope represents the higher flow rate purging that is accomplished by the vacuum - actuated valve . fig8 and 8a depict representative performance of an assembly such as that of fig6 . there are five plots of purge flow vs . duty cycle of a pulse width modulated signal applied to the solenoid - actuated valve for each of five different levels of manifold vacuum . each plot has a distinctive dual - slope character wherein the lesser slope represents the low flow rate purging accomplished by the solenoid - actuated valve and the greater slope represents the higher flow rate purging that is accomplished by the vacuum - actuated valve . however , unlike the pressure unregulated plots of fig7 and 7a , the pressure regulated plots of fig8 and 8a are substantially coincident showing the effect of pressure regulation . while a preferred embodiment of the invention has been illustrated and described , it should be appreciated that principles are applicable to other equivalent embodiments within the scope of the following claims .