It is known to utilize a spring in a valve to move a flow control member, such as a valve spool or poppet, between different operating positions within the valve. Conventional spring-biased check valves are one example of such valves. In some spring biased valves, the spring biases the valve spool or poppet to an "open" position until a pressure force overcomes the spring force and moves the valve spool or poppet to a closed position where a sealing force from a pressure differential produces a seal between the valve spool or poppet and the valve housing.
One problem associated with such valves is that the maximum spring force opposes the required seating force when the valve spool or poppet is in the closed position. This may prevent an adequate seal between the valve spool or poppet and the valve housing. This problem is particularly relevant to purge valves utilized in the fuel supply line of an engine. Commonly, purge valves utilize a valve spool or poppet that is spring biased towards a purge/fuel shut-off position where the fuel is to be purged from a purge port while preventing fuel flow to the engine. When the fuel system is activated, high pressure fuel in a fuel supply line acts against the spring bias to force the valve spool or poppet to a purge closed/fuel open position where the valve spool or poppet is seated against the purge port to prevent fuel leakage from the purge port while allowing fuel flow to the engine. For such purge valves, it is important that an adequate seal be formed in the purge closed/fuel open position to prevent fuel leakage from the purge port. However, as noted above, the bias force of the spring tends to reduce the sealing force, thereby potentially preventing an adequate seal. Further, because the spring biases the valve spool to the purge/fuel shut-off position, pressure fluctuations in the fuel supply line may cause the purge valve to inadvertently shut-off fuel flow to the engine. Such inadvertent shut-offs can be prevented by designing a sufficiently large "dead band" for the valve where the anticipated pressure fluctuations in the fuel supply line will not result in the valve spool moving to the purge/fuel shut-off position. In spring biased purge valves, such a "dead band" is typically provided by including a sufficiently large amount of travel for the valve spool between the open and closed positions. However, the required length of travel may result in a valve that cannot be conveniently packaged within the envelope for the fuel supply system, and may result in an unacceptably large response time for the valve.
A further problem associated with such valves is that the springs may add to the moving mass of the valve spool or poppet, thereby slowing the response time in the valve. Yet another problem associated with such valves is the potential for spring breakage or relaxation that may cause valve failure.
Thus, it can be seen that there is a need for a new and improved valve, and in particular a new and improved purge valve, that eliminates some, or all, of the above discussed problems associated with spring biased valves.