The present invention relates to evaporative emission control systems for vehicles and in particular to a purge valve that is adapted to be controlled by the engine management control system for regulating the supply of fuel vapors to the engine intake from the fuel tank vapor recovery system.
In order to meet current emission requirements, present day vehicles contain evaporative emission control systems which reduce the quantity of gasoline vapors emanating from the fuel tank of the vehicle. Generally, these systems include a charcoal canister which traps the vapors from the fuel tank, and a purge system which draws the vapors out of the canister and feeds them into the intake system of the engine when the engine is running. The fuel vapors are drawn into the engine intake manifold along with atmospheric air drawn through the canister.
The capability of the canister to trap vapors from the fuel tank is greatly dependent upon how thoroughly the vapors are purged from the canister when the vehicle was last operated. Accordingly, it is desirable to purge the canister as much as possible while the engine is running. However, the amount of vapor that can be drawn into the engine at any time is limited by the total airflow into the engine and the accuracy with which the purge flow can be controlled. At high speeds or under high engine loads, high purge flow rates can be easily handled. Under such conditions, however, the manifold vacuum is low which tends to limit the amount of fuel vapors and air which can be drawn from the canister into the engine intake manifold. In addition, when the engine is at idle, the airflow into the engine is low. Therefore, purgining at idle must be precisely controlled to prevent a rough idle. Moreover, due to the varying ratio of air to fuel vapors in the purge system, purging during idle can significantly impact the resulting air/flow ratio of the fuel mixture supplied to the engine. Consequently, purging at idle can easily result in a too rich or too lean fuel mixture causing excessive tailpipe emissions unless purging at idle is limited to low flow rates. Current emissions systems, therefore, do not generally purge the canister at idle to any substantial degree.
However, impending tighter emissions requirements and changes to the EPA testing procedures will require larger capacity canisters and therefore higher capacity purge systems. Moreover, the prospect of on-board refueling vapor recovery systems will only add to these system requirements. Accordingly, it is becoming imperative that such systems not only purge at idle, but that maximum flow rates be increased as well. This, of course, presents conflicting requirements for purge systems. Specifically, in order to purge at idle, the purge flow rate must be fairly low and accurately controlled by the engine control computer which monitors the resulting oxygen content of the exhaust gases from the engine. When a canister is saturated with fuel, and vapor is initially purged, the purge flow is very high in fuel vapor. After most of the fuel vapors are drawn out of the charcoal, the purge flow is almost pure air. Therefore, the purge control valve must be capable of allowing the engine control computer to precesely control small flow rates at idle while correcting the idle fuel-air ratio so that tailpipe emissions are not adversely affected. This type of precise flow control is best accomplished using a relatively small valve.
On the other hand, it is desirable to purge at very high flow rates when the engine is operating under high speed or heavy load conditions when it can efficiently consume significant quantities of fuel vapor and air with a minimum effect on fuel air ratios. In order to achieve large flow rates, it is necessary for the purge valve to provide a relatively large flow passage. This requirement, of course, is in direct conflict with the requirement for precise low flow rate control. Specifically, it is believed to be impractical to provide a valve large enough to satisfy the high flow requirements which at the same time is capable of precisely modulating the opening of the valve to meet the low flow requirements.
Accordingly, it is the primary object of the present invention to provide a two-stage purge control valve that is capable of providing both precise control at low flow rates and high flow capacity at low manifold vacuum pressures. In general, this is accomplished by providing a single assembly having two valves which control separate parallel flow paths. Low flow control is achieved with a small solenoid valve adapted to be driven by a pulse width modulated (PWM) signal from the engine control computer. High flow capacity is provided by a vacuum-controlled valve which opens at low manifold vacuum pressures. Because purge flow comprises a relatively small percentage of total air flow into the engine under the conditions when the high flow stage is open, precise control of the high flow capacity valve by the engine control computer is not required.
Accordingly, the purge valve according to the present invention allows the full range from 10% to 90% duty cycle control to be used to control low flow rates and opens the high flow valve only when the purge flow comprises a small portion of the total engine intake air flow. Moreover, the high flow valve is adapted to open gradually as engine manifold vacuum pressure decreases, thereby proportioning the purge flow to the total engine intake air flow. In addition, the engine control computer can still adjust the high purge flow rate to a degree by controlling the parallel flow through the PWM solenoid valve.
In the preferred embodiment of the present invention, the response and flow capacity of both the low and high flow control valves can be calibrated to meet the requirements of a particular engine family or purge system.