Frequency Valve: Description and Operation
The oxygen sensor frequency valve doesn't directly have anything to do with the oxygen sensor. Rather, the oxygen sensor indicates the exhaust gas oxygen content to the ECU which changes the duty cycle of the frequency valve which changes the control pressure of the fuel system which changes the air/fuel ratio which affects the exhaust gas oxygen content which starts the whole sequence over again. Other names for this device include:
Lambda Valve
Lambda Frequency Valve
Control Pressure Frequency Valve
Differential Pressure Frequency Valve
BASIC KNOWLEDGE REQUIRED
To fully understand the role of the oxygen sensor frequency valve, it is necessary to have a basic understanding of the fuel distributor, and how "control pressure" and "differential pressure" affect its operation.
In a fluid system, fluid will flow from one place to another only if there is a difference in pressure between where it is flowing from (high pressure) and where it is flowing to (low pressure). The difference between the two pressures is the differential pressure. For example, if the high pressure side of the system is at 5 bar (73.5 psi), and the low pressure side is at 4 bar (58.8 psi), then the differential pressure is 1 bar (14.7 psi, 1 bar = 1 atmosphere). there are several places in a CIS fuel system where pressure drops occur, and where ever there is a difference in pressure in a fluid system, there is a differential pressure. For our purposes, the term "differential pressure" will be used to describe the difference between system pressure and control pressure (explained below).
FUEL QUANTITY - METHODS OF CONTROL
The quantity of fluid (fuel) that flows can be controlled two ways. One way is to vary the cross-sectional area of the passage (metering slit) between the different pressure areas, the larger the passage, the greater the volume of flow (for a given pressure drop at the metering slit). This is the function of the air flow sensor plate and control plunger, see AIR FLOW SENSOR. The other way to control the quantity of flow is to vary the pressure drop at the metering slit. The greater the difference in pressure between the high and low sides, the greater the quantity of flow (for a given cross-sectional area of the metering slit). This is the function of the oxygen sensor frequency valve.
Lower Chamber Pressure and Oxygen Sensor Frequency Valve:
REGULATING PRESSURE DROP - CONTROL PRESSURE
Fuel enters two separate fluid circuits in the fuel distributor. One circuit (delivery circuit) delivers the fuel to the injectors, while the second circuit regulates the first. The two circuits are separated by a device (differential pressure valve) which operates as a dome fluid regulator. The control pressure works against this pressure valve that regulates the pressure drop across the metering slit. The high pressure (system pressure) side of the metering slit is maintained at a constant pressure by the fuel pump and primary pressure regulator. The pressure drop is regulated by varying the pressure on the control side of the differential pressure valve. Directly regulating the flow in the control circuit indirectly affects the flow in the delivery circuit.
In the fuel distributor, fuel passing through a metering slit enters a chamber (upper chamber, one for each cylinder) with a diaphragm valve (differential pressure valve) separating the upper chamber from a lower chamber (control pressure chamber, also one for each cylinder and connected by a common outlet). Since the volume of the upper chambers is relatively constant all the fuel that enters the upper chamber exits to the injector. By regulating the pressure at which fuel exits the upper chamber, the quantity entering the chamber is regulated.
The total pressure in the upper chamber (fuel delivery pressure + pressure due to spring) must exceed the total pressure exerted form the lower chamber side of the diaphragm (control pressure) before fuel can exit through the differential pressure valve and go to the injector (something like pressing your finger up against the differential pressure valve, if you could do so, see illustration). If the control pressure is low, then the delivery pressure is correspondingly low compared to the system pressure. This means that the pressure drop at the metering slit is large and a large quantity of fuel will flow through the metering slit (for a given position of the control plunger). If the control pressure is increased, the delivery pressure increases, and the pressure drop across the metering slit decreases, reducing the quantity of fuel that flows through the metering slit (for the same given position of the control plunger). If the control pressure is allowed to increase to the same as the system pressure, the pressure drop is reduced to nearly zero (delivery pressure = system pressure - spring pressure) and very little fuel will enter the upper chamber through the metering slit.
OXYGEN SENSOR FREQUENCY VALVE GENERAL PURPOSE
To regulate the control pressure (and in doing so, the pressure drop at the metering slit) fuel enters the lower chamber through a restrictor, flows through the lower chamber, then exits the lower chamber through the frequency valve back to the fuel tank. The frequency valve regulates how much fuel is allowed to drain back to the fuel tank.
Oxygen Sensor Frequency Valve (Lambda Valve):
OXYGEN SENSOR FREQUENCY VALVE OPERATION
Since fuel enters the lower chambers through a restriction (one for each chamber), if fuel is allowed to exit unrestricted, the pressure in the lower chamber (control pressure) is very low, and a large quantity of fuel will be allowed into the upper chamber and to the injector. If fuel exiting the lower chambers is restricted, the control pressure rises causing the upper chamber pressure to rise, and less fuel flows through the metering slit. The oxygen sensor frequency valve regulates how much pressure is retained in the lower chamber by cycling on and off. The duty cycle of the valve (on time/off time) determines the control pressure. A large duty cycle (on time much greater than off time) means the valve is open more than it is closed, control pressure is low, the pressure drop at the metering slit is large, and a large amount of fuel flows to the injector. As the duty cycle is decreased, the valve is closed more of the time, control pressure increases, the pressure drop at the metering slit is reduced, and fuel flow to the injector is reduced. During deceleration fuel cut-off, the duty cycle is greatly reduced. Deceleration fuel cut-off is employed when the following conditions exist simultaneously:
1. Coolant temperature above 30°C (86°F).
2. Engine speed greater than 1400 rpm.
3. Throttle valve in idle position (idle switch closed).
When the engine speed drops below approximately 1200 rpm, normal operation resumes and the duty cycle again depends on the signal from the oxygen sensor.