Fuel Mixture Regulation

MIXTURE CONTROL





In the past, the job of the steel diaphragm and the differential pressure valves in the fuel distributor was to maintain a constant pressure difference between the upper and lower chambers of the fuel distributor.

As long as this pressure difference was held constant, the amount of fuel flowing was determined only by the height of the control plunger. If the pressure in the lower chamber is varied, the fuel quantity to the injectors will change.

In the oxygen sensor system, the pressure in the lower chamber of the fuel distributor is controlled by the frequency valve.

The frequency valve resembles an electronic fuel injector which has been modified to accept the different fuel lines. The valve is connected between the lower chambers of the fuel distributor and the fuel return line.





When the oxygen sensor reports a rich mixture condition to the control unit, the control unit will close the frequency valve. This causes the pressure in the lower chamber of the fuel distributor to increase and press the steel diaphragm upward reducing the amount of fuel.





If the air fuel mixture is too lean, the frequency valve will open and cause the pressure in the lower diaphragm of the fuel distributor to decrease.

The reduced pressure allows the diaphragm to be deflected downward and the amount of fuel will be increased.

Actually, the frequency valve is opening and closing many times per second. The ratio of open to close cycles is what determines the average pressure and therefore the mixture.

For example if the frequency valve cycles at a ratio of 60:40 (open 60% of the time, closed 40%.) a richer mixture is created than would be found at a ratio of 45:55 (open 45%, of the time, closed 55%). This ratio is commonly called the "duty cycle".