Gasoline is predominately carbon and hydrogen. The exhaust products are a mixture of carbon monoxide, carbon dioxide, water,  unburned hydrocarbon molecules, oxygen, nitrogen and oxides of nitrogen. As well as all the other nasty additives that are included in fuel these days. The catalytic converter is used to complete the process and thus reduce pollution. Additional air is added prior to the converter to enable it to function.

How does the oxygen sensor work

      Nearly all automotive sensors are of the zirconia type. A ceramic bulb projects into the hot exhaust gas. The bulb is hollow and the inside surface is in contact with the atmosphere outside the exhaust pipe. Both surfaces are coated with platinum. Oxygen atoms are negatively charged and will attach to the platinum surfaces therefore building up a negative charge on each surface. When the hot exhaust contains little or no free oxygen then the charge on the bulb surface will be less than on the other surface. A voltage difference is produced and electrodes attach to each surface. The atmosphere side is connected to the vehicle earth, so the other side will be actually a positive going voltage since it has less negative oxygen atoms. It effectively forms a battery that only works once it has reached several hundred degrees, and the output voltage is dependent on the oxygen concentration of the exhaust gas.

From the Sensor Graph you can see that the output is not linear and actually changes abruptly about the stoichometric point. This is the point where the air/fuel ratio is optimum at 14.7 : 1 and should result in total combustion.
Lean conditions give close to zero volts out and a rich mixture will produce about 1 volt maximum.

      A very small change in the air/fuel ratio about the mid point will cause a large change in the sensor output. The computer can't always maintain a perfect mixture to keep the output exactly in the middle, so instead attempts to only maintain the average at that point. The sensor output will oscillate while driving. The rate of oscillation will vary according to how fast the sensor can react and how fast the computer reacts. Typically the rate is anywhere between 1 and 10 cycles per second. 

      The computer has 2 main modes of operation, open loop and closed loop. In open loop mode the oxygen sensor is ignored, with all the other sensors being used to calculate the required fuel quantity. Throttle position, air flow rate, air temperature, engine speed and others are measured and used for calculation. The calculation is quite accurate only if all the sensors are accurately adjusted, so adjustments of these is best left to the experts. The computer operates in open loop mode, ignoring the oxygen sensor when it suspects the sensor is not working. Remember the sensor must be hot to work so after start up the computer is waiting to see the sensor output go high first before it will change to closed loop mode. In this mode the other sensors are still used to make the main calculation, but the result is modified slightly according to the sensor output. Some computers learn as they go and are able to calculate a more accurate fuel requirement faster.

     There are other times also when the oxygen sensor is ignored. When accelerating hard with the throttle beyond about three quarters open, the mixture will be set rich to provide more power. When coasting down from high speed the mixture will go lean. If you install a dash mounted mixture display you can see this happen as you drive. We recommend this display because it helps tremendously to see exactly what the oxygen sensor is putting out at all times.

MIXTURE GRAPH

The Mixture graph displays the relationship between power, economy and air/fuel ratio.

      To control your mixture as you drive, it is a simple matter of intercepting  the oxygen sensor output before it reaches the computer and offsetting the signal slightly. You can adjust for leaner more economical driving or even richer mixture giving greater power. Using our Electronic mixture controller you have the ability to alter your air/fuel ratio as you drive. It is completely automatic in operation and the car can be driven normally. This is essential in a vehicle that will be driven by others as well as yourself.

      If you apply any mileage improving devices to a fuel injected vehicle, it is essential to also install the Electronic Mixture Controller to fully realize the gains available. Our controller has the additional benefit that it can speed up the reaction time of a particularly sluggish sensor. Even simple water or steam injection will increase the exhaust oxygen which the computer will attempt to compensate for. It is possible that water injection could result in worse mileage if you don't electronically fool the computer.