During engine operation, the 3 pollutants enter the catalytic converter. How efficiently they are converted, depends on the composition of the exhaust gases. And that depends on the air-fuel mixture sent into the cylinder for combustion.
If the air-fuel ratio supplied to the engine is too rich, then the nitrogen oxides are converted efficiently, but the carbon monoxide and the hydrocarbons are not.
If the air-fuel mixture supplied to the engine is too lean, the opposite occurs. Carbon monoxide and hydrocarbons in the exhaust gas are converted efficiently, but not the nitrogen oxides.
Highly efficient conversions of all 3 pollutants occur only in a narrow range of air-fuel ratios. This range occurs around the ideal air-fuel ratio by mass, of 14.7 parts of air, to 1 part of fuel. This is called the Stoichiometric Point. It’s also called the operating window of the 3-way catalytic converter.
If the mixture ratio falls outside this range, the efficiency of conversion of either the nitrogen oxides or the hydrocarbons and carbon monoxide will rapidly decrease.
Because of this strict requirement, vehicles with a 3-way catalytic converter have a feedback system, called looping. The ECU monitors the air-fuel ratio by using an exhaust gas oxygen, or EGO, sensor, also known as a lambda sensor. This sensor obtains information about the composition of exhaust gases. It’s located in the exhaust manifold, and it’s connected to the control unit.
Closed Loop can mean the control unit does receive feedback from the EGO sensor AND acts on it, to alter the injection setting.
Open Loop can mean there is feedback to the control unit, but it is ignored, and the fuel settings are then determined from the programmed memory. Alternatively, Open Loop means there is no feedback to the control unit. In that case too, fuel settings are determined from the programmed memory. During cold starting, this sensor is at too low a temperature to provide an output voltage to the control unit. So it operates in Open Loop.
The same applies until the sensor warms up. The control unit is in Open Loop. It is only when the EGO sensor reaches its operating temperature, that a voltage corresponding to the air-fuel ratio is sent to the control unit. During normal operation, Open Loop may also occur during idle, or under maximum power. It can also occur if there is a fault that causes the air-fuel ratio to be excessively rich for long periods. Similarly for a fault that makes the ratio too lean for long periods.
When the lambda sensor reaches its operating temperature, around 350°c or 662°F, it sends an output voltage to the control unit, to signal whether the mixture is richer or leaner than a lambda value of 1.00 - that is, the air-fuel ratio of 14.7 to 1, also called the stoichiometric point. When the mixture deviates from this, the output voltage changes sharply.
The voltage of the signal from the EGO sensor changes sharply when the air-fuel mixture changes from lean to rich. If the control unit sees the voltage as high, then the quantity of fuel injected is reduced.
Similarly if the mixture changes from rich to lean. If the voltage is low, more fuel is injected to enrich the mixture.
The control unit then adjusts the pulse width of the injector accordingly to ensure the most efficient operation of the catalytic converter.
During engine operation, this adjusting is continuous, and almost instantaneous, trying to maintain an air-fuel ratio for lambda equal to 1.
Source: CDX Global
