CIS-E

Lambda, a letter from the Greek alphabet, was chosen as the symbol for the optimum air/fuel ratio of an internal combustion engine.
The ratio is approximately 14:1 for gasoline engines. This means that 14 kilograms (kg) of air is required to completely burn 1 kg of gasoline.
When gasoline is burned in an internal combustion engine, essentially three pollutants are produced:
- Carbon monoxide
- Hydrocarbons
- Oxides of nitrogen
Air/fuel mixture
14 kg air required to burn 1 kg fuel





CARBON MONOXIDE (CO)
Is the product of incomplete combustion. Due to lack of oxygen, CO is formed instead of CO2 (carbon dioxide).
The richer the fuel mixture, the more CO content in the exhaust. Theoretically, complete combustion should occur when the air/fuel ratio is between .9 and 1.1. (See graph)
The richer the mixture, the higher the CO content in exhaust gas.





HYDROCARBONS (HC)
HC is unburned or only partially burned fuel. HC emissions can be caused by a rich fuel mixture (insufficient oxygen for complete combustion) or a lean mixture (incomplete combustion due to misfiring). (See graph)
There are numerous other causes for HC emissions including:
- Engine misfires due to fuel system faults, such as a faulty injector
- Ignition timing too far advanced
- Leaking exhaust valves
- Leaking spark plug cables
- Defective rotor or distributor cap
- Faulty spark plugs
The richer or leaner the fuel mixture, the higher the HC content.





OXIDES OF NITROGEN (NOx)
NOx consists of nitrogen and oxygen. Normally, nitrogen and oxygen will not be united, but because of the high temperatures and pressures produced in the engine's combustion chamber, the nitrogen will unite with oxygen to produce this pollutant.
High combustion temperatures caused by slightly lean fuel mixture and increased engine load will cause NOx to rise considerably. (See graph)
A Lambda value (1) most favorable for power output and fuel consumption will produce the highest NOx values.





THREE-WAY CATALYTIC CONVERTER
The three-way catalytic converter is so called because it can reduce the three major pollutants in the exhaust gases from a spark ignition engine.
- Carbon monoxide (CO)
- Hydrocarbons (HC)
- Oxides of nitrogen (NOx)

The catalytic converter consists of a honeycomb shaped ceramic core, which is covered with platinum and rhodium. As the exhaust gases containing HC and CO compounds are passed through the converter in the presence of oxygen, the platinum (catalyst) starts an oxidation (burning reaction) process. The HC and CO then unite with the oxygen to form a water vapor (H2O) and carbon (CO2). This process however does not have any effect on NOx emissions.
To control NOx, a separate reaction called reduction is necessary. A reduction reaction is the opposite from oxidation. It removes oxygen from a material. A reduction type converter uses rhodium as a catalyst to chemically promote the removal of oxygen from NOx. This process changes NOx to a harmless nitrogen (N2) and oxygen (O2).
With this type of catalyst, the pollutants can be reduced by up to 90%.

OXIDATION
The combining of an element with oxygen in a chemical process often produces extreme heat as a result.

REDUCTION
A chemical process in which oxygen is taken away from a compound.











OXYGEN SENSOR
The catalytic converter is most efficient when the air/fuel ratio is within the lambda window (.9-1.1). Therefore a "closed loop" system utilizing an oxygen sensor must be used.
The oxygen sensor is made of a ceramic material called Zirconium dioxide. The inner and outer surfaces of the ceramic material are coated with platinum. The outer platinum surface is exposed to the exhaust gas, while the inner surface is exposed to the outside air.
The difference in the amount of oxygen contacting the inner and outer surfaces of the oxygen sensor creates a pressure differential which results in a small voltage signal in the range of 100 to 1,000 mV. The amount of voltage that is produced is determined by the fuel mixture. A high voltage signal indicates a rich mixture, and if the mixture is lean, a low voltage signal is produced.
On some vehicles, the oxygen sensor is heated electrically to keep it at a constant operating temperature. This insures continuous and accurate reaction of the sensor during all operating conditions.
The heated oxygen sensor has three wires, two for the heating element (ground and power) and a signal wire for the oxygen sensor. Power is supplied to the heating element whenever the engine is running.











The oxygen system works like this. If the amount of oxygen in the exhaust system is high, the oxygen sensor will send a low voltage signal of 100 - 500 mV to the electronic control unit. The control unit will increase the current to the differential pressure regulator.
When the current to the differential pressure regulator is increased, the plate valve will deflect to the left to restrict the flow of fuel to the lower chamber. This reduces the pressure to the lower chamber causing the pressure regulating valves to deflect downward. The result is increased fuel flow to the injectors and an enriched fuel mixture.
If the amount of oxygen in the exhaust system is low, indicating a rich mixture, the sensor voltage will be high. This signal approx. 500 - 900 mV, is sent to the control unit. The electronic control unit then reduces the current signal to the differential pressure regulator.
When the current to the differential pressure regulator is reduced the plate valve will move to the right and allow more fuel into the lower chamber The pressure in the lower chamber will then increase and deflect the pressure regulating valve upward. The quantity of the fuel delivered to the injectors is then reduced and the fuel mixture is leaned.