3232853 - 1080B exhaust emission control system

The electronic control unit identifies the composition of the mixture (rich or lean) from the Lambda sensor output voltage.It adjusts the quantity of fuel injected to ensure the optimum composition of the mixture (l = 1) to create the ideal conditions for the treatment of the exhaust gases in the catalytic converter.If the mixture is too rich (lambda less than 1), then the quantity of fuel should be reduced and, if the mixture is too lean (lambda greater than 1), then the quantity of fuel should be increased. a, Rich mixture (lack of air) b, Weak mixture (too much air)

CONSTRUCTION FEATURES

Lambda sensors

Of the 'planar' type, two are mounted on the catalytic preconverters and two on the catalytic converters, and they inform the injection control unit about the combustion trend (stoichiometric ratio).The upstream sensors carry out the check on the mixture strength known as the 1st loop (upstream sensor closed loop).The sensors downstream of the catalyzer are used for the catalyzer diagnosis and for finely modulating the 1st loop control parameters.With this in mind, the adjustment of the second loop is designed to recover both production differences and those in the response of the upstream sensors which may occur as a result of ageing and pollution.This control is known as the 2nd loop (downstream sensor closed loop).The electronic control unit identifies the composition of the mixture (rich or lean) from the Lambda sensor output voltage.It adjusts the quantity of fuel injected to ensure the optimum composition of the mixture (? = 1) to create the ideal conditions for the treatment of the exhaust gases in the catalytic converter.If the mixture is too rich (? < 1), then the quantity of fuel should be reduced and, if the mixture is too lean (? > 1), then the quantity of fuel should be increased. a - Rich mixture (lack of air) b - Weak mixture (too much air) The Lambda sensor, placed in contact with the exhaust gases, generates an electrical signal whose voltage depends on the concentration of oxygen present in the gases.This voltage undergoes a sudden variation when the composition of the mixture moves away from the value ? = 1.The heating of the Lambda sensor is managed by the fuel injection control unit in relation to the exhaust gas temperature.This avoids thermal shocks on the ceramic casing due to the contact with condensed water, present in the exhaust gases when the engine is cold.The measuring cell and heater are built into the 'planar' (stratified) ceramic element, with the advantage of obtaining quick heating of the cell, thus permitting a closed loop' check (? = 1) within 10 seconds of engine start-up. 1 - Connecting cable 2 - Protective sleeve 3 - Planar sensor element 4 - Ceramic tube casing 5 - Sensor housing 6 - Ceramic seal 7 - Protective tube The operation of the Lambda sensor is based on the principle of an oxygen concentration cell with solid electrolyte.The surfaces of the measuring cell are coated with microporous layers of noble material. 1 - Exhaust gas 2 - Passage of reference air 3 - Heater 4 - Lambda sensor voltage

Catalytic converter

The three-way catalytic converter simultaneously reduces the three polluting gases present in the exhaust gases:

• unburnt hydrocarbons (HC);
• carbon monoxide (CO);
• nitrogen oxides (NOx).

Two types of chemical reactions take place in the converter:

• oxidation of the CO and HC, converted into carbon dioxide (CO2) and water (H2O)
• reduction of the NOx to convert them into nitrogen (N2).

The converter consists of a monolith, a metal mesh support for damping impacts and vibrations and an outer stainless steel housing resistant to high temperatures and atmospheric agents.The monolith consists of a honeycomb structure comprising a ceramic material coated with a very thin layer of catalytically active substances, platinum or rhodium. These accelerate the chemical decomposition of the harmful substances contained in the exhaust gases which, passing through the core cells at temperatures of over 300°-350° C, activate the catalyzers, thus starting the oxidoreduction reactions.To optimize the efficiency and duration of the catalyzer, a perforated plate metal cone improves the diffusion of the exhaust gases in the cells of the ceramic core. 1 - Ceramic monolith 2 - Metal support 3 - Outer casing 4 - Perforated plate metal cone

The noble metals contained in the catalytic converter, because of the high temperature, are chemically attacked if lead is present. For this reason the use of petrols containing lead should be avoided, otherwise the converter will be quickly and irreversibly damaged. Never use petrol containing lead, not even in an emergency or for a very short time.