3360113 - 1080B EXHAUST EMISSION CONTROL SYSTEM

Lambda sensors

CONSTRUCTION specifications

The Lambda sensors are the "planar" type and one is fitted straight after the exhaust manifold but before the catalytic converter ("PRE'') whilst the second one is fitted straight after the catalytic converter ("POST'').The two Lambda sensors inform the engine management control unit of the combustion progress; the pre catalytic converter one has the task of analyzing the stoichiometric ratio whilst the post catalytic converter one checks the operation of the catalytic converter informing the engine management control unit which carries out any fine adjustments to the stoichiometric ratio.To produce an optimum mixture the quantity of air drawn in by the engine must be equal to the theoretical quantity required to burn all of the fuel injected.Therefore the Lambda factor (?) is the ratio between the quantity of intake air and the theoretical quantity of air and the ideal value is 1.

Then:

  • ? = 1 ideal mixture
  • ? > 1 lean mixture
  • ? < 1 rich mixture
a, Rich mixture (not enough air)b, Lean mixture (too much air)

Composition and operation

The Lambda sensors are in contact with exhaust gases and generate an electrical signal with the voltage dependent on the oxygen level in the exhaust gases.Even if the Lambda sensors are the same, working in different gas conditions, they will send the engine management control unit a different signal; the signal coming from the pre catalytic converter sensor is calculated to correct the mixture strength when it differs from the value ? ( 1.At the same time, the post sensor checks to what extent the catalytic converter is keeping down the harmful substances in the exhaust gases and notifies the control unit whether these gases are more or less pollutant and the control unit uses this parameter to ensure the optimum Lambda value = 1.With the engine cold, to prevent thermal shocks to the ceramic casing due to the formation of condensation, the engine management control unit heats the pre and post catalytic converter Lambda sensors so that they reach the same temperature as the exhaust gases.The heater and the measuring cell are built into the "planar" (stratified) ceramic element which favours the rapid heating of the cell thereby allowing the closed loop control extremely quickly after the engine has been started.
1, Connecting element2, Protective sleeve.3, Planar sensor element.4, Ceramic support pipe5, Sensor seat6, Ceramic seal7, Protective pipeThe Lambda sensor works on the principle of an oxygen concentration chamber with solid electrolyte.The measurement chamber surfaces are coated with microporous layers of noble material.
1, Exhaust gas2, Reference air passage3, Heater4, Reference voltage

Electrical Properties:

  • Heater power supply. 12V
  • Heater resistance. 0.5 - 1 kO

EXHAUST EMISSION CONTROL SYSTEM

Construction specifications

The exhaust emission control system includes devices designed to reduce atmospheric levels of harmful emissions:

  • exhaust emissions
  • vapour/gas emissions from the crankcase

Catalytic converter

Construction and composition specifications

The catalytic converter is used to reduce emissions of the three main pollutants:

  • HYDROCARBONS = HC
  • NITROGEN OXIDES = NoX
  • CARBON MONOXIDE = CO
To sum up, the catalytic converter consists of a metal support on which a substrate is deposited which, in turn, contains chemically active material, which has the function of setting off and facilitating two chemical reactions, oxidation to decrease the CO and HC and reduction to decrease the NoX.The active elements are usually noble materials and RHODIUM or PALLADIUM is used for the reduction reaction, whilst PLATINUM is used for the oxidation.In addition, there are other chemical compounds present in the substrate that have the basic function of storing and release the oxygen required for the chemical reactions.This feature of the catalytic converters is known as oxygen storage.

Operation

The oxygen storage catalyzer in the substrate contains a material with chemical compounds that have the function of storing and releasing the oxygen required by the chemical reactions.The oxygen storage takes place in lean mixture conditions, whilst in rich mixture conditions the oxygen required for oxidation is released.This function is a feature of oxygen storage catalyzers.The chemical reactions in an old or deteriorated catalyzer take place less efficiently with a consequent increase in pollutant emissions; in a new catalyzer, the oxygen storage decreases correctly and the post catalyzer sensor provides an almost constant output voltage because the catalyzer uses all the oxygen for its chemical reactions.When the catalyzer ages, the oxygen storage effect gradually decreases and therefore the post catalyzer sensor will see an ever decreasing amount of oxygen giving rise to an ever increasing output voltage but with the same frequency as the post catalyzer sensor.
The nobile metals in the catalytic converter suffer chemical attack if lead is present, partly due to the high temperature inside the converter. For this reason, the use of leaded fuel should be be avoided, otherwise the converter will be rapidly and irreversibly put out of service. Never use leaded fuel, not even for a short time in an emergency.