628813 - Operation

LAMBDA SENSORS

These are planar type and located upstream and downstream of the catalytic converter.The upstream sensor determines mixture control, known as the first loop.The control unit controls and manages injection so that the air/fuel ratio is always sufficiently close to a stoichiometric level to maximise catalytic converter efficiency. The mixture is enriched to ensure maximum performance under conditions of full power and high use.The downstream sensor diagnoses the conversion efficiency of the converter/s.Then:

  • lambda = 1 ideal mixture.
  • lambda %gt; 1 lean mixture.
  • lambda %lt; 1 enriched mixture.
When it comes into contact with the exhaust gases, the lambda sensor generates an electrical signal with a voltage value dependent upon oxygen levels present in the gas. This voltages alters abruptly when mixture composition deviates from lambda = 1.The injection control unit manages lambda sensor heating in proportion with exhaust gas temperature.This prevents thermal shocks to the ceramic casing due to contact with condensed water present in the exhaust gases when the engine is cold.The measurement chamber and heater are built into a planar (laminated) ceramic element that offers the benefit of fast chamber heating. This allows closed loop (lambda = 1) control within 10 second of engine start-up.
The lambda sensor works on the principle of an oxygen concentration chamber with solid electrolyte. The measuring chamber surfaces are coated with microporous layers of noble metal.
Specifications:

  • Power supply: 12V
  • Internal resistance: 0.5 %divide; 1 kOhm.

CATALYTIC CONVERTER

The three-way catalytic converter makes it possible to keep down the levels of the three pollutant gases in the exhaust gases at the same time:

  • unburnt hydrocarbons (HC);
  • carbon monoxide (CO);
  • nitrogen oxides (NOx).
Two types of chemical reaction take place inside the converter:

  • oxidation of CO and HC to carbon dioxide (CO2) and water (H2O);
  • reduction of Nox to nitrogen (N2).
The converter consists of a structure, a metal gauze support to dampen impacts and vibrations and a stainless steel outer casing that is resistant to high temperatures and atmospheric agents. The honeycomb structure is made from a ceramic material covered in an extermely thin layer of catalytically active substances, platinum or rhodium, which accelerated the chemical decomposition of the harmful substances contained in the exhaust gases which, when passing through the core cells at temperatures above 300° ÷ 350°C, activate the catalyzers setting off the oxidation/reduction reactions.
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, even for a short time in an emergency.