939005839 - INTRODUCTION - EMISSION CONTROL SYSTEM
CONSTRUCTION SPECIFICATIONS
Lambda sensors
The two Lambda sensors upstream of the catalytic converters are the linear type whilst the downstream ones are the planar type and notify the engine management control unit of the combustion progress (stoichiometric ratio).The upstream sensors (? = 0.65) determine the mixture strength check (except during idling) known as the 1st closed loop (for the upstream sensors).The sensors downstream of the catalytic converter are used to for converter diagnostics and to fine-tune 1st loop control parameters.The second loop is therefore adaptive to make up for production discrepancies and slight drift that upstream sensor responses could experience due to ageing and contamination.This control is known as 2nd loop control (downstream sensor closed loop).The electronic control unit identifies mixture composition (lean or rich) from the lambda sensor output voltage.This adjusts injected fuel quantity to ensure optimum mixture composition (? = 1 to create ideal conditions for treatment of exhaust gas in the catalytic converter.If the mixture is too rich (? < 1) the quantity of fuel should be reduced and if the mixture is too lean (? > 1) the quantity of fuel should be increased.Planar Lambda sensors
a - Rich mixture (lack of air)b - Lean mixture (excess air)The Lambda sensors are in contact with exhaust gases and generate an electrical signal with voltage dependent on oxygen level in the gas.This voltage changes abruptly when mixture concentration deviates from a value of ? = 1.The injection control unit manages Lambda sensor heating in proportion with exhaust gas temperature.This avoids thermal shocks to the ceramic case due to contact with condensed water present in exhaust gas 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 (? = 1) control within 10 seconds of engine start-up.
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.A perforated steel cone improves the diffusion of the exhausts gases in the ceramic core cells to ensure the optimum efficiency and lifespan of the catalyzer.
1. Ceramic structure2. Metal support3. Outer casing4. Perforated steel cone
OIL VAPOUR RECIRCULATION SYSTEM (BLOW-BY)
The emission of oil vapours is controlled by means of an oil separator which collects the vapours coming from the crankcase and from the tappet cover; the condensed vapours return to the sump via a pipe whilst the uncondensed ones are sent to the intake air hose via a pipe.There is a regulation valve, on the inlet vapour delivery pipe, with a spring that in normal conditions acts on a diaphragm keeping it open, thereby allowing the flow of oil vapours coming from the oil separator.When the vacuum inside the pipe overcomes the calibration of spring, the actual vacuum moves the diaphragm thereby closing the duct from the oil vapour separator preventing the intake of oil from the sump.
EVAPORATION CONTROL SYSTEM
The vapours coming from the tank are sent to the canister (2), through the two float valves (1), where they are absorbed and stored by the active charcoal.
Canister
This is fitted to the left rear wheel arch and consists of an activated carbon filter element that absorbs fuel vapours from the separator.An opening in the canister allows the intake of air and is like a labyrinth to prevent the intake of water.The fuel vapour inlet and outlet pipes are thermowelded onto the canister and cannot therefore be dismantled.
1. From the multifunction valve2. To the solenoid valve3. Labyrinth opening
Fuel vapour solenoid valve
This is fitted on the lower part of the air chamber and is controlled by the injection control unit.The solenoid allows fuel vapours stored in the canister to flow through to the engine intake.If deactivated, the solenoid remains in open position. At key-ON, it closes ready for operation.Electromagnet (1) is excited and attracts plunger (2) that overcomes the load of spring pack (3) to close orifice (4) and prevent fuel vapours from passing through.
