Engine exhaust assembly diagram

ENGINE EXHAUST ASSEMBLY DIAGRAM

1. Exhaust manifold
2. Lambda sensor
3. Catalytic silencer
4. Silencers
5. Presa CO

The control of the mixture strength, which is the closed-loop type, is activated by the Lambda sensor which measures the oxygen content of the exhaust gases upstream of the catalytic silencer.

The Lambda sensor measurements allow the electronic control unit to continuously correct the quantity of petrol injected keeping the air/fuel ratio constant.

This ensures the control of the harmful exhaust emissions, a job completed by the three way catalytic converter (catalytic silencer).

The efficient operation of the catalytic silencer and consequently containing the toxicity of the exhaust gases depends on the air/fuel ratio which the engine receives.

The three-way catalytic converter makes it possible to simultaneously keep down the levels of the three pollutant gases present in the exhaust gases: unburnt hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx).

Two types of chemical reaction take place inside the converter:

• oxidation of the CO and the HC, converted into carbon dioxide (CO₂) and water (H₂O);
• reduction of the NOx, converted into Nitrogen (N₂).

The following would put the catalytic converter out of use and make it irreparable:

• the presence of lead in the petrol which lowers the degree of conversion to such levels that the presence of the catalytic converter in the system is useless;
• the presence of unburnt petrol in the converter: in effect, a flow of petrol for 30s in an ambient of 800 °C (temperature inside the silencer) is sufficient to cause the catalyzer to melt and break.

It is vital for the injection/ignitin system to be in perfect working order, therefore the spark plug leads should never be disconnected with the engine running and, the catalytic converter should be replaced with an equivalent section of pipe in the case of tests.