POLLUTION FROM SI ENGINES & THEIR CONTROL
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Major Sources:

Dirunal Emissions

Take place from fuel tanks and carburetor float bowls
(in engines fitted with carburetors) of parked vehicles.

It draws in air at night as it cools down

Expels air and gasoline vapour as it heats up during the day.

These could be up to 50g per day on hot days.


Running Losses

Gasoline vapours are expelled from the tank (or float bowl)
when the car is driven and the fuel tank becomes hot.

This can be high if the ambient temperature is high.


CO Formation
 
- Primarily dependent on the equivalence ratio.
 
-Levels of CO observed are lower than the maximum values
measured within the combustion chamber

- but are significantly higher than equilibrium values
for the exhaust conditions

-The processes which govern CO exhaust levels are
kinetically controlled

- The rate of re-conversion from CO to CO2 is slower than
the rate of cooling.

- This explains why CO is formed even with
stoichiometric and lean mixtures.