S07.52-3027-06
Task
Enhancing the efficiency of the catalytic converter
General
In emission control systems using lambda control and three-way catalytic converter, the mixture composition is adapted in line with the oxygen concentration in the exhaust, which is transmitted by the O2 sensor (R22) to the PEC control module (A13).
Design
1 Catalytic converter
A13 PEC control module
R22 O2 sensor (lambda sensor)
Y16-Y19 Fuel injectors
The pollutants are converted in the three-way catalytic converter by means of two chemical processes (oxidation and reduction). This necessitates the mixture composition constantly varying between "rich" (oxygen deficiency) and "lean" (oxgen excess) within in narrow bandspread around the stochiometric fuel-air ratio (λ = 1).
The stochiometric fuel-air ratio is the mass ratio of 1 kg fuel to 14.7kg air, which is theoretically required for complete combustion. The air index or the air ratio lambda [λ] indicates the extent to which the fuel-air mixture which is actually present, differs from that theoretically required.
Air ratio lambda (λ)
Air mass supplied
λ =
Theoretical air mass demand
Chemical processes in the three-way catalytic converter
Oxidation
CH + O2 CO2+H2O
CO + O2 CO2
Reduction
NOx + CO N2+CO2
Design/function
The O2 sensor (R22) reacts to the oxygen concentration in the exhaust and transmits an appropriate voltage signal to the PEC control module (A13), which thereupon extends or shortens the injection time of the injectors (Y16 - Y19). As a result, the mixture composition is altered. This process is constantly repeated (control loop).
Sequence with closed
control loop
injected quantity increased
rich mixture
O2 sensor voltage (V) >set value
control module produces leaner mixture
control module produces richer mixture
O2 sensor voltage (V) <set value
high O2 in exhaust
lean mixture
injected quantity reduced
low O2 in exhaust