Fiat Bravo Service Manual page 200

Bravo-Brava i f f i ^ Engine
Fuel system
10.
LAMBDA PROBE (NTK 0ZA112-A1)
This sensor measures exhaust gas oxygen content.
The sensor output signal is sent to the electronic control unit to adjust the air-fuel mixture in order to
maintain the ratio as close as possible to stoichiometric levels.
To obtain an optimum mixture, the quantity of fuel injected must be as close as possible to a theoretical
quantity that could be fully burnt for a given amount of air taken in by the engine.
In this case, Lambda factor (X) is said to be equal to 1;in fact:
INTAKE AIR QUANTITY
THEORETICAL QUANTITY OF AIR NECESSARY TO BURN ALL THE FUEL INJECTED
X= 1 Ideal mixture
CO levels are within legal limits
A. 5 = 1 Lean mixture
Excess air; CO levels tend to be low
X,«g 1 Rich mixture
Lack of air; CO levels tend to be high
NOTE While the coefficient X expresses an excess or lack of air supplied to the engine in relation to
the theoretical required level, the air-fuel mixture is a ratio between these two substances that
react chemically when combined. The stoichiometric proportions depend on fuel type used: this
ratio is 14.7-14.8 for present-day engines, which means they require 14.7 parts of air to burn 1
part of petrol fully.
The probe is fastened upstream of the converter.
It consists of a ceramic case (1) made up of a zirconium dioxide base covered by a light layer of plat­
inum. It is sealed at one end, enclosed in protective pipe (2), and housed in metal case (3) that provides
further protection and permits installation to the exhaust manifold. The outer part (B) of the ceramic
case is exposed to the exhaust gas flow while inner part (A) communicates with the outside air.
The probe works on the principle that when the temperature exceeds 300 °C, the ceramic material used
begins to conduct oxygen ions. Under these conditions, if the levels of oxygen at both ends (a and b) of
the probe are different, a voltage variation is set up between the two ends. This signal notifies the ECU
that the oxygen residues in the exhaust gas are not in proportions that will ensure lean burning of harm­
ful residues.
0.980
X 1.020
LEAN MIXTURES
0,7 0,8 0,9 1,0 1,1 1,2 1,3
AIR COEFFICIENT X
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