Lambda Probe (Ntk 0Za334-A1) - Fiat Bravo Service Manual

Engine
Fuel system
10.
LAMBDA P R O B E (NTK 0ZA334-A1)
This is the sensor w h i c h measures the oxygen content in the exhaust gases.
The sensor's output signal is sent to the electronic control unit to adjust the air-petrol mixture, in order to
maintain the mixture's stoichiometric ratio as close as possible to the theoretical value.
To obtain an optimum mixture, the quantity of fuel injected must be as close as possible to the theoreti-
cal quantity which w o u l d be required to be fully burnt in relation to the quantity of air drawn in by the
engine.
In this case the Lambda (X) factor is 1; in fact:
X=
QUANTITY OF THEORETICAL AIR REQUIRED TO BURN ALL THE INJECTED FUEL
VOLTAGE
m V
0.980
1000
800 -|
RICH MIXTURES
600
400¬
200
— i 1 1
0,7 0,8 0,9
NOTE While the coefficient X expresses the excess or shortfall of air supplied to the engine in relation
to the value theoretically required, the air-petrol mixture is a ratio between these two
stances which, combined together, react chemically. For today's engines, this means that they
require 14.7-14.8 parts of air to burn 1 part petrol.
The probe is mounted upstream of the catalytic converter.
This consists of a ceramic casing (1) based on zirconium dioxide, covered w i t h a thin layer of platinum
closed at one end, inserted in a protective pipe (2) and housed in a metal casing ( 3 ) , which provides fur-
ther protection and permits assembly onto the exhaust manifold. The outer part (B) of the ceramic is ex-
posed to the exhaust gas current, while the inner part (A) communicates w i t h the ambient air.
The operation of the probe is based on the fact that at temperatures exceeding 300°C, the ceramic mate-
rial becomes a conductor of oxygen ions. Under these conditions, if the quantitiy of oxygen on the t w o
sides (A and B) of the probe is in different percentages, a voltage variation between the t w o ends is gen-
erated, w h i c h is an indicator of the difference in quantity of oxygen in the t w o environments (air side
and exhaust gas side).
48
QUANTITY OF AIR DRAWN IN
1.020
W E A K
MIXTURES
t
*-r
1 1
T
1,0 1,1 1,2 1,3
A I R COEFFICIENT X
Bravo-Brava ^
X = 1 Ideal mixture
The CO is within legal limits
1 Weak mixture
Excess air; the CO tends to be low
X ^ . 1 Rich mixture
Lack of air; the CO tends to be high
16v
'98 range
sub-
Publication no. 506.670/14