Fiat 1998 Marea Weekend, Service Manual

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Engine
Fuel system
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Marea-Marea Weekend®^
1998 range
A stoichiometric ratio is achieved by using a hot lambda probe. This probe continually monitors the
amount of oxygen in the exhaust gas and then informs the ECU. This unit in turn uses mapped data to
correct the fuel air mixture on-line if proportions are no longer stoichiometric.
Idle speed, exhaust gas CO level and throttle valve sensor position cannot adjusted on this system.
Control of mixture concentration (retro-active control)
NOTE Mixture ratio is defined and indicated by the Greek letter oc (alpha) as follows:
quantity of air taken in by engine
a =
quantity of fuel injected
The following ratio is said to be stoichiometric and indicated by ast:
theoretical quantity of air required to burn all the fuel injected
ocst=
quantity of fuel injected
The following ratio defines mixture concentration and is indicated by the Greek letter X (lambda):
_ quantity of air taken in by engine
theoretical quantity of air required to burn all the fuel injected
Thus we easily find that
a/ast
-X.
Stoichiometric ratio depends on fuel type. For unleaded fuels in current use, this figure is 14.7 - 14.8,
which corresponds to lambda = 1. (a ratio of 14.8:1 means that 14.8 parts of air are required to burn 1
part of fuel).
The mixture is termed rich (or heavy) when the quantity if air is less than the stoichiometric level. In this
case, lambda equals < 1;
the mixture is termed lean (or light) when the quantity of air exceeds the stoichiometric level. In this
case, lambda equals > 1 ;
The strategy is designed to correct base pulse constants so that mixture concentration oscillates contin-
ually and at a high rate between 0.98 and 1.02.
This oscillation rate varies according to engine load and speed. It is in the order of a few Hertz (0.5 - 4
Hertz).
NOTE
1 Hz - 1 oscillation per second
Under conditions of:
- cut-off,
- throttle opening about 60% (lower at low speeds)
- engine temperature lower than 25 °C
the strategy is de-activated.
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Publication no. 506.763/09