Fiat 1995 Brava, Service Manual

Page: 153 / 544

Engine
Bravo-Brava
@ti6v
Fuel system
10.
Stoichiometric ratio is defined and indicated by the symboloc st as follows
theoretical quantity of air required to burn all the fuel injected
a s t
=
quantity of fuel injected
Mixture concentration is defined and indicated by the Greek letterX as the ratio::
quantity of air taken in by engine
^ theoretical quantity of air required to burn all the fuel injected
Thus we easily find that a / a st = k.
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 tens of Hertz.
NOTE
1 Hz = 1 oscillation per second
Under conditions of:
- cut-off,
- throttle opening greater than 70%
- engine temperature less than 25 °C
the strategy is de-activated.
Self-adaptability
The control unit features a self-adaptive function able to memorise deviations between basic maps and
corrections imposed by the lambda probe that occur persistently during operation. Such deviations (due
to system and engine component ageing) are saved permanently so that system operation can be adapt-
ed to gradual changes in engine and components with regard to original specifications.
The strategy is de-activated while the carbon filter flushing solenoid is open. If the E C U is replaced, car-
ry out a road test to enable the engine to warm up and the E C U autoadaptive strategy to come into play
(particularly important when idling at a standstill).
4
ittiicdliui) nu bUti b/'U