Fiat 1996 Brava Service Manual page 385

Engine Bravo-Brava ^
TD
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Fuel system
10.
Operating principle
The fuel, taken from the tank (11) and filtered by the filter (12), reaches the transfer pump (7) which has
the task of further increasing the pressure of the fuel.
Through the effect of the transfer pump pressure regulator valve (2), this pressure increases in a fairly
linear manner as the engine speed increases.
The valve (2) also has the task of shortcircuiting the transfer pump (7) during priming.
From the transfer pump the fuel reaches the meter (6) which determines the quantity of fuel to be sent to
the pumping element (4) and consequently the capacity for the injectors.
The position of the meter (6) is controlled by the speed regulator (13) and depends on the position of
the accelerator pedal.
The pumping element comprises two small opposing pistons which slide radially in the rotor and are op­
erated, by means of rollers and sliding plates, by the profile of a cam seal.
The pistons do not have return springs because they are thrust outwards by centrifugal force and the
pressure of the fuel entering the supply stage.
The rollers which operate the pistons come into contact with the cam profile at different points accord­
ing to the greater or lesser stroke during the supply stage.
This stroke depends on the quantity of fuel entering the pumping chamber, regulated by the meter and
the expansion time of the pumping pistons dictated by the rotation speed of the pump.
During the injection stage the piston control rollers come into contact with the cam seal cams; the pis­
tons themselves are thrust inwards causing an increase in pressure and consequently injection.
At the end of the injection stage, the pistons are thrust outwards creating a decrease in pressure, the fuel
is no longer supplied and the valve (5) closes the supply pipe.
In addition to closing the supply pipe, the valve (5) has the function of instantly lowering the pressure in
the pipe which is in contact with the injector (10) and therefore cuts off the injection promptly to pre­
vent the injector from dripping.
The maximum quantity of fuel which can be introduced into the pumping element (4) each cycle is de­
termined by a maximum flow rate regulator.
The pressure inside the pumping element (4) is kept at a constant value by the pressure valve (9).
The solenoid valve (8), operated by the electronic control unit, makes it possible to alter the advance for
the start of the pump delivery increasing it when the engine load exceeds a given level.
The engine cut out solenoid valve (electrostop) (3) is suitably protected by a special anti-tamper cover.
During the operation of the engine, the solenoid valve remains energized allowing the flow of fuel from
the transfer pump (7) to the supply duct.
When the key is turned to switch off the engine, the solenoid valve is de-energized cutting off the flow
of fuel; the meter supply chamber is no longer supplied and the engine cuts out.
6
IX-96 - Cancels and replaces
Print no. 506.670/08