Fiat 1996 Brava Service Manual page 206

Engine Bravo-Btava ty
TD
Fuel system
10.
Principle of operation
Fuel taken up from tank (11) and filtered by filter (12), flows to feed pump (7) which increases fuel
pressure.
Under the effect of the feed pump pressure regulator valve (2), the pressure increases in more or less lin­
ear fashion as engine rpm rises.
Valve (2) also by-passes the feed pump (7) during priming.
Fuel flows from the feed pump to metering device (6), which calculates the amount of fuel to be sent to
pumping element (4) and thus the effective output of the injectors.
The position of metering device (6) is controlled by speed regulator (13) on the basis of accelerator ped­
al position.
The pumping element consists of two opposing plungers which slide radially in the rotor and are operat­
ed by the profile of a ring with cams via rollers and sliding pads.
The plungers lack return springs because they are pressed outward by centrifugal force and the pressure
of the fuel entering during the supply stage.
The rollers which drive the plungers come into contact with the cam profile at different points according
to the length of travel during the fuel supply stage.
This travel is dependent upon the amount of fuel entering the pumping chamber, regulated by the me­
tering device, and upon plunger expansion time determined by pump rotating speed.
During the injection stage, the plunger control rollers come into contact with the cams and the plungers
are pushed inward to raise the pressure and bring about injection.
At the end of the injection stage, the plungers are pushed outward to lower the pressure. Fuel delivery
stops and valve (5) seals off the outlet pipe.
Valve (5) not only closes the delivery pipe but also instantaneously lowers the pressure in the pipe lead­
ing to injector (10). This quickly shuts off injection to prevent the injector dripping.
The maximum quantity of fuel which may be fed into the pumping element (4) during each cycle is de­
termined by a maximum flow regulator.
Pressure inside pumping element (4) is maintained at a constant level by pressurisation valve (9).
Turbocharging control valve (1) increases the fuel flow. Increased turbocharging pressure compresses a
membrane located inside the valve to move a piston which transfers feed pressure to the sliding pad ac­
tuator located inside pumping element (4). The sliding pads move to alter plunger travel and thus also
the quantity of fuel injected.
Solenoid (8), controlled by a electronic control unit, alters the pump delivery advance to bring about an
increase when engine load exceeds a given threshold.
The engine stop solenoid (3) is protected by a tamper-proof cover.
During engine operation, the solenoid remains active so that fuel may flow from feed pump (7) to the in­
take duct.
When the key is turned to switch off the engine, the solenoid is deactivated to shut off the fuel flow. The
metering device feed chamber is no longer supplied with fuel and the engine stops.
6
Publication no. 506.670/06