Hot-Film Air Flowmeter - Fiat 1998 Marea Service Manual

Engine
Fuel system
10.

HOT-FILM AIR FLOWMETER

This component measures the quantity of air drawn in by the engine. The electrical signal (proportional
to the air flow) which the flowmeter gives as an output is sent to the engine control unit. This is used to:
- calculate the quantity of fuel to be injected, so as to obtain the maximum fuel flow rate while main-
taining an acceptible level of smoke in the exhaust;
- check the quantity of gases recirculating through the EGR valve.
The check on the quantity of recirculating gases is based on the following equation:
Qam = quantity of theoretical air mapped in the control unit in a particular point of engine opera-
tion;
Qar = quantity of air which really passes through the flowmeter (and is drawn in by the engine) at the
same point of engine operation;
Qgr = the imbalance, compared with the theoretical value, of the quantity of
recirculating gases.
If the quantity of recirculating gases is correct for a particular point of engine operation, the quantity of
air drawn in by the engine must be the same as the mapped quantity. Thus Qgr must be zero. If Qam is
greater than Qar, this means that the quantity of air passing through the flowmeter is less than the theo-
retical quantity, i.e. the quantity of recirculating gases is higher than the specified value (Qgr > 0). In
this case the control system intervenes on the EGR valve to reduce the flow rate, so that the Qgr value is
cancelled.
The system behaves in the same way if Qar > Qam.
The "hot-film" air flowmeter.
The operating principle of these sensors is based on the heat exchange between a "hot" element and the
air flow enveloping it.
The heat exchanged is given by the electrical power supplied by the power supply to the hot element, to
keep its temperature constant.
8
Marea- Marea
Qam - Qar = Qgr
Weekend®
TD
'98 range
Publication no. 506.763/11