Pyrolysis

1. Support conveying air to hot wire
2. Platinum filament or hot wire
3. Resistance (constantan) for comparing hot wire temperature
4. Filter mesh
5. Supply terminal
6. Meter ECU in aluminium case

Direct measurement of the air mass taken in by the engine is carried out by a hot wire debimeter. Air taken in by the engine strikes a filament maintained constantly 100°C above intake air temperature by an electrical current, despite the cooling experienced by the filament when the intake air volume increases. The volume of air taken in by the engine can be established by the amount of this change. The hot wire current variation is proportional to intake air volume, density and speed. Barometric changes due to high altitude do not therefore affect intake air volume directly. Hot wire temperature and current changes are closely related because the hot wire forms part of a precision measuring circuit made up of a Wheatstone bridge and an electronic amplification system of which two components: hot wire (2) and constantan comparison resistance (3) are in contact with the mass of air taken into the engine, whereas the other parts of the circuit (resistances) are located in the top part (7) of the meter inside an aluminium container.

Pyrolysis

Whenever the engine stops after speeds in excess of 2400/min, filament (2) carries a current governed by the ECU for about 1 second that takes the temperature to 1000°C in order to burn off all the carbon deposits on the wire because otherwise these would alter the 100°C hot wire maintenance current and thus the exact air-fuel ratio calculated by the ECU.

If the hot wire is broken, the engine may continue to work (with very rich mixtures) using data stored in the ECU. This allows the vehicle to be driven to a repair workshop (RECOVERY function). Engines being recovered must not be allowed to cool because otherwise they will not be able to start again.