Hot-Wire Air Mass Sensor
A thin platinum wire is suspended inside the metering pipe. The inside metering pipe has 2 halves which hold a wire retaining ring, fixed resistance, and a compensating resistance. On top of the sensor housing is a hybrid electronic switching unit connected directly to LH control unit. A protective screen is mounted on either end of sensor housing to protect the hot-wire element. The protective screen also eliminates air turbulence and allows air to flow evenly around hot-wire.
The hot-wire compensating resistance wire and fixed resistance are combined into a bridge circuit. The hot-wire is a temperature dependent resistor, which increases its resistance with increasing temperature. See Fig 1 .
During engine operation, the hot-wire receives as much heating current as necessary to keep its temperature approximately 356°F (180°C) more than ambient temperature. This temperature is constantly maintained. Ambient temperature is detected by the compensating resistance wire located in front of the hot-wire and transmitted to the hybrid electronic switching unit.
If air mass increases, the hot-wire is cooled and its resistance increases. To maintain the constant hot-wire temperature more than ambient temperature, its heating current is immediately increased. The cooling and heating of the hot-wire happens in a few milliseconds. Since the heating current flows through a fixed resistor, current change due to air mass change can be detected as a voltage drop across the fixed resistor. This voltage drop serves as an input to LH control unit to detect intake air mass.
Different intake air temperature values are equalized by the compensating resistance wire so air mass signal remains unaffected. The hot-wire air mass sensor does not use moving parts and creates only a small airflow resistance in air intake.
If the hot-wire air mass sensor fails, fuel injection time is calculated from the absolute intake manifold pressure. Pressure conditions in intake manifold are recorded by pressure sensor in EZL/AKR ignition control unit. The pressure values are transmitted to the LH control unit through the Controller Area Network (CAN).