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About Airflow Sensors

By David Pertuz

(sorta) related to your thread, would you mind giving me a quick explanation on the mass air sensor and it's real purpose? (apart from the obvious of course...) How does it figure in the overall scheme of things in regard to engine operation?

I'm not mike but I can answer your question, so here is a try:

Most EFI systems around today use air mass flow in determining the mixture under given operating conditions. Typically there is also throttle position, intake manifold pressure and/or temperature, exhaust gas oxygen (EGO), etc. Some systems (most Hondas, I think) use volume air flow sensors rather than mass air flow sensors. One can set up a good EFI system with neither, using throttle position (TPS), manifold absolute pressure (MAP), engine speed (RPM) and an oxy sensor (EGO). The advantage to a mass air flow sensor over a volume flow sensor is that it allows the ECU to compensate for changes in atmospheric temperature, barometric pressure, and humidity. There are a variety of other sensors that the ECU reads to regulate things like fast (cold) idle, turning up the idle when the AC is on, etc. The mass air flow (MAF) sensor signal and engine speed are the primary (maybe only) determinants of fuel injector pulse length.

Older EFI systems like Bosch L-Jet used mechanical flaps, which could only sense volume flow. The flap was deflected against a spring, and a potentiometer converted the deflection into a voltage ratio to determine pulse width. Newer systems (e.g. LH-Jet, Motronic, others) use either hot-wire mass air flow sensors or thin-film MAF sensors.

In a hot-wire sensor, intake air flows over a thin wire heated by a small electric current. The wire is kept at a constant temperature by the current. with increased air mass flow (more volume, or colder denser, air) past the sensor, convective heat transfer from the wire to the air is increased, and more current is required to regulate the temperature. The current requirement thus acts as an index. It is converted to a voltage signal which is then used with engine speed as the main (but not only) inputs determining fuel injection.

Thin-film sensors work in a similar way, though I can't say anything about them without looking it up.