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AFM vs. MAF vs. MAP

Air flow SENSORS - HOW they WORK and how they DIFFER from each other

Today we’ll be exploring in detail the three different types of devices that measure the air coming into the engine and relay this important data to the ECU so that the ECU can inject the appropriate amount of fuel in order to obtain a desired air fuel ratio.

So let’s start things off with the oldest type of air flow measuring component the air flow meter.

The vane air flow meter gets it’s name because inside it you can find a moving vane, and the vane air flow meter measures incoming air by relying on the drag force generated by the air. The air pushes against the vane and opens it. The more air pushes against it the more the vane will open. The vane is connected to a variable resistor or a potentiometer. In essence the poentiometer allows voltage to “enter” it and exit only at the terminal that is proportional to the angle of the vane, telling the engine’s ECU how much the vane is open based on which the ECU determines the amount of fuel to be injected.

Now let’s look at another relatively old method of measuring airflow, but one that doesn’t noticeably restrict airflow. This one is known as the Karman Vortex Air Flow Meter. And it relies on the property of all fluids, air included of course, to generate vortices when they encounter an obstacle of a specific shape,usually a triangular rod, which is called a “vortex generator” in this case. The frequency or the number of the vortices within a certain time-frame is proportional to the intake air velocity. Compared to the vane air flow meter the Karman vortex generator is much less common on cars. It can be found mostly on cars from the 90s, most notably the DSM trio, as well as some Lexus, BMW and a bunch of Mitsubishi vehicles among others.

We have two main types of mass air flow sensors, the hot wire type and the hot film MAF sensor. A mass air flow sensor or MAF has a wire that sits in the incoming air stream. Because the mass air flow sensor directly measures air mass it also responds it won’t get confused by an increase in air density due to altitude changes or changes in temperature. Denser air will contain more air molecules which will cool the wire more and the MAF will always respond accurately to temperature or altitude changes. This is why a MAF usually doesn’t need an additional temperature sensor to be accurate. Despite this many MAF sensors do in fact contain a temperature sensor which is required by some engine management systems and can also be used as a secondary check to verify the the operation of the mass air flow sensor.

A hot film sensor takes things further than the hot wire because it’s able to measure not just the actual air mass but also the direction and the pulsation or reversion of incoming air mass. The hot film MAF senses direction by having two air mass sensing elements next to each other. They work the same way as the single hot wire but because there’s two of them the integrated electrical circuit can see which element experiences changes in resistance first, which it means it can tell the direction of the incoming air stream. When it comes to the location of all of our air flow measuring devices so far they’re almost always found right after your air filter, but our next device is different. and this tiny little thing here is a MAP or a Manifold Absolute Pressure sensor. And it’s name already tells us where it is and what it does.

It’s located on the intake MANIFOLD and it senses air pressure in the intake manifold. Most MAP sensors measure pressure by relying on piezo-electricity, which is electrical charge that accumulates in certain materials when they are exposed to mechanical stress. Most modern MAP sensors actually use micro-machined silicon pressure sensors a.k.a. silicon chips. The changing air pressure inside the manifold will flex the silicon chip to different extent creating measurable changes in the current that accurately correspond to the changes in the air pressure.

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