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Why do modern PCMs prefer to use pressure sensors to estimate the air charge going into an engine instead of air flow meters. Eg they typically use a MAP sensor and complex software to estimate air charge based on current cam positions when a simple MAF would avoid the need for any software to estimate this.

With infinitely variable exhaust and intake cams these airflow models are becoming extremely complex as your air mass depends on intake cam, exhaust cam (dynamic), cylinder volume*stroke (static), intake runner length (sometimes dynamic), humidity (dynamic), temperature (dynamic), pressure (dynamic). With all these variables you end up with a 4-6 dimensional lookup table. With a MAF you have a simple volts vs airmass lookup table, so much simpler and less sensors required.

MAF also allows for modifications without the airflow model being modified.

What is the advantage to the OEM/manafacturer by doing this?

  • Why do you think a MAF is simple? What does it measure? – Solar Mike Apr 2 '18 at 21:56
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    BTW a normally aspirated engine may use one or the other or sometimes both, while a forced induction engine, either turbo or supercharged, will normally have both. – Solar Mike Apr 2 '18 at 22:03
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    Welcome to Motor Vehicle Maintenance & Repair! – Pᴀᴜʟsᴛᴇʀ2 Apr 2 '18 at 22:19
  • Ford went to speed density(map) on their ecoboost line, and of course, Ford Racing offers upgraded tunes while still maintaining the factory warranty. So you can see the business advantage since it is harder for the average person to modify it without programming/tuning. As for the technical reasons, I thought this article explains it well enough - import-car.com/speed-density-engine-management-systems/2 – Milison Apr 3 '18 at 0:30
  • @Solar Mike a MAF is simple because it measures a signal that is proportional to air flow (hence air mass). This takes into account changes in volumetric efficiency (eg variable cams), humidity, altitude etc. A MAP sensors only measure pressure and temp, this means you need a volumetric efficiency map to calculate air mass. If you have variable cams this means you end up with a 4 dimensional lookup table which is complex to generate. The software behind MAP is far more complex and less forgiving to modifications. – rolls Apr 3 '18 at 3:38
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In the turbo applications I am familiar with, both a wire or film (better) type MAF is used, in conjunction with a MAP, IAT, ECT, etc.

Specifically on turbosupercharged Subarus, there's knowledge to be gained by also knowing intake pressure.

Ultimately, you are trying to reach the best AFR. Things change a bit when supercharging, whether mechanical or turbo.

Things also change with humidity - a hot wire gets cooled more with higher humidity, which doesn't represent denser air. Hot film type MAFs compensate for this somewhat. But you would still like to know manifold pressure after a turbo.

I certainly agree it's more complicated: With ROMRAIDER, I have seen there are dozens and dozens of "modifier" tables in a Subaru tune that ultimately affect injection and timing on top of the MAF table. Tuning is complex with so many vectors and map layers, it's difficult to get your head around.

However, I would have to think that this added complexity serves some benefit, especially when controlling engine operation under high levels of boost. While I know there are sophisticated speed-density systems out there, cars don't run on open loop anymore (at least not for long) and the more sensor information available, the better.

I think of it this way (it's what I tell my students): Air "flow" tells you one thing (like the vane meters) MAF tells you a bit more, and MAF+MAP tells you almost everything. It's just a simulation; a device to approximate.

In fact, the wire or film assumes a laminar flow in the rest of the tube, which may not be the case - especially if you put one of those "trick" $99 cold air intakes on. The power gains you may or may not see may be due to bends in the plumbing causing the sensor area to be in a less-dense part of the total flow... so the ECU is tricked into running lean. (See? "more power" for a mere $99 . . .)

Just like an O2 sensor doesn't sense hydrocarbons, A MAF doesn't count oxygen molecules. It's an important point. All you really care about is how many free (not part of water) oxygen molecules are getting into the intake - not what temperature they are, the speed they are moving, how much the other gasses weigh, or what the humidity is.

If you could make a sensor that would accurately tell how many oxygen molecules are entering the intake (and maybe what temperature they are), real-time, You could eliminate a lot of other sensors and complexity. A/F ratio would be a simple real-time division: mL per moles O2.

There is the empirical observation that a MAP sensor costs about $50, and a modern MAF about $500. A speed-density based system is cheaper to produce (in quantity) after the fleet of tuners gets their job done. But I think it hinges on the flexibility of the engine, providing sufficient power and economy while adhering to emissions rules. Higher performance applications and supercharging may suggest more complexity is needed.

And as Bart commented, it's likely the engine is mapped and calibrated with highly sophisticated mass-airflow instruments gathering real-time data, and then your complex 50-layer maps can be overlayed/reduced to a simple speed-density-load map with input only from the production MAP sensor.

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    Was unaware the cost difference was that great. It would certainly be cheaper for mass produced cars then. – rolls Apr 3 '18 at 8:25
  • Those are "replacement" costs, but the manufacturing cost is probably even a much greater difference. A modern MAF is a very complex beast, usually with other integrated sensors (like IAT), a small dedicated computer/controller power drivers (for heating in a controlled way) and a whole bunch of firmware embedded to make the response appear "linear". I'm certain you thermo experts can appreciate how it isn't linear (wire or film cooling against mass/temperature/velocity) and there's a lot more going on - beyond what I dream of understanding. – SteveRacer Apr 3 '18 at 21:46
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    As an aside to the complexity, here's a great example of me flexing the wrong muscle: mechanics.stackexchange.com/questions/30251/… There's part of this one that still haunts me. Still, it's obvious that Bosch felt a need to sense air flow direction - something that no MAP I'm aware of is capable.of. – SteveRacer Apr 3 '18 at 21:52
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    There is actually a couple of good reasons to detect direction. I just added a comment. – rolls Apr 3 '18 at 23:06
  • @rolls I'm not sure I agree with the "big cam" part (who cares; at idle the information has no value unless you are using it to slightly open a bypass or a throttle motor) ... BUT it exactly proves my point and ties it back to the original question: No MAP sensor can do that, at least any that I've ever seen. – SteveRacer Apr 7 '18 at 3:34

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