Discussion
Just something that I've wondered for a while as I plan to ditch mine... Why do most manufacturers seem to use MAF or AFM sensors as input to the ECU rather than a MAP sensor? Talking to a friend who used to work on the development of the early Rover MEMS ECUs, he seems to think it was just a damn good sales pitch by Bosch to sell more kit.
I don't know what the manufacturers' reasons were, but it seems to me that sensing the mass air flow directly is fundamentally better than measuring manifold pressure and trying to guess what air flow that corresponds to. At low revs the relationship between manifold pressure and mass air flow is pretty tenuous because of the amount of exhaust gas reversal. Although you can determine the relationship and use this to set up a map, the fact that the relationship is weak means that your map will be more vulnerable to variations between engines and changes that cause the engine's characteristics to change over time. If you want to know what mass of air is going into the engine, there's really no substitute for measuring that directly. Mass air flow sensors aren't ideal because they don't really measure the true mass, they aren't perfect at measuring a pulsing air flow. Still, they're a lot closer than measuring manifold pressure or throttle angle, which are the only real alternatives.
As emissions controls get more stringent it is necessary to use devices with less inherent error to permit a better fuelling & timing calculation. You will find on the latest engines quite a number of sensors around an engine to determine not just airflow in, but also EGR flow and in some cases cylinder pressure. An airflow meter is part of the equation to help get power with emissions control consistently.
A MAP sensor for most older cars could give adequate control to make power, but meeting the latest emissions regulations would be difficult.
A MAP sensor for most older cars could give adequate control to make power, but meeting the latest emissions regulations would be difficult.
Mr2Mike said:
I'm not convinced that most cars use a mass airflow sensor, there are an awful lot that use MAP.
Of course pretty much all modern diesels use a MAF sensor, because a MAP sensor is fairly useless without a throttle (though some diesels are actualy throttled).
Have you looked at a lot of modern cars ? Id say that around 80-90% + use an airflow meter.Of course pretty much all modern diesels use a MAF sensor, because a MAP sensor is fairly useless without a throttle (though some diesels are actualy throttled).
Why would this not be convincing ? I can think of a handful that dont, but it would only be a handful.
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