Mass air flow: Difference between revisions
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A mass air flow (or MAF) sensor placed just before the intake manifold measures the instantaneous rate of air entering the combustion chamber. This is done using a thin wire or film suspended in the air stream. |
A mass air flow (or MAF) sensor placed just before the intake manifold measures the instantaneous rate of air entering the combustion chamber. This is done using a thin wire or film suspended in the air stream. |
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===Hot wire MAF== |
===Hot wire MAF=== |
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The wire itself is a thermistor, which means that its resistance varies with its temperature. In addition, the more current flowing through the wire, the hotter it gets (similar to an electric heater). Since the MAF wire is a "positive k" thermistor, the hotter it gets, the higher its resitance. This sets an upper stable limit of current, since the higher the resistance of the wire, the less current flows through it. This "stable" point changes depending on how much air is flowing past the wire, providing cooling. The more air flowing past the wire, the more current the wire can allow through before it heats up enough to offset the current. Therefore, at any given time, the amount of air flowing through the MAF is proportional to the amount of current flowing through the wire in the sensor. |
The wire itself is a thermistor, which means that its resistance varies with its temperature. In addition, the more current flowing through the wire, the hotter it gets (similar to an electric heater). Since the MAF wire is a "positive k" thermistor, the hotter it gets, the higher its resitance. This sets an upper stable limit of current, since the higher the resistance of the wire, the less current flows through it. This "stable" point changes depending on how much air is flowing past the wire, providing cooling. The more air flowing past the wire, the more current the wire can allow through before it heats up enough to offset the current. Therefore, at any given time, the amount of air flowing through the MAF is proportional to the amount of current flowing through the wire in the sensor. |
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===Hot film MAF== |
===Hot film MAF=== |
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TBD |
TBD |
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Revision as of 00:33, 1 August 2006
General information
A mass air flow (or MAF) sensor placed just before the intake manifold measures the instantaneous rate of air entering the combustion chamber. This is done using a thin wire or film suspended in the air stream.
Hot wire MAF
The wire itself is a thermistor, which means that its resistance varies with its temperature. In addition, the more current flowing through the wire, the hotter it gets (similar to an electric heater). Since the MAF wire is a "positive k" thermistor, the hotter it gets, the higher its resitance. This sets an upper stable limit of current, since the higher the resistance of the wire, the less current flows through it. This "stable" point changes depending on how much air is flowing past the wire, providing cooling. The more air flowing past the wire, the more current the wire can allow through before it heats up enough to offset the current. Therefore, at any given time, the amount of air flowing through the MAF is proportional to the amount of current flowing through the wire in the sensor.
Hot film MAF
TBD
S4 specific information
The B5 S4's Bosch Motronic ME7.1 ECU uses a mass air flow based fuel injection system. During open loop (engine warm-up and wide open throttle) operation, the MAF tells the ECU how much fuel to deliver to keep the air fuel ratio at an optimal level (also known as stoichiometric). Note that during closed loop operation (idle and partial throttle), the O2 sensors are used to determine fueling, not the MAF.
B5 S4s came with two different types of MAF sensors - Hitachi and Bosch (part numbers?); both are film-type MAFs. Each has totally different characteristics and wiring. A-box cars (MY2000 to some of MY2001.0) came with Bosch MAFs. H-box (part way through MY2001 and later) cars came with Hitachi MAFs. To add to the confusion note that some MY2001.0 cars are A-box and some are H-box. M-box cars are generally considered MY2001.5.
Bosch vs. Hitachi pinout
| A-box/S8/RS4 aka Bosch |
H-box and M-box aka Hitachi | |||
|---|---|---|---|---|
| pin | color | wire | color | wire |
| 1 | blank/unused | none | violet/grey | +5V |
| 2 | green/yellow | +12V (ign/bat) | green/yellow | +12V (ign/bat) |
| 3 | black/green | ground | black/green | ground |
| 4 | violet/grey | +5V | green | signal |
| 5 | green | signal | blank/unused | none |
Temperature (IAT) effect on MAF
- how ecu compensates?
- hotwire MAFs dont need much compensation? hotter air == less dense air but less "cooling" of wire?
Trouble shooting/tuning
Logging
- max min values?
Bad MAF
- unplugging -> always closed loop (ECU uses O2 and MAP only)
How MAF housing affects readings
- larger housing -> smaller value
- ECU has fixed top end reading, so have to scale MAF if we want more HP (since more HP means more air will be passing through MAF)
- more air means we need more fuel - larger injectors, higher pressure FPR
- how to scale MAF housing diameter with injector size and fuel pressure.
MAF and fueling
- how lemmi fueling (pri/second/accell/decel)
- affects open loop fueling (MAF vs injector duty)
- affects closed loop fueling (LTFTs)
- see also lemmi doc.
MAF and BOV vs BPVs
- metering escaped air can lead to rich conditions and incorrect LTFTs