How does the MAF sensor know the difference between clean air and exhaust gasses?
How does the MAF sensor know the difference between clean air and exhaust gasses?
Alibro
Well-Known Member
Me too but nurse hasn't given me my medication yet.
I never professed to understand how the MAF in a diesel works so I'm watching this with genuine interest.
Recently Joe was talking about a simple home brew MAF amp, and produced a circuit diagram for it but if all the MAF does is control EGR gasses then what is the point of boosting the signal from it. With an EGR bypass it wouldn't do anything. I'm pretty sure the Ron box doesn't need a working EGR and I believe part of what it does is connected to the MAF.
Now I'm even more confused.
NURSE
NUUUUURSE
Diesel Do
Well-Known Member
Gaj1612
Active Member
with out getting involved in how a maf works (dont really care all that much) i unplugged mine a few weeks ago and there was no difference, other members on here suggested a new maf, so i changed it. made a huge difference.
this is the one i bought...
http://www.ebay.co.uk/itm/271154766925?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT
this is the one i bought...
http://www.ebay.co.uk/itm/271154766925?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT
GrumpyGel
Well-Known Member
From Rave (L Series text)....
Mass Air Flow (MAF) sensor
The MAF sensor is located in the air cleaner outlet pipe. The mass air flow is determined from the cooling effect of inlet air flowing over a Hot Film sensor. This signal is fed back to the ECM where it is used to monitor the recirculation of exhaust gases. An increase in the exhaust gas recirculation reduces the amount of intake air being drawn into the system, thereby reducing the cooling effect of the Hot Film resistor.
Manifold Absolute Pressure (MAP) sensor:
The MAP sensor is located on the bulkhead and is connected, via a pressure tube, to the outlet side of the turbocharger.
The MAP sensor is a strain gauge type sensor and monitors the intake air pressure. The sensor is connected electrically to the ECM.
The sensor comprises a plastic body containing a pressure detection chamber. The pressure detection chamber consists of four individual sensors made from glass panels with Silicon diaphragms. The Silicon diaphragms contain resistors which are bonded to the glass panels forming an enclosed vacuum.
When the intake air pressure changes, pressure in the detection chamber causes the diaphragms to deflect. The deflection of the diaphragms alters the length of each resistor, changing their resistance. This change in resistance is converted into an analogue output signal of between 0V and 5V within the pressure sensor. The ECM outputs a reference signal of 5V to the MAP sensor. The ECM compares the output from the MAP sensor to the reference signal and converts the difference into a pressure value.
Manifold absolute pressure, when used in conjunction with the signal from the IAT sensor and the CKP sensor, allows the ECM to accurately calculate the volume of oxygen in the air and adjust the quantity of fuel being injected to achieve optimum fuelling of the engine.
If the MAP sensor fails, the malfunction indicator lamp on the instrument panel is illuminated. The ECM uses a substitute value of 45.5 kPa (6.6 lbf/in 2 ) which limits the fuel quantity to 21 mg/stroke resulting in a reduction in engine performance.
Mass Air Flow (MAF) sensor
The MAF sensor is located in the air cleaner outlet pipe. The mass air flow is determined from the cooling effect of inlet air flowing over a Hot Film sensor. This signal is fed back to the ECM where it is used to monitor the recirculation of exhaust gases. An increase in the exhaust gas recirculation reduces the amount of intake air being drawn into the system, thereby reducing the cooling effect of the Hot Film resistor.
Manifold Absolute Pressure (MAP) sensor:
The MAP sensor is located on the bulkhead and is connected, via a pressure tube, to the outlet side of the turbocharger.
The MAP sensor is a strain gauge type sensor and monitors the intake air pressure. The sensor is connected electrically to the ECM.
The sensor comprises a plastic body containing a pressure detection chamber. The pressure detection chamber consists of four individual sensors made from glass panels with Silicon diaphragms. The Silicon diaphragms contain resistors which are bonded to the glass panels forming an enclosed vacuum.
When the intake air pressure changes, pressure in the detection chamber causes the diaphragms to deflect. The deflection of the diaphragms alters the length of each resistor, changing their resistance. This change in resistance is converted into an analogue output signal of between 0V and 5V within the pressure sensor. The ECM outputs a reference signal of 5V to the MAP sensor. The ECM compares the output from the MAP sensor to the reference signal and converts the difference into a pressure value.
Manifold absolute pressure, when used in conjunction with the signal from the IAT sensor and the CKP sensor, allows the ECM to accurately calculate the volume of oxygen in the air and adjust the quantity of fuel being injected to achieve optimum fuelling of the engine.
If the MAP sensor fails, the malfunction indicator lamp on the instrument panel is illuminated. The ECM uses a substitute value of 45.5 kPa (6.6 lbf/in 2 ) which limits the fuel quantity to 21 mg/stroke resulting in a reduction in engine performance.
But it only measures the air flow and density of the air passing over it, this is taken directly from the air filter. I don't see how operation of the EGR valve could affect this.
I have no doubt the MAF is used as a calculation within the ECU to determine EGR operation but it is absolutely not used solely to monitor the amount of exhaust gasses being introduced.
Alibro
Well-Known Member
And another thing. If all the MAF does is control the EGR valve how will that effect fuel economy? Will controlling the EGR change the pressure in the manifold and therefore have an effect on fueling? I've heard it said before that a dodgy MAF causes the car to be smoky and use more fuel. But if that is true then how come it still has an effect on fuel economy in a car with an EGR bypass?
BTW Wammers I'm not saying your wrong cause I don't know.
I'm just throwing out questions cause like I said earlier, I'm fick and confused.
Edit, Sorry Grumpy I had this written before I saw your post.
BTW Wammers I'm not saying your wrong cause I don't know.
I'm just throwing out questions cause like I said earlier, I'm fick and confused.
Edit, Sorry Grumpy I had this written before I saw your post.
Diesel Do
Well-Known Member
Very early systems were
The engine takes exhaust gas in preference to air when the valve opens as its forced in so the ecu sees a drop in air flow
On an L series not a TD4. Taken from tuning diesels website:
Q1: WHICH ROVERS ARE AFFECTED BY MAF FAULTS?
A: All ROVER 25, 45, 75, MG ZR, ZS ZT, & Freelander TD4 DIESELS. (Not to mention most turbo diesels from 1999/2000)
But NOT the 200/400/600 Sdi/Gsdi or SD. On these the maf is ONLY used to control the exhaust gas recirculation system. It is not used to modify the fuelling.
That is it's primary function.
Diesel Do
Well-Known Member
No it isn't. It used to be. You need to keep up with the times
The MAF sensor can influence fuel, but only in the sense that if a throttle request is made, were the fuel would exceed the available combustible air as measured by the MAF and relayed to the ECU. The ECU would not inject that fuel to go to the throttle demand until there was enough combustible air for clean burn.
GrumpyGel
Well-Known Member
OK from Rave TD4....
Mass Air Flow/ Inlet Air Temperature (MAF/ IAT) Sensor
The MAF/ IAT sensor is located on the engine intake air manifold, it combines the two functions into one unit.
The MAF sensor works on the hot film principle. The MAF sensor has two sensing elements contained within a film. One element is at ambient temperature e.g. 25 °C (77 °F) while the other is heated to 200 °C (392 °F) above this temperature e.g. 225 °C (437 °F). As air passes through the MAF sensor it has a cooling effect on the film. The current required to maintain the 200 °C (392 °F) differential provides a precise, although non-linear, signal of the air drawn into the engine. The MAF sensor output is an analogue voltage proportional to the mass of the incoming air. The ECM utilises this data, together with information from the other sensors and the fuelling maps, to determine the correct fuel quantity to be injected into the cylinders. It is also used as a feedback signal for the EGR system.
So according the LR, looks like MAF is used as an input to fuelling levels on TD4.
Mass Air Flow/ Inlet Air Temperature (MAF/ IAT) Sensor
The MAF/ IAT sensor is located on the engine intake air manifold, it combines the two functions into one unit.
The MAF sensor works on the hot film principle. The MAF sensor has two sensing elements contained within a film. One element is at ambient temperature e.g. 25 °C (77 °F) while the other is heated to 200 °C (392 °F) above this temperature e.g. 225 °C (437 °F). As air passes through the MAF sensor it has a cooling effect on the film. The current required to maintain the 200 °C (392 °F) differential provides a precise, although non-linear, signal of the air drawn into the engine. The MAF sensor output is an analogue voltage proportional to the mass of the incoming air. The ECM utilises this data, together with information from the other sensors and the fuelling maps, to determine the correct fuel quantity to be injected into the cylinders. It is also used as a feedback signal for the EGR system.
So according the LR, looks like MAF is used as an input to fuelling levels on TD4.
Back pedal
More refined answer easier for those who don't understand to take in. .
GrumpyGel
Well-Known Member
But exactly the same could be said of any sensor. At the end of the day, it is controlling the amount of fuel injected.
