Discussion
Hi Guys
I'm starting to try and educate myself on the mysteries of engine control, and have been re-reading various threads about the engine managemnt software, throttle pots and so on.
As far as I can see the lambda sensors, T pots, adaptives, etc all feed in to the ECU, and there is one of each for each bank. All seems logical to me so far.
So why is there only one air flow sensor?
I'm starting to try and educate myself on the mysteries of engine control, and have been re-reading various threads about the engine managemnt software, throttle pots and so on.
As far as I can see the lambda sensors, T pots, adaptives, etc all feed in to the ECU, and there is one of each for each bank. All seems logical to me so far.
So why is there only one air flow sensor?
There's no air flow sensor, that's sussed out from the throttle angle via the pots. There is an air pressure sensor, so you can trim the fuel as the air pressure/density and hence mass to be combusted changes with temperature, at altitude and so on.
Edited by Steve_T on Tuesday 24th August 15:06
Also, the adaptives don't feed into the ECU as such - ie they are not a separate input to the ECU. The adaptives are an internal table of correction values that the ECU maintains to account for discrepancies between the fuel map (which tells it how much fuel to deliver) and the readings of mixture strength from the lambda sensors. If the mixture appears too rich or too lean, an adjustment is calculated and that adjustment is stored in the adaptive table. The ECU tries that new value and looks at the lambdas again, and makes more corrections as necessary on a continuous basis. The ECU learns and refines its adaptives for each rev band when the lambdas are giving feedback, ie small to medium throttle openings. This helps the car keep itself in tune. When the throttle is open wider, the ECU ignores the lamdbas and adaptives and uses the map only.
Thanks, Rog. I vaguely understood that, but you've expressed it very clearly for me!
I think my question still stand though, in that understanding that the air pressure is the ECUs pre-emptive assessment of air, and that the lambdas provide a post bang assessment, why do we have a single air pressure, but two lambdas?
I think my question still stand though, in that understanding that the air pressure is the ECUs pre-emptive assessment of air, and that the lambdas provide a post bang assessment, why do we have a single air pressure, but two lambdas?
morebeanz said:
Thanks, Rog. I vaguely understood that, but you've expressed it very clearly for me!
I think my question still stand though, in that understanding that the air pressure is the ECUs pre-emptive assessment of air, and that the lambdas provide a post bang assessment, why do we have a single air pressure, but two lambdas?
Air pressure in the boxes must be assumed to be sufficiently similar that there's no need to measure it twice. This should be true if the banks are in balance. I think my question still stand though, in that understanding that the air pressure is the ECUs pre-emptive assessment of air, and that the lambdas provide a post bang assessment, why do we have a single air pressure, but two lambdas?
The barometric pressure is going to be the same at both air intakes, indeed anywhere within a few miles of the car! While the amount of air going into the engine is going to be (slightly) affected by both the barometric pressure and the air temperature (and therefore the air density) it is largely a function of the angle of throttle opening and rpm. The wider the throttle and the higher rpm, the greater the volume of air that flows into the engine in a given time - how dense the air is determines how much oxygen there is in that volume.
The lambda sensors are effectively reporting on the efficiency of combustion in each bank of cylinders, which may be quite different, for example if the throttles are not perfectly balanced.
The lambda feedback is only used to tweak the fuelling (adaptives), where the major inputs of throttle angle and rpm are used to set the base fuelling.
The lambda sensors are effectively reporting on the efficiency of combustion in each bank of cylinders, which may be quite different, for example if the throttles are not perfectly balanced.
The lambda feedback is only used to tweak the fuelling (adaptives), where the major inputs of throttle angle and rpm are used to set the base fuelling.
Not wanting to argue, but while the static pressure may be constant, it is entirely possible that you would get uneven air flow from the air intake through the pipes to the throttle bodies, particularly in a cross-wind on the mtorway or at speed.
This would change the air flow to each bank, even if barometric pressure were constant.
Hence my musing on seperate sensors...
This would change the air flow to each bank, even if barometric pressure were constant.
Hence my musing on seperate sensors...
The barometric pressure sensor is only reading the static pressure - not the air flow. Also bear in mind that the air boxes are fed from a common plenum which has an air filter between it and the open atmosphere. The engine is sucking air in so effectively the ECU knows the demand from the rpm and the aperture from the throttle position and therefore the volume/time. The barometric pressure and temperature are only relevant as a calculation of density i.e how much air there is in that volume.
Edited by Tanguero on Wednesday 25th August 09:10
Don't forget that the intakes are effectively joined together because they draw air from a common space behind the air filter. They have equal opportunity to draw air from that space. It doesn't matter much what's going on on the other side of the filter.
If the air filter becomes more restrictive, the pressure in both intake boxes would drop, and the ECU can compensate for this via the baro sensor (and adaptives).
If the air filter becomes more restrictive, the pressure in both intake boxes would drop, and the ECU can compensate for this via the baro sensor (and adaptives).
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