Thread: Calculating MPG from VSS and MAF from OBD2

Volumetric efficiency varies with RPM, as noted by an earlier post. Its not just a inversely proportional relationship, as there are sympathetic harmonic effects in the intake and exhaust which can and do apply.

I have gotten the impression that the PCM contains a table which has the VE as function of RPM. But I have never come across a means for accessing it.

Regardless, I'm wondering if the instantaneous VE is reflected in the intake Manifold pressure (MAP). For example, let say @ 1000 RPM, the VE is 70% but drops to 65% at 2000 RPM. Is the MAP value different than it would be if the VE was a constant?
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Given that the previously defined equation for calculating MPG based on MAP, is it enough to make the VE a constant at, in this case, 70% and then rely on the MAP to factor in VE changes with RPM ?

So in a MAP based calculation:

pseudoMAF= (IMAP/120)*(VE/100)*(ED)*(MM)/(R)

Therefore, if VE increases,.. pseudoMAF increases?

I thought the engine sucks less fuel if it has higher volumetric efficiency?

Not quite.

I think you are thinking another type of efficiency, perhaps thermal. VE is the engine's ability to move air through the engine i.e. a ratio of displacement to air coming in. If it can move more air, it can move more fuel.

Originally Posted by jyaron

So in a MAP based calculation:

pseudoMAF= (IMAP/120)*(VE/100)*(ED)*(MM)/(R)

Therefore, if VE increases,.. pseudoMAF increases?
I thought the engine sucks less fuel if it has higher volumetric efficiency?

VE refers to how completely the cylinder can be filled with each intake stroke. Because of the restrictive nature of the intake path and the inertia of the air/fuel mixture, the cylinder can not fill completely before it begins its up stroke. So if it only filled with 80% of the air that it could have, it has a VE of 80%
The IMAP above is absolute. So if the engine can "suck" better, the IMAP would be less (more of a vacuum). So if we could adjust VE with a knob we might see that as VE increased, IMAP decreased. VE ^, IMAP V.
Furthermore, with increased VE, the throttle need not be opened as much, as the same air/fuel is needed, so perhaps the IMAP would decrease. Or may be its the same

VE is not a constant, it varies with RPM. Ignoring the potential harmonic effects, one would expect that as the RPM increases, the cylinders have less time to fill, hence the VE drops. As VE drops, IMAP would increase (less vacuum). But since RPM is increasing, there are more intake strokes per second to suck more air and hence IMAP may decrease.
Hell, ultimately I'm not sure if IMAP provides the compensation for variations in VE. Hence my question.
Has any come across a chart of VE versus IMAP. Though such a chart would likely be different for each engine design, a single chart would at least provide some insight into the relationship.

good project. thanks for share

Originally Posted by split63

VE refers to how completely the cylinder can be filled with each intake stroke. Because of the restrictive nature of the intake path and the inertia of the air/fuel mixture, the cylinder can not fill completely before it begins its up stroke. So if it only filled with 80% of the air that it could have, it has a VE of 80%
The IMAP above is absolute. So if the engine can "suck" better, the IMAP would be less (more of a vacuum). So if we could adjust VE with a knob we might see that as VE increased, IMAP decreased. VE ^, IMAP V.
Furthermore, with increased VE, the throttle need not be opened as much, as the same air/fuel is needed, so perhaps the IMAP would decrease. Or may be its the same

VE is not a constant, it varies with RPM. Ignoring the potential harmonic effects, one would expect that as the RPM increases, the cylinders have less time to fill, hence the VE drops. As VE drops, IMAP would increase (less vacuum). But since RPM is increasing, there are more intake strokes per second to suck more air and hence IMAP may decrease.
Hell, ultimately I'm not sure if IMAP provides the compensation for variations in VE. Hence my question.
Has any come across a chart of VE versus IMAP. Though such a chart would likely be different for each engine design, a single chart would at least provide some insight into the relationship.

Been there...done that. As it turns out, this is a very "doable" project if you happen to have an OBD-II compliant vehicle with a MAF sensor---plus MAP, IAT and RPM sensors (almost 100% of OBD-II vehicles have these latter three). The MAF sensor gives you air-flow directly. The other parameters can be used to get a value that is directly proportional to volumetric efficiency (VE)---using the methods I've discusses earlier in this thread. The MAF sensor's "air flow" reading allows you to solve for VE. Then you can plot VE vs. RPM.

You will likely find that VE is in fact *not* a constant and does vary somewhat with RPM. And, depending upon the exact vehicle, the precise relationship between RPM and VE may be "complex", for all the reasons you discussed.

However, if you just want long-term MPG, you will find that using a constant for VE likely gives you a number that is "good enough". Part of the reason for this is that during "normal driving" a vehicle's engine is operated in a relatively narrow range of RPMs, and therefore with a more or less constant VE.

Best regards,

Bruce D. Lightner
lightner@lightner.net
http://www.lightner.net/lightner/bruce/

If as Bruce says:..." MAP, IAT and RPM sensors (almost 100% of OBD-II vehicles have these latter three)" what percentage of OBD-II vehicles have these three PIDs available(I've learned that just because there's a sensor,doesn't mean there's a PID offered in some cases).-Keith

Originally Posted by Keith55555

If as Bruce says:..." MAP, IAT and RPM sensors (almost 100% of OBD-II vehicles have these latter three)" what percentage of OBD-II vehicles have these three PIDs available(I've learned that just because there's a sensor,doesn't mean there's a PID offered in some cases).-Keith

Excluding IAT, I have not come across a car that did not support some combination of Speed, RPM, MAP or MAF so that Ii could compute MPG. I have only not found IAT on some 1997 and older vehicles.