Discrepancy in Volumetric Efficiency Calculator.

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https://www.otctools.com/ve

With my truck on the highway at WOT, 2033 rpm, 68F IAT, MAF is reporting 66.1 g/s. Doing the calculations - swept volume of 4.3l every two revs gives 72.85 l/s, air at 68F weighing 1.204 g/l yields air flow of 87.7 g/s. 66.1 is 75.4% of 87.7, yet the OTC VE Calculator yields 65%.

What am I missing?
 
How do you know how accurate the on-board air temp and MAF are?

Also, is the engine's displace actually 4.3L ... most manufacturers round up when designating an engine displacement label, so the actual engine displacement might be a bit less than 4.3L.
 
Nothing is exactly as how it seems to be. Engines efficiency varies at different RPMs do to the intake, Exhaust and everything in between. Some where in the engines operating range should be a "sweet spot" that will give the best numbers.Though not 100% unless the engine is supercharged then maybe not.
 
Every one of the replies skirted my question.
1) The actual displacement really does not matter, whether it is 4.25l or 4.34l. 4.3 is the figure used in my calculations and in the OTC app, so it is the exact same figure in both.
2)Pumping losses would explain why the efficiency is less than 100%, but that was not my question. All I am doing is questioning the discrepancy between my figures and the app's. Ditto "sweet spot", "supercharging" etc.
3) The air temp and Maf are the values reported by a scantool, but, again, all immaterial to my question.

WHY IS THERE A DISCREPANCY BETWEEN MY FIGURES AND THE OTC APP?
 
Originally Posted By: George7941
Every one of the replies skirted my question.
1) The actual displacement really does not matter, whether it is 4.25l or 4.34l. 4.3 is the figure used in my calculations and in the OTC app, so it is the exact same figure in both.
2)Pumping losses would explain why the efficiency is less than 100%, but that was not my question. All I am doing is questioning the discrepancy between my figures and the app's. Ditto "sweet spot", "supercharging" etc.
3) The air temp and Maf are the values reported by a scantool, but, again, all immaterial to my question.

WHY IS THERE A DISCREPANCY BETWEEN MY FIGURES AND THE OTC APP?


Hard to say how they are calculating VE in their on-line calculator. The VE is simply the ratio of how much air the engine actually takes in vs. how much it could 100% ideally take in based on displacement. Pumping losses and intake/exhaust restriction are what makes VE less than 100% and should already be taken into account when using intake mass flow (g/s) and intake air temp vs. what the 100% ideal intake based on displacement could be.

Seems it should match your numbers better as it should be a pretty simple calculation as you've shown. Maybe they have an error in their calculation model, or their are accounting for something not obvious.

You should email them and show them the issue to see if they look into it and respond why the discrepancy.
Email: [email protected]
 
Originally Posted By: George7941
https://www.otctools.com/ve

With my truck on the highway at WOT, 2033 rpm, 68F IAT, MAF is reporting 66.1 g/s. Doing the calculations - swept volume of 4.3l every two revs gives 72.85 l/s, air at 68F weighing 1.204 g/l yields air flow of 87.7 g/s. 66.1 is 75.4% of 87.7, yet the OTC VE Calculator yields 65%.

What am I missing?


The VE is defined with air at "standard temperature and pressure" ATM conditions. At IUPAC standard temperature and pressure (0 °C and 100 kPa), dry air has a density of 1.2754 kg/m3 (same as 1.2754 g/l).
https://www.google.com/#q=air+density+at+standard+temperature+and+pressure

As a check of their calculator, use the standard 1.2754 g/l at 32F to get 72.85 l/s times 1.2754 = 92.9 g/s if the engine was 100% VE, and when using those numbers in their calculator it shows 98% (close). So the standard temp, and also probably standard press, have a factor in their calculator. 100 kPa = 14.5 PSI which is slightly less than 14.7 PSI.

 
What does the absolute load value say? It's based on VE and the MAP/MAF. Does it seem low (<80% at peak torque)?
 
Looks like the OTC Tools calculator is using 0 deg C (32 deg F) and 1 ATM air density for their "standard" air temp/press, which would be a density of 1.2922 g/l (94.14 g/s at 2033 RPM). That gives a VE of 100% if the 4.3L engine was ingesting air at the "standard" 0 deg C and 1 ATM to it's full displacement.



 
Originally Posted By: SHOZ
What does the absolute load value say? It's based on VE and the MAP/MAF. Does it seem low (div>


Not sure. The Actron 3130 scanner I was using only gave calculated load and gave it as 95.6%. My Tech2 has a engine load pid but I do not know if it is the absolute load.
 
Thanks, ZeeOSix for your explanation. So the OTC calculator gives the right result when IAT is 32F, but the results seem to be off when the temp is not 32F.
 
Originally Posted By: George7941
Thanks, ZeeOSix for your explanation. So the OTC calculator gives the right result when IAT is 32F, but the results seem to be off when the temp is not 32F.


My example with 32 deg F shows that their calculator is using a standard baseline of 32F and 1 ATM at 101.325 kPa (14.7 PSI) to define the "standard air conditions" for measuring VE. So their calculator is correct, and if the air temp and pressure isn't at the standard conditions then a NA engine will never hit 100% VE. In other words, the ATM conditions have an effect on the VE.
 
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