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"Noob Question" Why VE? 
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LQFP112 - Up with the play
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Ok, smack some sense into me if this is a completely stupid question but...

:!: Why does everyone want to use VE tables? (please tell me it is not just because MS uses them)

If we are mapping engines to run at different AFR's in different parts of the map and engines have constantly changing VE's through out the map where is the advantage. Is it a simpler formula than taking the number (as pulse time) in the map and applying the corrections required? Would a pulse time make more sense to look at when tuning than a VE number which has been tweeked anyway?


Wed Aug 06, 2008 1:06 pm
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MS only claims to use VE, currently their idea of VE is not exactly close to the truth. Especially not with Kens map multiply turning off feature... With that off it's not VE at all. For most users the best way to tune a MS is to ignore that it's VE and max out the table values up around 255. Real VE values are close to 100% hence ours will max out at 128% giving the average user the best possible experience without being hacky at all.

The reason I want to use VE is to stick as closely as possible to a true physical model of what is happening. If all aspects of operation are included in the calculations then VE, AFR, fuel properties, physical properties, air density etc all combine together nicely in a clean true to life non hacky way.

A true VE map will tell you quite a bit about your engine and setup too. A fake VE map won't tell you much at all though.

Fred.

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Wed Aug 06, 2008 1:44 pm
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My opinion:

It's all about transformations/mappings really. The general model is basically one big transformation/mapping (sometimes with a feedback mechanism), which takes a number of inputs (MAP, CLT, TPS, etc) and spits out final outputs like injection duration and spark advance angle. Furthermore, the units, scale or even linearity(to an extent) of the inputs doesn't matter much as long as the transformation/mapping is determined and calibrated experimentally (what we call 'tuning'), with some kind of reliable feedback (WBO, seat of pants, common sense) and the resolution and dynamic range of the transformation/mapping and inputs' signals are sufficient for the desired accuracy of the outputs.

VE tables: personally, i see no reason to have real/pure Volumetric Efficiency numbers in the VE table. IMO, VE table, as it was popularized by MS, is a misnomer. The table is a big part of the MS implementation of the transformation/mapping discussed above, and IMO, is fine to have all kinds of other factors 'embedded' into it, that affect the final behavior/tune, or even some fudge-factors to improve resolution/dynamic range. Of course one can have a pure VE table + other tables for those extra factors and just combine them during the calculation of the transformation/mapping, but IMO it may very well be a waste of cpu time, memory and will complicate the tuning process. It's not like one could read the real VE values from the engine's manual and punch them into the VE table. So, IMO, value of having real VE values is zero.

I could be wrong
Alex

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Fri Aug 08, 2008 4:30 pm
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Quote:
I could be wrong

Surely not ;-)

ababkin wrote:
Furthermore, the units, scale or even linearity(to an extent) of the inputs doesn't matter much as long as the transformation/mapping is determined and calibrated experimentally (what we call 'tuning')

I disagree. The reason you want all your parts separate and as they actually are in real life is two-fold. Number one, you want to be able to conceptualise and adjust each parameter independently of the others. Number two, in order to produce a clean software solution you need the same modularity and clear defined boundaries of what each thing is and does. By far the most logical way to lay these out is to follow mother nature and do it as the physics really is.

Some parameters can be combined without harm, such as VE and AFR are on MS in most installations, however it would be very difficult to combine some others. Consider not splitting injector dead time out for example, how would you model that in software? How would you "tune" it in a VE table? You need separate control of that parameter for very well defined reasons.

Splitting the AFR OUT of the VE table is a very good move as it removes the guess work required when you change your mind about what sort of AFR you want at XxY load point. VE remains the same and only the AFR actually changes. VEMS does it this way and I believe it is the correct way to do it.

Quote:
IMO, VE table, as it was popularized by MS, is a misnomer.

Absolutely, it doesn't reflect VE at all in most MS installations for a large variety of reasons.

Quote:
[it] is fine to have all kinds of other factors 'embedded' into it, that affect the final behavior/tune, or even some fudge-factors to improve resolution/dynamic range.

As I mentioned above it entirely depends on which one that is exactly. If you try and blend the wrong ones in it will actually be totally impossible to ever reach a tune that always works. Others you can include with no consequences except those of conceptual understanding of what it is you are doing. However, that is a BIG loss IMO.

Quote:
but IMO it may very well be a waste of cpu time, memory and will complicate the tuning process.
Allow me to fix that for you :
Quote:
but IMO it may very well be a waste of cpu time, memory and will simplify the tuning process.

That depends on what you are trying to achieve. My goal is to create a physics realistic model with a clean implementation that is easy to comprehend and adjust. I believe segregating the code into real world physics is the best way to achieve both a clean implementation AND ease of use.

Quote:
It's not like one could read the real VE values from the engine's manual and punch them into the VE table.

Maybe not, but after you are done it is nice to get a visual representation of how your head, cams manifolds and other plumbing flow at various RPM. A real VE table WILL give you that fairly accurately if the system is implemented and setup correctly. MS tables will not at the moment for various reasons.

Quote:
So, IMO, value of having real VE values is zero.

And you are entitled to it :-)

Fred.

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Fri Aug 08, 2008 9:41 pm
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I found out the hard way. I took the ve table from my GM ECU and plugged it into my MS.

BARELY RAN! Even though all my other constants were good. I was pissed.

I would like to be able to look at a tuned ve table and say damn that engine runs good, or what a POS. With my GM ecu my VE tables and power curve were almost right on when you compared it to a dyno graph.

-sean

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Thu Aug 14, 2008 3:22 am
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Aside from all the geeky reasons to go with VE over something pulse width, I'll say from a usability stand-point that VE is quite a bit better to tune fuel with. I've done both (VE with VEMS and pulse width with AEM) and here's the way I see it.

Swapping injectors on either system is just about as easy. You either grab the entire pulse width map and multiply by a constant or you adjust your injector type/settings. Tie.

Tuning fuel on AEM involves setting up your target air/fuel map (for EGO correction) and tuning the pulse width map to match the target air/fuel map. Tuning fuel on VEMS involves setting up your target air/fuel map (for EGO correction and injector pulse width) and tuning the VE map to match the target air/fuel map. Tie.

Adding something that changes VE like a cam or similar involves a re-tune. With AEM you head to the pulse width map and re-tune and on VEMS to hit the VE table and re-tune. Tie.

Adjusting your desired air/fuel ratio on AEM involves changing the target map (to update the EGO correction) and changing the pulse width map to get the desired air/fuel ratio (involving a bunch of seat time). Adjusting your desired air/fuel ratio on VEMS involves changing the target map only (to update the EGO correction and injector pulse width), no need to touch the VE map (and no seat time). Advantage: VE Table.

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Fri Aug 15, 2008 7:31 pm
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BenFenner wrote:
Aside from all the geeky reasons to go with VE over something pulse width, I'll say from a usability stand-point that VE is quite a bit better to tune fuel with. I've done both (VE with VEMS and pulse width with AEM) and here's the way I see it.

Swapping injectors on either system is just about as easy. You either grab the entire pulse width map and multiply by a constant or you adjust your injector type/settings. Tie.

Tuning fuel on AEM involves setting up your target air/fuel map (for EGO correction) and tuning the pulse width map to match the target air/fuel map. Tuning fuel on VEMS involves setting up your target air/fuel map (for EGO correction and injector pulse width) and tuning the VE map to match the target air/fuel map. Tie.

Adding something that changes VE like a cam or similar involves a re-tune. With AEM you head to the pulse width map and re-tune and on VEMS to hit the VE table and re-tune. Tie.

Adjusting your desired air/fuel ratio on AEM involves changing the target map (to update the EGO correction) and changing the pulse width map to get the desired air/fuel ratio (involving a bunch of seat time). Adjusting your desired air/fuel ratio on VEMS involves changing the target map only (to update the EGO correction and injector pulse width), no need to touch the VE map (and no seat time). Advantage: VE Table.


Bingo - this is the main reason to use VE tables rather than say injector duration maps. The idea is that you can tune safe - ie set your target to say 12:1 or 11:1 and then you can do lean cruise etc later by just changing the target map in the areas required, then go for a run down the freeway to check your AFR's EGT's and coolant temp. If it heats up too much or EGT's go way out of range, then all you need to do is change the target map and go for another run to see if its fixed. If you have injector maps rather than VE you need to spend time on the dyno again. Injectors are NOT linear - VE tables allow you to have injector slopes etc in software so you can change injectors and not have massive changes in AFR requiring a retune etc. VE tends to win from a lot of perpectives.


Wed Aug 20, 2008 6:31 pm
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