TPS correction when on partial boost

[Nitroking]

Hello,

I will try to describe a situation and if it is not clear please ask any question..

1)Let's say I have my boost controler (whatever brand it is) set to 2 boost levels, one at 150kpa(abs) and one at 200kpa.
2)I go out to tune the car and tune it at 11.5AFR at WOT for 150kpa and 11.5AFR as well for 200Kpa.

Now what happens is that if I choose the high boost level of 200Kpa, when I am at partial throttle, ie 40% my car makes boost of 150kpa. At this very time, the AFR is around 10.7-10.8, maybe 11.1. If I choose the low boost level and go WOT and 150kpa it is 11.5AFR correctly.

That makes sense to me. Less throttle, same amount of boost=we need less fuel. Full throttle, same amount of boost=we need more fuel. But we tune for WOT to be safe. MAP load is not the only variable we need to take care of. And the above situation happens all the time especially when cruising in high gear on the highway and you want to change lane then you press the throttle a bit, it makes boost and the AFR goes rich.

I was thinking that we need a TPS correction map like the one I created on a Motec M400 ECU a while ago (numbers are examples but you get the point, 100% means VE Table intact):


So lets say ms outputs that in 150Kpa you should have a 10ms injector pulse. If you are at 40% tps then it should be 10*0.87=8.7ms which would result to an AFR of 12.2 instead of 10.7-10.8, which is good for partial throttle and boost..

I've seen the blend mode for VE Table 1 and VE Table 2 but it uses RPM on the one axle and is not what we need here. Or else instead of creating a new map we could change the RPM in VE Table 2 and make it MAP Load (KPA) optionally. This would work I think.

The above function would result to cleaner engines, less emissions, less fuel used and better engine performance, protection as the oil film will not get disolved by the rich mixtures as it is now..

Looking forward to your comments,
Nick

[wes kiser]

That's already there if you want it, you simply need to enable some form of secondary load, and make this aplha-n, multiplicative. I would tend to accept it, as it doesn't cause much of a "problem" and is typically transient, but the tools already exist in the firwmware to dial it in.

[tutuur]

I always believed this was true what you're stating above, however other people had me convinced it wasn't.

They say, the amount of boost depends on the amount of air, so no matter what throttle opening, if you reach a certain amount of boost that's how much air is going in. (As boost reflects the restriction etc.)

Now i'm confused again!

[Nitroking]

I know exactly what you 're saying.. Been told this a lot. But it wouldn't make sense to me..

The fact that It is NOT true is also proved by the results of the experiments below:

1)If you are at WOT AFR is 11.5 at 150Kpa boost. If you are at 50%tps AFR is 10.8 or whatever RICHER than the WOT's AFR (11.5), again at 150Kpa boost.
What does this say to us? That LESS air is going into the engine but we inject the same amount of fuel so AFR gets richer..
2)WOT at 150Kpa, you reach the redline from 3000rpm in lets say 5 seconds. 50% tps at 150Kpa you may not even reach the rev limiter or will do that hell later (maybe 10 sec total, 5s more).
What does this say to us? LESS Power than when we were at WOT. Less power with the same amount of fuel means LESS air. And pig rich afr as we inject the same amount of fuel.

I asked my mechanic last night and he told me that it is not about pressure only but about FLOW as well.. And flow is hugely restricted by a 50% tps. So less air is going into the engine and so we need a tps correction map to reduce fuel according to the throttle position.

Waiting for your comments.

Thanks

[tutuur]

Maybe it depends on where you measure boost?

It's contradictionary really as boost is basically the restriction of the engine to breath

[Nitroking]

I measure boost at manifold of course. But the throttle is an obstacle and it struggles the airflow. That's why the engine can't breath as it can at WOT.

[wes kiser]

I would hardly say it "wasn't true," as even in your case NO CORRECTION at all results in perfectly acceptable operation. While I understand wanting to ensure your AFR target is roughly achieved, you are speaking of a transient condition you don't spend much time in. Your improvement in fuel economy or oil dilution would not be measureable, and the engine will run just fine at 10.5 vs. 11.5

The speed density model depends on many things, one of which is a good manifold pressure reference, the other is the notion that engine VE is constant/close to constant with changine throttle position.

As you throttle the engine (while still making boost), there is a pressure drop across the butterfly that the turbo must overcome (IE, exhaust pressure will be higher at 150kp WOT vs. a lower throttle opening), this will reduce engine VE at this point. I also don't know about you, but I typically have small fluctuaitons in pedal movement when trying to hold a light (but still substantial) amount of throttle under any acceleration at all, which can induce AE events.

There is also the very real possibility that your MAP sensor reference point behaves slightly differently in terms of how respresenative it is of actual manifold pressure at different butterfly openings.

With common plenum engines, the pure speed density model does fairly well, but there are a great many sources of error/assumptions (the above are two specific ones that likely apply to your specific instance). For most of us, we accept this error as being not significant in terms of overall operations. There is abosolute nothing wrong with wanting a better approximation in that specific instance. If this is the case for you, it can be made "perfect" with a secondary load table, and easily tuned. This is already there, and will work perfectly for what you are trying to achieve.

[Nitroking]

Hi, regarding this situation, I am not building boost, I have stable boost while I have partial throtte. This can happen easily on my setup. Especially if there is a hill area on the highway and I can keep the boost stable at 150Kpa with partial throttle. So it is not that this only happens for 1-2 seconds, it can happen for 10 or 15 or more easily.. And that's the magic of turbo, it can give you loads of torque on hilly areas and you don't have to be at WOT to make it happen..

The map sensor is the stock on board of MS2 extra..

You mentioned earlier that the functionality is there, could you please point it? I only see TPS vs RPM as second load, not TPS vs MAP which is what I'm asking for..

Thanks again,
Nick

[CSXRT4]

We know that by the ideal gas law we can calculate mass of air getting to each cylinder per intake event. This is what the speed density system is. If we know air temperature, pressure, and volume of the container we can calculate air mass and subsequently how much fuel to inject for a given air:fuel ratio. This has to be adjusted by a volumetric efficiency multiplier for different engine running conditions. This is why throttle position does not matter, because for a given pressure in the manifold, temp of the air, and ve of the engine at that rpm/pressure, the mass of air getting into the cylinders will be constant.

The only way I could see throttle position effecting things is if it had a significant effect on air velocity which could alter engine ve. But a change in air velocity would most likely cause a change in manifold pressure given the same engine operating conditions.

The likely cause of your issue is density correction based on the air temperature, when you are at part throttle you probably have a higher pressure in the intercooler pipe than in the manifold. This puts the turbo at a different operating efficiency which changes the output temperature, if you are not getting accurate air temperature readings and/or compensations then it will throw off fueling. Do you have an intercooler?

Also make sure you don't have any tip-in enrichment in effect when this is happening.



Edit: looks like wes touched on most of these points already. Good thought about pre-turbo back pressure being varied which could cause a change in ve.

[Nitroking]

I have an intercooler. And I have the GM IAT mounted just before the throttle body and as much as possible protected from the radiator fans to not getting heat soaked. MAP sensor is on the intake manifold plenum.

As for the air entering the engine being the same when the pressure is the same then how do you explain the fact that the AFR is richer? IAT is ok. You mentioned air velocity. We can all agree that this changes when the throttle is at 50% as it creates a big obstacle.. So the cylinder may not get filled as well as it would if it was 100% open..

Plus I will say something more.. As long as the air entering the engine has less speed then the gases may also have less speed and the turbine wheel will have less speed so turbine acts as a restrictor as well so the flow between intake and exhaust gets struggled. That's another why less air is entering the engine..

[wes kiser]

It intrinsically gives you what you want. Keep primary load as speed density. Select alpha N for secondary load, and keep this multiplicative.

Keep your speed density load table as it. Then use the secondary load table to modify it. Your "maximum" need for a modifier is only 15%, so don't overthink it. It will also be very speed dependant, so I promise you don't really want it set up verses MAP, that is just the way you are inclined to think about it.

I would populate your alpha N table with the bottom number being the abosulte minimum throttle position you can build any real boost at, and the top number being 100%. Up until your boost threshold rpm (speed at which you can build substantial boost at 100% throttle), I would set all values to 100% (I think this means zero modification with multiplicative). The top (100% throttle row) will alway equal 100%. As you go down in throttle position, the multiplier would decrease. Try it this way before you suggest there is a need for a different approach, because this approach actually attempts to "model" what is going on, verses you simply being aware of a need for a correction at a specific operating condition.

[Nitroking]

Thanks for your suggestion, it is an approach that could work. I will try it as soon as I can.

The reason however I was suggesting a TPS vs MAP table is that you can build for example 140kpa at 3500rpm on 2nd gear with 50% tps but on 5th gear with 50% you may build 155Kpa of boost. So same RPM, different MAP, we lost it.

The same can happen even in the same gear, same TPS but different road conditions (straight level vs hill ie). That's why TPS vs MAP would only be the best solution. Do you disagree?

[wes kiser]

MAP is already there in the primary. What you are seeing, is VE is changing with throttle position, for some reason or another (real VE change due to drive pressure, or potentially MAP reading differently depending on butterfly angle and flow phenomenon). This will vary every bit as much with RPM as it does with MAP vs. TP. You are just seeing it as MAP vs. TP because of specific operational circumstances.

There is always a time dependance with anything that depends on boost from a turbo (IE, your gear dependant analogy). How would viewing this as MAP vs. Throttle position without any tie back to what is achieveable (RPM at leaves gives you that) treat this any differently?

The other piece of this is the transient nature of what you are describing. When at WOT, but at a similar boost pressure, you are typically accelerating faster (IE, low gear, faster sweep rate) than you are when you are "holding it back" with the throttle in a tall gear pulling a hill. You may very well be seeing a need for accelleration enrichment under these circumstances. The EAE scheme would provide this.

[whittlebeast]

Conventional thought is MAP in the intake and RPM are the only two big factors that go into airflow. Sure there is IAT and a few other things but MAP and RPM are the big ones. The thing is a motor is a huge pipe organ. Anything that changes the effective length of the pipes in this organ can mess with the basic calculations. As you go from part throttle to full throttle the effective length transitions from a closed end pipe organ to and open end pipe organ. This is when the a TPS correction comes into this stuff.

I see this all the time as I mat be working with an ITB 9000 rpm car on one weekend and a supercharged jetski the next.

Listen to this this video as I roll into and out of the throttle as the sound of the motor changes with throttle position. The turbo motors tend to be less finicky but still have the issues.

http://www.youtube.com/watch?v=e56GgdFs ... zdUjy3KJuQ

Hope this helps

Andy

[whittlebeast]

This may help put a little light on the subject....

http://www.ncs-stl.com/tuning/

[tutuur]

As you throttle the engine (while still making boost), there is a pressure drop across the butterfly that the turbo must overcome (IE, exhaust pressure will be higher at 150kp WOT vs. a lower throttle opening), this will reduce engine VE at this point.

This explains perfectly what is happening, cleared that up!

So basically when tuned for 150kpa as a dd setting and 200kpa for full boost 150kpa will always be richer on part throttle.

[Nitroking]

So whittlebeast how would you outtake that problem? Do you think a TPS-MAP correction is needed?

[wes kiser]

As stated several times, viewing it as TPS vs. MAP will only allow you to fix it at a single operating condition. The trend will be very speed dependant.

Are you sure a portion of this (the fact you mention behavior vs. gear, plus conventional wisdom of how most cars behave in different gears makes me certain at least some of it is) sweep speed dependant, and actually should be partially handled via an acceleration enrichment routine?

[Nitroking]

The only way to be sure is to have map. This is also the reason why we tune turbo cars using map and not tps. Because we need to know map.

I know that TPS vs RPM can do some job but will not be able to handle the problem perfectly. We disagree and that's tottaly ok, it is different points of view.

[tutuur]

I know ego correction isn't recommended on full load but at 150kpa i think it should be able, so basically the solution is already there....

Guess you could always set the permission low so when a sensor fails it won't f*** up the engine