Dynamic Cylinder Air...?

[AKSSS]

Hopefully this is the right section, and not suppose to be in "forced induction"

I need an "simple" explanation of the "dynamic cylinder air"? I have been working with Mr. Wheatley on tuning my truck with my new setup(we are making great headway concidering my weather right now!). As I understand it now, the dynamic cylinder air is a calulated formula that the ecm puts together from references of engine sensors, to figure out the load? or the airflow into the cylinders?

 

So here is my example and a question all in one.

 

I have recently gone from a sts turbo setup running a gt-67 turbo running at 9-10 psi. Truck ran pretty good. NOW, I have a new front mount setup from kbracing and I got a new mp t-76 turbo. If I remember right(I have lost my old logs when my computer died), with my old setup at 10psi, the dynamic cylinder air(dca) was only about a max of 1.20-1.25 g/cyl. Shoot. The other night at only about 7psi with the new setup, I saw over 1.44 g/cyl. Granted, the air was alot more dense since it was -20 below.

 

So my two questions are what is dynamic cylinder air? And maybe an understanding of the pressures from the turbos? If you have a small turbo at 5 psi, and big turbo at 5 psi, the manifold is at 5psi. Is the bigger turbo putting more air in? I get that the larger turbo moves more cfm than the smaller one. But if you are running 5psi. You are at 5psi, no matter the turbo size? Can you put more air into the motor with the larger turbo at 5psi? I don't see it, since the manifold can only get so much cfm into the volume to reach the 5 psi. I get that the larger turbo is more efficent at the same 5psi as the smaller turbo because it is not spinning as fast to reach the psi, not including the drop in intake air temps!

 

Hopefully, this all made sense.

Edited January 5, 2011 by Aksss (see edit history)

[Krambo]

I could take a stab at your question however I am not fully clear on this myself. HOWEVER, I do have a question for you that may shed some light on your observations:

 

Are you doing a SD tune now and your previous tune was MAF???

[misterp]

You're asking the right questions. At the risk of over-simplifying:

 

"Dynamic Cylinder Air" is simply the amount of air the PCM thinks is inside the cylinder at any given moment. Period. Another correct term is - "air mass".

 

There are two ways to determine how much air is in the motor - you can calculate it, or you can measure it.

 

To calculate the amount of air, you use a basic chemistry formula called The Ideal Gas Law: if you know what the temperature of the air is (IAT sensor), and the pressure that air (MAP sensor) and how well the engine breathes at the current RPM (Volumetric Efficiency, or VE table) then you can very accurately calculate how many grams of air are in the cylinder. This approach is called SPEED-DENSITY.

 

To directly measure how much air is entering the motor, you fit it with a carefully calibrated laboratory sensor which measures the volume of air entering the throttle body (MAF sensor) and reports that number to the PCM.

 

Either way, once the amount of air in the engine has been figured it is reported as "Dynamic Cylinder Air".

 

Air is stretchy stuff; heat makes it expand (i.e. less dense), pressure makes it more dense (duh!). So you can have hot air at 10-psi and arrive at a final 1.2 grams of air in the cylinder, or you can alternatively get cold air at 5-psi and *still* arrive at the same 1.2 grams of air in the cylinder - either way, in the end the motor makes the same torque either way because it ingested the exact same amount of air by weight. (Sidenote - even though it's the same amount of air, the ignition timing required will be different because of the density of that air in the motor, that's outside this discussion but my point is it's not apples-to-apples the same).

 

The PCM has one PRIMARY job - and that simply is: determine how much oxygen is in the motor at any given moment. Everything else (like how much spark advance to use, or how long to hold the injectors open, etc) is all "downstream math". Only when you know how much oxygen is in the cylinder can you then determine how much gasoline to add (hence the term 'air to fuel ratio') and when to light it so as to not grenade the motor.

 

The more air in the motor (by weight) the more gas you must add - and the more power you generate. "Dynamic Cylinder Air" is basically your "torque meter" - the more air the motor is taking in at any given moment, the more torque it will be making; in fact that relationship is one-to-one. It's just that simple. The real reason we mod our engines is to breathe/process more air; when you add a set of headers, or CAI, or swap heads, or change cam etc the dynamic cylinder air value should go up or you will be very unhappy because your mod added no additional power (because it added no additional air mass).

 

I encourage you to continue your thinking about 'pressures' - when you get your head wrapped around exactly what pressures are happening when & where in an engine then you start to master it's tuning and in turn how to mod it effectively. Master cam designers think strictly in terms of air mass, they can tell you at any given instant in the 4-stroke cycle what the air mass/pressure is in the intake, cylinder, and header and this fundamental understanding is *critical* in turbo apps.

 

Mr. P.

[Krambo]

Here is the relationship in the most basic form I can type:

 

Airmass (Dynamic Cylinder Air) = (The VE value in a particular cell of the VE table MULTIPLIED BY MAP Value at that moment) DIVIDED BY temperature value

 

This applies only to GM VE tables as they are a composite of pressure and temperature and the temperature value is actually a blend of IAT and ECT depending on airflow. Think about it, the air will be cooler if it flows more through the hot engine than less flow but we do not measure the temp when it is IN the cylinder. There is some function formula for this Temperature value that I have not been able to uncover. I am sure the code writers at EFI Live and HPTuners are well aware of this function but they do not want to share

 

If you want to break it down even further, Volumetric efficiency (VE) is mearly REAL AIRFLOW divided by THEORETICAL AIRFLOW.

 

Simple right? So what if you wanted to use that Ideal gas law (P*V=n*R*T) that Steve mentioned and not use the GM VE values? No problem:

 

First you need to calculate true VE (or absolute VE):

 

True VE = (VE value of the cell in your VE tableMULTIPLIED BY the "R" value constant in the ideal gas law) DIVIDED BY the cylinder volume.

 

Now that you have the True VE value, you can calculate AirMass or Dynamic Cylinder Air:

 

Air Mass = (cylinder volume MULTIPLIED BY True VE value MULTIPLIED BY MAP value) DIVIDED BY (Temperature value MULTIPLIED BY the "R" value constant in the Ideal Gas Law

 

 

...Man it is hard to type formulas in a forum!

 

 

EDIT: The reason I asked if you went to a speed density tune from a MAF tune is because if you still log your MAF values while running a SD tune, there is a good chance the dynamic cylinder air calculation is screwed up in the log if you didn't delete that PID in your configuration. The calculation will factor in a "0" for the MAF value and effectively cut your dynamic cylinder air value in half.

Edited January 5, 2011 by Krambo (see edit history)

[SSplaytoy]

Damn I love physics

[AKSSS]

You're asking the right questions. At the risk of over-simplifying:

 

"Dynamic Cylinder Air" is simply the amount of air the PCM thinks is inside the cylinder at any given moment. Period. Another correct term is - "air mass".

 

I encourage you to continue your thinking about 'pressures' - when you get your head wrapped around exactly what pressures are happening when & where in an engine then you start to master it's tuning and in turn how to mod it effectively. Master cam designers think strictly in terms of air mass, they can tell you at any given instant in the 4-stroke cycle what the air mass/pressure is in the intake, cylinder, and header and this fundamental understanding is *critical* in turbo apps.

Mr. P.

 

Well, I am starting to understand what the pressures are doing(or the result from them). It is definitly alot to take in.

 

...Man it is hard to type formulas in a forum!

 

EDIT: The reason I asked if you went to a speed density tune from a MAF tune is because if you still log your MAF values while running a SD tune, there is a good chance the dynamic cylinder air calculation is screwed up in the log if you didn't delete that PID in your configuration. The calculation will factor in a "0" for the MAF value and effectively cut your dynamic cylinder air value in half.

 

 

Lol! Your formulas are beginning to make sence, I think. Both explanations are excellent! And thank you both. I have a much better understanding of what the cylinder pressures are meaning now.

 

The only thing that I am trying to figure out is my increased cylinder pressures? They must have just been from the cold outside air(iat's around -20). SO I thought. Today on my way to work, it warmed up outside to about 20 above 0, so I stepped on it on my low boost setting. I still had cylinder pressures around 1.40g/cyl at 6psi. My old setup at 10 psi would only get around 1.20. So, is this a result of the larger turbo stuffing more air in? Or from the increased effiencey of the turbo and exhaust?

What kind of cylinder pressures are you guys seeing? Especially in a fi application.

 

Sorry guys. Just trying to get a handle on what the heck I am doing? I think that I am a long way from understanding everything, but I am getting there. Thanks for the help guys!

 

edit-oh, yeah. I was running a 2bar sd cl setup. I did remember to erase the original map sensor pid. Here is my cfg file if you would like to look at it. I just tried to create a boost pid also. We will see if I got it right later tonight. Nothing special about it.

SSS-Boost.cfg

Edited January 6, 2011 by Aksss (see edit history)

[misterp]

The only thing that I am trying to figure out is my increased cylinder pressures? They must have just been from the cold outside air(iat's around -20). SO I thought. Today on my way to work, it warmed up outside to about 20 above 0, so I stepped on it on my low boost setting. I still had cylinder pressures around 1.40g/cyl at 6psi. My old setup at 10 psi would only get around 1.20. So, is this a result of the larger turbo stuffing more air in? Or from the increased effiencey of the turbo and exhaust?

What kind of cylinder pressures are you guys seeing? Especially in a fi application.

All great questions for discussion. I am VERY happy to hear your truck project is finally going in the right direction!

 

I cannot tell you exactly what improved your airmass, because you changed too many things at once to be able to nail the answer down accurately.

• Lower outside air temps have increased density. Guys living in Phoenix, AZ are at a disadvantage.
  
• Outside air at lower altitudes will have increased density. Guys living in Boulder, CO are at a disadvantage.
  
• More efficient compressor designs move air without beating it to a froth and raising IATs, thus increasing charge density. Radix and STS guys are at a disadvantage.
  
• Better intercooling will decrease IATs, thus increasing charge density. Guys without an intercooler are at a disadvantage.
  

Keep in mind that for discussion we are only talking about air mass, other IAT-related topics like detonation and octane sensitivity etc are all valid but a whole 'nuther bag of snakes.

 

I am NOT saying you cannot make good power with a Radix, or living in AZ or CO - just that there are extra hurdles to overcome that those running high-dollar turbos with custom high-dollar intercooling setups at Atco Speedway (cold, sea-level) don't have!

 

The end answer is, in your project you are seeing higher DCA because you lowered IATs through a combination of several means.

 

A gearhead statistic for you - every 10-degrees increase in IAT will loose you 1+ % torque; if your IATs are 150-degrees versus 50-degrees then your observed torque output (ft-lbs) will be off 10% (ballpark figure).

 

I don't have my EFILive logs here with me right now, if I remember I will look at them tonight. But from memory I am thinking that 1.40g/cyl at 6-psi is about right in line with my expectations, not great but not bad either, just about typical. The 1.20 g/cyl @ 10-psi metric is SAD. I bench race it this way: I have a bone stock LQ9, and it's 6.0L and makes ballpark about 350-hp; all things being equal, if that motor can breathe/process 12.0L of air (double your DCA from stock, double your power!) then you will be making ballpark about 700 horsepower. If I remember right, a stock motor under best conditions only has a logged DCA of 0.90 g/cyl, and I think that's being generous; assuming for discussion that number is correct, then at 1.20 g/cyl (ignoring boost MAP or IAT or how the hell it got into the chamber in the first place) that poor engine is only breathing/processing 8.0L of air (make that 8L of blistering hot air!), I come by that number as 1.2 / 0.9 = 1.33 (i.e. you are processing 33% more air mass than stock) soooo 6.0L * 1.33 = your engine will behave (and make power!) like an 8.0L motor. Basically. With the numbers you are seeing now (DCA = 1.40 g/cyl) your LQ9 is acting like it's actually a 9+ litre motor, because it's breathing/processing that much gross airmass! It will be interesting to see what your DCA is at 10-psi.

 

Again I am assuming the max observed typical DCA for a LQ9 is 0.90 g/cyl... I will have to go home and actually verify that number, I could be way off. But I hope you get the gist of the discussion.

 

Totally off subject, but when you log RPM, MAP, DCA you can take those numbers and plot a line directly on the compressor map for your turbo and tell exactly where in it's efficiency range the turbo is operating. FWIW.

 

Mr. P.

Edited January 6, 2011 by Mr. P. (see edit history)

[AKSSS]

I don't have my EFILive logs here with me right now, if I remember I will look at them tonight. But from memory I am thinking that 1.40g/cyl at 6-psi is about right in line with my expectations, not great but not bad either, just about typical. The 1.20 g/cyl @ 10-psi metric is SAD. I bench race it this way: I have a bone stock LQ9, and it's 6.0L and makes ballpark about 350-hp; all things being equal, if that motor can breathe/process 12.0L of air (double your DCA from stock, double your power!) then you will be making ballpark about 700 horsepower. If I remember right, a stock motor under best conditions only has a logged DCA of 0.90 g/cyl, and I think that's being generous; assuming for discussion that number is correct, then at 1.20 g/cyl (ignoring boost MAP or IAT or how the hell it got into the chamber in the first place) that poor engine is only breathing/processing 8.0L of air (make that 8L of blistering hot air!), I come by that number as 1.2 / 0.9 = 1.33 (i.e. you are processing 33% more air mass than stock) soooo 6.0L * 1.33 = your engine will behave (and make power!) like an 8.0L motor. Basically. With the numbers you are seeing now (DCA = 1.40 g/cyl) your LQ9 is acting like it's actually a 9+ litre motor, because it's breathing/processing that much gross airmass! It will be interesting to see what your DCA is at 10-psi.

 

Again I am assuming the max observed typical DCA for a LQ9 is 0.90 g/cyl... I will have to go home and actually verify that number, I could be way off. But I hope you get the gist of the discussion.

 

Totally off subject, but when you log RPM, MAP, DCA you can take those numbers and plot a line directly on the compressor map for your turbo and tell exactly where in it's efficiency range the turbo is operating. FWIW.

 

Mr. P.

 

 

Well I am kinda curious also what this thing is going to do also. Something to keep in mind about my truck. Compression is at 8.8. That will make a Huge diffrence on what the pressures are compared to the boost levels I would think. I am planning on turning this thing up around 15psi. If you have ZERO issues getting it up there(no detonation or anything), I might try a little higher except then I would have to go to a 3bar tune. That is a cool explanation of the engine processing of the air. I like that way of thinking....yeah, I have a 9L. Just don't look under the hood.

[AKSSS]

I'm bumping this up. There is alot of great information that I learned from some guys in this discussion and is great for new tuners. I read it once in a while to refresh it for me.

[GI.SS]

this is very good info!

[canadianVRss]

Yup, excellent read for sure.