Available in the latest VCM Suite beta is a new calculator for the Ford Speed Density model. This calculator is quite sophisticated, so an understanding of the process is vital to help get the best results from the calculator. The calculator is capable of giving you the quadratic coefficients including blowthrough, with the goal being to hasten/simplify the process of calibrating this model.
Our generated table is shown as load. As will be explained, a classical sense of VE % doesn't necessarily fit the calculator.
Speed Density Refresher
A quick refresh on the goal of Speed Density: convert various sensor measurements of air pressure, temperature, and other conditions to get an estimation of the amount of air in a cylinder when combustion occurs.
Typically, you're looking at Manifold Absolute Pressure (MAP), Manifold Charge Temperature (MCT), Exhaust Manifold Absolute Pressure (ExMAP), etc. to give you a good estimation based on the ideal gas law.
Starting with the basic equation, we can refine it to suit our needs:
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To convert from "moles" of stuff, to an actual mass, we use a specific gas constant for our gas (air at our reference conditions)
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And then we arrive at our final equation for estimating cylinder aircharge using the pressure, volume, and temperature for the gas in our cylinder! Easy!
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This would be the amount of air in the cylinder if we had those ideal conditions. Unfortunately, measuring the pressure and temperature of the air once the intake valve is closed is non-trivial, so instead we estimate it based on manifold conditions.
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But, that's in the ideal world. In reality, the cylinder may not have enough time to fill, some of the air may be pushed back out into the intake manifold due to valve timing, our temperature may not be an accurate representation, etc, so a "fudge factor" known as "volumetric efficiency" is introduced. This makes our final equation roughly:
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The basics of calibrating speed density revolve around getting volumetric efficiency right in as many conditions as you can.
How is Ford different?
It isn't really, it still bases its model off of measured/inferred manifold pressure, but it of course has a wrench thrown into it to improve accuracy in many varying conditions.
You may have heard the model is quadratic in nature. This is only partly true. The real model is actually multi-faceted, and has several parts.
There are several terms we need to define:
Maximum Trapped Aircharge - This is the maximum amount of air that can be in a cylinder for a given cylinder condition (pressure and temperature). By definition, you cannot get more air in there, you would need a higher pressure, lower temperature, etc. This is defined by physics.
Standard Aircharge - This is the maximum amount of air that can be in the cylinder at STANDARD conditions. (For most Ford vehicles, this is 29.95inHg of pressure, 60*F air temperature). This is the value labelled "Engine Displacement" in HPTuners.
Blowthrough - This is a condition where we are flowing more air through the cylinder than the maximum trapped aircharge. This happens when the intake and exhaust valve are both open at the same time, and MAP > ExMAP. Also known as scavenging.
Load - This is a ratio of the current aircharge in the cylinder to the standard aircharge.
As such maximum trapped aircharge is like our 100% VE aircharge amount.
So, how does the ECU calculate maximum trapped aircharge? First, takes the standard aircharge calibrated under known standard conditions, and corrects this for current conditions. Some of these can be the effect that engine coolant temperature and air temperature might have on charge density (see table below), as well as others. The idea is to get close to the best guess at in-cylinder conditions and use the ideal gas law to get this aircharge.
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So, rolling this all into one value, which Ford simply calls "C", probably for "Constant at all the conditions we just took out", we can now calculate the maximum trapped aircharge simply as:
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What about greater than 100% VE?
This can happen, when conditions in the cylinder are better than the raw equation might suggest. Typically, the weak link is our use of manifold pressure to estimate cylinder pressure. Certain conditions can actually cause our cylinder pressure to be higher than measured manifold pressure, for example, well tuned "resonant" intake runners can make use of the energy of the air moving to help pack more air into the cylinder.
To correct for this, Ford has the "Aircharge Multiplier" tables. These directly multiply the calculated C value from above, to allow you to compensate for these effects (whether helpful >1, or harmful <1). Remember, this compensates the actual in-cylinder aircharge. If that air is going into the cylinder, but not taking part in combustion due to leaving through the exhaust valve for example (aforementioned blowthrough), that does NOT become a multiplier to the table.
Can I shove more air in than this 100%? What is blowthrough?
You may be thinking, "with boost I have VE greater than 100%!", this would be true if your equation DIDN'T already account for this with its dependence on MAP. So what happens if we DID somehow setup a way to get more air in, say through some nice overlap with the exhaust cam, with our intake MAP > exhaust MAP?
Well, you have to evaluate: Is this going to increase the air in the combustion chamber ABOVE my theoretical physics calculation? Will this air stay in the cylinder, or will it blowthrough it because of overlap?
If it stays in, Aircharge Multiplier adjustments are needed.
If it blows through and into the exhaust, blowthrough tuning is needed.
Why does Ford track blowthrough?
Well, fresh air in the exhaust messes up our oxygen sensor readings. We now have excess oxygen in the exhaust, so our measured exhaust lambda is now leaner than what actually happened in the cylinder. However, if we know how much fresh air made it into the exhaust, we can compensate our measured exhaust lambda to get an idea of the in-cylinder lambda. This means we can preserve closed loop fueling in these conditions.
It should be noted, blowthrough is really only calibrated by Ford on the Ecoboosts, a lot of the other vehicles have it disabled as it is unneeded. These applications didn't have the ability to dynamically vary overlap with older VCT, or they were NA applications that couldn't necessarily hit the conditions to have enough blowthrough to worry about. Roush and Whipple calibrations dont enable blowthrough logic even though they could see it, as they often aren't run in conditions where it would make enough of an effect that would warrant it. GT500 didn't have VCT and had a set overlap without blowthrough even under boost, so its uncalibrated as well.
Look below for a section on whether or not you need to consider calibrating for it in your application.
Okay, so enough about max trapped aircharge, what about how it actually calculates the air in the cylinder?
We needed to get to the blowthrough part of the talk so that we could finally answer how the quadratic terms and blowthrough slope play together.
Ford came up with this model after extensive measurement. Using the figure from their patent on this model with blowthrough (US 20130111900 A1):
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The line representing our maximum trapped aircharge is line 202. 220 represents actual measured points of MAP and total aircharge. They noticed once blowthrough begins, the points start to follow a linear line, and so blowthrough has a single "slope" value. The points below that are quadratic in nature, and so have a quadratic fit (204).
So, once you've reached conditions where blowthrough can happen (you'll notice it usually conveniently starts around when MAP > ExMAP), our in-cylinder aircharge becomes our max aircharge, and our extra airflow that fools the oxygen sensor is tracked via blowthrough slope.
So, we kind of have three calculations going on:
- Maximum trapped aircharge
- If we are flowing more than maximum trapped aircharge, incylinder airflow is capped to the maximum aircharge. Blowthrough air is tracked via the slope from the point of where it started
- If we are flowing less than the maximum trapped aircharge, incylinder airflow follows the quadratic curve. Blowthrough airflow is zero.
As well, when solving the quadratic by inputting MAP and getting out aircharge, there are several cases:
- If both roots are real and negative, or our quadratic term is zero, fall back to the linear model. (Solve the linear equation of MAP = Slope*Aircharge + Offset)
- If both roots are real and positive, we use the smaller root.
- If both roots are real and only one is positive, we use the positive root.
- If both roots are imaginary, airflow is set to the maximum trapped airflow.
As well, some of you may be aware that the quadratic term is "orthogonal". This means that if you zero it, you should still have a decent enough fit for the data. The quadratic term actually affects the slope and offset of the function while finding the quadratic solution. As such you cant just take a "normal" quadratic solution to the numbers you see, they need to be adjusted with respect to the quadratic as well.
Once we do all this, we have a nice calculated aircharge, but its uncorrected. So, we correct for the same effects we did the maximum trapped aircharge. Air density, ExMAP (which is another model entirely that may need calibration), coolant temp, etc.
This final corrected value is then our aircharge.
This is really complicated
We agree. That's why we made the calculator to handle all the background math the same way the ECU does, and calculate the correct "orthogonal" quadratic curve for you.
Calibration
Why do I need to calibrate this?
The following systems use this to help predict aircharge/infer MAP, and as such dictate the importance of this calibration:
- Manifold Filling/Transient - Inferred MAP/Aircharges use these values extensively
- Throttle Body Control - Inferred MAP and aircharges are used to predict throttle airflow, as such are direct players in the throttle body control
- Injector Flow Rate - Inferred MAP on Port Injected motors directly goes into the flow rate calculation
- Sensor Failure - Even if your MAF fails, this acts like the modern Load With Failed MAF, amongst other things
- Actual Airflow - On Speed Density applications like Ecoboost, this is the air model. You don't have a MAF to feed input into all the models, you have this.
How-To
You'll notice the table is RPM vs. MAP vs. Load. The idea is, you hit a target RPM and MAP in steady state, and adjust the load to either the actual load if you're a MAF car, or adjust it based on fuel trims assuming your fuel model is correct (You are using good injector data aren't you?) if you're an SD car.
Lock your car in a given Mapped Point, or use the logged parameters to make sure you update the correct mapped point! It's really that straight forward.
For example, if we're on a MAF Equipped Coyote, and we hit steady state, we may notice the load in our table is off from what we actually have measured via our installed MAP sensor and actual load. If we dial this in, we improve airflow anticipation and throttle control as the ECU can more accurately calculate airflows downstream of the MAF. By remaining in steady state, we aim to take out the effect any transient corrections based on the current model may have. Without a physical MAP sensor, this calibration is also just guess and check at that point. You cannot use the inferred MAP to try to correct it, as that's what you're trying to correct!
For SD cars, this is all you have to estimate cylinder airflow! You need this dialed in as it is the vehicles air model, which everything else based on load is based off of. Its the only way the engine knows how much air is actually entering the cylinders.
Blowthrough
Calibration of blowthrough is more complicated. First, you need to decide if blowthrough is something that can happen on the vehicle, and if its effect is great enough that it warrants need. If it can, you need to enable it.
How do you know? Well you have a good MAF curve / SD calibration, but you notice during conditions that could cause blowthrough, that your fuel trims go lean. This cant be MAF error (you dialed that in!)
If you start to see this happen and you're solid on your MAF curve and its only happening for these airflows when your cam positions could be causing it, that's how you know its time to enable it.
On SD, you calibrate blowthrough just like normal, its just more important your maximum aircharge line is dead on so its not actually injecting more fuel, but simply correcting the readings for blowthrough.
Then, just adjust your load values in those cells with the percent you're off like normal, and you should notice it goes away. To be sure it's calculating it as "blowthrough" you can log InjPW or Fuel Mass injected to verify that's constant. You're seeing the effect of blowthrough "fixing" your readings.
To properly calibrate this, we should mention, the load you see from the scanner WILL NOT match the load in the calculator during blowthrough. The Load in the scanner is incylinder aircharge, where as the calculator is showing a load based on total aircharge (including blowthrough)! The reason to do this is it allows you to still make the same percentage changes to get the desired effect and allows us to keep it all in one table.
Caveats
Why does my load suddenly spike or go negative at low MAP values?
What you see is what the ECU would output in that given condition. Typically, what is happening is you are below the offset value. I.e. the MAP it shows would never be reached by the vehicle running at that RPM and cam position. Depending on whether it devolves to the slope based model (which will give you negative loads!) or caps it to max trapped airflow (suddenly larger loads), the calculation is still correct as to what the ECU would infer the load to be for those conditions (which wouldn't realistically be reached anyways!). Don't worry about calibrating these points unless you think you will have a valid value for them. The calculator attempts to intelligently work around them!
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Originally Posted by Nader
