Formula for Exhaust Gas Speed?

[TGP-37]

UPDATE Jan 02 2010: I am such a goof, lol. I just figured out comparing Injector ms to Wideband and subtract the time it takes for the RPMS to cycle from injector firing to exhaust valve open gives me the time it takes for exhaust gasses to travel from exhaust valve to Wideband. The Mass out is equal to the Mass in always, it is the volume that changes due to heat differences. All of this can be calculated easily in an excel table and thus, allows me to pinpoint the most likely wideband reading to the originating event time. It won't be a perfect calculation but it will be far better then simply sliding wideband readings back a cell or two. Since Exhaust Flow Velocity will surely speed up at WOT. PSI of exhaust is already determined per intake psi.

This will be a huge project but should it work out well the benfits for tuning would be profound, in my opinion. Anything done mathematically with a "good" formula which works will benefit.

Should all work out I'll post up the spreadsheet.

EDIT: Anyone?! Even just a simple velocity at a specific rpm such as idle. Even just a hint...lol

I'm trying to figure out a formula to calculate the speed of exhaust gasses.

I have a formula which calculates velocities of a given gas. But I need to find out how to calculate exhaust gas CFM from Airflow lb/min, Wideband AFR and possibly EGT.

Would anyone happen to know if there is a standard in the auto industry regarding exhaust gas speeds throught the manifolds, downpipe, ect?

Maybe even calculating cylinder pressures at exhaust valve open after combustion. Knowing the airflow in and area of combustion chamber at exhaust valve open, knowing the pressure could give me something to get a CFM of exhaust flow.

Maybe I am making much harder then it has to be. Can anyone chime in?

I wish to use this data in excel to match precise oxygen sensor measurements back to the combustion event that was responsible for it. Basically offsetting the O2's to match engine operation as if there was no distance from exhaust valve to oxygen sensors. Hopefully I can find usefull data to better fine tune my PCM.

[MikeGyver]

I've always wondered how fast oxygen sensors can react. Is that common knowledge?

[TGP-37]

I've always wondered how fast oxygen sensors can react. Is that common knowledge?

As in latency processing the signal or time from exhaust valve to the sensor?

Latency IDK but would like to. Similar to fuel injectors there should be a latency time for the sensor to react to the environmental changes. That should be a very quick process.

What I am after is timing the exhaust flow mathematically. Once I get the exhaust pipe diameters, lengths worked out it should be easy math from there.

If the exhaust piping measured 100 cubic inches to the wideband (which is 3 feet) then... the adjusted MAF volume by exhaust temp will = the exhaust volume. If it is example, 200 cubic inches per second then the exhaust gasses will travel 6 feet per second.

So the exhaust would take 0.5 seconds to reach the wideband. Meaning the wideband signal can be retarded by 0.5 seconds to get a more precise measurement.

greater precision = more power and greater control

Still working on it.

[jakebdb56]

I've always wondered bout a few small things like this while I'm tuning. Props to you for having the time to sit down and really think about it lol

[swebld]

I think Greg Banish mentioned this in his book. That you may have to tune for the change in volume in the exhaust system, or the changed distans to O2 sensors when putting headers on.

So, Greg. When will we learn more about this ??

[Alvin]

I'll have to look back in my notes. I remember in powerplants class we had a similar problem on a test. "Calculate thrust from exhaust on a top fuel car." Thrust is just mass * velocity. Its not something simple to find at all and its going to depend on RPM, heat added, energy extracted, etc. Better brush up on your chemistry and thermo.

[TGP-37]

I'll have to look back in my notes. I remember in powerplants class we had a similar problem on a test. "Calculate thrust from exhaust on a top fuel car." Thrust is just mass * velocity. Its not something simple to find at all and its going to depend on RPM, heat added, energy extracted, etc. Better brush up on your chemistry and thermo.

I found a simple method of calculating exhaust mass. Exhaust Mass = Intake Air Mass. The only difference is expansion by heat. Volume calcs utilizng temps will provide exhaust volume/density ect.

Beyond that I need the volume of exhuast piping from the exhaust valve to the sensor.

[Alvin]

I found a simple method of calculating exhaust mass. Exhaust Mass = Intake Air Mass. The only difference is expansion by heat. Volume calcs utilizng temps will provide exhaust volume/density ect.

Beyond that I need the volume of exhuast piping from the exhaust valve to the sensor.

That's wrong on so many levels.

Intake + fuel is going to = what comes out the back in terms of mass.


In terms of volume you'll need to look at the molar expansion across the reaction. Half of those processes are constant volume, other are constant pressure which makes a huge difference in the pressure and temperature at the end of the cycle. You can not use P=nVRT or any form of it ie (p/v=p2/v2). You need to know how much energy was release, what went into expansion, what when into heat, what was extracted by the engine as useful work, what went into waste energy like heating oil, heating coolant etc. All of these things is going to make a difference in your exhaust gas velocity.