# Exhaust flow

Is there a way you can calculate exhaust flow using intake flow and exhaust temp? Maybe I would need fuel quantity as well. I just need a rough idea.. It's for on a diesel.

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mfb
Mentor
Mass is conserved, if you measure flow as mass per time it is easy.
If you measure volume per time: the ideal gas law should give a good approximation.

I need more specifics. I'm slow..

mfb
Mentor
Your question was "is there a way", and my answer is an extended "yes", together with a rough idea how (ideal gas law).

If you have more questions, feel free to ask them.

I mean up to this point I know how it all works before fuel is injected. I can get the engine flow and everything, but if you inject and ignite fuel, I don't know how to calculate how much heat is added and how much flow results or is the exhaust temp all that matters? Actually, when fuel is injected, the air is just heated way up so the only thing that changes is pressure right? Density stays the same since the moles of air in there is the same as before, they are just much hotter. Is all that right? So when the exhaust valve opens, temperature drops as pressure drops. But I guess the exhaust gas temp is an average temp and would work. Is this all correct?

mfb
Mentor
Exhaust temperature and added heat are related, and determined by the type of fuel, its amount and the initial temperature.
the air is just heated way up so the only thing that changes is pressure right?
The chemical composition changes, too.
Density stays the same since the moles of air in there is the same as before
The number of moles can change, but density has to stay the same (if the volume is constant).
But I guess the exhaust gas temp is an average temp and would work.
Average of what?

Average temp of the gas flowing out of the engine. With that temp you can do all the flow calculations I assume. How does the number of moles change? I realize the diesel adds some but does it add a significant amount?

mfb
Mentor
Average temp of the gas flowing out of the engine.
Okay.
How does the number of moles change? I realize the diesel adds some but does it add a significant amount?
Diesel is mainly a source of carbon and hydrogen, and hydrogen reacts as ##O_2 + 4 H \to 2 H_2 O##, so you get an additional molecule. ##O_2 + C \to CO_2## does not change the number of molecules. Each diesel molecule gives several of those reactions before it is gone.

Using the Ideal Gas Law I end up with the same CFM going into the engine as going out. However, putting in the different air temperatures mean the pressure going out is much higher. Does this sound right? I am assuming nothing about injected fuel, etc, just incoming 100F air and outgoing 1000F air. Comparing 1000F to 100F the only thing that changes is pressure. Just confirming here.

mfb
Mentor
Using the Ideal Gas Law I end up with the same CFM going into the engine as going out.
Should be a bit more due to the reaction I posted.
However, putting in the different air temperatures mean the pressure going out is much higher.
Right

I didn't take fuel into account at all. I have enough trouble figuring out how to calculate it all without the fuel..