? about Ideal Gas Law

[dordtrecht5]

Okay ... just a question or two about Ideal Gas Law in regards to turbocharging.

Is the Ideal Gas Law the same with gas AND diesel?

I am using PV=nRT equation to figure ideal air mass for my engine. Don't wanna get too complicated, I am just more or less playing because I desire to learn more about turbocharging, especially map reading and A/R.

P=Absolute Pressure
V=Volume
n relates to mass
R is a constant
T=Air Temp in Rankine

Is this the right equation?

Will anyone with EFI Live who logs frequently tell me what IATs you are getting when you drag race or sled pull? I am thinking 130-140 deg.F.

So ... what I have come up with so far to find out n(lbs.min) at 35 psi of boost and 3600rpm is 95.33612 lb/min. (THAT seems awfully high.) I do not want peak hp at 3600 rpm of course, but that is just a hypothetical number that I am using for MAX. rpm

Am I close?

The equation I used to find lb/min is

n(lb/min)=(14.7+psig) x Vcfm x 29 / 10.73 x Rankine

 

[dordtrecht5]

several looks, but as much as everybody talks like they are turbo gurus, not any replies:spit: ... any replies?

Anybody?

 

[[email protected]]

I would have to look back at my maf flow on my stock turbo. I can tell you that air mass per cylinder is going to drop pretty fast after about 2800-3000rpm.

At speeds over about 30mph my IAT's will stay within about 5-10degrees of ambient. But once you slow down to idle heat soak can push them past 130 even in cold weather heak soak will still push iat's into the 110 area.

I don't have my logs on this computer to see my maf readings. But I guess you have to remember ideal gas law is for an ideal gas. And there are a lot of other factor such as heat to deal with.

 

[dordtrecht5]

Thanks for all the help and input, guys.

 

[DAVe3283]

I believe your values are right, IF you could maintain 35 psi of boost at 3600 RPM, which I don't think is likely with a stock turbo.

I just looked at a log of my truck, and I log 62 lb/min at 3000 RPM, 37.0 PSI MAP, and 97 degree F intake.

Using your formula, and 350 CFM @ 3000 RPM (based on 100% VE), I calculate 62.8 CFM, which is darn close to the logged value.

SO: I believe you have it right, assuming you can get 35 PSI boost at 3600 RPM.

 

[TrentNell]

I believe your values are right, IF you could maintain 35 psi of boost at 3600 RPM, which I don't think is likely with a stock turbo.

I just looked at a log of my truck, and I log 62 lb/min at 3000 RPM, 37.0 PSI MAP, and 97 degree F intake.

Using your formula, and 350 CFM @ 3000 RPM (based on 100% VE), I calculate 62.8 CFM, which is darn close to the logged value.

SO: I believe you have it right, assuming you can get 35 PSI boost at 3600 RPM.

The intake air temp value you logged is at the MAF sensor and not the temp of the air entering the engine.

 

[DAVe3283]

The intake air temp value you logged is at the MAF sensor and not the temp of the air entering the engine.

That is correct, the air gets hotter when it is compressed, than cooled through the intercooler.

Overall, the lb/min into the engine is the same. The *density* and *velocity* are very different at the air intake vs. the intake manifold, but the same *mass* of air per second goes through both.

The CFM calculation does not reference temperature, so I believe everything still works OK.

 

[TrentNell]

That is correct, the air gets hotter when it is compressed, than cooled through the intercooler.

Overall, the lb/min into the engine is the same. The *density* and *velocity* are very different at the air intake vs. the intake manifold, but the same *mass* of air per second goes through both.

The CFM calculation does not reference temperature, so I believe everything still works OK.

I am not saying it wont work out on "paper", I just don't think its a reliable way to spec a charger , IMOP its loose data , to many "assumptions" and variables and does not reflect real world , even your logs are only as accurate as the voltage # GM decided to place there in reference to X flow , it does not have to be accurate to what the stock turbo flows, just accurate to GM's tuning goals, but I have never seen a Map of an IHI so that is an assumption too ................... but you get what i am saying.

I still trust experience over any other method , I look at Maps every day , figure PR's , ect, always looking for the perfect fit and new idea's , but it never leaves my head that the data is very loose and a turbo manufacture can scew a map very very easily to suit there needs , they get you close though.

another problem with a Diesel is there is way to wide of a gap on A/F/R , Its not really a problem it give us alot of room for error when comparing to a "gasser" , but also look at where "stoich" ( 15 ) is on a diesel , that is smoking like a freight train , lots more fuel than most run for a given charger size ......................................

 

[dordtrecht5]

Thank you for responding,
this is interesting that you mention the ever-changing variables. Heck, i was just looking into Barometric pressures and how much they can throw off the Rankine.

Out of curiosity I am wondering if anybody knows what the Brake Specific Fuel Consumption is on an LBZ? I understand that there are a LOT of variables in this as well, but I have seen numbers on the web from .38 (Garrett site) to one guy used a tech scan tool and got .67 on an LB7. That is HUGE difference when trying to determine actual airflow. Based on a 400hp engine there is a calculated difference of a little over 43.5lb/min.