? about Ideal Gas Law

dordtrecht5

dordtrecht5

Mr. Having Fun
Joined
Nov 9, 2010
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46
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, or any data logger 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 for my particular application, but that is prolly tittlywinks to some of you.) 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
 
joefarmer said:
Loaded IAT is likely 2-4x that amount.
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YOU ARE WRONG!!! :hehe:

I checked a few EFI runs. 474hp dyno run, stage 3 CP3, unknown 50hp injectors, 2003 LB7, OAT 22F, IAT 72F peak.

Stock 2001 LB7, factory everything. 74F OAT, 117F IAT.

Now I wish I had an aluminum headed cummins that flowed like a dmax head. :(
 
nice to know ... so I am not all that far off ... on IAT. So on race day, on a track in the Midwest, track temp being at least 110 on a 100 deg. day .... LOL A good intercooler might keep it near 140.

So, for the Volume ... do you suppose I should be using the RPM at maximum, or should the RPM be at peak HP? I have been looking for the answer to this, but it is coming on any searches so far.

Does 96 lb/min sound high or just about right? Oh, and another, that number is at 100% efficiency, I dont know an engine that is that efficient. 80 to 90 might be a little closer, but tuning can compensate with this figure.
 
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I think you're getting close to understanding the VE of your engine once you figure out that it's not using 100% of the 96 lb/min. I'm not sure, but on the 474hp dyno run the MAF said 62.3 lb/min. That would make his truck's VE ~60%. It's also quite possible that I'm not understanding any of this.
 
joefarmer said:
I think you're getting close to understanding the VE of your engine once you figure out that it's not using 100% of the 96 lb/min. I'm not sure, but on the 474hp dyno run the MAF said 62.3 lb/min. That would make his truck's VE ~60%. It's also quite possible that I'm not understanding any of this.
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Now I may be way off base here, but from the info I gleaned from Garrett's turbo 101 stuff, VE has no affect on power vs. mass flow. That would be a mass flow vs. pres. issue.

From the spreadsheet I had from Garrett's formulas, for 474hp at a 21:1 air/fuel ratio, you'd need 63 lbs/min. Using this, with a VE or 90% for a 5.9 at 3k RPM at 140F, you'd need 49 PSIA. At 60%, all else the same, 75 PSIA.
 
A good rule of thumb is 8 HP at the wheels per lb/minute of airflow on turbo diesel engines. I wouldn't necessarily use this rule of thumb to size turbos, but I does give you a close estimate of real world results.

Using this rule of thumb, 95 lbs per minute should equate to roughly 760 rear wheel hp.

Also note, the symbol n in the ideal gas law stands for moles of gas. Use wikipedia and read up a little bit more on the ideal gas law. To figure out the mass of air, you'd have to add up the atomic weights of the elements in various common molecules in atmospheric air and multiply that info by the number of moles to get the actual mass flow of air.
 
joefarmer said:
YOU ARE WRONG!!! :hehe:

I checked a few EFI runs. 474hp dyno run, stage 3 CP3, unknown 50hp injectors, 2003 LB7, OAT 22F, IAT 72F peak.

Stock 2001 LB7, factory everything. 74F OAT, 117F IAT.

Now I wish I had an aluminum headed cummins that flowed like a dmax head. :(
Click to expand...

74 deg. IAT is when you're going down the highway...put that truck on the sled track and see where it's at. Just get the engine up to operating temp then leave it set and idle, check the IAT after a few minutes.
 
Big Blue24 said:
Also note, the symbol n in the ideal gas law stands for moles of gas. Use wikipedia and read up a little bit more on the ideal gas law. To figure out the mass of air, you'd have to add up the atomic weights of the elements in various common molecules in atmospheric air and multiply that info by the number of moles to get the actual mass flow of air.
Click to expand...

Good to know ... thanks!
 
Tate said:
Now I may be way off base here, but from the info I gleaned from Garrett's turbo 101 stuff, VE has no affect on power vs. mass flow. That would be a mass flow vs. pres. issue.

From the spreadsheet I had from Garrett's formulas, for 474hp at a 21:1 air/fuel ratio, you'd need 63 lbs/min. Using this, with a VE or 90% for a 5.9 at 3k RPM at 140F, you'd need 49 PSIA. At 60%, all else the same, 75 PSIA.
Click to expand...

I see your point, however, I think that the VE and the formula does have something to do with power v. mass flow. When making reference tomass flow, in this equation, it is what the engine is making BEFORE a turbo is placed on the engine so that the builder can know what size of turbo he will need in order to make the require amount of power. With that said, the equation is based on gas engines, not diesel. That is why I asked the question in regards to gas in the original post. So MAF readings on data logs do me no good really IMO. Why I say that is because there are too many variables to account for when reading maf once a trubo is installed on the engine, AND I am not that smart!!:hehe:

The reason I am asking about this stuff is because I wanna build my own turbo on the cheap. My plans are a nice, efficient single for hot rodding, drags and sleds (WS) and then build another S400ish for a compound system (when the budget allows for that). Just trying to learn more about turbocharging so I have decent knowledge before I build. Wheels size, A/R, compressor, turbines ... the whole nine yards.
 
dordtrecht5 said:
I see your point, however, I think that the VE and the formula does have something to do with power v. mass flow. When making reference tomass flow, in this equation, it is what the engine is making BEFORE a turbo is placed on the engine so that the builder can know what size of turbo he will need in order to make the require amount of power. With that said, the equation is based on gas engines, not diesel. That is why I asked the question in regards to gas in the original post. So MAF readings on data logs do me no good really IMO. Why I say that is because there are too many variables to account for when reading maf once a trubo is installed on the engine, AND I am not that smart!!:hehe:

The reason I am asking about this stuff is because I wanna build my own turbo on the cheap. My plans are a nice, efficient single for hot rodding, drags and sleds (WS) and then build another S400ish for a compound system (when the budget allows for that). Just trying to learn more about turbocharging so I have decent knowledge before I build. Wheels size, A/R, compressor, turbines ... the whole nine yards.
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You need two things to make power, fuel and air. If you can cram 100 lbs/min air into a 4 banger at 10k rpm at X fuel ratio, or 100 lbs/min into a 16 cylinder 1000 rpm at X fuel ratio, your power out put (given the same thermal efficiency) will be the same.

VE on an NA engine will determine how much air it can take in per stroke, which will affect the lbs/min, which at X fuel ratio, will affect the power. Since we are cutting straight to the mass flow, the VE has nothing to do with the power output. It will affect the the pressure required to fill the cylinders with the required mass flow.

Like Big Blue said, the Garrett site does have lots of good info, and does deal with diesel in a couple sections.
 
Tate said:
You need two things to make power, fuel and air. If you can cram 100 lbs/min air into a 4 banger at 10k rpm at X fuel ratio, or 100 lbs/min into a 16 cylinder 1000 rpm at X fuel ratio, your power out put (given the same thermal efficiency) will be the same.

And that is the reason there are variables in the equations in relation to engine speed, displacement, and horsepower levels.

VE on an NA engine will determine how much air it can take in per stroke, which will affect the lbs/min, which at X fuel ratio, will affect the power. Since we are cutting straight to the mass flow, the VE has nothing to do with the power output. It will affect the the pressure required to fill the cylinders with the required mass flow.

Like Big Blue said, the Garrett site does have lots of good info, and does deal with diesel in a couple sections.
Click to expand...

Got about the same numbers NOT using Air/Fuel numbers. It is good to have this information though. I like the Tech articles on the Garrett site. One person noted on another site that thre is a huge amount of variables involved with this stuff. One of the things that made the variable stand out the most, IMO, is the Brake Specific Fuel Consumption. I am trying to find a good average number for the LBZ Duramax, but am having a hard time because it really depends on so many other variables like load, IAT, PSIA, etc. For example, on an LB7 one guy said he figured .67BSFC with a light load at 60mph. Garrett uses a BSFC of .38 for a Duramax. The difference of lb/min for actual airflow, just between those numbers right there, on a 400hp Duramax is 98.26 lb/min (using Garretts theory and at .67 BSFC) as opposed to a 55.73 lb/min @ .38 BSFC. It is easy enough to calculate BSFC using the Pounds Per Hour divided by HP. But again, this equation is going to be different through the RPM cycle.

I am not trying to brain f*** this whole thing, just trying to get some base numbers to use.
 
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If you're trying to figure out what turbo to use, I've always gone about it like this....

displacement / 3456 x rpm = theoretical airflow

Theoretical Airflow x Engine VE (guessing 80% at best with a dmax) = closer to real airflow

Closer to real airflow x pressure ratio = best guess turbo cfm.
 
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