Quote:
Originally Posted by J.Fast
The initial starting point is where I'm hung up... I'll try and do the math and hopefully you guys can double check it for me and then once it can be confirmed it's right we'll go from there. So to do this I ineed to calc the volumetric airflow requirement, the stage 1 turbocharger compressor outlet temperature, the stage 1 density ratio, the stage 2 compressor outlet temperature, the stage 2 density ratio, the combined density ratio, and the intercooler density change?
Volumetric airflow(cfm) = RPM (X) Displacement (X) .5 (X) (volumetric efficiency/1728)
Absolute compressor outlet temp = Compressor inlet temp (*R) + (((Compressor inlet temp (x) Pressure Ratio) - Compressor inlet temp) / Compressor Efficiency)
Density ratio = (Inlet absolute air temp(*R) / Outlet temperature(*R)) (X) Pressure Ratio
Combined density ratio = 1st stage density ratio (X) 2nd stage density ratio
Intercooler density change = (Stage 2 compressor outlet / Stage 1 compressor outlet) - 1
Used my thermodynamics book for this so hopefully this equation setup is right.
Jeremy
I'll reiterate again that I'm not sure I know as much about this as I think I know, so I may be off and take what I write as an opinion. It's certainly not a definitive lesson on the topic.
So how I would start is the same way you have: by figuring the cfm of the engine. I'll just go ahead and ASSume 7000rpm and 90%ve for that, which equals 345cfm.
Now the question is what's the cfm I need to make 1000hp? I figure on 1lb/min equaling 10hp, so that's 100lbs/min. And 1lb/min equals 14.47cfm at 1 atmosphere (14.7psia) so we want to flow 1,447cfm to make 1,000hp.
1,447/345=4.19 round to 4.2
take the square rt of 4.2 = ~2.05 <-that's the pressure ratio I want each turbo to hit.
To find the flow rate needed for the second stage, I would just use this calculator:
Not2Fast: Turbo Calculator
After plugging in all the engine variables and using 15.4psi, (roughly equal to 2.05pr,) it tells me the small turbo needs to draw 655cfm, or the equivalent of 49lb/min, if it were drawing from atmosphere. Although it's not drawing from atmosphere, I think it should work when looking at compressor maps.
So, first stage turbo should flow 100lb/min of air, and second stage should flow 49lb/min of air when looking at compressor maps. Both should be just above the 2:1pr line. Looking at Garrett maps, a GT4718 and a GT3582 would work well. As for the turbines, I don't even know where to start haha.
None of this takes into account the effect that heat has on the density of the charge, but there's no question that a GT4718 can make the power, and the 3582 would be a decent match to the motor as well. So if you were to use these turbos I'd plan on maybe having to gun a little more boost than what I've figured here. If I were to change anything I'd maybe go a little larger on the secondary turbo, but not by much.
Hope this helps you, and I look forward to seeing what you do with this. It's really interesting and I think I probably stand to learn a thing or two.
