Request for help calculating choke flow for Rally application (34mm restrictor)

[Inn-Tune]

Some of you may already know me as I've helped a few FSAE teams with parts and engine management system tuning. Today I need your help.

I need the choke flow on a 34mm restrictor to help properly design a turbocharged system that will be run on a rally car this year. The inlet restrictor will be 34mm x 3mm as per the current Rally America rules:
"The restrictor shall be placed before and within 50mm of the nearest impeller blade. The inlet restrictor shall have a single, circular opening through which all inlet air passes. There shall be no other provision for airflow to the pressure unit. The inlet restrictor installation shall include provision for sealing by the scrutineering staff. The restriction shall have a parallel area maintained for 3mm."

I found a link on this site to the KTH paper entitled "Simulation of turbocharged SI-engines - with focus on the turbine" which seems to include the formulas required for this calculation. Unfortunately, the units aren't listed and I'm too rusty on my high school/college math from back in the day to figure it out.

If anyone can help me calculate the choke flow on a 34mm restrictor I would greatly appreciate it. I see you've found the choke flow on the 20mm restrictor at 135 CFM so I'm sure someone can help me out here.

Thank you for your time and if I can shed some light on your engine management system tuning questions in return I'm happy to help.

-Mike

[Inn-Tune]

Some of you may already know me as I've helped a few FSAE teams with parts and engine management system tuning. Today I need your help.

I need the choke flow on a 34mm restrictor to help properly design a turbocharged system that will be run on a rally car this year. The inlet restrictor will be 34mm x 3mm as per the current Rally America rules:
"The restrictor shall be placed before and within 50mm of the nearest impeller blade. The inlet restrictor shall have a single, circular opening through which all inlet air passes. There shall be no other provision for airflow to the pressure unit. The inlet restrictor installation shall include provision for sealing by the scrutineering staff. The restriction shall have a parallel area maintained for 3mm."

I found a link on this site to the KTH paper entitled "Simulation of turbocharged SI-engines - with focus on the turbine" which seems to include the formulas required for this calculation. Unfortunately, the units aren't listed and I'm too rusty on my high school/college math from back in the day to figure it out.

If anyone can help me calculate the choke flow on a 34mm restrictor I would greatly appreciate it. I see you've found the choke flow on the 20mm restrictor at 135 CFM so I'm sure someone can help me out here.

Thank you for your time and if I can shed some light on your engine management system tuning questions in return I'm happy to help.

-Mike

[Superfast Matt McCoy]

Couldn't you just scale up the 135cfm by the ratio of the cross sectional areas of the two restrictors? In other words:

135cfm/(pi*20mm^2/4) = Xcfm/(pi*34mm^2/4)

X = 390 cfm

This is assuming your maximum velocity is the same. Of course your maximum flow might be a bit lower because of the 3mm restrictor length and the 50mm difuser length.

[Inn-Tune]

Looking at the equation I found it appears to be a linear relationship. I ran the numbers that way myself and considering the power output of rally cars on this restrictor it's at least in the right ballpark.

Does anyone know for sure?

[Jersey Tom]

You'll want to get your hands on ASME standard MFC-7M, "Measurement of Gas Flow by Means of Critical Flow Venturi Nozzles". Has all the stuff on how to calculate isentropic mass flow rates and design good choked-flow venturi-style restrictors. Cheap too I believe, we picked up a copy last year but I don't have it handy.

The super super important thing is inside surface finish, which will have such a big effect on your isentropic efficiency. Can change where your restrictor chokes by a good couple tenths of a mach number.

[Inn-Tune]

I ended up with a range now based on varying temperatures and specific gravity values which will help me set up the rest of the system.

Tom I will certainly look into that when reviewing restrictors. I was looking at the Raetech units just now as we may not bother making our own. I'll give them a ring to get some more info on Tuesday.

http://www.raetech.com/Restrictors/Inlet_Restrictors.php

Thanks guys!

[Jersey Tom]

Looking at their website, the shapes they use look pretty much dead on ASME standard. Good stuff. Expensive, understandably for billet parts.

Ohh I love scrounging stock and free machine time..

[rjwoods77]

http://fsae.com/eve/forums/a/tpc/f/125607348/m/26110633...10633531#26110633531 (http://fsae.com/eve/forums/a/tpc/f/125607348/m/26110633531?r=26110633531#26110633531)