Throttle body Valve (butterfly type) sizing

[uptick]

Hello,
I am new here. I have a customer asking how to size a correct throttle body for their engine? This TB will control the fuel (NG) line, not on the mixer line. What information do I need from them and how do I calculate the size of the TB?
Is there any relationship between the Cv data and the throttle flow area?
Please help. Thanks in advance
Utick

 

[JimCasey]

If I remember from my gearhead days reading HotRod magazine, carburetor flow rates are rated with 1"Hg loss through the throttles. Therefore a Holley 650 flows 650 CFM with atmospheric pressure at the inlet and 1" Hg subatmospheric down.

Arbitrarily selecting a 345.6 cubic inch engine for simplified math (I hope): it will theoretically inhale 0.1 cubic feet of air per revolution. ( there's the simplified math part)
If you spin it up to 4000 rpm, there's a 400 cfm requirement.(Assuming volumetric efficiency of 1.0, which is unlikely unless using tuned intake)

THat would require a valve with a Cv of 157, which would be really close to a 3" diameter throttle plate.

Put in your motor parameters and ratio to this.
Throttle Cv would be ratioed, also.
Cv is proprtional to the AREA of the plate, so
d=SQRT(Cv/17.4)

 

[uptick]

Jimcasey. Thanks very much.
1. Can you care to share how to get Cv of 157?
2. Assume d is the throttle diameter. Eqn d=SQRT(Cv/17.4) applies to any gas?
Thanks
Uptick.

 

[JimCasey]

1. Can you care to share how to get Cv of 157?
I plugged the flow parameters in to the Flowserve sizing program to calculate the Cv. You can download the Flowserve program, and well as programs from most any valve manufacturer. There is an equation, but it is ugly.

2. Assume d is the throttle diameter. Eqn d=SQRT(Cv/17.4) applies to any gas?

Well, yeah, but the Cv required will change if the gas specific gravity changes. Are you taking this engine to a place with a non-air atmosphere? Here on Earth, I expect that Natural gas will be metered in below the throttle body and will not flow through the throttle plate.

 

[uptick]

Thanks Jim,

Yes, I understand the gas type is a factor in Cv equation. In eqn d=SQRT(Cv/17.4)will the constant 17.4 change with different gas used?
Do you have any data / table of these value for use? Thanks

 

[JimCasey]

17.4 is just 157/3^2 ; solving for the scale factor of the throttle valve. It is a rough approximation, but sizing of a throttle valve does not have to be superduper precise. More precision would require a lot more information than has been offered.

Yes the Cv calculated WILL change with different gases because the Cv calculation has gas density as a parameter.

The Cv equation for compressible flows has nine (9) variables in it, some of which are calculated from other parameters. you really don't want to crunch through it manually. Cv is inversely related to the square root of Molecular weight. Effective MW for air is 29.

Still, I contend that the throttle plate will/should only see air. Your natural gas flow should be regulated as a function of the actual or projected mass flow through the throttle plate at its range of openings and over the range of engine speeds. NG or Propane carburetors exist that do this well, and althought I was not specific, it was my intent that you use the scaling guideline I offered to find an appropriately sized carburetor to meet your needs.

To find the gas flow you will need to determine the stoichiometric ratio of the fuel to the air, and then determine how much of the combined gases the engine can inhale. THe fuel gas will limit the amount of air that the engine ingests by displacing a part of the incoming charge.

Example: So since oxygen is 20% of air, if you burn pure methane
CH4+2O2> CO2 + 2 H2O
If the methane is warmed to the same temp as the incoming air, you would need one part methane to 2 parts O2 +8 parts N2 and other atmospheric components. 11 parts total. The necessary gas would be 9% gas by volume in this example.

DOn't know at what pressure the methane is being stored, WHst the initially regulated pressure of the methane, whether it is flowing thru a hx to normalize the temps, so it's darned hard to guess what size device would be there to regulate the fuel gas.