| Author | Topic |
|---|
Location:
Montrose, VIC
Registered:
May 2002
|
|
Flow vs. Pressure
|
Mon, 03 January 2005 11:27
|
 |
|
I've been through a 900 page thermodynamics book with minimal success, and had a look on the net without much luck yet. To save me a trip to the library, who can point me at a formula that can help me work out the difference in flow through an injector that changing fuel pressure will make? Someone must have an idea!
|
|
|
Location:
Brisbane CBD
Registered:
September 2004
|
|
Re: Flow vs. Pressure
|
Mon, 03 January 2005 12:57
|
|
|
its to do with the area of the injector valve thing vs the flow start from there sorry verry late just throwing stuff
|
|
|
Location:
Perth
Registered:
June 2002
|
|
Re: Flow vs. Pressure
|
Mon, 03 January 2005 14:39
|
|
OK, I don't know if this will help at all, and it may be a little deep for what you are wanting to work out, but try anyway.
The units are a little funny but if you're good with maths you should be right.
Also I can't type the formula out so it looks right on the screen, or I would have done that!
1. Multiply the specific gravity of the fuel (which will change with temperature) by 12.
2. Divide into this figure the differential pressure across the injector in inches of water.
3. Multiply this figure by 64.34.
4. Square root this figure.
5. Multiply what you get by the cross-sectional area of the injector in square feet (1mm squared is equal to 1.076391 x 10 to the power negative 5 square feet).
6. The figure you now have is the volumetric flow through the injector in cubic feet per second! 
This formula of course doesn't take into consideration the viscosity of the fuel, velocity of approach to the injector or frictional losses, but these will make bugger all difference in the real world anyway!
Hope it helps!
Gabe
|
|
|
Location:
Perth
Registered:
June 2002
|
|
Re: Flow vs. Pressure
|
Mon, 03 January 2005 15:04
|
|
Just with that, you will need to divide down the cubic feet per second figure you come up with, to the duty cycle of your injector.
Also, this formula assumes the injector is instantly open. But in the real world it takes time to open then close again. So the answer won't be spot on, however, you can compensate for the difference when working the formula through!
Gabe
|
|
|
Location:
Adelaide
Registered:
June 2002
|
|
Re: Flow vs. Pressure
|
Mon, 03 January 2005 20:19
|
|
To find the new flow rate if you have increased the rail pressure do as follows-
SQRT(new press/old press) X old flow rate = new flow rate
It means of course that increasing rail pressure by 100% only gives a 41% flow increase-f***en laws of physics!
I think that comes from Bernoullis stuff if my memory serves correctly.
Have a look at the RC Engineering site-lots of useful stuff.
http://www.rceng.com/technical.htm
Cheers
Sean
|
|
|
Location:
Perth
Registered:
October 2003
|
|
Re: Flow vs. Pressure
|
Mon, 03 January 2005 23:55
|
|
Just looking at it quickly id say its just a simple case of Bernoullis as sean suggested.
After cancelling all the commons out like density and area of injector and subbing in flowrate formula you end up with:
P(1) + Q(1)^2 = P(2) + Q(2)^2
Therefore
Q(2)= SQRT(P(1) - P(2) + Q(1)^2)
This gives a different answer from seanns formula.
But now you know where you are ended (start with bernoullis) it should be easier. May have made a mistake somewhere.
|
|
|
I supported Toymods
Location:
melbourne
Registered:
June 2002
|
|
Re: Flow vs. Pressure
|
Tue, 04 January 2005 00:15
|
|
mrshin, id just work it out experimentally (set duty cycle, measure pressure, hose petrol into a easuring glass for a set period of time), as i dont think it will increase linearly with pressure, and the outlet nozzles will cause issues too.
I dont think you can really get shitloads more flow from just increasing pressure, but it is worth a try.
|
|
|
Location:
Newcastle
Registered:
April 2004
|
|
Re: Flow vs. Pressure
|
Tue, 04 January 2005 00:41
|
|
Try looking in a fluid dynamics or momentum transfer textbook. Thermodynamics is about heat.
I cant help with any formulas, but bernoullis would be an approximation. It doesnt take into consideration viscosity (which is a function of temp so make sure you make any measurenents at the same temp) or pressure drops due to bends or change in area of the injector or the pipe.
|
|
|
Location:
Montrose, VIC
Registered:
May 2002
|
|
Re: Flow vs. Pressure
|
Tue, 04 January 2005 10:49
|
|
I was leaning towards bernoullis theorum, good to know I've got a few other people to blame as well if it doesn't work 
I was using the thermodynamics book simply because I had it handy, leftover from uni. The change in viscosity and density with temperature are being compensated seperately with a temp sensor.
I need to be able to do this simply because with what I'm doing, there are enormous changes in fuel pressure which I can't do anything about. I already have a temp and a pressure sensor in the fuel rail, I'm just trying to make up some tables to shove into the Motec, no doubt they'll need a little tidying up on the dyno later anyway. Thanks guys!
|
|
|
Location:
Sydney
Registered:
January 2004
|
|
|
