Ratio of psi vs. remaining fuel?

[John Mitton]

I know CNG is not linear in terms of pressure vs. what % of fuel is left in the tank. Does anyone have this curve? For now our experience is something like this:

Full tank @ 3600 psi
3/4 tank @ 2600 psi
1/2 tank @ 1500 psi
1/4 tank @ 800 psi

Thanks!

[freedml]

Re: Ratio of psi vs. remaining fuel?

It should be linear except for 1) the amount of fuel left in the system that can't be used, and 2) temperature changes.

[Steve]

Re: Ratio of psi vs. remaining fuel?

In addition to what freedml has said, here is Boyle's Law and its equation:

http://en.wikipedia.org/wiki/Boyle%27s_law#Equation

No exponents so as pressure decreases, volume (or the mas of the methane) decreases linearly.

[jetboatjohnny]

Re: Ratio of psi vs. remaining fuel?

Ok so it's linear, then how about this scenario. I fill up and usually get at least 3600psi. After the gas cools, the pressure is around 3,000psi (from top off experience). I drive around 175 miles or so, and the light comes on and I am at 1,000psi. Does that mean I have 1/3 tank left? The temperature has remained 70 degrees F the whole time.

[John Mitton]

Re: Ratio of psi vs. remaining fuel?

Right, we are going to assume constant temperature here.
Two of my kids have older Civic GX's ('98 and '99) and it seems the fuel gauge drops faster toward the top of the scale than towards the bottom. Our newer '07 drops almost linearly so I am guessing Honda incorporated Boyle's curve into the fuel gauge driver for the newer models? Does anyone know if they included temperature compensation too?

[John Mitton]

Re: Ratio of psi vs. remaining fuel?

Johnny, this is an older model GX, right?
Do you have a separte pressure gauge installed on the car?

[jetboatjohnny]

Re: Ratio of psi vs. remaining fuel?

I think it must compensate for temperature, because I noticed Thursday morning I topped off and the pressure was about 2500psi and the gauge was still above the full line (before filling). It took about 1.3 gge, temp was about 65. The night before I filled and it was about 85 degrees. I have noticed when it is warmer the gauge is at the full mark right around 2800-3000psi.
I can usually go 80 miles from the full mark (highway) to the 1/2 mark, then about 50 miles untill the light comes on, and this is regardless of the temperature out.
The pressure readings I am getting are off the pump when I connect the nozzle, and I am using the same pump on a daily basis. Topping off I can get as many as 70 miles before I hit the full mark.

[Curtis]

Re: Ratio of psi vs. remaining fuel?

Right, we are going to assume constant temperature here.
Two of my kids have older Civic GX's ('98 and '99) and it seems the fuel gauge drops faster toward the top of the scale than towards the bottom. Our newer '07 drops almost linearly so I am guessing Honda incorporated Boyle's curve into the fuel gauge driver for the newer models? Does anyone know if they included temperature compensation too?

My understanding of the GX fuel guage is that it is a temperature compensating pressure guage.

[lcovelli]

Re: Ratio of psi vs. remaining fuel?

Here is how we can test this..

Go to a pump that has 3K and 3.6K psi
Fill full with the 3K psi first and jot the pressure gauge and volume of gge
Then use the 3.6K pump and mark the same info.
if the 3600 psi tops off at at a rate that is not equivocal to linear calculated rate then you have your answer.
This would be fun.. I might try this.

L

[Adrian]

Re: Ratio of psi vs. remaining fuel?

In addition to what freedml has said, here is Boyle's Law and its equation:

http://en.wikipedia.org/wiki/Boyle%27s_law#Equation

No exponents so as pressure decreases, volume (or the mas of the methane) decreases linearly.

I agree with John, the relationship is not linear.

It's been a while since undergraduate Thermodynamics, but I remember that no gas behaves as an "ideal gas". "Real" gas behavior approaches "ideal gas behavior" as described by the ideal gas law (Boyle's law is a sub part) at low pressures only. The compressibility of a gas varies based on the interaction at a molecular level, which can either increase or decrease the actual pressure when compared to an ideal gas pressure. Please see this link to wikipedia for the progression of knowledge on real gas behavior, starting with Boyle's law (1662.) Our knowledge has progressed significantly in this area since then.

http://en.wikipedia.org/wiki/Equation_of_state

In a modern software simulation program one can pick the equation of state known to be most accurate for the chemicals in the process modeled. I've used HYSYS, I'm sure other fellow Chemical Engineers might have experience with other software as well.

To accurately develop the curve John is asking for, one would also have to pick a fixed composition (%methane, ethane, propane, air, etc.) for the natural gas in addition to the fixed temperature, then use Peng-Robinson or a similar equation of state.

Anyone else remember this stuff from undergrad?

[afvman]

Re: Ratio of psi vs. remaining fuel?

John et al,

Per your request, attached are the P-T charts from the Gas Research Inst. (formerly IGT). You'll see that they're all straight lines.

Hope this helps.

afvman

[Steve]

Re: Ratio of psi vs. remaining fuel?

I only had one thermodynamics class and I didn't enjoy it that much. But, I found a web site that can calculate the volume of a gas using different equations. I used 300 moles of CH4 which is about 5 KG (I think), 294 K which is 70 F. Enjoy:

http://antoine.frostburg.edu/chem/senese/javascript/realgas.shtml

[freedml]

Re: Ratio of psi vs. remaining fuel?

So, John, my first questions would be:

How did you measure the pressure (what kind of gauge, how is it mounted in your system)?

How did you KNOW you were at Full, 3/4, 1/2, 1/4?

Did you measure the temperature? Where? How? Was it the same each time?

[freedml]

Re: Ratio of psi vs. remaining fuel?

I personally have given up on trying to calculate CNG mileage, which is a very similar problem. I am very happy to pay 1/2 the price of gasoline (I pump at home) and get 'about' the same mileage.

[John Mitton]

Re: Ratio of psi vs. remaining fuel?

So, John, my first questions would be:

How did you measure the pressure (what kind of gauge, how is it mounted in your system)? How did you KNOW you were at Full, 3/4, 1/2, 1/4? Did you measure the temperature? Where? How? Was it the same each time?

Mechanical psi gauge, known range of vehicle vs. miles traveled at each benchmark, fairly constant temperature in Phoenix at each benchmark reading.