3000 vs 3600 psi question

[mattamhop]

My local "Greed Energy" CNG pump has had the 3600 side down now for well over 3 months (they have the part they say and each time I call, the tech is surprised that it is not yet up and running... but they manage to get out to the pump regularly to bump the price up). So we use the 3000 side, and I don't understand what is happening. When I fill on 3000 my van (2001 Chevy, IMPCO Bi-fuel) it shows a "full" tank (needle all the way over) and I am getting just over 8gge in the system (specs say that it is a 10 gge system). I have never gotten a full 10 gge into my van and typically the best I can get is just over 9 gge. How is it that I am getting nearly the same volume of gas, the needle shows full, and the pressure gauge on the pump is reading 3000psi?
Matt

[siai47]

Re: 3000 vs 3600 psi question

Because the gauge doesn't tell quite the truth. On both my Cavailers and my Van, the gauges will go to full on a 3000 psi slow fill. On a 3600 psi slow fill the gauges go past full. You have to drive quite a few miles for the gauge to come off full and then (in the case of the van for sure) it drops like a rock. If you were to put a scan tool on the vehicle, you would be able to read percent of fill. The gauges reach full around 90% and the scan tool reports 100% somewhere around 3400 psi on a 70 degree day. I have seen the van listed as 10 gge and also 9.5 gge in various GM literature. I know the tanks are slightly different in length so maybe the 9.5 gge is more accurate. In any event, because the tanks never go to zero pressure before changeover to gasoline, it would be impossible to get the fully rated gge into the tank.

[John Mitton]

Re: 3000 vs 3600 psi question

I moved the posts regarding a Chinese refueling appliance over to the Refueling forum to keep this one on topic. Please follow the discussion over there:

http://www.cngchat.com/forum/showthread.php?t=6046

[Tom Stoflet]

Re: 3000 vs 3600 psi question

Hi Mettamhop:

I’m not sure I’m following the correct procedures for responding on this thread.

But here are my comments to address your concerns regarding using 3000 psi CNG fuel vs. 3,600 psi CNG fuel, from the perspective of one who has designed and operated one of the largest CNG stations is Northern CA.

As you may realize, CNG is a gas. So the more it can be compressed, the more “mass” or weight of the fuel can be supplied into your tank, as opposed to a liquid fuel which whose gallon fills are more “constant”.

However CNG fueling is also affected by temperature. The more well designed CNG station have temperature compensators which try to balance out the variations of CNG fuel dispensed at high temperature vs. low temperatures. What is meant is that because CNG is a gas, it expands at high temperatures, and contracts at low temperatures, so you will get more fuel in your tank if you fuel at times where the ambient temperatures are very low, unless these stations provide temperature compensators at the dispensers, which most do not.

So regardless of what the bill states, the actual GGE you get may get may be much lower at high temperatures than low.

Further, as you can imagine 3,600 psi CNG fuel provides about 20% more fuel (mass) ,than 3,000 psi fuel. So if your vehicle can accept 3,600 psi fuel, do not fuel from 3,000 psi dispensers. That 20% less fuel may explain why your MPG is much lower.

Hope this helps.

.

[Alaska GX]

Re: 3000 vs 3600 psi question

Tom

I was with you all the way to the last paragraph.....how does the tank pressure affect the mileage..per GGE ????

The cars do not run on 3000 psi...they drop the pressure ( cooling off ) to less than 100 psi.


As I understood ......a GGE is a given voulme at a given temp and pressure and the only variable was the BTU rating of the gas.

[Curtis]

Re: 3000 vs 3600 psi question

It really has no effect on your mpg, although incomplete fills do make it nearly impossible to reasonably calculate it. This leads many to overestimate the amount of fuel they've taken on and thereby reduces the mileage numbers when you try to figure them out.

The best way to calculate your mileage, aside from installing a Scan Guage, is to note GGE's and mileage over an extended period of time and average it out.

[Tom Stoflet]

Re: 3000 vs 3600 psi question

Hi Again to ALL:

Without sounding condescending, I did not want to get too technical on my last thread on this matter.

All CNG fueling stations should have mass flow meters that monitor and dispense a standard amount of "mass or weight" of CNG fuel for each gallon of CNG that is dispenses. These dispensers are to be calibrated and certified by applicable regulatory agencies. In the state of the CA where I live, it is the "Weights & Measures Dept."

What I was trying to suggest is that 3000 psi CNG dispenses about 20% less fuel than 3,600 psi. That is why it is better to have vehicles that can handle 3,600 psi fuel because they get more fuel per fill.

Anyone who has tracked their MPG on using 3600 psi CNG fuel vs. 3000 psi fuel under similar driving conditions should observe this.