FMQ2-36 Rate of compression time.

[rgliedt]

Is the .9 gallon per hour an average rate of compression and is the rate of
compression relative to the back pressure in the tanks that are being filled? In other words does it take longer to pump in a gallon against 3000 psi than say 500 psi?

Also is it harder on the equipment to constantly keep topping off tanks when they are 3/4 full than to run them all the way down to 1/4 or 1/8 of a tank and then fill up.

Has anyone done a chart that shows the gge relative to tank pressure of say
a 10 gal tank. My understanding is that you can't just divide 3600 psi by 10 and get your gallon per psi as it isnt a linear formula. It would be nice to know more precisely the amount of fuel you have, especially when you live in a area as I do with no fueling infrastructure.

[CNG MOTORS]

Re: FMQ2-36 Rate of compression time.

We had the GX down to a science at one time we figured out that in a GX you can travel .66 miles per 1 psi. I currently have a fmq2 that pretty much runs 24 hours a day fueling 3 vehicles and a cascade. When we were running the PHIL with it we notice we were not getting much output from the FMQ so we had our gas company install a larger reg, now that we do not use our PHIL we are getting 1.25 gge's/ hour out of the FMQ regardless of backpreassure.

The man in the know is Dave Clement though, maybe he'll chime in.

[freedml]

Re: FMQ2-36 Rate of compression time.

The pumping rate doesn't fall off much with tank pressure. I let my tanks get lower anyway to reduce the number of cycles, resulting in less wear and tear on the whole system.

[Amfuel]

Re: FMQ2-36 Rate of compression time.

A Phill system compresses at the rate of .6gge/hr and drops to .42gge/hr during the last 3% of the fill so there is a diminishing rate as the pressure rises. The small Q is essentially a larger version of the same compressor concept. FM lists the small Q at .9 gge/hr which should be the lowest rate of compression. I didn't look up the small Q specs but the physics would be the same.

[freedml]

Re: FMQ2-36 Rate of compression time.

From my personal testing, Phill starts out closer to 1/3 gge/hr and drops down to 1/4 gge/hr near 3600psi.

[CraziFuzzy]

Re: FMQ2-36 Rate of compression time.

Is the .9 gallon per hour an average rate of compression and is the rate of
compression relative to the back pressure in the tanks that are being filled? In other words does it take longer to pump in a gallon against 3000 psi than say 500 psi?

Also is it harder on the equipment to constantly keep topping off tanks when they are 3/4 full than to run them all the way down to 1/4 or 1/8 of a tank and then fill up.

Has anyone done a chart that shows the gge relative to tank pressure of say
a 10 gal tank. My understanding is that you can't just divide 3600 psi by 10 and get your gallon per psi as it isnt a linear formula. It would be nice to know more precisely the amount of fuel you have, especially when you live in a area as I do with no fueling infrastructure.

Actually, the mass to pressure relationship in the cylinder is in fact linear, assuming a constant temperature. Also, the fuel gage in most factory CNG vehicles is actually temperature compensated to account for this. Therefore, if the system is calibrated correctly, 1/2 tank on the gage would in fact be 5 gge (assuming that 10gge tank).

To point out though, the average vehicle fuel gage is far from 'calibrated' and usually far from linear.

As for compressor output, it will certainly be lower with a higher discharge pressure, and will also be lower with a lower suction pressure. 'Overcharging' the suction side would have a significant improvement in fill-rate, which is why Murphy saw such an improvement by getting a larger regulator installed. Caution must be had though, as a higher suction pressure results in higher stress loads on the pistons, rods, crank, and motor, as well as greatly increases heat of compression. Definitely keep the suction pressure within the factory's specs, but as high WITHIN THE SPECS as you can to maximize fillrate.

[freedml]

Re: FMQ2-36 Rate of compression time.

I think we all agree that compressor output is higher with higher inlet pressure and lower with higher outlet pressure. The question is, 'how much?' I find it is a big percentage difference with Phill and not very noticeable with FMQ. The difference probably becomes larger as the ratio of tank size to compressor capacity increases.

[rgliedt]

Re: FMQ2-36 Rate of compression time.

So can I simply divide 3600 psi by 10 gal and figure that I burn a gallon every 360 psi? It was implied to me that the top end of the pressure drop was less per gallon and that the lower end was more per gallon. Is that simply due to
a temperature difference?

[freedml]

Re: FMQ2-36 Rate of compression time.

That's right, the pressure change per 'gallon' stays the same. You can't use the last 40psi or so as the engine will die below that pressure.

[CraziFuzzy]

Re: FMQ2-36 Rate of compression time.

I think we all agree that compressor output is higher with higher inlet pressure and lower with higher outlet pressure. The question is, 'how much?' I find it is a big percentage difference with Phill and not very noticeable with FMQ. The difference probably becomes larger as the ratio of tank size to compressor capacity increases.

I think it probably has more to do with compressor design than tank size. Parameters such as number of stages, which would affect the compression ratio. This will determine how much of the stroke is 'wasted' which is what determines how differential pressure affects flowrate.

[John Mitton]

Fuel vs. psi not linear

Roger, you are correct. The top end of the pressure drops faster than the bottom end as the vehicle drives down the road. This is an approximation but due to Boyle's law:

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

I started a discussion thread here on the topic:
http://www.cngchat.com/forum/showthread.php?t=3797

I know one small volume manufacturer who is busy developing a Boyle's-compensated fuel gauge driver for a sedan retrofit... should be available for existing prototype vehicles to evaluate next week.

[Franz]

Re: FMQ2-36 Rate of compression time.

One thing that has not been mentioned is the advantage to slow filling. It is possible for the manufacturer to ramp up the filling rate by uprating the inlet pressure and changing the compressor outlet rates, but at a cost, both expense, and longevity.

Also, a slow fill allows for the heat generated during compression to dissipate, allowing for a stable and better fill with no pressure settling. A fast fill will generate more heat and usually trigger a premature pressure rise and corresponding dispenser shutdown.

The fill is based on pressure, and if the pressure is generated by heat, it will shutdown quicker. Overfilling a tank and allowing a pressure settle is dangerous.

On the GX I used several years ago, the last 50 or so miles were iffy and unpredictable for performance. I was told to refill before I got to the 1/4 tank mark even if a refueling station was nearby.

Franz

[freedml]

Re: Fuel vs. psi not linear

I'm sorry, John. This is just crazy. It certainly doesn't derive from Boyle's Law.

top 1/4 - 1000
next 1/4 - 1100
next 1/4 - 700
bottom 1/4 - 800

Where did these numbers come from? This is either wrong, gauge error, temperature is involved, or there's some weird effect I can't imagine.

[CraziFuzzy]

Re: Fuel vs. psi not linear

I'm sorry, John. This is just crazy. It certainly doesn't derive from Boyle's Law.

top 1/4 - 1000
next 1/4 - 1100
next 1/4 - 700
bottom 1/4 - 800

Where did these numbers come from? This is either wrong, gauge error, temperature is involved, or there's some weird effect I can't imagine.

I think the odds are it is all just gage error. No car gage, even the speedometer, is designed for accuracy, and all have significant linearity problems. I wouldn't be surprised if it was much more linear if you went off of a good quality pressure gage, and compensated for temperature yourself.

[John Mitton]

Re: FMQ2-36 Rate of compression time.

I think Phoenix heat might be the culprit. I'm going to do another run off a full tank today but this time in the Utah cold. Overnight I time filled to temperature-adjusted 3600 psi with my FM-Q2-36 (should be about 2410 psi @ 32F) and then it's steady cruise control on the highway. If I time the run from around 11am to 5pm this variable should be somewhat out of the equation as the forecast is from 34F to 39F during that period.
http://www.findlocalweather.com/hour...ican+fork.html

We have a very accurate transducer connected to a volt meter in the car so I will record the voltage and temperature at each 10 mile interval and put it on a spreadsheet. Will try to plan the run so the vehicle is almost out of fuel when I get home and then let it idle until empty to take the final reading.