Re: LPG in cars

[Tom Jennings <tomj@xxxxxxx>]

On Sun, 8 May 2005, Jim Blair wrote:

> A: Sounds like you guys are mixing up LPG (Low Pressure Gas) and CNG
> (Compressed Natural Gas)

Not to be pedantic, but LPG stands for

Liquified
Petroleum 
Gas

It's 80% propane, 20% butane and other -anes. About 80% of the
BTU/liquid gallon of gasoline. Since it's pumped into the tank as
a liquid, it's very close to a gasoline filling station
experience.  Tank fills in a few minutes.

CNG is
Compressed
Natural
Gas.

Natures farts. Mostly methane. The stuff in your stove also
contains CO2 and water plus other crap depending on which fart
hole (well) it came from. The water's gotta be removed for vehicle
use. It's about 60% the BTU rating of gasoline.

It takes TREMENDOUS pressure to liquify it, then it requires
cooling to keep it liquid. Therefore, motor fuel is a 3000 PSI gas
cylinder. It takes FOREVER to fill the @!@#$!! things.



>   LPG is very cold when liquid (it starts to "boil" at -45C) and vaporizes
> as it warms up, but sits at less than 100 lbs pressure in a tank while
> liquid.


Once liquified, LP remains a liquid at most human-survivable
temperatures.

Gas to liquid to solid (and back) is called a phase change.  To
change 32-degree liquid water to 32-degree solid ice takes energy,
even though the temperature stays the same!

To liquify propane, you cool it below -44F; at that temp it'll
remain liquid with zero pressure. If you raise the temperature it
starts to boil, unless you keep it pressurized. Same as a
radiator.



The one problem I have predictably (if extreme) is filling the
tank when partially full in extremely hot weather (like 110F) and
after driving a long distance on hot asphalt.

The tank lives a foot away from a road surface that's probably 130
- 150 F.  This raises the pressure in the tank to 150 psi. Because
of the phase-change business, even cooling the tank down with a
cold water hose won't lower the pressure! I have to bleed fuel off
with the bleeder for 20 minutes before I can fill it.

This happens ONLY at little rinky-dink filling stations (mom'n'pop
BBQ fillers) with old worn-out pumps. It's not a problem at big
tank yards.  And only when it's extremely hot out, like Death
Valley in June, or Grand Canyon area in July.

Once, in 18 years, I had a tank vent liquid out the
saftey-overflow.  Unnerving! On route I10, I filled up around
Blythe CA (HOT!!!) heading to LA. With a full tank, it got REAL
HOT, and pressures rose... since the tank was full, there was
little "air space" in the tank, so pressures exceeded the popoff
and it hissed LP until the pressure dropped. Sounded like a huge
leaking tire.