A: ERF! I think I was sleep typing! (Liquified Petroleum Gas) As for the mix, it's different in different parts of the country. Butane is generally cheaper so they try to mix as much in as possible in some areas, but it has less get up and go and less economy than propane. (see the Shell link I sent last night while sleep typing) CNG is stored in those massively heavy, high pressure tanks as a gas and NOT liquid! In order for it to remain liquid, it would have to be kept under high pressure and at -100 degrees C (I think I re-read it last night, but I was tired. Used to be a "had to know" thing to get certified for installations, but I was last licensed to do so before my head injury in '95) When the CNG tanks are filled on a cold day and the vehicle is parked in direct sunlight w/o driving far, the resultant gain in pressure from 50 degrees to 95F is sometimes enough to cause the blow off valve to exhaust almost 1/2 the tank before shutting off. (I had that unnerving thing happen to me in '93 as I was walking past one of the CNG powered delivery trucks that had been fuelled the night before in Calgary at -40F and the temp jumped up to 55 the next day due to spring Chinook warm up. Good thing I wasn't smoking!) From: Tom Jennings <tomj@xxxxxxx> To: mail@xxxxxxxxxxxx Subject: Re: LPG in cars Message-ID: <20050509005031.T10273@localhost> 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.