mcosman wrote:

We get 15 in a van that is always fully loaded with tools all the time.
There is supposedly 10 gallons on board but we go about 120 miles on 7.5
gallons of CNG.  I am trying to understand the conversion tables between
compressed CNG and uncompressed as to how it relates to a gallon of regular
fuel.

Matt Cosman
Microcosman.com

I would be interested in Knowing just what they consider a "gallon" of natural gas.
Natural

gas is primarily methane with a bit of other hydrocarbons and some inerts. The
heating value of a standard cubic foot is just a bit over 1000 btu and 
at std press and temp that is .042 lbs.

That makes the btu per lb about 24,000 as compared to gasoline at roughly 20,000
btu/lb.

Pressurizing to 1470 psia (100 atmospheres) raises the density to 5.11 lbs/cu ft,
still well short of the approx 44 lbs/cu ft of gasoline. 

As I recall one unit of sale for  natural gas is the
"therm" which is 100,000 btu. Actually this is pretty close to the heating value of a
gallon of gasoline 20,000 times 5.9 lbs 118,000 btu, so maybe 
that is what they are calling a gallon.

Even if cryogenically liquified, natural gas is only about 26.4 lbs/cu ft at roughly
-260 F.

We looked at methane as a rocket fuel as a cheaper and denser replacement for liquid
hydrogen. It is quite a bit easier to liquify (harder than oxygen tho) and much
cheaper, but the performance and combustion properties were significantly 
poorer, and the work was tabled. I do not know what the effective "octane" 
characteristics are. You could make  a CNG airplane fly, but the range 
and payload resulting from large heavy tank versus btu
equations would not be very satisfactory. 

LNG would improve this a bit, but tank insulation,
and boil off would be rather a pain. LPG is generally a mix 
of propane and butane type petroleum  gases
that are liquid at room temperature at modest pressures, but if 
you are really interested in alternate fuel propulsion, ethanol, and 
methanol are much better. 

However, for your car the concept of a compressor 
hooked to the gas tap, and topping up the car every night
with low  cost - no road tax fuel is rather attractive, and would beat 
a ton of lead acid batteries by a whole  bunch - Vance

Heck Vance! Anything would beat lead acid batteries including rubber bands :-)

But I like that municipal gas supply idea. Any plan to deny the gov 
tax money is a good plan. P&W compresses municipal gas supplies real time to
run their power generating gas turbines in the Hartford Conn. high altitude test cell
facility. Would my 3 HP Sears air compressor work? How about a filament wound
composite storage tank to save weight? I am also curious about how weight 
efficient a bunch of long thin wall aluminum tubes would be as a storage tank.

Paul Lamar
 
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