timster2
Platinum Member
The BTU content of CNG varies depending on the source. Typically its above 950BTU/CuFt. When the source is a landfill it could be much less. The gas we were using was consistently 980BTU. You need about 127 CuFt of CNG to equal a gallon of gasoline. CNG is about 130 Octane. Engines can be optimized to take advantage of this. Modified engines use a Miller cycle not an Otto cycle for volumetric efficiency and lower emissions.
LNG has a whole set of its own problems as a motor fuel. Its a cryogenic liquid and has to be kept cold or the storage tanks will vent to prevent over pressurization. Usually if the vehicle is constantly used this is not a problem, however, if the vehicle is out of service for more than a few days the fuel system needs to be drained. Definitely not something you want to deal with unless you are set up to handle the process. It is colorless and odorless, the odorant present in CNG, methanethiol, cannot be used.
Early CNG cylinders were steel over wrapped with fiberglass, heavy and not much fuel volume per cuft of storage area. Newer cylinders are carbon fiber much lighter but still big for the amount of fuel stored. CNG cylinder operating pressure is either 3000 or 3600 Psi. It was available in two pressure ratings when it first was available. Now the standard pressure rating is 3600psi. Because of the pressure storage tank shape is limited to a cylinder or sphere thus making it hard to fit into smaller vehicles. Like all high pressure cylinders they have a limited life span per DOT.
CNG is actually safer than gasoline as a fuel. Harder to ignite and lighter than air any leaks float up and away. It does not pool under a vehicle in the event of a wreck. Excess flow valves stop fuel flow at the cylinder valve in the event of a ruptured line such as in an accident like stopping the fuel pump in a liquid fueled vehicle. Ever try to light a propane torch with a match, not easy, you have to get the air/fuel ratio just right to ignite it. The fuel ratio needed is five to fifteen percent gas to air or it wouldn't ignite.
CNG needs little refining just needs to have the moisture content lowered and inert gasses at a low enough level to maintain a constant BTU output. If the moisture level is too high problems arise in the fuel pressure regulators, they freeze the water vapor and plug up.
I spent a lot of time working with CNG vehicles.
LNG has a whole set of its own problems as a motor fuel. Its a cryogenic liquid and has to be kept cold or the storage tanks will vent to prevent over pressurization. Usually if the vehicle is constantly used this is not a problem, however, if the vehicle is out of service for more than a few days the fuel system needs to be drained. Definitely not something you want to deal with unless you are set up to handle the process. It is colorless and odorless, the odorant present in CNG, methanethiol, cannot be used.
Early CNG cylinders were steel over wrapped with fiberglass, heavy and not much fuel volume per cuft of storage area. Newer cylinders are carbon fiber much lighter but still big for the amount of fuel stored. CNG cylinder operating pressure is either 3000 or 3600 Psi. It was available in two pressure ratings when it first was available. Now the standard pressure rating is 3600psi. Because of the pressure storage tank shape is limited to a cylinder or sphere thus making it hard to fit into smaller vehicles. Like all high pressure cylinders they have a limited life span per DOT.
CNG is actually safer than gasoline as a fuel. Harder to ignite and lighter than air any leaks float up and away. It does not pool under a vehicle in the event of a wreck. Excess flow valves stop fuel flow at the cylinder valve in the event of a ruptured line such as in an accident like stopping the fuel pump in a liquid fueled vehicle. Ever try to light a propane torch with a match, not easy, you have to get the air/fuel ratio just right to ignite it. The fuel ratio needed is five to fifteen percent gas to air or it wouldn't ignite.
CNG needs little refining just needs to have the moisture content lowered and inert gasses at a low enough level to maintain a constant BTU output. If the moisture level is too high problems arise in the fuel pressure regulators, they freeze the water vapor and plug up.
I spent a lot of time working with CNG vehicles.
