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CNG vs LPG
No Natual Gas and LPG (liquified petroleum gas) are quite different.
Natural gas is usually stored as Compressed Natural Gas (CNG) on a vehicle and is mostly methane (95%+). CNG is still a gas at the high pressures (20 MPa/3000 psi) that the tanks get filled to. Because of this high pressure the tanks are thick steel (expensive and heavy) or a composite (very expensive but lighter). A conversion kit to fit CNG to a car looks similar in principle to an LPG kit but all the components on the high pressure side, such as the tank, fuel lines, and regulator are very different. In Australia there are only about 2 or 3 public refuelling stations, with most of the other refuelling facilities catering for captive fleets such as forklifts and buses. Natural Gas can be liquified, but only at very low temperatures, about -162°C. The storage containers in vehicles for LNG are complex cryogenic containers. LNG is the way natural gas is shipped to Japan and China from off the North West Shelf. LNG plants are big and expensive and need to produce heaps of LNG to be economically viable.
LPG is a mixture of propane and butane (with such other stuff occasionally thrown in). The actual ratio of the two varies depending on where it was sourced from. LPG liquifies under pressure, hence its name, and therefore is stored on the vehicle as a liquid and usually converted to a vapour before being mixed with the intake air to supply the engine. Australia has heaps (about 3000 or more I think) refuelling stations around Australia.
Because LPG is a liquid you can store a lot more of it in a tank than you can with CNG for the same volume. A 90 litre tank on an LPG Falcon would get you about 600 - 700 km whereas a 90 litre CNG Falcon would get about 150 - 220 km.
CNG does have some advantages though. Usually it is cheaper than the other fuels (per kJ) and its price is very stable, about as stable as your household price for gas. There are also some companies developing vehicle refuelling appliances for the home. These are small compressors that take the gas from the household and compress it into the car, the only problem with this is it takes about 8 hours to refuel. Public CNG refuelling stations store high pressure gas on site (at 25 MPa / 3600 psi) and can fill a car in a few minutes. CNG also has the potential to have fewer emissions than LPG or petrol and also produce less CO2 emissions, but like all things the engine has to be optimised to take full advantage of the properties of CNG. The most obvious difference is the octane rating, methane has an octane rating of about 135 - 140 (if the scale could go that high) and as such the compression ratio can be increased dramatically.
Jimbo
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