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Compressed natural gas
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Compressed Natural Gas (CNG) is a fossil fuel substitute for gasoline (petrol), diesel, or propane fuel. Although its combustion does produce greenhouse gases, it is a more environmentally clean alternative to those fuels, and it is much safer than other fuels in the event of a spill (natural gas is lighter than air, but disperses quickly when released).
CNG is made by compressing natural gas (which is mainly composed of methane [CH4]), to less than 1% of its volume at standard atmospheric pressure.
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Encyclopedia
Compressed Natural Gas (CNG) is a fossil fuel substitute for gasoline (petrol), diesel, or propane fuel. Although its combustion does produce greenhouse gases, it is a more environmentally clean alternative to those fuels, and it is much safer than other fuels in the event of a spill (natural gas is lighter than air, but disperses quickly when released).
CNG is made by compressing natural gas (which is mainly composed of methane [CH4]), to less than 1% of its volume at standard atmospheric pressure. It is stored and distributed in hard containers, at a normal pressure of 200–220 bar (2900–3200 psi), usually in cylindrical or spherical shapes.
CNG is used in traditional gasoline internal combustion engine cars that have been converted into bi-fuel vehicles (gasoline/CNG). Natural gas vehicles are increasingly used in Europe and South America due to rising gasoline prices.
In response to high fuel prices and environmental concerns, CNG is starting to be used also in light-duty passenger vehicles and pickup trucks, medium-duty delivery trucks, transit and school buses, and trains.
CNG's volumetric energy density is estimated to be 42% of LNG's (because it is not liquefied), and 25% of diesel's.
Technology
CNG can be used in Otto-cycle (gasoline) and modified Diesel cycle engines. Lean-burn Otto-cycle engines can achieve higher thermal efficiencies when compared with stoichiometric Otto-cycle engines at the expense of higher NOx and hydrocarbon emissions. Electronically-controlled stoichiometric engines offer the lowest emissions across the board and the highest possible power output, especially when combined with exhaust gas recirculation, turbocharging and intercooling, and three-way catalytic converters, but suffer in terms of heat rejection and fuel consumption. A suitably designed natural gas engine may have a higher output compared with a petrol engine because the octane number of natural gas is higher than that of petrol as this would allow for an engine design with a higher compression ratio.
CNG may be refueled from low-pressure ("slow-fill") or high-pressure ("fast-fill") systems. The difference lies in the cost of the station vs. the refueling time. There are also some implementations to refuel out of a residential gas line during the night, but this is forbidden in some countries. Fueling a vehicle from a home natural gas fuel line is becoming more popular in the United States, especially in California and New York, and tax credits are available for installing the necessary appliance.
CNG cylinders can be made of steel, aluminum, or plastic. Lightweight composite (fiber-wrapped thin metal "ISO 11439 CNG-3"/fibre-wrapped plastic "ISO 11439 CNG-4") cylinders are especially beneficial for vehicular use because they offer significant weight reductions when compared with earlier generation steel and aluminum cylinders, which leads to lower fuel consumption. The CNG cylinders bundled with safety-valve generally follow the ISO 11439 standard.
The equipment required for CNG to be delivered to an Otto-cycle engine includes a pressure regulator (a device that converts the natural gas from storage pressure to metering pressure) and a gas mixer or gas injectors (fuel metering devices). Earlier-generation CNG conversion kits featured venturi-type gas mixers that metered fuel using the Venturi effect. Often assisting the gas mixer was a metering valve actuated by a stepper motor relying on feedback from an exhaust gas oxygen sensor. Newer CNG conversion kits feature electronic multi-point gas injection, similar to petrol injection systems found in most of today's cars.
Drawbacks
Compressed natural gas vehicles require a greater amount of space for fuel storage than conventional gasoline power vehicles. Since it is a compressed gas, rather than a liquid like gasoline, CNG takes up more space for each gasoline gallon equivalent (GGE). Therefore, the tanks used to store the CNG usually take up additional space in the trunk of a car or bed of a pickup truck which runs on CNG. This problem is solved in factory-built CNG vehicles that install the tanks under the body of the vehicle, thanks to a more rational disposition of components, leaving the trunk free (eg. Fiat Multipla, New Fiat Panda, Volkswagen Touran Ecofuel,Chevy Taxi (sold in countries such as Peru) etc). CNG-powered vehicles are considered to be safer than gasoline-powered vehicles.
CNG cars
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