Car Technology: (Alternative Fuels:: Natural Gas CNG)

[ar]

Natural-gas Vehicle Design (NGV)
Natural-gas vehicles use the same basic principles as gasoline-powered vehicles. In other words, the fuel (natural gas in this case) is mixed with air in the cylinder of a four-stroke engine and then ignited by a spark plug to move a piston up and down. Although there are some differences between natural gas and gasoline in terms of flammability and ignition temperatures, NGVs themselves operate on the same fundamental concepts as gasoline-powered vehicles.

Fuel Storage

Most NGVs operate using compressed natural gas so the fuel takes up less space. At a fueling station, gas is compressed to 3,000-3,600 pounds per square inch before being pumped into high-pressure, tube-shaped cylinders that are attached to the rear, top or undercarriage of the vehicle. The storage tanks of early NGVs were bulky and took up much of the vehicle's cargo space, but newer, more lightweight cylinders have been developed. Called Integrated Storage Systems (ISSs), these all-composite cylinders are contained within a fiberglass shell and impact-absorbing foam to protect them in the event of a crash. The cylinders also have a smaller diameter so that three of them can be housed together in a size and shape resembling a conventional gasoline tank.

Engine Modifications
When the engine in an NGV is started, natural gas flows from the storage cylinders into a fuel line. Near the engine, the natural gas enters a regulator to reduce the pressure. Then the gas feeds through a multipoint gaseous fuel-injection system, which introduces the fuel into the cylinders. Sensors and computers adjust the fuel-air mixture so that when a spark plug ignites the gas, it burns efficiently. A natural-gas engine also includes forged aluminum, high-compression pistons, hardened nickel-tungsten exhaust valve seats and a methane-specific catalytic converter.


Differences in Refueling
Refueling a natural-gas vehicle may also be a little different. The fueling point is typically at the front of the vehicle, although in some NGVs, such as the Honda Civic GX, the fueling point is in the rear. An NGV can be fueled at a "fast-fill" pump in about the same time it takes to fuel any gasoline- or diesel-powered vehicle. Alternatively, an NGV can be fueled in five to eight hours using a "slow-fill" method. The home refueling stations offered by Honda are of the slow-fill variety, requiring car owners to refuel their vehicles overnight

Advantages and Disadvantages
The biggest advantage of NGVs is that they reduce environmentally harmful emissions. natural-gas vehicles can achieve up to a 93 percent reduction in carbon monoxide emissions, 33 percent reduction in emissions of various oxides of nitrogen and a 50 percent reduction in reactive hydrocarbons when compared to gasoline vehicles. NGVs also rate higher in particulate matter 10 (PM10) emissions. PM10 particles transport and deposit toxic materials through the air. NGVs that operate in diesel applications can reduce PM10 emissions by a factor of 10.

NGVs are safer. The fuel storage tanks on an NGV are thicker and stronger than gasoline or diesel tanks. There has not been an NGV fuel-tank rupture in more than two years in the United States.

Natural gas costs are lower than gasoline. On average, natural gas costs one-third less than gasoline at the pump.

Natural gas is convenient and abundant. A well-established pipeline infrastructure exists in the United States to deliver natural gas to almost every urban area and most suburban areas. There are more than 1,300 NGV fueling stations in the United States, and more are being added every day.

Natural gas prices have exhibited significant stability compared to oil prices. Historically, natural gas prices have exhibited significant price stability compared to the prices of petroleum-based fuels. This stability makes it easier to plan accurately for long-term costs.

NGVs have lower maintenance costs. Because natural gas burns so cleanly, it results in less wear and tear on the engine and extends the time between tune-ups and oil changes.

Disadvantages
One of the biggest complaints about NGVs is that they aren't as roomy as gasoline cars. This is because NGVs have to give up precious cargo and trunk space to accommodate the fuel storage cylinders. Not only that, these cylinders can be expensive to design and build -- a contributing factor to the higher overall costs of a natural-gas vehicle compared to a gasoline-powered car.