The following does have jargon! I will try to put up some pictures to assist in better understanding what is what.
Cars that use CNG 
(Compressed Natural Gas) are also called NGVs (Natural Gas Vehicles). According to the International Association for Natural Gas Vehicles, Pakistan has the second-largest number of natural gas vehicles. Recently Landi Renzo of Italy has set up a production subsidiary in Karachi to cater to the growing demand of CNG Kits in Pakistan. OEM’s like Toyota Pakistan and Suzuki Pakistan is producing company fitted CNG cars.
Usage: CNG (Compressed Natural Gas) can be used in Petrol and modified Diesel engines. 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. In Pakistan, this is unheard of! Fueling a vehicle from a home natural gas fuel line is becoming more popular in the United States though.
CNG cylinders: CNG cylinders can be made of steel, aluminum, or plastic. Lightweight composite 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. For the most part, CNG cylinders are made of steel. There is a proper standard for safety for CNG cylinders (the ISO 11439 standard)
The Equipment: The equipment required for CNG to be delivered to an Petrol 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 of CNG: Compressed natural gas (CNG) vehicles require a greater amount of space for fuel storage than conventional petrol powered vehicles. Since it is a compressed gas, rather than a liquid like petrol, 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 can be 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. This is done by companies like Fiat and Volkswagon, but these configurations are unheard of in Pakistan. While CNG-powered vehicles are considered to be safer than petrol-powered vehicles, there are concerns about how best to fight fires involving CNG vehicles.
But is CNG safe? That may be a valid question, given that car’s CNG tanks actually exploding is not unheard of. Although stricter action by the Pakistani government is still required to catch people who make sub-standard CNG tanks/cyclinders, the choice lies with you, the consumer. CNG is safe as long as the cyclinder is quality checked and approved. Here is a little excerpt about CNG safety:
Natural gas vehicles are very safe, for not only do they have all the same standard safety equipment as conventional cars (passive restraints, air bags, head restraints and anti-lock brakes), but they are subjected to the same crash safety tests as well.
Because compressed natural gas (CNG) fuel systems operate at pressures in excess of 3000 PSI, the fuel tank and associated plumbing have to be incredibly rugged and strong enough to contain that pressure. The on-board tanks are made of steel up to one half-inch thick and often wrapped in protective reinforced fiberglass sheathing. Plus, newer tanks are constructed of polymers and composites that are stronger than steel.
Contrast this with standard gasoline and diesel tanks in regular vehicles. These tanks are usually made from stamped steel shell halves, just a few sixteenths of an inch thick, that are welded or crimped together. In the event of a traffic accident, the ability of rugged, durable CNG tanks to withstand rupture or puncture certainly exceeds that of simple stamped steel.
But the safety of NGVs doesn’t just stop with the robust construction of the fuel tank. To take it a step further, most CNG systems have automatic release valves. In a situation of excessive heat or pressure build-up, the valve will open and release the gas to the atmosphere—and since it is lighter than the surrounding air, it will rise and dissipate. The low threshold is set well above ambient temperatures and the high threshold is set well below the ignition temperature of the gas. In the event of a fire, the fuel is safely evacuated from the car before it ever has a chance to catch fire. Gasoline and diesel vehicles simply can’t do that.
A manual shut-off valve also exists just downstream from the CNG tank to allow user intervention if the need arises. A neat side benefit of this valve might be the ability to use it as an anti-theft device. Shut off the fuel supply and any would-be thief wouldn’t get much further than a mile down the road and off shuts the engine. That is the epitome of clever thinking.
So fear not, the use of compressed natural gas as a motor fuel— from compression, storage and fueling to vehicle manufacturing—must meet stringent industry and government standards. And it’s all in the name of clean, efficient and utterly safe motoring.
- Source: About.com (Link to full article)
