From city buses to three-wheel taxis, these guides follow natural gas from the dispenser nozzle to the engine, with the units, standards and station hardware in between.
Natural gas pressed into steel or composite cylinders at 200–250 bar powers more than twenty million compressed natural gas vehicles worldwide, from Delhi buses to Italian hatchbacks. Here is what the fuel is and how it behaves.
A CNG station is a compressor plant with a dispenser attached. Its layout depends on one question: do vehicles arrive at random during the day, or park at the same depot every night?
A gasoline engine needs surprisingly little hardware to burn methane. The difficult parts are the cylinder package, the emissions paperwork and the arithmetic that decides whether the conversion ever pays for itself.
Small two-stroke engines were among the dirtiest vehicles on Asian streets. Natural gas cleaned up Delhi's auto-rickshaws within a few years, but fitting a cylinder to a motorcycle proved much harder.
A compressed natural gas vehicle carries its fuel at up to about 250 bar in one or more cylinders, drops it to a few bar in a regulator and burns it in a spark-ignition engine. Most of what makes these vehicles different sits in that short chain, and in the station that fills them. A fast-fill dispenser can fill a car in a few minutes from high-pressure storage; a time-fill post lets a depot fill a whole fleet slowly overnight from a compressor.
What is CNG covers the fuel itself, the gasoline and diesel gallon equivalents used to price it and the fleets that run on it. CNG fueling stations walks through compressors, dryers, storage cascades and dispensers. NGV conversion explains what a conversion kit contains and how certification works, and two- and three-wheelers looks at the small vehicles where gas made the largest difference to urban air.
To size a cylinder for a given range, the tank capacity calculator converts water volume and pressure into kilograms and gasoline gallon equivalents.
One gasoline gallon equivalent of CNG is 2.567 kilograms of gas. At 250 bar and 15 °C that mass fills about 13 liters of cylinder, against 3.8 liters for the gallon of gasoline it replaces, so a compact car using about 4 kg of gas per 100 kilometers needs roughly 80 liters of cylinder at 250 bar for a 400-kilometer range, several times the space of a gasoline tank. Buses and refuse trucks solve the space problem on the roof or behind the cab, which is why they were among the first fleets to switch.
Cylinders on vehicles follow strict rules. In the United States, light-duty tanks are inspected every 36 months or 36,000 miles and after any fire or crash, and every cylinder carries a fixed end-of-life date on its label, typically 15 to 20 years after manufacture. Pressure relief devices vent the gas in a fire before heat can weaken the vessel.
Conversions are the other path into gas. A kit adds a regulator, injectors, an electronic control unit and the cylinders, and in the United States it has to be certified for the specific engine family. The details are in the NGV conversion guide.
The station design follows the way the vehicles are used. Taxis, delivery vans and private cars arrive one at a time and need to leave quickly, so public stations keep gas in high-pressure storage banks and fill from them in a cascade. Buses and refuse trucks return to the same depot every night, and a slow fill directly from the compressor uses smaller equipment and fills the cylinders more completely, because the gas heats less during a slow fill. The fueling station guide describes both layouts.
At a fast-fill station a car fills in a few minutes, about as long as with gasoline. Time-fill posts fill fleet vehicles slowly over several hours, usually overnight at the depot.
The amount of natural gas with the same energy as a gallon of gasoline, set at 2.567 kilograms (5.66 pounds) of gas. CNG is sold by this unit in the United States so that prices can be compared with gasoline.
In the United States, light-duty vehicle cylinders are inspected every 36 months or 36,000 miles, whichever comes first, and after any fire or accident. Each cylinder also carries a fixed expiry date on its label.
Only with a kit certified for that engine family. Gasoline engines convert most easily because they already use spark ignition; diesel engines need either a dual-fuel system or a rebuild to spark ignition.
Policy and cost. In Delhi, a 1998 Supreme Court order required buses, taxis and auto-rickshaws to move to CNG, and the switch was largely made in the early 2000s. Similar programs followed in other South Asian cities where gas was cheaper than gasoline.