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CNG cylinder
A certified high-pressure cylinder (or tandem pair) with a defined service life, stamped with manufacture and test data.
Bi-fuel vehicles
Petrol when you want it, CNG when you need it
A converted vehicle is bi-fuel: it can run on petrol or on CNG and switch between them while driving. The petrol system stays intact, so the vehicle can still be driven if CNG is unavailable.
CNG is stored in a certified cylinder and delivered to the engine at low pressure through a reducer, where it is mixed with air or injected under the control of a dedicated electronic unit.
For fleet operators, the main considerations are fuel cost per kilometre, access to refuelling, cylinder certification and the discipline of keeping inspection dates.
History
How Bangladesh went to gas
The shift to CNG was not a single decision but a two-decade transition driven by fuel economics, air quality and policy.
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Late 1990s
Gas comes to the fleet
As domestic natural gas production grew, policy encouraged vehicle owners to convert from petrol to CNG. Conversion workshops appeared alongside the first filling stations.
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Early 2000s
Auto-rickshaws lead the change
Directives against older, more polluting two-stroke auto-rickshaws and taxis pushed operators towards cleaner vehicles. Converted three-wheelers became the cheapest way to run, and the word “CNG” entered everyday speech for the vehicles themselves.
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Mid-2000s onward
A nationwide network
The refuelling network spread beyond Dhaka and Chittagong to most district towns, and conversion, maintenance and cylinder re-testing became an industry in their own right.
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Today
A fleet in transition
CNG remains the backbone of urban passenger transport while the country weighs gas supply, emissions rules and newer alternatives. Older cylinders reaching re-test and retirement age keep re-testing centres busy.
Process
What a conversion involves
A professional conversion follows a repeatable sequence. The exact components depend on the engine and the kit used.
- Vehicle assessment The vehicle is inspected for engine condition, available mounting space and suitability for a bi-fuel conversion.
- Kit installation A CNG conversion kit is fitted: reducer/vaporiser, gas injector rail or mixer, ECU, wiring and changeover switch.
- Cylinder mounting A certified cylinder is mounted securely, usually in the boot or under the chassis, with correct valve and piping.
- Calibration & test The system is calibrated for the specific engine, leak-tested and a road test confirms performance and changeover.
- Documentation Registration of the conversion and the cylinder, with the first inspection date scheduled for periodic re-testing.
Components
What sits in the conversion kit
Eight parts do the work of turning high-pressure gas into a controlled fuel supply for the engine.
Cylinder valve
The manual shut-off valve on the cylinder, opened only when the vehicle is running on gas.
High-pressure piping
Stainless or copper-nickel line from cylinder to engine bay, secured and protected from chafe and heat.
Filling valve
A non-return filling point, usually in the engine bay, where the station dispenser couples to the vehicle.
Reducer / vaporiser
Reduces cylinder pressure to low pressure and uses engine coolant to warm the gas and prevent icing.
Mixer or injector rail
Adds gas to the air charge: a mixer on older carburettor engines, an injector rail on modern multipoint engines.
Gas ECU
A dedicated controller that manages gas quantity and petrol changeover, working alongside the petrol ECU.
Changeover switch
The dashboard switch showing fuel mode and the gas level, and switching between petrol and gas automatically.
Safety
Cylinders and periodic re-testing
Certified cylinders
Vehicle cylinders are manufactured and certified to a recognised standard, with the manufacture date and test pressure stamped on the cylinder neck or label. Only cylinders approved by the relevant authority should be fitted.
Service life
Cylinders have a defined service life and must pass hydrostatic re-testing at the required interval. An expired test means the vehicle should not run on CNG until the cylinder has been tested and certified again.
Everyday checks
Drivers should watch for the smell of gas, hissing near the cylinder or regulator, warning lights, and any damage to mounting brackets or piping. A cylinder valve should be closed when the vehicle stands for long periods.
Refuelling
At the filling station
At a CNG station the dispenser couples to the vehicle's filling valve and fills the cylinder to its rated pressure. Stations normally fill from cascade storage in several pressure steps, so the flow slows down towards the end of the fill — a full fill simply takes a few minutes.
- Switch the engine to petrol and switch off before filling
- Do not smoke or use open flame in the filling area
- Stay with the vehicle; never leave a fill unattended
- If the filler or hose shows damage or a smell, refuse the fill
- Watch the station's pressure gauge: a weak fill can indicate a station running low on storage pressure
The same cylinder valve discipline applies after filling: the valve is opened for driving and closed when the vehicle is parked for extended periods.
Running costs
What the fuel actually costs
The saving from CNG depends entirely on the prices of CNG, petrol and diesel at the time. Those prices change with tariff decisions, so this site shows them as configurable placeholders rather than quoting figures that would go stale.
- CNG price per litre equivalent
- set in includes/config.php once verified
- Petrol price per litre
- set in includes/config.php once verified
- Diesel price per litre
- set in includes/config.php once verified
- Figures last checked
- not yet verified
How to use these figures. Enter verified prices in
includes/config.php (FIGURES) and they will
appear here automatically. Until then, treat every cost comparison
you read elsewhere as a snapshot with a date on it — and
remember that conversion cost, cylinder re-testing and maintenance
belong in any honest comparison, not just the price at the pump.
Environment
Environmental effects
What improves
- No soot or particulate emissions from the exhaust
- Much lower carbon monoxide and unburnt hydrocarbons than petrol, when the engine is correctly converted
- Lower carbon dioxide per unit of energy than petrol or diesel, because methane burns efficiently
- Quieter combustion than diesel engines
What to be honest about
- CNG is still a fossil fuel — carbon dioxide is still produced
- Old or badly calibrated kits release unburnt methane, a potent greenhouse gas
- A worn engine or faulty injector rail can eliminate the emissions advantage entirely
- Leakage at stations and on vehicles adds to the methane footprint if maintenance is neglected
Maintenance
Keeping a converted vehicle safe
- Service the kit at the interval recommended by the kit manufacturer — filters, hoses and clamps wear too
- Leak-test the high-pressure side whenever the cylinder or piping is disturbed
- Keep the cylinder test date current and carry the certification documents
- Replace hoses and lines at their service interval, not when they fail
- Book re-testing before the due date — an untested cylinder means the vehicle cannot legally run on gas
If you smell gas. Switch to petrol, close the cylinder valve, stop safely and have the system checked before continuing. Do not investigate with a flame, and avoid operating electrical switches in a gas-filled space.
Workshops matter. Conversion quality decides both fuel economy and safety for years. See the conversion workshop and re-testing centre pages for what a professional service covers.
Looking for a conversion workshop or re-testing centre?
See the Products & Services overview for the four service areas.