CNG and LPG may seem the same: both are gases that enable more efficient travel, with lower emissions than petrol or diesel. But they have less in common than it may first appear. In this clear, fully informative guide, you will learn how they differ, when each one is most suitable, and what to consider before making your choice.
What is CNG?
CNG (compressed natural gas) is natural gas, primarily methane (CH₄), stored at high pressure (typically 200–250 bar) in a gaseous state.
- Origin: natural gas deposits or purified biomethane.
- Form of storage: in cylindrical steel or composite cylinders.
- Typical use: cars, vans, and urban fleets (taxis, delivery, buses) that make repeat journeys.
What is LPG?
LPG (liquefied petroleum gas) is a mixture of propane and butane that liquefies at low pressure (around 5-10 bar, depending on temperature).
- Origin: by-product of oil refining or natural gas processing.
- Storage: in liquid state inside a moderate-pressure tank.
- Typical use: cars converted to LPG (autogas), inter-urban fleets, and vehicles that alternate between city and motorway driving.
Composition and properties
- CNG: high methane content → higher octane rating and cleaner combustion (fewer particles).
- LPG: propane/butane mixture → more energy per litre when liquefied, which translates into greater range with similar tanks.
Pressure, energy density, and range
- Working pressure: CNG (very high) vs. LPG (moderate).
- Energy density: similar on a per-kilogram basis, although per litre LPG (being a liquid) stores more energy than gaseous CNG.
- Range: with the same tank volume, LPG usually offers more kilometres. CNG partly makes up for this by using higher‑pressure or additional tanks.
Fuel and system cost
- Price per kg/litre: varies by market, but both are usually cheaper per kilometre than petrol.
- Conversion/vehicle:
- CNG: requires high-pressure cylinders, specific valves, and pipework → somewhat more expensive, heavier systems.
- LPG: toroidal or cylindrical tank and specific injection rail → more widespread conversions and often more economical.
- Maintenance: similar to a petrol vehicle, with periodic inspections of the gas system and tank according to regulations.
Performance and driving experience
- CNG: smooth operation, good urban performance, slight loss of power compared to petrol in some engines, which can be compensated for by calibration.
- LPG: very close to petrol in terms of performance; smaller perceived power loss than CNG in equivalent conversions.
Emissions and sustainability
- CO₂: both reduce emissions compared to petrol; CNG usually has an advantage in CO₂ per km due to its higher hydrogen content in methane.
- NOx and particulates: significantly lower than diesel.
- Biofuels:
- CNG can use biomethane (bio‑CNG) to further cut CO₂ emissions (achieving an even lower net carbon footprint when the biogas is produced from waste).
- Renewable LPG (bio-LPG) exists, but its availability is more limited.
Safety
Both fuels are safe if the facility is approved and well maintained.
- CNG: lighter than air; in the event of leaks, it tends to disperse upwards. Very robust tanks, with overpressure valves.
- LPG: heavier than air; it can accumulate in low areas in the event of a leak, leading to ventilation systems and detectors being strictly implemented in workshops and car parks, as required by regulations.
Refuelling infrastructure
- CNG: specific stations with high-pressure compressors; coverage concentrated in urban and industrial areas.
- LPG (autogas): extensive network in many countries, often with broader coverage than for CNG, enabling long-distance travel.
Space and weight on board
- CNG: high-pressure cylinders take up more space and add weight; may reduce boot space unless specific designs are used.
- LPG: toroidal tank can go in the spare wheel well, encroaching less on the boot space.
Regulations and inspections
- Require approved components and periodic roadworthiness inspections of the gas system and tank (intervals set by local regulations).
- Access to low-emission zones: in many cities, CNG and LPG vehicles are eligible for a low emissions sticker, although it is advisable to check the local regulations.
Quick comparison table
| Criterion | CNG (Compressed Natural Gas) | LPG (Liquefied Petroleum Gas) |
| Composition | Methane (CH₄) | Propane/Butane |
| State in tank | Gas at 200-250 bar | Liquid at 5-10 bar |
| Range (similar volume) | Lower | Higher |
| Price per km | Very competitive | Very competitive |
| Conversion cost | Medium–high | Medium |
| Weight/space of system | More weight and volume | Lower impact (toroidal) |
| Performance vs. petrol | Slightly lower | Very close |
| CO₂ emissions | Very low; better with biomethane | Low; variable |
| Charging network | Less widespread, urban/industrial | More extensive (autogas) |
| Safety in case of leaks | Gas rises and disperses | Gas can accumulate in low areas |