LPG Compared to CNG – Methane – Compressed Natural Gas
CNG is Compressed Natural Gas (methane) in storage cylinders. The biggest difference between CNG and LPG cars is the fuel itself. CNG is methane and LPG is propane and/or butane. CNG is stored as a gas and LPG is stored as a liquid.
CNG gas is compressed natural gas storage – methane. LPG is liquefied petroleum gas – propane.
LPG – Liquefied Petroleum Gas or Liquid Petroleum Gas, also called just propane or butane, are both flammable hydrocarbon gases used as fuel for heating, cooking and vehicular fuel.
LPG can be compressed into a liquid, increasing its energy density.
CNG cylinders have a tare weight approximately 3x heavier than comparable capacity LPG cylinders.
There are many similarities between LPG and CNG, as well as some major differences…
CNG Acronym – CNG Fuel Meaning
CNG is an acronym abbreviation for Compressed Natural Gas.
CNG fuel meaning is compressed natural gas, which is primarily methane.
What are the Differences Between LPG vs CNG Gas (Natural Gas)?
Difference Between CNG vs LPG – Properties
What is the Difference Between CNG vs LPG? |
||
| Properties | CNG – Methane | LPG – Propane |
| Chemical Formula | CH4 | C3H8 |
| Energy Content | 9MJ/L | 25MJ/L |
| Storage Pressure | 20 – 25 MPa | 2 MPa |
| Air:Gas Combustion Ratio | 10:1 | 25:1 |
| Operating Pressure | 1.1 kPa | 2.75 kPa |
| Density (vs Air) | .5537:1 | 1.5219:1 |
| Cylinder Weight | 1 | ≈3x |
| State | Gas | Liquid or Gas |
• LPG (propane) has a higher energy content than CNG (methane), with 25MJ/L versus 9MJ/L, respectively.
• Storage pressure of LPG is less than 2 MPa whilst CNG compressed natural gas storage is 20 – 25 MPa.
• For proper combustion, LPG requires an air to gas ratio of approximately 25:1 whilst the air to gas ratio for natural gas is a 10:1 ratio.
• LPG and natural gas appliances operate at different pressures – 2.75 kPa vs 1.1 kPa, correspondingly.
• LPG (propane) is denser than air at a relative density of 1.5219:1 vs natural gas (methane) at 0.5537:1, which is lighter than air.
• CNG – compressed natural gas storage cylinders have a tare weight approximately 3x heavier than comparable capacity LPG cylinders.
• LPG can be compressed into a liquid, increasing its energy density whilst CNG is stored as a gas.
Natural Gas Formula – Chemical Formula for Methane – CNG is CH4
The chemical formula of CNG – CH4 – is also the chemical formula for Natural Gas and is also the chemical formula for Methane.
A methane molecule contains one carbon and 4 hydrogen atoms and is the simplest of the flammable hydrocarbons.
This also means that CNG has the smallest carbon footprint of any hydrocarbon.
The chemical composition of CNG – chemical formula of CNG is CH4 – is actually the chemical formula for methane, which is the primary constituent of refined compressed natural gas (CNG).
To be accurate, this natural gas formula is actually the chemical formula for methane, which is the primary constituent of refined natural gas and CNG.
Raw natural gas, straight out of the ground, also contains natural gas liquids and various impurities.
Chemical Composition of CNG (Compressed Natural Gas) vs LPG
LPG (propane) and natural gas (methane) have different chemical compositions or formulas.
CNG chemical formula is CH4, which is methane.
The chemical composition of LPG is C3H8, which is also the chemical formula of LPG (propane).
What are the Key Similarities between LPG vs CNG Gas?
• Both LPG and CNG gas can be used in traditional home and business applications, as well as for vehicle fuel.
• Both are domestically produced energy sources, reducing reliance on imported oil.
• LPG and CNG both have greatly reduced greenhouse gas emissions, compared to petrol and diesel.
• Both CNG and LPG are stored in cylinders or tanks, making them transportable fuels.
Compressed Natural Gas Storage – CNG Storage vs LPG Storage
Compressed natural gas storage – CNG storage – is mostly methane stored under very high pressure of 20 – 25 MPa vs LPG at 2 MPa. CNG remains gaseous under pressure vs LPG that liquefies. LPG is also stored in gas cylinders but in its liquid form versus CNG, which is stored as gas.
Compressed natural gas (CNG) storage can also be in larger vessels or vehicle fuel tanks.
LPG can be stored in very large LPG tanks, called ‘bullets’, or even in giant spheres, known as Horton Spheres.
Transportation is typically by truck for both.
Compressed natural gas storage can be used for CNG vehicles or domestic applications.
Uses of LPG and CNG Gas
The uses of LPG and CNG gas are similar with both used as fuel in vehicles, to power appliances and fuel industrial processes.
They both can be used in place of petrol (gasoline) or diesel fuel in cars, trucks and stationary power generation.
Other uses of LPG – liquefied petroleum gas – and CNG – compressed natural gas (methane) – include cooking, heating and hot water.
CNG can also be used in place of piped natural gas, as can LNG.
LPG & CNG can also be used as substitutes for each other, with adjustments to the vehicle or appliance.
LPG can be used as an aerosol propellant whilst CNG cannot, as methane released into the atmosphere is a greenhouse gas.
Difference Between CNG and LPG Cars
The difference between CNG and LPG cars is storage pressure, with CNG at 10 times the pressure with 20-25 MPa vs 2 MPa for LPG. LPG has a higher energy content so a much greater cruising range with the same size tank.
There are many more LPG dealers, too.
The biggest difference between CNG and LPG cars is the fuel itself.
CNG (Compressed Natural Gas) is methane (CH4) and LPG (Liquefied Petroleum Gas) is propane (C3H8) and/or butane.
One of the other key differences between CNG and LPG cars relates to energy density.
CNG – compressed natural gas storage has a lower energy density than LPG.
This means you typically lose more boot space with CNG because you need a larger tank capacity to offset the difference in compressed natural gas storage .
Even with this, you may still have a reduced cruising range.
Another difference between CNG and LPG cars is the tank pressure. LPG pressure is less than 2 MPa whilst CNG – compressed natural gas storage is 20 to 25 MPa.
This means high pressure CNG tanks are heavier and more expensive than LPG tanks.
State-of-the-art direct liquid injection is only available in LPG, providing a potential performance advantage over CNG.
You should also check local fuel availability before choosing, as it may vary between the two fuels.
Petrol and diesel vehicle owners contemplating conversion to CNG and LPG should also be aware of the advantages.
Both LPG and CNG are much cleaner fuels than petrol or diesel.
They both have fuel systems that are sealed. This avoids any leakage or evaporative losses.
Both CNG and LPG also have lower maintenance costs, including greatly reduced fouling of the spark plugs.
Similar Sources
LPG and CNG gas both come from drilling oil wells and gas wells, and are both hydrocarbon fuels.
LPG (propane) and CNG (methane) are typically found together.
They must be separated before they can be used commercially.
LPG is also produced during natural gas processing and petroleum refining and is also referred to as natural gas liquids – NGL.
LPG processing involves the separation from raw natural gas or collection of the gas from its petroleum base.
Major Differences
The big difference is in volumetric energy density.
LPG has almost 3x the energy density with 25MJ/L for LPG versus only 9MJ/L for CNG – compressed natural gas storage.
Another major difference is that compressed natural gas storage pressures can be more than 10x that of LPG.
LPG pressure is less than 2 MPa whilst CNG – compressed natural gas storage is 20 – 25 MPa.
In other words, for comparable capacity cylinders, the LPG cylinders will last more than twice as long.
This much higher compressed natural gas storage pressure requires a much heavier and more expensive cylinder or tank.
This also means it is impractical to make them large enough to offset the energy density deficit, due to the size and weight required.
Density of CNG in kg/M³ vs Density of LPG in kg/M³
Density of CNG in kg/M³ is 0.656 kg/M³ at 25°C or 0.04095lb/ft³ at 77ºF.
Density of LPG in kg/M³ is 1.899 kg/M³at 15ºC.
Air to Gas Ratio for Natural Gas (CNG) vs LPG
The air to gas ratio for natural gas (CNG) is lower than for LPG.
For proper combustion, the air to gas ratio for natural gas is a 10:1 ratio.
LPG requires an air to gas ratio of approximately 25:1.
So, LPG requires 2½ times the air vs the air to gas ratio for natural gas.
Which Natural Gas is More Powerful, CNG or LPG?
If by more powerful, you mean which one contains more energy, the answer is LPG.
As previously mentioned, CNG only has about 1/3 the energy density of LPG (propane) with 9MJ/L versus 25MJ/L, respectively
LPG can also be compressed into a liquid, increasing its energy density.
Why Can’t We Use CNG Gas Instead of LPG as Home Gas?
Due to the lower energy capacity of a CNG gas cylinder, they would need to be replaced more than twice as often.
This means the homeowner would have the inconvenience of checking the cylinders and placing orders much more frequently.
The homeowner would also have to absorb the extra delivery costs involved for the extra deliveries and because high pressure CNG cylinders weigh more almost 3x more.
For example, an empty 45kg LPG cylinder only weighs approximately 35kg.
An empty CNG gas cylinder, with comparable volume capacity, is about 108kg.
This also makes it virtually impossible to increase the cylinder size to hold an equivalent amount of energy, as the cylinder would then be too large and too heavy to transport for exchange purposes.
Higher cylinder rental is also likely, as the heavier CNG gas cylinders cost more.
Finally, the equipment to fill the CNG gas cylinders is very expensive – much more than LPG.
The supplier would need to offset these higher costs in the cost of the gas.
Ultimately, it is the volumetric energy density and pressure that influence the comparable convenience and affordability of use.
Different Gas Regulators, Piping and Fittings
Natural gas and LPG appliances operate at different pressures, with 1.1 kPa and 2.75 kPa, respectively.
The incoming pressure to the regulator is also very different, as CNG – compressed natural gas storage has 10x higher pressure.
This means the homeowner must replace the home LPG gas regulator to use CNG.
The gas piping may or may not be suitable for both gases, as the volume of natural gas required is much higher.
If the gas fitter determines that the pipe sizing is incompatible, it would also need to be replaced.
Various fittings would also require replacement, including the gas bottle connections.
Appliance Conversion
There is also the cost of converting the gas appliances, if you switched from LPG to CNG.
LPG (propane) and natural gas (methane) appliances are not interchangeable — they must be converted.
As with the regulators, the gas appliances operate at different pressures.
LPG also requires an air (oxygen) to gas ratio of approximately 25:1 whilst natural gas is 10:1, for proper combustion.
To achieve this difference, LPG is typically provided in a smaller quantity but at a higher pressure (2.75 kPa vs 1.1 kPa), drawing more oxygen with it into the burner.
Conversion requires the purchase of a conversion kit from the manufacturer and the labour cost for having a gas fitter make the changes.
Gas appliances conversions should only be done by qualified technicians or licensed gas fitters.
This assumes that there is even a conversion kit available.
Conversion kits are not available for many older gas appliances, necessitating the purchase of a new gas appliance.
CNG Price & Lack of Competition
The CNG price is obviously linked to the local cost of natural gas.
The cost of the more expensive high pressure compressed natural gas storage cylinders are also likely to increase the CNG price of cylinder rental, when compared to LPG.
The rarity of CNG gas suppliers for home gas means there is little, if any, competitive pressure to keep CNG prices reasonable.
So, an initially attractive CNG price may vanish quickly, after the homeowner is locked in by the cost of conversion.
Can Natural Gas be Stored in a Tank? Natural Gas Bottles & CNG Tank
Natural gas can be stored in a tank as either CNG – Compressed Natural Gas – or as LNG – Liquefied Natural Gas.
Natural gas bottles contain the CNG – compressed natural gas storage.
The compressed natural gas storage is at the very high pressure pressure of 20–25 MPa (2,900–3,600 psi).
However, the energy density of CNG gas is relatively low, so the fuel tanks take up more space than LPG.
The cost and size of the high pressure compressed natural gas storage gas bottles (CNG tank), as well as the expensive compressor equipment, make a CNG filling station a significant investment.
How to Fill a Propane Tank with Natural Gas?
You could fill an LPG – propane – tank with natural gas but it would need to be at a lower pressure than with compressed natural gas storage – a CNG gas tank.
LPG tanks are just not as heavy and strong as CNG tanks because they are not normally required to sustain high pressure contents.
The end result is that there will be less CNG contained in the LPG tank, as the pressure and resultant energy density will be lower.
Cost of CNG Service Stations
The high cost of the refilling gear is the major barrier to wider adoption of CNG gas as vehicle fuel.
This is why it is commonly used for vehicles operating out of central terminals, as is the case with buses.
The very high and captive usage rate makes the investment viable.
Different Cruising Ranges & Boot Space
Assuming they have the same sized fuel tanks, LPG has a much greater cruising range, due to the higher energy content.
CNG gas fueled vehicles typically have a much larger and heavier tank because of the compressed natural gas storage, usually infringing on boot space. (see accompanying image)
Even with a larger tank, many CNG vehicles still have a diminished cruising range.
In contrast, the new toroidal LPG tanks are doughnut shaped and fitted within the spare wheel well, with no boot (trunk) space lost. (image below)
Availability for Vehicular Fueling
LPG – Autogas – has much more availability than CNG gas.
CNG gas refueling stations are sparse, to say the least.
This is particularly true outside of metropolitan areas.
In combination with the lesser cruising range, this makes trips to certain country areas very difficult, if not impossible.
One could get very anxious when the fuel warning light comes on, when outside of a metro area.
Home Refueling is Not Viable
If you are thinking that you can refuel a CNG gas car at home, you may wish to reconsider that concept.
Natural gas in a home is under very low pressure.
To fill from home would require a system that is a combination pump and compressor.
The process is very slow, so usually done overnight.
There are two other problems, aside from the refilling time.
The first drawback is the cost of the refueling system, at upwards of $10,000.
The second is fuel contamination.
Honda stopped selling its home CNG refueling unit because of moisture and other contaminants found in many ordinary natural gas supplies.
Final Thoughts
While CNG gas is an excellent choice for vehicle fleets that operate out of terminals, availability and low energy density make it a very questionable choice for cars or home use.
The existing and extensive LPG infrastructure makes LPG a much better choice for cars, in addition to the cruising range advantage.
The much greater energy density of LPG also make it the superior solution for home bottled gas users, with longer lasting cylinders.
Eric Hahn has over a decade of experience in the LPG industry and has become a key figure in shaping how LPG is marketed and communicated. As the creator of one of the world’s first LPG blogs, Eric has authored over 200 technical posts, helping educate and inform the industry and consumers.
Eric has served on multiple committees for Gas Energy Australia and the World LPG Association (WLPGA), contributing to important discussions and advancements within the global LPG sector. He has presented at WLPGA Annual Conferences, sharing his expertise as a speaker, panellist, and panel chair.
- Elgas to the Rescue with Helicopter LPG Delivery - November 14, 2024
- Gas Patio Heater Guide: Outdoor Gas Heaters – Outdoor Mushroom Heater - September 22, 2024
- LPG Meaning – LPG Means: What Does LPG Stand For - August 31, 2024
Steve Reynolds
Technical Consultant
Steve Reynolds is a leading expert in the LPG industry with over 22 years of experience. As part of the national management team at ELGAS, Steve ensures the safe and efficient storage, handling, and transportation of LPG. He serves as the lead investigator for incidents and collaborates with authorities on industry developments.
Steve is a technical advisor to Standards Australia and Gas Energy Australia (GEA), and an active member of the World LPG Association (WLPGA), contributing to global standards and technical reviews. He holds a BSc. (Hons) in Industrial Chemistry from UNSW and has held senior safety and technical roles at ELGAS, making him a trusted authority in LPG safety and standards.