Can LPG be Used in CNG Cars?
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. The biggest difference between CNG and LPG cars is the fuel itself. CNG is methane and LPG is propane and/or butane.
CNG is compressed natural gas – methane.
LPG is liquefied petroleum gas – propane.
There are many similarities between LPG and CNG, as well as some major differences.
They are not interchangeable as automobile fuel…
What are the Differences between LPG vs CNG (Natural Gas)?
• LPG has a higher energy content than CNG, with 25MJ/L versus 9MJ/L, respectively.
• Storage pressure of LPG is less than 2 MPa whilst CNG is 20 – 25 MPa.
• For proper combustion, LPG requires an air to gas ratio of approximately 25:1 whilst natural gas requires a 10:1 ratio.
• 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 cylinders have a tare weight approximately 3x heavier than comparable capacity LPG cylinders.
• LPG can be compressed into a liquid, increasing its energy density.
• LPG (propane) and natural gas (methane) have different chemical formulas: Methane is CH4.
• Propane is C3H8.
Difference Between CNG and LPG Cars
The biggest difference between CNG and LPG cars is that CNG cars are runnuing on compressed methane whilst LPG cars are running on LPG - Liquefied Petroleum Gas.
Another 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.
In other words, for comparable capacity cylinders, the LPG cylinders will last more than twice as long.
The third major variance is that CNG is stored at pressures that can be more than 10x that of LPG.
LPG pressure is less than 2 MPa whilst CNG is 20 – 25 MPa.
This much higher pressure requires a much heavier and more expensive fuel 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.
CNG vs LPG for Cars and other Vehicles
Petrol and diesel vehicle owners contemplating conversion to CNG and LPG should also be aware of the differences.
Both LPG and CNG are much cleaner fuels than petrol or diesel.
They both have fuel systems are sealed.
This avoids any leakage or evaporative losses.
They both also have lower maintenance costs, including greatly reduced fouling of the spark plugs.
Can LPG be Refilled with CNG in Running Cars?
No, absolutely not.
As explained above, they are different gases with different combustion properties.
They are not interchangable in the filling of cars.
For proper combustion, LPG requires an air to gas ratio of approximately 25:1 whilst natural gas requires a 10:1 ratio.
This alone would prevent the car from running properly and could create a very dangerous safety threat.
Cost of CNG Service Stations
The high cost of the refilling gear is the major barrier to wider adoption of CNG 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.
However, CNG fuelled vehicles typically have a much larger and heavier tank, 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 Fuelling
LPG – Autogas – has much more availability than CNG.
CNG refuelling 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 Refuelling is Not Viable
If you are thinking that you can refuel a CNG 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 refuelling system, at upwards of $10,000.
The second is fuel contamination.
Honda stopped selling its home CNG refuelling unit because of moisture and other contaminants found in many ordinary natural gas supplies.
While CNG 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.
Comments, questions or feedback?
The information in this article is derived from various sources and is believed to be correct at the time of publication. However, the information may not be error free and may not be applicable in all circumstances.