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Which is better alternative fuel (CNG-NGV vs LPG) for us all?



Which one is the better alternative fuel (CNG-NGV vs LPG) for us all? This is the question most people are asking and seeking an absolute answer.  We strongly believe the answer depends on the availability of such alternative fuel in your  country. Countries blessed with natural gas (or crude oil reserves) are able to tap and domestically produced CNG (or LPG) as the alternative fuel source. The best means to know whether your country offers CNG or LPG is through a check at your local Pump Stations. Alternatively, you can ask whether your local public transport companies, like taxicabs or buses, are  using CNG or LPG as a fuel to power their fleet. 


It is important to recognise that 'readily available' is the key factor for you. There are many Alternative Fuel sources, but many are not made available at local pump stations. In most circumstances, your ambition to convert your vehicle to operate Alternative Fuel (LPG or CNG) is restricted by the availability of either LPG or CNG at your pump stations.




CNG (Methane) is used as a clean alternative to other automobile fuels such as (petrol) gasoline and diesel fuel. As of 2005, the countries with the largest number of natural gas vehicles were Argentina, Brazil, Pakistan, Italy, India, Bangladesh and eventually PR China. The CNG energy efficiency is generally equal to that of petrol (gasoline) engines, but lower compared with modern diesel engines, partially due to the fact that natural gas engines function using the Otto-cycle, but research is on its way to improve the process (Westport Cycle - http://www.cumminswestport.com). LPG (a propane and butane blend) is also used to power vehicles.


LPG and CNG vehicle fuel systems are NOT compatible. CNG also requires higher pressure tanks (withstanding 3,000 psi refilling pressure) which are typically much heavier than those used for LPG (200psi or more refilling pressure). Most accidents happen when it is mistakenly believed that CNG tanks are similar (same) to LPG storage technologies.





For the auto industry today, designing, developing and marketing "green" cars has not been an easy task for all alternative fuel participants. This is the underlining reason why petrol (gasoline) and diesel powered vehicles still rule the road and fossil fuels still account for almost 75 percent of the world's energy consumption. As crude oil prices soar and concerns over harmful emissions increase, alternate fuel sources are becoming enormously important. A natural-gas vehicle, or NGV, is the perfect example of such a car as it is fuel-efficient, environmentally friendly and offers an immediate alternative as a Bi-Fuel vehicle.  


Natural Gas (fossil-methane) is created from the decay of ancient organisms underneath the earth, as like fossil-based crude oil. This process commonly occurs in association with crude oil. Alternatively, it can be derived from both landfill, aquatic organic matters and forms above or below oil deposits layer. Raw natural gas comes primarily from any one of three types of gas wells: crude oil wells, gas wells, and condensate wells. Natural gas that comes from crude oil wells is typically termed associated gas. This gas can exist separate from the crude oil in the underground formation, or dissolved in the crude oil. Natural gas from gas wells and from condensate wells, in which there is little or no crude oil, is termed non-associated gas. Gas wells typically produce only raw natural gas, while condensate wells produce raw natural gas along with a very low density liquid hydrocarbon called natural gas condensate (sometimes also called natural gasoline or simply condensate. Raw natural gas can also come as methane deposits in the pores of coal seams. Such gas is referred to as coalbed gas and it is also called sweet gas because it is relatively free of hydrogen sulfide.


Picture Source: www.naturalgas.org

Natural Gas Well



Natural Gas (bio-methane) typically refers to a (biofuel or biogas) gas produced by the anaerobic digestion or fermentation of organic matters including manure, sewage sludge, municipal solid waste, biodegradable waste or any other biodegradable feedstock, under anaerobic conditions. Biogas is comprised primarily of bio-methane and carbon dioxide. If biogas is cleaned up sufficiently, biogas has the same characteristics as natural gas. In this instance, the producer of the biogas can utilize the local gas distribution networks. The gas must be VERY clean to reach pipeline quality. Water (H2O), hydrogen sulfide (H2S) and particulates are removed if present at high levels or if the gas is to be completely cleaned. Carbon dioxide is less frequently removed, but it must also be separated to achieve pipeline quality gas. If the gas is to be used without extensive cleaning, it is sometimes cofired with natural gas to improve combustion. Biogas cleaned up to pipeline quality is called renewable natural gas. In some cases, landfill gas contains siloxanes. During combustion of biogas containing siloxanes, silicon is released and can combine with free oxygen or various other elements in the combustion gas. Deposits are formed containing mostly silica (SiO2) or silicates (SixOy) in general, but can also contain calcium, sulphur, zinc, phosphor… as indicated by the analysis piston scrapings from biogas-fired engines. These (mostly white) deposits can ultimately build to a surface thickness of several millimetres and are difficult to remove by chemical or mechanical means. In internal combustion engines, deposits on pistons and cylinder heads are extremely abrasive and even a small amount is sufficient to cause enough damage to the engine to require a complete overhaul at 5,000 hours or less of operation. The damage is similar to that caused by carbon build up during light load running of diesel engines. Deposits on the turbine of the turbocharger will eventually reduce the charger’s efficiency. Stirling engines are more resistant against siloxanes, though deposits on the tubes of the heat exchanger will reduce the efficiency.


Picture Source: www.prometheus-energy.com 

Biogas Landfill




Key safety features of a CNG-NGV conversion includes systems that automatically shut off the CNG cylinder and CNG regulator when the ignition is turned off, or when the engine stalls, or when fuel pipes are damaged. The on-board CNG cylinder is also a major key to safety. Accordingly, all CNG cylinders are rigorously tested and designed to withstand high pressure refilling, high impact and extreme temperatures. In fact, CNG cylinders must comply with ISO11439 or NZ5454 standards for Unfired Pressure Vessel. All CNG cylinders are designed to a pressure limit of 350 bar. At this high pressure regime, CNG cylinders can be built in one shape (tubular or cylindrical), but in different sizes to hold different capacities.


Picture Source: www.mjcengineering.com

CNG cylinders is commonly known as seamless cylinder (or zero-welding cylinder).

 Seamless CNG Cylinder



Picture Source: www.dynetek.com

This images show the CNG cylinders that's installed on-board the vehicles with various configuration.

NGV Cylinder Installation 3

 Rear Boot Mounting for Bi-Fuel Vehicle (After Market Conversion)

NGV Cylinder Installation 2

 Undercarriage Mounting for Bi-Fuel Vehicle (OEM or Factory Fitted)








When LPG is used to fuel internal combustion engines, it is often referred to as Autogas. In some countries, it has been used since the 1940s as an alternative fuel for spark ignition Otto-engines. More recently, it has also been used in diesel engines.


In highly purified form, a blend of the LPG constituents propane and iso-butane is used to make hydrocarbon refrigerants, which are increasingly being used in servicing mobile air conditioning systems. This is because of concerns about the global warming effect of the widely used HFC 134a (Global Warming Potential 1410), and because hydrocarbons are more energy efficient, run at lower pressure and are generally cheaper than HFC 134a.


About 60% of the world's supply of LPG comes from the separation of natural gas products, and 40% is a by-product from the refining of crude oil. In the past, LPG has been considered as waste and flared off; now it is recognised as a major energy source. It is produced in vast quantities by petroleum companies and exported worldwide as Autogas as well as utility gas. It is particularly abundant in the North Sea’s ‘wet’ crude oil and offers security of fuel supply for many years.




Key safety features of a conversion include systems that automatically shut off the LPG tank when the ignition is turned off, or when the engine stalls, or when fuel pipes are damaged. The on-board tank is also the key to safety. Accordingly, all Autogas tanks are rigorously tested and designed to withstand high impact and extreme temperatures. In fact, LPG tanks can survive far greater impact than diesel or petrol tanks. Most LPG tanks are designed to a pressure limit of 400 psi (as compared to CNG tank at 3,000psi). At this low pressure regime, LPG tank can be built in various sizes and shapes (whereas CNG tanks are tubular or cylindrical in shape and must be zero-welded).



Picture Source: http://www.lpg-vehicles.co.uk

This image shows the vehicle gas tanks which come in a variety of shapes and sizes.

LPG Tank

These are known as “Torpedo” and “Donut” tanks.

Warning: LPG Tanks should NEVER be substituted as a CNG Cylinder.

LPG Tanks will EXPLODE at CNG refillingl!.


The main engine components in a LPG Autogas vehicle.

LPG Engine Kit

Clockwise from left: Gas Injectors, Vaporizer Unit & Engine Control Unit.

Here we see the gas tank installed into the boot of the vehicle – in this case in the spare wheel well.

LPG Boot


The dash board switch to alternate between gas and petrol.

LPG Switchover

On this model, there’s also an indicator to show how full the tank is.


The refuelling point at the rear of the vehicle, which is usually situated near the existing petrol point.

LPG Refilling valve


After all the components have been fitted, the LPG system is calibrated and tuned to fully integrate with the standard engine control unit.


Other Website Resources:

(a) http://en.wikipedia.org/wiki/Natural_gas

(b) http://www.naturalgas.org

(c) http://www.prometheus-energy.com

(d) http://www.lpg-vehicles.com.uk










Compiled and presented by Dr. Xander Thong and Alex Theng

(If you have noticed any error or inaccuracy, kindly email and notify to drxander@ngvcommunity.com)






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